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 reord ering process through annealing. Structural changes due to the mechani cal treatment are correlated to the appearance of a strong magnetizati on in the disordered phase. In the absence of significant Fe contamina tion, this magnetic property is attributed to the existence of magneti c 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 disord ered phase are presented. The random distribution of atoms combined wi th 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 or dering transformation as a function of annealing time and temperature. The changes of spin-lattice relaxation rates (T-1(-1)) and Knight shi fts during transformation are also examined. The overall ordering beha vior as observed by NMR is described in terms of a stretched exponenti al for the time dependence of the untransformed fraction, implying a t ime-dependent transformation rate. The activation energy of the Ni vac ancy migration mechanism responsible for the transformation was determ ined 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-wal k model. [S0163-1829(98)09317-5].