Elsevier

Acta Materialia

Volume 201, December 2020, Pages 425-434
Acta Materialia

Tailoring magnetocaloric effect in all-d-metal Ni-Co-Mn-Ti Heusler alloys: a combined experimental and theoretical study

https://doi.org/10.1016/j.actamat.2020.10.013Get rights and content

Abstract

Novel Ni-Co-Mn-Ti all-d-metal Heusler alloys are exciting due to large multicaloric effects combined with enhanced mechanical properties. An optimized heat treatment for a series of these compounds leads to very sharp phase transitions in bulk alloys with isothermal entropy changes of up to 38 J kg1K1 for a magnetic field change of 2 T. The differences of as-cast and annealed samples are analyzed by investigating microstructure and phase transitions in detail by optical microscopy. We identify different grain structures as well as stoichiometric (in)homogenieties as reasons for differently sharp martensitic transitions after ideal and non-ideal annealing. We develop alloy design rules for tuning the magnetostructural phase transition and evaluate specifically the sensitivity of the transition temperature towards the externally applied magnetic fields (dTtμ0dH) by analyzing the different stoichiometries. We then set up a phase diagram including martensitic transition temperatures and austenite Curie temperatures depending on the e/a ratio for varying Co and Ti content. The evolution of the Curie temperature with changing stoichiometry is compared to other Heusler systems. Density Functional Theory calculations reveal a correlation of TC with the stoichiometry as well as with the order state of the austenite. This combined approach of experiment and theory allows for an efficient development of new systems towards promising magnetocaloric properties. Direct adiabatic temperature change measurements show here the largest value of -4 K in a magnetic field change of 1.93 T for Ni35Co15Mn37Ti13.

Introduction

Energy-efficient technologies are necessary to slow down climate change and the depletion of natural energy resources. Since the global power demand for cooling will likely exceed the energy consumption for heating in the second half of this century [1], it is pivotal to develop new energy-efficient, inexpensive and environmentally friendly cooling technologies, which will result in reduced CO2 emissions [2]. The magnetocaloric effect can provide a solution with an increased energy-efficiency compared to conventional vapor-compression technology [2], [3], [4].

Various material systems such as La-Fe-Si, Fe-Rh, and Fe2P-type alloys show attractive magnetocaloric properties [5], [6], [7], [8], [9], [10]. A versatile class is represented by the huge family of Ni(-Co)-Mn-X Heusler alloys, with X being a main group element, e.g. Ga, In, Sn, Sb or Al [11], [12], [13], [14], [15]. They crystallize in the Heusler structure (L21) with four interpenetrating face-centered cubic sublattices [16]. The sublattices are occupied according to the valency with Ni on the A- and C-sites, Mn on the B-site and the X element on the D-site sublattice. A disorder between B- and D-sites is very common and leads to the B2 structure. Typical for these alloys is the occurrence of a martensitic transition with a tunable transition temperature Tt. This transition can be coupled to a magnetic phase transition leading to a first-order phase transition with a conventional or inverse magnetocaloric effect. Since the (local) transition temperature is sensitively depending on the chemical composition, sharpest transitions are obtained for chemically homogeneous compounds. Thus, an optimized sample preparation process is of utmost importance. So far, the most promising Heusler alloy for magnetocaloric purposes is Ni-Co-Mn-In with an adiabatic temperature change ΔTad of -8 K in a magnetic field of 2 T [17]. Since In is an expensive and critical element, it is worth searching for other Heusler alloys with a comparable or even superior performance.

Recent studies on all-d-metal Heusler alloys with X being a transition metal occupying the D-sites of the Heusler lattice showed a magnetostructural phase transition of first-order type for Ni-rich and Mn-rich Ni-Co-Mn-Ti systems [18], [19], where highest isothermal entropy changes of 10 J kg1K1 in magnetic fields of 2 T are reported for bulk Ni35Co15Mn35Ti15 [18]. An increasing amount of Ti stabilizes the austenitic phase, whereas Co substitution additionally increases the austenitic Curie temperature TCA. For a certain amount of Co atoms on the Ni sites, the desired martensitic transformation from ferromagnetic austenite to weak-magnetic martensite is enabled. One drawback of many magnetocaloric compounds is the brittleness leading to mechanical degradation even after a small number of magnetocaloric cycles [20], [21], [22], [23]. The all-d-metal Heusler alloys are suggested to solve this problem since the strength of hybridized dd bonding leads to a higher mechanical stability [18], [24] making them very interesting for magnetocaloric [18], [19], [25], barocaloric [26] and elastocaloric [27], [28] purposes. Enlarged magnetocaloric effects have been shown recently by isothermal entropy changes of up to 27 J kg1K1 in magnetic field changes of 2 T for mechanically stable melt-spun ribbons [29]. However, the performance of the all-d-metal Heusler alloys has not been reported in terms of direct measurements of the adiabatic temperature change for the magnetocaloric effect.

We analyze the magnetocaloric effect of the Ni50xCoxMn50yTiy sample series by systematically varying the Co content as well as the Ti content. We use this stoichiometric series to set up a phase diagram. The results are then compared to other Heusler systems and correlated with Density Functional Theory (DFT) calculations, which describe the experimental findings accurately. As already shown in [30], the annealing conditions need to be chosen carefully for the production of magnetocaloric Heusler alloys. In a systematic study, we optimize the heat treatment and improve the magnetocaloric properties of bulk samples significantly. This underlines that the key to sharp phase transitions is a good stoichiometric homogeneity, which we study by correlating the temperature-dependent magnetization with the respective microstructure for differently annealed samples. Finally, we investigate the magnetocaloric effect for this system by direct measurements of the adiabatic temperature change in order to assess the cyclic performance of all-d-metal Heusler alloys for magnetic-field induced cooling applications.

Section snippets

Material and Methods

Nominal Ni50xCoxMn50yTiy compositions were prepared by arc melting in Ar atmosphere for three series, each with fixed Co content (x=13,15,17) and varying Ti concentration. Due to evaporation losses of Mn during melting, an excess of 3% Mn was added. In the following, samples will be denoted by their nominal composition. An overview of the samples with actual stoichiometries determined by energy-dispersive X-ray spectroscopy (EDS) and selected properties from this work are shown in Table 1.

Optimization of the heat treatment

In order to produce Heusler samples with sharp phase transitions leading to large magnetocaloric effects, the homogenizing heat treatment has been optimized. Fig. 1 (a) shows that the phase transition for a heat treatment for 96 h at 1173 K and below is very broad for the exemplarily shown sample. This broad transition behavior leads to reduced magnetocaloric effects due to the diminished dMdT. For higher annealing temperatures, the phase transitions of the samples become sharper indicating a

Conclusions

We present a systematic study on the optimization of the heat treatment procedure for improved magnetocaloric properties of Ni50xCoxMn50yTiy all-d-metal Heusler alloys. With an annealing step at 1323 K for 96 h followed by rapid water quenching, very sharp martensitic phase transitions can be achieved with isothermal entropy changes of up to 38 J kg1K1 in a magnetic field change of 2 T for Ni37Co13Mn34Ti16 with a transition temperature of around 250 K. Microstructural investigations show

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowlgedgments

We acknowledge the financial support by the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (Grant No. 743116 - project ”Cool Innov”), by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) over the CRC ”HoMMage” Project-ID 405553726 TRR 270, by the Helmholtz Association via the Helmholtz-RSF Joint Research Group with the Project No. HRSF-0045, and by the HLD at HZDR, a member of the European Magnetic Field Laboratory

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