Formation mechanism and effect on the mechanical properties of TiSi phase for Ti-5Al-5Mo-5Cr-3Nb-2Zr alloyed by silicon

Abstract

To investigate the formation mechanism and evaluate the silicide effect on the microstructure and mechanical properties of Ti–5Al–5Mo–5Cr–3 Nb–2Zr–xSi (Ti–xSi, wt%, x = 0, 0.2, 0.4, 0.6, 0.8) alloys, they are melted via vacuum arc melting and observed by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results indicate that their microstructure comprises grain boundary α (αGB), secondary α (αs), primary α (αp), and residual β phases when the Si content is less than 0.2 wt%. With an increase in the Si content from 0.4 to 0.8 wt%, the content of the TiSi phase precipitated in αGB phase increases. Moreover, its size increases from 40 nm to 0.8 µm. The diffusion of Si atoms from their high concentration in the β to the α phase of the supersaturated solid solution is hindered by the phase boundary, resulting in the precipitation of silicides. The high energy at the interface and the diffusion of Si accelerate this precipitation. The elongation and fracture toughness increase at 0–0.2 wt% Si and then decrease at 0.4–0.8 wt% Si, peaking at Ti–0.2Si (7.1% and 66.4 MPa·m1/2, respectively). The strength is improved owing to the Si solid solution strengthening, precipitation of Ti5Si3 and TiSi phases, and strengthening effect caused by the difference in crystal plane spacing between the TiSi and the αGB phases. A higher silicide content leads to a gradual transformation from an intergranular ductile fracture to intergranular brittle fracture.