Open Access
Outstanding role of the magnetic entropy in arrested austenite in an ordered Ni45Mn36.7In13.3Co5 metamagnetic shape memory alloy
(Elsevier, 2019) Pérez de Landazábal Berganzo, José Ignacio; Recarte Callado, Vicente; Sánchez-Alarcos Gómez, Vicente; Jiménez Ruiz, M.; Cesari, Eduard; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias
The relevance of the entropy and in particular the outstanding role of the magnetic contribution is analyzed in a non-equilibrium arrested austenite phase in a Ni 45 Mn 36.7 In 13.3 Co 5 metamagnetic shape memory alloy. The Debye and Bragg-Williams approximations have been used to analyze and quantify the vibrational and magnetic contributions respectively, to the total entropy change linked to the martensitic transformation. The sign on the entropy change associated to the forward austenite to martensite transformation is different depending on whether it occurs at low or at high temperature as a consequence of the different vibrational and magnetic contributions.
Open Access
Entropy change caused by martensitic transformations of ferromagnetic shape memory alloys
(MDPI, 2017) L'vov, Victor A.; Cesari, Eduard; Kosogor, Anna; Torrens Serra, Joan; Recarte Callado, Vicente; Pérez de Landazábal Berganzo, José Ignacio; Física; Fisika
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.
Open Access
Routes for enhanced magnetism in Ni-Mn-In metamagnetic shape memory alloys
(Elsevier, 2019) López García, Javier; Sánchez-Alarcos Gómez, Vicente; Recarte Callado, Vicente; Pérez de Landazábal Berganzo, José Ignacio; Fabelo, O.; Cesari, Eduard; Rodríguez Velamazán, José Alberto; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias
The authors provide in-depth physical insight into the enhancement of the magnetic properties of metamagnetic shape memory alloys produced by thermal treatment and cobalt doping. They use neutron scattering to study the atomic order and magnetic structures in the austenitic phases of Ni50Mn34In16 and Ni45Co5Mn37In13 alloys in two different states induced by thermal treatments. The increase of the magnetization in the austenite phase, particularly by cobalt doping, is explained by the enhanced ferromagnetic coupling between the magnetic moments located in octahedral sites. The spin density maps obtained from polarized neutron diffraction reveal the magnetic interaction pathways responsible for this coupling scheme.
Open Access
Long-range atomic order and entropy change at the martensitic transformation in a Ni-Mn-In-Co metamagnetic shape memory alloy
(MDPI, 2014) Sánchez-Alarcos Gómez, Vicente; Recarte Callado, Vicente; Pérez de Landazábal Berganzo, José Ignacio; Cesari, Eduard; Rodríguez Velamazán, José Alberto; Física; Fisika
The influence of the atomic order on the martensitic transformation entropy change has been studied in a Ni-Mn-In-Co metamagnetic shape memory alloy through the evolution of the transformation temperatures under high-temperature quenching and post-quench annealing thermal treatments. It is confirmed that the entropy change evolves as a consequence of the variations on the degree of L21 atomic order brought by thermal treatments, though, contrary to what occurs in ternary Ni-Mn-In, post-quench aging appears to be the most effective way to modify the transformation entropy in Ni-Mn-In-Co. It is also shown that any entropy change value between around 40 and 5 J/kgK can be achieved in a controllable way for a single alloy under the appropriate aging treatment, thus bringing out the possibility of properly tune the magnetocaloric effect.
Open Access
Giant direct and inverse magnetocaloric effect linked to the same forward martensitic transformation
(Springer Nature, 2017) Pérez de Landazábal Berganzo, José Ignacio; Recarte Callado, Vicente; Sánchez-Alarcos Gómez, Vicente; Beato López, Juan Jesús; Rodríguez Velamazán, José Alberto; Sánchez Marcos, J.; Gómez Polo, Cristina; Cesari, Eduard; Fisika; Institute for Advanced Materials and Mathematics - INAMAT2; Física
Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.
Open Access
Experimental observation of vacancy-assisted martensitic transformation shift in Ni-Fe-Ga alloys
(American Physical Society, 2019) Unzueta, Iraultza; Alonso de Robador Lorente, Daniel; Cesari, Eduard; Sánchez-Alarcos Gómez, Vicente; Recarte Callado, Vicente; Pérez de Landazábal Berganzo, José Ignacio; García, José Ángel; Plazaola, Fernando; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias
Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni55Fel7Ga28 alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2(1) and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V-Ni is identified as the most probable vacancy present in Ni55Fe17Ga28. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.