A systematic study of hot deformation mechanisms in La–Fe–Co–Si alloys and the mitigation of defects in hot rolling process

Abstract

Although hot-rolled La(Fe, Co, Si)₁₃-based alloys are promising magnetocaloric materials for solid-state cooling with near-net shaping capabilities, their underlying hot deformation mechanisms remain largely unexplored. In this study, a comprehensive and systematic investigation was conducted, by encompassing the analysis of hot deformation mechanisms, along with the microstructure evolution and magnetocaloric properties of hot-rolled La–Fe–Co–Si alloy. The La_1.05Fe_11.2Co_0.7Si_1.38 alloy was examined using multiscale mechanical analysis to assess the effects of temperature. A series of macroscale hot compression and microscale nanoindentation tests were performed to access global and local mechanical properties, including variations in hardness and indentation modulus of the primary α-Fe and secondary 1:1:1 phases up to 800 °C. A significant decrease in hardness and elastic recovery of the secondary phase was observed between 600 and 800 °C, above half of its melting point (1113 °C), suggesting pronounced flow softening in both the α-Fe and 1:1:1 phases. Additionally, a novel multi-step annealing process was introduced for hot-rolled La–Fe–Co–Si alloys, involving partial transient liquid-phase diffusion in the 1:1:1 phase to address deformation-induced defects, such as residual α-Fe and lattice distortions in the 1:13 phase, which have not been previously reported. As a result, a primary La(Fe, Co, Si)₁₃ phase with a volume fraction of 97.5% was achieved after multi-step annealing, compared to 87.5% using conventional annealing. Correspondingly, the magnetocaloric properties were restored, with the Curie temperature (T_C) recovering from 276 to 268 K and the maximum magnetic entropy change (ΔS_M) increasing from 7.56 to 8.67 J kg⁻¹ K⁻¹ under a 2 T magnetic field.

Graphical Abstract