Lattice Transformations Related to Unique Mechanical Effects

JEFF PERKINS

Athermal and s t r e s s - i n d u c e d m a r t e n s i t i c t r a n s f o r m a t i o n s a re examined in va r ious al loys of the la rge fami ly which exhibit the unique " m e m o r y " and /o r " s u p e r e l a s t i c " shape m e m - ory effects (SME). Such mechan ica l effects a re found to be in t imate ly r e l a t ed to deta i ls of the m a r t e n s i t i c and p r e m a r t e n s i t i c r eac t ion paths in each sys tem. A common fea ture of va r ious " u n c o m m o n " sy s t ems is that the usual phenomenological c rys t a l log raph ic ana ly - s is cannot comple te ly desc r ibe the m a r t e n s i t i c t r a n s f o r m a t i o n in these s y s t e m s . Addi- t ional f ea tu res r e p r e s e n t e d by la t t ice " s h u f f l e s " or low-wavelength la t t ice waves, and the mechan i s t i c ro le of t r a n s f o r m a t i o n d is loca t ions a re examined. A common th read in v a r i - ous sy s t ems such as TiNi, CuZn, AuCd, In-T1, and so forth, is viewed in t e r m s of evidence r e l a t ed to a l loying (e lec t ronic entropy) effects on la t t ice ins tab i l i ty of the pa ren t phase. In- s tab i l i ty re f lec ted by p r e m o n i t o r y phenomena can be given cons ide rab le gene ra l i t y when r e l a t ed to obse rva t ions in s y s t e m s which exhibit s i m i l a r dynamic la t t ice t r ans i t i ons , such as the s e c o n d - o r d e r " a t h e r m a l o m e g a " la t t ice t r a n s i t i o n in Group W - b a s e sy s t ems . The impor tance of r e v e r s i b i l i t y in m a r t e n s t t i c t r a n s f o r m a t i o n of SME al loys is emphas ized . Compar i sons with more common non-SME m a r t e n s i t i c a l loys a re made.

T H E R E have been numerous reports of alloys possess- ing unique mechanical effects involving shape recovery, the best known being the "memory" or "shape-memory" effect in near-equiatomic TiNi alloys. In this case, and in the case of other systems and kindred effects, the be- havior has been related to martensitic transformation, and the effects will be referred to here generally as shape memory effects (SME). It is a general feature of ordered bcc (B2 or fl') phases to transform martensit- ically during cooling, and for this transformation to be quite sensitive to strain. In some systems, such as those based on the Cu-Zn/3' phase, the martensitic phase may form thermoelastically, during which pro- cess the martensite plates grow as the temperature is lowered, and shrink back upon heating, with little or no hysteresis, x

In other systems, a similar form of martensite de- velops and grows during continuous application of stress, and disappears when the load is removed. This reversible "elastic" (because no permanent strain is involved) martensite results in so-called "rubber-like" properties, and the phenomenon has been termed "superelasticity" or "pseudoelasticity." Superelastic behavior is known to occur in such martensitic/3' sys- tems as In-T1, z Au-Cd, 3 Cu-AI-Ni, 4 Cu-Zn-Si, and Cu- Zn-Sn. 5

The memory effect is much like the superelastic ef- fect except that upon release of stress the deformation- induced marteiasite does not revert until activated by heating above some reversion temperature. Such re- version, however, carries with it a return to the origi- nal (predeformation) shape, provided that the "plastic" strain is below a certain limit (the same sort of limit applies to the superelastic effect). The memory effect

JEFF PERKINS is Assistant Professor, Mechanical Engineering De- partment, Naval Postgraduate School, Monterey, Calif. 93940. This paper is based on a presentation made at a symposium on "Phase Trans- formations in Less Common Metals: A Dialogue," held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.

METALLURGICAL TRANSACTIONS

has been reported in such systems as Cu-AI-Ni, 6 Ni- A1, ~ and Cu-Zn, 8 as well as in the much publicized Ni- tinol alloys (TiNi), and informally alluded to for vari- ous other systems. Historically, as early as 1932, Olander 9 noted the striking rubberlike behavior of Au- rich Au-Cd martensite, which was followed in the 1950's by similar initial observations on In-T1, 2 Au-Cd, 3 Cu- Zn, I~ and Cu-A1-Ni. 4 In several Cu-Zn-X ternaries, reversible pseudoelastic strains of as much as 15 pct can be achieved. ~ Therefore, the plastic deformation recovery behavior of TiNi, discovered in the early 1960's at NOL, and often described as "unique," is ac- tually a type of phenomenon which is much more gen- eral than originally or recently suspected. However, the alloy systems involved in SME behavior are appar- ently diverse and in many cases less common. It will be attempted to present the common threads linking behavior in such systems, and to compare with mar- tensitic alloys which do not exhibit SME.

THERMOELASTICITY, SUPERE LASTICITY, AND SHAPE MEMORY

The occurrence of the various types of SME are in- terrelated, in that several alloy systems exhibit more than one of the effects, depending especially on compo- sition and thermomechanical history. For example, in several systems, on cooling, a thermoelastic marten- sitic reaction ("slow growth" martensite) is followed by a burst-type martensitic reaction which can be re- lated to superelastic behavior, s Other systems may exhibit either superelastic or shape memory behavior in different composition or temperature ranges, de- pendent on history, z'3 The three classifications are also not always mutually exclusive. As pointed out re- cently 6'12 all known shape-memory martensitic trans- formations apparently have thermoelastic character.

The nomenclature of the effects under discussion is varied, colorful, and ever-increasing. The most widely used terms are "shape memory" (or simply "mem- ory"), "superelastic," and "thermoelastic." Particu- larly in connection with the superelastic effect, a mul-

VOLUME 4, DECEMBER 1973-2709

Table I. Terminology of the Shape Recovery Effects

Shape Memory Superelastic 4 Thermoelastlc

Memory Rubberlike Slow-growth martenslte Marmen~2 Ferroelastic~3

Pseudoelasnc s Metaelastic ~a Anelastic Is

(Each term is usually applied prior to the word, "effect"). Note that in the his- torical development of these terms, the "rubberlike" and "ferroelasUc" terms were applied originally to the immediate-recovery effect in martensltic alloys, while "superelastic" and "pseudoelastic" referred only to alloys austeniUc before deformation. In either of these cases, the phenomenon is effectwely the same, but the mechanism apparently differs. The effects have been often referred to generally as shape memory effects, or simply SME.

t i p l i ca t ion of t e r m s has o c c u r r e d in the l a s t few y e a r s . Table I s u m m a r i z e s the t e r m i n o l o g y that has been used in connect ion with the v a r i o u s c l a s s e s of unique m e c h a n - i ca l e f fec t s . F o r example , b e c a u s e the s t r e s s - s t r a i n c u r v e s dur ing s u p e r e l a s t i c behav io r a r e not l i ne a r , and do not r e p r e s e n t the t rue e l a s t i c modulus of the m a t r i x , Pops ~ sugges t ed the t e r m " p s e u d o e l a s t i c " as m o r e a p - p r o p r i a t e than the o r i g i n a l t e r m coined by Rach inge r , 4 i . e . , " s u p e r e l a s t i c . " Of c o u r s e , the t e r m s s u p e r e l a s t i c and p s e u d o e l a s t i c could a l so both be app l ied with some jus t i f i ca t ion to the s h a p e - m e m o r y effect . Conve r se ly , a l l the e f fec t s involve " m e m o r y " of a p r ev ious cond i - t ion o r condi t ions to some d e g r e e . Because of the ana logy among the growth of f a v o r a b l y o r i en t ed (i) f e r - r o m a g n e t i c doma ins dur ing app l i ca t ion of a magne t i c f ie ld , (ii) f e r r o e l a s t i c domains dur ing app l ica t ion of an e l e c t r i c f ie ld , and (iii) t w i n - r e l a t e d domains dur ing a p - p l i c a t i on of s t r e s s , L i e b e r m a n 13 uses the t e r m f e r r o - e l a s t i c i t y for the s u p e r e l a s t i c effect . The re may, how- e v e r , be m o r e g e n e r a l i t y for the m e c h a n i s m on which F. C. F r a n k o r i g i n a l l y b a s e d th is c l e v e r t e rmino logy . Well known in Au-Cd, the growth of f a v o r a b l y - o r i e n t e d m e m b e r s of t w i n - r e l a t e d domains by the mot ion of m o - b i l e twin i n t e r f a c e s m a y be a g e n e r a l m e c h a n i s t i c f ea - t u re in shape r e c o v e r y e f fec t s exh ib i ted by m a r t e n s i t i c a l l oys . As poin ted out r e c e n t l y by Wayman, 8 a l l m a r - t e n s i t e s in which these e f fec t s have been r e p o r t e d , and in which the m a r t e n s t t i c m o r p h o l o g i e s a r e wel l -known, a p p a r e n t l y have i n t e rna l twinning r a t h e r than s l ip as the mode of l a t t i c e - l n v a r i a n t de fo rma t ion . Another t e r m sugges t ed for r u b b e r l i k e behav io r is m e t a e l a s - t i c i ty , '4 b e c a u s e the shape of the load -un load s t r e s s - s t r a i n c u r v e s t yp i ca l l y r e s e m b l e s the magne t i za t ion c u r v e s of m e t a m a g n e t i c m a t e r i a l s . Was i l ewsk i , who has been ac t ive in the f ie ld , ~5 p r e f e r s to r e f e r to the s u p e r e l a s t t c ef fec t a s " a n e l a s t i c t t y , " b e c a u s e of the s t r e s s h y s t e r e s i s e f fec t s involved. It would s e e m that th is t e r m is much too g e n e r a l to be useful . Among the v a r i o u s t e r m s app l i ed to the r u b b e r l i k e effect (which is an e x a g g e r a t i o n i t se l f ) , " s u p e r e l a s t i c i t y " and " p s e u - d o e l a s t t c i t y " f ind the m o s t usage at th is t ime .

