Study On Microstructure And Mechanical Properties Of Molybdenum Alloy Bars Reinforced By Different Oxides

Molybdenum and its alloys are widely used in the fields of metallurgy,metal processing,aerospace and nuclear energy due to their high melting point,good electric and thermal conductivity,low expansion coefficient,good thermal shock resistance and wear resistance.However,the low strength and plasticity of pure molybdenum limit its further application as a structural material.Metal oxides such as Al₂O₃,ZrO₂ and La₂O₃ have high hardness and melting point,which are potential reinforcing phases to improve the mechanical properties of molybdenum alloy.In this paper,Al₂O₃ doped,ZrO₂ doped,La₂O₃ doped molybdenum alloys powders were prepared by the liquid-liquid doping process,with(NH₄)₂Mo₄O₁₃·2H₂O,Al(NO₃)₃·9H₂O,Zr(NO₃)₄·3H₂O,La(NO₃)₃·6H₂O as raw materials.And molybdenum alloy bars with diameter of 8 mm were prepared by pressing forming,non-pressing sintering and rotary forging.The compressive and tensile properties of molybdenum alloys at room temperature and different annealed temperatures and high temperature compression properties at different temperatures and strain rates were tested.And the effects of different oxide doping on the mechanical properties of molybdenum alloys were analyzed.In the powder preparation stage,the phase transformation process of molybdenum powder is(NH₄)₂Mo₄O₁₃×2H₂O?MoO₃×(H₂O)_0.333?h-MoO₃?b-MoO₃?a-MoO₃?MoO₂?Mo.The addition of the three oxides can refine the powder.With the increase of the oxide content,the size of composite powder decreases gradually and agglomeration phenomenon appears.The doping of Al₂O₃ and ZrO₂ had no effect on the morphology of molybdenum powder,and molybdenum powders are spherical particles.The doping of La₂O₃ changed the morphology of molybdenum powder,which is polyhedral.Nano-sized or submicron sized oxide particles are dispersed in the molybdenum matrix,and refine molybdenum grain size and improve the density and hardness of molybdenum alloy.The relative density of molybdenum alloy reaches 98.5%when the content of Al₂O₃ is 2.0vol%,and the hardness is the highest.The density and hardness of La₂O₃ doped molybdenum alloy are the smallest.After annealing at 850?,the microstructure of swaged molybdenum alloy did not change significantly.After annealing at 1200?,the pure molybdenum was completely recrystallized,while the grain size of the doped oxide molybdenum alloy was fine,which was just recrystallized or not recrystallized.The doping of three oxides can improve the compressive yield strength of molybdenum alloy,and the effect is related to the content and temperature.The compressive yield strength of molybdenum alloys of swaging and annealing at 850?increase first and then decrease with the increase of Al₂O₃ content.When the content of Al₂O₃ is 1.2vol%,the compressive yield strength of molybdenum alloy is the largest.The compressive yield strength of Mo-ZrO₂ alloys increase with the increase of ZrO₂content.When the content of ZrO₂ was 2.0vol%,the compressive yield strength of molybdenum alloy was the largest.With the increase of La₂O₃ content,the compressive yield strength of molybdenum alloy show a gentle trend after reaching the maximum,which indicates that the doping of trace La₂O₃ can improve the compression performance of molybdenum alloy,further increase of La₂O₃ content,and the improvement of molybdenum alloy's compression performance at room temperature is limited.When annealing temperature was 1200?,the compressive yield strength of molybdenum alloy increased with the increase of oxides content.When oxides content was 2.0vol%,the compressive yield strength of molybdenum alloy doped with three oxides reached maximum,which was 56.03%,61.53%and33.76%higher than that of pure molybdenum,respectively.The strengthening mechanism in compression process is the joint action of fine grain strengthening,deformation strengthening and dislocation strengthening.In the compression process,the high-density dislocation walls(DDWs)is formed due to dislocation tangle,and the larger molybdenum grains are divided into subgrains,which also helps to improve the yield strength.The particle size of La₂O₃ in the matrix is larger than that of Al₂O₃ and ZrO₂,so the fine grain strengthening of La₂O₃ doped molybdenum alloy is not as good as that of Al₂O₃ and ZrO₂ doped,and the yield strength is lower on the macro.The tensile strength of molybdenum alloy can be improved by the doping of three oxides,but the strengthening effect is different.For molybdenum alloys of swaged,annealed at 850? and annealed at 1200?,the tensile strength of Mo-Al₂O₃ alloy first increases and then decreases with the increase of Al₂O₃ content.When the content of Al₂O₃ is 1.2vol%,the tensile strength of alloy reaches maximum,which is 726.3MPa,679.60MPa and 662.88MPa,respectively.When the ZrO₂ content was 2.0vol%,the tensile strength of Mo-ZrO₂ alloy reached maximum,which was 743.27MPa,717.40MPa and 702.87MPa,respectively.When the content of La₂O₃ is 0.4%,the tensile strength of Mo-La₂O₃ alloy reaches maximum,which is 743.26MPa,641.51MPa and 626.95MPa,respectively.It was found that the doping of ZrO₂significantly improved the tensile properties of molybdenum alloy at room temperature.The fractures of pure molybdenum,doped trace Al₂O₃ and ZrO₂ exhibited brittle fracture,and those changed to ductile fracture with the increase of the content of Al₂O₃and ZrO₂.The molybdenum alloy doped with trace La₂O₃ showed ductile fracture.When the contents of three oxides reach maximum,the fractures of molybdenum alloys are a mixture of toughness and brittleness.The doping of three oxides can all improve the high temperature mechanical properties of molybdenum alloys,and their compressive strength increases with the increase of oxide content.The most significant improvement in the high temperature compression performance of molybdenum alloy is achieved when the content of ZrO₂is 2.0vol%.The stress of high temperature compressive deformation of Mo-2.0vol%ZrO₂ alloy decreases with the increase of temperature and the decrease of strain rate.Compared with pure molybdenum,the introduction of ZrO₂ reduces the thermal activation energy of molybdenum alloy,thus improving the plasticity of molybdenum alloy.