The dynamic behavior of metals is governed by collective dislocation motion and interactions that strongly depend on the applied strain rate. Metals exhibit weak strain rate sensitivity (SRS) below a certain threshold, followed by a distinct SRS upturn at higher loading rates. While this upturn is typically attributed to increased glide resistance at high dislocation velocity due to mechanisms such as phonon drag, the role of strain-ratedependent dislocation multiplication and microstructural evolution under these extreme conditions remains elusive. Here, we decouple these two strengthening effects and show that, while dislocation velocity primarily governs the SRS upturn, the hardening due to microstructure evolution depends strongly on the initial dislocation density. Our investigation of hardness evolution across ten decades of strain rates in a quenched and tempered martensitic low-carbon steel (LCS) using laser-induced projectile impact tests (LIPIT) and nanoindentation reveals SRS upturn at similar to 107 s- 1. By performing in situ re-indentation of the formed craters, we probe the contribution of dislocations generated during initial deformation at different strain rates. We show that while dislocation multiplication plays a negligible role in fine-grained LCS with high dislocation density, a pronounced dislocation multiplication contributes to the hardness increase in pure iron with lower initial dislocation density. Our results show that, depending on the initial microstructure of metals, dislocation multiplication can significantly govern high-strain-rate plasticity, in addition to dislocation velocity effects.

Eurasian mammoths (Mammuthus) underwent substantial modifications in molar morphology as later-diverging species evolved progressively thinner enamel and increased enamel crest complexity. These features have been hypothesized to reduce whole-tooth wear and extend dental longevity as increasingly graze-dominated diets evolved within the lineage. This hypothesis has yet to be directly tested. Here, we developed an in-silico wear model using experimentally derived wear rates from fossil and extant proboscidean dental tissues. The models revealed that shifts in tissue topology do not affect whole-tooth wear rate, as inverse trends in lamellar frequency and enamel thickness preserve a consistent surface enamel area fraction; the determining factor of wear. Rather, topological shifts produce a wear-emergent secondary occlusal surface with greater numbers of triturating crests that create a regular, low-relief, file-like shearing pavement. These changes in occlusal architecture likely directly impacted the mastication capacity of Mammuthus dentitions, facilitating their dietary expansion to incorporate fibrous, lower-nutrient graze. STATEMENT OF SIGNIFICANCE: Eurasian mammoths evolved progressively thinner enamel and more complex enamel crests which have been hypothesized to reduce whole-tooth wear and extend dental longevity as grazing intensified, but this idea has not yet been directly tested. Here, we test this long-standing hypothesis using mechanical testing and in-silico wear models on fossil Woolly mammoth and modern Indian Elephant molars. We demonstrate that wear relevant material properties can be preserved in fossil dentitions and show that the addition of enamel plates through evolutionary time does not decrease whole-tooth wear rate as previously hypothesized, but rather topological shifts facilitate the emergence of a regular, low-relief, occlusal surface with an increasing number of triturating crests that likely improved mastication capacity of fibrous, lower nutrient graze.

Accurately quantifying the sub-granular mechanical response of bcc metals remains challenging because grainlevel deformation is governed by complex interactions between crystallographic orientation, dislocation activity, and uncertainties (indent size effect) inherent to nanoindentation. This study addresses these challenges by conducting a systematic, multi-scale spherical nanoindentation investigation of single- and polycrystalline tantalum across three indenter sizes (5, 25, and 150 mu m) and a wide range of grain orientations. A well-designed, comprehensive statistical analysis of indentation stress-strain (ISS) responses revealed clear orientation dependence in indentation yield strength (Yind), modulus (Eind), and strain-hardening coefficients (Kind and Kind-p). Smaller tips provided reliable post-elastic characterization results but produced frequent pop-ins (unreliable Yind), whereas the 150 mu m tip minimized pop-in events and provided the most reliable Yind measurements, but unreliable Kind given test system load capacity. Consistent with bcc slip activity, the (111) orientation exhibited the largest Eind (190.3 GPa) and Yind (0.84 GPa), while the (001) orientation showed the smallest Eind (181.1 GPa) and Yind (0.76 GPa). Notably, an apparent secondary hardening regime, Kind-p, was observed in several polycrystalline orientations. Because this feature is inferred from ISS curve shape, it is more of a phenomenological post-pop-in response and a probable evidence of a specific dislocation mechanism, reflecting the complex interplay of dislocation motion and potential dislocation avalanches in highly annealed large grains of the polycrystalline sample. Finally, a systematic statistical reliability assessment further shows that the required number of indents is parameter-dependent: approximately 80-90 tests are required for stable significance in Kind and Kind-n, while approximately 110-120 tests are required for nind in orientation-property relationships for the present dataset. Together, these results provide a statistically grounded indentation dataset for orientationdependent elastic-plastic response in BCC Ta and clarify the reliability limits of different spherical indenter radii, linking statistics of crystallographic orientation, dislocation-mediated deformation, and statistical confidence, advancing the use of spherical nanoindentation for high-fidelity mechanical characterization of BCC Ta.

