REORDERING KINETICS AND MAGNETIC-PROPERTIES OF MECHANICALLY DISORDERED NANOCRYSTALLINE LL2-TYPE NI3AL+FE ALLOYS

The paramagnetic disordered high temperature states of L1(2) or gamma'-type Ni3Al-Fe alloys become ferromagnetic at low temperatures if retained in metastable disordered (gamma) state. Using mechanical attrition (milling) which also transforms the material into nanocrystalline form, various have been prepared in metastable-gamma form and their magnetisations M(gamma) and M(gamma) and Curie temperatures T(c)(gamma) and T(c)(gamma) measured. Fe atoms possess a giant magnetic moment in these alloys and polarise neighbouring Ni atoms but this polarisation effect is lower in the disordered state due to a lower number of Fe-Ni nearest neighbours resulting in M(gamma) < M(gamma') and T(c)(gamma < T(c)(gamma'). Currently available expressions for magnetic ordering energies based on Bragg-Williams nearest neighbour interactions are found to be inadequate for these giant-moment-type alloys. Isothermal ordering kinetics of the metastable-gamma-state which is also nanocrystalline indicate that the ordering reaction cannot propagate across the disordered nanograin boundaries and each grain must obtain its own gamma' nucleus. This results in a Johnson-Mehl-Avrami exponent n almost-equal-to 1.