Effect of cyclic heat treatment on microstructures and mechanical properties of directionally solidified Ti-46Al-6Nb alloy

The Ti-46Al-6Nb (mole fraction, %) ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 degrees C in the a phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 mm, nevertheless the lamellar spacing increases from 0.71 mu m to 1.38 mu m. Extending the holding time and increasing the cyclic times of heat treatment eliminate the beta-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.