NMR study of ordering kinetics in Ni3Al alloys

Al-27 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 fee L1(2) Ni3Al 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 mu(B) (mu(B) = Bohr magneton) localized at the Ni sites. Al-27 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 Al-27 NM 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(-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 eV +/- 0.2 eV. The average distance covered by the atoms during the ordering is estimated by means of a simple random-walk model. [S0163-1829(98)09317-5].