Engineering topics

Demkowicz, Michael J.

Publications and source records attributed to Demkowicz, Michael J..

At least 19 records

DOE OSTI2024

Deformation and Delamination of Dynamically Bulging Bilayered Films

We employ finite element simulations to study the dynamic bulging of films under shock tube conditions and compare these responses with quasi-static bulging. Both scenarios subject the film to time-varying pressure. Two film types are considered: uniform material films and bilayers with a relatively soft interior layer and a hard exterior layer. The interface between the materials in the bilayer cases is modeled as a cohesive zone that follows a bilinear normal traction-separation cohesive law. Additionally, bilayer films with preexisting cracks in the interior layer are analyzed. Parametric studies are also conducted to explore the effects of loading rate and specimen size. The mechanical response of all materials is assumed to be strain-rate independent, focusing solely on the inertial effects in the response of the films under dynamic loading, and the absence of such effects under quasi-static loading. The results indicate that quasi-static bulging exhibits a predictable plate bending like deformation. In contrast, depending on the loading rate and specimen size, dynamic bulging can give rise to elastic waves, and a sequence of deformation processes including initial uniform acceleration, followed by bending-like deformation, and finally through-thickness biaxial stretching. Dynamic loading also leads to larger bulge formation and greater deformation than quasi-static loading. A key finding is the role of preexisting cracks in driving delamination under dynamic loading, which is absent in quasi-static loading. This work enhances our understanding of dynamic bulging and motivates further research on characterizing delamination in layered films using dynamic bulge tests.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Role of slip in hydrogen-assisted crack initiation in Ni-based alloy 725

We conduct in situ tensile straining experiments to investigate the role of hydrogen and slip in crack initiation in nickel-based alloy 725. Our experiments reveal no tendency for hydrogen to enhance localized slip and no necessity of slip for crack initiation. We use electrochemical charging to introduce hydrogen into samples, melt extraction to measure hydrogen content, and digital image correlation to analyze localized plastic strains during in situ tensile tests in a scanning electron microscope. Cracks initiate both in regions with and without nearby localized slip. Moreover, the fraction of cracks initiating with no nearby slip is greater at higher hydrogen content. Slip-assisted crack initiation generally occurs at locations where intergranular slip is arrested, especially at intersections of slipping coherent twin boundaries with thin twin lamellae. Cracks that initiate without nearby slip occur at a wider variety of microstructural features, including inclusions, triple junctions, and surface flaws.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Mobility of twinning dislocations in copper up to supersonic speeds

Understanding the mobility of twinning dislocations is important for multiscale modeling of crystal plasticity, especially at high strain rates, where such dislocations may reach transonic or supersonic speeds. Here, we used molecular dynamics simulations to investigate the relationship between dislocation velocity and the applied resolved shear stress of an edge twinning dislocation in copper up to supersonic speeds. The twinning dislocation mobility relation is composed of two branches separated by a band of forbidden velocities. The lower velocity branch is limited by the first transverse sound speed ~2000 m/s while the upper branch stretches from ~3500 m/s in the transonic regime to supersonic velocities. Twinning dislocations cannot undergo uniform steady-state motion at velocities within the forbidden band. Our simulation results also reveal that edge twinning dislocation motion in copper is kink-mediated. We discuss the implications of our findings for the motion of twins, twinning dislocations, and twinning dislocation kinks in copper.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Permeation of niobium through grain boundaries in copper

Low mutual solubility is expected to avert interdiffusion in layered composites of phase-separating metals. However, we show that Nb diffuses through polycrystalline Cu—despite the minimal bulk solubility of these elements—due to short circuit transport of Nb along certain grain boundaries in Cu. Atomistic modeling demonstrates that Nb-permeable Cu grain boundaries exhibit negative enthalpy of mixing of Nb, resulting in an enthalpically-stabilized solution of highly mobile Nb atoms that easily diffuse through the boundary. By contrast, Mo, which also has minimal solubility with Cu, has positive enthalpy of mixing at Nb-permeable Cu grain boundaries and does not diffuse through them. Furthermore, our findings suggest material selection and grain boundary engineering as pathways for designing improved diffusion barriers and thermally stable laminate composites.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Comparing theoretical predictions of radiation-free velocities of edge dislocations to molecular dynamics simulations

