1997 TMS Annual Meeting: Wednesday Abstracts

FUNDAMENTALS OF GAMMA TITANIUM ALUMINIDES: Session VI: Microstructure/Property Relationships--Creep and Environmental Effects

Session Chairpersons: Patrick L. Martin, Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360; Kevin J. Hemker, Dept. of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218-2686

1:55 pm OPENING REMARKS

2:00 pm INVITED

REDUCTION OF PRIMARY CREEP IN TiAl ALLOYS BY PRESTRAINING: Jian N. Wang, L.M. Hsiung, T.G. Nieh, Lawrence Livermore National Laboratory, L-370, P.O. Box 808, Livermore, CA 94551-9900

In some engineering applications, TiAl components are limited to undergo less than 0.3-0.5% creep distortion. However, existing fully lamellar TiAl alloys deform to this strain within a short period of time during primary creep. Thus, the reduction of primary creep has become a key issue. Present experiments show that this creep can be significantly reduced by prestraining. A sample prestrained at a high stress could undergo no forward deformation at a subsequent low stress even after a long period of time. Microstructural examination indicates that the motion of misfit dislocations along interfacial and thermal twin boundaries is the dominant mode of deformation at low stress. The mobility of these dislocations after prestraining and the physical origin of this effect will be discussed.

2:30 pm

CHANGES IN MICROSTRUCTURE DURING PRIMARY CREEP IN A Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si ALLOY: D.Y. Seo, T.R. Bieler, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226; D.E. Larsen, Howmet Corporation, Whitehall, MI 49461

Cast gamma titanium aluminides are gaining acceptance as potential replacements for superalloy and steel components in many applications. One particular alloy with W, Mo and Si additions has shown exceptional primary creep resistance. Quantitative microscopic comparisons were made between microstructures in the grip sections and the deformed gage section in creep specimens deformed to 0.5% strain, using optical, SEM and TEM techniques. The lamellar spacing in lamellar grains systematically decreased after creep deformation. Precipitates containing refractory elements nucleated and grew at a faster rate in the deformed part of the specimen, as compared to the deformed region, and the precipitates were generally smaller and more homogeneously nucleated. Precipitates have been identified to be silicides or beta phase. These observations indicate that strain assisted nucleation of precipitates accounts for much of the excellent creep resistance.

2:50 pm

EVOLUTION OF MICROSTRUCTURE DURING THE ELEVATED TEMPERATURE CREEP OF SINGLE-PHASE GAMMA Ti47Al51Mn2 ALLOY: Min Lu, Kevin J. Hemker, Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218

Mechanical experiments have shown that single-phase gamma TiAl does not exhibit steady-state creep. Instead, the microstructure evolves along the creep curves. To investigate microstructure/property relations, constant stress creep tests on single-phase polycrystalline gamma Ti47Al51Mn2 were conducted at temperatures below and above the peak temperature (600°C). Primary creep, minimum strain rate regions and tertiary creep were investigated. Creep tests conducted at 550°C, 550°C and 700°C were stopped at different stages and specimens were prepared for TEM observations. No evidence of the subgrain formation has been found in any of the creep tests. Faulted dipoles and superdislocations, as well as a few ordinary dislocations, were present in the microstructure of the early stage of the creep tests. At longer creep times, faulted dipoles were annihilated and superdislocations were found to form "roof" like barriers. TEM observations of faulted dipoles and superdislocations were correlated, where appropriate, with computer simulations. The density and the degree of bowing of the "cusps" on ordinary dislocations increased dramatically with increasing creep strain. The configuration of the "cusps" formed on the ordinary dislocations were studied with weak-beam TEM and were compared to those found in the yielding and the prestraining studies of the same alloy.

3:10 pm

MICROSTRUCTURE EVOLUTION AND CREEP DEFORMATION OF A MO-CONTAINING NEAR-GAMMA ALLOY: Eric Ott, Tresa Pollack, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

The addition of Mo in near-gamma alloys promotes the formation of the beta phase. The presence of the beta phase stabilized by Mo as well as other alloy additions including Nb and Cr has been shown to result in good hot working characteristics. Microstructural evolution of a Ti-46.4Al-2Nb-2Cr-0.8Mo fine grained wrought alloy during annealing and subsequent heat treatment will be reported with a specific emphasis on the effects of the beta/B2 phases on the development of gamma and lamellar microstructures. Results of creep testing and subsequent microscopy of near-gamma and duplex microstructures will also be discussed and compared to that of a similar Ti-48Al-2Nb-2Cr type alloy containing no Mo.

