Anisotropic growth of nano‐precipitates governed by preferred orientation and residual stress in an Al‐Zn‐Mg‐Cu alloy

Through an understanding of diffusion, precise control of the size distribution of nano-precipitates can be essential to developing superior properties in precipitation-strengthened alloys. Although a significant influence of crystallographic orientation on the diffusion process is known to exist in low-symmetry hexagonal close-packed alloys, such anisotropic diffusion is still unidentified in high-symmetry cubic alloys. In this work, we reveal the diffusion-controlled coarsening induced anisotropic growth process of nano-precipitates in an Al-Zn-Mg-Cu alloy. Our experimental and theoretical studies demonstrate that with an increase in the residual stress, the diffusion-controlled coarsening rate is slow along the 〈112〉 fiber texture in the alloy matrix with smaller grain sizes. As such, we find that the diffusion activation energy will be increased along the preferred orientation with largest residual stress, which leads to a reduced diffusion-controlled coarsening rate. Specifically, we demonstrate that the increase in the volume fraction of nano-precipitates originates from the rapid grain-boundary controlled coarsening of the grain-boundary precipitates. Based on these results, an underlying microstructural design strategy is proposed, involving the crystallographic orientation, the residual stress and the grain boundaries to manipulate the precipitate size distribution in this class of alloys.