Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy

TL;DR

This study investigates how different heat-treatment conditions affect the microstructure and magnetic properties of an Alnico alloy, revealing optimal processing parameters for enhanced magnetic performance.

Contribution

It provides detailed microstructural analysis and identifies optimal heat-treatment conditions to improve Alnico alloy's magnetic properties.

Findings

Optimal spinodal morphology achieved at ~840°C for 10 min

Remanence increased by 40-70% after magnetic annealing

Maximum energy product of 5.9 MGOe with coercivity of 1845 Oe

Abstract

Further property enhancement of alnico, an attractive near-term, non-rare-earth permanent magnet alloy system, primarily composed of Al, Ni, Co, and Fe, relies on improved morphology control and size refinement of its complex spinodally decomposed nanostructure that forms during heat-treatment. Using a combination of transmission electron microscopy and atom probe tomography techniques, this study evaluates the magnetic properties and microstructures of an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu (wt.$\%$) alloy in terms of processing parameters such as annealing temperature, annealing time, application of an external magnetic field, as well as low-temperature "draw" annealing. Optimal spinodal morphology and spacing is formed within a narrow temperature and time range ($\sim 840 \unicode{x2103}$ and 10 min during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed $\sim 40$ nm diameter (Fe-Co)-rich rods ($\alpha_1$ phase) embedded in an (Al-Ni)-rich ($\alpha_2$ phase) matrix. A Cu-enriched phase with a size of $\sim$ 3-5 nm is located at the corners of two adjacent $\{110\}$ facets of the $\alpha_1$ phase. The MA process significantly increased remanence ($B_\text{r}$) ($\sim$ 40-70 $\%$) of the alloy due to biased elongation of the $\alpha_1$ phase along the $\langle100\rangle$ crystallographic direction, which is closest in orientation to the applied magnetic field. The optimum magnetic properties of the alloy with an intrinsic coercivity ($H_\text{cj}$) of 1845 Oe and a maximum energy product ($BH_\text{max}$) of 5.9 MGOe were attributed to the uniformity of the mosaic structure.