Ultrastrong medium entropy alloy with simultaneous strength-ductility improvement via heterogeneous nanocrystalline structures

Medium Entropy Alloy (MEA) has been of great research interests in materials science. However, the strength and ductility, like most metals, cannot be obtained in MEAs simultaneously so far, without changing their chemical compositions. Here, we present a new way to fabricate gradient heterogenous nanograined structure for overcoming strength-ductility trade-off of the equiatomic Medium Entropy Alloy (MEA) at multiple length scales. Ultrasonic assisted cyclic thermal dynamic solid-state physical process was developed to fabricate ultrastrong gradient heterogenous nanograined FeCoNi MEA structures with heterogenous nanocrystalline (NC). The microstructure and mechanical behaviors of the gradient heterogenous nanograined MEA are investigated. We studied the mechanical responses of the FeCoNi MEAs with designed crystalline structures. The yield strength of the heterogenous nanograined FeCoNi MEA (1.45 GPa) was increased by 8 times compared to that of coarse-grained FeCoNi MEA, while ductility was increased from 33% to 45% after the processing. The strengthening mechanism in the NC FeCoNi MEA is found to be dominated by grain refinement and heterogenous grain structure. We also found an identical strain rate sensitivity exponent for the FeCoNi medium entropy alloy at multiple length scales, implying that dislocation mediated deformation mechanism dominates in the heterogenous FeCoNi MEA. This study provides a new way to fabricate novel gradient heterogenous nanograined structures to achieve both high strength and ductility in MEAs and provide fundamental understanding of the mechanical strengthening and deformation mechanisms of the nanograined MEAs.