The designed high-performance PM Ti-6Al-4V alloy via two-step pressureless sintering

Abstract

Sintering is a critical step to obtain the desired mechanical properties for titanium alloys. However, the residual porosity and the coarse microstructure caused by traditional pressureless sintering adversely affect the mechanical properties. Herein, we proposed a novel two-step pressureless sintering route to achieve sintering densification and grain refinement. The high density of 99.0% and the refined parent β grains with a size of 90.57 µm for Ti-6Al-4V alloy are acquired ultimately, which is among the best pressureless sintering practice reported in the literatures. The selection of two-sintering temperature (< 1250 °C) is confirmed to be the key to improving density and reducing grain size. Benefitting from grain refinement, the increased number of grain boundaries provides more channels for vacancy diffusion, and the formation of small α colonies with multiple orientations makes the Widmanstätten microstructure more uniform. During plastic deformation, the abundant dislocation cross slips and nanoscale twins of face-centered cubic phase within these colonies relieve the stress accumulation of phase interfaces, thereby delaying the formation of cracks and promoting the large plastic flow with the high-stress level. Compared with standard sintering, two-step sintered samples exhibit better mechanical properties (ultimate tensile strength of 968 MPa, yield strength of 871 MPa and elongation of 16.1%), achieving the strength-ductility synergy. This work casts light on developing high-performance and low-cost titanium and its alloys.