We present an investigation of Fe-14Cr-3W-0.4Y-0.4Zr-0.18Ti (wt.%) as a reduced-activation oxide-dispersion strengthened (ODS) ferritic steel, an alternative to the "14YWT" structural alloys designed for nuclear energy applications. Gas atomization reaction synthesis (GARS) was used to produce these Zr-modified powders with a non-equilibrium (metastable) phase that enable a heat treatment (delayed) route for producing nanoscale oxidedispersion strengthening. In situ synchrotron X-ray diffraction and transmission electron microscopy were used to analyze phase evolution as a function of temperature and time, revealing formation of highly dispersed oxide nanoprecipitate phases at temperatures above those needed for powder consolidation and shaping of components. This would allow fabrication of net-shape parts using conventional powder-processing methods prior to thermal activation of a reaction producing nanocrystalline Y-(Ti, Zr)-O particles, with a size of 20 +/- 7 nm. These results can inform processing of tubes, cladding, sheets, and plates for use in advanced fission and fusion reactors.

We utilize high-throughput nanoindentation using spherical and Berkovich indenters at varying strain rates in conjunction with detailed microstructural characterization using XRD, EBSD, and TEM, to assess the local nanomechanical responses in low-carbon martensitic steels as a function of their local morphology and crystallography. Our findings reveal an orientation dependence in the local properties measured using spherical nanoindentation of the as-received low-carbon martensitic steels, with the near-(111) and near-(100) oriented blocks showing the highest and lowest indentation yield strengths, respectively, while the trend was reversed for the strain hardening under indentation. Correspondingly, only a weak orientation dependence was observed in the Berkovich hardness values. Our study also analyzed the effects of block size, distance from the block boundary, misorientation of the block boundary and indentation strain rate on the measured indentation properties. These findings provide new insights into the role of microstructural hierarchy in controlling both microscale and macroscale deformation behavior in low-carbon martensitic steels.

Experiments and crystal plasticity finite element (CPFE) simulations were carried out on four Ta single crystal micropillars to identify active slip systems under compression across different crystal orientations at four temperatures from room to 133 K. Slip trace analyses were employed to identify the activated slip systems accommodating the initial yielding and subsequent initial hardening under compression of micropillars. Fixed slip modes consisting of slip planes and directions characteristic of body-centered cubic (BCC) metals and a maximum resolved shear stress plane (MRSSP) per characteristic direction modeling approaches were employed within CPFE to elucidate the activated slip systems. Simulation results were compared with experimental observations to investigate orientation- and temperature-dependent slip activity in Ta. The MRSSP approach allowing slip planes to deviate from fixed crystallographic families toward maximizing resolved shear stress showed more accurate prediction of the orientation- and temperature-dependent deformation of Ta. Analyses of activated slip systems revealed pronounced occurrence of {123} slip that decreased with lowering temperature at the expense of {110} slip dominance, indicating a transition towards a more planar slip behavior at low temperatures.

Experiments and corresponding crystal plasticity finite element (CPFE) simulations of spherical nanoindentation were performed to determine yield stress under indentation of fifteen Ta single crystals randomly distributed in the orientation space. Agreement between the measured and simulated indentation yield stresses and initial hardening slopes demonstrated accuracy of the model. Moreover, simple compression simulations were performed for the same crystals to study the differences in compressive versus indentation yielding. Ratios of the indentation to compressive yield stress were found to vary with crystal orientation in the range from 2.6 to 3.6. The simulations allowed us to reveal underlying deformation mechanisms accommodating the yielding in indentation and simple compression. It is found that more crystallographic glid mechanisms activate under indentation than simple compression owing to the more complex state of stress and strain in indentation than in compression. Owing to the activation of more glide systems in indentation than in simple compression, the indentation yield stress is less anisotropic than the simple compression yield stress. The modeling framework, simulation setups, results, and insights from the results are presented and discussed in this paper.

