The high-cycle fatigue life of nanocrystalline and ultrafine-grained Ni-Fe was examined for five distinct grain sizes ranging from approximately 50–600 nm. The fatigue properties were strongly dependent on grain size, with the endurance limit changing by a factor of 4 over this narrow range of grain size. The dataset suggests a breakdown in fatigue improvement for the smallest grain sizes <100 nm, likely associated with a transition to grain coarsening as a dominant rate-limiting mechanism. The dataset also is used to explore fatigue prediction from monotonic tensile properties, suggesting that a characteristic flow strength is more meaningful than the widely-utilized ultimate tensile strength.
Bias. It’s a word that makes most of us squirm. Bias implies to us that we are “bad people” and are being accused of deliberately discriminating against others. Yet, if you ask a social scientist, you will find that it doesn't mean that at all; implicit bias is a neurologically based, energy-saving short cut. Our brains apply mental models to make thousands of quick decisions every day: which brand of milk to buy at the store or when to turn the wheel to avoid a traffic accident. Lastly, we form our implicit biases subconsciously over time, influenced by our upbringing, societal norms, and life experiences.
Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10−9 mm3 N−1 m−1, comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.
In the absence of pre-existing failure-critical defects, the fracture or tearing process in deformable metals loaded in tension begins with the nucleation of internal cavities or voids in regions of elevated triaxial stress. While ductile rupture processes initiate at inclusions or precipitates in many alloys, nucleation in pure metals is often assumed to be associated with grain boundaries or triple junctions. This study presents ex situ observations of incipient, subsurface void nucleation in pure tantalum during interrupted uniaxial tensile tests using electron channeling contrast (ECC) imaging, electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Instead of forming at grain boundaries, voids initiated at and grew along dislocation cell and cell block boundaries created by plastic deformation. Most of the voids were associated with extended, lamellar deformation-induced boundaries that run along the traces of the {110} or {112} planes, though a few voids initiated at low-angle dislocation subgrain boundaries. In general, a high density of deformation-induced boundaries was observed near the voids. TEM and TKD demonstrate that voids initiate at and grow along cell block boundaries. Two mechanisms for void nucleation in pure metals, vacancy condensation and stored energy dissipation, are discussed in light of these results. The observations of the present investigation suggest that voids in pure materials nucleate by vacancy condensation and subsequently grow by consuming dislocations.
Niobium and niobium nitride thin films are transitioning from fundamental research toward wafer scale manufacturing with technology drivers that include superconducting circuits and electronics, optical single photon detectors, logic, and memory. Successful microfabrication requires precise control over the properties of sputtered superconducting films, including oxidation. Previous work has demonstrated the mechanism in oxidation of Nb and how film structure could have deleterious effects upon the superconducting properties. This study provides an examination of atmospheric oxidation of NbN films. By examination of the room temperature sheet resistance of NbN bulk oxidation was identified and confirmed by secondary ion mass spectrometry. As a result, Meissner magnetic measurements confirmed the bulk oxidation not observed with simple cryogenic resistivity measurements.
Ductile rupture in metals is generally a multi-step process of void nucleation, growth, and coalescence. Particle decohesion and particle fracture are generally invoked as the primary microstructural mechanisms for room-temperature void nucleation. However, because high-purity materials also fail by void nucleation and coalescence, other microstructural features must also act as sites for void nucleation. Early studies of void initiation in high-purity materials, which included post-mortem fracture surface characterization using scanning electron microscopy (SEM) and high-voltage electron microscopy (HVEM) and in-situ HVEM observations of fracture, established the presence of dislocation cell walls as void initiation sites in high-purity materials. Direct experimental evidence for this contention was obtained during in-situ HVEM tensile tests of Be single crystals. Voids between 0.2 and 1 μm long appeared suddenly along dislocation cell walls during tensile straining. However, subsequent attempts to replicate these results in other materials, particularly α -Fe single crystals, were unsuccessful because of the small size of the dislocation cells, and these remain the only published in-situ HVEM observations of void nucleation at dislocation cell walls in the absence of a growing macrocrack. Despite this challenge, other approaches to studying void nucleation in high-purity metals also indicate that dislocation cell walls are nucleation sites for voids.
