Achieving strength–ductility synergy in novel paramagnetic Fe-based medium-entropy alloys through deep cryogenic deformation

Cryogenic pre-deformation treatment has been widely used to effectively improve the comprehensive mechanical properties of steels and novel metals. However, the dislocation evolution and phase transformation induced by different degrees of deep cryogenic deformation are not yet fully elucidated. In this study, the effects of multiple cryogenic pre-treatments on the mechanical properties and deformation mechanisms of a paramagnetic Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 medium-entropy alloy (MEA) were investigated, leading to the discovery of a pretreated MEA that exhibits exceptional mechanical properties, including a fracture strength of 3.0 GPa, plastic strain of 26.1% and work-hardening index of 0.57. In addition, X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses revealed that multiple cryogenic pre-deformation treatments significantly increased the dislocation density of the MEA (from 9 × 10¹⁵ to 4 × 10¹⁶ m⁻² after three pre-treatments), along with a transition in the dislocation type from predominantly edge dislocations to mixed dislocations (including screw- and edge-type dislocations). Notably, this pretreated MEA retained its paramagnetic properties (μ_r < 1.0200) even after fracture. Thermodynamic calculations showed that cryogenic pretreatment can significantly reduce the stacking fault energy of the MEA by a factor of approximately four (i.e., from 9.7 to 2.6 mJ·m⁻²), thereby activating the synergistic effects of transformation-induced plasticity, twinning-induced plasticity and dislocation strengthening mechanisms. These synergistic effects lead to simultaneous strength and ductility enhancement of the MEA.