Achieving excellent strength-ductility combination in Ti–6Al–4V alloy by spark plasma sintering technology using large-diameter PREP spherical powder

Powder metallurgy (PM) technology is expected to provide a promising strategy for the cost-effective and highly efficient utilization of spherical Ti–6Al–4V powders with particles size of large than 150 μm to reduce material waste. However, how to overcome the problem of low sinterability and poor ductility of coarse lamellar microstructures of large-diameter spherical powders are still remains a significant challenge. In this study, Ti–6Al–4V alloys were prepared by spark plasma sintering (SPS) at different sintering temperatures with plasma rotating electrode process (PREP) large-diameter spherical powders (∼156 μm). The microstructure evolution, grain structure, mechanical properties and fracture mechanisms were systematically investigated. Results demonstrate that the acicular martensite structure inside the unmelted particles changed to a uniform and fine α/β lamellar microstructure with the increased sintering temperature. Consequently, the Ti–6Al–4V alloys exhibit excellent strength-ductility at 925 °C. (ultimate tensile strength of 881 MPa and elongation up to 14.8 %). Fundamentally, the excellent strength-ductility was attributed to the uniform and dense fine α/β lamellar structure that enhanced the effective bearing area during deformation. The stacked dislocations at the α/β phase interface provided high strength. In addition, the high-density α/β phase interface in the matrix reduced the mean free path of the dislocation, which resulted in a high strain hardening rate of the high-density geometrically necessary dislocations (GNDs) and delayed the premature fracture. The intergranular brittle fracture mode of the alloys changed to the ductile fracture mode. These findings have important implications for recycling super-large diameter spherical powders.