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Deformation-induced grain-growth in nanocrystalline materials is a widely-reported phenomenon that has been attributed to grain boundary (GB) processes. In this paper, we report on the opposite phenomenon, wherein a stable nanocrystalline (NC) Cu-Ta alloy undergoes a further refinement of the nano-grains during severe plastic deformation (SPD). SPD up to 250% results in a significant grain-size reduction despite the 350°C increase in temperature caused by the deformation process. Experiments and atomistic-simulations show that this unexpected grain-refinement is a direct result of well-dispersed Ta-nanoclusters throughout grain centers and along GBs acting as kinetic-pinning agents and suppressing GB processes that occur during recrystallization.

Thermomechanical behavior of an ultrafine-grained nickel-yttrium alloy has been investigated through quasi-static (10−4 s−1) and high temperature creep experiments under uniaxial compression along with post-deformed transmission electron microscopy (TEM) characterization. While the alloy possesses a quasi-static flow stress of ~1255 MPa at room temperature, the flow stress drops by about 80% at 873 K. TEM analysis showed negligible average grain growth over the entire temperature range tested. Furthermore, the alloy showed exceptional steady–state creep behavior owing to the relative stability of the grain size and interactions between dislocations/grain boundary and inclusions/dispersoids.

Nanocrystalline (NC) materials possess excellent room temperature properties, such as high strength, wear resistance, and toughness as compared to their coarse-grained counterparts. However, due to the excess free energy, NC microstructures are unstable at higher temperatures. Significant grain growth is observed already at moderately low temperatures, limiting the broader applicability of NC materials. Here, we present a design approach that leads to a significant improvement in the high temperature tensile creep resistance (up to 0.64 of the melting temperature) of a NC Cu-Ta alloy. The design approach involves alloying of pure elements to create a distribution of nanometer sized solute clusters within the grains and along the grain boundaries. We demonstrate that the addition of Ta nanoclusters inhibits the migration of grain boundaries at high temperatures and reduces the dislocation motion. This leads to a highly unusual tensile creep behavior, including the absence of any appreciable steady-state creep deformation normally observed in almost all materials. This design strategy can be readily scaled-up for bulk manufacturing of creep-resistant NC parts and transferred to other multicomponent systems such as Ni-based alloys.

Nanocrystalline materials are known to possess excellent radiation resistance due to high fraction of grain boundaries that act as defect sinks, provided they are microstructurally stable at such extreme conditions. In this work, radiation response of a stable nanocrystalline Cu-Ta alloy is studied by irradiating with 4 MeV copper ions to doses (close to the surface) of 1 displacements per atom (dpa) at room temperature (RT); 10 dpa at RT, 573 and 723 K; 100 and 200 dpa at RT and 573 K. Nanoindentation results carried out for samples irradiated till 100 dpa at RT and 573 K show exceptionally low radiation hardening behavior compared to various candidate materials from literature. Results from microstructural characterization, using atom probe analysis and transmission electron microscopy, show a stable nanocrystalline microstructure with minimal grain growth and a meagre swelling in samples irradiated to 100 dpa (~0.2%) and 200 dpa at RT, while no voids in those at 573 K. This radiation tolerance is partly attributed to the stability of Ta nanoclusters due to phase separating nature of the alloy. Additionally, the larger tantalum particles are observed to undergo ballistic dissolution at doses greater than 100 dpa and are eventually precipitated as nanoclusters, replenishing the sink strength, which enhanced material's radiation tolerance when exposed to high irradiation doses and elevated temperatures.

The microstructures of materials typically undergo significant changes during shock loading, causing failure when higher shock pressures are reached. However, preservation of microstructural and mechanical integrity during shock loading are essential in situations such as space travel, nuclear energy, protection systems, extreme geological events, and transportation. Here, we report ex situ shock behavior of a chemically optimized and microstructurally stable, bulk nanocrystalline copper–tantalum alloy that shows a relatively unchanged microstructure or properties when shock compressed up to 15 GPa. The absence of shock-hardening indicates that the grains and grain boundaries that make up the stabilized nanocrystalline microstructure act as stable sinks, thereby annihilating deformation-induced defects during shock loading. This study helps to advance the possibility of developing advanced structural materials for extreme applications where shock loading occurs. Shock loading of materials alters the microstructure and considerably degrades mechanical performance. Here, shock loading of a nanocrystalline Cu–Ta alloy is found to induce minor changes to microstructure and mechanical performance, attributed to the annihilation of defects during deformation.

