Microstructure control and high temperature properties of TiAl base alloys

An equiaxed fine grain structure, a gamma grain structure with the precipitated alpha(2) laths, and a fully lamellar structure were obtained by the microstructure control using thermomechanical processing and heat treatment. The key to obtaining the equiaxed fine grain structure using isothermal forging is to decompose the lamellar structure and then produce the fine grain microstructure through dynamic recrystallization. TiAl base alloys consisting of fine equiaxed grains, in particular, Ti-39Al-9V consisting of the gamma and B2 phases exhibited superplastic elongation of more than 600% at 1423 K. Creep rupture properties of TiAl binary alloys with various microstructures were studied in purified He in the temperature range from 1073 to 1373 K. Above 1173 K the precipitated alpha(2) phase improved the steady state creep rate and creep life. At 1023 K, the alpha(2) phase did not improve the creep rate, although the steady state creep rate decreased and the creep life increased as the gamma grain size increased. (C) 1998 Elsevier Science Limited. All rights reserved.