Microscopic Phase-Field Simulation Coupled With Strain Energy For The Precipitation Process Of Nickle Base Alloys

Based on the microelasticity theory and the microscopic phase-field kinetic model, the software of microscopic phase-field including the elastic strain energy was explored. It can be applied to the simulation of the whole precipitation process of nickel base model alloy, binary nickel base alloy and ternary nickel base alloy on the atomic scale, the simulation results are exacter than those using the microscopic phase-field model without considering of the strain energy. The main conclusions were summarized as follows:When the strain energy is zero, the coarsening process follows LSW mechanism. When the strain energy is higher, the coarsening process follows the orientation controlled mechanism of growing and coarsening along the elastic soft direction. When the strain energy is intermediate, the coarsening process follows the mixture mechanism determined by the size and orientation of precipitation phase.With the increase of the elastic strain energy, the composition fluctuations of the precipitation phase become smaller and the protruding width of the composition curves become narrower at the same time step, the curve profiles become scattered more and more, which indicates that the hindrance effect of the strain energy on the precipitation and growth becomes obvious with the strain energy increasing.The precipitation process of higher composition Ni-A1 alloys considering the elastic strain energy possess the distinct congruent ordering stage, its precipitation mechanism is congruent ordering followed by spinodal decomposition. However, the precipitation process of these alloys neglecting the elastic strain energy does not possess congruent ordering stage.Simulation results indicate that the different aggregation modes among theγ' phases in Ni-A1 alloys of different concentration are formed in the later precipitation process.γ' phases of lower concentration Ni-A1 alloys keep isolated; four "splitting" patterns amongγ' phases of intermediate concentration Ni-AI alloys are formed, such as L-shaped pattern, a doublet, a triplet and a quartet. The aggregation mode among theγ' phases of higher concentration Ni-A1 alloys is mainly the doublet.For Ni-A1 alloy, the coarsening rate constant decreases with increase in volume fraction for small volume fractions. It remains constant for intermediate volume fractions and increases with increase in volume fractions for large volume fraction.For NioCr-A1 alloy, the effects of strain energy on the precipitation dynamic of L1₂ phases and DO₂2 phases are similar. The coarsening rate time exponential of DO₂2 phases of lower A1 concentration alloy is bigger than that of L1₂ phases. However, the situation of higher A1 concentration alloy is opposite from that for lower A1 concentration. The coarsening rate time exponential of DO22 phases of intermediate A1 concentration alloy is close to that of L12 phases.For Ni-Cr-A1 alloy, with the increase of A1 concentration, the precipitation of L1₂ phases become more easier, their precipitation mechanism is also gradually varied from the non-classical nucleation and growth to the congruent ordering＋spinodal mechanism. At the same time, the precipitation of DO₂2 phases becomes more difficult, their precipitation mechanism is gradually varied from the spinodal mechanism to the non-classical nucleation and growth.For Ni-A1 alloy and Ni-Cr-A1 alloy, their coarsening processes follows the mixture mechanism determined by the size and orientation of precipitation phase. The initial irregular shaped, randomly distributed precipitation phases is gradually transformed into cuboidal shape, regularly aligned along the elastic soft directions ([100] and [001]) and highly preferential selected microstructures are formed during the later stage of the precipitation.