# Unveiling the Stacking Fault-Driven Phase Transition Delaying Cryogenic Fracture in Fe-Co-Cr-Ni-Mo-C-Based Medium-Entropy Alloy

## TL;DR

This study explores how a specific alloy maintains strength and ductility at extremely low temperatures by analyzing its deformation mechanisms and phase transitions.

## Contribution

The paper reveals the role of stacking fault energy and martensitic transformation in enhancing cryogenic mechanical properties of a medium-entropy alloy.

## Key findings

- At cryogenic temperatures, martensitic transformation-induced plasticity becomes the main strengthening mechanism.
- Reduced stacking fault energy promotes twinning and martensitic transformations, improving ductility.
- The Mo4C1 alloy achieves optimal strength–ductility at cryogenic-to-room temperatures.

## Abstract

In this work, the tensile deformation mechanisms of the Fe55Co17.5Cr12.5Ni10Mo5−xCx-based medium-entropy alloy at room temperature (R.T.), 77 K, and 4.2 K are studied. The formation of micro-defects and martensitic transformation to delay the cryogenic fracture are observed. The results show that FeCoCrNiMo5−xCx-based alloys exhibit outstanding mechanical properties under cryogenic conditions. Under an R.T. condition, the primary contributing mechanism of strain hardening is twinning-induced plasticity (TWIP), whereas at 77 K and 4.2 K, the activation of martensitic transformation-induced plasticity (TRIP) becomes the main strengthening mechanism during cryogenic tensile deformation. Additionally, the carbide precipitation along with increased dislocation density can significantly improve yield and tensile strength. Furthermore, the marked reduction in stacking fault energy (SFE) at cryogenic temperatures can promote mechanisms such as twinning and martensitic transformations, which are pivotal for enhancing ductility under extreme conditions. The Mo4C1 alloy obtains the optimal strength–ductility combination at cryogenic-to-room temperatures. The tensile strength and elongation of the Mo4C1 alloy are 776 MPa and 50.5% at R.T., 1418 MPa and 71.2% in liquid nitrogen 77 K, 1670 MPa and 80.0% in liquid helium 4.2 K, respectively.