PHENOMENOLOGICAL INTERPRETATIONS

The g e n e r a l q u a s i e l a s t i c behav io r ( s u p e r e l a s t i c o r shape m e m o r y ) in v a r i o u s s y s t e m s is a r e s u l t of the fac t that they undergo m a r t e n s i t i c t r a n s f o r m a t i o n s (often conven ien t ly in the v i c in i t y of room t e m p e r a t u r e ) which can r e v e r s e t h e m s e l v e s along the s ame path and can be a c t i v a t e d to do th is r e l a t i v e l y ea s i l y . Intui t ively,

2 7 1 0 - V O L U M E 4, DECEMBER 1973

one deduces that the t r a n s f o r m a t i o n mus t be r e l a t i v e l y s i m p l e , and f r ee of deve lopmen t s which could i m p a i r r e v e r s i b i l i t y . With the except ion of In-T1 and Cu-A1- NI, the high t e m p e r a t u r e s t r u c t u r e of the a l loys (and a l so TINi) is the bcc ~ -pha se b a s e d on B2 (CsCl) o r d e r and o c c u r s in the v i c in i ty of the equ la tomlc c o m p o s i - t ion. The high t e m p e r a t u r e phase of the In-T1 a l l oy is fee 16 and Cu-A1-NI has the DO3 s t r u c t u r e . 17 Known p a r e n t - p r o d u c t s t r u c t u r e s have been s u m m a r i z e d r a t h e r c o m p r e h e n s i v e l y in o the r r e c e n t r e v i e w s . ~2 The conclus ion is that t h e r e is no spec i f i c s t r u c t u r a l p r e - r e q u i s i t e for the p a r e n t phase . However , t h e r e a r e def in i te m e c h a n i s t i c r e q u i r e m e n t s for the path along which p a r e n t is conve r t ed to m a r t e n s i t e .

In the ca se of Cu-Zn ~'~8 C u - Z n - X a l loys 5'~9 and Cu- A1-Ni 4 the M s t e m p e r a t u r e is somewhat below room t e m p e r a t u r e and the f o r m a t i o n of m a r t e n s l t e is t h e r - moelastic. 4,5,18,2~ In this case, either a decrease in temperature below M s or application of stress causes formation and growth of martensite plates, and tem- perature or stress reversal results in shrinkage of the plates with little or no hysteresis. Thus the pseu- doelastic behavior in Cu-Zn-X alloys and Cu-A1-Ni is attributed to the stress-induced formation of marten- site from the high temperature B2 phase and almost immediate reversion when the stress is released. In effect , the A f t e m p e r a t u r e is below the M d t e m p e r a - t u r e .

A somewhat d i f fe ren t m e c h a n i s m is r e q u i r e d to e x - p la in the s u p e r e l a s t i c behav io r in AuCd and In-T1. Both AuCd z and In-T1 ~6 t r a n s f o r m m a r t e n s i t l c a l l y above r o o m t e m p e r a t u r e , and a r e c o m p l e t e l y m a r t e n s i t i c at r o o m t e m p e r a t u r e . Thus the s t r e s s - i n d u c e d f o r m a t i o n of m a r t e n s i t e used to d e s c r i b e the p s e u d o e l a s t i c b e - hav io r in Cu-Zn-X systems and Cu-A1-Ni cannot hold. AuCd martensite can exhibit several crystal structures, depending on composition or heat treatment, the struc- ture of the Au-rich "rubberlike" Au-Cd martenslte is orthorhomblc, 21 while that of fl' martenslte, also rubber- like, is trigonal; the martensitic structure of In-T1 is tetragonal. ~6 In both cases the microstructure consists of f ine t w i n - r e l a t e d " d o m a i n s of ma r t e ns i t e . " 2~ '

Chang and Read 3 f i r s t sugges t ed that the r u b b e r l i k e behav io r of Au-Cd might r e s u l t f r om the r e v e r s i b l e growth of f a v o r a b l y o r i e n t e d domains at the expense of l e s s - f a v o r a b l y o r i e n t e d domains under the a p p l i c a - t ion of s t r e s s . This mode l has s e e m i n g l y been con- f i r m e d both for Au-Cd 24 and In-T1. ~ It is i n t e r e s t i n g to note that In-T1 shows p s e u d o e l a s t i c i t y not only by the " f e r r o e l a s t i c " m e c h a n i s m wel l below the M s t e m - p e r a t u r e , but a l so by s t r e s s induced t h e r m o e l a s t i c t r a n s f o r m a t i o n above the M s .25

Under c e r t a i n c i r c u m s t a n c e s Au-Cd and In-T1 do not show s u p e r e l a s t t c behav io r but i n s t ead show the " s h a p e - m e m o r y " effect . Chang and Read s noted that i m m e d i a t e l y a f t e r the cubic - - o r t h o r h o m b i c t r a n s f o r - ma t ion Au-Cd does not exhibi t r u b b e r l i k e behav io r but r a t h e r t akes a p e r m a n e n t se t when bent . Then, upon h e a t - ing back through o r t h o r h o m b i c ~ cubic r e v e r s i o n t e m - p e r a t u r e , th is " p l a s t i c " de fo rma t ion is e r a s e d and the s a m p l e r e t u r n s to i ts o r ig ina l shape . S imi l a r behav io r was noted by Bas in sk i and C h r i s t i a n for In-T1. 2s As ment ioned e a r l i e r , In-T1 is s u p e r e l a s t i c both i) above the M s t e m p e r a t u r e , and ii) wel l be low M s (when c o m - p l e t e ly m a r t e n s i t i c ) . In the r eg ion j u s t below Ms, how- eve r , the m a t e r i a l behaves p l a s t i c a l l y r a t h e r than e l a s -

M E T A L L U R G I C A L TRANSACTIONS

t i c a l l y when d e f o r m e d , so that a s h a p e - m e m o r y effect is r e a l i z e d when hea t ing th rough the r e v e r s i o n t e m p e r - a tu r e . Although not c o m p l e t e l y unders tood , the m e m o r y b e h a v i o r of Au-Cd and In-T1 is b e l i e v e d to r e s u l t f r om the mot ion of twin b o u n d a r i e s as in the " f e r r o e l a s t i c " d e s c r i p t i o n . What a p p a r e n t l y is involved is a t ime o r t e m p e r a t u r e dependent twin bounda ry " s t a b i l i z a t i o n " p r o c e s s involving de fec t s of some kind. In th is de - s c r i p t i on , if the twin b o u n d a r i e s a r e s t a b i l i z e d , they wil l i m m e d i a t e l y r e t u r n to t h e i r p r e f e r r e d pos i t i ons on r e l e a s e of s t r e s s and r e s u l t in s u p e ~ e l a s t i c i t y ; if not s t ab i l i z ed , the r e s u l t is appa ren t p l a s t i c d e f o r m a - t ion, and the shape m e m o r y effect o c c u r s dur ing t h e r - m a l l y a c t i v a t e d r e v e r s i o n .

V e r y r e c e n t l y the s h a p e - m e m o r y has been o b s e r v e d in a l a r g e number of " n e w " a l loys by i n v e s t i g a t o r s at Osaka Un ive r s i t y . It has been r e p o r t e d in Cu-A1-Ni by Otsuka and Shimizu 6 in an a l loy compos i t i on and hea t t r e a t m e n t p rov id ing an M s above r o o m t e m p e r a t u r e . ~7 Again, the m e c h a n i s m is a s c r i b e d to the mot ion of twin- r e l a t e d domain b o u n d a r i e s f i 'a6 Enami and Nello 7 have noted the m e m o r y effect in o f f - s t o i c h i o m e t r i c NiA1, and Nagasawa ~7,a8 has r e p o r t e d i t in F e - N i , Co, Ti, Zr , Cu-A1, and Co-Ni . Nagasawa and Kawachi 28 have gone so f a r a s to a rgue that any m a t e r i a l that exhib i t s a m a r t e n s i t i c t r a n s f o r m a t i o n may be capab le of showing a m e c h a n i c a l m e m o r y effect .

