Multiscale Design of Α-Al, Eutectic Silicon and Mg2Si Phases in Al-Si-Mg Alloy Manipulated by in Situ Nanosized Crystals

A new design method of Al-Si-Mg alloy manipulated by in situ nanosized crystals was proposed. Starting from source design of the microstructure manipulation agent with a high crystallographic matching relationship with the alpha-Al, eutectic Si and Mg2Si phases in Al-Si-Mg alloy, the interatomic misfit (f(r)) between NiTi(B2) and alpha-Al or Mg2Si is 5.0% or 5.3%, respectively, and that between NiTi(B2) and Si is 9.9%, corresponding to good crystallographic matching. Based on calculations, in situ nanosized crystals could be considered as heterogeneous nucleation sites of alpha-Al, eutectic Si and Mg2Si, thus, uniformly fine alpha-Al grains, near-spherical eutectic Si and fine precipitated Mg2Si were obtained. The solidification behavior analysis also highly corresponded to the E2EM calculations. In the alloy manipulated by 0.05 wt% NiNbTi manipulation agent, the average sizes of alpha-Al, eutectic Si and Mg2Si were greatly decreased by nearly 8, 2 and 20 times, respectively, thus achieving comprehensive multiscale manipulation of the microstructure of the alloy in one step. Specially, the huge energy stemmed from the B2 to B19' phase transformation will generate a large number of crystal defects, which provides nucleation sites and driving force for the formation of Mg2Si and greatly refines the Mg2Si.Moreover, the yield stress, tensile stress and fracture strain of the manipulated alloy were simultaneously improved by 34.1%, 20.4% and 35.0%, respectively. The excellent strength and plasticity could be ascribed to grain-refinement strengthening, Orowan strengthening and eutectic Si refinement and spheroidization, which reduced the stress concentration during deformation. The comprehensive microstructure manipulation of Al-Si-Mg alloy using nanosized crystals formed by in situ physical crystallization, especially the manipulation of the precipitated phase, is an innovative work that not only avoids the "poisoning" effect between multiple manipulation agents but also realizes comprehensive manipulation of the overall microstructure, consequently broadening further industrial applications.