Grain Growth and Its Kinetics of Nanophase Niobium Aluminide Produced by Mechanical Alloying

Grain growth and its kinetics studies were conducted on an intermetallic compound, NbAl3 powders produced by mechanical alloying of elemental Nb and Al powders with 5.5 mass% Ti in a ball mill under Argon atmosphere for 1800 ks at ambient temperature. The initial grain size of as-MAed powders and grown grain size of annealed powders were measured from X-ray line broadening. Isochronal annealings of MAed powders were conducted from 653 to 1473 K, and it was found that a significant grain growth takes place only in a temperature range of 973 to 1373 K and temporarily ceases at 1473 K by reaching approximately 78 nm regardless of anneal time. Accordingly isothermal annealings were carried out at 1048, 1073 and 1173 K to deduce the grain growth kinetics that is described byln(dD⁄dt)=ln(r⁄4)−3.0ln(D)where D is the measured grain size and r a constant. This r depends on temperature of anneal in the following manner:r=r′exp[−Q⁄kT]where Q is the activation energy for grain growth and k and T are the Boltsmann constant and the anneal temperature, respectively. Arrhenius plots of r against the reciprocal of T yielded a straight line, from which slope the activation energy Q for grain growth was deduced to be 201±2 kJ/mol, which is greater by 45 kJ/mol than 162 kJ/mol previously obtained for pure NbAl3 powders similarly prepared by mechanical alloying. Addition of 5.5 mass%Ti to NbAl3 influences not only on the activation energy but also on the exponent of the kinetic equation.