Acoustic emission of Ni–Mn–Ga magnetic shape memory alloy in different straining modes
Introduction
Ni–Mn–Ga alloys with compositions close to the stoichiometric Ni2MnGa compound undergo a thermoelastic martensitic transformation and constitute a family of magnetic shape memory alloys. The magnetic shape memory effect (MSM effect) or magnetic-field-induced strain (MFI strain) up to 9.5% has been observed in the martensitic phase in a magnetic field lower than 1 T [1], [2], [3], [4], [5].
The parent phase of the alloys is L21 cubic and transforms to martensite during cooling. Depending on the composition, the crystal structure of the Ni–Mn–Ga martensite is tetragonal with a long c-axis, seven-layered modulated orthorhombic, or five-layered modulated tetragonal with a short c-axis [2], [6]. There may occur other transformation(s) between martensites (intermartensitic) during further cooling [7]. So far, the MSM effect in the Ni–Mn–Ga alloys has been observed in five-layered modulated [2], [3] and seven-layered modulated martensites [5]. The MSM effect occurs in magnetic field of about 0.1–1 T. The five-layered modulated martensite (5M) seems to be the best candidate for a practical use. This martensite possesses uniaxial magnetic anisotropy with the short c-axis, which is the direction of easy magnetization [8]. There are three possible martensitic variants with orientation of c-axis approximately parallel to the 〈1 0 0〉 directions of the parental cubic phase.
The mechanism of the MSM effect is a motion of martensitic twin boundaries in magnetic field [1], [2], [3], [4] leading to the redistribution of twin variants. This causes macroscopic shape change. The twin boundary motion can be also induced by external stress. To some extent, the effect of the external stress and magnetic field can be considered equivalent and thus we can use the external stress instead of magnetic field to study twin boundary motion [9].
The high mobility of martensitic twin boundaries of the martensitic structure is a basic precondition for the existence of the MSM effect. The reason for the high mobility of the martensitic twin boundaries has not been explained yet, but it seems to be connected with modulated martensitic structure since no MSM effect occurs in non-modulated tetragonal structure with long c-axis. The complete understanding of the mechanism of the twin boundaries motion may lead to the new generation of MSM alloys with extremely mobile twin boundaries.
It is well known fact that small part of the energy involved in thermoelastic martensitic transformation in solids is dissipated in a form of acoustic waves spreading through the crystal. Also the lattice rearrangement is acoustically active process [10]. Additionally, Planes et al. [11] used the acoustic emission (AE) technique for studying pre-martensitic transformation in Ni–Mn–Ga.
In our work, we use the acoustic emission technique to study different structure changes in Ni–Mn–Ga magnetic shape memory alloys. Our primary aim was to investigate the motion of the twin boundaries during stress-induced martensitic variant reorientation using acoustic emission technique. The same set-up was used for studying stress-induced transformation. Further we show that the technique is a suitable probe for monitoring the thermally-induced martensitic transformation.
Section snippets
Experimental
Single crystals of Ni–Mn–Ga alloy were grown by a modified Bridgman method by Adaptamat, Finland. The composition of the investigated alloy determined by energy dispersive spectroscopy was Ni49.7Mn29.1Ga21.2 (numbers indicate at.%). The measurements of structural, mechanical, magnetoelastic and shape memory properties of the samples from the same alloy were quite thoroughly discussed in our previous work [9], [12], [13]. The samples used for acoustic measurement were single crystalline,
Stress-induced reorientation of the martensitic variants
Typical stress–strain curve of martensite reorientation is shown in Fig. 1. The sample initially contains only one variant with the short c-axis perpendicular to the stress. During transition to the induced variant with the short c-axis along the stress, the stress level remains practically constant and then rapidly increases when the reorientation is finished and the martensite is loaded elastically. The reorientation of the 5M martensite is relatively easy, the stress necessary for transition
Conclusions
The AE of transformation processes in Ni–Mn–Ga single crystals was experimentally investigated during stress-induced reorientation of martensite and during stress-induced and thermally-induced martensitic transformations.
Following phenomena were observed:
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Austenite remains acoustically inactive during loading and unloading as well as during thermal cycling.
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Martensite exhibits high AE during stress-induced reorientation, remains acoustically active during loading and unloading, and shows lower AE
Acknowledgements
This work has been supported by Grant Agency of the Czech Republic under the project No. 106/01/0396, by Finnish Academy in the form of the exchange grant, and by the National Technology Agency of Finland (TEKES) and its industrial partners Nokia Research Center, Metso Oyj, and Adaptamat Ltd.
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