Investigation of three-step heat treatments on the microstructure and steady-state creep behaviors of (TiB + TiC + Y₂O₃)/-Ti composite

The microstructure and steady-state creep behaviors of as-cast and heat-treated (TiB + TiC + Y2O3)/& alpha;-Ti composites were systematically investigated in this study. The relevant properties of the matrix alloy are also provided for comparative analysis. The matrix alloy exhibits the coarse Widmannstatten structure. After heat treatment, the microstructure of the composite transforms from the basket-weave structure to a trimodal structure with spheroidized TiC and Y2O3 particles and some merged TiB whiskers. Compared to the matrix alloy, the steady-state creep rates of the as-cast and heat-treated composites are decreased by 29% and 40%, respectively. After creep, serious & beta;-Ti dissolution in the matrix alloy and the as-cast composite contributes to the instability of & alpha;/& beta; interfaces and precipitated silicides. The morphology of & beta;-Ti phases in the heat-treated composite remains stable during creep and many dislocations are observed inside the secondary & alpha;-Ti phases, which is the main reason for the improvement of the creep performance. Extensive dynamic recrystallization occurred during the steady-state creep stage, and the dislocation structure undergoes significant evolution, along with a large number of sub-grains and dislocation walls. The coarse reinforcements play a role in the particle stimulated nucleation, while the small TiC and Y2O3 spherical particles provide little contribution to dynamic recrystallization. Dislocation climbing is the main creep control mechanism, and the creep rate keeps the constant through dislocation rearrangement and dislocation annihilation.