The friction behavior of ultra-nanocrystalline Ni-W coatings was investigated. A critical stress threshold was identified below which friction remained low, and above which a time-dependent evolution toward higher friction behavior occurred. Founded on established plasticity models we propose a correlation between surface grain size and applied stress that can be used to predict the critical stress separating the two friction regimes. This interpretation of plasticity models suggests that macro-scale low and high friction regimes are respectively associated with the nano-scale mechanisms of grain boundary and dislocation-mediated plasticity.
The widely recognized property improvements of nanocrystalline (NC) materials have generated significant interest; yet, they have been difficult to realize in engineering applications due to the propensity for grain growth in these interface-dominated systems. Although traditional pathways to thermal stabilization can slow the mobility of grain boundaries, recent theories suggest that solute segregation in NC alloys can reduce the grain boundary energy such that thermodynamic stabilization is achieved. Following the predictions of Murdoch et al., here we compare for the first time the thermal stability of a predicted NC stable alloy (Fe-10 at.% Mg) with a predicted non-NC stable alloy (Fe-10 at.% Cu) using the same processing and characterization methodologies. Results show improved thermal stability of the Fe-Mg alloy in comparison with the Fe-Cu, and thermally-evolved microstructures that are consistent with those predicted by Monte Carlo simulations.
Recent studies have shown the potential for nanocrystalline metals to possess excellent fatigue resistance compared to their coarse-grained counterparts. Although the mechanical properties of nanocrystalline metals are believed to be particularly susceptible to material defects, a systematic study of the effects of geometric discontinuities on their fatigue performance has not yet been performed. In the present work, nanocrystalline Ni-40 wt%Fe containing both intrinsic and extrinsic defects were tested in tension-tension fatigue. The defects were found to dramatically reduce the fatigue resistance, which was attributed to the relatively high notch sensitivity in the nanocrystalline material. Microstructural analysis within the crack-initiation zones underneath the defects revealed cyclically-induced abnormal grain growth (AGG) as a predominant deformation and crack initiation mechanism during high-cycle fatigue. The onset of AGG and the ensuing fracture is likely accelerated by the stress concentrations, resulting in the reduced fatigue resistance compared to the relatively defect-free counterparts.
The goal of this SAND report is to provide guidance for other groups hosting workshops and peerto-peer learning events at Sandia. Thus this SAND report provides detail about our team structure, how we brainstormed workshop topics and developed the workshop structure. A Workshop “Nuts and Bolts” section provides our timeline and check-list for workshop activities. The survey section provides examples of the questions we asked and how we adapted the workshop in response to the feedback.
It is well known that nanostructured metals can exhibit significantly improved properties compared to metals with conventional grain size. Unfortunately, nanocrystalline metals typically are not thermodynamically stable and exhibit rapid grain growth at moderate temperatures. This severely limits their processing and use, making them impractical for most engineering applications. Recent work has shown that a number of thermodynamically stable nanocrystalline metal alloys exist. These alloys have been prepared as powders using severe plastic deformation (e.g. ball milling) processes. Consolidation of these powders without compromise of their nanocrystalline microstructure is a critical step to enabling their use as engineering materials. We demonstrate solid-state consolidation of ball milled copper-tantalum nanocrystalline metal powder using cold spray. Unfortunately, the nanocrystalline copper-tantalum powder that was consolidated did not contain the thermodynamically stable copper-tantalum nanostructure. Nevertheless, this does this demonstrates a pathway to preparation of bulk thermodynamically stable nanocrystalline copper-tantalum. Furthermore, it demonstrates a pathway to additive manufacturing (3D printing) of nanocrystalline copper-tantalum. Additive manufacturing of thermodynamically stable nanocrystalline metals is attractive because it enables maximum flexibility and efficiency in the use of these unique materials.
