Investigation On Dual-scale Microstructure Design And Mechanical Properties Of Mo-12Si-8.5B-La₂O₃ Alloy

To further improve the operating efficiency of aerospace engines and industrial gas turbines,designing and developing new structural materials which can withstand higher operating temperature is the hot spot in present studies.Among them,Mo-Si-B alloy is one of the candidate materials because of its high melting point,excellent creep resistance and oxidation resistance at high temperature.However,poor room-temperature fracture toughness of this alloy restricts its application in industrial field.The ?-Mo is the only ductile phase in Mo-Si-B alloy.The volume fraction,size and distribution of ?-Mo significantly affect the fracture toughness and strength of Mo-Si-B alloy.Therefore,the mechanical properties of Mo-Si-B alloy can be effectively optimized by designing and controlling the microstructure of the ?-Mo.In this paper,the Mo-12Si-8.5B-0.57wt%La2O3 alloy with micrometer/sub-micrometer dual-scale ?-Mo was designed,and a series of alloys with different dual-scale distributions of ?-Mo grains were prepared by adjusting the volume fraction and size of micrometer scale ?-Mo grains.By characterizing quantificationally the microstructure of alloys and testing the room/high-temperature mechanical properties and oxidation resistance of alloys,the corresponding relationship between microstructure and properties was systematically studied,the optimal microstructure characteristic parameters for the good combination of strength and toughness were explored,the microscopic mechanisms of room/high-temperature strengthening and toughening were revealed.The dual-scale Mo-Si-B alloys were composed of ?-Mo,Mo₃Si and Mo₅SiB₂ phases.Among them,the ?-Mo exhibited a micrometer/sub-micrometer dual-scale microstructure,Mo₃Si and Mo₅SiB₂ were in submicron scale,and nanoscale La2O3 particles were uniformly distributed in the grain boundary and grain interior of three phases.When the volume ratios of micron-scale?-Mo to submicron-scale ?-Mo in the alloys were in the range of 0.24-0.57,the microstructure of alloys showed that Mo₃Si and Mo₅SiB₂ were distributed uniformly in the continuous dual-scale ?-Mo matrix.With the volume fraction of micron-scale ?-Mo increasing,the fracture toughness of alloys gradually increased.When the volume ratio of micron-scale ?-Mo to submicron-scale ?-Mo in the alloys was 0.57,the fracture toughness of alloys increased gradually with the increase of grain size of micron-scale ?-Mo.When the volume ratio of micron-scale ?-Mo to submicron-scale ?-Mo in the alloy was increased to 0.71,the micron-scale ?-Mo grains aggregated,the dual-scale ?-Mo phase was no longer continuous,and the strength and toughness of alloy decreased.The quantitative analysis of strengthening and toughening mechanisms of the dual-scale alloy at room temperature showed that grain boundary strengthening and crack trapping were the most important strengthening and toughening mechanisms,respectively.When the dual-scale ?-Mo phase in the alloy was continuous,the increasement of grain size or volume fraction of the micro-scale ?-Mo grains promoted crack trapping and toughened alloy.The formation of microcracks induced by La2O3 particles distributed in micro-scale ?-Mo grains,interface debonding and crack deflection played additional toughening effects.However,the increasement of grain size or volume fraction of the micro-scale ?-Mo grains decreased the contribution of grain boundary strengthening,resulting in the decrease of room-temperature yield strength of dual-scale alloys.The alloys with micrometer/sub-micrometer dual-scale ?-Mo structure were heat treated at 1700? and 1800?,respectively.After heat treatment,the ?-Mo matrix showed micro-scale bimodal microstructure,and the Mo₃Si/Mo₅SiB₂ were distributed in the continuous ?-Mo matrix.With the heat treatment temperature increasing,the grain sizes of three phases increased,the volume fraction of ?-Mo phase increased,the distribution of three phases became more uniform,the fracture toughness of alloys increased significantly,and the yield strength decreased slightly.In particular,after heat treatment at 1800?,the fracture toughness of the alloy with micron-scale bimodal ?-Mo structure reached 15.4 MPa·m1/2,which was increased by 164%compared with that of the alloy with sub-micrometer single-scale ?-Mo structure.In the compression tests at 1200-1400?,the compressive strengths of the alloy with micron-scale bimodal ?-Mo structure and the alloy with micrometer/sub-micrometer dual-scale ?-Mo structure were higher than that of the alloy with sub-micrometer single-scale ?-Mo structure.At high temperature,because of the less grain boundaries in the alloys containing micron-scale microstructure,the proportion of deformation caused by grain boundary sliding was low,and thus the strengths of alloys containing micron-scale microstructure were high.Besides,the obvious work hardening effect induced by the large plastic deformation of the ?-Mo grains in the micron-scale microstructure contributed to the improvement of strength of alloys.In addition,the La2O3 particles could strengthen alloy by inhibiting dislocation slip and pinning grain boundaries.Compared with the alloy with sub-micrometer single-scale ?-Mo structure,the oxidation resistance of the alloy with micrometer/sub-micrometer dual-scale ?-Mo structure decreased slightly at 1200-1300?.The MoSi2-based silicide coating was synthesized on the dual-scale alloy surface via Si pack cementation,which improved the oxidation resistance of alloy surface without decreasing the excellent mechanical properties of alloy.