On the fast kinetics of B2–L2₁ ordering in Ni-Co-Mn-In metamagnetic shape memory alloys

B2–L2₁ configurational order resulting from thermal processing of Ni–Co–Mn–In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2–L2₁ ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L2₁ ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. These predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order.