HOT-COMPRESSION BEHAVIOR AND MICROSTRUCTURE EVOLUTION OF PRE-ALLOYED POWDER COMPACTS OF A NEAR-GAMMA TITANIUM ALUMINIDE ALLOY

The hot-compression behavior and microstructure evolution of pre-alloy ed powder compacts of the near-gamma titanium aluminide alloy Ti-48at. %Al-2at.%Cr-2at.%Nb were determined and compared with results for the same alloy processed via ingot metallurgy methods. Gas-atomized powder was by consolidated hot isostatic pressing at a low temperature (1010 -degrees-C) to retain a fine microstructure. Samples of this material were upset isothermally at temperatures between 1000 and 1260-degrees- C and strain rates between 10(-3) and 10(-1) s-1. The stress-strain cu rves revealed moderate amounts of flow softening which were attributed primarily to dynamic recrystallization and secondarily to deformation heating (at the highest strain rate studied). The absence of large la mellar colonies in the powder metallurgy (PM) material was deduced to be the reason for peak flow stresses which were much lower than those previously noted for cast plus hot isostatically pressed Ti-48at.%Al-2 at.%Cr-2at.%Nb. As for ingot metallurgy near-gamma titanium aluminides , the flow stress and grain size showed a strong dependence on tempera ture and strain rate. The steady state flow stresses for the PM alloy were almost identical with those for cast plus hot isostatically press ed and for cast plus hot isostatically pressed plus isothermally forge d Ti-48Al-2Cr-2Nb. The PM materials also revealed a noticeable degree of microstructure non-uniformity which persisted even after hot-compre ssion testing at various sub-a transus temperatures.