Atomistic Structural Evolution and Glass Transition in Ni-Mn Alloy under Different Cooling and Annealing Conditions

In this study, molecular dynamics simulations were used to examine glass formation in equiatomic Ni.Mn alloy across system sizes of 4000–16384 atoms, cooling rates of 5×10 12 –10 14 K s –1 , and isothermal annealing at 600, 700, and 800 K. Increasing the model size improves the statistical sampling of local structural ordering and reduces finite-size fluctuations, although the degree of local structural order does not increase monotonically with system size. An apparent glass transition temperature of approximately 708 K was determined from the energy-versus-temperature curve at a cooling rate of 10 13 K s –1 , with the data-fitting window spanning roughly 680–780 K; indicating that this value represents a protocol-dependent kinetic crossover rather than an equilibrium glass-transition temperature. The Warren–Cowley parameter reaches α 1 ≈ –0.09 at 300 K, indicating a moderate yet persistent tendency toward the formation of unlike Ni–Mn bonds—a tendency that becomes kinetically arrested as the transition temperature is approached. Common-neighbor analysis shows that the combined fraction of FCC/HCP/BCC-like environments increases from ≈ 3% under rapid quenching (10 14 K s –1 ) to ≈ 48% under slower cooling, corresponding to an approximately sixteen-fold increase in the fraction of CNA-identified locally ordered environments. Meanwhile, the bond-orientational order parameter (〈Q 6 〉 ≈ 0.15–0.17) remains substantially below that of ideal crystals, indicating enhanced short-range ordering without long-range crystallization. These findings demonstrate that system size, thermal history, and cooling rate collectively regulate local structural ordering and glass formation in binary Ni–Mn alloys.