Elsevier

Acta Materialia

Volume 155, 15 August 2018, Pages 95-103
Acta Materialia

Full length article
Effect of Fe doping and magnetic field on martensitic transformation of Mn-Ni(Fe)-Sn metamagnetic shape memory alloys

https://doi.org/10.1016/j.actamat.2018.05.052Get rights and content

Abstract

In this work we report the elaboration of a family of metamagnetic shape memory alloys with composition Mn49Ni42-x FexSn9 (x = 0, 2, 3, 4, 5 and 6 at.%) and the systematic study of their structure, martensitic transformation (MT) behavior and functional characteristics as a function of the Fe doping and magnetic field, up to 12 T. Regarding the influence of the magnetic field, we have tentatively divided the alloys into two groups: group I, alloys with x = 0, 2, 3 and 4 and group II, alloys with x = 5 and 6. Group II alloys exhibit an appreciable amount of dispersed γ-phase and a field-induced arrest of MT. The alloys from group I show a large magnetization drop at MT that proceeds steeply. This group also displays a monotonous evolution of all studied properties with pronounced metamagnetic effect and large magnetostrain effect, reaching 0.3%, in the alloy with x = 4.

Introduction

Heusler Ni-Mn-X (X = In, Sn, Sb) metamagnetic shape memory alloys (MetaMSMAs) have been extensively studied during the last decade due to their remarkable physical properties and large functional response associated to a first order martensitic transformation (MT) [[1], [2], [3]]. The strong magnetovolume coupling and concurrent ferromagnetic – antiferromagnetic interactions present in these compounds allow to control MT by means of magnetic field or pressure resulting in different multifunctional properties such as the giant inverse caloric effects (magnetocaloric, elastocaloric or barocaloric) [[4], [5], [6], [7]], very large magnetoresistance [8,9] and the metamagnetic shape memory effect [1,10,11]. All these unusual effects open up novel possibilities for technical applications.

The magnetic driving force responsible of the metamagnetic transformation depends directly on the difference of magnetization value between the austenitic and martensitic phases at the MT. According to the Clausius-Clapeyron relationship:-dTM/d(μ0H) = ΔM/ΔS(where μ0H is the external applied magnetic field, TM is the martensitic transformation temperature, ΔM and ΔS are the magnetization and entropy change at MT, respectively) a large ΔM together with a reduced ΔS are necessaries to facilitate a field-induced MT. Besides, MT with low hysteresis width near room temperature is desired for high performance [12,13]. While tuning ΔS is hardly possible, ΔM and TM can be tailored by modifying the composition and doping [[14], [15], [16]].

It is well-known that in the magnetic shape memory Heuslers the magnetization depends mainly on the magnetic moment on Mn atoms and their exchange interactions, which depend on the Mn-Mn interatomic distances [[17], [18], [19], [20]]. So, increasing Mn content is promising to enhance the ΔM value in these compounds. In fact, systematic studies of Mn-based Mn50Ni50-xInx alloys reported by Xuang et al. [21] verified the possibility to develop the two-way magnetic field induced MT for 9 < x < 11 compounds, due to their low hysteresis of MT and high ΔM (6 K and 60 Am2kg−1, respectively, for x = 10). Thus, these alloys present remarkable functionalities that make them competitive candidates for technical applications [21,22].

Mn-Mn exchange interactions can be affected also by the doping with a fourth element. Specifically, doping with transition metals, like Co or Fe, promotes the ferromagnetic (FM) coupling of the nearest neighbors Mn atoms in the austenitic phase raising the magnetization saturation of this phase whereby triggering enhancement of ΔM at MT. In the case of the Mn50Ni40In10 alloys, a 3 at.%Co addition increases ΔM from 40 Am2kg−1 [23,24] to 89 Am2kg−1 [25]. This doping also promotes the formation of secondary phases which improve the mechanical properties of these materials, in particular they mitigate the intrinsic brittleness of the MetaMSMAs [15,26,27].

In the present work, we perform a systematic study of the effect of Fe addition and applied magnetic field on MT and related functional properties in a series of the polycrystalline MetaMSMAs with a composition of Mn49Ni42-xFexSn9 (x = 0, 2, 3, 4, 5). Particularly, we report the influence of the Ni substitution by Fe on MT, structure and microstructure, transformation characteristic temperatures and magnetization change at the transformation. The spontaneous strain accompanying MT has been also investigated, providing new details about the mechanism of the magnetic field induced strain effect observed in these alloys.

Section snippets

Experimental procedure

Polycrystalline ingots of Mn49Ni42-xFexSn9 with x = 0, 2, 3, 4, 5 and 6 were prepared by induction melting of high purity metals (>99.9%) and then heat treated at 1173 K during 3 days under argon atmosphere for homogenization. Small pieces were cut from ingots and heat treated during 0.5 h at 1173 K with subsequent water quenching. After the heat treatment, the samples were mechanically polished to examine their microstructure by secondary electron imaging in a Hitachi TM300 table-top scanning

Crystal structure and microstructure

X-ray diffraction patterns of the bulk samples, depicted in Fig. 1 , show several phases at room temperature depending on the Fe content. The alloy with x = 0 is a single phase martensite with the orthorhombic structure. In the alloys x = 2, 5 and 6, two phases were found: the B2 cubic phase and some fcc γ-phase precipitates resulting from the Fe addition [26,29,30]. On the other hand, a mixture of three phases (austenite, martensite and γ-phase) is observed for x = 3 and 4. The cell parameters

Summary and conclusions

In this work, we have developed a family of the metamagnetic shape memory alloys with compositions of Mn49Ni42-xFexSn9, where x was varied from 0 to 6. Systematic studies were carried out to reveal the crystal structure, transformation behavior, some magnetic characteristics and related functional properties as a function of Fe content and high magnetic fields.

All compounds undergo a martensitic transformation, MT, from the B2 cubic structure in the austenite phase to an orthorhombic one in the

Acknowledgements

This work has been carried out with the financial support of the Spanish Ministry of Economy and Competitiveness (project MAT2014-56116-C4-3-4-R) and Basque Government Department of Education (project IT711-13). The authors thank for technical and human support provided by SGiker of UPV/EHU and European funding (ERDF and ESF).

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