The Interfacial Reaction And Mechanical Properties Of SiC_f/?-TiAl Composites Prepared By Suction Casting

SiCf/TiAl composites are considered to be the most promising high temperature structural material due to their excellent comprehensive mechanical properties.The application of such materials can significantly improve the fuel efficiency and thrust-to-weight ratio especially in the aerospace areas which has higher requirements for lightweight.However,due to the poor room-temperature ductility and high hot isostatic pressing temperature of ?-TiAl alloy,the traditional methods of preparing SiCf/y-TiAl composites were complex and expensive which limits the application of this kind of composites.In this paper,SiCf/TiAI composites were prepared by vacuum suction casting method.During the process of preparation,the fast filling of liquid alloy and the rapid solidification rate of the composite played an important role in preventing interfacial reaction.Two kinds of SiC fibers were selected as reinforcement.One is the traditional SiC fiber,and the other is the SiC fiber coated with additional layer of titanium alloy.In the prepared composites,the fiber and matrix alloy were well bonded,and no microcracks were found around the interface.The interfacial products of those two as-prepared composites were mainly composed of fine equiaxed TiC layer and coarse equiaxed TiC layer.The thermal exposure treatment were carried out at 700? 800? and 900? for the purpose of investigating the growth law of the interfacial products and the thermal stability of the interfacial reaction layer.Results showed that the growth of the interfacial reaction layer of the two composites followed the Arrhenius law.The activation energy Q of SiC(C)f/?-TiAl composite was 152.8 kJ/mol,and that of SiC(C/Ti)f/?-TiAl composite was 239.8 kJ/mol.The latter has better themal stability of interfacial reaction.During thermal exposure,the diffusion of elements around the interface were more sufficient,new interfacial reaction products were formed.After thermal exposure at 800? for 200h,the interfacial reaction products of SiC(C)f/?-TiAl composites from the fiber side to the matrix side were fine grain TiC+Ti3Si layer,coarse grain TiC layer,Ti2AlC layer,TisSi4 layer and Ti3AlC layer respectively.The interfacial reaction products of SiC(C/Ti)f/TiAl composites from the fiber side to the matrix side were fine grain TiC layer,coarse grain TiC layer,Ti3Sn+Ti5Si4 layer and Ti2AlC layer,respectively.Tensile tests of the as-prepared SiC(C/Ti)f/y-TiAl composites with better interfacial thermal stability were carried out at room temperature and 800?.The results exhibited that the tensile strength of the composite was higher than that of the TiAl alloy at both room temperature and 800?.At room temperature,those fibers with moderate interface bonding strength had the reinforcement effect.The tensile strength of the composites was increased by about 7%(53MPa)compared with that of TiAl alloy.The main fracture mechanisms of the composite at room temperature consisted of cleavage fracture and translamellar fracture of the matrix alloy,interface debonding of the fiber,crack deflection,and fiber pull-out.While almost all the fibers contributed to the strengthening effect of the composites at 800?,and a double increase of tensile strength 14%(111 MPa)was obtained.The main fracture mechanisms of the composite at 800? consist of translamellar fracture of the matrix alloy,plastic deformation of titanium alloy coating,interface debonding of the fiber,crack deflection,fiber pull-out,tungsten core pull-out and plastic deformation.It was also found that,the titanium alloy coating on the fiber played a role in inhibiting the propagation of cracks in the matrix to the fiber at 800?.Howere,the tensile strength of the composites decreased compared to that of the matrix alloy at both room temperature and 800? after hot isostatic pressing treatment at 1000?/180MPa/2h.The interfacial of the composites grew rapidly after hot iso static pressing and there were many microcracks in the interfacial.Under tensile load,these cracks propagated into the matrix and fiber,which accelerated the fracture of the matrix alloy and caused the fiber to lose the load-bearing capacity prematurely,so that the strength of the composites was lower than that of the matrix alloy.Compared with the strength of ?-TiAl matrix alloy,the tensile strength of the composites decreases by about 18.9%(89MPa)and yield strength decreased by about 8.7%(33MPa)at room temperature,while at 800?,the tensile strength decreased by about 3.7%(18MPa)and the yield strength decreased by about 7.5%(22MPa).The creep fracture behavior of the SiC(C/Ti)f/?-TiAl composites were investigated under different creep stresses at 800?.Results showed that at high creep stress condition(250 MPa and 300 MPa),the crack initiated from the interfacial reaction layer and then propagated rapidly to the inside of the fiber and the matrix alloy,at this condition,the fiber and the matrix alloy fractured almost at the same time.So the creep properties of the composites were similar to those of ?-TiAl matrix alloy.However,under low creep stress(150MPa,200MPa),the crack initiated at the interfacial reaction layer was not easy to propagate,and the load was transferred from the matrix alloy to the fiber,then stress relaxation occured in the matrix alloy around the fiber.When the rate of load transfer from matrix to fiber and the rate of the load from the fractured fiber redistribution to matrix are balanced,the creep will reache a steady creep stage.So in this condition the existence of fibers prolongs the steady-state creep stage of composites,which greatly increased the creep life of composites.