Abstract
We measured the change in the average hyperfine field strength of several high entropy alloys in relation to small compositional deviations from the equiatomic alloy, FeCoNiCuMn. Mössbauer spectra of four psuedo-binary systems, in which Mn content is increased and another element was decreased in equal measure, reveal several discrete peaks in the hyperfine field distribution that show evidence of the discrete exchange interactions between magnetic elements in the alloy. A simple linear regression modelling the perturbation of the average hyperfine field when the composition is altered calculates the contribution of each atom to the overall average. The average hyperfine field is linear with Tc, so these values allow us to estimate Tc for alloys with more complex compositional variation within the window of linearity (<24% Mn based on other alloys). The results were confirmed experimentally by calculating Tc of two new alloys, Fe19Co20Ni19Cu19Mn23 and Fe19Co20Ni19Cu20Mn22.
Highlights
The multiple sized atoms mixed in relatively equiatomic amounts in these alloys lead to a 2nd order magnetic transition, which is broadened due to the distributed exchange interactions arising from the disorder,10–14 and allows Tc tuning and control of the refrigeration capacity, which leads to a broader transition range than seen in traditional alloys
The four pseudo-binaries probed, Cu-Mn, Ni-Mn, Co-Mn, and Fe-Mn, produce a range of alloys with Tc spanning from 400K to 265K, and the average hyperfine field of these alloys decreased with Tc
We present Mössbauer spectra obtained for four psuedo-binary alloy systems branching from equiatomic FeCoNiCuMn
Summary
Magnetocaloric refrigeration at room temperature is a topic of great interest due to the fact that it has been shown to be up to 20% more efficient than conventional vapor compression refrigeration, and it has the additional advantage of being environmentally friendly because ozone depleting and warming refrigerants are not used. Much work has been done to explore materials with transition temperatures around room temperature, but the majority of them contain rare earth (RE) metals, the scarcity and high price of which is prohibitive for large scale production. Our past work has explored the RE-free transition metal-based high entropy alloy (HEA) system FeCoNiCuMn for this reason, building on the work of Lucas et al on FeCoCrNi alloys. The multiple sized atoms mixed in relatively equiatomic amounts in these alloys lead to a 2nd order magnetic transition, which is broadened due to the distributed exchange interactions arising from the disorder, and allows Tc tuning and control of the refrigeration capacity, which leads to a broader transition range than seen in traditional alloys. The multiple sized atoms mixed in relatively equiatomic amounts in these alloys lead to a 2nd order magnetic transition, which is broadened due to the distributed exchange interactions arising from the disorder, and allows Tc tuning and control of the refrigeration capacity, which leads to a broader transition range than seen in traditional alloys This is a novel approach to increasing the refrigeration capacity of a magnetocaloric material that does not rely on physical processing (ball milling, cold rolling, etc) to broaden the temperature range. The Mössbauer spectra obtained for our alloys were taken using a 57Co gamma ray source embedded in a Rh matrix This emitter nucleus’ gamma rays will be absorbed by Fe atoms, so all spectra and hyperfine field distributions are explorations of the local environment surrounding Fe atoms in our alloys. These alloys were previously confirmed to be single phase using both x-ray diffraction (XRD) and electron dispersive spectroscopy (EDS) for compositional mapping. Cu tends to phase separate at large concentrations19–we postulate that a combination of extended solubilities due to entropic effects in multicomponent FCC solid solutions and the lower concentration of Cu in quinternary HEAs versus quaternary HEAs allows us to retain a single phase
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