Bone marrow examination is an important tool for the diagnosis of various hematological disorders. It involves the use of bone marrow aspiration (BMA) and bone marrow biopsy (BMB). To compare concordance and discordance rate between bone Objectives: marrow aspiration and trephine biopsy ndings in making etiological diagnosis in pancytopenia patients. A cross Material And Methods: sectional prospective study was conducted in department of pathology MAMC, Agroha on 36 cases of pancytopenia to compare the ndings of bone marrow aspiration and bone marrow biopsy. The overall concordance and discordance rate between BMA and BMB wa Results: s 63.8% and 36.2% respectively. Conclusion: It was concluded in our study that BMA and BMB are important, useful complementary diagnostic procedures which gives a higher diagnostic yield when performed simultaneously.

The potential benefits of nanocrystalline (NC) alloys for use in various structural applications stem from their enhanced mechanical strengths. However, deformation-induced grain growth in NC materials reduces the strength and is a widely reported phenomenon occurring even at low-temperatures. Controlling such behavior is critical for the maturation of bulk nanocrystalline metals in various advanced engineering applications. Here, we disclose the mechanism by which grain boundary sliding and rotation are suppressed when a NC material is truly thermo-mechanically stabilized against grain growth. Unlike in any other known nanocrystalline metals, the absence of sliding and rotation during loading, at extreme temperatures, is related to short-circuit solute diffusion along the grain boundaries causing the formation of solute clusters and thus a significant change of the grain boundary structures. The departure of this unusual behavior from the well-established norm leads to a strong enhancement of many mutually exclusive properties, such as thermo-mechanical strength, creep resistance, and exceptionally high electrical/thermal conductivity. This work demonstrates that Cu-based nanocrystalline alloys can be used in applications where conventional Cu-based polycrystalline materials are not viable.

This work investigates macroscopic plastic behavior in polycrystalline Ti with varying oxygen concentrations using both experiments and a crystal plasticity finite element framework applied to a representative volume element (RVE) of the microstructure. The proposed multiscale framework makes use of parameters such as critical resolved shear stress (CRSS) ratios obtained from lower length scale first-principle calculations and strain hardening rates obtained from experiments. Generalized stacking fault energy calculations (GSFE) in combination with the Peierls-Nabarro model and a temperature dependent phenomenological description of CRSS were utilized to compute the CRSS ratios for different slip systems in Ti with and without oxygen. Experimentally measured stress-strain responses along the rolling direction for polycrystalline Ti with different oxygen concentrations were used to obtain the strain hardening rates. The crystal plasticity framework was then calibrated using the computed CRSS ratios and the strain-hardening rate for Ti with various oxygen concentrations. The calibrated model was then used to predict the macroscopic response of Ti under different loading conditions and orientations with different oxygen concentrations. Without tuning the fitting parameters for the crystal plasticity framework, we show that the model is able to simulate the experimental responses within the experimental uncertainty. Thus, a systematic calibration procedure is presented to capture the macroscopic homogeneous responses of Ti with various oxygen concentrations.

Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocation-precipitate interactions scenarios for both θ′ and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ′ Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as un-shearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts.