We utilize elevated temperature physical vapor deposition (PVD) techniques to design metal/MAX multilayered nanocomposite thin films with alternating nanoscale metallic (Nb, Ti) and MAX phase (Ti2AlC) layer thicknesses. These metal/MAX nanolaminate architectures attempt to exploit a unique hierarchical topology - as interfaces between the layers are expected to be in direct competition with the internal interfaces within the MAX layers, to drive their tunable macroscopic mechanical behavior. Two metal/MAX nanolaminates - Nb/Ti2AlC and Ti/ Ti2AlC - were deposited. The Nb/Ti2AlC metal/MAX system showed highly diffused layer interfaces with distinct Ti - rich and Nb-Al - rich layers, with the presence of MAX phase alongside TiC and other Ti-Al and Nb-Al intermetallic phases. The Nb/Ti2AlC system possessed a layered architecture, though the MAX phases were not found to be continuously present in each alternating layer. The second Ti/Ti2AlC system showed a non-lamellar nanocomposite microstructure and the formation of mixed Tin+1AlCn phases (a mix of n = 1, 2), and no indication of layering. Diffusion occurring between the metal/MAX layers in both cases, likely due to the elevated temperatures during the deposition process, is speculated as the likely cause of these resultant microstructures. The mechanical properties of both systems were evaluated using micromechanical (nanoindentation and micropillar compression) techniques, which demonstrated high strengths for both systems (Nb system: yield and instability strengths of 4.88 +/- 0.1 GPa and 5.57 +/- 0.03 GPa, Ti system: yield and instability strength of 5.61 +/- 0.28 GPa and 6.21 +/- 0.25 GPa). This work highlights the promising mechanical properties of metal/MAX multilayered depositions and summarizes the challenges in PVD synthesis of metal/MAX multilayered nanolaminates.

Beavers (Castor) stand out among mammals for their unique capacity to fell trees using their large, ever-growing incisors. This routine consumption of resistant fodder induces prodigious wear in the lower incisors, despite this blunting effect the incisors maintain a remarkably sharp cutting edge. Notably, the enamel edges of their incisors show a highly complex two-part microstructure of which the biomechanical import is unknown. Here, using fracture analysis, nanoindentation, and wear testing on North American beaver (C. canadensis) incisors we test the microstructure's possible contribution to maintaining incisal sharpness. Although comparable in hardness, the inner enamel preferentially fails and readily wears at 2.5 times the rate of the outer enamel. The outer microstructure redirects all fractures in parallel, decreasing fracture coalescence. Conversely, the inner microstructure facilitates crack coalescence increasing the wear rate by isolating layers of enamel prisms that readily fragment. Together these two architectures form a microstructurally driven self-sharpening mechanism contained entirely within the thin enamel shell. Our results demonstrate that enamel microstructures exposed at the occlusal surface can markedly influence both enamel crest shape and surface texture in wearing dentitions. The methods introduced here open the door to exploring the biomechanical functionality and evolution of enamel microstructures throughout Mammalia. STATEMENT OF SIGNIFICANCE: Enamel microstructure varies significantly with the diversity of diets, bite forces, and tooth shapes exhibited by mammals. However, minimal micromechanical exploration of microstructures outside of humans, leaves our understanding of biomechanical functions in a nascent stage. Using biologically informed mechanical testing, we demonstrate that the complex two-part microstructure that comprises the cutting edge of beaver incisors facilitates self-sharpening of the enamel edge. This previously unrecognized mechanism provides critical maintenance to the shape of the incisal edge ensuring continued functionality despite extreme wear incurred during feeding. More broadly, we show how the architecture of prisms and the surrounding interprismatic matrix dictate the propagation of fractures through enamel fabrics and how the pairing of enamel fabrics can result in biologically advantageous functions.

While bimetallic nanocomposites have demonstrated extraordinary – three to even ten-fold – gains in strength with decreasing layer thickness, their strengths tend to plateau beyond a critical layer thickness. More disappointingly, such increases in strength are almost always accompanied by a decrease in their strains to failure (ductility). In this work we report simultaneous improvements in both strength and mechanical stability of Nb/Mg nanolaminates with decreasing layer thicknesses, a trend seldom reported in nanolaminates consisting of pure metals. Using micro-pillar compression and nanoindentation experiments we show that physical vapor deposited (PVD) Nb/Mg nanolaminates that contain a body center cubic (bcc) Mg pseudomorphic phase demonstrate a >60% increase in strength and a >80% increase in strain to failure over those containing the hexagonal close packed (hcp) Mg phase. Instead of a strength plateau, the hcp-to-bcc phase transition in Mg results in a renewed strengthening regime in the nanolaminate caused by the change to a coherent interface from an incoherent one, along with a concurrent increase in strain-to-failure due to the introduction of a more plastically isotropic bcc material from an anisotropic hcp structure. Using high resolution transmission electron microscopy (HR-TEM) we also demonstrate the presence of a thin layer of bcc Mg at the Nb/Mg interface at larger layer thicknesses when Mg is predominantly hcp. Our results suggest that the increases in strain to failure in the Nb/Mg nanolaminates with decreasing layer thicknesses can be corelated to the approximate volume fraction of the pseudomorphic bcc Mg present in the layers.

