Microscope Phase-Field Simulation Of Early Precipitation Process Of Ni-Cr-Al Alloy

The Ni-Cr-Al alloys are studied in this paper. The interaction energies of atoms are only the input parameter, and the evolution of atom morphology, the long range order(lro) parameter and concentration, the averaged lro parameter and concentration and volume fraction can be gotten based on the microscope phase-field model, and atomic clustering and ordering during the precipitation process of alloy could be obtained. The precipitation mechanism and precipitation sequence with different composition and temperature, the substitution behavior of Cr atoms, and the influence of progressive aging to the size and volume fraction of ordering phases are investigated.The L1₂ and D0₂₂ phases are precipitated by non-classical nucleation and growth mechanism during the precipitation process of Ni-Cr-Al alloy with lower Al and Cr concentration at 873K. With the increasing of Cr content, the precipitation mechanism of D0₂₂ phases transforms from non-classical nucleation and growth to the mixture of growth and spinodal decomposition gradually and convert to congruent ordering+spinodal decomposition at last, the precipitation mechanism of L1₂ phases transit to the mixture of growth and spinodal decomposition. When Al content increase, the precipitation mechanism of L1₂ phases change to the mixture of growth and spinodal decomposition gradually and convert to congruent ordering+spinodal decomposition at last. D0₂₂ phases disappear gradually. L1₂ and D0₂₂ phases precipitate by spinodal decomposition mechanism when Al and Cr contents are higher.In the temperature range of 873K to 1373K, the precipitation mechanisms of L1₂ and D0₂₂ phases transit from congruent ordering+spinodal decomposition to mixture mechanism and to non-classic nucleation with increasing of temperature. The shapes of precipitates become spheroid from slice shape.The incubation period of non-classical nucleation and growth mechanism is the longest and that of spinodal decomposition is the shortest, mixture mechanism is mediacy. The incubation period is short with increasing Al and Cr contents and become long gradually with increasing of temperature.Nonstoicheometric L1₂ phases are precipitated with congruent ordering+spinodal decomposition mechanism when the temperature is lower than 1223K, then Cr atoms gather at phases boundaries of L1₂ phases, D0₂₂ phases are formed gradually. The other of D0₂₂ phases appear inside of L1₂ phases. The volume of D0₂₂ phases decrease with the temperature rising. At 1373K, L1₂ and D0₂₂ phases precipitate simultaneously by non-classical nucleation and growth mechanism, the particles of D0₂₂ phases diminish and disappear gradually, L1₂ phases grow and single L1₂ phases are formed at last.The ordering of both Al and Cr atoms takes place simultaneously during the precipitation process of Ni₇₅Al_25-xCr_x alloy. Both atoms occupyβsites together, and complex L1₂ phases(Ni₃Al_1-xCr_x) are formed. Cr atoms substitute Al atoms at the boundary of L1₂ phases, and then the D0₂₂ phases(Ni₃Cr) are formed. The limits of occupation probabilities of Cr atαandβ-lattice in L1₂ phases reached when Cr fraction exceeds 3% in Ni_75-xAl₂₅Cr_x alloy, the concentration of Cr rises at L1₂ phases boundaries, and D0₂₂ phases are formed.The occupation probabilities of Cr atoms atαandβlattice in L1₂ phase increase with the temperature increasing in the range of 873K to 1173K, the volume of L1₂ phase increase and that of D0₂₂ phases decrease.The non-classical nucleation and growth mechanism is present at the stage of middle heat treatment, and precipitation mechanism transit to congruent ordering+spinodal decomposition at the stage of aging(973K～1373K). The size and volume of L1₂ phases increase with the rising of temperature of aging, and they decrease with the decreasing of temperature of middle heat treatment. The size and volume of ordering phases in re-aging and three step aging are longer than those in single stage aging.