Research On Organization And Performance Of Refractory Metal Powder By High-pressure Torsion

With high strength hardness, good thermal conductivity, excellent wear and corrosion resistance, refractory metal and its alloys were widely used in aerospace, nuclear power, military and medical diagnosis. The traditional preparation of refractory metal mainly adopts its powder compacting and forming, this method will bring more pore and coarse grains, raise the ductile brittle transition temperature and reduce the temperature toughness, increase the difficulty of material processing. High-pressure torsion can using its high hydrostatic pressure and large shear deformation to close the internal porosity of powder materials in a lower temperature, improve the organization density and refine matrix grain size, produce a large number of non-equilibrium large angle grain boundary to promote grain boundary slip and dislocation deformation, solving the low plasticity and strong brittleness of refractory metal essentially. It can improve the traditional refractory metal forming methods and material performance and promote the development of refractory metal forming industry in China.According to the forming characteristics of refractory metal molybdenum powder, pure molybdenum powder was successfully directly compacted into the dense body material by improved high-pressure torsion at 350℃. Surface morphology, particles deformation, pore evolution, microstructure and microhardness of molybdenum billet in loose conglomeration, machine preloading, powder compacting and HPT were investigated by optical telescope, scanning electronic telescope, microhardness-testing device and X-ray diffractometer, explore the pure molybdenum powder compact by HPT in strengthening and toughening mechanism. The results indicate that the molybdenum powder mechanical mesh each other by particles movement and deformation under high pressure and shear force, the large chain pores in particles were welded effectively after high-pressure torsion; The microhardness indentation neat with clear boundary at room temperature and indentation four corners without crack defect, showing a good strength and toughness; The cold hardening and dislocation multiplication caused by grain refinement and microstrain increment can strengthen the matrix organization.Microstructure characterization and performance of molybdenum under different deformation parameters showing that:high-pressure torsion will not change the internal crystal structure type and plastic deformation of molybdenum, the plastic deformation of molybdenum is mainly based on the sliding deformation of{110} crystal plane, the twinning deformation based on the twins face of{112} crystal plane will increase obviously to supplement the sliding deformation when the deformation is large; Pressures and laps can improve the relative density and microhardness of molybdenum billet, the distribution of microhardness along radius direction were uneven, the maximum microhardness of molybdenum billet were on the center distance of 3～4 mm; Pressures and laps can improve the deformation storage energy and reduce the recrystallization temperature of molybdenum billet, the recrystallization behavior will more obvious after DSC testing; Although increase laps under high pressure will induce dynamic recrystallization behavior, increase microstructure grain size and reduce the proportion of large angle grain boundary slightly, but the subgrain size refinement and the microstrain increment by laps will improve the dislocation multiplication to strengthen the matrix organization.