The mechanical properties of Ti alloys are significantly affected by the introduction of solute or impurity elements. Thus, the role of impurities on the hardening or softening behavior of α-Ti was investigated through first principles calculations. In particularly, to provide a comprehensive electronic and atomic basis of solute addition in α-Ti, the effect of metallic (V and Al) and non-metallic (C and O) impurities on the ideal shear strength (ISS) and generalized stacking fault energy (GSFE) across different slip systems of Ti were probed. The results revealed that the addition of V atom reduces both ISS and unstable stacking fault energy across various slip systems of α-Ti, whereas Al addition increases the ISS of Ti. Further, the addition of O atom decreases ISS for most of the slip systems while C solute atom increases ISS across all slip systems of α-Ti. To illustrate the underlying factors influencing the observed softening/strengthening behavior, the electronic density of states and valence charge transfer were determined. The electronic density of states calculations showed that the contribution from the d states of V atom decreases the stability of Ti-V solid solution, thereby leading to a decrease in the plastic anisotropy of α-Ti. Finally, the shearability parameter and critical resolved shear stress (CRSS) ratios across different slip systems of Ti solid solutions were calculated to understand the macroscopic effects of impurity addition on the deformation behavior of α-Ti at ambient conditions. Overall, the first-principles study provides an insight into the electronic basis of strengthening/softening effect of solute addition in α-Ti and assesses their implications on the deformation behavior of α-Ti alloys.

Here, the Cottrell–Stokes ratio in a microstructurally-stable Cu-3Ta (at.%) nanocrystalline alloy is examined from the standpoint of changes in deformation mechanisms. Toward this, uniaxial compression experiments were performed in the temperature range of 113 K – 273 K. The Cottrell-Stokes ratio at the lowest temperature tested was ~1.3, and the material exhibited a very low strain-rate sensitivity at cryogenic-temperatures. Transmission electron microscopy (TEM) characterization showed negligible average grain size coarsening and a transition in the deformation mechanism toward athermal activation processes such as twinning with the reduction in the testing temperature.

An investigation of the mechanical properties of high-purity niobium single crystals is presented. Specimens were cut with different crystallographic orientations from a large grain niobium disk and uniaxial tensile tests were conducted at strain rates between 10-4 and 103 s-1. The logarithmic strain rate sensitivity for crystals oriented close to the center of a tensile axis inverse pole figure (IPF) is ~0.14 for all strain rates. The strain at failure (ranging from 0.4 to 0.9) is very sensitive to crystal orientation and maximal at ~10-2 s-1 for crystals oriented close to the center of an IPF. The high anisotropy observed at quasi-static strain rates decreased with increasing strain rate. The activation of multiple slip systems in the dynamic tests could account for this reduction in anisotropy. A transition from strain hardening to softening in the plastic domain was observed at strain rates greater than approximately 6 × 10-2 s-1 for crystals oriented close to the center of a tensile axis IPF. Shear bands were observed in specimens with orientations having similarly high Schmid factors on both { 110 } and { 112 } slip families, and they are correlated with reduced ductility. Crystal rotations at fracture are compared for the different orientations using scanning electron microscopy images and EBSD orientation maps. A rotation toward the terminal stable [101] orientation was measured for the majority of specimens (with tensile axes more than ~17° from the [001] direction) at strain rates between 1.28 × 10-2 and 1000 s-1.

Both engineered and natural materials may be subjected to extremes of elevated pressure, temperature, and strain rate due to shock loading; examples include ballistic impact of projectiles on armor, planetary impacts, and high-speed machining operations. Experimental techniques for ascertaining the macroscopic response of materials to shock loading are well established, but insight into fundamental mechanisms of deformation requires the ability to characterize the evolution of microstructure in real time. Experiments in which specimens are recovered and characterized after shock loading have been widely used to understand structure-property relationships. But these shock recovery experiments only reveal information at the end state of the material, from which the evolution of the structure during unloading must be inferred. There is always the possibility that the structure of the material continues to evolve during unloading, leading to a potential misunderstanding of the structural evolution during shock compression and release. In this paper we describe the results of shock recovery and time-resolved in situ x-ray diffraction studies of deformation twinning in an extruded fine-grained AMX602 magnesium alloy. The samples were shock compressed along the plate normal and extrusion directions, then released back to ambient conditions. Analysis of the microstructure before and after shock loading indicates a substantial change in crystallographic texture reflecting substantial deformation twinning. Texture evolution from in situ synchrotron x-ray diffraction measurements show significant twinning during shock compression followed by detwinning during stress release. These results not only provide insight into the complex twinning-detwinning behavior of this particular alloy, but also illustrate the utility of in situ characterization for bridging the knowledge gap between shock recovery experiments and the transient behavior of materials during shock loading more generally.

