Effect Of Heat Treatment On Microstructure And Mechanical Properties Of GH80A Alloys

The GH80A is a kind of nickel-based superalloy with good room temperature and high temperature strength,high temperature creep resistance,oxidation resistance and corrosion resistance,which is widely used in aerospace,electric power industry and other important fields.The Chongqing Materials Research Institute Co.,Ltd.provided all the materials used in this study.The materials are mainly used to manufacture high-end automotive engine valves.For nickel-based superalloys,heat treatment is an important means to control the microstructure and optimize the properties.In this paper,GH80A alloys were taken as the research object and a series of heat treatment process parameters were designed.X-ray diffraction(XRD),metallographic microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to characterize and analyze the microstructure of the material and their mechanical properties were tested.The influence of different heat treatment regimes on the microstructure and properties of alloys were explored,which provides a scientific theoretical basis for further research on the microstructure regulation and strengthening and toughening of alloy.It was found that the phase composition of different heat-treated GH80A alloys are the same,and the alloys microstructure are mainly composed of?matrix and the second phases.The main strengthening phases in the alloy are?'phase and carbides.The?'phase is in the interior of grain and the carbides with irregular shape mainly distributes interruptedly at the grain boundaries.With the increase of solution treatment temperature,the amount of carbides on the grain boundaries of solution-treated alloy decreases gradually.After single-stage aging and double-stage aging treatments,the amount of carbides on the grain boundaries increases greatly.TEM results show that a large number of fine?'phases are uniformly distributed in the matrix with spherical shape and exhibit a certain coherent relationship with?matrix after double-stage aging heat treatment,which is(100)?'?(100)?and[001]?'?[001]?.The double-stage aging treatment regime makes the?'phase present multi-dimensional distribution,and the spherical?'phase has the tendency to change into cubic shape.The carbide Cr₂₃C₆in the double-stage aging heat-treated alloys has a face-centered cubic structure and the carbide Cr₇C₃ has a face-centered orthogonal structure,and there is an ordered structure in Cr₇C₃.The change of solution temperature has no significant effect on the hardness,room temperature and high temperature tensile properties of the alloy.Compared with properties at room temperature,the high temperature tensile strength and elongation of solution-treated alloy decreased,while the yield strength increased.The hardness and room temperature strength of single-stage aged alloy and double-stage aged alloy increased significantly compared with solution-treated alloy,among which the room temperature tensile strength and yield strength of stabilized alloy exceeded 1100 MPa and 700 MPa respectively.The room temperature tensile strength and yield strength of double-staged alloy and 700?-aged alloy exceeded 1200 MPa and 800 MPa respectively.The room temperature elongation of single-stage and double-stage aged alloys both decreased compared to solution-treated alloy.Due to the effect of equal strength temperature,there is no significant difference in the high temperature tensile strength of different heat-treated alloys.It is speculated that the equal strength temperature of the alloy is near 750?,and there is no significant difference in the high temperature tensile plasticity of different heat-treated alloys.Due to the mixed crystal structure exist in the alloy solid-solution-treated at 1080? and the 700? aged alloy after solid-solution-treated at 1080?,the high temperature elongation of the alloys decreased significantly compared with other alloys,which are 9%and 14.5%respectively.The fracture surface of heat-treated alloys after room temperature or high temperature tensile test are composed of a large number of equiaxed dimples.The fracture morphology of the fracture specimen shows typical plastic fracture characteristics.The difference is that the cracks propagate through the grains to the interior when stretched at room temperature and the alloy is in the form of transgranular fracture.The cracks propagate along the grain boundaries when stretched at high temperature and the alloy is in the form of intergranular fracture.In order to obtain a better combination of microstructure and properties,the recommended heat treatment regime is 1020?1050?/0.5 h solution treatment+845^oC/24 h+700?/16 h.