Abstract
The effect of Al substitution on microstructure, martensitic transformation and magnetocaloric properties in Ni48Mn39.5Sn12.5−xAlx (x = 0, 1, 2, 3) alloys is reported. At room temperature, depending on Al concentration, the alloys have typical Heusler L21 austenite structure and/or orthorhombic martensite structure with Pmma space group. A secondary Ni-Mn-Al phase also appears already for low Al concentrations (x ≥ 1). On cooling, irrespective of Al substitution, all the samples show ferromagnetic type ordering below 303 K in the austenite phase. The martensitic transition temperature varies with Al content. All the alloys undergo magnetic field-induced reverse martensitic transformation giving rise to an inverse magnetocaloric effect. The largest magnetic entropy change (8.5 J·kg−1·K−1) is observed near 280 K for the Ni48Mn39.5Sn12.5 alloy.
Highlights
Heusler Ni-Mn-based magnetic shape memory alloys undergoing a first order reversible martensitic transformation (MT) have aroused considerable attention owing to their various functional properties [1,2] including a magnetocaloric effect (MCE) [3]
Some special focus has been given to the metamagnetic shape memory alloys (MSMA); i.e., Ni-Mn-X (X = Sn, In, Sb) [7] capable of the reverse martensitic transformation (RMT) under magnetic field, which is geared by the difference in the Zeeman energy (μ0∆martensite start (Ms)·H) between martensite and austenite [8,9,10]
Regardless of the Al composition all the alloys feature Mn-rich phase, as confirmed by Scanning Electron Microscopy (SEM)-Energy Dispersive X-ray (EDX) analysis, which is visible on SEM-backscattered electron (BSE) images as spots with dark contrast (Figure 1a). The amount of this phase estimated from the results of SEM-BSE analysis is below 1% and its appearance may be due to a segregation effect similar to the one reported by other authors in Ni-Mn-Sn alloys [17,33]
Summary
Heusler Ni-Mn-based magnetic shape memory alloys undergoing a first order reversible martensitic transformation (MT) have aroused considerable attention owing to their various functional properties [1,2] including a magnetocaloric effect (MCE) [3]. In previous letters the current authors discussed the influence of Sn replacement with Al on structure and magnetocaloric properties in Ni48Mn39.5Sn12.5 alloys produced by melt spinning technique [26,27,28], which offers significant advantages over conventional metallurgy originating in microstructure refinement, enhanced chemical homogeneity, and extended solid solubility. This becomes ever more important when considering newly developed multicomponent Ni-Mn-based systems [29]. The functional properties of bulk and ribbon alloys are cross examined clearly showing that melt spinning is the preferable method when producing chemically homogenous multielement Ni-Mn-Sn-based alloys
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