In virtue of Auger electron spectroscopy,the grain boundary concentrations of phosphorus in Ni-Cr-Fe superalloy are measured after solution treatment at 1180 ℃ for 45 min.The results show that a peak of phosphorus concentration occurs at about 180 min during isothermal ageing at 500 ℃,and a maximum concentration of phosphorus appears also at about 500 ℃ for all specimens aged for 20 min at temperatures of 200,400,500,700 and 800 ℃.The results are analyzed with the laws of nonequilibrium grain boundary segregation.It is found from the analysis that peaks are related to critical time for nonequilibrium grain boundary segregation of phosphorus.
Finding the internal-friction peak of grain boundary anelastic relaxation was one of the important breakthroughs in the study of internal friction in the last century.But the micro-mechanism of grain boundary anelastic relaxations is still obscure.Based on the observations of the grain boundary seg-regation or depletion of solute induced by an applied stress,the following micro-mechanism was suggested:grain-boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted,inducing grain-boundary depletion or segregation of solute,respectively.The equations of vacancy and solute con-centrations at grain boundaries were established under the equilibrium of grain-boundary anelastic relaxation.With these the kinetic equations were established for grain boundary segregation and depletion during the grain boundary relaxation progress.
The research progress of non-equilibrium grain-boundary segregation theories in the last 20 years is reviewed. Based on studies by the present authors, the critical time of non-equilibrium segregation and its impact on the development of non-equilibrium segregation theories are described. Quasi- thermo- dynamics and kinetics for thermal non-equilibrium grain boundary segregation are detailed along with a non-equilibrium grain boundary cosegregation model. The experimental validation of the theories and their application to the reversible temper embrittlement of steels and the intermediate temperature brittleness in metals and alloys are also addressed.
XU TingDong1, WANG Kai1 & SONG ShenHua2 1 Central Iron and Steel Research Institute, Superalloy Department, Beijing 100081, China
The data obtained by bending tests for intergranular embrittlement after 45 h and 450 h exposure to Strauss solution have been reported for 304 stainless steel.The results show that an embrittlement peak appears at 650 ℃ for all samples quenched from 1260 ℃ and then sensitized for 150 h at 480,565,650,730,815 and 900 ℃ respectively.The temperature corresponding to the embrittlement peak is decreased to 565 ℃ when the sensitizing time is prolonged to 1500 h.In this paper,these data are analyzed with an isothermal kinetic model of nonequilibrium grain boundary segregation,indicating that the embrittlement peak is related to the critical time for nonequilibrium grain boundary segregation of sulfur.
