Grain growth kinetics and thermal stability in a nanocrystalline multiphase mixture prepared by low-energy ball milling

Abstract

A multiphase powder mixture consisting of approximately 40 vol.% Nb 2 Al, 30 vol.% Nb, 28 vol.% Nb 3 Al 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, Nb 2 Al, Al and Nb 3 Al, their volume fractions being 3.9, 4.8 and 91.3 vol.% and their ultimate paniculate 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 Nb 2 Al and Nb 3 Al 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 Nb 3 Al below and above 1173 K, respectively. Likewise, the activation energy for grain growth in Nb 2 Al was found to be 62.8 ± 2 and 83.9 ± 7 kJ mole below and above 1173 K, respectively.