Relative contributions of solutes and forest dislocations to thermally activated plasticity in hydrogen-alloyed Fe-Cr-Ni austenitic steels: analyzing stress-dependence of Haasen plot

Yuhei Ogawa; Akinobu Shibata

doi:10.1016/j.actamat.2026.122577

Journal article Relative contributions of solutes and forest dislocations to thermally activated plasticity in hydrogen-alloyed Fe-Cr-Ni austenitic steels: analyzing stress-dependence of Haasen plot

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Yuhei Ogawa, Akinobu Shibata. Relative contributions of solutes and forest dislocations to thermally activated plasticity in hydrogen-alloyed Fe-Cr-Ni austenitic steels: analyzing stress-dependence of Haasen plot. Acta Materialia. 2026, 317 (), 122577. https://doi.org/10.1016/j.actamat.2026.122577

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A competitive relationship between solute atoms and forest dislocations in determining the kinetics of thermally activated plasticity was studied for a Type310S (Fe-24Cr-19Ni) austenitic steel containing ∼8500 at ppm hydrogen (H), where solute H promotes solid solution-hardening through an increase in the effective stress. The investigation was performed by constructing Haasen plot (i.e., strain rate sensitivity (SRS) vs. flow stress) based on stress relaxation tests, and by analyzing its evolution in relation to the decay of effective stress. In the regime of high effective stress, alloying solute elements—whose overcoming by dislocations is retarded by the presence of H—acted as primary rate-controlling obstacles to dislocations, giving rise to a relatively large and strain-independent SRS. In terms of Haasen plot, these manifested as an increase in the intercept and a decrease in the slope. Beyond the phenomenological findings of these H-induced characteristic changes, the present study elaborates their physical origin and transient nature. As the effective stress decreased and dislocation density increased, such predominance of solutes was progressively taken over by forest dislocations, leading the Haasen plot for H-charged specimens to approach that for non-charged specimen. These new insights further establish how H, alloying elements, and forest dislocations sequentially compete as rate-controlling obstacles depending on mechanical loading condition and internal state of the material.

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