Continuous strengthening in nanotwinned high-entropy alloys enabled by martensite transformation

Strengthening by nanotwins in metals has been widely reported to have a limit, i.e., softening occurs when the twin thickness is reduced below a critical value, similar to the Hall-Petch breakdown in nanocrystalline materials. Here we report based on atomistic simulations that such a limit can be eliminated in high-entropy alloys (HEAs) by tuning their composition and the corresponding stacking fault energies. By using nanotwinned ${(\mathrm{CoCrFeMn})}_{1\text{\ensuremath{-}}x}\mathrm{N}{\mathrm{i}}_{x}$ HEA as a model system, our results indicate that continuous strengthening can be enabled in this HEA at Ni concentrations below 44% by martensite phase transformation even when the twin thickness approaches the theoretical minimum (i.e., three-atomic-layers thick). In contrast, at Ni concentration above 44%, a strengthening-softening transition appears in the HEA due to detwinning when the twin thickness decreases, which is similar to that in conventional fcc metals with nanoscale twins. A simple parameter based on the ratio between the energy barriers for detwinning and martensite transformation has been proposed to explain these findings, which should shed some light on a generalized deformation map in nanotwinned metals.