An efficient scheme to tailor the magnetostructural transitions by staged quenching and cyclical ageing in hexagonal martensitic alloys. (1st August 2019) - Li, Yong
Abstract: The hexagonal MM'X alloys with giant magnetocaloric effects have become an important magnetic phase-transition alloy family. Among the studies on this family, various methods, including composition engineering, have been developed to tailor the magnetostructural transitions. In this study, we present an alternative scheme to tailor the magnetostructural transitions by applying high-temperature staged quenching and low-temperature cyclical ageing on the hexagonal MnCoGe-based alloys with unchanged compositions. The experimental results show that the martensitic transition temperature decreases due to the increasing number of thermal vacancies produced in the increasing annealing temperature, which indicates the martensitic transition is strongly dependent on the staged quenching from high temperatures that can tune the concentration of equilibrious thermal vacancies in alloys. As a result, a Curie temperature window is established, in which the coupling of magnetostructural transitions is successfully realized, showing tunable, large magnetocaloric effects. By further applying cyclical ageing at low-temperature region, the magnetostructural transitions and magnetocaloric effects can be tuned to higher temperatures again due to the release of the trapped thermal vacancies, showing a high tunability of the magnetostructural transitions. During these heat treatments, furthermore, the transitions are robust against the high temperatures up to 550 °C, with no decoupling of the structural and magnetic transitions. The present work provides an important, efficient scheme to tailor the magnetostructural transitions, multiple magnetic/structural phase states, and the related functional properties of hexagonal phase-transition alloy family, without changing the chemical composition of the materials. Graphical abstract: (a) The high-resolution (HR) images of austenite and martensite phases. (b) Ageing duration dependence of structural transition temperature. The data curves at 250 °C are fitted. (c) Temperature dependence of magnetic entropy change (ΔSm ) at different states for MnCo0.958 Fe0.042 Ge with HT-973, HT-1073, HT-1123, respectively. Image 1