Sintering behavior and densification mechanisms of ultrafine molybdenum powders under varying compaction and sintering conditions

Abstract

The sintering behavior of ultrafine molybdenum powders with a particle size of approximately 1 μm was systematically investigated. The impact of cold isostatic pressing (CIP) and sintering parameters on the densification of molybdenum was investigated. A range of CIP pressures from 50 MPa to 250 MPa was employed before the sintering process. It indicates that the densification is predominantly influenced by the CIP pressure and sintering temperature. Substantial dislocations are introduced after CIP. The dislocation density is significantly reduced in the early-stage of sintering, accompanied by the formation of substructures and sub-grains within the particles. The minimal orientation difference between the adjacent sub-grains provides a prerequisite for in-situ recrystallization. As the temperature increases, most substructures evolve into sub-grains due to the rapid diffusion and rearrangement of atoms. With prolonged holding time at 1400 °C, sub-grains within large primary grains coalesce through the degradation of sub-grain boundaries. Higher sintering temperatures further promote grain boundary migration, leading to continuous grain growth and improved densification. A relative density of 98.83 % is achieved after sintering at 1800 °C for 4 h, whereas the highest hardness of 183.60 HV_1.0 is obtained after sintering at 1600 °C for 8 h. The activation energy for sintering and grain boundary migration are 383.49 kJ/mol and 3.29 kJ/mol, respectively.