In mos t of the s y s t e m s d e s c r i b e d above, work has been done p r i m a r i l y on the s u p e r e l a s t i c b e h a v i o r and r e l a t i v e l y l i t t l e on the m e m o r y effect . The oppos i te is the c a s e for T i -N i . A lmos t a l l i n t e r e s t and r e s e a r c h has been on the m e m o r y effect and only v e r y r e c e n t l y has the f i r s t o b s e r v a t i o n of s u p e r e l a s t i c b e h a v i o r been r e p o r t e d , by Was i l ewsk i . ~s

H i s t o r i c a l l y , the f i r s t o b s e r v a t i o n of the m e m o r y e f - fec t in TI -Ni was made at the U.S. Naval Ordnance L a b o r a t o r y (N.O.L., thus N i - T i - N O L , Nit inol) in about 1962, ag's~ and s ince that t ime much e f for t has been ap - p l i ed to unde r s t and and apply the ef fec t to eng inee r ing p u r p o s e s . Unlike the e a r l y m e m o r y ef fec t a l loys Au- Cd and In-T1, TiNi p o s s e s s e s a po t en t i a l l y v e r y useful combina t ion of p r o p e r t i e s such as duc t i l i ty , t en s i l e s t r eng th , c o r r o s i o n r e s i s t a n c e , high damping capac i ty , and v e r y low magne t i c p e r m e a b i l i t y .

The shape m e m o r y effect in TiNi is qua l i t a t i ve ly s i m i l a r to that d e s c r i b e d above for Au-Cd, In-T1, and Cu-A1-Ni, and involves m a r t e n s i t i c t r a n s f o r m a t i o n . In the v i c in i t y of r oom t e m p e r a t u r e n e a r - e q u i a t o m i c T i - Ni a p p e a r s to be soft and duct i le , and can be r e a d i l y f o r m e d into v a r i o u s shapes . Upon hea t ing above the m a r t e n s l t e r e v e r s i o n t e m p e r a t u r e (< 100~ the " p l a s - t i c " d e f o r m a t i o n is e r a s e d and the o r i g i n a l undefo rmed shape is r e g a i n e d . The exac t m e c h a n i s m for th i s b e - hav io r i s not ye t fu l ly unders tood . Bal l , et al. ~ ob- s e r v e d at l e a s t p a r t i a l m e m o r y r e v e r s i o n in a Ti -52 at . pc t Nt a l loy that was c o m p l e t e l y in the high t e m p e r - a t u r e phase p r i o r to d e f o r m a t i o n and a t t r i bu t ed this b e h a v i o r to the f o r m a t i o n and r e v e r s i o n of de fo rma t ion m a r t e n s i t e . DeLange and Z i j d e r v e l d ~ s i m i l a r l y a t t r i b - u ted the m e m o r y behav io r in a T i -50 at . pc t Ni a l loy ( l a r g e l y m a r t e n s l t i c at r o o m t e m p e r a t u r e ) to d e f o r m a - t ion induced t r a n s f o r m a t i o n of and t h e r m a l r e v e r s i o n to some r e t a i n e d h i g h - t e m p e r a t u r e phase . The " t e x - t u r e " of the m a r t e n s i t e so f o r m e d was o b s e r v e d to d i f fe r depending on whether the app l i ed s t r e s s was t en - s i l e o r c o m p r e s s i v e , i .e . , c e r t a i n v a r i a n t s of m a r t e n -

s i t e be ing p r e f e r r e d depending on the s ign and d i r e c - t ion of the s t r e s s r e l a t i v e to the c r y s t a l l o g r a p h i c axes of the high t e m p e r a t u r e phase . E s s e n t i a l l y the s a m e model of d e f o r m a t i o n induced m a r t e n s i t e f o rma t ion and t h e r m a l r e v e r s i o n was independent ly p r o p o s e d by Sas t r t and Marc inkowsk i . a~

Was i l ewsk i is has r e c e n t l y a rgued that the s c h e m e of s t r e s s - i n d u c e d ~ ~ m a r t e n s i t e t r a n s f o r m a t i o n is not an adequate exp lana t ion for the behav io r of a l loys which a r e c o m p l e t e l y m a r t e n s i t i c at r o o m t e m p e r a t u r e . He p r o p o s e d a mode l involving a s t r e s s - i n d u c e d m a r - t ens i t e ~ ~ r e v e r s i o n , p o s s i b l y fo l lowed by conve r s ion to another m a r t e n s i t e of d i f f e ren t o r i en t a t i on ; the c o m - p lex d e f o r m a t i o n b e h a v i o r of TiNi is d e s c r i b e d on the b a s i s of s p e c i m e n t e m p e r a t u r e r e l a t i v e to such t e m - p e r a t u r e s as Ms, M f , Md, As , A f , and Ad, and t h e i r va lues r e l a t i v e to each o the r ( s ize and shape of the m a r t e n s i t i c h y s t e r e s i s loop). T h e r e is no e x p e r i m e n - t a l ev idence to con f i rm th is double t r a n s i t i o n p r o c e s s . Mos t r e c e n t l y , i t was sugges t ed tha t the m e m o r y effect r e s u l t s f r om the r e v e r s i b l e mot ion of m a r t e n s i t i c twin b o u n d a r i e s , s4 i .e . , the m e c h a n i s m d i s c u s s e d e a r l i e r for Au-Cd, In-T1, and Cu-A1-Ni . This mode l has been r e - cent ly d i scounted by Was i l ewsk t is and suppo r t ed by Wayman. 8,1a It is s u r p r i s i n g that th is concept was not app l i ed e a r l i e r to d e s c r i b e the shape m e m o r y behav io r of TiNi, in view of the fact that i t was used many y e a r s ago to d e s c r i b e s i m i l a r e f fec ts in Au-Cd and In-T1.

The TiNi m a r t e n s t t e s a r e known to be i n t e r n a l l y twinned; for example , Fig. 1 shows the s t r u c t u r e of a s ingle m a r t e n s t t e p la te in a thin foi l of p a r t i a l l y t r a n s - f o r m e d Tt -50 .0 at . pc t Ni (M s ~- 50~ when o b s e r v e d

Fig . 1 - - T w i n n e d m a r t e n s i t e s t r u c t u r e in a th in foil of p a r t i a l l y transformed Ti-50 at. pct Ni; observed at room temperature (Ms ~ 50~

METALLURGICAL TRANSACTIONS VOLUME 4, DECEMBER 1973-2711

Fig. 2--Fringe patterns produced by diffraction contrast at in- terfaces of twin lamellae in martensite of Ti-50 at. pct Ni (M s

50~ thin foil observed at room temperature.

at r o o m t e m p e r a t u r e . The In t e r f ace s of twin r e l a t e d doma ins have been s tud ied in de ta i l in ful ly deve loped p l a t e s ; an exampl e is shown in Fig . 2. When su i t ab ly t i l t ed , f r inge p a t t e r n s a r e o b s e r v e d at i n t e r f a c e s of twin l a m e l l a e . Ind ica t ions a r e that the twin domains may be s l igh t ly tw i s t ed with r e s p e c t to each o ther , o r s l igh t ly s t r a i n e d at the i n t e r f ace . Dis loca t ion s t r u c - t u r e s have not been o b s e r v e d in the i n t e r i o r of ful ly deve loped m a r t e n s i t e s , but a r e usua l ly o b s e r v e d in r e g u l a r a r r a y s a f t e r the p l a t e s r e v e r t . This wi l l be d i s c u s s e d fu r t he r in a l a t e r sec t ion .

MECHANISTIC INTERPRETATIONS

It is likely that there is a basic mechanism underly- ing the various martensite-related SME effects in the various alloy systems. Efforts to reduce these reac- tions to their fundamental components, or to join them with a common thread have been limited. It is also al- most certain that the potentially useful memory effect belongs to systems other than presently known; it would be useful to isolate the conditions under which such ef- fects are emphasized.

