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ION RESISTANT MATERIALS/COMPOSITES . .. , . . .. , . ., .

By STANLEY ABKOWITZ, President PAUL F. WEIHRAUCH, Director of Research

and SUSAN M. ABKOWITZ, Director, Engineering & Quality Dynamet Technology, Inc. Burlington, MA 01803

Editor's Note: The cl~ip process, cornbi~ring cold artd \lot isostatic pressing, has been applied to the nranufacture of nretal matrix composites trtilizing particulate reinforce- ments. Tliese nlicrocomposites demon- strate significant increase in the strength, ~ m d ~ r l r ~ s and use terrrperature of advanced structurnl alloys. This article which is froiir a paper presented nt MPIFIAPMI 7993 irrternatiorial Conference and Exhibition on Powder Metalllrrgy and Particrrlate Materials i n Nashville, T N , Mn!/ 16-19, describes the mantrfacture and. properties of these M M C S with particular ntlorliorr to tllc significant inrproveirlents iri ~irodultrs and strength at both room and elevated tenlperatures.

A family of titanium MMC's, CermeTiO", has been devel- oped with superior properties . .*

by combining cold and hot isostatic pressing (CHIP) manufacturing tech- nology commercialized for monolithic titanium alloys2. The initial successful development involved the use of titani- um carbide (Tic) as the particle rein- forcement3. Subsequent work has involved the use of titanium diboride (TiB,) as the reinforcement. Recent efforts have introduced the intermetal- lic compound titanium aluminide (TiA1) as the reinforcement phase. Most of the work has been done with Ti-6Al-4V as the matrix alloy; however,

'Cerme Ti@ is a registered trademark a t the office of U.S. Palenb and Trademarks and is the subject of U.S. and foreign patents assigned to Dynamet Technology, Inc. ,

other matrices have been successfully employed. These include Ti-6A1-6V- 2511 (for higher strength), Ti-6A1-2%- 4Zr-2Mo and its silicon containing vari- ant (for elevated temperature strength) and Ti-3AI-2V (for greater machinabili- ty and workability).

All these materials have been identi- fied as "CermeTi" compositions. To distinguish among various materials a generic designation system has been adopted identifying each microcom- posite as CermeTi-X-Y, where X identi- fies the particulate, Y, its loading in weight percent. The matrix alloy is assumed to be Ti-6.41-4V unless other- wise identified. The X designations are identified as follows:

Representative microstructures ol Tic and TiB, containing microconipos- ites are shown in Figs. 1 and 2. In all cases the particle/matrix interfaces are clearly established having maintained the integrity of the particulate within the Ti-6A1-4V matrix. This is evident throughout the niicrostructure nf thc part and from part to part processed by this technology. The particulate is essentially homogeneously dispersed and the microcomposite has achieved virtually 100% density.

MANUFACTURING PROCESS The CHIP process, which pl-ovides

an economical manufacturing roule for near net shape component production, also proviclcs a prodt~cl~on ~ ~ p t i ~ v i I ~ I mill product form manufart i l~c. ( 8 1

unique composite niatcrial- 1 schematic uf this prorrss 15 ~ I ~ . I I i l l

Fie. 3. " Thus, a rnicrocornpositc consisting of a This process consists of blendin:; clr- 10 wt% addition of Tic in a Ti-6AI-4V mental and master alloy powdew cold matrix is designated CermeTi-C-10. isostatic pressing (CIP) to a pn1forn1

Fig. 2 Uniform microstruc- ture of 10 wf% TiE$ in Ti- 6AMV. CHIPpmcess.

