An analysis of cavitation occurring in near-&gg titanium aluminide during superplastic deformation
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General Background
THE potential for producing superplastic near-␥ titanium aluminides exists because relatively stable microstructures containing fine two-phase mixtures of ␣2-Ti3Al and ␥-TiAl can be developed via thermomechanical processing.[1] Results to date show that these materials do indeed exhibit good superplastic characteristics, i.e., they can achieve tensile elongations of several hundred percent or more and have strain-rate sensitivity values (m) greater than 0.3,[2,3] where m is defined as m⫽
d(log )
冢d(log ˙ )冣
[1]
T,
where is the stress, is the strain, ˙ is the strain rate, and T is the temperature. A number of near-␥ compositions (which range in composition from 43 to 49.5 pct aluminum) have been tested by researchers; the window of superplastic behavior is found to be dependent of composition and grain size. While some parallels can be drawn to other superplastic two-phase titanium alloys such as Ti-6Al-4V, one notable exception is that near-␥ titanium aluminides show a propensity to cavitate under appropriate superplastic testing conditions, which leads to premature failure.[3]
C.M. LOMBARD, Materials Research Engineer, formerly with the Metals Branch, Metals, Ceramics and Nondestructive Evaluation Division, Air Force Research Laboratory/Materials Laboratory (AFRL/MLLM), is with the Processing and Fabrication Branch, Manufacturing Technology Division, AFRL/MLMP, Wright-Patterson Air Force Base, OH 45433-7739. A.K. GHOSH, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/ML, WPAFB, OH 45433-7817. Manuscript submitted August 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
The current research centered on developing an understanding of cavitation behavior in near-␥ titanium aluminide under tensile superplastic forming conditions. A superplastic material undergoing tensile deformation will neck down to an almost negligible cross-sectional area when it does not fail prematurely due to cavitation. Control of cavitation during deformation can be difficult if the material behavior is not understood prior to superplastic forming. Increasing or decreasing the temperature or strain rate may exacerbate cavitation if the trends of either increasing or decreasing m values are not known. An increase in temperature or a decrease in strain rate would be the first choices, as these would decrease the stress and, thus, increase the critical radius of a stable cavity. On the other hand, these changes could also enhance dynamic grain growth and, thereby, result in greater accommodation distances and higher flow stresses. B. Cavity Initiation Cavity formation between grains is seen as the intermediate step of the grain-boundary sliding process. Under optimum forming conditions, a transfer of matter occurs by diffusion and/or dislocation processes into the regions betwe
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