| Ni-Cr-W based alloy is a nickel based wrought superalloy which derives its strength from solid-solution and dispersed carbides.The alloy has excellent mechanical strength,good oxidation and corrosion resistance,good thermal structure stability,deformation performance and weld ability at 9001000°C.It can be used in manufacturing of intermediate heat exchanger in high-temperature gas-cooled reactor,combustor and tail pipe in aerospace engine,anti-corrosive components in chemical process industries.The alloy has a broad application prospect.The investigations of solid solution strengthened Ni-Cr-W based superalloy mostly concentrated on the precipitation microstructure of rich Cr M23C6 carbides,the stability of primary M6C and so on.However,the researches on grain boundary segregation,secondary precipitation behavior and tensile deformation behavior of the different distributed carbides at elevated temperature are not enough in depth.The key problems,which is about the distribution of elements in grain core and on the grain boundary before the carbides precipitated at grain boundary,the morphology of the carbides after aging heat-treatment,and the effects of carbides on the tensile behavior,are still needed to be deeply discussed.Therefore,the Ni-20Cr-18W-1Mo superalloy was selected in this study.Systematic research works on above-mentioned problems have been carried out.The purpose of this paper was to investigate the effect of grain boundary segregation on precipitation mechanism and evolution behavior of carbides.The effect and mechanism of the carbides on tensile properties were also studied.Meanwhile,the effect of plastic deformation and stress on carbide precipitation and phase transformation at elevated temperature was investigated.The main research contents and contribution of this paper are as follows:The segregation behavior of Cr,W and Mo in Ni-20Cr-18W-1Mo superalloy was investigated by SEM,EPMA and so on.It is indicated that the grain boundary concentration of Cr decreases with the temperature increasing during the thermal exposure at temperatures between 600°C and 1000°C.On the contrary,the grain boundary concentration of W and Mo increases with the temperature increasing.The value of element W equilibrium segregation energyQW increases with the aging temperature increasing,and the increasing trend of element Mo equilibrium segregation energyQMo is not obvious.The diffusion coefficient of element W ranges from7.208.69?10-2 0m2 s-1.The diffusion coefficient of element Mo,which is sensitive to temperature,increases with aging temperature increasing.The effect of aging time and temperature on grain boundary segregation behavior during aging treatment was studied by SEM and EPMA technology.It can be found that increasing of aging temperature has a conspicuous effect on the concentration distribution between the grain boundary and grain core at low temperature stage（25 to600°C）.Grain boundary concentration of phosphorus increases with increment of testing temperature until a peak value obtained 400°C.At the temperatures range from600 to 1000°C,the type of carbide changes from M23C6-type to M6C-type.Meanwhile,the elements Cr,W and Mo segregate and de-segregate in a wide range.After the Ni-20Cr-18W-1Mo superalloy aged at 1100°C for different times,the intergranular and intracrystalline particles are W-rich M6C-type carbide.The grain size of samples with 60%defermation degree has no obvious change.The morphology changing process of the intergranular carbides is:granular→chains→plate-like→bone shaped.with the exposure time increasing at 1100°C.W segregated and Cr desegregated in a layer about 10μm in width on the grain boundary.With the increasing of aging time,the W atoms segregate at both sides of grain boundary with high grain boundary energy and Cr atoms diffuse from grain boundary to grain core.The W atoms near the grain boundary are depleted to forming carbide with C atoms.During the thermal exposure at 1100°C,there are new granular carbides precipitated in the samples with 90%defermation degree.The morphology of carbides changes from granular to plate-like and needle-shaped.With the aging time increasing,plate-like and needle-shaped carbides break and decomposed in the grain core.The W atoms segregate to vacancy and form M6C-type carbide with C atoms.Cr depletion zone forms near the carbides.The ultimate tensile strength（UTS）and ductility（elongation）of samples with 60%defermation degree increase dramatically,termed as peak stress.Then the UTS decreases dramatically.After that,the UTS and elongation have a bit variation and remain in stable state.The peak stress is 989 MPa and peak elongation is 36.4%.With increasing aging time,the deformation of the alloy occurs by crossing and parallel slipping band.The fractographs of the alloy exhibite ductile dimple fracture pattern and transgranular cracking.It also contains small amounts of intergranular fracture.The UTS and elongation of samples with 90%defermation degree also increase dramatically.The values of UTS and elongation reach peak two times with increasing aging time.The peak stress is 1120 MPa and peak elongation is 49.9%.The fracture initiates preferentially from the carbide particles of the specimen,which indicates that the failure is probably related with intergranular carbide precipitates.The fracture mode is dominated by an transgranular fracture mode.During thermal compressing at 900°C,the amount of granular M6C carbides obviously increases in the experimental alloy.After hot compressing deformation at1000°C,the morphology changing process of the intergranular and transgranular carbides is:granular→block→granular.Stress-induced phase transformation can be confirmed and the stress and elevated temperature provide energy to accelerate the carbide growth rate in grain boundaries.With the 40 MPa tensile stress aging for 5 and20 h at 900°C,the morphology of intergranular carbides appears granular and continuous type.The cellular discontinuous precipitation precipitates from grain boundary to grain core.The primary M6C carbide has a polygon morphology after tensile stress aging.With the 150 MPa tensile stress aging for 5 and 20 h at 900°C,the grain has been lengthen.The granular M6C carbides precipitate at grain boundary.Primary M6C carbide and small size granular carbides distribute in the grain core.Lots of small size granular carbides also precipitate on the surface of primary M6C carbides. |
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