Evolution of microstructural and mechanical properties of nanocrystalline Co 2 FeAl Heusler alloy prepared by mechanical alloying

Abstract

Mechanical alloying (MA) has been used to fabricate the Co₂FeAl Heusler alloy with a nanocrystalline structure. The formation mechanism of the alloy has been investigated. Rietveld analysis showed that all the samples that were milled for >15 h had an L2₁ structure with a space group of Fm3m. The crystallite size and internal strain of the samples were calculated using the Williamson–Hall equation. The crystallite size of Co₂FeAl increased to ∼22 nm by increasing the MA time from 15 to 20 h, and after 20 h of MA, the crystallite size decreased. In contrast, the internal strain has increased with the increase in milling time. The powder obtained after 20 h of MA was split into three parts and separately annealed at 300, 500 and 700°C for 5 h. A considerable increase was observed in the hardness value of powder particles with the increase in annealing temperature up to 500°C. However, the hardness value of the sample annealed at 700°C decreased. It seems that this feature is related to parameters such as increase in crystallite size, enhancement of lattice ordering, change in density of defects and impurities and non-stoichiometric effects.