Abstract
Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.
Highlights
Shape memory alloys reveal the fascinating ability to recover their initial shape after deformation upon heating
In literature different approaches can be found to solve this issue ranging from the improvement of the chemical composition of the chosen material [16], the enhancement of the compatibility between martensitic and austenitic phase by applying a hydrostatic pressure [17] to just using the minor loops of the thermal hysteresis [18]
We have demonstrated that the thermal hysteresis could be decreased by preparing off-stoichiometric NiCoMnAl multilayer systems, which consist of alternating grown martensite intercalations (MIs) and active transforming austenitic layers (ALs) [19]
Summary
Shape memory alloys reveal the fascinating ability to recover their initial shape after deformation upon heating Their electrical and thermal properties depend strongly on temperature and mechanical stress, which opens different fields of applications [1,2]. The discovery of the shape memory effect in ferromagnetic Ni2 MnGa by Ullakko et al in 1996 [3], offers new application fields of shape memory alloys, because the martensitic transition cannot only be triggered by temperature, mechanical strain or hydrostatic pressure and by external magnetic fields [4,5,6,7]. In literature different approaches can be found to solve this issue ranging from the improvement of the chemical composition of the chosen material [16], the enhancement of the compatibility between martensitic and austenitic phase by applying a hydrostatic pressure [17] to just using the minor loops of the thermal hysteresis [18]
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