Evolution of microstructural and mechanical properties of nanocrystalline Co2FeAl Heusler alloy prepared by mechanical alloying

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 L2(1) 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 similar 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 degrees 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 degrees C. However, the hardness value of the sample annealed at 700 degrees 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.