The phenomenological theories of martensite crys- tallography have been most successful in correlating the crystallographic features of twinned martensltes, such as are usually observed in connection with shape recovery effects, and have been used to examine sev- eral pertinent martensites specifically. Transmission microscopy has also identified the internal structure in numerous martensites, thus providing a physical basis for the lattice invariant deformation inherent in the formal analyses. When, in fact, details of shape recovery martensites become known in a number of cases, it will be revealing to compare the common features, on a crystallographic basis, for the marten- sltic and premartensitic structural transitions, and to

2712-VOLUME 4, DECEMBER 1973

extend the f indings to o ther c a s e s . In many m a r t e n s i t i c t r a n s f o r m a t i o n s , the hom oge -

neous l a t t i ce s t r a i n s that f o rm a p a r t of the f o r m a l t h e o r i e s conver t only a f r ac t ion of the a t o m s f r o m the i r pos i t i ons in the p a r e n t s t r u c t u r e to those in the p roduc t s t r u c t u r e s . An addi t iona l , unspec i f i ed se t of " s h u f f l e s " is r e q u i r e d to a t ta in the f inal s t r u c t u r e . In s e v e r a l s y s t e m s that r e q u i r e t hese add i t iona l a tom movemen t s , such as TiNi and Au-Cd, diffuse d i f f r a c - t ion ef fec ts or t r a n s i t i o n s t a t e s have been de t ec t ed at t e m p e r a t u r e s above M s .as,as These o b s e r v a t i o n s have led to the r e c e n t concept that the shuff les in i t ia te at a t e m p e r a t u r e above M s and occu r un i fo rmly throughout the m a t r i x a s a h ighe r o r d e r phase t r a n s i t i o n , a5 Spe- c i f i ca l ly , i t is sugges t ed that unique phonon modes a r e accen tua ted in a c e r t a i n t e m p e r a t u r e r ange , that these occu r in t he i r v a r i o u s p o s s i b l e c r y s t a l l o g r a p h i c v a r i - an ts in a s ingle gra in , and that the a tom movemen t s involved a r e subsequen t ly i n c o r p o r a t e d in the f inal m a r t e n s i t e s t r u c t u r e , fol lowing homogeneous d e f o r - ma t ion of the usual type imagined .

In th is s cheme , which beg ins to i so l a t e d i s t i nc t p h y s - i ca l s t eps in c e r t a i n m a r t e n s i t i c r e a c t i o n s , the M s t e m - p e r a t u r e is p r o p o s e d to r e p r e s e n t that t e m p e r a t u r e at which l a t t i ce s t r a i n s p roduced by loca l a tomic d i s p l a c e - men t s b e c o m e suf f i c ien t ly s e v e r e to t r i g g e r the l a t t i ce inva r i an t de fo rma t ion . If th is "phonon" o r " d y n a m i c " m e c h a n i s m of m a r t e n s i t e nuc lea t ion is va l id and gen- e r a l , it would make i t u n n e c e s s a r y to s e a r c h fu r the r for d i s t inc t m a r t e n s i t t c e m b r y o s analogous to those which a r e known for d i f fus lonal nuclea t ion and growth t r a n s f o r m a t i o n s . The p e r t i n e n t point of i n t e r e s t is that a l l m e m o r y m a r t e n s i t e s (with the appa ren t except ion of Cu- Z n - b a s e d a l loys and FeaPt) involve "shuf f le s , ' ' and that the s t r a i n s a s s o c i a t e d with t hese shuff les a r e c o n s i d e r e d i n t eg ra l to the m e m o r y m e c h a n i s m s . L i e b - e r m a n and c o w o r k e r s a7 have r e c e n t l y comple t ed a de - t a i l e d a t o m i s t i c a n a l y s i s showing that the r e l a t i o n s h i p be tween shuff les and mobi le twin bounda ry movemen t in f e r r o e l a s t i c Au-Cd m a r t e n s i t e s g ives r i s e to the ac tua l f o r c e of shape r e c o v e r y . It is l i ke ly that the L i e b e r m a n app roach wil l p rove useful in ana lyz ing o the r m a r t e n s i t i c shape r e c o v e r y e f fec t s .

Wayman a has r e c e n t l y d i s c u s s e d the fact that m e m - o r y effect m a r t e n s i t e s a p p a r e n t l y a lways have t h e r m o - e l a s t i c (slow growth) c h a r a c t e r , and it has been shown that the m a r t e n s i t e r e a c t i o n is d i scont inuous a round the s t a r t point , p rov ing that the r e a c t i o n is f i r s t o r d e r . T r a n s m i s s i o n o b s e r v a t i o n s of fo i l s , including i n s i t u

heat ing and cool ing, have r e c e n t l y r e v e a l e d i n t e r e s t i n g in fo rmat ion with r e s p e c t to both these poin ts . Wayman r e p o r t s that t h e r m o e l a s t i c behav io r is shown by the slow and cont inuous growth of indiv idual m a r t e n s i t e p l a t e s as the t e m p e r a t u r e is lowered , b a s e d on l ight m i c r o s c o p e obse rva t ion . I n s i t u o b s e r v a t i o n s of thin fo i l s by the author has shown that p l a t e s in fo i l s grow by s e r i e s of d i s c r e t e jumps , on a s ca l e be low the r e - so lv ing power of l ight m i c r o s c o p y .

Embryon ic m a r t e n s i t i c f e a t u r e s in TiNi fo i l s have a unique a p p e a r a n c e . Fig . 3 shows the v a r i e t y of f ea - t u r e s which can t yp i ca l l y be o b s e r v e d in a s ing le foi l , where admi t t ed ly m a t r i x r e s t r a i n t s on the m a r t e n s i t e a r e r e l axed . In fo i l s o b s e r v e d jus t a round M s , as in the 50.5 at . pet Ni TiNi a l loy at r oom t e m p e r a t u r e , some of t he se e m b r y o n i c f e a t u r e s have an i n t e r e s t i n g " o p e n - e n d e d " a p p e a r a n c e as seen in Fig . 4. S i m i l a r

METALLURGICAL TRANSACTIONS

w n," I.L

AF* A 'F* B2 --,.TP TP --,.M

DISPLACEMENT PARAMETER(AMPLITUDE) Fig. 5--Schematic free energy curve for a near equiatomic TiNi alloy just above the M s temperature (after Sandrock44): free energy as a function of amplitude of the lattice wave. B2 is the CsCl-ordered bec structure; TP is the transition phase (static displacement wave) ; M is the martensite s truc- ture.

Fig. 3--Embryonic martensitic features in Ti-50.5 at. pct Ni thin foil (M s ~ 25~ observed at room temperature.

region (between M d and M s ) and a re absent f rom all other foi ls .

A c lear unders tand ing of the m e c h a n i s m s of va r ious shape r e c o v e r y effects r e q u i r e s deta i led c ry s t a l l o - graphic knowledge. The r ecen t work of Schmerl ing and L iebe rman , 37 and of Sandrock and Hehemann ~'44 r e p r e s e n t major s teps in this d i rec t ion . Sandrock and Hehemann ~ have identif ied these shuffles as a c l ea r ly independent step in the reac t ions , oc c u r r i ng as a so- cal led p r e m o n i t o r y event to the c l a s s i ca l homogeneous deformat ion of a mar t ens i t f c t r ans fo rma t ion . Figs. 5 and 6, taken f rom the work of Sandrock and Hehemann, indicate r e spec t ive ly the apparent t he rmodynamic s i tu - ation in the v ic in i ty of M s , and the c rys ta l lograph ic s teps which occur in TiNi m a r t e n s i t e . (Note that the s teps compr i s e a model r a the r than a t rue set of d i s - c re te motions.) Schmer l ing and L i e b e r m a n s7 have r e - cent ly out l ined a s i m i l a r c rys ta l log raph ic scheme for Au-Cd f e r r o e l a s t i c m a r t e n s i t e . These la t te r workers have also shown how the mot ion of mobile twin bounda- r i e s in the shuffled m a r t e n s i t e gives r i s e to the r u b b e r - llke shape r e c o v e r y forces . Whether the me chan i sm is appl icable in m e m o r y effects p e r se is not yet c lear .

Fig. 4--"Open-ended" martensitic features in Ti-50.5 at. pct Ni thin foil (M s ~ 25~ observed at room temperature.

f ea tu res have been seen dur ing in s i t u observa t ion of Au-Cd embryon ic mar tens f t e . 36 These should not be confused with thin foil effects that s o m e t i m e s have s i m i l a r appea rances ; the fea tu res obse rved here a re p r e s e n t only when foils a re observed in the t r a n s i t i o n

1. The Role of Dis locat ions in Shape Recovery Effects

One a r ea that has r ece ived l i t t le a t tent ion in the study of shape r e c o v e r y effects is the ro le of d is loca t ions . Ball, et al . 31 f i r s t noted the genera t ion of high d i s loca - t ion dens i t i es by cycl ing the s t r e s s - i n d u c e d m a r t e n - s i t ic t r a n s f o r m a t i o n in TiNi, which has been used to explain the high fatigue r e s i s t a n c e of TiNi. Sandrock and coworkers s5 subsequent ly found that t e m p e r a t u r e cycl ing through the a the rma l t r ans fo rma t ion can also genera te d is loca t ions , with the complexi ty of the t an - gles i n c r e a s i n g with the number of " i n c o m p l e t e " cy- c les over the M s - A s range . ( see Fig. 7). This cycl ing p rocedure also produces a peak in the r e s i s t i v i t y - t e m p e r a t u r e curve , which has been co r r e l a t ed with the degree of the shape m e m o r y effect. Thus it is ob- vious that d i s loca t ions play an impor tan t role in shape

METALLURGICAL TRANSACTIONS VOLUME 4, DECEMBER 1973-2713

~)

o _~[01T] 02

ONi ATOMS ix~ w . , f u i

eTi ATOMS 3 . 0 / ~ t ~ 0 ~ (b) = e

~' X~" [Ol l]B2 [lOO]

O]o

; [oo,] [,OO]o o y O'.

