Kinetics of order-disorder transformation of L12 FeNi3 in the Fe-Ni system

The Fe-Ni system has seen a resurgence of interest for the development of high magnetocrystalline anisotropy alloys. The kinetics of the chemical order-disorder phase transformation of the ferromagnetic test-bed compound L12 FeNi3 were quantified using differential scanning calorimetry data obtained from annealed meltspun Fe-75 at% Ni ribbons. The isothermal transformation kinetics are found to be well described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation with a determined Avrami exponent of unity, consistent with one-dimensional growth of the chemically-disordered A1 phase from the chemically-ordered L12 FeNi3 parent phase. The determined activation energy for the transformation from the chemically-ordered L12 structure to the disordered A1 structure in FeNi3, 3.1 ± 0.1 eV, is equal to that reported for the L10-to-A1 order-disorder phase transformation of the FeNi phase that is found in meteoritic specimens. These results confirm that the L12 phase of FeNi3 is a suitable model phase for investigation of formation aspects of the tetragonal L10-type FeNi phase, also known as tetrataenite, which exhibits extremely sluggish formation kinetics but possesses appreciable magnetocrystalline anisotropy and excellent saturation magnetization that is of interest for sustainable permanent magnet applications.