Elastic strain-induced amorphization in high-entropy alloys.

Elastic stability is the basis for understanding structural responses to external stimuli in crystalline solids, including melting, incipient plasticity and fracture. In this work, elastic stability is investigated in a series of high-entropy alloys (HEAs) using in situ mechanical tests and atomic-resolution characterization in transmission electron microscopy. Under tensile loading, the HEA lattices are observed to undergo a sudden loss of ordering as the elastic strain reached ∽ 10%. Such elastic strain-induced amorphization stands in intrinsic contrast to previously reported dislocation-mediated elastic instability and defect accumulation-mediated amorphization, introducing a form of elastic instability. Together with the first principle calculations and atomic-resolution chemical mapping, we identify that the elastic strain-induced amorphization is closely related to the depressed dislocation nucleation due to the local atomic environment inhomogeneity of HEAs. Our findings provide insights for the understanding of the fundamental nature of physical mechanical phenomena like elastic instability and incipient plasticity.

Mechanical quenching phenomenon in diamond.

The structure of dislocation cores, the fundamental knowledge on crystal plasticity, remains largely unexplored in covalent crystals. Here, we conducted atomically resolved characterizations of dislocation core structures in a plastically deformed diamond anvil cell tip that was unloaded from an exceptionally high pressure of 360 GPa. Our observations unveiled a series of nonequilibrium dislocation cores that deviate from the commonly accepted "five-seven-membered ring" dislocation core model found in FCC-structured covalent crystals. The nonequilibrium dislocation cores were generated through a process known as "mechanical quenching," analogous to the quenching process where a high-energy state is rapidly frozen. The density functional theory-based molecular dynamic simulations reveal that the phenomenon of mechanical quenching in diamond arises from the challenging relaxation of the nonequilibrium configuration, necessitating a large critical strain of 25% that is difficult to maintain. Further electronic-scale analysis suggested that such large critical strain is spent on the excitation of valance electrons for bond breaking and rebonding during relaxation. These findings establish a foundation for the plasticity theory of covalent materials and provide insights into the design of electrical and luminescent properties in diamond, which are intimately linked to the dislocation core structure.

In Situ Observation of Fracture along Twin Boundaries in Boron Carbide.

The observation of fracture behaviors in perfect and twinned B4 C crystals via in situ transmission electron microscopy (TEM) mechanical testing is reported. The crystal structure of the synthesized B4 C, composed of B11 C icosahedra connected by boron-deficient C-▫-C chains in a chemical formula of B11 C3 , is determined by state-of-the-art aberration-corrected scanning TEM. The in situ TEM observations reveal that cracking is preferentially initiated at the twin boundaries (TBs) in B4 C under both indentation and tension loading. The cracks then propagate along the TBs, thus resulting in the fracture of B4 C. These results are consistent with the theoretical calculations that show that TBs have a softening effect on B4 C with amorphous bands preferentially nucleated at the TBs. These findings elucidate the atomic arrangement and the role of planar defects in the failure of B4 C. Furthermore, they can guide the design of advanced superhard materials via planar defect control.

Sintering Behaviors of Supported Nanoparticles Related to Spatial Location by a Quasi-Four-Dimensional TEM.

The performance degradation via sintering phenomenon is a critical issue for the application of supported nanoparticles in industrial catalysis. However, the challenges to combine in situ stimulation and three-dimensional (3D) characterization hinder a profound understanding of sintering behaviors, thus the effect of spatial location on nanoparticles sintering has long been neglected. Herein, based on a homemade holder integrated with in situ Joule heating and electron tomography, a quasi-four-dimensional (4D) transmission electron microscope characterization approach is developed to reveal the spatial location of supported nanoparticles and its pronounced impact on size distribution and sintering behaviors. The results of 3D visualization and statistical analysis demonstrate a strong location-dependent sintering behavior of supported nanoparticles, where external nanoparticles sinter via migration coalescence, and internal nanoparticles sinter via Ostwald ripening. The quasi-4D methods developed in this work can also be extended to the study on 3D configuration evolution of other nanomaterials under an external stimulus.

Elastic ice microfibers.

Ice is known to be a rigid and brittle crystal that fractures when deformed. We demonstrate that ice grown as single-crystal ice microfibers (IMFs) with diameters ranging from 10 micrometers to less than 800 nanometers is highly elastic. Under cryotemperature, we could reversibly bend the IMFs up to a maximum strain of 10.9%, which approaches the theoretical elastic limit. We also observed a pressure-induced phase transition of ice from Ih to II on the compressive side of sharply bent IMFs. The high optical quality allows for low-loss optical waveguiding and whispering-gallery-mode resonance in our IMFs. The discovery of these flexible ice fibers opens opportunities for exploring ice physics and ice-related technology on micro- and nanometer scales.

Approaching diamond's theoretical elasticity and strength limits.

Diamond is the hardest natural material, but its practical strength is low and its elastic deformability extremely limited. While recent experiments have demonstrated that diamond nanoneedles can sustain exceptionally large elastic tensile strains with high tensile strengths, the size- and orientation-dependence of these properties remains unknown. Here we report maximum achievable tensile strain and strength of diamond nanoneedles with various diameters, oriented in <100>, <110> and <111> -directions, using in situ transmission electron microscopy. We show that reversible elastic deformation depends both on nanoneedle diameter and orientation. <100> -oriented nanoneedles with a diameter of 60 nm exhibit highest elastic tensile strain (13.4%) and tensile strength (125 GPa). These values are comparable with the theoretical elasticity and Griffith strength limits of diamond, respectively. Our experimental data, together with first principles simulations, indicate that maximum achievable elastic strain and strength are primarily determined by surface conditions of the nanoneedles.

Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase.

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (Fe_{50}Mn_{30}Co_{10}Cr_{10}, at. %) activates three types of dislocations, i.e., ⟨a⟩, ⟨c⟩, and ⟨c+a⟩. Specifically, nonbasal ⟨c+a⟩ dislocations occupy a high line fraction of ∼31% allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a c/a ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal ⟨c+a⟩ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the c/a ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the c/a ratio of hcp phases into regimes promoting nonbasal slip activation.

Unconventional Deformation Behaviours of Nanoscaled High-Entropy Alloys.

The bulk high-entropy alloys (HEAs) exhibit similar deformation behaviours as traditional metals. These bulk behaviours are likely an averaging of the behaviours exhibited at the nanoscale. Herein, in situ atomic-scale observation of deformation behaviours in nanoscaled CoCrCuFeNi face-centred cubic (FCC) HEA was performed. The deformation behaviours of this nanoscaled FCC HEA (i.e., nanodisturbances and phase transformations) were distinct from those of nanoscaled traditional FCC metals and corresponding bulk HEA. First-principles calculations revealed an obvious fluctuation of the stacking fault energy and stability difference at the atomic scale in the HEA. The stability difference was highlighted only in the nanoscaled HEA and induced unconventional deformation behaviours. Our work suggests that the nanoscaled HEA may provide more chances to discover the long-expected essential distinction between the HEAs and traditional metals.