O c ~ e (d)

"[oo,] [,oo]M M

.40,0]

(e)

['oo]M Fig. 6 - -Crys ta l lograph ic " s t e p s " for the B2 ~ M t r a n s f o r m a - t ion in TiNi (af ter Sandroek and Hehemann43). Note that the s teps indicated c o m p r i s e a model r a t h e r then t rue d i sc re t e s tages , (a) B2 ce l l s ; (b) _B2 - FCT cel l ; (c) o r thorhombic d i s to r t ion ; (d) (100)B2 [011]B2 s h e a r to monoclinic; (e) (011)B 2 [01i]B2 p lanar shuffle.

recovery effects. It is appropriate at this point to in- troduce recent evidence supporting this view, although the interpretations are as yet incomplete.

Transmission electron microscopy studies have pro- vided the most direct insights to date into the mecha- nisms underlying such propert ies as the memory effect in TiNi. Examination of thin foils after various thermal treatments have provided support for an important role for dislocations in these reactions. When an initially bcc specimen is cooled below Ms, then reheated to above Ms, regular a r rays of aligned dislocations are observed (Fig. 8) which were not present pr ior to mar - tensitic transformation. The dislocations are evenly spaced on the bcc slip planes. Multiple cycling of this sort increases the complexity of these a r rays (Fig. 7). The development of increasingly dense dislocation a r - rays is coincidental with the development of a peak in res is t iv i ty- temperature curves for multiple-cycled specimens. Wayman TM has shown that such a peak de- velops irrespective of whether the cycles through the M s temperature are "comple te" (i.e., t raverse Mf and Af during each cycle) or " incomplete ." However, the maximum memory effect in TiNi is known to be asso-

Fig. 7--Tangled d is locat ions genera ted in Ti-50.5 at. pct Ni (Ms ~ 25~ thin foil cycled twice in bulk through Ms, thinned, and observed at room t e m p e r a t u r e (Sandrock44).

ciated with this condition. R is clear that each cycle below M s creates an a r ray of dislocations, such as seen in Fig. 8 for a single excursion below Ms, During successive cycles, martensite forms on different habit variants, causing increasingly complex tangles between the intersecting " fo re s t s . " It is apparent flint s imilar dislocation a r rays develop during either athermal or s tress- induced martensit ic formation. Dislocations have also been reported in Au-Cd alloys cycled through Ms .ao

R is probably the back-s t ress created by these dis- location structures (when in a piled-up configuration at martensite: parent boundaries) which creates the mechanical force for shape recovery. In the ease of the memory effect, this occurs during heating through an As-A f temperature range (see Fig. 9), during which this force is released as martensite reversion is suf- ficiently activated. In the case of the related super- elastic (rubberlike) effect, which can be produced as an alternative to the memory effect in several alloys depending on prior thermomechanical history, heating is not required for activation, and the martensitic structures formed by deformation rever t immediately upon release of the applied s t ress . Whether disloca- tions are also involved in the superelastic mechanism is not known. As mentioned, Lieberman has devised an alternative scheme giving r ise to the recovery forcefl ~ It has been suggested that these alternatives (memory effect vs superelastic effect) can be accom- plished by manipulation of history such as to vary the

2714-VOLUME 4, DECEMBER 1973 METALLURGICAL TRANSACTIONS

relative M d and A f temperatures . 15 Whether or not this is an appropriate way to view these phenomena, it is probable that the fundamental mechanistic differ- ence lies in the role played by and the nature of dis- location a r rays that participate in or accompany the growth of the respective martensit ic s tructures.

The simple aligned dislocation a r rays of Fig. 8, character is t ic of a reverted structure after a single incomplete cycle, consist of evenly spaced disloca- tions, lying on evenly spaced parallel f~ phase planes. Trace analysis shows that these dislocations are con- sistent with a r rays lying on the expected slip planes of the bee structure. Whether or not these regular s t ructures can be associated with any crystallographic feature of the martensite (such as the twin boundaries) is not yet resolved. The uniform distribution of de- fects exhibited by the reverted structure would not be expected to exert a part icularly high back-s t ress . How- ever, it is reasonable to speculate that a piled-up con- figuration existed before heating above A s ; this would explain the observation of significant shape recovery after a single s t ress- induced excursion, and the asso- ciated observation of increased capability for shape r e - covery after multiple cycling.

Dislocation pileups cannot account for all of the ob- served recovery energy of the shape memory effect. A twin-boundary interaction process , such as de-

(b)

(a) Fig. 8--Aligned dislocat ion a r r ays in Ti-50.5 at. pct Ni quenched to just below M s (quenched in iced water , M s ~ 35~ and observed in thin foil at room tempera tu re . (a) a rea con- raining severa l r eve r t ed pockets, (b) same area, tilted, d i s lo - cations in bes t contras t , viewed end-on, and (c) same area, t i l ted fur ther .

METALLURGICAL TRANSACTIONS

(c)

VOLUME 4, DECEMBER 1973-2715

I00

t r-

O"

(a) (b) 1,, !(b)

I '~'. ~", i I

Ad Md Temperature--~

Fig. 9--Schematic representation of hysteresis loops for mar - tensitic transformation. Depending on the temperature "width" of the loop, slope of sides, and spacing of M d and A d relative to the loop, various thermomechanical responses are expected because various sequences of cri t ical temperatures are pos- sible. The two representative situations here are: (a) Mf. < M s <A d < M d <A s <Af, and (b) A d < M f < Ms <As <24f < M d,

s c r i b e d e a r l i e r in connect ion with the f e r r o e l a s t i c e f - fect , is o p e r a t i n g as wel l . Dis loca t ion p i l eups at au - s t e n i t e - m a r t e n s i t e i n t e r f a c e s p r o b a b l y account for r e - p o r t e d p a r t i a l (or impe r f ec t ) m e m o r y effects in pu re m e t a l s and d i s o r d e r e d a l loys , which should not have a m e m o r y on the b a s i s of ex i s t i ng c r i t e r i a and mode l s .

Re laxa t ion of r e s t r a i n t s in thin fo i l s p lagues any a t - t empt to do exac t c r y s t a l l o g r a p h i c a n a l y s i s of a m a r - t e n s i t i c t r a n s f o r m a t i o n as in th is ca se . However , ex - t ens ive in s i t u hea t ing and cool ing work has been done in o r d e r to at l e a s t qua l i t a t i ve ly a s s e s s the p r o g r e s - s ion of the a t h e r m a l f o r m a t i o n and r e v e r s i o n of m a r - t en s i t e in n e a r - e q u t a t o m i c a l l oys . Fig . 10 d e m o n s t r a t e s the r e v e r s i o n of m a r t e n s i t e p l a t e s dur ing in s i t u h e a t - ing of a T i -50 at . pc t Ni thin fol l (M s ~- 50~ in i t i a l ly p a r t i a l l y t r a n s f o r m e d to m a r t e n s i t e ) . Pho tography of s t r u c t u r e s under these condi t ions i s made e x t r e m e l y dif f icul t by t h e r m a l f luc tua t ions in the foil , which cause changes in d i f f r ac t ion c o n t r a s t condi t ions even when focus can be ma in ta ined . The examina t ion of thin fo i l s mus t be app roached with s p e c i a l c a r e ; b e c a u s e of the p r o x i m i t y of M s at r o o m t e m p e r a t u r e , the bulk m a t e - r i a l a n d / o r foi l m a y be sub jec t to " m i n i c y c l e s " through M s and A f . This " a g i n g " can give r i s e to n o n r e p r e s e n - t a t ive s u b s t r u c t u r e ; Fig . 11 shows what is p r o b a b l y an example of such s t r u c t u r e in a thin foi l of Ti -50 .5 at . pc t Ni cyc led in bulk once through M s (~ 25~ dur ing the in i t ia l quench. A f a i r l y dense a r r a y of d i s loca t i ons is o b s e r v e d in an i s o l a t e d r eg ion where a m a r t e n s i t e p la te o r p l a t e s have r e v e r t e d ; on t i l t ing , a l ignment of some of the d i s l o c a t i o n s along p a r a l l e l p l anes can be p e r c e i v e d .

