Structural hierarchy defeats alloy cracking

Bidirectional Phase Transformations in Multi-Principal Element Alloys: Mechanisms, Physics, and Mechanical Property Implications

The emergence of multi-principal element alloys (MPEAs) heralds a transformative shift in the design of high-performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi-based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face-centered-cubic (FCC) matrix into a hexagonal-close-packed (HCP) phase and the reverse HCP-to-FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual-phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation-induced plasticity (B-TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B-TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B-TRIP mechanism and the mechanistic design of next-generation high-performing MPEAs.

Room-temperature super-elongation in high-entropy alloy nanopillars

Nanoscale small-volume metallic materials typically exhibit high strengths but often suffer from a lack of tensile ductility due to undesirable premature failure. Here, we report unusual room-temperature uniform elongation up to ~110% at a high flow stress of 0.6-1.0 GPa in single-crystalline <110>-oriented CoCrFeNi high-entropy alloy nanopillars with well-defined geometries. By combining high-resolution microscopy and large-scale atomistic simulations, we reveal that this ultrahigh uniform tensile ductility is attributed to spatial and synergistic coordination of deformation twinning and dislocation slip, which effectively promote deformation delocalization and delay necking failure. These joint and/or sequential activations of the underlying displacive deformation mechanisms originate from chemical compositional heterogeneities at the atomic level and resulting wide variations in generalized stacking fault energy and associated dislocation activities. Our work provides mechanistic insights into superplastic deformations of multiple-principal element alloys at the nanoscale and opens routes for designing nanodevices with high mechanical reliability.

Porous gold with three-level structural hierarchy

Facilitating the mass transfer and enlarging the active surface area are two mutually exclusive demands in porous materials, while structural hierarchy could settle this issue by constructing continuous channels with different length scales. However, it is a great challenge to fabricate porous metallic materials with three or more geometrically similar hierarchy levels. Herein, a novel strategy combining vapor phase dealloying with electrochemical dealloying is proposed to achieve nanoporous gold (NPG) with three-level nested hierarchy (N³PG), in which the length scale covers micron (5866.8 ± 1445.5 nm), submicron (509.9 ± 106.0 nm), and nanometer (20.1 ± 3.0 nm) for each level. Notably, the structural superiority of such material is manifested by its faster charge transfer behaviors, as benchmarked with unimodal and bimodal NPG (N¹PG and N²PG). The present strategy is of great potential to fabricate other hierarchically porous metals with enhanced functional and structural properties.

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N³PG with three-level structural hierarchy was fabricated based on VPD and ECD

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The ligament distribution of N³PG covers nanometer, submicron and micron scales

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The structure superiority of N³PG is manifested by its faster charge transfer rate

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The strategy is of great potential to fabricate other hierarchically porous metals