Crack growth kinetics in crystalline materials is examined both from the point of continuum mechanics and discrete dislocation dynamics. Kinetics ranging from the Griffith crack to continuous elastic-plastic cracks are analyzed. Initiation and propagation of incipient cracks require very high stresses and appropriate stress gradients. These can be obtained either by pre-existing notches, as is done in a typical American Society of Testing and Materials (ASTM) fatigue and fracture tests, or by in situ generated stress concentrations via dislocation pile-ups. Crack growth kinetics are also examined using the modified Kitagawa&ndash, Takahashi diagram to show the role of internal stresses and their gradients needed to sustain continuous crack growth. Incipient crack initiation and growth are also examined using discrete dislocation modeling. The analysis is supported by the experimental data available in the literature.

In this work, a nanocrystalline (NC) Cu-10at.%Ta alloy is irradiated with helium at different temperatures to assess the stability and effectiveness of Ta nanoclusters in trapping helium and suppressing swelling. Advanced microstructural characterization of the room-temperature irradiated specimens indicated the presence of small He-bubbles (∼1-2 nm) at the peak damage depth mainly at the core and along the interface of Ta nanoclusters with Cu matrix. Few bubbles were found along grain boundaries, with much smaller bubbles homogenously distributed within the copper lattice. High-temperature irradiation exhibited bubbles of ∼3-5 nm, which were primarily associated with nanoclusters as compared to other locations, with no observed faceting of the bubbles. Atom probe analysis confirmed helium partitioning to the Ta nanoclusters indicating the effective entrapment of these He atoms.

Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically, ~103 s−1, resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (~1356 K)., Metals deformed at very high rates experience a rapid increase in flow stress due to dislocation drag. Here, the authors stabilise a nanocrystalline microstructure to suppress dislocation velocity and limit drag effects, conserving low strain-rate deformation mechanisms up to higher strain rates and temperatures.

Solution strengthening or softening is an effective way to enhance mechanical properties, especially in magnesium based alloys due to their inability to activate adequate non-basal deformation mechanisms at the room temperature. Hence, using first-principles calculations, the effects of several different alloying elements on the ideal shear resistance across various slip systems of Mg were investigated. The results reveal that the addition of a Ce or Zr solute atom decreases the ideal shear resistance (softening); whereas, the substitution of a Sn, Li or Zn atom increases the ideal shear resistance of Mg (strengthening). The dominant slip system in Mg was found to change from the basal partial (0001) [ 10 1 ¯ 0 ] to prismatic (10 1 ¯ 0)[11 2 ¯ 0] with the addition of a Ce or Zr solute atom; whereas, the addition of a Sn, Li or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer, which provides a quantum mechanical insight into the underlying factors influencing the observed softening/strengthening behavior, was probed. For instance, the electronic density of states calculations show that the contribution from d states of Ce and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. In the end, theoretical analyses were performed and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys.

Mechanically alloyed 91Fe-8Ni-1Zr (at%) powders were fabricated through high energy ball milling of elemental powder and subsequently consolidated via equal channel angular extrusion (ECAE) at 800 °C and 1000 °C. The resulting microstructure was fine grain with a nano-dispersion of Zr-oxide within the matrix, which was spherical for the 800 °C. ECAE and plate-like (and volumetrically larger) for the 1000 °C ECAE conditions. Atom probe tomography confirmed trace levels of C, N, and Cr impurities within the alloy making it similar to a low-carbon steel. By performing mechanical testing at a quasi-static strain rate (10−3 s−1) and at high strain rate (103 s−1) at room temperature and 473 K, a load drop was noted after yielding. In general, this load drop became more pronounced with increasing strain rate and temperature and has been shown to be a result of dynamic strain aging in the ODS alloy.

