The Microsturcture And Mechanical Property Of Minim Silicon Strengthened Molybdenum Sheet Alloys

The pure molybdenum and molybdenum alloy sheets with different silicon concentration were fabricated by powder-metallurgical processing. The effect of annealing treatment on phase composition and microstructure of molybdenum alloys were studied via those sheets anealed at different temperature (850℃,950℃,1050℃,1150℃,1250℃,1350℃,1450℃,155O℃). The sheet specimens with stress-relief annealing and grain coarsening treatment of molybdenum alloy were tensile at room and elevated temperature (300℃,800℃and 1200℃) to investigate the influence of grain size and test temperature on tensile mechanical property. The rules of fracture mode and microstructure changes with test temperature increased were also studied through fracture surface scanning and transmission electron microscopy observation of sheet specimens. Finally, the room and elevated temperature strengthening mechanism and the individual contributions from solid solution strengthening, particle dispersion strengthening and fine-grain strengthening to the strength of Mo-Si alloy sheet were discussed through the microstructure and mechanical properties.The presence of Si can effectively refine the grain size and improve the recrystallization temperature of molybdenum alloys. With the increasing of annealing temperature, the grains of molybdenum alloy sheets are gradually widening and transition from fibrous structure to equiaxed grain, the density of dislocation inside of grain decreased, the sub-grain grows up through combination and the grain boundaries become thinner and straighter. Addition of Si can also promote the tensile strength and yield strength of molybdenum alloys sheet, at the same time, it reduces its ductility. With the increasing of tensile test temperature, the strength of the sheets decreased dramatically, the elongation shows a increasing trends before 300℃and then decreasing at elevated temperature. The fracture mode of molybdenum alloy sheets transform from transgranular cleavage fracture at room temperature to dimple fracture at 300℃and 800℃, and then to intergranular fracture at 1200℃. Because of grain boundaries sliding, the sheets can obtain higher deformation than room temperature before failures at 1200℃tensile. During the elevated temperature tensile, the molybdenum alloy sheets with the coarse grain (annealed at 1350℃) shows a better ductility and poor strength than the sheets with fine grain structure(annealed at 1050℃). With the increasing of test temperature, the decreased rate of yield strength of sheets with fine grain was faster than the sheets with coarse grain. The analysis results of microstructure and mechanical properties indicated that the strengthening of molybdenum alloy sheet contribute from fine-grain strengthening, particle dispersion strengthening and solid solution strengthening except the matrix strength, meantime, the fine-grain strengthening plays the main role at room temperature. While, at elevated temperature, the strengthening includes three components:solid solution strengthening, particle dispersion strengthening and grain boundary strengthening, meantime, the grain boundary strengthening dominated influence the strength of sheet.