Pressureless two-step sintering of ultrafine-grained refractory metals: Tungsten-rhenium and molybdenum

• Pressureless two-step sintering of W-10Re and Mo has been demonstrated. • 98.4% dense W-10Re with an average grain size of 260 nm can be pressurelessly sintered below 1200 °C. • 98.3% dense Mo with an average grain size of 290 nm can be pressurelessly sintered below 1120 °C. • W-10Re and W have similar grain boundary diffusivity at the same homologous temperature and similar activation energy. • Mo has more sluggish grain boundary diffusivity and larger activation energy. • Pressureless two-step sintering to be a general method to produce dense ultrafine-grained refractory metals and alloys. The challenge of sintering ultrafine-grained refractory metals and alloys to full density is hereby addressed by pressureless two-step sintering in tungsten-rhenium alloy and pure molybdenum. Using properly processed nano powders (∼50 nm average particle size), we are able to sinter W-10Re alloy to 98.4% density below 1200 °C while maintaining a fine grain size of 260 nm, and sinter molybdenum to 98.3% density below 1120 °C while maintaining a fine grain size of 290 nm. Compared to normal sintering, two-step sintering offers record-fine grain sizes and better microstructural uniformity, which translates to better mechanical properties with higher hardness (6.3 GPa for tungsten-rhenium and 4.0 GPa for molybdenum, both being the highest in all pressurelessly sintered samples of the respective material system) and larger Weibull modulus. Together with our previous demonstration in tungsten, we believe that two-step sintering is a general effective method to produce high-quality fine-grained refractory metals and alloys, and the lessons learned here are transferable to other materials for powder metallurgy.