Spinodal decomposition influence of austenite on martensitic transition in a Mn-13 at %Cu alloy

Abstract The effect of ageing regimes at temperatures in a range from 400 °C to 520 °C and time for up to 120 h on the spinodal decomposition of austenite in an as-quenched Mn–13Cu alloy is investigated using small-angle neutron scattering and in situ neutron diffraction. The as-quenched from 850 °C alloy has a 100% antiferromagnetic martensitic structure that can be described as a face-centered tetragonal (f.c.t.) lattice at room temperature (RT). In the early stage of the ageing process (for 8 h at 400–480 °C or for the first 16 h at 440 °C), the decomposition of the high-temperature face-centered cubic (f.c.c.) phase into an Mn-rich f.c.c. matrix and Cu-rich clusters takes place. Higher ageing temperature or longer ageing time leads to the formation of α-Mn inclusions with a body-centered cubic (b.c.c.) structure, retained after cooling to RT. Martensitic transition temperatures strongly depend on the ageing temperature and time, i.e., on the degree of chemical decomposition in austenite. The linear relationship between phase transition temperatures and Mn content in the matrix is quantitatively established. The thermal hysteresis between a direct and reverse phase transition does not change with ageing treatment. According to the in situ neutron diffraction results, structural and magnetic transitions occur at the same temperature during heating and cooling.