Increasing the achievable state of order in Ni-based Heusler alloys via quenched-in vacancies

The ability to tune the degree of L2₁ order is of utmost importance for the magneto-mechanical properties of Ni-Mn-based Heusler alloys, e.g., the appearance of a martensitic phase in the Ni₂MnAl system. Here, differential scanning calorimetry is established as a convenient tool for determining the state of order by way of its effect on the magnetic transition temperature, and it is used for studying the low-temperature ordering kinetics in Ni₂MnAl. A significant acceleration of ordering kinetics due to excess vacancies retained after high-temperature quenching is demonstrated. Using this effect, equilibrium of order could be attained at temperatures as low as 623K, where ordering under equilibrium vacancy concentration would take unpractically long.