Gradient-cell-structured High-entropy Alloy with Exceptional Strength and Ductility

Author: LU Lei , PAN Qingsong

In past decades, the emerging multi-principal-element high-entropy alloys (HEAs) with a near-infinite multicomponent phase space have received the growing attention of the materials community due to their unprecedented mechanical properties, such as good ductility, and exceptional damage tolerance at low temperature. However, most multicomponent HEAs lose ductility with increasing strength, owing to the similar full dislocation mediated plastic deformation in conventional materials. Still, the extraordinary low-hanging fruits that the HEA field may offer, such as novel deformation mechanism, as expected, are not seen yet.

Recently, by integrating a simple yet efficient cyclic-torsion treatment without any surface tooling, we controllably introduced a novel sample-level gradient nano-scaled low-angle dislocation-cell structure (GDS) in one stable single-phase face-centered-cubic HEA, with a length scale spanning 6 magnitudes from millimeter to nano scale. In particular, the initial grain structure, including grain size and morphology, is unchanged from the surface to the core, which is fundamentally distinct from conventional nanostructure with severely-refined grain size. A significantly-enhanced yield strength with steady work-hardening, exceptional strength and ductility combinations, is achieved in the GDS HEA, which has not been possibly achieved in homogenous or heterogeneous structured and most existing metals and alloys with gradient nanograins or nanotwins.

By combining in-situ and ex-situ characterizations, a novel shockley partial associated with stacking-faults-induced plasticity mechanism is observed in GDS HEA (Fig. 1). After an initial tensile strain as small as 3%, highly dense micrometer-length SF bundles, composed of sub-10 nanometer SFs and twins, are progressively activated to mediate the plastic deformation of GDS HEA, nucleating from abundant low-angle dislocation cells. In addition, they also progressively refined three-dimensional SF/cell networks acting as strong obstacles to dislocation slip and sustainable sources for high-density dislocation storage during uniaxial tension, thereby contributing to extra strengthening, work-hardening and high tensile plasticity. Such a novel strengthening and ductilizing deformation mechanism, endowed by both chemical and structural features of the GDS HEA, is fundamentally distinct from the most commonly observed full dislocation activity in numerous single-phase HEAs and conventional metals.

Fig. 1. Mechanical property and deformation mechanism of the GDS Al0.1CoCrFeNi HEA. (A) Tensile engineering stress-strain relations. (B) The product of strength and ductility versus yield strength normalized by Young’s modulus, compared with the counterparts with homogenous and gradient-grained structures and other metals and alloys with gradiently-distributed nanograins and nanotwins reported in the literature. GNG and GNT denote the gradient nanograin and nanotwin, respectively; TWIP denotes twinning-induced plasticity. (C-D) Highly dense micrometer-length SF bundles in the GDS HEA at 3% strain ?, which are composed of numerous sub-10 nanometer SFs and tiny TBs (D). The inset in ? is the corresponding selected area electron diffraction patterns containing parallel streaks from SFs.

Our findings advance the fundamental understanding of the intrinsic deformation behavior of HEAs and offer a new promising paradigm for achieving better performance of many other metallic systems, through tailoring gradient-dislocation cells at the nanoscale (See details in Science 374 (2021) 984).