PROCESSING OF A TiAl ALLOY BY POWDER METALLURGY AND HOT WORKING

Titanium aluminides have superior high temperature properties and are attractive for gas turbine applications. Cast titanium aluminides have large grain sizes which reduce hot workability. Alternately, powder metallurgy technique is explored by mixing of elemental powders for 2 hrs to prepare 46Ti-46Al-4Nb-2Cr-2Mn followed by Hot Isostatic Pressing (HIP) at 1150 degrees C for 4 hrs under a pressure of 150 MPa. In addition to the formation of TiAl solid solution, the microstructure showed a small volume fraction of Nb-rich phase. The HIP'ed billets were compressed at temperatures and strain rates in the ranges of 850-1050 degrees C and 0.0001-10 s(-1). The flow curves exhibited flow softening leading to a steady-state flow at strain rates lower than 0.1 s(-1) while fracture has occurred at higher strain rates. The hot workability was good at temperatures higher than 950 degrees C and the flow stress at different temperatures and strain rates follows the kinetic rate equation with a stress exponent of 4.4 and an apparent activation energy of 387 kJ/mole which is close to that for self-diffusion of Al in gamma TiAl.