Transonic defect motion is of interest for high strain-rate plastic deformation as well as for crack propagation. Ever since Eshelby's 1949 prediction [J. D. Eshelby, Proc. Phys. Soc. A 62, 307 (1949)] in the isotropic limit of a “radiation-free” transonic velocity $v$ RF = $\sqrt{2}$c T , where shock waves are absent, there has been speculation about the significance of radiation-free velocities (if they truly exist) for defect mobility. Here, in this work, we argue that they do not play any significant role in dislocation dynamics in metals, based on comparing theoretical predictions of radiation-free velocities for transonic edge dislocations with molecular dynamics simulations for two face-centered cubic metals: Ag, where theory predicts radiation-free states, and Cu, where it does not.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Effect of free surfaces on localized plastic deformation in single-crystal nickel containing helium bubbles and radiation-induced self-interstitial atom clusters

This paper presents molecular dynamics (MD) simulations of plastic deformation in single-crystal Ni models containing internal distributions of helium (He) bubbles and self-interstitial atom (SIA) clusters corresponding to experimental characterization of ex-service Inconel X-750 components. Here, we simulate single crystals with and without free surfaces at room temperature and 588K, the latter temperature corresponding to the Inconel X-750 service conditions. Both models are deformed under multiple slip to a maximum deviatoric strain of ~10 %. A dislocation slip band forms in the model without free surfaces, but not in the model with free surfaces. However, the latter exhibits nano-twin nucleation from surfaces. Helium (He) bubbles that overlap with a slip band or a nano-twinned region deform more severely than other bubbles in the model. Nevertheless, the bubbles do not link up and do not form configurations that resemble incipient cracks.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Minor titanium addition markedly improves the co-deformability of copper-tantalum composites

We investigated the microstructures of equal channel angular extrusion (ECAE) processed sintered composites of copper (Cu) and tantalum (Ta). These composites were produced with nominal Ta volume percentages ranging from 25% to 75%. Additional composites of the same nominal Ta content were prepared with a minor amount of titanium (Ti) added to facilitate interphase bonding. Ti addition was found to markedly improve the strength and co-deformability of the composites, allowing reliable extrusion of Cu-Ta composites with novel microstructures. A major influence of the Ti appears to be to soften the Ta, which contributes to improved co-deformability by reducing the flow stress mismatch between the composite constituents. Here, a change in the fracture surface morphology in the Ti-modified composite compared to the unmodified composite suggests that the Ti may also improve cohesion between Cu and Ta by reducing Ta surface oxides.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Implications of Fatigue-Crack Healing in Nanocrystalline Metals [Slides]

Under high-cycle fatigue conditions, a fatigue crack in nanocrystalline Pt was observed to undergo healing. The healing appears to occur by cold welding, facilitated by grain boundary migration, and also by local closure stresses. The healing may help explain several observations: role of air (or vacuum) on fatigue life, impeded subsurface fatigue cracking, apparent flaw healing in sub-critical cycling of ceramics, the existence of a fatigue threshold, and the role of vacuum on the fatigue threshold.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Fragmentation of the edge of a terminated Cu nanolayer within a Nb matrix upon annealing

