Ultra-strong metal/high entropy alloy nanolaminates: Utilizing size constraining effects on phase transformation

In this work, the phase stability of non-equiatomic Fe 50 Mn 30 Co 10 Cr 10 and Fe 50 Mn 30 Co 10 Ni 10 high entropy alloys (HEAs) under strong constraining conditions as well as its effect on the mechanical properties were comparatively studied in the magnetron sputtered Cu/Fe 50 Mn 30 Co 10 Cr 10 and Cu/Fe 50 Mn 30 Co 10 Ni 10 nanolaminates with the layer thickness h ranging from 5 to 150 nm. During the deposition process, the size-driven hexagonal close packed (HCP) to face-centered cubic (FCC) phase transformation occurs in the Fe 50 Mn 30 Co 10 Cr 10 layers as h < 25 nm due to the nanolayer constraining and template effects. Meanwhile, the stress-driven HCP-to-FCC phase transformation also occurs in the Cu/Fe 50 Mn 30 Co 10 Cr 10 nanolaminates during the indentation deformation, which is attributed to the nucleation of stacking faults that could serve as the nuclei for phase transformation. However, the Fe 50 Mn 30 Co 10 Ni 10 layers maintain stable microstructure without size-driven nor stress-driven phase transformation. With reducing h , both the Cu/HEA nanolaminates exhibit a transition from h -independent to h -dependent ultrahigh hardness, as elucidated by the partial dislocation-mediated mechanisms. In particular, the normalized hardness of Cu/Fe 50 Mn 30 Co 10 Cr 10 nanolaminates represented by the ratio of measured hardness to predicted hardness from the rule-of-mixture is more superior to conventional Cu-based bimetal nanolaminates. These findings provide a new perspective to tailor the phase transformation of HEAs and thereby enhance their strength and plasticity.