The ro l e of d i s l oca t i on s t r u c t u r e in these r e a c t i o n s d e s e r v e s f u r t he r s tudy. Of p a r t i c u l a r i n t e r e s t is the m e c h a n i s m of gene ra t ion , and the r e l a t i o n of the d i s - loca t ions to mode l s of the m e m o r y effect , such as the m e c h a n i s m r e c e n t l y p r o p o s e d by Wayman and c ow ork - e r s , ~4 involving the growth of t w i n - r e l a t e d domains of m a r t e n s i t e . The p r e s e n t v iew of the ro l e of d i s l o c a - t ions can be s u m m a r i z e d as fo l lows (for TiNi): t r a n s - m i s s i o n m i c r o s c o p y shows that t h e r m a l cyc l ing through M s o r app l i ca t ion of s t r e s s be low Md, ac t s a s a f o rm of " c o l d w o r k " in that p e r m a n e n t d i s loca t ion s u b s t r u c - tu re is deve loped ; t he se d i s l o c a t i o n s appa ren t l y ac t as a b a r r i e r to the f o r m a t i o n of m a r t e n s i t e p l a t e s , and a r e r e s p o n s i b l e for the " s t a b i l i z a t i o n " of the high t e m -

2 7 1 6 - V O L U M E 4, DECEMBER 1973

p e r a t u r e phase ( i . e . , the o b s e r v e d lower ing of the M s t e m p e r a t u r e with i n c r e a s e d cyc les ) . A r e l a t e d effect is the tendency to fo rm d i f fe ren t d i s t r i bu t i ons of m a r - t ens i t e p l a t e s f rom cyc le to cyc le . In s i t u d e f o r m a t i o n work has not ye t been done. It would be of p a r t i c u l a r i n t e r e s t to see if t h e r e is any s ign i f i cance to the c lo se co inc idence of the d e f o r m a t i o n m a r t e n s i t e ( M d - M s ) t e m p e r a t u r e r ange with the m e t a s t a b l e phonon range o b s e r v e d above M s (next sec t ion) .

2. Me ta s t ab l e Phonons

The s t ab i l i t y of /3-brass type s t r u c t u r e s has often been d e s c r i b e d in t e r m s of en t ropy s t a b i l i z a t i on at high t e m p e r a t u r e s ; the bcc s t r u c t u r e is m e c h a n i c a l l y un- s t ab le to c e r t a i n t ypes of e l a s t i c de fo rma t ion at low t e m p e r a t u r e s . The p a r a m e t e r c l a s s i c a l l y examined is the a n i s o t r o p y r a t io , A, e x p r e s s e d in t e r m s of s h e a r

1 coef f i c ien t s , A = C44/~ (C 1, - C,~). In th is e x p r e s s i o n , the t e r m C44 r e f e r s p h y s i c a l l y to a s h e a r a c r o s s the (100) p lane in an a r b i t r a r y d i r ec t i on , while the t e r m ' ( C , , - C , 2 ) r e f e r s to a s h e a r a c r o s s the (110) p lane in the [ l i 0 ] d i r ec t i on . As f i r s t d e s c r i b e d by Z e n e r ? 8 a high va lue of th is r a t io , as in the ca se of /3 phase s , ind ica tes that the bcc s t r u c t u r e is uns tab le with r e - spec t to a ( l l 0 ) [ l i 0 ] s h e a r which wil l lower the e n e r g y of the s y s t e m by the m a x i m u m amount . This i n s t a b i l - i ty is often ma n i f e s t e d by m a r t e n s i t i c t r a n s f o r m a t i o n v i a th is s h e a r mode when the bcc s t r u c t u r e is quenched. The a n i s o t r o p y r a t i o can a l so be a p p r o x i m a t e l y r e l a t e d to a r a t i o of s ingle c r y s t a l e l a s t i c modul i , as E(~lt > / E<,oo>. A high o r low value of e i the r r a t i o t h e r e f o r e l eads to the concept of " s o f t " (100> o r (111> d i r e c - t ions r e s p e c t i v e l y .

Fundamenta l ly , c o n s i d e r a t i o n s of spec i f i c e l a s t i c behav io r of c r y s t a l s should be r e l a t e d to i n t e r a t o m i c bonding and p a r t i c u l a r l y to the spa t i a l d i s t r i bu t i on of bonding. Few m a t e r i a l s have pure bonding of a s ingle type; some d e g r e e of mixed bonding usua l ly e x i s t s . The bcc s t r u c t u r e , be ing n o n - c l o s e - p a c k e d , i m p l i e s the p r e s e n c e of d i r e c t i o n a l bonding, and the p a r a m e t e r (C t~ - C,2) can be c o n s i d e r e d to r e p r e s e n t the d e g r e e of cova len t c h a r a c t e r in the bonding of a p a r t i c u l a r bcc a l loy . Since a sof tening of �89 ( C t , - C~2) is l ike ly on coo l - ing toward M s in TiN! for example , th is r e p r e s e n t s a d e c r e a s e in the cova len t component of bonding p r i o r to o r dur ing the l a t t i ce t r a n s f o r m a t i o n . This is s e n s i b l e if one c o n s i d e r s the s t r u c t u r e jus t at some t r a n s i t i o n point to be " u n d e c i d e d , " and in fact , f luctuat ing b e - tween p a r e n t and p roduc t s t a t e s . The s a m e concept a l so app l i e s if t h e r e is a t r a n s i t i o n r ange p r i o r to M s , p e r h a p s e n c o m p a s s i n g a second o r d e r (gradual) t r a n - s i t ion f rom one s t r u c t u r e to ano ther . In th is concept , the sugges t ion of d e c r e a s e d cova lency does not imply i n c r e a s e d m e t a l l i c c h a r a c t e r (in t e r m s of i n c r e a s e d c a r r i e r dens i ty) , but r a t h e r d e c r e a s e d bonding d i r e c - t i ona l i ty a r i s i n g f rom the dynamic na tu re of bonding at a c e r t a i n t e m p e r a t u r e o r in a c e r t a i n t e m p e r a t u r e r ange . Because the p a r t i c u l a r soft phonon modes op - e r a t e only ove r a l im i t e d t e m p e r a t u r e range , they can be c o n s i d e r e d m e t a s t a b l e with r e s p e c t to the high or low t e m p e r a t u r e s tab le s t r u c t u r e s . T h e r e f o r e , the p r e m a r t e n s i t i c r ange , such as in TiNi, t y p i c a l l y show- ing a nega t ive t e m p e r a t u r e coef f ic ien t of r e s i s t i v i t y , m a y a r i s e f r o m e i the r i) e l e c t ron -phonon s c a t t e r i n g due to soft modes which develop, o r ii) l oca l e l a s t i c

METALLURGICAL TRANSACTIONS

Fig. 10- -Revers ion of m a r t e n s i t e plates during in situ heat ing of Ti-50 at. pct Ni thin foil ( M s ~ 50~ (a) room tempera- ture, (b) 40~

s t ra ins due to the atomic motions. It s eems l ikely that both fac tors contribute to the development of a r e s i s - t ivi ty peak,in TiNi at approximate ly the M s t e m p e r a - ture when al loys a re cycled through M s and A s without complete annealing of the /3 phase. As pointed out e a r - l i e r , such cycling has been observed to generate p e r - manent defect networks of increas ing density, as will be d iscussed la te r . Wayman a~ has recen t ly suggested that this r e s i s t i v i t y peak is not d i rec t ly re la ted to the memory effect. However it is c lea r that the peak is in t imately r e l a t ed to some feature of the mar tens i t i c t ransformat ion in memory effect a l loysf l s

Experimental ly , metas table phonons are evidenced by specif ic observat ions in the var ious cases where they occur. The observat ions a r e genera l ly s imi l a r , however, for such cases as the bcc w t ransi t ion, and

the p r emar t ens i t i c t rans i t ion in B2 TiNi and other m a t e r i a l s . There a r e two genera l manifestat ions of metas table phonons, namely i) diffuse diffraction sca t - te r ing in a cer ta in t empera tu re range, having a spec i - fic dis t r ibut ion of intensi ty in r ec ip roca l space; and ii) r egu la r "mot t l ing" of t r ansmis s ion e lec t ron mi - croscopic images due to phonon per turba t ions of nor- mal diffraction contras t . For example, a "s ingle phase" ~ s t ruc ture often appears to contain finely di- vided prec ip i ta te pa r t i c l e s . 4~

An example of the r egu la r dis t r ibut ion of diffuse e lec t ron sca t te r ing is seen in Fig. 12, obtained from a thin foil of Ti-51.0 at. pct Ni ( M s ~- 0~ observed at room tempera tu re . The al loy at this t empera tu re exis ts in the center of the t rans i t ion range of about 50~ over which metas table phonons are accentuated.

METALLURGICAL TRANSACTIONS VOLUME 4, DECEMBER 1973-2717

(a)

Fig. l l - -D i s loca t i on a r r a y s assoc ia ted with r e v e r t e d m a r t e n - s i te in Ti-50.5 at. pct Ni thin foil subjected to " m i n i c y e l e s " thru M s near room temperature.