Industrial Healing - Seplernbei 199.3

Ftg 3 Schemat,c diagram of the CHiPprocess.

shape, vacuum sintering to a closed porosity, followed by containerless (i.e. without the need for expensive wpendable tooling ) hot isostatic press- i n ( I ) Alternate processing routes i~,<Ihldr forging, extrusion, or rolling i n m i l l~e I'/M preform as appropriate I ) . L ~ , Y ~ LIPOII the final part configura- tiim

Incorporating the particulate addi- tions is readily accomplished during (he powder blend preparation. Care mnst be taken to assure uniformity not only in the blend but also during load- ing and cold pressing. In processing the composite further, slight variations are made in sintering and HIPing para- meters as necessary to avoid parti- cle/matrix interaction, and to achieve

high density. The CHIP process has been demonstrated as a practical man- ufacturing process route for near-net shape manufacture of Tic particulate reinforced composite materials. Work is continuing on achieving higherfrac- tions of Tic addition, further refining processing parameters for TiB, and TiAl composites, containing and incor- porating alternate titanium alloy matri- ces to further enhance properties.

CermeTi materials have been forged successfully at ALCOA4 and have been shown to behave much like titanium alloys during forging, with metal flow achieved without any deleterious effect on the reinforcement. Corresponding data with Tic reinforced Ti-6A1-4V has been presented by the University of

Fig. 5 Micm- composite combined with monolithic Iita- nium alloy to form a macro- composite lntegraiiy bonded single malarial. High hardness of gear teeth is combined with ductile inner structure.

Birmingham (T.P. Johnson et aU5. A* o u ~ r b w t h of this technology

involves alternately layering compos- ites with monolithic alloys to create selectively reinforced components with unique properties. Since a common alloy matrix is generally used, no dilu- tion of either phase occurs. These micro-macro composites have since been termed Common Matrix, Micro- Macro Composites or CM3Cs. Fig. 4 illustrates CM3C concept, in this instance showing a T i c reinforced microcomposite with a monolithic tita- nium alloy. As a result of the self-dif- fusion of the titanium alloy, no dis- cernible "bond line" exists at the origi- nal interface of the MMC with the tita- nium alloy. Fig. 5 illustrates a dual hardness gear application for CM3C.

The potential design flexibility offered by CM3C technology is illus- trated by the cross-section of the "pie extrusion" shown in Fig. 6. The pie pieces consist of 20 wt% Tic reinforced Ti-6A1-4V micro-composite wedges alternating with wedges of Ti-6A1-4V alloy. The detailed configuration was produced in a 1/2 inch diameter bar using the CHIP process followed by the extrusion process.

PHYSICAL PROPERTIES Tensile Strength

Among this family of CermeTi mate- rials, the 10 wt % Tic in a Ti-6A1-4V matrix fabricated by the CHIP process

Fig. 6 CM3C pie exlrusion consisting of alternating wedges of20% TiCtTi- 6Ai.4 V microcomposite and monolithic Ti-6AI-4V.

Industrial Heating - September, 1893

s 10% TIClTI.6AI-4V. Forged Ti.6Al-4V Band a

€. 6 110 750 6 6 R eo z

i LI

70 500 F

60 RT BM 1 W

Temperature (S)

with and without subsequent forging or extrusion has been the most com- pletely characterized. Comparison of tensile properties of this MMC with those of the monolithic matrix alloy indicates that the room temperature yield and tensile strengths are increased 10.15% and the strength differential is maintained to 1200 O F (GOT). Reason- able engineering ductility is retained throughout the temperature range with elongation of up to 3.5% measured at room temperature6. At 1200 OF the duc- tility of the composite and that of the matrix material do not differ significant- ly. As expected, strength increases and ductility decreases with increased parti- cle loading.

Further improvement in tensile properties has been achieved through microstructural refinement from hot working as well as from a change in particle type and loading level. Fig. 7 shows the improvement in elevated temperature strength after beta forging of 10% and 15% Tic reinforced Ti-6AI- 4V', and Fig. 8 shows the correspond- ing effect from extrusion processing, as well as the even more effective strengthening effect of the TiB, particu- late over the Tic.

Modulus The most dramatic property

improvement of the particle reinforced CermeTi-C-10 material compared to

Fig. 7 Strength of 10% Tic and 15% TiC reinlorced Ti-6AI- 4V after beta lorgig.

Fig. 8 Tensile strength 01 Tic, TiB, and M I reinforced composites alter exlmsion.

monolithic Ti-6A1-4V alloy is in the elastic modulus. Room temperature modulus values are about 15% higher on average than Ti-6A1-4V (Fig. 9). Furthermore, the modulus advantage persists over the temperature range to

1200 'F (650•‹C) and represents as much as 600 OF (315 "C) improvement7.

