Elsevier

Acta Materialia

Volume 125, 15 February 2017, Pages 481-489
Acta Materialia

Full length article
Rare-earth high-entropy alloys with giant magnetocaloric effect

https://doi.org/10.1016/j.actamat.2016.12.021Get rights and content

Abstract

In this paper, we report the development of rare-earth high-entropy alloys (RE-HEA) with multiple principle elements randomly distributed on a single hexagonal close-packed (HCP) lattice. Our work demonstrated that it is the entropy, rather than other atomic factors such as enthalpy, atomic size and electronegativity, that dictates phase formation in the current rare-earth alloy system. The high configuration entropy stabilized the crystalline structure from phase transformation during cooling, whereas a second-order magnetic phase transition occurred at its Neel temperature. The quinary RE-HEA exhibited a small magnetic hysteresis and the largest refrigerant capacity (about 627 J kg−1 at the 5 T magnetic field) reported to date, along with respectable mechanical properties. Our analysis indicates that the strong chemical disorder resulted from the high configuration entropy makes magnetic ordering in the HEA difficult, thus giving rise to a sluggish magnetic phase transition and enhanced magnetocaloric effect. Our findings evidenced that RE-HEAs have great potential to be used as magnetic refrigerants and the alloy-design concept of HEAs can be employed to develop novel high-performance magnetocaloric materials.

Introduction

Compared with conventional gas refrigerants, magnetic refrigerants based on the magnetocaloric effect (MCE) have advantages of being both highly efficient and environmental friendly [1], [2], [3], [4]. The magnetocaloric effect can be characterized by the field-induced entropy change (ΔSM) due to the alignment of its magnetic spins that occurs under an external magnetic field [5]. According to the involved magnetic phase transitions, magnetic refrigerants can be divided into two categories, i.e., the first and the second order magnetic phase transition materials. Materials in the former category usually show large MCE in a narrow temperature range, but their large thermal and magnetic hysteresis and easy occurrence of cracking and fatigue limited their widespread use as magnetic refrigerators [6]. In contrast, the MCE materials in the second category normally possess gradual and continuous magnetization variation, exhibiting broader peaks with no thermal and magnetic hysteresis. Currently, this type of MCE materials is considered to be an optimal choice for magnetic refrigerants [7]. Up to now, however, very few alloys in this specific category simultaneously having large magnetic entropy change and refrigerant capacity (RC) have been developed and fabricated. Exploring novel alloys concurrently possessing large magnetic entropy change and RC through innovative routes is paramount for enabling practical applications of magnetic refrigerants.

In recent years, a new alloy-design concept, termed as high-entropy alloys (HEAs), was proposed [8], [9], [10]. Generally, HEAs contain multiple principal elements in an equimolar or near-equimolar ratio, which induces formation of disordered solid-solution phases with simple structures due to the high entropy of mixing, such as body-centered cubic (BCC), face-centered cubic (FCC) or hexagonal close-packed (HCP) structures. Formation of ordered crystalline intermetallic phases that often contain structurally complex giant unit cells is usually suppressed [11]. The HEA crystallographic structure is characterized by a topologically ordered lattice with an exceedingly high chemical disorder. Such a configurational disorder in MCE materials might hinder thermal motion of magnetic atoms or ions due to the so-called entropy stabilization, which could result in increased heat flow from the ambient environment and lead to large MCE. Rare-earth (RE) elements are known to possess unique magnetocaloric effects, therefore, it is interesting and imperative, both scientifically and technologically, to apply the HEA concept into developing high-performance RE-MCE materials. Recently, Feuerbacher et al. [12] first reported the development of a HCP HEA using pure RE metals, suggesting that our approach is feasible. In this paper, we report design and fabrication of several RE-HEAs which simultaneously have large MCE and RC, and the underlying mechanisms will also be explored.

Section snippets

Experimental

Alloys with a nominal composition of Gd20Dy20Er20Ho20Tb20, Gd25Er25Ho25Tb25, Dy25Er25Ho25Tb25 and Er33.33Ho33.33Tb33.34 were prepared by arc-melting a mixture of high-purity Gd (99.9%), Dy (99.9%), Er (99.9%), Ho (99.9%) and Tb (99.9%) in a Ti-gettered high-purity argon atmosphere. The ingots were re-melted at least six times to ensure chemical homogeneity and subsequently drop-cast into a copper mold with a dimension of Φ10 mm × 60 mm. Phase constitutions were identified by X-ray diffraction

Microstructure

Fig. 1 a shows XRD patterns of the as-cast Gd20Dy20Er20Ho20Tb20, Gd25Er25Ho25Tb25, Dy25Er25Ho25Tb25, and Er33.33Ho33.33Tb33.34 alloys. For the quinary Gd20Dy20Er20Ho20Tb20 alloy, all the XRD peaks can be indexed to a HCP structure. The corresponding SEM image of this alloy is shown in Fig. 1b in which irregular-shaped grains are seen, indicating formation of a mostly single crystalline phase. While for the quaternary Gd25Er25Ho25Tb25 and Dy25Er25Ho25Tb25 alloys, and ternary Er33.33Ho33.33Tb33.34

Phase formation

To evaluate the effect of mixing entropy on phase stability in the current RE-HEA, the parameter Ω was calculated [20]:Ω=TmΔSmixΔHmixWhere Tm, ΔSmix and ∆Hmix are the average melting temperature, the entropy and enthalpy of mixing of the alloy system, respectively, which are calculated separately as:Tm=i=1nci(Tm)i ΔSmix=Ri=1n(cilnci) ΔHmix=i=1,ijnΩijcicj=4i=1,ijnΔHijmixcicjwhere (Tm)i is the melting point of the ith element, R(=8.314 J K−1mol−1) is the gas constant, ci is the molar ratio

Conclusions

Phase formation of multiple principle-element rare-earth alloys was investigated and single phase structure was only found in the quinary alloys with high entropy of mixing of 1.61R, suggesting that it is entropy, rather than other factors such as enthalpy, atomic size and electronegativity, that dominates phase formation in the current rare-earth system. The quinary RE-HEA exhibited large MCE (8.6 J kg-1 K-1 at the 5T magnetic field) and the largest RC reported so far (about 627 J kg-1 at the

Acknowledgements

This research was supported by National Natural Science Foundation of China (Nos. 51531001, 51671018, 51422101, 51371003 and 51271212), 111 Project (B07003), International S&T Cooperation Program of China (2015DFG52600) and Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R05). YW acknowledges the financial support from the Top-Notch Young Talents Program and Fundamental Research Fund for the Central Universities (Nos. FRF-TP-15-004C1). The work of Tong was

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