This report documents the development, demonstration and validation of a mesoscale, microstructural evolution model for simulation of zirconium hydride {delta}-ZrH{sub 1.5} precipitation in the cladding of used nuclear fuels that may occur during long-term dry storage. While the Zr-based claddings are manufactured free of any hydrogen, they absorb hydrogen during service, in the reactor by a process commonly termed ‘hydrogen pick-up’. The precipitation and growth of zirconium hydrides during dry storage is one of the most likely fuel rod integrity failure mechanisms either by embrittlement or delayed hydride cracking of the cladding. While the phenomenon is well documented and identified as a potential key failure mechanism during long-term dry storage (NUREG/CR-7116), the ability to actually predict the formation of hydrides is poor. The model being documented in this work is a computational capability for the prediction of hydride formation in different claddings of used nuclear fuels. This work supports the Used Fuel Disposition Research and Development Campaign in assessing the structural engineering performance of the cladding during and after long-term dry storage. This document demonstrates a basic hydride precipitation model that is built on a recently developed hybrid Potts-phase field model that combines elements of Potts-Monte Carlo and the phase-field models. The model capabilities are demonstrated along with the incorporation of the starting microstructure, thermodynamics of the Zr-H system and the hydride formation mechanism.
Crystallographic slip planes in body centered cubic (BCC) metals are not fully understood. In polycrystals, there are additional confounding effects from grain interactions. This paper describes an experimental investigation into the effects of grain orientation and neighbors on elastic–plastic strain accumulation. In situ strain fields were obtained by performing digital image correlation (DIC) on images from a scanning electron microscope (SEM) and from optical microscopy. These strain fields were statistically compared to the grain structure measured by electron backscatter diffraction (EBSD). Spearman rank correlations were performed between effective strain and six microstructural factors including four Schmid factors associated with the <111> slip direction, grain size, and Taylor factor. Modest correlations (~10%) were found for a polycrystal tension specimen. The influence of grain neighbors was first investigated by re-correlating the polycrystal data using clusters of similarly-oriented grains identified by low grain boundary misorientation angles. Second, the experiment was repeated on a tantalum oligocrystal, with through-thickness grains. Much larger correlation coefficients were found in this multicrystal due to the dearth of grain neighbors and subsurface microstructure. Finally, a slip trace analysis indicated (in agreement with statistical correlations) that macroscopic slip often occurs on {110}<111> slip systems and sometimes by pencil glide on maximum resolved shear stress planes (MRSSP). These results suggest that Schmid factors are suitable for room temperature, quasistatic, tensile deformation in tantalum as long as grain neighbor effects are accounted for.
We present the results of a three-year LDRD project focused on understanding microstructural evolution and related property changes in Zr-based nuclear cladding materials towards the development of high fidelity predictive simulations for long term dry storage. Experiments and modeling efforts have focused on the effects of hydride formation and accumulation of irradiation defects. Key results include: determination of the influence of composition and defect structures on hydride formation; measurement of the electrochemical property differences between hydride and parent material for understanding and predicting corrosion resistance; in situ environmental transmission electron microscope observation of hydride formation; development of a predictive simulation for mechanical property changes as a function of irradiation dose; novel test method development for microtensile testing of ionirradiated material to simulate the effect of neutron irradiation on mechanical properties; and successful demonstration of an Idaho National Labs-based sample preparation and shipping method for subsequent Sandia-based analysis of post-reactor cladding.
The goal of this LDRD project is to develop a rapid first-order experimental procedure for the testing of advanced cladding materials that may be considered for generation IV nuclear reactors. In order to investigate this, a technique was developed to expose the coupons of potential materials to high displacement damage at elevated temperatures to simulate the neutron environment expected in Generation IV reactors. This was completed through a high temperature high-energy heavy-ion implantation. The mechanical properties of the ion irradiated region were tested by either micropillar compression or nanoindentation to determine the local properties, as a function of the implantation dose and exposure temperature. In order to directly compare the microstructural evolution and property degradation from the accelerated testing and classical neutron testing, 316L, 409, and 420 stainless steels were tested. In addition, two sets of diffusion couples from 316L and HT9 stainless steels with various refractory metals. This study has shown that if the ion irradiation size scale is taken into consideration when developing and analyzing the mechanical property data, significant insight into the structural properties of the potential cladding materials can be gained in about a week.
