Evolution of the microstructure and hardness of the Ti–Si alloys during high temperature heat-treatment

Samples of Ti–8Si, Ti–13.67Si and Ti–23S (at.%) alloys were prepared by vacuum ar melting technique and heat-treated for 2 h at 1373 K and 1473 K in a vacuum environment. The as-cast and annealed samples were examined by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX) and hardness test. During high temperature heat-treatment, the quasi-continuous network-shaped Ti5Si3 + Ti phases in the as-cast Ti–8Si alloy finally converted to near-equiaxed Ti5Si3 grains dispersed in the Ti matrix. For Ti–13.67Si alloy, the Ti5Si3 grains finally also transformed to the near-equiaxed ones compared with the lamellar grains in the as-cast alloy. Moreover, compared with the as-cast alloy, Vickers hardness of Ti–8Si and Ti–13.67Si alloys was increased by the annealing. However, the microstructure of the Ti–23Si alloy could not change significantly. So, adversely Vickers hardness decreased after high temperature heat-treatment. The microstructural evolution of Ti–Si alloys is related with the diffusion of Si atoms in the Ti/Ti5Si3 interface during high temperature heat-treatment producing as much as possible homogeneous distribution of Ti5Si3 particles in a continuous α-Ti matrix. The task was not achievable for Ti–23Si alloy because of the stability of the primary Ti5Si3 phase particle during heat-treatment.