Unified Model for Size-Dependent to Size-Independent Transition in Yield Strength of Crystalline Metallic Materials

Ultrastrong and ductile steel welds achieved by fine interlocking microstructures with film-like retained austenite

The degradation of mechanical properties caused by grain coarsening or the formation of brittle phases during welding reduces the longevity of products. Here, we report advances in the weld quality of ultra-high strength steels by utilizing Nb and Cr instead of Ni. Sole addition of Cr, as an alternative to Ni, has limitations in developing fine weld microstructure, while it is revealed that the coupling effects of Nb and Cr additions make a finer interlocking weld microstructures with a higher fraction of retained austenite due to the decrease in austenite to acicular ferrite and bainite transformation temperature and carbon activity. As a result, an alloying design with Nb and Cr creates ultrastrong and ductile steel welds with enhanced tensile properties, impact toughness, and fatigue strength, at 45% lower material costs and lower environmental impact by removing Ni.

A novel method of high-throughput micro-and nanomechanical sample fabrication

There is an increasing interest in understanding materials' mechanical properties at small length scales. Mechanical testing from nano- to meso-scale has seen a rapid development over the last decade, leading to a high demand of sample fabrication. In the present work, a novel method of micro-and nanomechanical sample preparation is introduced based on a new technique combining femtosecond laser and focused ion beam (FIB), namely LaserFIB. The new method greatly simplifies the sample preparation workflow by taking advantage of the fast milling-rate of femtosecond laser and the high precision of FIB. It significantly improves the processing efficiency and success rate, allowing for the high-throughput preparation of reproducible micro- and nanomechanical specimens. The novel method has far more advantages: (1) it allows for site-specific sample preparation based on scanning electron microscope (SEM) characterization (lateral and depth direction of bulk material) (2) following the new workflow, mechanical specimens are still connected to the bulk by its natural bonding, yielding more reliable mechanical testing results; (3) it extends the processable sample size to meso-scale while still remaining high precision and high efficiency; (4) the seamless transfer between laser and FIB/SEM chamber greatly reduces the risk of sample damage and is very friendly for environmental sensitive materials. The new method solves critical problems for high-throughput multiscale mechanical sample preparation, greatly contributing to the development of nano to meso-scale mechanical testing by making sample preparation efficient and convenient.

Exploration of Hierarchical Metal-Organic Framework as Ultralight, High-Strength Mechanical Metamaterials

Due to the extraordinarily high surface to volume ratio and enormous structural and chemical diversities, metal-organic frameworks (MOFs) have drawn much attention in applications such as heterogeneous catalysis, gas storage separation, and drug delivery, and so on. However, the potential of MOF materials as mechanical metamaterials has not been investigated. In this work, we demonstrated that through the concerted effort of molecular construct and mesoscopic structural design, hierarchical MOFs can exhibit superb mechanical properties. With the cutting-edge in situ transmission and scanning electron microscope (TEM and SEM) techniques, the mechanical properties of hollow UiO-66 octahedron particles were quantitatively studied by compression on individual specimens. Results showed that the yield strength and Young's modulus of the hierarchical porous framework material presented a distinct "smaller is stronger and stiffer" size dependency, and the maximum yield strength and Young's modulus reached 580 ± 55 MPa and 4.3 ± 0.5 GPa, respectively. The specific strengths were measured as 0.15 ± 0.03 to 0.68 ± 0.11 GPa g^-1 cm³, which is comparable to the previously reported state-of-the-art mechanical metamaterials like glassy carbon nanolattices and pyrolytic carbon nanolattices. This work revealed that MOF materials can be made into a new class of low-density, high-strength mechanical metamaterials and provided insight into the mechanical stability of nanoscale MOFs for practical applications.

Surface Triple Junctions Govern the Strength of a Nanoscale Solid

Surface triple junctions (STJs), i.e., the termination lines of grain boundaries at solid surface, are the common line defects in polycrystalline materials. Compared with planar defects such as grain boundaries and surfaces, STJ lines are usually overlooked in a material's strengthening although abundant atoms may reside at STJs in many nanomaterials. In this study, by in situ compression of coarse-grained and nanocrystalline nanoporous gold samples in an electrochemical environment, the effect of STJs on the strength of nanoporous gold was successfully decoupled from grain-boundary and surface effects. We found that the strength of nanoporous gold became sensitive to STJ modification when ligament size was decreased to below ∼100  nm, indicating that STJs started to influence ligament strength at sub-100 nm scale. This STJ effect was associated with the emission of dislocations from STJs during plastic deformation. Our findings strongly suggest that the structure and chemistry at STJs should be considered in understanding the mechanical response of sub-100 nm scale materials.

Theoretical Justification of Single-Ended Dislocation-Source-Controlled Deformation of Micropillar fcc Crystals

It was established at the beginning of the 21st century that the critical resolved shear stress of small-sized (diameter from 50 nm to 10  μm) metallic crystals fabricated from bulk crystals increases drastically with decreasing specimen diameter. Dou and Derby [Scr. Mater. 61, 524 (2009)SCMAF71359-646210.1016/j.scriptamat.2009.05.012] showed that, the critical shear stresses of small-sized single crystals of various fcc metals obeyed a universal power law of specimen size with an exponent of -0.66. In this study, we succeeded in reproducing almost perfectly the above universal relation without any adjustable parameters, based on a deformation process controlled by the operation of single-ended dislocation sources.

Thermo-mechanical correlation in two-dimensional materials

Two-dimensional (2D) materials have received tremendous attention from the research community in the past decades, because of their numerous striking physical, chemical and mechanical properties and promising potential in a wide range of applications. This field is strongly interdisciplinary, requiring efficient integration of knowledge with different insights. In this review, we summarize the up-to-date research on the thermal and mechanical properties and thermo-mechanical correlation in 2D materials, including both theoretical and experimental insight. Firstly, the mechanical properties of 2D nanomaterials are discussed, in which the underlying physics is summarized. Then, we discuss the impacts of thermal fluctuation on the mechanical properties. Next, from experimental points of view, we present the methods to introduce strain in 2D materials experimentally and the experimental tools to measure the degree of strain. Finally, we discuss the fundamental phonon and thermal properties of 2D materials, including the strain effects on phonon dispersion, phonon hydrodynamic behavior, phonon topological feature, ballistic thermal conductance and diffusive thermal conductivity. This article presents an advanced understanding of the mechanical and thermal properties of 2D materials, which provides new opportunities for promoting their applications in nanoscale electronic, optoelectronic, and thermal functional devices.

Size Effect on Mechanical Properties and Deformation Behavior of Pure Copper Wires Considering Free Surface Grains

The size (grain size and specimen size) effect makes traditional macroscopic forming technology unsuitable for a microscopic forming process. In order to investigate the size effect on mechanical properties and deformation behavior, pure copper wires (diameters range from 50 μm to 500 μm) were annealed at different temperatures to obtain different grain sizes. The results show that a decrease in wire diameter leads to a reduction in tensile strength, and this change is pronounced for large grains. The elongation of the material is in linear correlation to size factor D/d (diameter/grain size), i.e., at the same wire diameter, more grains in the section bring better plasticity. This phenomenon is in relationship with the ratio of free surface grains. A surface model combined with the theory of single crystal and polycrystal is established, based on the relationship between specimen/grain size and tensile property. The simulated results show that the flow stress in micro-scale is in the middle of the single crystal model (lower critical value) and the polycrystalline model (upper critical value). Moreover, the simulation results of the hybrid model calculations presented in this paper are in good agreement with the experimental results.