GRAIN-GROWTH KINETICS AND THERMAL-STABILITY IN A NANOCRYSTALLINE MULTIPHASE MIXTURE PREPARED BY LOW-ENERGY BALL-MILLING

A multiphase powder mixture consisting of approximately 40 vol.% Nb2Al, 30 vol.% Nb, 28 vol.% Nb3Al and 2 vol.% Al with their crystallite sizes of approximately 5 to 8 nm was subjected in vacuum to elevated temperatures. The anneal at 1373 K for 25.2 ks produced a three phase mixture of Nb, Nb2Al, Al and Nb3Al, their volume fractions being 3.9, 4.8 and 91.3 vol.% and their ultimate particulate sizes being 8.2, 13.8 and 36.0 nm, respectively. The detailed analysis of isothermal annealing from 973 to 1373 K for 3.6, 10.8 and 25.2 ks revealed complex grain growth kinetics for Nb2Al and Nb3Al phases with an exponent of n=1/8 in D=kt(n) where D is grain size, k the growth constant and t annealing time. The analysis of grain growth data yielded a dual-stage activation energy of 68.2+/-4 and 91.3+/-6 kJ/mol for grain growth in Nb3Al below and above 1173 K, respectively. Likewise, the activation energy for grain growth in Nb2Al was found to be 62.8+/-2 and 83.9+/-7 kJ/mole below and above 1173 K, respectively.