Preparation of low-oxygen and high-density ultrafine-grained molybdenum with two-step method of hydrogen deoxidation pre-sintering and spark plasma sintering

High-density ultrafine-grained molybdenum was prepared using ultrafine molybdenum powder (0.40 μm) with the two-stage sintering method of hydrogen deoxidation pre-sintering and spark plasma sintering (SPS). It is found that by pre-sintering at 1000 ℃ in hydrogen for 2 hours, the mass fraction of oxygen in ultrafine molybdenum powder decreased from 0.237% to 0.066%. After pre-sintering, a large number of sintering necks are formed, but the grain size does not significantly increase. Then, the pre-sintered samples sintered by SPS at 1200 ℃ under 50 MPa for 10 min undergo the densification sintering and the relative density reaches 99.2%. Due to the relatively low sintering temperature (1200 ℃), the average grain size is 0.88 μm, the growth of grain size is not significant. Benefiting from the low oxygen content, high relative density, and small grain size, the Vickers hardness and bending strength of the Mo samples are HV 300 and 523 MPa, respectively. However, reducing the SPS sintering temperature to 1100 ℃ may result in the insufficient sintering with the relative density being only 91.3%, and the Vickers hardness and bending strength dropping to HV 215 and 343 MPa, respectively. Increasing the SPS sintering temperature to 1300 ℃ may caused the grain size to increase significantly to 6.95 μm, and the Vickers hardness and bending strength decreases to HV 197 and 467 MPa, respectively.