The effects of changing loading level and particle type on elastic modulus is shown in Fig. 10. These data indicate that TiB, is a more effective stiffener than the Tic, with a 10% TiB, loading level providing approximately the same level of modulus increase as a 20% loading of Tic. The 10% TiB, and 20% Tic microcomposites, with modu- lus values of over 21 at RT and of over 17 at 1000 O F (540 OC), provide greater than a 2 Msi improvement over the 10% Tic microcomposite, which shows a similar 2 Msi improvement over the Ti-6A1-4V alloy. The 15% TiB, addition provides an additional 1.5 Msi, yield- ing a modulus value of over 23 Msi at room temperature and of approximate- ly 20 Msi a t 1000•‹F (540•‹C). These improvements are achieved with essentially no sacrifice in weight since the CermeTi density is close to that of the titanium alloy. The density, modu- lus, and specific modulus (modulus to density ratio) of these materials is shown in Table I.

Fracture Toughness Even with limited room temperature

ductility, these particle reinforced tita- nium composites provide useful levrls of fracture toughness.

Improvements have now becn lnde

Fig. 9 Dynamic (elastic) modulus vs. temperature 01 10% Tic rein- forced Ti-6AC4V.

Fig. 10 Elastic modulus of CermeTi materi- als.

Industrial Healing - September, 1lW.'-

to the fracture toughness of this class of materials, resulting primarily from the upgrade of the Ti-6A1-4V matrix mater- ial to the higher purity extra-low chlo- ride grade of titaniumR. With this change, KIc values of 40 and 29.1 ksi din have been reported for 10% Tic and 20% Tic additions, respectively. The TiB, reinforced composites also provide a useful level of fracture toughness, with values of 27.6 and 24.2 ksi din for additions of 10% TiB, and 15% TiB,, respectively (see Fig. 11).

Hardness and Wear Resistance Ceramic particles embedded in the

titanium matrix increases hardness of the resulting composite material. The

flexibility of the CHIP process permits the increased hardness, shown in Fig. 12, to be applied in specified locations and to any required depth of the mate- rial. More significant than hardness itself, however, is the resulting wear resistance of the composite materials in a variety of operating and environmen- tal conditions, where the conventional baseline titanium alloys are known to possess poor tribological properties.

Results from two completely differ- ent wear tests demonstrate the poten- tial for improvement in widely varied applications. Fig. 13 shows the sig- nificant improvement of CermeTi over Ti-6AI-4V in pin-on-flat wear testing against ultra-high molecular

Fig. 1 1 Fracture Toughness (KJ of Tic and TiB, reinforced Ti-6Al-4V micro- composites measured by four-point bend testing of unnotched specimens.

weight polyethylene (UHMWPE) for a biomedical application9. While not yet achieving the resistance of the Co- Cr alternative, the significant improvement was achieved with only a 10% Tic addition, with the expecta- tion that further improvement could be achieved with higher particulate loading.

The 10% Tic reinforced Ti-6A1-4V composite was also tested in a simulat- ed engine test to determine durability and effect of the composite material on valve guide wear. In this test a com- puter was used to cycle the engine through a speed range of 6000-8000 rpm with a typical Winston Cup Stock Car 500 mile race profile. After testinfi, neither the Tic containing valve stem, nor the standard valve guide showed measurable wear. These results indi- cated significant improvement over conventional titanium which requires a protective coating for use in such valve applications; the embedded ceramic particles apparently were successful in both protecting the titanium and not degrading the guide material.'0

.ig. 13 Pin on flat wear testing iwsuits comparing Cerme Ti C-10 with Ti-6AI- Vand Co-Cr against UHMWPE.

Rotating Bending Fatigue f=60Hz at R.T.

. (Ti4AI-4V)+ $j 600 Ill mars% Tic

(I133MPe)

. q 4 O 0 "MI.'" ... 2 ---Q- --..-.-. 2-.. ..-. 3: 300

@PIMP.) % ZOO P . . ....... . . ....

l oa lo' l o5 l o 6 10' l o 8

Fig. 12 Measured hardness values for various Cerme Ti materials. Fig. 14 Fatiguepmpetiies of Ti-6Ai-4V+ 10% Tic.

