Contributions to the Transformation Entropy Change and Influencing Factors in Metamagnetic Ni-Co-Mn-Ga Shape Memory Alloys

Abstract

Ni-Co-Mn-Ga ferromagnetic shape memory alloys show metamagnetic transition from ferromagnetic austenite to paramagnetic (or weak-magnetic) martensite for a limited range of Co contents. The temperatures of the structural and magnetic transitions depend strongly on composition and atomic order degree, in such a way that combined composition and thermal treatment allows obtaining martensitic transformation (MT) between any magnetic state of austenite and martensite. The entropy change ΔS measured in the magnetostructural transition comprises a magnetic contribution which depends on the type and degree of magnetic order of the related phases. Consequently, both the magnetization jump across the MT (ΔM) and ΔS are composition and atomic order dependent. Both ΔS and ΔM determine the effect of applied magnetic fields on the MT, hence knowledge and understanding of their behavior can help to approach the best conditions for magnetic field induced MT and related effects. In previous papers, we have reported findings regarding the behavior of the transformation entropy in relation to composition and atomic order in Ni50−xCoxMn25+yGa25−y (x = 3-8, y = 5-7) alloys. In the present paper we will review our recent results, summarizing the key findings and drawing general conclusions regarding the magnetic contribution to ΔS and the effect of different factors on the magnetic and structural properties of these metamagnetic alloys. OPEN ACCESSEntropy 2014, 16 5561

Effect of Stress-Induced Martensite Stabilization on Acoustic Emission Characteristics and the Entropy of Martensitic Transformation in Shape Memory Ni51Fe18Ga27Co4 Single Crystal

“…In addition, it seems to be less probable that defects introduced by the SIM aging would be responsible for the observed entropy change: the martensite stabilization results in more ideal martensite microstructure as compared to the one in the quenched state (e.g., by annealing out of the frozen-in vacancies by quenching). This assumption is also in accordance with the arguments used in [16]: "the only effect of such stabilization is a homogeneous decrease of the chemical free energy of the sample's material". On the other hand, the fact that the austenite stabilization has a minor effect is in accordance with arguments published in [12,19,22], i.e., the change of the long-range order in the austenite, after the heat treatments applied, has a moderate effect in this alloy too.…”

“…Nevertheless, the observed increase of the width of the hysteresis indicates that the entropy change cannot be connected with this since it alone would cause a much smaller effect. It is worth adding that in ferromagnetic shape memory alloys, there is a magnetic contribution to ∆s [16,34,37,38] and its possible change due to SIM-aging calls for further investigations. It can be added that since the transformation entropy can be sensitive to the changes in the atomic and magnetic order [16], it can be assumed that SIM-aging can have a similar effect as the change of the slope of free energy versus the temperature.…”

“…It is worth adding that in ferromagnetic shape memory alloys, there is a magnetic contribution to ∆s [16,34,37,38] and its possible change due to SIM-aging calls for further investigations. It can be added that since the transformation entropy can be sensitive to the changes in the atomic and magnetic order [16], it can be assumed that SIM-aging can have a similar effect as the change of the slope of free energy versus the temperature. Regarding the possible contribution of the change of the elastic energy to the entropy change, we remind that after a few thermal cycles, the SIM-aged sample was broken into small pieces because of the very large local stresses due to the large shape change accompanied with the transformation.…”

“…This exceeded by about a factor of two, the maximum compression transformation strain during the B2-L1 o martensitic transformation, and thus, it is very attractive for applications in different actuators. In addition, it was also found that in NiFeGaCo alloys, depending on the content and on the stabilization thermal treatments, martensitic transformations between any magnetic state of austenite and martensite can be achieved (see e.g., as a review, [16]), which can be important in tailoring magnetocaloric properties. In these alloys, stabilization thermal treatments of the austenite and martensite phases can also result in a change of the transformation entropy.It is clear that such SIM-aging, giving preference to a certain martensite variant, should have important effects on the details of the martensite/austenite transformation.…”

Optimizing the Caloric Properties of Cu-Doped Ni–Mn–Ga Alloys

“…Consequently, the magnetic contribution to entropy is also the highest. By the way, the equivalent criteria has been successfully used to analyze the composition dependence of ΔS of Co-doped Ni-Mn-Ga alloys; as they undergo Aferro⟶mweak MT, Δ S decreases with increasing (TC A − TMT), obviously owing to an increase of the magnetic contribution to the entropy change as the magnetic order of austenite at the MT improves [14]. The temperature hysteresis of the MT, computed as ΔT = AP − MP, as a function of x, is also shown in Figure 4.…”