In s e a r c h i n g fo r the fundamenta l o r ig in of a p a r t i c u - l a r mode of fi phase i n s t ab i l i t y , t he re a r e many c r i t e - r i a that can be s e l e c t e d . F o r e x a m p l e , the w in s t ab i l i t y of bcc t r a n s i t i o n m e t a l a l l oys has been r e l a t e d to e l e c - t ron changes which occu r in the indiv idual a t o m s as a function of t e m p e r a t u r e ; while /3 -brass t r a n s f o r m a t i o n s at low t e m p e r a t u r e s have h i s t o r i c a l l y (s ince Zener ) been m o r e c lo se ly connec ted with inheren t bcc m e c h a n i c a l in - s t ab i l i t y that cu lmina t e s in a m a r t e n s i t i c s h e a r co l l apse of the s t r u c t u r e . Recent r e s u l t s sugges t that t h e r e may be s ign i f i can t s i m i l a r i t i e s between the i n s t a b i l i t i e s d e m o n s t r a t e d by s u p e r s a t u r a t e d noble m e t a l fl p h a s e s and that exh ib i ted by the t r an s i t i on m e t a l a l l oys . Diffuse d i f f r ac t ion e f fec t s have been seen in s e v e r a l noble m e t a l a l loys in the s ingle /3-phase f ie ld , and have been r e c e n t l y i n t e r p r e t e d as due to s c a t t e r i n g due to spec i f i c s h o r t - wavelength l a t t i ce waves which a r e d i s t inc t f rom t h e r m a l diffuse s c a t t e r i n g ; " o m e g a ' r e f l e c t i o n s and s c a t t e r i n g have been o b s e r v e d in s e v e r a l b r a s s e s . 4~

E l e c t r o n i c en t ropy t e r m s can p l ay an impor t an t r o l e in phase s t ab i l i t y . Although it i s appa ren t that changes in bonding accompany s t r u c t u r a l t r a n s f o r m a t i o n s , e l e c - t ron ic t r a n s i t i o n s in the a t o m s of e l e m e n t s or a l loys have not often been c i ted as the cause of phase changes . An excep t ion has a r i s e n f r o m the r e c e n t s tud ies of Hehemann and c o w o r k e r s of the o2 t r an s i t i on in the m e t a s t a b l e fl phase of Group IV b a s e d bcc t r a n s i t i o n m e t a l a l l oys . 4' This is known to be a h igher o r d e r t r a n s i t i o n and was f i r s t o b s e r v e d in numerous a l loys of Ti group e l e m e n t s with t r a n s i t i o n me ta l s . In the

(b) Fig. 12--Room t e m p e r a t u r e SAD's near (111)/32 or ienta t ion f rom thin foil of Ti-51 at. pct Ni quenched f rom l l00~ (M s

0~ (a) r igh t on ( l l l ) B 2 or ientat ion, (b) s l ight ly t i l ted.

ca se of the w r eac t ion , bcc phase i n s t ab i l i t y man i f e s t s i t se l f on cool ing be low a c r i t i c a l t e m p e r a t u r e . This t akes p lace not by the homogeneous s h e a r s of m a r t e n - s i t i c t r a n s f o r m a t i o n , but by the homogeneous ope ra t i on of c e r t a i n l a t t i ce v i b r a t i o n modes throughout the bcc s t r u c t u r e . ~ The p a r t i c u l a r phonon modes a r e unique and can be r e l a t e d to e l e c t r o n i c changes in the a t o m s of the a l loys which cause the a toms of the unit ce l l to f luc tua te toward new spa t i a l a r r a n g e m e n t s (angles and d i s t ances ) c h a r a c t e r i z i n g a new bonding na tu re . In the case of the w t r ans i t i on , th is can be mos t s i m p l y d e - s c r i b e d as the in t roduct ion of m o r e covalent c h a r a c t e r into the bond hybr id s as the s t r u c t u r e changes f rom fi phase to o2 phase .

Another example of th is type of g e n e r a l l a t t i c e i n s t a - b i l i t y c h a r a c t e r i z e d by v i b r a t i o n waves o c c u r s in the CsC1 (B2) l a t t i ce of equ ia tomic TiNi a l l oys when cooled below a c r i t i c a l t e m p e r a t u r e f l s In th is case , the phonon range is c l i m a x e d by a s h e a r t r a n s f o r m a t i o n of the (112)(111> type, which is b e l i e v e d to be i n t i m a t e l y r e - l a ted to the a tom mot ions involved in the p r e c e d i n g unique l a t t i ce modes . ~ It is i n t e r e s t i n g to note that the m e t a s t a b l e phonon range in TiNi c o r r e s p o n d s ap - p r o x i m a t e l y to the p u r e l y d e f o r m a t i o n m a r t e n s i t e

2718-VOLUME4, DECEMBER 1973 METALLURGICAL TRANSACTIONS

(M d - M s ) t e m p e r a t u r e r ange one usua l ly o b s e r v e s above M s in a m a r t e n s i t i c a l loy .

/%phase i n s t ab i l i t y at low t e m p e r a t u r e s can be d e m - o n s t r a t e d in ways that cannot be c l a s s i f i e d into con- ven t iona l phase t r a n s f o r m a t i o n c a t e g o r i e s . Mechan- i ca l s t a b i l i t y a r g u m e n t s such as put fo r th i n i t i a l l y by Zener a r e not a p p r o p r i a t e for p r e d i c t i n g the way in which fl p h a s e s wi l l change s t r u c t u r e when e l e c t r o n i c changes in the a t o m s a r e involved. In th i s case , it i s p a i r w i s e a tomic i n t e r a c t i o n s that b e c o m e impor t an t , and l a t t i c e v i b r a t i o n s of v e r y s h o r t wavelength mus t be c o n s i d e r e d to be m o r e s ign i f i can t than the long wavelength e l a s t i c waves or m a c r o s c o p i c d e f o r m a t i o n s c o n s i d e r e d in s i m p l e m e c h a n i c a l s t a b i l i t y d i s c u s s i o n s . F o r e x a m p l e , the l a t t i c e i n s t ab i l i t y in bcc t r a n s i t i o n me ta l a l l oys ment ioned above involves the ope ra t ion of t r a n s v e r s e l a t t i ce v i b r a t i o n s of wave lengths on the o r d e r of the bcc unit ce l l s i ze , u sua l ly p o l a r i z e d in (111) d i r e c t i o n s . This i m p l i e s the coo rd ina t ed m o v e - ment of the ne ighbor ing c l o s e - p a c k e d (111) chains in bcc with r e s p e c t to each o ther , but does not n e c e s s a r - i ly fol low f r o m c o n s i d e r a t i o n of the e f fec t s of m a c r o - scop ic s h e a r s on the e n e r g y of c r y s t a l s .

(a)

COMMON AND UNCOMMON FEATURES OF SME ALLOYS

T h e r m a l r e v e r s i o n of s t r e s s - i n d u c e d m a r t e n s i t e is not unusual , of c o u r s e ; however , r e c o v e r y of the "au- s t e n i t i c " - s h a p e - p r i o r - t o - d e f o r m a t i o n dur ing th is r e - v e r s i o n i s unique to c e r t a i n m a r t e n s i t i c a l l oys , as a r e o the r spec i f i c types of SME phenomena . One might a sk what i t i s about the p a r t i c u l a r s h a p e - m e m o r y - e f f e c t (SME) a l loys tha t give them t h e i r unique behav io r . C l e a r l y , the answer l i e s in the spec i f i c f e a t u r e s of the m a r t e n s i t i c t r a n s f o r m a t i o n m e c h a n i s m . Wayman ~2 has s u m m a r i z e d the p r e r e q u i s i t e s for SME behav io r a s i) t h e r m o e l a s t l c m a r t e n s i t e , ii) o r d e r i n g in the p a r e n t phase , i i i) i n t e r n a l twins a s the s u b s t r u c t u r a l a c c o m - modat ion mode in the m a r t e n s t t e phase . An except ion to th is l a s t point may be m a r t e n s t t e s of the C u - Z n - b a s e ~ (bcc) phase , which a r e known to be i n t e r n a l l y faul ted r a t h e r than twinned (Fig. 13), yet show SME ef fec t s .

Such c r i t e r i a exc lude a v e r y l a r g e useful group of common f e r r o u s m a r t e n s t t i c a l loys , among o the r s , due to d i s l o c a t e d m a r t e n s i t i c s t r u c t u r e s and d i s o r d e r . This by chance has l i m i t e d SME a l l oys in many c a s e s to " u n c o m m o n " s y s t e m s , that i s , u n i n t e r e s t i n g eng i - n e e r i n g s y s t e m s . Excep t ions to th is a r e TiNi, which has an e x t r e m e l y i n t e r e s t i n g combina t ion of e n g i n e e r - ing p r o p e r t i e s , C u - Z n - b a s e b r a s s e s and s e v e r a l o the r p r o m i s i n g s y s t e m s . AuCd and In-T1 would s e e m un- l i ke ly to e v e r see w i d e s p r e a d app l i ca t i ons in the m a r - ke tp l ace , and a r e t h e r e f o r e " u n c o m m o n . " What is in - t e r e s t i n g i s the appa ren t d i v e r s i t y of SME s y s t e m s that ex i s t among uncommon and r e l a t i v e l y common a l loy s y s t e m s .

