Effect of wet environment on hardness and yield stress of B2 Fe–Al alloys

Effects of wet environment on hardness and yield stress of B2 Fe-Al alloys at room temperature (RT) were investigated. First, change in micro Vickers hardness was measured for annealed Fe-18 mol% Al and Fe-25 mol% Al alloys as a function of the duration spent in: (1) distilled water at RT after a drying treatment; and (2) dry air at 427 K after a wet treatment. An age-hardening-like phenomenon appears for the former case and an age-softening-like phenomenon takes place for the latter case. Such changes in hardness caused by environmental treatments are reversible. Second, room-temperature yield stress and hardness were measured for annealed and vacancy-introduced Fe-44 mol% Al single-crystal alloys, where specimens were kept in various environments for a long time in order to achieve their equilibrium states. It is concluded that, with absorbing hydrogen, annealed Fe-Al alloys are hardened and vacancy-introduced Fe-Al alloys are softened. A new method is proposed for estimating the diffusivity of a solute. Hydrogen diffusivity (DH) at RT is estimated to be around 10–12 m2/s for Fe18Al and 10–12 m2/s for Fe25Al, suggesting that DH is lowered with increasing Al content in Fe-Al alloys.