Martensite formation kinetics of substitutional Fe–0.7 at.%Al alloy under uniaxial compressive stress

Differential dilatometry was applied to investigate the effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → martensite (α′) transformation in the substitutional Fe–0.7 at.%Al alloy upon isochronal cooling/quenching with constant rate. All imposed stress levels are below the yield stresses of γ and α′ phases in the temperature range of the martensite formation. Albeit the start temperature of the γ → α′ transformation remains approximately constant in the range of stress explored, the overall transformation temperature range increases significantly with the increase of the uniaxial compressive stress. A modular phase transformation model, adopting a model for continuous nucleation and an anisotropic thermally-activated growth model, yielding a corresponding impingement correction, was employed to extract the nucleation rate and the γ/α′-interface velocity during the transformation. The kinetic analysis suggests that athermal nucleation and thermally activated growth govern the martensite transformation under the uniaxial compressive stress. More driving force is required when a larger uniaxial compressive stress is imposed, and the thus obtained velocity of the γ/α′-interface as function of temperature indicates a thermally activated growth governed by a relatively low activation energy.

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