Superplastic behavior in a powder-metallurgy TiAl alloy with a metastable microstructure

摘要

Superplasticity in a powder-metallurgy TiAl alloy (Ti-47Al-2Cr-2Nb) with a metastable microstructure has been studied. Samples were tested at temperatures ranging from 650 to 1100(degrees)C, and at strain rate ranging from 10(sup -6) to 10(sup -4) s(sup -1). An elongation value of over 300 obtained at a strain rate of 2 x 10(sup -5) s(sup -1) and at a temperature as lo as 800(degrees)C, which is close to the ductile-to-brittle-transition temperature. This is in contrast to the prior major observations of superplastic behaviors in TiAl in which typical temperatures of 1000(degrees)C have usually been required for superplasticity. It is proposed that the occurrence of superplasticity at 8000(degrees)C in the present alloy is caused by the presence of a B2 phase. During superplastic deformation (grain boundary sliding), the soft P grains accommodate sliding strains to reduce the propensity for cavitation at grain triple junctions and, thus, delays the fracture process. The final microstructure consists of stable, equiaxed y+a(sub 2) grains.