J. SINGI~ and S. RANGAHATHAN: On the Mechanism of F.C.C. -, H.C.P. Transformat.ion 243

phys. stat. sol. (a) 73, 243 (1981)

Subject classification: 1.2 and 1.4; 21.1; 21.1.1

Department of Metallurgical Engineering, Bunaras Hindu University, Varanasil)

On the Mechanism of F.C.C. -+ H.C.P. Transformation

BY J. SINGH and s. RANGANATHAN

Electron microscopic studies of thermally induced f.c.c. to h.c.p. transformation in co-5y0 Be alloys are interpreted in favour of the fault agglomeration model. Little or no evidence is found for the role of grain boundaries in this transformation. An earlier model based on grain boundary dislocations is shown to suffer from severe crystallographic limitations.

Elektronenmikroskopische Untersuchungen des thermisch induzierten Ubergangs k.f.z. zu h.d.p. in co-5Y0 Fe-Legierungen werden erfolgreich mit dem Defekt-Agglomerationsmodell interpretiert. Keine oder nur geringe Hinweise werden fur den EinfluS von Korngrenzen bei diesem Tfbergang gefunden. Es wird gezeigt, daS ein fruheres Model1 auf der Grundlage von Korngrenzenversetzungen durch schwerwiegende kristallographische Beschrankungen beeintrgchtigt ist.

1. Introduction The f.c.c. + h.c.p. transformation requires the passage of cl/S [112] type Shockley partials on alternate { 111} planes. The original model suggested by Christian [l] that partials could get reflected a t the surface is now only of historical significance. At the moment there is an agreement that lattice defects play an important role in this structural change [ 2 to 101.

The proposals due to Basinski and Christian [ 2 ] and Seeger [3, 41 are inspired by the Cottrell-Bilby mechanism for deformation twinning in b.c.c. crystals [ll]. If a small h.c.p. nucleus exists already, a major dislocation emerging from it may dissociate in the cubic region as follows:

The third dislocation is the transformation dislocation. As it rotates about the pole dislocation, the h.c.p. region is extended by two atomic planes for every complete resolution. The model is attractive for its simplicity. However, there is no clear ex- perimental evidence for the existence of pole dislocations. It does not explain the early stages of transformation, as it presupposes the existence of h.c.p. nuclei. The growing h.c.p. regions will be perfect on the basis of this model. There is, on the contrary, abundant experimental evidence that the h.c.p. regions are faulted.

Bollmann [5] proposed a model based on his studies of cobalt. It is a reformulation of Christian's original idea, whereby new stacking faults are formed not a t the crystal surface but a t pre-existing faults on differently oriented planes. Bollmann's mecha- nism has the merit of explaining the multivariant nature of the transformation, the retention of the cubic phase between transformed regions and the large number of faults found in the h.c.p. regions.

Varanasi-221005, India.

16"

244 J. SINGH and S. RANGANATHAN

Fujita and Ueda [6, 71 have proposed an alternative model based on their in- vestigations of austenitic stainless steels. Stair-rod cross-slip plays a crucial role. A Shockley partial dissociates into a stair-rod dislocation and another Shockley partial gliding on a cross-slip plane under the influence of high stress on t,he intersecting slip plane. If t,his process is repeated on slip planes separated by two layers, an h.c.p. region is generated.

Mahajan et al. (8, '31 have carried out careful electron microscopic investigations of Co-Fe alloys. They proposed that stacking faults on different planes agglomerate to produce four- or six-layer 2i.c.p. regions. It is conceivable that two a/2(110) disloca- tions of different but coplanar Burgers vectors can interact leading to the formation of six-layer h.c.p. regions. When such six-layer nuclei located at different levels grow into each other, a macroscopically observable h.c.p. region is formed. It is also possible that f.c.c. twins could also result depending on the relative locations of the nuclei. The presence of occluded f .c.c. twins in transformed regions lends significant support to this proposal. Faults in h.c.p. regions can also be explained on the basis of this model. This appears to be the most successful model advanced so far.

