Formation mechanisms of NaZn13-type phase in giant magnetocaloric La-Fe-Si compounds during rapid solidification and annealing

The promise of the La-Fe-Si alloys for energy efficient magnetic refrigeration devices stems from a strong magneto-structural transition in the La(Fe,Si)13 (1:13) phase near room temperature. The formation of the 1:13 phase during rapid solidification was compared in detailed microstructural studies of the wheel-side and free-side surfaces of melt-spun ribbons. On the free-side, clusters of similarly-oriented crystallites are observed; chemical segregation of La, Fe, and Si leads to a nanoscale texturing of α-Fe and LaFeSi. In contrast, equiaxed 1:13 grains ∼100-400 nm form the microstructure of the wheel-side surface, with a minor α-Fe phase precipitated in the matrix. Upon annealing, the 1:13 phase grows via dissolution of the α-Fe phase on the wheel side and a peritectoid reaction from the free side. For longer annealing times, the completion of the peritectoid reaction improves the magnetic entropy change under a magnetic field change of 1.5 T from 12 J/kg K (2 min) to 17 J/kg K (2 h), and increases the Curie temperature of the ribbons from 189 K to 201 K.