Interdiffusion in Fe–Fe 2 Al 5 ( η ) and Fe 2 Al 5 ( η )–Al couples

Abstract

The growth kinetics and morphology of FeAl₂(ζ) and FeAl₃(θ) were investigated for Fe–Fe₂Al₅(η) and Fe₂Al₅(η)–Al diffusion couples. An electron-probe microanalyser was used to identify the compounds formed and the thickness of compound was measured by an optical metallograph. An FeAl₂(ζ) layer was formed between Fe and Fe₂Al₅(η)and increased in thickness with time and temperature. The parabolic growth-rate constant K _p of FeAl₂(ζ) was expressed as p, follows in the temperature range 740°−985°C; K _p = 3·16×l0⁻⁴ exp (−28·14/RT), cm²s⁻¹, where the activation energy is 28·14kcal mol⁻¹. The rate of growth was orders of magnitude less than the formation rate of Fe₂Al₅(η) as observed in hot-dip aluminizing. This phase was so small that it was difficult to detect. FeAl₃(θ) was not found for the Fe₂Al₅(η)−Al (solid) couple but was identified for the couple with molten aluminium, and occurred as a collection of small particles at the interface, which became dispersed throughout the molten aluminium as a sludge.