Enhanced Magnetocaloric Response of High-Entropy Alloy Microwires due to Induced Nanocrystals by Current Annealing

Abstract

High-entropy alloys (HEAs), whose new alloy design concept with multiple principal elements, attracted a lot of attention for structural application and functional aspects. Since their first report in 2004, their compositions have developed from first generation with quinary equiatomic single-phase to second-generation multi-phase structure with non-equiatomic compositions. For magnetocaloric HEAs, their limitations have been recently found to be overcome by using non-equiatomic HEA compositions: GdTbCoAlFe (with Curie temperatures up to 108 K) overcomes the low temperature limit of the rare-earth-containing HEAs (typically saturates around < 60 K) [1,2] and the first-order magnetostructural phase transition in FeMnNiGeSi surpasses the smeared out magnetocaloric effect (MCE) of rare-earth-free HEAs [3,4]. In this work, we further enhance the MCE of non-equiatomic Gd34.9Tb19.4Co19Al23.3Fe3 HEA through microstructural control, in which the alloy microwires are annealed using the current density annealing technique. Induced nanocrystals in the amorphous matrix observed from transmission electron microscopy (TEM) are found to increase and grow with the current density magnitude (Fig. 1). This leads to an increase in the working temperature span of the HEA microwires and at the same time offers a relative cooling power (RCP) comparable to many reported amorphous MCE alloys, both conventional alloys and HEAs (Fig. 2). Our findings pave a pathway for further optimizing MCE through appropriate processing methods on top of careful alloy design selection.  Fig. 1 TEM results of the (a) as-cast and annealed Gd34.9Tb19.4Co19Al23.3Fe3 microwires using current density values of (b) 50 and (c) 100 x 106 A m-2.  Fig. 2 Literature comparison of RCP as a function of full-width at half maximum (FWHM) for studied microwires (star symbols) versus amorphous conventional alloys (triangles) and HEAs (squares).