Optically-levitated dielectric particles can serve as ultra-sensitive detectors of feeble forces and torques, as tools for use in quantum information science, and as a testbed for quantum coherence in macroscopic systems. Knowledge of the structural and optical properties of the particles is important for calibrating the sensitivity of such experiments. Here we report the results of nanomechanical testing of silica nanospheres and investigate an annealing approach which can produce closer to bulk-like behavior in the samples in terms of their elastic moduli. These results, combined with our experimental investigations of optical trap lifetimes in high vacuum at high trapping-laser intensity for both annealed and as-grown nanospheres, were used to provide a theoretical analysis of the effects of porosity and non-sphericity in the samples, identifying possible mechanisms of trapping instabilities for nanospheres with non-bulk-silica-like properties.

In this work, we present a nanostructure-sensitive crystal plasticity model for the deformation response of nanolaminate composites. The model is applied to investigate the strength of Mg/Nb nanocomposites, wherein the Mg phase has either a hexagonal close-packed (HCP) or a bodycentered cubic (BCC) crystal structure. To account explicitly for the effects of layer thickness and biphase interface on crystallographic slip, the model features a hardening law, called dualmode confined layer slip (CLS). The model is applied to a suite of stress-strain measurements made on Mg/Nb nanocomposites, varying layer thickness, texture, and interface structure. Experiments show that the BCC/BCC Mg/Nb nanocomposites achieve substantially higher strength than the HCP/BCC nanocomposites. Apart from the finer layer thicknesses, the model indicates that the pseudomorphic BCC Mg phase contributes to strength by increasing the slip strengths of the (111) slip modes compared to the (a) slip modes in HCP Mg. It also suggests that the coherent interface poses less resistance to dislocation motion than the incoherent interface. It is, therefore, found that the BCC/BCC composite strength benefited from both the confinement on dislocation motion imposed by the reduced layer thickness and higher inherent strength of its BCC phase, but that it would be even higher if the interface was not a sharp coherent interface.

We report phenomenal yield strengths—up to one-fourth of the theoretical strength of silver—recorded in microcompression testing of initially dislocation-free silver micro- and nanocubes synthesized from a multistep seed-growth process. These high strengths and the massive strain bursts that occur upon yield are results of the initially dislocation-free single-crystal structure of the pristine samples that yield through spontaneous nucleation of dislocations. When the pristine samples are exposed to a focused ion-beam to fabricate pillars and then compressed, the dramatic strain burst does not occur, and they yield at a quarter of the strength compared to the pristine counterparts. Regardless of the defect-state of the samples prior to testing, a size effect is apparent—where the yield strength increases as the sample size decreases. Since dislocation starvation and the single-arm-source mechanisms cannot explain a size effect on yield strength in dislocation-free samples, we investigate the dislocation nucleation mechanisms controlling the size effect through careful experimental observations and molecular statics simulations. We find that intrinsic or extrinsic symmetry breakers such as surface defects, edge roundness, external sample shape, or a high vacancy concentration can influence dislocation nucleation, and thus contribute to the size effect on yield strength in initially dislocation-free samples.

We studied the lamellar level correlations between the local composition and local mechanical properties in the femur of two inbred strains of mice (A/J and C57BL/6J (B6)), with known differences in the average mineralization and long-bone mechanical properties, to gain insights into how their extracellular matrix is mineralized. The local elastic moduli and indentation yield strengths were determined using spherical nanoindentation stress-strain analysis, while Raman spectroscopy was used to determine the local composition around the indents in a total of 11 samples. Our results show a significant difference in the mineral-to-matrix ratio of the two strains of mice, with the A/J mice showing an overall higher mineral-to-matrix ratio and lower carbonate substitution in the mineral. These differences are prominent in the newer bone and become less significant as the bone matures. Additionally, local mineral-to-matrix ratio was found to be a good indicator of the local mechanical properties.

We examined the high temperature indentation response of physical vapor deposited Cu-TiN multilayered nanocomposites with layer thicknesses ranging from 5 nm to 200 nm. A decrease in hardness with increasing temperature was observed, along with a strong correlation between the hardness and the nanometer-level TiN grain sizes, rather than layer thickness. The apparent activation energies calculated from the high temperature indentation experiments indicated that, for all but the smallest layer thicknesses, the deformation of copper in the nanolaminates dominate the plastic response in these composites. In the finest layer thicknesses, a decrease in the apparent activation energy value indicated possible co-deformation of Cu and TiN.

