A systematic study of the microstructure, phase formation and magnetocaloric properties in off-stoichiometric La-Fe-Si alloys

In this work, the influences of chemical composition and annealing processing on the phase morphology, structural evolution and associated magnetocaloric effect have been investigated in off-stoichiometric La1+xFe13-ySiy (x = 0.2-1.2 and y = 1.1-1.8) alloys. A peculiar microstructure, constituting of the facet-like La(Fe,Si)13 phase grains and surrounding La5Si3 secondary phase, is present in the composition range x = 0.7-1.2. A large magnetic entropy change of 16 J/kg K at 2 T is obtained in these La-rich bulk samples subjected to a relatively short annealing time of 3 days. The phase formation mechanism is studied by means of the diffusion couple technique. The transition phase La1Fe1Si1 that appears as the lamellar morphology in La-rich alloys plays the key role in accelerating the inter-diffusion with alpha-Fe, thus eventually forms the La(Fe,Si)(13) functional phase at fast rate. Our finding indicates that off-stoichiometric La-Fe-Si composition alloys can be a promising magnetic refrigerant suitable for time-and energy-saving mass production. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.