Computation And Simulation Of Microstructure And Phase Transformation In Cu-Ni-Si Alloy

The Cu-Ni-Si alloy is an excellent leadframe material with high strength and good electric conductivity, its performance mainly depending on the shape, size and distribution ofδ-Ni₂Si precipitates. The alloy has been systematically investigated using first-principles and phase-field method because of the urgent demand of its development and performances to further improve. The thermodynamic stability and electronic structure of the precipitates were calculated and the spinodal decomposition structure (SDS) was theoretically determined for the first time. Phase-field models for spinodal decomposition, precipitation and recrystallization grain growth were established based on Cahn-Hilliard (CH)Allen-Cahn(AC)equations in Cu-Ni-Si alloy, which directly coupling of CALPHAD (Computer Coupling of Phase Diagrams Thermochemistry) thermodynamics and dynamics calculation, meanwhile taking into account the elastic energy and interface energy anisotropy. And computer simulation programs of which were phase firstly worked out at home. The mechanisms of spinodal decomposition and precipitation kinetics, and recrystallization grain growth law in the system containing dispersedδ-precipitates were interpreted. The main computational and simulation results were confirmed by comparing our and other experimental results, and therefore they may provide theoretical guidance and help for the actual processes in some way. Meanwhile, the application of phase-field to the filed of solid-state phase transition is enriched.Based on first-principles pseudopotential plane wave method, the formation heat, cohesive energy and electronic properties of the precipitates and the SDSs were calculated in Cu-Ni-Si alloy, and the crystal structure of SDS and sequence of precipitates stability were theoretically determined for the first time. The calculated formation heat and cohesive energy of the Do22-I are -0.975eV·atom^-1 and -5.388eV·atom^-1, respectively, with its negative value the greatest among all possible SDSs (L12, DO22 and DO23). Therefore, the structure is most likely to be the DO22-I at the early stage of spinodal decomposition process in Cu-Ni-Si alloy. The calculated lattice constants of theδ-Ni₂Si,γ-Ni5Si2 andβ-Ni3Si are in good agreement with experimental results, with all relative errors less than 1%. The calculated results of the cohesive energy and the formation heat indicate that these three kinds of precipitates are all thermodynamically stable in structure, their structural stability following the order ofδ-Ni₂Si >γ-Ni5Si2 >β-Ni3Si. Electronic structures analysis show that the three precipitates with stable structures are mainly due to strong hybridization between Ni-3d and Si-3pAgeing process of Cu-Ni-Si alloy was investigated using the phase-filed model of ternary alloy spinodal decomposition established by ourselves. The simulated results show that the spinodal decomposition takes place at the early stage of aging in the alloy at 723 K and 673 K. When ignoring the elastic strain energy, the SDS is two-phase mixture of rich Cu and rich Ni and Si, in the semi-interconnected labyrinth-like distribution. Under the influence of the elastic strain energy, the SDS manifest the obvious anisotropic characteristics, along the crystallographic orientation of [10] and [01] in distribution. The calculated concentration profiles and correlation function analysis indicate that the modulation wavelength is between 10.4 and 12.6 nm. With constant concentration, the modulation wavelength decreases slightly with the lowering of aging temperature; and with constant aging temperature, the modulation wavelength increases slightly with the rising of concentration of Ni and Si.The eigenstrain and the coherent as well as semi-coherent interfacial energies (227mJ/m2, 492mJ/m2) were theoretically calculated for the disc-shapedδ-Ni₂Si precipitate. The relationships were built between the model parameters and thermodynamic and kinetic data. A pioneering phase-field model was established for precipitation phase transformation in multi-component alloy, which incorporates the interfacial energy and elastic energy anisotropy. The precipitation and growth process of disc-shapedδ-precipitates was simulated for the first time by computer based on our model. Furthermore, the precipitation kinetics equations of Cu-Ni-Si alloy were obtained based on the Avrami equation form. For a singleδ-precipitate, the elastic energy anisotropy accelerate growth along its length, the interfacial energy anisotropy accelerate growth along its thickness, and under the influence of both interfacial energy and elastic energy anisotropy,δ-Ni₂Si precipitates were presented in disc-shaped. For a number ofδ-precipitates, when one precipitate hits another precipitate with a different orientation, it stops growing, consequently forming a"T"-shape precipitate configuration. When two precipitates with the same orientations grow and hit each other, they connect or coarsen only if the spacing between the precipitates is very small. Therefore, the coarsening behavior of disc-shaped precipitate should be completely different from that of spherical precipitates.The phase-field model was worked out for the effect of second-phase particles on grain growth. And the specific effect ofδ-Ni₂Si precipitates on the recrystallization grain growth has been simulated during re-aging process of Cu-Ni-Si alloy. The results show that the finely dispersed pre-agingδ-Ni₂Si particles exert a strong pinning effect on the recrystallization grain boundaries. The initial grain growth corresponds to the power growth law with n between 0.15 and 0.35, obviously less than that of pure single phase system ( n = 0.5), followed by a gradual transition to growth stagnation. The exponent n strongly relates to the number of particles in per unit area in the system, and the more the particles, the smaller n value will be. The final average grain size follows a Zener relation ofβ= 1.41 and b = 0.49.