36 Industrial Heating - September, 199.7

Fatigue and Creep I:;lkigue properties of a 15% TiC/Ti-

hiZ1-4V microcomposite were found to be comparable to Ti-6A1-4V castings. Fatigue fracture strength wi th 10% T i c below lo6 cycles w a s even higher (Fig. 14).

In regard to creep properties, initial evaluations of the 15 wt% T i c contain- ing Ti-6A1-4V h a v e s h o w n d ramat i c i m p r o v e m e n t o v e r c o n v e n t i o n a l w r o u g h t Ti-6A1-4V. W h i l e speci f ic c r e e p d a t a h a s y e t t o b e pub l i shed , exce l l en t c r e e p p r o p e r t i e s c a n b e expected from this class of materials.

CONCLUSIONS M a n u f a c t u r e of t i t a n i u m m e t a l

matrix composites utilizing economical p a r t i c l e r e i n f o r c e m e n t h a s b e e n demonstrated by the P/M processing ~ O I I I C . These composites exhibit homo- :;,,n<.t)1rs microstructures wi th clearly i , i~ .~ l> l i shed part icle/matrix interface.

I i l , ~n i l tn i c a r b i d e ( T i c , t i t a n i u m cl ibor ide (TiB,), a n d t i t a n i u m a l u - m i n i d e (TiAI) re inforced Ti-6A1-4V have been produced by P / M a n d with

subsequent forging or extrusion, a t var- i o u s l e v e l s of r e in fo rcement . Properties, particularly room and ele- vated temperature strength a n d modu- lus, offer s igni f icant improvement , w h i l e u s e f u l f r ac tu re toughness h a s been maintained.

These material combinations, w i th the near net shape P / M manufacturing route, offer a viable alternative for eco- nomical titanium MMC application, for both military and commercial applica- ti0ns.V

References 1. S. Abkowitz and P. Weihrauch, "Trimming the Cost of MMC's," Ad7,nnced Mnterinls and Processes, (1989), pp. 31-34. 2. 5. Abkowitz, "Production of Complex Full Density P/M Titanium Shapes by the CHIP Process," APMJ Nnfionnl Powder Metnlltrrgy Conference, Philadelphia, PA, 1981. 3. S Abkowitz, P. Weihrauch and 5: Kraus, "Development and Evaluation of the Metallurgy and Shape Fabrication of Titanium Matrix CermeTi Composites for High Modulus, High Temperature Applications," Final Report, Navy Contract No. N69021-86C-0222, October 1988.

4. G. Kuhlman, A. Charkrabarti and S. Abkowitz, "Properties of P IS Titanium Forgings", Aeromnt Conference, Long Beach, CA, May 1990. 5. T. P. Johnson, M. M. Loretto, M. J. Walker, M. W. Kearns, "The Forging Characteristics of Two Ti-6A1-4V Based Particulate Composites." 6. H. Fujii, T. Yamazaki, T. Horiya and K. Takahashi, "High Performance Ti-6A1-4V + Tic Alloy by Blended Elemental Powder Metallurgy", Is1 Pacific Rim Internotionnl Conference on Advanced Mnterinls, Hangzhou, China (1992). 7. S. Abkowitz, P. Weihrauch, S. M. Abkowitz. "Advanced P/M Ti Alloy Matrix Composites Reinforced with Ceramic and Intermetallic Particles", 7th lnternnlionnl TMS/TDA Conference on Titanium, S a n Diego, CA, 19921 8. S. Abkowitz. P. Weihrauch. S. M Abkowitz, M.G. wells, "Low cos t P/M Manufacture of Titanium alloy Components for Fatigue Critical Applications", 3rd Internntionnl MPIF Conference on PIM in Aerospace, San Diego, CA, 1993. 9. D. Schroeder, Biomet, Inc., unpublished data. 10. J.W. David, North Carolina State University, unpublished data.