In situ neutron diffraction study of a new type of stress-induced confined martensitic transformation in Fe22Co20Ni19Cr20Mn12Al7 high-entropy alloy

We successfully prepared an Fe22Co20Ni19Cr20Mn12Al7 alloy consisting of a face-center cubic (fcc) phase and body-center cubic (bcc) phase that exhibits an outstanding combination of true strength of 1430 MPa and ductility of 19.9% at room temperature (RT). The micromechanical behavior at RT and 77 K for the studied alloy during tensile deformation was investigated using in situ time-of-flight (TOF) neutron diffraction in combination with synchrotron-based high-energy X-ray diffraction (HE-XRD). Here, the striking finding of a large elastic strain of 7.0% and 5.6% is reported for the {200} bcc crystal plane, which was achieved at RT and 77 K, respectively. Such a large lattice distortion observed in the bcc phase was attributed to a new type of stress-induced confined martensitic transformation. We attributed the physical origin of this specific martensitic transformation to the intrinsic microstructural feature of a nano-scale continuous distribution of an ordered-to-disordered crystal structure within the bcc phase, i.e., the disordered A2-structure was distributed continuously in the ordered B2-structure matrix. The stress-induced martensitic transformation from the metastable nano-sized disordered A2-phase was confined by the stable B2-ordered matrix. The new findings in this study provide additional understanding of the deformation mechanisms of high-entropy alloys and insights into alloy design for further enhancement of the mechanical properties of high-performance structural materials used at cryogenic temperatures.