GRAIN-GROWTH IN NANOCRYSTALLINE NBAL3 PREPARED BY MECHANICAL ALLOYING

Grain growth and its kinetics were studied on an intermetallic compound, NbAl3 powder prepared by mechanical alloying of elemental Nb and Al powders for 1.8 Ms in an argon atmosphere at ambient temperature. The initial and grown grain sizes were measured from the X-ray line broadening of as-alloyed and annealed powders. Isochronal annealing of mechanically alloyed powders from 573 to 1373 K indicated that substantial grain growth occurs only in a temperature range of 1048 to 1173 K and ceases at 1273 K regardless of anneal time. Accordingly isothermal annealing of 1.8 to 18 ks was carried out at 1048, 1073 and 1098 K to obtain the grain growth kinetic that is described by In(dD/dt) = In(r(o)/3) - 2.0 InD where D is the measured grain size and r(o) a constant. This r(o) depends on temperature according to r(o) = r(o)' exp (- Q/kT) where Q is the activation energy for grain growth, k the Boltzmann constant and T the absolute temperature. Arrhenius plots of r(o) against the reciprocal of temperature yield a straight line, from whose slope the activation energy for grain growth is deduced to be 162 +/- 2 kJ mol-1. Of significance is the fact that the ultimate grain size at 1273 K is approximately 70 nm, which will not grow by further annealing even at 1373 K.