Typically, austenitic stainless steels (e.g., 316 alloy category) are among the most high-performance alloys in a hydrogen environment, due to their resistance to hydrogen embrittlement (HE). Nonetheless, the mechanical performance of these alloys is not among the top steel grades. Generally, strength and HE resistance have a trade-off relationship in metals. The mechanical performance can be improved by solid solution hardening, but its effect on HE behaviour is not well understood. In this study, we investigate the effect of solid solution hardening on the HE behaviour of 316 steel by modifying its chemical composition. The high entropy alloy (HEA), or multi-principal element alloy (MPEA) concept is used for alloy modification/design. MPEAs, with their supersaturated solid solution structure, possess enhanced mechanical properties and durability in various harsh conditions, including hydrogen environments. We investigate the impact of increasing the entropy in 316 alloys by adjusting the content of Ni and Cr to maintain the austenitic structure of 316 while increasing Mn and Mo to increase the entropy of the alloy while maintaining the enthalpy of formation as close as possible to zero. The inclusion of other elements, such as Al, will be studied for the possibility of enhancing hydrogen solubility and reducing alloy density.

The alloy modification and screening were done by utilizing the in-house developed high-throughput CALPHAD, combined with empirical models used for designing MPEAs or multi-principal element alloys. The designed “Fe-based MPEA” was subjected to experimental processing and characterization to benchmark the HE performance with 316 steel.

Metal casting (vacuum induction melting/casting) is used for alloy production followed by optimized heat treatment. Microscopy evaluation (SEM/EBSD) confirmed the anticipated (supersaturated) FCC lattice structure that can offer HE resistance. Cathodic charging was used for Hydrogen charging. Thermal desorption analysis (TDA) was used to measure the hydrogen content in the MPEA and benchmarked with the reference 316 alloy. Slow strain rate tensile (SSRT) testing was conducted on the pre-charged samples to compare their resistance against HE. The designed MPEA showed a comparable HE resistance to the 316 alloy, while possessing higher strength. This offers a solution for breaking down the strength-HE resistance trade-off.