Entropy Change Caused by Martensitic Transformations of Ferromagnetic Shape Memory Alloys

Abstract

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 ∆S 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 ∆S and Q values, which were experimentally determined for Ni-Mn-Ga and Ni-Fe-Ga alloys with different Curie temperatures T C and MT temperatures T M , are theoretically analyzed. It is shown that for Ni-Mn-Ga martensites with T M < T C , the ratio of elastic and magnetic contributions to the entropy change may be greater or smaller than unity, depending on the temperature difference T C -T M .

Dissipation losses limiting first-order phase transition materials in cryogenic caloric cooling: A case study on all-d-metal Ni(-Co)-Mn-Ti Heusler alloys

“…The compensation temperature of 300 K in Ni 33 Co 17 Mn 50-y Ti y also clearly indicates that arrested martensitic transformations are of thermodynamic and not kinetic origin. Transition entropy changes of 65 J(kgK) -1 at ≥ are substantially larger compared to classical Heusler alloys with typical values of approximately 30 J(kgK) -1 for Ni-Mn-Ga [77,78], 45 J(kgK) -1 for Ni-Mn-In [49] and Ni-Mn-Sn [79], 50 J(kgK) -1 for Ni-Co-Mn-Sn [80] and 55 J(kgK) -1 for Ni-Co-Mn-In [49].…”

Influence of NiTi Spring Dimensions and Temperature on the Actuator Properties

“…The temperatures used to study its effect on our simulated equiatomic NiTi actuator were (5,10,15,20,25,30,35,40,45,50 For every (force-deformation) hysteresis loop, the points of transformation increased primarily during the loading cycle (upper plateau) and decreased during the unloading cycle (lower plateau). At 5℃: for the upper plateau, the point transformation starts with deformation equal to 2.02 mm and spring force equal to 2.40 6 N; these values increase along the upper plateau to become 𝑓 which has deformation equal to 19.8mm and spring force 6.007 N, respectively.…”

“…There are many applications based on the elasticity of shape-memory alloys where large recoverable deformations are required. Most applications based on superelasticity are in biomedical applications [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”