A radically different approach has been advocated by Horsewell et al. [lo]. All the models discussed above depend upon imperfections within the grain. Horsewell et al. visualize a special role for grain boundaries. The structure of high angle grain bound- aries can be understood on the basis of the coincidence site lattice model of Brandon et al. [la]. Bollmann [13] has given a comprehensive account of this model as well as its extension by the 0-lattice. The most important feature of concern here is that grain boundaries contain dislocations. Horsewell et al. [ 101 have proposed that Shock- ley partials can be emitted from the grain boundary in a regenerative manner, thereby accomplishing the change in stacking sequence necessary to effect the transformation. The proposal is interesting but has been found by us to contain several serious flaws. This is discussed later.

2. Experimental Procedure

co-50/, Fe alloy was received from the Cobalt Information Centre, Belgium in the form of rods 15 em long and 2 om in diameter. The specimens for microstructural observations were prepared by hot forging followed by cold rolling. Intermediate anneals a t 1073 K were carried out.

The final treatment consisted of solution a t 1473 K for 1 h in vacuum sealed silica tubes. The tubes were quenched in water and the specimens recovered. Thin foils for electron microscopic observation were prepared by the jet technique using a solution of 50?/, orthophosphoric acid in water. The foils were examined in Philips EM-300 electron microscope operating a t 100 kV.

3. Results

Fig. 1 shows a transmission electron micrograph of Co-50/, Fe alloy aged a t 473 K for G h. Stacking fault intersection as visualized by Bollmann [5] can be clearly seen. Fig:2 a to c shows micrographs from the as-quenched alloy. Big. 2a shows a number of fault clusters similar to those reported by Mahajan et al. [8]. Pig. 2b shows a region containing h.c.p. crystals. The stacking faults are in contrast in the cubic region and out of contrast in the h.c.p. region. This contrast is similar to that reported by Four- deux et al. [14] in their study of Co-lO(%, Cr alloys. Fig. 2c shows the role of grain boundaries in this transformation. The faults appear to originate froin a coherent twin boundary. Due to overlapping of faults, light fringes can be seen in some areas.

On the Mechanism of F.C.C. -+ H.C.P. Transformation 245

Fig. 1. Transmission electron micrograph of co-5% Fe alloy showing intersection of faults

4. Discussion

As described in the Intaroduction, Horsewell et al. [lo] have made a novel suggestion that grain boundary dislocations could provide Shockley partials in a regenerative fashion to convert an f.c.c. crystal in a h.c.p. region. Unfortunately their paper con- tains errors in the handling of DSC vectors. These are pointed out below. A more serious objection may be levelled against their treatment about retention of Shockley partials a t the boundary. The correction of this concept appears to invalidate their mechanism a t least in the form in which it is developed in their paper.

Horsewell e t al. [lo] have considered the special case of a coincidence-site lattice (CSL) with 2 = 5, where L' is the reciprocal of the density of coincidence sites. They have given the DSC vectors for this CSL as

a b - - [310],; l - 20

a 20 -- pie], ,

a 20 - ~301,

The vectors of the DSC lattice may be arrived at both by geonietric construction as well as the use of matrices. These have been tabulated in the literature [15, 161. The

246 J. SINGH and s. R.4NGARATHAN

Fig. 2. Transmission electron micrographs from as-quenched co-5y0 F e alloy showing a) a number of fault clusters, b) bands of h.c.p. material in the f.c.c. matrix. The stacking faults in the 1i.c.p. region are o u t of contrast, c ) stacking faults, originating from a coherent twin boundary

correct values are

a a a 10 10 10

b - - [130], , b, = - [i251A . 2 - b, = [310], ,

It iiiay be noted that the correct b, and b, values are twice as long. Their b3 is a lattice vector and not a DSC vector. Pig. 2 of their paper is reproduced as Fig. 3. The cor- rected version by us appears as Fig. 4. Their error is due t,o the fact that they have considered the projection of atom positions a t levels 0 and 1/2 and tried to link them up by DSC vectors.

They have considered the association of grain boundary dislocations to form matrix dislocations and given the reaction as

On the Mechanism of F.C.C. --t H.C.P. Transformation 247

Fig. 3 Fig. 4

Fig. 3. The DSC lattice for a ,E = 5 grain boundary. The formation of an u/2[110]~ is illustrated. o atoms of lattice A at height 0; atoms of lattice A a t height 1/2; atoms of lattice B at height 0; rn at,oms of lattice B a t height 1/2 (from Horsewell e t al. [lo])