One of the tenets of nanotechnology is that the electrical/optical/chemical/biological properties of a material may be changed profoundly when the material is reduced to sufficiently small dimensions - and we can exploit these new properties to achieve novel or greatly improved material's performance. However, there may be mechanical or thermodynamic driving forces that hinder the synthesis of the structure, impair the stability of the structure, or reduce the intended performance of the structure. Examples of these phenomena include de-wetting of films due to high surface tension, thermally-driven instability of nano-grain structure, and defect-related internal dissipation. If we have fundamental knowledge of the mechanical processes at small length scales, we can exploit these new properties to achieve robust nanodevices. To state it simply, the goal of this program is the fundamental understanding of the mechanical properties of materials at small length scales. The research embodied by this program lies at the heart of modern materials science with a guiding focus on structure-property relationships. We have divided this program into three Tasks, which are summarized: (1) Mechanics of Nanostructured Materials (PI Blythe Clark). This task aims to develop a fundamental understanding of the mechanical properties and thermal stability of nanostructured metals, and of the relationship between nano/microstructure and bulk mechanical behavior through a combination of special materials synthesis methods, nanoindentation coupled with finite-element modeling, detailed electron microscopic characterization, and in-situ transmission electron microscopy experiments. (2) Theory of Microstructures and Ensemble Controlled Deformation (PI Elizabeth A. Holm). The goal of this Task is to combine experiment, modeling, and simulation to construct, analyze, and utilize three-dimensional (3D) polycrystalline nanostructures. These full 3D models are critical for elucidating the complete structural geometry, topology, and arrangements that control experimentally-observed phenomena, such as abnormal grain growth, grain rotation, and internal dissipation measured in nanocrystalline metal. (3) Mechanics and Dynamics of Nanostructured and Nanoscale Materials (PI John P. Sullivan). The objective of this Task is to develop atomic-scale understanding of dynamic processes including internal dissipation in nanoscale and nanostructured metals, and phonon transport and boundary scattering in nanoscale structures via internal friction measurements.
The size effect in body-centered cubic metals is comprehensively investigated through micro/nano-compression tests performed on focused ion beam machined tungsten (W), molybdenum (Mo) and niobium (Nb) pillars, with single slip [2 3 5] and multiple slip [0 0 1] orientations. The results demonstrate that the stress-strain response is unaffected by the number of activated slip systems, indicating that dislocation-dislocation interaction is not a dominant mechanism for the observed diameter dependent yield strength and strain hardening. Furthermore, the limited mobility of screw dislocations, which is different for each material at ambient temperature, acts as an additional strengthening mechanism leading to a material dependent size effect. Nominal values and diameter dependence of the flow stress significantly deviate from studies on face-centered cubic metals. This is demonstrated by the correlation of size dependence with the material specific critical temperature. Activation volumes were found to decrease with decreasing pillar diameter further indicating that the influence of the screw dislocations decreases with smaller pillar diameter.
In-situ transmission electron microscopy (TEM) straining experiments provide direct detailed observation of the deformation and failure mechanisms active at a length scale relevant to nanomaterials. This presentation will detail continued investigations into the active mechanisms governing high purity nanograined pulsed-laser deposited (PLD) nickel, as well as recent work into dislocation-particle interactions in nanostructured PLD aluminum-alumina alloys. Straining experiments performed on nanograined PLD free-standing nanograined Ni films with an engineered grain size distribution revealed that the addition of ductility with limited decrease in strength, reported in such metals, can be attributed to the simultaneous activity of three deformation mechanisms in front of the crack tip. At the crack tip, a grain agglomeration mechanism occurs where several nanograins appear to rotate, resulting in a very thin, larger grain immediately prior to failure. In the classical plastic zone in front of the crack tip, a multitude of mechanisms were found to operate in the larger grains including: dislocation pile-up, twinning, and stress-assisted grain growth. The region outside of the plastic zone showed signs of elasticity with limited indications of dislocation activity. The insight gained from in-situ TEM straining experiments of nanograined PLD Ni provides feedback for models of the deformation and failure in nanograined FCC metals, and suggests a greater complexity in the active mechanisms. The investigation into the deformation and failure mechanisms of FCC metals via in-situ TEM straining experiments has been expanded to the effect of hard particles on the active mechanisms in nanograined aluminum with alumina particles. The microstructures investigated were developed with varying composition, grain size, and particle distribution via tailoring of the PLD conditions and subsequent annealing. In order to develop microstructures suitable for in-situ deformation testing, in-situ TEM annealing experiments were performed, revealing the effect of nanoparticle precipitates on grain growth. These films were then strained in the TEM and the resulting microstructural evolution will be discussed. In-situ TEM straining experiments currently provide a wealth of information into plasticity within nanomaterials and can potentially, with further development of TEM and nanofabrication tools, provide even greater investigative capabilities.