The effect of heat treatments on microstructures and primary creep deformation of investment cast titanium aluminide alloys and polysynthetically twinned (PST) crystals

Several heat treatments were developed and applied to several investment cast duplex (equiaxed+lamellar) TiAl alloys (Ti-45∼47Al-2Nb-2Mn+0.8v%TiB 2XDTM, Ti-47Al-2Nb-2Mn+0.8TiB2XDTM with interstitial elements, Ti-47Al-2Nb-2Cr, and Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si) in an effort to enhance creep properties. Creep behavior in polysynthetically twinned (PST) crystals with different orientations was also investigated.; Primary creep resistance of W-Mo-Si alloys can be improved by about 10 times with heat treatment, and the XD alloys with additions of interstitials can be improved by about 7 times, and 47-2-2 can be improved by about 3 times, and the XD alloys can be improved slightly, or not at all when the Al level is lower. The variation in creep resistance with heat treatment can be explained by differences observed in the microstructures and textures produced by the various heat treatments. The XD alloys with high O show large lamellar volume fraction (40%), since oxygen is an alpha stabilizer. Observations indicate that strain assisted nucleation and/or growth of precipitates accounts for much of the excellent creep resistance of the W-Mo-Si alloy.; The lamellar spacing in lamellar grains systematically decreased by 15--35% with increasing stress, during the first 0.2--0.5% strain at the early stage of primary creep. More refinement of lamellar spacing occurs at lower temperature and higher stress. The refinement process is a consistent microstructural feature during primary creep deformation in all investment cast TiAl alloys. The refinement occurred by mechanical twinning (easy mode deformation) or/and alpha 2 shear transformation parallel to lamellar boundary.; The stress exponent and activation energy for creep in the lamellar microstructure depend on the direction of the stress tensor. In comparing activation energies of the early process in primary creep in the W-Mo-Si alloy with creep in the soft orientation in a PST crystal, the activation energies are small, near 150 kJ/mol. This indicated that the early stage deformation in TiAl alloys correlates closely with PST crystal creep in easy mode deformation. From simulations of lamellar refinement, computed local shear strains are similar or higher than effective shear strain. There are some correlations between the computed local shear strain and microstructural changes during primary creep deformation. (Abstract shortened by UMI.)...