Abstract
Iron and steels are extensively used as structural materials, and have three primary phase structures: Body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal closed-packed (hcp). Controlling phase stabilities, especially by the use of interstitials, is a universal method that provides a diverse variety of functional and mechanical properties in steels. In this context, hydrogen, which can act as an interstitial species in steels, has been recognized to promote phase transformation from fcc to hcp. However, we here report a dramatic effect of interstitial hydrogen that suppresses this hcp phase transformation. More specifically, the fraction of hcp phase that forms during cooling decreases with increasing diffusible hydrogen content. This new finding opens new venues for thermodynamics-based microstructure design and for development of robust, strong, and ductile steels in hydrogen-related infrastructures.
Highlights
Hydrogen is a key resource for next-generation green energies[1], which creates new demands of hydrogen-compatible infrastructures
Phase stability has been recognized as a critical factor, because it causes a diffusionless transformation at ambient temperature to bcc or hcp structures
The hcp phase acts as metastable state of bcc, or in other words, the relative phase stability of fcc compared to hcp affects hcp transformation and bcc transformation in fcc steels[15,16]
Summary
Hydrogen is a key resource for next-generation green energies[1], which creates new demands of hydrogen-compatible infrastructures. The alloy shows a thermally-induced hcp transformation, with the hcp content increasing with decreasing temperature. As shown, the specimen with the higher hydrogen content more distinctly exhibits a clear suppression of the thermally induced hcp transformation.
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