Enhanced superplasticity and strength in modified Ti-6AI-4V alloys
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INTRODUCTION
ALLOYTi-6A1-4V is superplastic at temperatures between approximately 1150 K and 1220 K. 1 Full-scale components of this alloy are frequently formed at temperatures near 1200 K, where the a- and/3-phases coexist in approximately equal volume fractions. Partitioning of the substitutional alloying elements between the a- and/3-phases slows grain coarsening when substantial volume fractions of both phases are present. 2 At higher temperatures, the volume fraction of a-phase is lower, allowing more rapid grain coarsening. As a result of rapid grain coarsening, superplastic properties of Ti-6AI-4V degrade as the forming temperature increases above 1200 K. At temperatures substantially below 1200 K, the deformation processes active in superplastic deformation do not proceed sufficiently rapidly to produce superplastic behavior. Consequently, superplasticity in Ti-6AI-4V occurs only in the temperature range where the a- and/3-phases are present in similar volume fractions and where deformation processes proceed at reasonable rates. Similar observations have been made for other alloys with duplex microstructures. 3-6 Lower superplastic forming temperatures would be desirable for a number of reasons, such as reduced oxidation, shorter forming cycle times, lower cost die materials, and increased die life. Alloy modifications designed to lower the optimum superplastic forming temperature of Ti-6A1-4V must alter two characteristics of the alloy. 7 First, a substantial volume fraction of/3-phase is required for superplastic deformation of titanium alloys. 8'9 Consequently, alloy additions should stabilize the/3-phase, thereby increasing the J. A. WERT is with Rockwell International Science Center, P.O. Box 1085, Thousand Oaks, CA 91360. N.E. PATON is with Rockwell International Corporation, 600 Grant Street, Pittsburgh, PA 15219. Manuscript submitted May 2, 1983.
METALLURGICAL TRANSACTIONS A
volume fraction of r-phase at temperatures below the conventional superplastic forming temperature. Second, the creep processes essential to superplastic deformation proceed too slowly at temperatures substantially below 1200 K for superplasticity to occur at reasonable strain rates. Alloy additions with high diffusivity accelerate the rates of processes that contribute to superplastic deformation.4 Based on these arguments, additions of strong r-stabilizers with rapid diffusion rates in Ti-6A1-4V are expected to lower the optimum superplastic forming temperature of this alloy. Previous investigations of lowering superplastic forming temperatures for titanium alloys have been conducted by Paton and Hall7 and by Hammond. 4 Paton and Hall studied the effect of Fe additions to Ti-6A1-4V on superplastic properties, as well as room temperature strength. Their results showed, for example, that addition of 2 wt pct Fe to Ti6AI-4V lowered the flow stress required for deformation at 1144 K and at a strain rate of 2 x 10-4 s -~. Conversely, they also found that addition of the slowly diffusing element Mo to Ti-6A1-4V substantially
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