Lattice defects and their influence on the deformation behavior of single crystals of TiAl

The precipitation behavior of the Al5Ti3 phase in the TiAl matrix as well as its influence on the mechanical properties of TiAl has been investigated for single crystals with compositions of Ti–54 and –56 at.% Al. The precipitation tendency for the Al5Ti3 phase is largest at approximately 40 °C for both Ti–54 and –56 at.% Al. The precipitation behavior of the Al5Ti3 phase is unique since the Al5Ti3 phase once precipitated dissolves in the TiAl matrix upon prolonged annealing, resulting in the initial and final phases being the same TiAl phase. Al5Ti3 precipitates and short-range-ordering of the Al5Ti3-type increase the flow stress of TiAl with the extent of increase in flow stress being larger for 〈110〉 ordinary slip than for 〈011〉 superlattice slip. Both the flow stress and the deformation structure are reversible with respect to changes in deformation temperature, if Cottrell–Stokes type experiments are conducted very carefully so as to minimize the time duration for which the specimen is exposed to the temperature of 400–500 °C where the tendency for Al5Ti3 precipitation is largest.