Here, we investigate the evolution of the tip of a terminated Cu nano-layer embedded within a Nb matrix upon annealing at 800°C. The tip of the terminated Cu layer fragments progressively into a series of single-crystal Cu particles upon continued annealing. This finding shows that layer fragmentation is the second step, after layer pinchoff, in the thermal coarsening of nanolayered metal composites. Comparison to prior phase-field modeling permits us to infer that the mobility at 800°C of Cu-Nb, as modeled by the Cahn-Hillard equation, is M(c)=(M 0 = 7.3 ± 3.6 nm 5 / s • eV) |1 – c 2 |, where the order parameter, c, is 1 for Cu and -1 for Nb.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Outgassing of implanted He via short circuit transport along phase and grain boundaries in vapor co-deposited Cu-W nanocomposites

We investigate the response of physical vapor co-deposited copper (Cu)-tungsten (W) nanocomposites to helium (He) implantation. Nuclear reaction analysis (NRA) reveals that a substantial fraction of He implanted into these materials escapes during implantation. Analysis of nanocomposite microstructure shows that the He loss is likely due to its transport out of the material by diffusion along phase and grain boundaries. Lastly, our findings suggest that solid-state interfaces such as phase and grain boundaries are short circuit diffusion pathways for transport of He.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Persistence of negative vacancy and self-interstitial formation energies in atomistic models of amorphous silicon

We use classical potential atomistic modeling to demonstrate that negative vacancy and selfinterstitial formation energies are inherently present in amorphous silicon (a-Si). Our conclusions are based on comprehensive calculations of vacancy and self-interstitial formation energies in nine different models, ranging in size from 1000 to 2744 atoms and formed by quenching liquid Si at rates between 10 10 and 10 12 K/s. We account for a range of possible reference energies corresponding to different choices of reservoirs for atoms inserted into or removed from our models. Moreover, we show that successive addition of either vacancies or self-interstitials does not deplete the number of negative formation energy sites. Instead, it further lowers defect formation energies. Furthermore, these findings lead us to conclude that a-Si is not a structure at or near a local enthalpy minimum nor can it be brought to a local enthalpy minimum through the introduction of constitutional defects.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Copper-Tantalum Metal Matrix Composites Consolidated from Powder Blends by Severe Plastic Deformation

We investigated the effectiveness of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of metal powders into metal matrix composites. Equal volumes of copper (Cu) and tantalum (Ta) powders were consolidated at ambient temperature via different ECAE routes. Composites processed by ECAE routes 4E and 4Bc were also processed at 300 °C. The resulting materials were characterized by scanning electron microscopy (SEM) and compression testing. Processing by route 4Bc at 300 °C resulted in the highest compressive strength, lowest anisotropy, and least strain rate sensitivity. We conclude that the superior properties achieved by this route arise from mechanical bonding due to interlocking Cu and Ta phases as well as enhanced metallurgical bonds from contact of pristine metal surfaces when the material is sheared along orthogonal planes.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Toughening of interface networks through the introduction of weak links

The resistance of polycrystalline materials to intergranular fracture depends on the toughness of the interfaces between neighboring grains. Using finite element simulations on model, two-dimensional microstructures, we demonstrate that the steady-state crack growth resistance of a polycrystal that contains exclusively high toughness interfaces is suboptimal: its crack growth resistance may be increased by introducing a minority fraction of weak interfaces at random locations within the interface network. Furthermore, the weaker are these interfaces, the greater is their toughening effect. These weak interfaces toughen the material through crack deflection and branching, nucleation of secondary cracks, and inelastic dissipation within intact interfaces. Furthermore, our findings suggest that fracture resistance-oriented material development should not aim to eliminate all weak interfaces, but rather judiciously incorporate a low fraction of them.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Healing of nanocracks by collision cascades in nickel

We use molecular dynamics simulations to investigate the interaction between collision cascades and pre-existing nano-cracks in nickel (Ni). We find that collision cascades cause the cracks to heal whenever a cascade initiates a thermal spike whose core overlaps with the crack. Moreover, crack healing affects the distribution of radiation-induced defects generated during a collision cascade: defect clusters, such as dislocations and stacking fault tetrahedra, dominate in the presence of crack healing, while isolated point defects dominate in its absence. Furthermore, this work indicates that irradiation may reduce the number of pre-existing crack-like flaws in metallic components, potentially improving their mechanical resilience.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Persistence of crystal orientations across sub-micron-scale "super-grains" in self-organized Cu-W nanocomposites

