Atomic-Scale Computer Simulation For Alloy During Early Precipitation Process

With the microscopic phase-field model, the atomic-scale computer simulation programs of the binary and ternary Ni-based alloys were firstly worked out based on the microscopic diffusion equation and nonequilibrium free energy. It can be applied to the whole precipitation process and composition range. Any a prior assumptions on the new phase structure or transformation path was unnecessary, the possible nonequilibrium phases, atomic clustering and ordering could be described automatically, and atomic pictures, order parameters and volume fractions of precipitates were obtained. Computer simulation was performed systematically on the precipitation mechanism, precipitation sequence and influence of coherent elastic strain energy in complicated system with ordering and clustering simultaneously. The main simulation results were confirmed by comparing the experimental results and theories of others, meanwhile, the phenomenon and laws were also obtained, which are difficult or unable to discover in experiments at present. The main conclusions were summarized as follows:Non-classical nucleation and growth mechanism of y' ordered phase occurs in the lower composition Ni-Al alloys. Ordering and clustering occurs simultaneously and no GP zones are found. With the increase of Al composition, there is more characteristic of Non-classical nucleation and growth, and the order parameter values of critical ordered nucleus decrease. The precipitation sequence is:Supersaturated solid solution Non-classical NG Nonstoicheometric ordered phasestoicheometric ordered phaseGrowthFor higher composition Ni-Al alloys, the precipitation mechanism of y' phase is congruent ordering followed by spinodal decomposition, and ordering occurs prior to clustering. By congruent ordering, a nonstoicheometric single ordered phase is produced with the ordered domains being separated by antiphase domain boundaries (APBS). Spinodal decomposition occurs predominantly at APBS, which results in the replacement of the APBS by the two-order/disordered interfaces. Spinodal decomposition at APBS promotes the decomposition in domains, however, the disordered phase in domains disappear as time follows. The precipitation sequence is as follows:Supersaturated solid solution Congruent ordering Nonstoicheometric ordered phaseSpinodal decomposition stoicheometric ordered phaseNo incontinuous transition of precipitation mechanism has been found in middle composition Ni-Al alloys. With the increase of Al composition, the precipitation characteristic transforms from non-classical nucleation and growth to Spinodal decomposition. Ordering occurs earlier and quicker than clustering; the composition in critical nucleus is lower and the size is larger; Spinodal decomposition occurs in ordered phase. The precipitation mechanism near lower composition alloys zone is non-classical nucleation and growth mechanism, which is similar to that of lower composition alloys. Non-classical nucleation and growth + Spinodal decomposition mechanism occurs in alloys near higher composition zone, which is similar to that of higher composition alloys. The precipitation sequence is as follows:Supersaturated solid solution Non-classical NG Nonstoicheometric ordered phaseSpinodal decomposition stoicheometric ordered phaseFor Ni75AlxV25-x alloy with lower Al composition, ordered phase precipitated earlier than y' ordered phase by congruent ordering + spinodal decomposition mechanism and thus produced a nonstoicheometric single ordered phase. Then, the nonstoicheometric r' phase precipitated by a non-classical nucleation and growth mechanism at the APBS of phases. With the increase of Al composition, the congruent ordering process of phase be more slowly, and the composition in critical r nucleus is lower and the size is larger. Meanwhile, the occupation percentage of r increase. Meanwhile, both of them transformed to stoicheometric ordered phases.For Ni75AlxV25-x alloy with middle Al composition, and y' ordered phase precipitated at the same time. With the...