Hard nanocrystalline gold materials prepared via high-pressure phase transformation

Abstract

As one of the important materials, nanocrystalline Au (n-Au) has gained numerous interests in recent decades owing to its unique properties and promising applications. However, most of the current n-Au thin films are supported on substrates, limiting the study on their mechanical properties and applications. Therefore, it is urgently desired to develop a new strategy to prepare n-Au materials with superior mechanical strength and hardness. Here, a hard n-Au material with an average grain size of ~ 40 nm is prepared by cold-forging of the unique Au nanoribbons (NRBs) with unconventional 4H phase under high pressure. Systematic characterizations reveal the phase transformation from 4H to face-centered cubic (fcc) phase during the cold compression. Impressively, the compressive yield strength and Vickers hardness (H V ) of the prepared n-Au material reach ~ 140.2 MPa and ~ 1.0 GPa, which are 4.2 and 2.2 times of the microcrystalline Au foil, respectively. This work demonstrates that the combination of high-pressure cold-forging and the in-situ 4H-to-fcc phase transformation can effectively inhibit the grain growth in the obtained n-Au materials, leading to the formation of novel hard n-Au materials. Our strategy opens up a new avenue for the preparation of nanocrystalline metals with superior mechanical property.nanocrystalline Au, high hardness, high strength, high-pressure forging, 4H Au nanoribbons

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

“…Surface stress may also trigger the transformation from hcp to fcc phases during the plastic deformation process . These phenomena in phase transformation were also observed for metals with morphologies other than 2D nanostructures , or in the bulk form …”

“…254 These phenomena in phase transformation 255 were also observed for metals with morphologies other than 2D nanostructures 256,257 or in the bulk form. 258 Silver may form thick and triangular 2D nanostructures because of the formation of twin defects. In this anisotropic growth, Ag tends to grow in the [100] direction with increasing thickness.…”

Structural Transformation of Unconventional-Phase Materials

“…As a result, vertically aligned graphene sheets homogeneously covered the surface of diamond crystals, leading to the formation of graphene–diamond heterostructures with covalent interfacial bonding (Figure e). Very recently, in cooperation with Zhao, Tian, and other co-workers, we reported the high-pressure synthesis of nanocrystalline Au bulk materials with a grain size of ∼40 nm by using unconventional 4H-phase Au nanoribbons as a starting material (Figure f) . The irreversible 4H-to-fcc phase transformation of Au occurred during the compression process in a DAC, which effectively inhibited the grain growth of Au and thus favored the formation of Au bulk materials with a nanoscale grain size (Figure g,h).…”

“…(h) HRTEM image of n-Au sample prepared at 29.8 GPa, denoted as n-Au-29.8 GPa. Reproduced with permission from ref . Copyright 2022 Springer Nature.…”

“…Very recently, in cooperation with Zhao, Tian, and other co-workers, we reported the high-pressure synthesis of nanocrystalline Au bulk materials with a grain size of ∼40 nm by using unconventional 4H-phase Au nanoribbons as a starting material (Figure 5f). 90 The irreversible 4H-to-fcc phase transformation of Au occurred during the compression process in a DAC, which effectively inhibited the grain growth of Au and thus favored the formation of Au bulk materials with a nanoscale grain size (Figure 5g,h). As a comparison, the compression of conventional fcc-Au foil would lead to the formation of a microcrystalline Au bulk phase composed of micrometer-sized crystalline domains.…”