Microstructure And Mechanical Properties Of Molybdenum Alloys Reinforced By Carbides（Mo₂C,Mo₂TiAlC₂ And Mo₂TiC₂）

The brittleness of molybdenum at room temperature leads to its poor machinability,which seriously restricts the expansion of its application field.At the same time,strength of molybdenum alloy at ambient/elevated temperature must be further improved to meet the higher requirements of modern industrial development for high temperature structural materials.Molybdenum alloys added with carbides have good strength at room temperature,ductility and creep resistance,which have been used as structural materials and die materials in aerospace,military,nuclear energy and metallurgy fields.However,with the increasingly demanding requirements for material properties in various application fields,the properties of traditional carbide reinforced molybdenum alloys,especially ductility at room temperature and strength at high temperature,have been difficult to meet the requirements of modem technology applications.Therefore,the main purpose of this paper is to explore and study the second phase particle reinforced molybdenum alloys,which is different from traditional carbides,and to provide new ideas for obtaining molybdenum alloys with high strength and toughness.Therefore,in this paper,in-situ Mo2C particles and Mo2TiAlC2(MAX phase)and Mo2TiC2(MXene phase)particles added directly were used as reinforcements,respectively,and molybdenum alloys with different amounts of carbide were prepared by powder metallurgy technology.The microstructure and mechanical properties of molybdenum alloys at room and high temperature were studied,and the effect of different carbide particles on microstructure,mechanical properties at room and high temperature was revealed.By correlating microstructure,deformation mechanism and mechanical properties,a quantitative analytical model of strengthening mechanism of in-situ Mo2C reinforced molybdenum alloys was established,and the toughening mechanisms of molybdenum alloys reinforced by Mo2TiAlC2 and Mo2TiC2 particles were elucidated.The results show that in molybdenum alloys reinforced by Mo2C particles,the Mo2C particles are nearly spherical,and the size is mainly micron,a few are submicron and nanometer.With the increase of Mo2C content,the relative density of alloys increases gradually,while the grain size decreases gradually.At the same time,the volume fraction of intergranular particles increases gradually,and that of intragranular particles decreases gradually.When the content of Mo2C is 15 wt%,grain size of the alloy decreases to 8 μm,and the Mo2C particles are mainly distributed on grain boundaries.The inhibiting effect of Mo grain boundaries and Mo2C particles on the dislocations movement results in the yield strength of alloy at room temperature increases by 100%,compared with that of pure Mo.However,the introduction of Mo2C particles reduces the toughness of molybdenum alloys obviously.In molybdenum alloys reinforced by Mo2TiAlC2 and Mo2TiC2 particles,the size of second phase particles is submicron and the particles are mainly distributed within molybdenum grains.There is a meshing interface layer with the thickness of nanometer between the particles and molybdenum matrix.With the increase of the addition amount of Mo2TiAlC2 and Mo2TiC2 particles,the relative density of alloys increases gradually,while the grain size decreases gradually.When the addition of Mo2TiAlC2 particles is 2 wt%,grain size of the alloy decreases to 6 μm.Compared with pure molybdenum,yield strength and tensile strength of the alloy at room temperature are increased by 28%and 43%,respectively.Meanwhile,the elongation(27.8%)and fracture toughness(102.1 MPa-m1/2)of the alloy are 1.6 times and 2 times those of pure molybdenum,respectively.The addition of Mo2TiAlC2 particles improves the strength and ductility of molybdenum alloys simultaneously.Compared with Mo2TiAlC2 particles,the effect of Mo2TiC2 particles on the molybdenum grain refinement is less,but the addition of Mo2TiC2 particles introduces more low-angle grain boundaries and larger phase boundary area into the molybdenum matrix,which further improves the plastic deformation coordination ability of molybdenum alloys.When the addition of Mo2TiC2 particles is 2 wt%,grain size of the alloy is 12 μm.Although strength of the alloy is not significantly improved,the elongation and fracture toughness of the alloy reach 46.5%and 123.3 MPa-m1/2,which are increased by 172%and 143%,respectively,compared with that of pure Mo.The toughening effect of Mo2TiC2 particles on molybdenum alloys is the best.The strength improvement of Mo alloys with Mo2C particles is mainly due to the fine grain strengthening and dislocation packing induced by Mo2C particles.The quantitative analysis result shows that the latter has a greater contribution to the yield strength of the alloys.The fracture mode of molybdenum alloys at room temperature is transformed from the brittle fracture of pure molybdenum,which is dominated by intergranular fracture,to the ductile fracture of microvoids agglomeration.The analysis of the toughening mechanism shows that the high ductility of molybdenum alloys reinforced by Mo2TiAlC2 and Mo2TiC2 particles is mainly due to the synergistic effects of the following four factors:high density of the alloys,micro-scale plastic deformation of the second phase particles which loosens the stress concentration at the phase interfaces,Mo grain refinement and high strain hardening rate of the alloys.The high fracture toughness of Mo alloys with Mo2TiAlC2 and Mo2TiC2 particles is mainly due to the following four aspects:high density of the alloys,local plastic deformation of Mo matrix,crack deflection and particle pullout,and the generation of secondary cracks.In particular,the gaps in Mo2TiC2 particles result in the increase of phase boundary area and low-angle grain boundary proportion in molybdenum matrix,which further improves the coordinated plastic deformation ability of molybdenum alloys,and makes the molybdenum alloys reinforced by Mo2TiC2 particles have excellent ductility and toughness.At 1000℃,1100℃ and 1200℃,strength at elevated temperature of the molybdenum alloys is improved by the addition of three kinds of carbides.The yield strength of molybdenum alloy with 15 wt%Mo2C is more than twice that of pure molybdenum.The yield strength of alloy with 2 wt%Mo2TiAlC2 is 1.9～2.2 times that of pure molybdenum.The yield strength of molybdenum alloy with 2 wt%Mo2TiC2 was not significantly improved,but the compression strength was increased by 35%-58%compared with pure molybdenum.Compared with pure molybdenum and molybdenum alloy with 15 wt%Mo2C,the molybdenum alloy with 2 wt%Mo2TiAlC2 has the greater strain hardening ability and the Schmid Factor changes to a smaller value after recrystallization,resulting in the latter shows the greater deformation resistance after yielding during compression at high temperature.