3:30 pm BREAK

3:50 pm

CREEP DEFORMATION OF NEAR/FULLY LAMELLAR Ti-48Al-BASED ALLOYS: G.B. Viswanathan, V.K. Vasudevan, Dept. of Mat. Sci. &Eng., Univ. of Cincinnati, Cincinnati, OH 45221

In this study, the creep properties of the binary Ti-48Al and quarternary Ti-48Al-2Cr-2Nb alloys are investigated for the near and fully lamellar microstrutures in the temperature range of 700-815°C and stress of 103-240 MPa. The creep behavior of all the tested microstructures is best described by the power law creep. Activation energy (Q) between 350-370 KJ/mole and stress exponent (n) in the range of 5.0-8.0 was obtained for the binary Ti-48Al alloy. The corresponding values for the Ti-48Al-2Cr-2Nb alloy were ~315 KJ/mole and ~5.0. The effect of grain size and the lamellar spacing on the creep behavior is discussed. Defect analysis through weak beam dark field imaging was carried out to elucidate the dislocation mechanisms in the crept samples. Observations of the crept samples with the fully lamellar structure in the binary alloy indicate that both 1/2<110] and 1/2<112] dislocations are active, that the deformation is highly anisotropic and that the lamellar interfaces obstruct slip (depending on the orientation). The combination of low creep rates with a high stress exponent for the fully lamellar structures is thought to be caused by the combined effect of anisotropic deformation and lamellar boundaries obstructing slip; the dislocations that are piled up at the interfaces overcome these obstacles by local climb, which is reflected in the Q values near that of self-diffusion of Ti in TiAl. The formation of substructures within the laths and the lamellar interface structures before and after creep will be discussed.

4:10 pm

DEVELOPMENT AND EVALUATION OF AN OXIDATION-RESISTANT COATING ALLOY FOR GAMMA TITANIUM ALUMINIDES: M.P. Brady, W.J. Brindley, J.L. Smialek, MS 106-1, NASA Lewis, Cleveland, OH 44135

A two-phase (TiAl) + Laves (Ti[Cr,Al]2) oxidation-resistant coating alloy, Ti-51.25Al-12.25Cr at.%, has been developed for the + 2 class of titanium aluminides. The composition of the coating alloy was selected so that the microstructure consists of the g phase, and a minor volume fraction of the oxidation-resistant Laves phase. By basing the microstructure on the g phase, the mechanical properties and substrate compatibility are optimized. The volume fraction of the Laves phase is kept to a minimum because it is extremely brittle. The Ti-51.25Al-12.25Cr coating alloy was applied to the + 2 alloy, Ti- 48Al-2Cr-2Nb, by low pressure plasma spray (LPPS). Oxidation tests at 800°C and 1000°C in air indicated that the coating successfully protected the substrate from oxidation. The oxidation behavior and mechanical properties of the coating alloy will be discussed.

4:30 pm

VERY LONG TERM OXIDATION OF Ti-48Al-2Cr-2Nb AT 704°C IN AIR: I.E. Locci, M.P. Brady, R.A. MacKay, J.W. Smith; MS106-5, NASA Lewis, Cleveland, OH 44135

The alloy Ti-48Al-2Cr-2Nb was exposed from 500h to 9000h at 704°C in air. A complex, mixed alumina/titania scale, rather than a protective alumina scale, was formed. After 9000h, this scale was approximately 15 microns thick. A continuous Ti-rich nitride layer was formed at the alloy/scale interface. The alloy ahead of this nitride layer was depleted in Ti and enriched in Al. Comparisons with the oxidation behavior of binary TiAl alloys, and implications of the scale microstructure for mechanical properties will be discussed.

4:50 pm

RECENT DEVELOPMENTS ON THE OXIDATION OF TiAl: R. Wheeler, R. Banerjee, H.L. Fraser, Department of Mater. Sci. and Eng., Ohio State University, Columbus, OH 43210

The high level of Al in TiAl based alloys does not always relate to slow oxidation kinetics. While protective alumina scales are formed during high temperature exposures in an oxygen environment, mixed Al2O3/TiO2 scales are formed when similar tests are conducted in air. In this latter case, the surface corrosion products form a complex layered structure which governs oxidation rates. In a Ti-48Al-2Cr (at.%) alloy oxidized in air at 800°C for 1 hour, two distinct layered microstructures are observed. One is composed of an Al depleted underlayer and a mixed Al2O3/TiO2 outerlayer. The other is composed of the same depleted layer with a multicomponent, multilayered outerscale. This second microstructure is characteristic of a nitrogen effect found to be associated with increased oxidation kinetics. Detailed TEM analyses has been conducted to identify the reaction products present in the scale. The deduced reaction paths suggest a rate behavior based on both interfacial reactions and the inward and outward anion and cation diffusion.

5:10 pm

CLOSING REMARKS: Kwai S. Chan, Southwest Research Institute, San Antonio, TX 78228-0510