Microstructure And Mechanical Properties Of CoCrW Matrix Composites With Network Structure

CoCrW alloy is widely used in structural components of aero-engine and industrial gas turbine due to its good high temperature stability,strong temperature bearing capacity,long creep life,good wear resistance and high hardness.In order to further improve the properties of CoCrW alloy,in this paper,Co₂₅Cr₅W₅Mo₅spherical powder,Ti powder and B powder were used as raw materials,combined with low energy planetary milling and vacuum hot pressing sintering process,CoCrW matrix composites with network structure characteristics were successfully prepared.Scanning electron microscopy(SEM),X-ray diffraction(XRD),electron probe microanalyzer(EPMA)and transmission electron microscopy(TEM)were used to characterize and analyze the microstructure and phase of the material.The mechanical properties at room temperature and high temperature were tested.The effects of sintering process and network structure on the microstructure and properties of the material were studied.The strengthening mechanism of the material was discussed combined with fracture analysis.Taking Co₂₅Cr₅W₅Mo₅ spherical powder,Ti powder and B powder as raw materials,the effects of ball milling process(ball milling speed,ball milling time,ball-to-powder ratio)on the distribution uniformity of Ti powder and B powder on the surface of CoCrW spherical particles were studied.The experimental results show that high ball-to-powder ratio(5:1)can make most Ti powder and B powder bonded on the surface of spherical matrix.With the increase of milling speed(180-220 rpm)and milling time(4-6 h),the added Ti and B powders can be uniformly adhered to the surface of CoCrW powder,and the matrix can maintain the original spherical appearance.The optimum ball milling process was determined as follows:ball milling speed 220 rpm,ball milling time 4 h,ball to material ratio 5:1.The results of sintering process show that the density of CoCrW alloy and composite increases with increasing sintering temperature.The best sintering process of CoCrW alloy is 1250°C for 1h.The best sintering process of CoCrW matrix composites is1220°C,holding2h.In other words,the addition of reinforcement can reduce the sintering densification temperature of the composite by 30°C compared with that of the alloy.With the extension of holding time,the density of the material is further improved,and the density of CoCrW matrix composite can reach more than 96%.The room temperature tensile results show that the material prepared at the optimum sintering temperature has the highest strength and elongation at the same time,which is due to the high density and reinforcement network structure distribution.With the increase of holding time,the strength and toughness of CoCrW matrix composites are further improved,and the maximum tensile strength can reach 967.2 MPa,which is 11.5%higher than that of the alloy.The fracture analysis shows that the reinforcement in the network structure can bear the stress well and hinder the deformation of the matrix,so as to improve the strength of the material.The friction and wear test results show that the friction coefficient(0.61)and wear amount(2.7 mg)of CoCrW matrix composites are lower than those of CoCrW alloy under the same test conditions(0.73)and wear amount(4.2 mg).The results of composition and morphology analysis of wear scars and debris reveal that the wear mechanisms of composite materials are mainly abrasive wear and oxidation wear,and the wear mechanisms of alloy materials are mainly oxidation wear.The wear resistance mechanism of composites is mainly that the reinforcement particles can better bear the load in friction and wear.The high temperature tensile test shows that the network structure can make the material have higher strength at 650-750°C.The tensile strength of CoCrW matrix composites can reach 729.4 MPa at 650°C,which is40.9%higher than that of the alloy.The analysis of high temperature tensile fracture shows that the fracture reason of the material is that the high temperature leads to the softening of CoCrW matrix,and the resistance of crack propagation along the matrix is smaller,and finally transgranular fracture.In order to further improve the mechanical properties of CoCrW alloy and CoCrW matrix composites,the two materials were subjected to thermal deformation treatment.The density of the treated alloys and composites increased to more than 98%,and the maximum hardness of the composites reached 42.7 HRC.Microstructure analysis showed that thermal deformation did not significantly change the internal network structure of the composites.XRD scanning results showed that the internal?-Co content of the material increased significantly after deformation,which was due to the excessive cooling rate during thermal deformation.The tensile test results show that the thermal deformation has a great influence on the yield strength of CoCrW matrix composites,and the maximum yield strength is increased to 1047 MPa,which is nearly double that before deformation.