Crack initiation and mechanical properties of TC21 titanium alloy with equiaxed microstructure

Deformation and crack initiation of equiaxed microstructure were investigated using in-situ scanning electron microscopy (SEM). With αs sizes of 0 μm and 0.1 μm, the cracks initiated along αp/β and αp/βtr boundaries (αp, primary alpha phase; βtr, transformation microstructure), respectively. When the size of coarsened αs (αs, secondary alpha phase) was about 0.5 µm, the crack initiated along αp/β and αs/β boundaries. The results show that the crack growth was put off by continuous slip across the coarsened αs phase. The coarsening of αs leads to increase in ductility of the alloy, while it has a slight effect on the strength. Based on the in-situ observations, the morphology of coarsened αs was further optimized by heat treatment procedure. According to mechanical tests performed, it was seen that the best property combination could be obtained by adjusting the sizes and morphologies of αp and αs phases.