Research On The Preparation And Properties Of Alumina Reinforced Fe-Cr-Ni Composites By In-situ Method

Heat-resisting alloy is a kind of metal material based on iron,nickel and cobalt,which can work at high temperature up to 600? and under certain stress for a long time.It has excellent high temperature strength,good oxidation resistance and thermal corrosion resistance.It was called "super alloy".Heat-resisting alloy matrix composites with different ceramic reinforcement can be flexibly designed by in-situ reaction which can improve the comprehensive properties of heat-resisting alloy matrix composites such as specific strength and specific modulus wear resistance and dimensional stability at high temperature.It is widely used in aerospace,automotive and equipment manufacturing industries.In this paper,Al₂O₃ particles(fibers)reinforced Fe-Cr-Ni composites were prepared by pressure-free reaction sintering method.Al₂O₃ and MgO particles reinforced Fe-Cr-Ni composites were prepared by reactive hot pressing.The effect of heat treatment on Al₂O₃ particle reinforced Fe-Cr-Ni composites was studied as well.The effects of aluminum particle size(fiber-to-diameter ratio),sintering temperature and holding time on the phase composition,microstructure and mechanical properties of the samples were discussed.X-ray diffraction,thermogravimetric-differential thermal analysis,scanning electron microscopy,microhardness testing and Archimedes drainage method were used to analyze the composition,structure and properties of the samples testing?NiO,Fe,Cr,Al reinforced porous Fe-Cr-Ni composites were prepared by pressureless reaction sintering using micrometer Al₂O₃ powder and Al fiber as raw materials.It was found that the optimum sintering temperature was 700 ? and the holding time was 2h.The surface of the sintered sample had metallic luster and the phase composition of the sintered sample were Al₂O₃ and Fe-Cr-Ni alloys.The porosity increased at first and then decreased with the increasing of the ratio of length to diameter of aluminum fiber,which was about 40%⁵0%.The grain size distribution of the sample was uniform and the metal matrix formed a certain continuous structure.Dense particles reinforced Fe-Cr-Ni composites with different Al₂O₃ and MgO contents were prepared by reactive hot pressing using Al powder,mg powder mixed with metal and oxide powder(FeCrNiO)respectively.It is found that the samples are mainly composed of Al₂O₃,MgO,mg-Al spinel,Fe-Cr brittle phase and Fe-Cr-Ni alloy phase.The hardness,bending strength and fracture toughness of the composites increased with the increase of Al₂O₃ content due to the residual stress between the metal matrix and ceramic particles caused by the different ceramic phase contents in the samples.When MgO n·Al₂O₃,n = 1.78,the mechanical properties,hardness,bending strength and fracture toughness of the composites are 5.59GPa,244MPa and11.66MPa ·m^1/2,respectively.Dense nano-Al₂O₃ particles reinforced Fe-Cr-Ni composites were prepared by reactive hot pressing using nano-Cr₂O₃,Fe,Ni and Al powders as raw materials,and heat-treated at 1000? for 2h.The phase composition and microstructure of the composites were studied.It was found that the composite samples sintered by hot pressing were composed of Al₂O₃,Cr_0.7Fe_0.36Ni_2.9,Cr_0.19Fe_0.7Ni_0.11,Fe-Cr phase.After heat treatment at high temperature,austenite transformation occurred in Fe-Cr phase and grain size of metal matrix increased.In addition,the Vickers hardness decreased and the bending strength and fracture toughness of the composites increased,which were 2.98GPa,459.3MPa and 12.8MPa ·m^1/2,respectively.Al₂O₃ reinforced Fe-Cr-Ni matrix composites were successfully prepared by reactive hot pressing.The phases are Cr0.19Fe0.7Ni0.11,Cr0.7Fe0.36Ni2.9 alloy phase,Fe-Cr phase and Al₂O₃ phase.With the decrease of Al₂O₃ content,the Vickers hardness of the composites remained unchanged,about 4.28 GPa.The flexural strength and fracture toughness increase accordingly.This is due to the decrease of porosity,resulting in an increase in the flexural strength of the composite.The composites made of Al,Ni2O3,Fe and Cr have the highest flexural strength and the highest resistance.The bending strength is 529MPa.The results of scanning electron microscopy(SEM)and electron energy dispersive spectroscopy(EDS)show that the composites are composed of Al₂O₃ particles,a small amount of brittle Fe-Cr phase and a large amount of Fe-Cr-Ni alloy matrix.The fine Al₂O₃ particles distribute uniformly in the metal matrix.The main fracture mode of the composites is transgranular fracture.The fracture toughness of the composites prepared by Al,Ni2O3,Fe and Cr is the highest,and the fracture toughness value is 15.2MPa ·m1/2.