Effect of heat treatment on microstructure evolution and magnetocaloric properties of droplet melted Mn–Fe–P–Si alloys

Abstract

Practical implementation of Mn–Fe–P–Si alloys requires not only excellent magnetocaloric but also mechanical properties. Although these properties can be improved by droplet melting process, the effect of heat treatment on droplets has not been clarified. An investigation has been carried out on the microstructure and magnetocaloric properties of Mn1.20Fe0.75P0.45Si0.55 droplets annealed at different conditions. Experimental results show that the as-cast sample is composed of Fe2P and (Mn,Fe)3Si phases. Tuning the annealing condition leads to fluctuations in Fe2P phase fraction and compositions, thereby affecting the magnetocaloric properties. Increasing the annealing temperature from 1273 to 1373 K, Curie temperature (TC) increases from 204.0 to 277.0 K, thermal hysteresis (ΔThys) reduces from 2.5 to 0.5 K, and isothermal entropy change (ΔSm) doubles to 18.5 J/(kg·K) under a 5 T field. The increase in annealing time leads to the 34.0% increase in TC to 301.5 K, the 66.7% decrease in ΔThys to 1.0 K, but the slight decrease in ΔSm to 11.9 J/(kg·K) at an annealing time of 8 h. The larger relative cooling power (RCP) at higher temperatures and longer times are related to the increase in the Fe2P fraction. Moreover, optimizing the heat treatment condition to 1373 K and 4 h makes the alloy lay at the ideal border of the first-order and second-order magnetic transition, thus obtaining high ΔSm (18.5 J/(kg·K)), large RCP (508.8 J/kg), and extremely low ΔThys (0.5 K) around room temperature. Consequently, this optimal processing produces a competitive candidate for near room temperature magnetic refrigeration.