Fig. 4. Corrected version of Fig. 3

The corrected version is

Table 1 gives the corrected version of the table in their paper. A more serious objection must be made to their statement that ~ / 6 [ 1 2 I ] ~ can be

retained in the boundary. This is not a DSC vector and as such cannot be contained in the boundary. They have fallen into this error by assuming that Q,/6[121]~ is

T a b l e 1

Association of grain boundary dislocations to form matrix dislocations for ,E = 5 CSL boundary

U - [oil] 0 -1 1

- [ 1011 -1 1

- [ O l l ] 1 2 2

-[loll 2 0 2

-[110] 2 1 2

2

2

1 a - 2

1 U

1

0

U

U

a[ i io l -1 2 0

248 J. SINGH and S. RANGANATHAN: On the Mechanism of F.C.C. -t H.C.P. Transformation

a coincidence site lattice vector, as a ,Z = 5 grain boundary can be described by a rota- tion of 101.54’ about [112]. This is incorrect as is their statement that cooperative regeneration of partial dislocations into abutting grains occurs for 2 values of 3, 5 , 7, 11, and 15 characterised by rotation around [112].

It appears from a consultation of tabulated values of DSC vectors [ 161 that Shockley partials appear as possible grain boundary dislocations for ,Z values of 3 and 9. T t is also possible to generate Shockley partials by association of grain boundary disloca- tions whenever the 2 value is divisible by 3. Thus only those grain boundaries which have these special 2 values will be capable of retaining Shockley partials in them. Since the high 2 value boundaries are of limited significance, i t may be concluded that t,he proposed mechanism will operate only when boundaries are twin related. 2 values of 3 and 9 correspond to first and second order twins. Mahajan et al. [8] have given an example where non-coherent twin boundaries can provide partials for transforma- tion. Fig. 2c depicts another example from our study.

5. Conclusions 1. A study of thermally induced transformation in c0-57~ Fe alloy allows us to infer that the agglomeration of faults, as proposed by Mahajan et al. [8] , is the likely mode for transformation from f.c.c. to h.c.p. configuration.

2. The model of Horsewell et al. [lo], ascribing a special role to grain boundary dislocations, has been critically examined. It is contended that a general grain bound- ary cannot be a source of Shockley partials. Non-coherent twin boundaries alone appear to be capable of providing Shockley partials, which can help driving the f.c.c. to h.c.p. transformation. Grain boundaries with higher ,Z values, divisible by 3, can also retain Shockley partials but their role is not considered to be significant.

A cknowledgements

The authors are grateful to Prof. T. R. Anantharaman for encouragement. They wish to express their sincere thanks to Dr. B. Ralph and Dr. S. Lele for many stimulating discussions.

References [ l ] J. W. CHRISTIAN, Proc. Roy. SOC. London A206, 51 (1951). [2] Z. S. BASINSKI and J. W. CHRISTIAN, Phil. Mag. 44, 791 (1953). 131 A. SEEOER, Z. Metallk. 47, 247 (1953). [4] A. SEEGER, Z. Metallk. 47, 653 (1953). [5] W. BOLLMANN, Acta metall. 9, 972 (1961). [6] H. FUJITA and S. UEDA, Acta metall. 20, 759 (1972). [7] S. UEDA and H. FUJITA, Trans. Japan. Inst. Metals 18, 169 (1977). [8] S. MAHAJAN, M. L. GREEN, and D. BRASEN, Metallurg. Trans. 8A, 283 (1977). [9] S. MAHAJAN, D. BRASEN, and T. WAKIYANA, Metallurg. Trans 9A, 1817 (1978).

[lo] A. HORSEWELL, B. RALPH, and P. R. HOWELL, phys. stat. sol. (a) 29, 587 (1975). [ll] A. H. COTTRELL and B. A. BILBY, Phil. Mag. 42, 573 (1951). [12] D. G. BRANDON, B. RALPH, S. RANOANATHAN, and M. S. WALD, Acta metall. 12, 813 (1964). [13] W. BOLLMANN, Crystal defects and orystalline interfaces, Springer-Verlag, Berlin 1970. [14] A. FOURDEUX, R. GEVERS, and S. AMELINCKX, phys. stat. sol. 24, 195 (1967). [15] Y. ISHIDA and M. MCLEAN, Phil. Mag. 27, 1125 (1973). [l6] B. ROY and S. RANGANATHAN, Trans. Indian Inst. Met& 29, 62 (1976).

(Receiced June 10, 1981)