Unraveling the cryogenic formability in high entropy alloy sheets under complex stress conditions

This work investigates how temperature and microstructural evolution affect the formability of face-centered cubic (fcc) structured CoCrFeNiMn_0.75Cu_0.25 high entropy alloy (HEA) sheets under complex stress conditions. Erichsen cupping tests were conducted to quantitatively evaluate the deformation capacity at room temperature (298 K) and cryogenic temperatures. The findings reveal a strong temperature dependence on the formability of the HEA. A decrease in the deformation temperature from 298 to 93 K causes a significant increase in both the Erichsen index (IE) (from 9.8 to 12.4 mm) and the expansion rate (δ) of surface area (from 51.6% to 76.3%), as well as a reduction in the average deviation (η) of thickness (from 55.1% to 44.4%), signifying its ultrahigh formability and uniform deformation capability at cryogenic temperature. This enhancement is attributed to the transition in the deformation mechanism from single dislocation slip at 298 K to a cooperative of plastic deformation mechanisms at 93 K, involving dislocation slip, stacking faults (SFs), Lomer-Cottrell (L-C) locks and multi-scale nanotwins. The lower stacking fault energy of the alloy facilitates these deformation mechanisms, particularly the formation of SFs and nanotwins, which enhance ductility and strength by providing additional pathways for plastic deformation. These mechanisms collectively contribute to delaying plastic instability, thereby improving the overall formability. This work provides a comprehensive understanding of the underlying reasons for the enhanced formability of HEAs at cryogenic temperatures, offering valuable insights for their practical use in challenging environments.