In-situ observation of the tensile deformation and fracture behaviour of powder-consolidated and as-cast metastable beta titanium alloys

The plastic deformation processes and fracture mechanisms of as-consolidated powder metallurgy (PM) and as-cast ingot metallurgy (IM) metastable beta titanium alloys (Ti-5Al-5V-5Mo-3Cr) were thoroughly investigated by room temperature tensile tests with in-situ scanning electron microscopy (SEM) observations and further microstructure characterizations including transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results suggested that IM alloy has a better compatible deformation capability than PM alloy characterized by varying slip deformation mechanisms and the constantly changing slip modes. The microcracks in PM alloy were observed to be originated at the interfaces of α″/β and grain boundary α/β. Whereas, the microcracks in IM alloy were observed on the grain boundaries and the intracrystalline α precipitation areas, where the slip steps were generated. Particularly, the common defects in PM (residual pores and microvoids) and IM (casting cavity) alloys did not exhibit significant negative effects, contrary to previous awareness. Finally, the coalescence of the grain boundary α and the widened α″/β microcracks led to the sudden brittle transgranular-intergranular mixed fracture of PM alloy. For IM alloy, the serious cracking initiated the V-shape notch made its way through the microcracks at the α/β slip steps, resulting in the gradual transgranular fracture with better ductility.