Resolving the sintering conundrum of high-rhenium tungsten alloys

Tungsten-rhenium (W-Re) alloys with high-Re contents are the preferred refractory metal materials in many applications because of the improved ductility and processability over pure W and low-Re tungsten alloys. However, the sintering concurrently becomes increasingly more difficult with increasing Re contents. Here we proposed that the sintering conundrum is caused by the lowered crystal symmetry and the wider dihedral angle distribution when body-center-cubic (BCC) W is alloyed with more hexagonal-close-packed (HCP) Re, which results in inefficient pore removal in the final stage sintering. We showed that the conundrum can be resolved by pressureless two-step sintering (TSS) which suppresses accelerating final-stage grain growth, and our proposal is supported by the data of the critical density ρc that is required to start the second step for successful TSS at different W-Re compositions. Dense ultrafine-grained W-Re alloys with ∼300 nm average grain size and up to 25 wt% Re were successfully produced. Our work demonstrates the unique opportunities offered by two-step sintering to advance the scientific understanding and technological practices in powder metallurgy and related fields.