An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-based alloys: Part II. Precipitation under various loadings

An elastoplastic phase-field model, developed in Part I, was employed to study stress-assisted diffusional phase transformation, taking plastic deformation into consideration. Different loading conditions were applied to a single crystal of Mg-Al-based alloys during precipitation, aiming at investigating the effects of elastoplastic deformation associated with both phase transformation and external loadings. The simulation results show that even when external loadings are below the materials' yield limits, plastic deformation may occur due to stress accumulation triggered by the volume expansion of & beta;-Mg17Al12 precipitates and the lattice mismatch between the precipitate and the matrix. Under both axial and shear loading conditions, plasticity plays a significant role in the morphological evolution of & beta;-Mg17Al12 precipitate with Burgers orientation. In contrast to the lath morphology observed in the absence of external loadings, a rhombic-like precipitate is formed when applying an external tensile loading of 30 MPa along [1120]& alpha; direction, while a mirror-symmetrical-parallelogram-like morphology emerges under a shear loading of + 30 MPa. Additionally, plasticity plays a similar dual role during precipitation under different loading conditions, which would promote the growth of the precipitate at the initial stage but tend to retard it afterward. These findings illustrate that the plastic strain generated under external loadings has a great impact on the precipitate microstructure, which provides guidelines for designing the precipitate microstructure to obtain better mechanical properties.