In g e n e r a l , any f ea tu r e which i n t roduces i r r e v e r s i b l e p r o c e s s e s into the t r a n s f o r m a t i o n m e c h a n i s m p r e - c ludes SME. It has been sugges t ed for example that the r e q u i r e m e n t for o r d e r i n g in the p a r e n t phase has to do with the ab i l i t y to a c c o m m o d a t e d i s t o r t i o n ( m i s - match) at the m a r t e n s i t e - a u s t e n i t e i n t e r f ace . Such d i s t o r t i o n is m o r e l i ke ly to exceed the m a t r i x e l a s t i c l i m i t ( t he re fo re in t roduc ing an i r r e v e r s i b l e s tep in the

METALLURGICAL TRANSACTIONS

(b) Fig. 13--(a) Internal structure in Cu-36.5 at. pct Zn-l .5 at. pct A1 stress-induced martensite, (b) Overlapping faults in (a) ob- served at high magnification.

f o rm of t r ue p l a s t i c flow) in a d i s o r d e r e d m a t r i x . S im- i l a r l y , SME m a r t e n s i t i c p h a s e s should not contain m o - b i l e d i s l oca t i ons b e c a u s e of i r r e v e r s i b l e shape changes th is would cause . It is p o s s i b l e that i n t e r e s t i n g f e r r o u s s y s t e m s can be found involving the twinned �9 m a r t e n - s i t e s t r u c t u r e . These s y s t e m s wil l a l so by p r e s e n t s t a n d a r d s not be common a l loys , and wil l l i ke ly have to be ful ly m a r t e n s i t i c o r exhib i t a p p r o p r i a t e s t r e s s - induced t r a n s f o r m a t i o n . Deve lopment of SME effec ts in f e r r o u s s y s t e m s wil l l i ke ly s h a r e the c r i t e r i a a l - r e a d y e s t a b l i s h e d for the n u m e r o u s non fe r rous s y s t e m s .

F r o m the s tandpoin t of phenomeno log ica l m a r t e n s i t e t r a n s f o r m a t i o n t heo ry , s c h e m e s of the WLR or B-M type, e m p h a s i z i n g inva r i an t p lane s t r a i n r e q u i r e m e n t s have been r e c e n t l y po in ted out to s o m e t i m e s be inade - quate . 4a,49 While some m e a s u r e of d e g r e e of c r y s t a l l o - g raph ic m i s m a t c h c l e a r l y d e t e r m i n e s the t r a n s f o r m a - t ion h y s t e r e s i s and can be i n t e r p r e t e d in t e r m s of in- v a r i a n t p lane s t r a i n , m o r e e f fec t ive i n t e r p r e t a t i o n s should be b a s e d on v o l u m e t r i c m i n i m i z a t i o n of s t r a i n

VOLUME 4, DECEMBER 1973-2719

F i g . 1 4 - - D e t a i l s o f i n t e r s e c t i o n of two f a u l t e d s t r e s s - i n d u c e d m a r t e n s i t e p l a t e s i n C u - 3 6 . 5 a t . p c t Z n - l . 5 a t . p c t A1.

ene rgy . 49 This o b s e r v a t i o n b e c o m e s c l e a r l y the key c r i t e r i o n (which could be added to, but m o r e p r o p e r l y , subs t i tu ted for those ment ioned e a r l i e r ) for SME. A m a r t e n s i t i c t r a n s f o r m a t i o n with low enough h y s t e r e s i s is b e h a v i o r a l l y t h e r m o e l a s t i c ; if v o l u m e t r i c s t r a i n en - e r g y is low enough, t h e r e a r e no r e m a n e n t e f fec ts of t r a n s f o r m a t i o n ; th is s u g g e s t s that t h e r e may not a lways be a need for p a r e n t phase o r d e r i n g to r e s i s t y ie ld ing. This would account for the b e h a v i o r of d i s o r d e r e d SME In-T1 a l l oys . Ex tens ion of a r g u m e n t s b a s e d on th is vo l - u m e t r i c s t r a i n e n e r g y concept can account for cyc l ing e f fec t s in c e r t a i n a l loys ( loss of m e m o r y , o r the " a m - n e s i a e f f ec t " ) , SME in pu re m e t a l s , and " p a r t i a l " m e m o r y e f fec t s in c e r t a i n a l l oys .

Stated in mos t g e n e r a l t e r m s , the common c r i t e r i a for a l l oys exhib i t ing SME should be c o n s i d e r e d to be :

t) M a r t e n s i t i c t r a n s f o r m a t i o n which l i m i t s v o l u m e t - r i c s t r a i n ene rgy to be low a c r i t i c a l va lue b a s e d on the m a t r i x y ie ld s t r eng th ;

ii) La t t i ce i nva r i an t m a r t e n s i t i c a ccommoda t i on which i s l i m i t e d to r e v e r s i b l e p r o c e s s e s .

SUMMARY AND CONCLUSIONS

O b s e r v a t i o n s f r o m r e c e n t e x p e r i m e n t a l work: i) T h e r m a l m a r t e n s i t e in TiNi g e n e r a t e s d i s loca t ion

s u b s t r u c t u r e in the p a r e n t phase which may cont r ibu te to the f o r c e of the m e m o r y effect r e v e r s i o n ;

ii) The M s - M d t e m p e r a t u r e r ange in TiNi c o r r e - sponds c l o s e l y with a t r a n s i t i o n or p r e m o n i t o r y range in which m e t a s t a b l e phonons a r e accen tua ted ;

l i t) M e t a s t a b l e phonon modes a r e a l so o b s e r v e d in m o d i f i e d / 3 - b r a s s e s which can exhibi t the shape m e m - o r y and s u p e r e l a s t i c e f fec t s ;

iv) Phonon nuc lea t ion of m a r t e n s i t i c t r a n s f o r m a t i o n a p p e a r s l i ke ly in TiNi and o the r m e m o r y a l l oys .

At l e a s t t h r e e s ign i f i can t a r e a s d e s e r v e fu r the r in- ves t iga t ion , inc luding:

i) D e t e r m i n a t i o n of the ro l e of d i s loca t ions in the m a r t e n s i t i c t r a n s f o r m a t i o n s , and in the s h a p e - r e c o v - e r y e f fec t s , inc luding the mode of gene ra t ion of such

d i s l oca t i ons . It is not c l e a r whether t he se d i s l oca t i ons a r e g e n e r a t e d by e x t e r n a l l y - a p p l i e d s t r e s s , o r by the m a r t e n s i t i c t r a n s f o r m a t i o n i t se l f , and whether o r not they a r e p a r t of the in t e rna l s t r u c t u r e of the m a r t e n - s i t e p e r s e .

il) Exact d e t e r m i n a t i o n of v a r i o u s m a r t e n s i t e m o r - pho log ies and c r y s t a l l o g r a p h i e s wil l a l low m o r e c o m - p le te mode l ing of the r e a c t i o n s f rom a m e c h a n i s t i c point of view, as wel l a s s e r v i n g to t e s t the phenom- eno log lca l t h e o r i e s .

i t i) More comple t e d e t e r m i n a t i o n of the ro l e of e l e c - t r on i c en t ropy con t r ibu t ions to i n s t ab i l i t y in the high t e m p e r a t u r e p a r e n t phase wi l l a l so be of i n t e r e s t .

ACKNOWLEDGMENTS

The continued interest and encouragement of Pro- fessor T. B. Massalski and the Metal Physics Group at Mellon Ins t i tu te is g r a t e fu l l y acknowledged. Thanks a r e p a r t i c u l a r l y due to Dr. Ga ry Sandrock of Inco Re- s e a r c h and P r o f e s s o r R. F. Hehemann for the c o n t r i - but ion of r e s e a r c h r e s u l t s and m a n u s c r i p t r ev iew. F inanc ia l suppor t ove r the c o u r s e of th is r e s e a r c h has been p rov ided by s e v e r a l agenc i e s , inc luding the U. S. A r m y R e s e a r c h Office, Durham, N .C . (Cont rac t No. D A - A R O - D - 3 1 - 1 2 4 - G l 1 2 1 ) and the Office of Naval R e s e a r c h (Naval P o s t g r a d u a t e School Foundat ion Re - s e a r c h Grant P r o g r a m ) .

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M E T A L L U R G I C A L TRANSACTIONS VOLUME 4, DECEMBER 1 9 7 3 - 2 7 2 1