In this work we apply N+ ion irradiation on vertically aligned carbon nanotube (VACNT) arrays in order to increase the number of connections and joints in the CNT network. The ions energy was 50 keV and fluence 5 × 1017 ions cm-2. The film was 160 μm thick. SEM images revealed the ion irradiation altered the carbon bonding and created a sponge-like, brittle structure at the surface of the film, with the ion irradiation damage region extending ∼4 μm in depth. TEM images showed the brittle structure consists of amorphous carbon forming between nanotubes. The significant enhancement of mechanical properties of the irradiated sample studied by the cyclic nanoindentation with a flat punch indenter was observed. Irradiation on the VACNT film made the structure stiffer, resulted in a higher percentage recovery, and reduced the energy dissipation under compression. The results are encouraging for further studies which will lead to create a class of materials-ion-irradiated VACNT films-which after further research may find application in storage or harvesting energy at the micro/nanoscale.

Microstructural analyses and micro-pillar compression were conducted on 14YWT nanostructured ferritic alloy (NFA) to compare different processing pathways: hydrostatic extrusion and Pilger processing with varying annealing temperatures into thin walled tubing, and after hot extrusion and cross-rolling into a plate. Hydrostatic extrusion at 815 degrees C resulted in the smallest grain sizes and highest yield strength of 1.20 GPa. Pilger processing with annealing at 800 degrees C had fine grained regions and bands of coarse grains, leading to a large variation in yield strength of 0.9-1.40 GPa. Higher annealing temperatures of 1200 degrees C after pilger processing significantly increased the grain size and lowered the yield strength to 1.01 GPa. These tubes showed a stronger < 111 > crystallographic texture in the normal direction and elongated grains in the extrusion direction. Characterization of the nano-oxides using TEM reveals more numerous, smaller oxides present in tubing processed at lower temperatures. This work shows NFA tubing after hydrostatic extrusion and pilger processing can lead to fine grained microstructures and texturing leading to higher yield strengths at lower annealing temperatures (e.g. 800 degrees C).

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation.

At ambient temperature, pressure, and sufficiently fine layer thicknesses, pure body-centered cubic (bcc) Mg can exist as a pseudo-morphic phase when coherently bonded with a substantially stiffer bcc metal, such as Nb. Compared to the hexagonal close-packed Mg/bcc Nb nanocomposite that exists in the larger layer thicknesses, the bcc Mg/bcc Nb nanocomposite was recently shown to exhibit significantly higher yield stresses and strains to failure. However, because of the morphological, spatial, and crystallographic constraints imposed by nanolayered architecture, the elastic constants of the individual bcc Mg phase cannot be directly measured experimentally. Lack of this fundamental property stands in the way of theoretical and computational modeling of the mechanical properties of the pseudo-morphic bcc phase of Mg. In this work, we employ density functional theory calculations and a strain-energy-based elasticity method to calculate the lattice and elastic constants of pure bcc Mg. For validation of these constants, we combine a set of micropillar compression experiments and microstructurally explicit finite element simulations for the fully bcc Mg/bcc Nb nanolaminate system. We conclude that (i) for the stress-free bcc Mg, the lattice parameter a(0) is 3.581 angstrom, and the three independent elastic constants C-11, C-12, and C-44 are 39.64 GPa, 34.14 GPa, and 31.38 GPa, respectively, and (ii) for the laminated bcc Mg (i.e., a(0) = 3.347 angstrom), the three elastic constants are 84.68 GPa, 56.68 GPa, and 61.4 GPa, respectively.

The mechanical properties of multilayer films consisting of alternating layers of magnesium and niobium are investigated through micropillar compression experiments across a broad range of temperatures. The data collected from the variable temperature micropillar compression tests and strain rate jump tests are used to gain insight into the operative deformation mechanisms within the material. At higher temperatures, diffusion-based deformation mechanisms are shown to determine the plastic behavior of the multilayers. Diffusion occurs more readily along the magnesium–niobium interface than within the bulk, acting as pathway for magnesium diffusion. When individual layer thicknesses are sufficiently small, diffusion can remain the dominant deformation mechanism down to room temperature. Multilayer strengthening models historically rely solely on dislocation-based arguments; therefore, consideration of diffusion-based deformation in nanolaminates with low melting temperature components offers improved understanding of multilayer behavior.