Genetic Characteristic And Damage Mechanical Behavior Of Non-metallic Inclusions In P/M Superalloy

The genetic characteristic of three kinds of typical non-metallic inclusions—Al₂O₃, SiO₂ and their mixture—mullite, in two widely used superalloy—FGH95 and FGH96, was investigated. The effect of non-metallic inclusions on damage mechanical behavior of the matrix materials was studied by experimental tesing and computer simulation. Simultaneously, the methods of removing the non-metallic inclusion from the superalloys were advanced.The results indicate that the three kinds of non-metallic inclusions show different behavior characteristic in FGH96 superalloy after hot-working processes. The morphology, size and composition of Al₂O₃ are not changed due to its hardness and thermal stability. While, SiO₂ reacts with Al and Ti elements in FGH96 superalloy, and the thinγ′phases zone is formed in the matrix around the inclusion. Theγ′phases around SiO₂ particles become smaller in size, and few in amount. Because of the synchronous effect of high temperature and pressure during HIP, the mixed inclusions––Al₂O₃ and SiO₂––are turned into mullite. The mullite is apt to be soften at high temperature to form arc boundary. The mullite also reacts with the matrix and has the same thing with SiO₂, but the width of reaction zone is narrower than that of SiO₂'s. The micro-cracks usually initiate at interface between the matrix and the inclusion after being quenched—oil quenched or salt quenched, because of their difference in thermal expanding modulus. The effect of aging treatment on the characteristic of the tow kinds of reacting inclusions—SiO₂ and mullite, is not evident. As the high hardness, low ductility and coefficient of thermal expansion, the three kinds of non-metallic inclusions could not be deformed with the matrix during HIF simultaneously, and tend to be broken into pieces. Al₂O₃ particle is completely broken as a chain along deforming direction; SiO₂ particle is partly broken due to the protection of reaction zone. And the mullite becomes narrow like film. The characteristic of the three kinds of inclusions in the two kinds of superalloys—FGH95 and FGH96, is the same although the difference in compositions and processes between them. Thus, the above characteristic could be applied in all PM superalloys because of their similarity in compositions. The cracks tend to be initiated at inclusions during in-situ tension and in-situ fatigue experiments. The cracks are usually initiated at the interface between inclusion and the matrix, and seldom at the inclusion itself. At the beginning, the crack propagates into the matrix from the corner of the inclusion about 45°with the main stress axis, and then, normally until the matrix is ruptured. The fracture surface shows special characteristic because of the reaction zone generated by SiO₂ and mullite. The effect of non-metallic inclusions on the mechanical properties is rather complicated, and lies on the location and size of the inclusion. The smaller in size and the further from the surface, the less effect of non-metallic inclusion on the mechanical properties of the matrix is.The influence of non-metallic inclusion on damage mechanical behavior of the matrix was studied by computer simulation. The distribution of stress field around a non-metallic inclusion will be changed accordingly when the shape , location and orientation of the inclusion are varied. Especially, during LCF. The diagnostic parameter of the inclusion—deformable capability at different temperature, the size variety due to phase transformation other than shape and location, should be introduced to improve the veracity of the simulation results.The actual non-metallic inclusions in superalloys are oxides of Al, Si, Ti, Mg and Ca those come from original alloy melting and argon atomization. The three kinds of artificial non-metallic inclusions may be on behalf of the actual inclusions in PM superalloys. And the characteristic shown by them could be applied in all the inclusions in PM superalloys. Remove and discriminate methods for the inclusions are advanced for respective process according to the source and different characteristic with the metal powders. The criterion of damage tolerance can be established based on the characteristic of the non-metallic inclusions in PM superalloy applied with computer simulation technology.