Phase transformation in an n-type FeSi2 processed by mechanical alloying

N-type Fe_0.98Co_0.02Si₂ powders have been produced by mechanical alloying process and consolidated by vacuum hot pressing. As-milled powders were in a metastable state and fully transformed to β-FeSi₂ phase by subsequent isothermal annealing. However, as-consolidated iron silicides consisted of untransformed mixture of α-Fe₂Si₅ and ε-FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting β-FeSi₂ phase. The transformation behavior of β-FeSi₂ was investigated a modified TGA, SEM and XRD analyses. Isothermal annealing at 830°C in vacuum led to the β-FeSi₂ phase transformation, but some residual metallic α- and ε-phases were unavoidable. Microstructures of iron silicides were investigated using SEM. Microharness before and after isothermal annealing was evaluated and correlated with the phase transformation as well as thermoelectric properties.