The effect of heat treatments on microstructures and primary creep deformation of four investment cast titanium aluminide alloys

Several heat treatments were developed and applied to investment cast Ti-45Al-2Nb-2Mn(at%)+0.8v%TiB2 XD(TM), Ti-47Al-2Nb-2Mn(at%) + 0.8v%TiB2 XD(TM), Ti-47Al-2Nb-2Cr(at%), and Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si alloys in an effort to enhance creep properties and decrease heat treatment time compared to current practices. Results show that the creep resistance of WMoSi can be improved by 10 times with heat treatment, and 47-2-2 can be improved by 3 times, and the XD alloys can be improved slightly, or not at all with lower Al level. The variation, or lack of variation, in creep resistance with heat treatment can be explained by differences observed in the microstructures and textures produced by the various heat treatments. Quantitative microscopic comparisons were made between microstructures in undeformed and deformed regions in creep specimens deformed to strains between 0.1 and 1.5% strain, using optical, SEM and TEM techniques. The lamellar spacing in lamellar grains systematically decreased by 15-35%, with increasing stress, during the first 0.1-2% strain. Precipitates containing W, Mo, and/or Si nucleated and grew at a faster rate in the deformed part of the specimen, as compared to the grip section, with approximately twice the volume fraction in the deformed region, and the precipitates were generally smaller and more homogeneously dispersed. These observations indicate that strain assisted nucleation of precipitates accounts for much of the excellent creep resistance of the WMoSi alloy.