Preparation of low-oxygen, high-density ultrafine-grained molybdenum by a two-step method: hydrogen deoxidation pre-sintering + spark plasma sintering

High-density ultrafine-grained molybdenum was prepared using ultrafine molybdenum powder (0.40 μm) as the raw material by a two-stage sintering method of hydrogen deoxidation pre-sintering and spark plasma sintering (SPS). The research 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 were formed, but the grain size did not significantly increase. Then, the pre-sintered samples were sintered by SPS at 1200 ℃, with a sintering pressure of 50 MPa for 10 min. During the spark plasma sintering process, the sample underwent densification sintering and the relative density reached 99.2%. Due to the relatively low SPS sintering temperature (1200 ℃), the average grain size was 0.88 μm, which is not grow significantly. Benefiting from the low oxygen content, high relative density, and small grain size, the Vickers hardness and bending strength of the Mo metal prepared in this work were HV 300 and 523 MPa, respectively. However, reducing the SPS sintering temperature to 1100 ℃ would result in insufficient sintering of the sample, 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 ℃ caused the grain size to increase significantly to 6.95 μm, and the Vickers hardness and bending strength decreased to HV 197 and 467 MPa, respectively.