Spinodal Decomposition in Fe-25Cr-12Co Alloys under the Influence of High Magnetic Field and the Effect of Grain Boundary

Abstract

Fe-Cr-Co alloys precipitate nanosized α₁ particles through spinodal decomposition, and their magnetic performance is susceptible to influence by the shape and arrangement of α₁ particles. We studied spinodal decomposition during the heat treatment of Fe-Cr-Co alloys by both experimental and numerical simulation. Fe-Cr-Co alloys were fabricated first by directional solidification, followed by thermomagnetic treatment in a high magnetic field (HMF) and step aging. The experimental results show a spinodally decomposed structure consisting of nanosized α₁ particles. The applied HMF caused the α₁ phase to change into a rod-like shape. Moreover, a feather-like structure was observed near the grain boundary (GB), with slim α₁ rods regularly arranged along the direction perpendicular to the GB. With the shape change and alignment of the α₁ phase in the HMF, Fe-Cr-Co alloys show magnetic coercivity that is superior to those of samples without an HMF. To reveal the influence of HMF on phase transformations and the effect of GB, we conducted phase-field simulations of spinodal decomposition in the Fe-Cr-Co alloy. A migrating GB contributes to the elongation and arrangement of the α₁ phase in the regions where the GB has passed. Thus, the α₁ phase is arranged as parallel rods that are perpendicular to the GB. This GB effect consists of the effect of enhanced atomic mobility and the elastic energy. The α₁ rods are elongated along the direction of HMF. The simulation results indicate that the feather-like structure is caused by a combined effect of both the GB and HMF. It is shown that the model generates microstructures which are qualitatively similar to those observed experimentally.