Influence of microstructure on the tensile and creep properties of titanium aluminides processed by powder metallurgy

The microstructural development of prealloyed, hot isostatically pressed Ti48Al3Cr and Ti48Al2Cr2Nb-powder was studied as a function of heat treatment condition. It is shown that a wide range of microstructures from fine, duplex (20μm) to coarse, fully lamellar (600μm) can be obtained which in turn controls mechanical properties. Best tensile ductility was obtained with a duplex microstructure containing approximately 30%--50% lamellar grains (100μm). In this heat treatment condition, up to 2.1% plastic elongation was measured at room temperature which is similar to values reported for cast materials. The reduction of oxygen from 1,600ppm to 1,050ppm showed a significant improvement in room temperature ductility for duplex as well as for fully lamellar microstructures, whereas a further reduction in oxygen seemed not to affect tensile properties. The creep properties of duplex and fully lamellar microstructures were continuously measured at 750 C and 250MPa in air and compared with TiAl-alloys processed by ingot metallurgy as well as nickel-base superalloys using Larson-Miller extrapolations. If a Larson-Miller parameter realistic for TiAl-alloys (C = 18) is used, it becomes apparent that fully lamellar microstructures are needed in order to exceed the specific creep strength of nickel-base superalloys at long service times