Functionally Graded Medium-Entropy Alloy Coatings for Hydrogen-Assisted Cryogenic Fatigue Resistance of Stainless Steel

Abstract

Hydrogen-facing stainless-steel components are subjected to hydrogen ingress, cryogenic deformation, cyclic loading, and thermal gradients that challenge uniform coatings. A route to distribute interfacial compliance, hydrogen management, and surface durability is offered by compositionally graded CoCrFeNiMo architectures. The coupled effects of CoCrFeNiMo grading on phase stability, hydrogen transport, and cryogenic fatigue damage in 316L stainless steel have not been resolved. The objective of this work is to establish a composition–structure–hydrogen–fatigue framework for a directed-energy-deposited graded CoCrFeNiMo/316L system. Thermodynamic descriptors, CALPHAD-informed phase analysis, DED thermal modelling, diffusion–trapping formulations, and fatigue-mechanics relationships were integrated with literature-constrained validation. Six CoCrFeNiMo compositions were evaluated while unsupported measurements were excluded. Configurational entropy rose from 11.526 to 13.381 J mol K, atomic-size mismatch rose from 0.302% to 3.671%, and VEC shifted from 8.250 to 7.800. A nominal linear DED energy input of 228.0 J mm was obtained from the reference process condition. An FCC-rich inner region, an intermediate transition, and a Mo-rich surface region are supported as a mechanistically consistent graded architecture. Compositionally resolved design for hydrogen-assisted cryogenic fatigue service is enabled by the framework. Future research is directed toward experimental validation of hydrogen trapping, phase connectivity, residual stress, and fatigue crack evolution.