Structure and inverse magnetocaloric effect in Ni-Co-Mn-Sn(Si) Heusler alloys

Abstract

In the presented work, a systematic study of crystal structure, microstructure, magneto-structural behavior and inverse magnetocaloric effect in the Ni44Co6Mn39Sn11-xSix (x = 1, 2 at.%) Heusler alloys, obtained by conventional casting and rapid solidification process, have been performed. All alloys, independently of the chemical composition, i.e. different addition of Si and fabrication process (melt-spinning and induction melting) were fully martensitic at ambient temperature. This was the case in spite of the large difference in the mean grain size of ribbons compared to bulk. Interestingly, the microstructure of ribbons consists of larger grains of about 5–20 μm in diameter with martensitic relief and smaller cells of about 1 μm. The crystal structure of both ribbons and bulk was identified as modulated six-layered (12 M) martensite with five additional spots between main reflections in the reciprocal space. The characteristic temperatures of the martensitic transformation were lower for melt-spun ribbons with respect to bulk. This may be connected with the grain refinement, internal stresses and high density of dislocations caused by rapid solidification process. Moreover, the addition of Si enhances the transformation temperatures. The calculated values of magnetic entropy change were higher for bulk alloys than ribbons.