Effects of time-dependent low-temperature aging on Ni-Co-Mn-Sn alloys via analysis of atomic order and defect evolution

Research on the effects of low-temperature heat treatment on Ni-Co-Mn-Sn alloys is limited. Most existing studies primarily rely on superlattice diffraction peaks from XRD to assess changes in atomic order and elucidate the effects of heat treatment, often without fully revealing the underlying mechanisms. This study integrated fitting calculations and experimental investigation to systematically investigate the effects of low-temperature aging treatment at 473 K for different time durations on the phase transition behavior, mechanical properties, and magnetic properties of Ni-Co-Mn-Sn powders, while also exploring the associated mechanisms. The experimental results demonstrated that as aging time increased, martensitic transformation temperatures consistently decreased, whereas the austenite Curie temperature continuously increased. After aging at 473 K for 168 h, the mechanical properties of the powder exhibited a reduction in hardness and modulus, while the magnetic properties improved. Dark-field images obtained from TEM revealed an expansion of the ordered L2₁ region after aging. Additionally, XRD-Rietveld full-spectrum fitting results indicated a reduction in anti-site defects, which accounts for the enhanced ferromagnetic coupling, thus the widening of the ferromagnetic austenite phase region, the increase in magnetization and the optimization of magnetocaloric effect. Moreover, both the Williamson-Hall method calculations and bright field images from TEM indicated the decrease in dislocation density, which also contributed to the reduction in hardness and modulus, in addition to the reduction of anti-site defects. This work enhanced our understanding of the mechanisms underlying low-temperature aging, with the aim of utilizing heat treatment to regulate and optimize the performance of shape memory alloys.