Microstructure And Mechanical Properties Of CoCrW Matrix Composites Prepared By Hot Pressing

CoCrW alloy has excellent wear resistance,corrosion resistance and heat strength,and is widely used in wear-resistant and corrosion-resistant environments,such as aerospace,ship,oil and gas,nuclear industry and so on.However,CoCrW alloys have higher brittleness,which greatly limits its application.Considering that the strengthening mechanism of CoCrW alloy is similar to metal matrix composites,this study attempts to add different scale reinforcing phases to CoCrW alloy to improve the comprehensive performance of this materials.CoCrW alloy powder with an average particle size of 30μm,WC particles size of10μm,and Cr₃C₂ particles size of 1μm were used as raw materials in this experiment.Low energy ball milling and hot pressing sintering were used to prepare CoCrW alloys and composites.By adjusting the sintering temperature,the addition of the reinforcing phase content,the holding time,the solution aging temperature and time the CoCrW composites under different process conditions were prepared.The toughening mechanism and wear resistance mechanism were analyzed by three-point bending test,room temperature compression test,high temperature compression and friction and wear test.The microstructure analysis shows that the phase of CoCrW alloy isγ-Co,Cr₄Ni₁₅W,M₂₉C(Cr₂₅Co₂₅W₈C₂),and the phase of CoCrW matrix composite isγ-Co,M₂₃C₆(Cr₂₃C₆),M₆C.（Co₂W₄C）,WC,M₂₉C(Cr₂₅Co₂₅W₈C₂),the added Cr₃C₂ in the material completely reacts with the matrix material to form M₂₃C₆(Cr₂₃C₆),which is mainly distributed at the boundary of the original particles.The part of WC still remains in the material,partially reacting with the matrix to form Co₂W₄C,and the layer interface reaction layer of WC and the matrix material is Cr₂₅Co₂₅W₈C₂.The mechanical properties test results show that the flexural strength,compressive strength and toughness of CoCrW-based composites are better than that of CoCrW alloys with increasing sintering temperature,but the yield strength and hardness are lower than that of CoCrW alloys.When the sintering temperature is 1150℃,the flexural strength and compressive strength of CoCrW-based composites can reach 1900MPa and 2250MPa,which is 11%and 6%higher than that of CoCrW alloy.The toughness of composites is48%higher than alloys.The microstructure analysis of the fracture joint shows that the addition of the reinforcing phase makes the material change from brittle fracture to ductile fracture.With the increase of the added phase content,the fracture strength and fracture strain of the composites decreased continuously,but the fracture toughness increased continuously in the room temperature compression test,and the fracture strain increased from 15%to 17%.With the prolonged holding time,the flexural strength and strain at break of CoCrW matrix composites are slightly improved.The high temperature compressive strength of the 800℃composite material can reach 1000MPa or more.Since the material reacts with the graphite mold at a sintering temperature of 1200℃,the optimum sintering process is 2 hour at 1150℃.The room temperature compression test results show that the yield strength and compressive strength of the composites increase after solution treatment.When the solution temperature is 1250℃,the yield strength of the material reaches 1900MPa and the compressive strength reaches 2400MPa.After aging treatment,the yield strength and compressive strength of the composite material are further improved.When the aging temperature is 950℃,the Rockwell hardness value reaches 55HRC,and the compressive strength is about 2500MPa.The friction and wear test results and microstructure analysis results show that in the composite material,the new phase M₆C type carbide formed by the addition of the reinforcing phase is not tightly bonded to the matrix,and the M₆C type carbide will peel off from the matrix during the friction and wear process.The wear mode of the composite material changes from surface fatigue wear to abrasive wear,which reduces the wear performance of the composite.Based on the results of the microstructure characterization and the mechanical properties,the addition of the reinforcing phase will cause the alloy elements such as Cr,Ni,Fe in the material to diffuse into the enhanced phase and react with the added reinforcing phase to form new phases M₆C and M₂₃C₆.The content of solid solution elements in the matrix material is lowered,resulting in a decrease in the solid solution strengthening effect of the material matrix,a decrease in yield strength,and an increase in fracture toughness.The M₂₃C₆ and M₆C type carbides at the boundary of the original particles can effectively inhibit the grain boundary slip and improve the compressive strength of the material.