We use precession electron diffraction to investigate the crystallographic character of copper (Cu)-tungsten (W) nanocomposites fabricated via physical vapor co-deposition at 400 oC. We observe sub-micron-scale regions, where apparently disconnected Cu and W grains have near identical crystallographic orientations. This persistence of grain orientations suggests Cu and W grains within these regions are interconnected in 3-D when they first form and may be considered as intercalated, sub-micron-scale "super-grains". Indeed, atom probe tomography provides direct evidence of 3-D interconnectivity of W domains. Our findings shed light on the structure and self organization mechanisms of nanocomposites formed by spontaneous phase separation of co-deposited metals.

Yadav, Digvijay↗

DOE OSTI2020

The effects of nanoscale confinement on the behavior of metal laminates

By reducing the layer thickness, h, in metal multilayers from micrometer- to nanometer-levels, scale-dependent transitions in behavior are observed. The role of interface properties in these transitions has been extensively investigated. However, reducing h to the nanoscale regime also imposes a physical geometric confinement on defect-related processes occurring within the layers, an effect that is relatively less explored. Herein, we present illustrative examples of the influence of nanoconfinement on the strength, structure, and radiation response of multilayers. These examples show that nanoconfinement and interface structure and topology act cooperatively in the nanoscale limit, affecting behaviors that cannot be explained based on either factor alone.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Vacancy and interstitial interactions with crystal/amorphous, metal/covalent interfaces

In this work, we use atomistic simulations to investigate the interaction of vacancies and interstitials with interfaces between a crystalline metal and an amorphous, covalently-bonded solid. We select the gold (Au)/silicon (Si) binary system as a model material and construct interface models along two different facets of crystalline Au and with amorphous Si (a-Si) created at three different quench rates. We compute formation energies of vacancies, self-interstitials, and interstitial impurities as a function of position relative to the interface and find that they have markedly lower values near the interface than in the interior of the adjoining phases. We conclude that crystal/amorphous, metal/covalent interfaces may be as effective at removing radiation-induced point defects as interfaces in polycrystalline metals composites. Moreover, irrespective of interface character, the average formation energies of all point defects at all the Au/a-Si interfaces we investigated are comparable. Thus, unlike in polycrystalline metals, where an interface’s crystallographic character has a marked effect on its interactions with point defects, all interface types in crystal/amorphous, metal/covalent composites may be equally effective at absorbing all radiation-induced defects.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Comparative study of helium bubbles in a Ti-Ta alloy and a Ti/Ta nanocomposite

Here, we investigated the size, density, and distribution of helium (He) bubbles in a Ti-Ta single-phase alloy and a Ti/Ta dual-phase nanocomposite using transmission electron microscopy. The Ti/Ta nanocomposite was fabricated via phase separation during high-temperature annealing of the single-phase Ti-Ta alloy. He ion implantation in the Ti-Ta single-phase alloy leads to the formation of nano-scale He bubbles (~1.7 nm in size, ~2.6×10 5 /μm 3 in volumetric density). He bubbles were found to segregate to grain boundaries. Under identical implantation conditions, numerous He bubbles also formed in the Ti/Ta nanocomposite. Closer inspection revealed that He bubbles in the Ti-rich phase are smaller and of higher volumetric density (~1.2 nm, ~8.4×10 5 /μm 3 ) than those in the Tarich phase (~3 nm, ~1.8×10 5 /μm 3 ) and the single-phase alloy. He bubbles were also observed to decorate phase boundaries, but they were generally smaller than those on grain boundaries. These results suggest that Ti/Ta phase boundaries are less conducive to the growth of large He bubbles than grain boundaries in the Ti-Ta single phase alloy.

36 MATERIALS SCIENCE↗