NMR study of ordering kinetics inNi3Alalloys

Abstract

${}^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) and relaxation has been applied to the study of the effects of ball milling on the long-range order (LRO) of fcc ${L1}_{2}$ ${\mathrm{Ni}}_{3}\mathrm{Al}$ as well as to the subsequent microscopic reordering process through annealing. Structural changes due to the mechanical treatment are correlated to the appearance of a strong magnetization in the disordered phase. In the absence of significant Fe contamination, this magnetic property is attributed to the existence of magnetic moments of the order of $0.23{\ensuremath{\mu}}_{B}$ (${\ensuremath{\mu}}_{B}=\mathrm{Bohr}\mathrm{ }\mathrm{magneton}$) localized at the Ni sites. ${}^{27}\mathrm{Al}$ NMR spectra in both the ordered and the disordered phase are presented. The random distribution of atoms combined with the magnetic properties in the disordered phase cause a substantial broadening of the ${}^{27}\mathrm{Al}$ NMR line. The linewidth is proportional to the fractional change of disorder and is therefore used to monitor the ordering transformation as a function of annealing time and temperature. The changes of spin-lattice relaxation rates ${(T}_{1}^{\ensuremath{-}1})$ and Knight shifts during transformation are also examined. The overall ordering behavior as observed by NMR is described in terms of a stretched exponential for the time dependence of the untransformed fraction, implying a time-dependent transformation rate. The activation energy of the Ni vacancy migration mechanism responsible for the transformation was determined to be $E=1.8\mathrm{}\mathrm{eV}\ifmmode\pm\else\textpm\fi{}0.2\mathrm{}\mathrm{eV}.$ The average distance covered by the atoms during the ordering is estimated by means of a simple random-walk model.