Achieving superior toughness in austenitic high-manganese steel at extreme cryogenic temperatures via Al alloying

The global transition toward a carbon-neutral economy has accelerated the deployment of liquid hydrogen (LH 2 ) infrastructure, demanding structural materials that retain resistance to brittle fracture at extreme cryogenic temperatures. However, plastic deformation and fracture become increasingly difficult to accommodate as dislocation mobility is reduced at low temperature. Here, we investigate the impact behavior and deformation mechanisms of Fe–24Mn–0.5C–4Cr and Fe–24Mn–0.5C–4Cr–5Al austenitic steels at −196 °C and −269 °C. While the Al-free steel shows a modest decrease in impact energy from 122 J to 115 J upon cooling to −269 °C, the Al-containing steel retains a high impact energy of 163 J at −196 °C and 170 J at −269 °C. Thermodynamic calculations and microstructural characterization show that the higher calculated SFE in the Al-containing steel is accompanied by a transition in the dominant deformation substructure, from more localized planar-slip-dominated deformation in Al-free steel to deformation with abundant nanoscale twinning in Al-containing steel. Molecular dynamics simulations using a simplified Fe–Mn model show that a twin-containing matrix sustains broader shear-strain distribution and higher Shockley partial dislocation activity than a twin-free matrix under cryogenic shock loading, providing atomistic support for twin-assisted plastic strain accommodation. These results indicate that the excellent cryogenic toughness retention of Al-containing steel is associated with a transition toward more distributed plastic deformation upon Al addition.