Excellent mechanical properties and strengthening mechanisms in ultrathin foils of CoCrNi medium-entropy alloy via accumulative rolling

Ultrathin foils are used in multiple fields, including aerospace, and precision instrument design. In this paper, CoCrNi medium-entropy alloy (MEA) ultrathin foils with terminal thicknesses of 77 mu m were prepared by accumulative cold rolling at room temperature. The quasi-static tensile properties of the rolled ultrathin foils were tested. The influence of the reduction ratio and thickness values on their mechanical properties was studied through EBSD and TEM. The results showed that the CoCrNi MEA ultrathin foils exhibited excellent tensile performance when the thickness reached 160 mu m, in which the tensile strength reached 1.69 GPa at a tensile elongation of 7 %. Microstructure characterization revealed that the dominant strengthening mechanism of the CoCrNi ultrathin foils were composed of grain boundary and dislocation strengthening. Specifically, when the thickness exceeded 160 mu m, enhanced dislocation and grain size strengthening cause ' work hardening ', and the strength gradually increased with decreasing thickness. When the thickness was 160 mu m, the strengthening effect produced by dislocation banding, twinning, and multistage twinning delivery significantly influenced the mechanical properties of the foils. When the thickness was less than 160 mu m, this process mainly occurred due to the interactions of dislocation proliferation and annihilation in the grain, causing the strength to decrease gradually with decreasing thickness, thereby resulting in ' work softening '.