Abstract
In this paper, our most recent findings on the influence of magnetic order on the main transformational caloric and elastic properties of shape memory alloys (SMAs) are reviewed. It is argued that ferromagnetic order has a strong influence on the temperature interval of martensitic transformation (MT), the characteristics of stress-induced MT, and the shear elastic modulus of SMA. The problem of separation of the magnetic contributions to the entropy change ΔS and heat Q exchanged in the course of martensitic transformation (MT) of SMA is considered in general terms, and theoretical formulas enabling the solution of the problem are presented. As an example, the ΔS and Q values, which were experimentally determined for Ni-Mn-Ga and Ni-Fe-Ga alloys with different Curie temperatures TC and MT temperatures TM, are theoretically analyzed. It is shown that for Ni-Mn-Ga martensites with TM < TC, the ratio of elastic and magnetic contributions to the entropy change may be greater or smaller than unity, depending on the temperature difference TC – TM.
Highlights
Magnetic shape memory alloys exhibit a thermoelastic martensitic transformation (MT) from the high-symmetry phase to the low-symmetry state and, related to this transformation, display several functional properties
We review our most recent findings on the influence of magnetic order on the main transformational properties of ferromagnetic shape memory alloys (FSMAs), citing as an example the Ni-Mn-Ga alloy system, and we discuss both the measured and theoretically evaluated magnetic contributions to the entropy change
We have argued above that the elastic properties of shape memory alloys in the ferromagnetic phase depend on magnetic order; we will consider that ∆Smag is the part of ∆S that depends on spontaneous magnetization arising below TCA
Summary
Magnetic shape memory alloys exhibit a thermoelastic martensitic transformation (MT) from the high-symmetry phase (austenite) to the low-symmetry state (martensite) and, related to this transformation, display several functional properties. Large strains are observed under application of a magnetic field to the martensite due to the reorientation of martensite variants in the so-called “conventional” ferromagnetic shape memory alloys (FSMAs), like Ni-Mn-Ga and. MT possible, which is referred to as the magnetic shape memory effect, or the metamagnetic behavior of the alloy [4]. Due to large differences in magnetization values between the phases involved in the MT, the metamagnetic alloys show interesting and promising properties, such as high values of magnetoresistance [5] and inverse magnetocaloric effect [6]. We review our most recent findings on the influence of magnetic order on the main transformational properties of FSMAs, citing as an example the Ni-Mn-Ga alloy system, and we discuss both the measured and theoretically evaluated magnetic contributions to the entropy change.
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