The mechanical properties of the weld regions of a 6061-T6 resistance spot welded lap joint are determined. The change in mechanical properties resulting from RSW are linked to the changes observed in the microstructure. Processing currents and strain rates are varied to probe the effects of processing temperature at strain rates from 10−3 to 103 s−1. Results show that material strength decreases within the heat affected zone (HAZ) and fusion zone due to precipitate dispersion. Further, decreased ductility results at quasi-static strain rates from accelerated crack growth arising near voids formed during weld formation, but the short time scale at higher strain rates limits the ability for crack growth from these voids allowing the material to exhibit higher ductility. Overall, significant changes in the mechanical behavior across the weld resulting from a change in microstructure congruent with precipitate dispersion are apparent for all processing conditions.

Nanocrystalline Cu-Ta alloys are emerging as a new class of structural materials preserving the nano-scale grain size up to the melting point of Cu. This extraordinary structural stability is caused by the strong pinning of grain boundaries (GBs) by Ta nano-clusters precipitating from the unstable solid solution after mechanical alloying. Many aspects of the Ta stabilization effect remain elusive and call for further experimental and simulation work. In previous atomistic computer simulations of stress-driven GB migration [JOM 68, 1596 (2016)], the GB–cluster interactions in Cu-Ta alloys have been studied for several different compositions and GB velocities. The results have pointed to the Zener pinning as the main mechanism responsible for the grain stabilization. This paper extends the previous work to the motion of individual GBs driven by capillary forces whose magnitude is similar to that in real nanocrystalline materials. Both the impingement of a moving GB on a set of Ta clusters and the GB unpinning from the clusters are studied as a function of temperature and alloy composition. The results demonstrate a quantitative agreement with the Zener pinning model and confirm the “unzip” mechanism of unpinning found in the previous work. In the random Cu-Ta solid solution, short-circuit Ta diffusion along stationary and moving GBs leads to the nucleation and growth of new GB clusters, which eventually stop the GB motion.

Here a time dependent numerical model aimed to investigate the role of mechanical deformation on the corrosion behavior of galvanic joint is developed. The influence of mechanical loading on the corrosion behavior of the AE44 (Magnesium alloy) and mild steel galvanic joint immersed in a 1.6 wt% NaCl solution is explored across a wide range of combined mechanical and electrochemical conditions. It is shown that the onset of plastic deformation during mechanical loading greatly accelerates the galvanic corrosion behavior. The overall numerical approach developed here provides a robust framework for understanding the role of mechanical deformation on the corrosion behavior.

The presence of complex states-of-stress and strain-rates directly influence the dominant deformation mechanisms operating in a given material under load. Mg alloys have shown limited ambient temperature formability due to the paucity of active slip-mechanisms, however, studies have focused on quasi-static strain-rates and/or simple loading conditions (primarily uniaxial or biaxial). For the first time, the influence of strain-rate and stress-triaxiality is utilized to unravel the active deformation mechanisms operating along the rolling, transverse- and normal-directions in wrought AZ31-alloy. It is discovered that the activation of various twin-mechanisms in the presence of multiaxial loading is governed by the energetics of the applied strain-rates. IMPACT STATEMENT It is shown for the first time that the higher deformation energy associated with dynamic strain-rates, coupled with high-triaxiality, promotes detwinning and texture evolution in HCP alloys with high c/a ratio.

In this work, various alloy compositions of immiscible copper-tantalum (Cu–Ta) are systematically studied to understand the interplay between cluster stability, precipitation hardening, and the overall stability of the matrix's average grain size. An alloy composition of Cu-3at.%Ta is found to exhibit dramatic improvements relative to other neighboring Ta contents (both higher and lower) only after exposures of more than 300 h at 800 °C. An extremely low steady-state growth rate, i.e., 3.1 nm per 100 h exposure, speaks to the extreme kinetics which allow this composition to retain its high mechanical strength. The key attribute responsible for the NC Cu-3at.%Ta alloy exhibiting such behavior is a high cluster density tailored through the Ta solute content to achieve the best combination of stability without sacrificing strength by limiting Orowan coarsening of the clusters compared to other Ta concentrations. In general, the growth kinetics of this Cu-3at.%Ta alloy are so sluggish that it places it among the most thermally resistant alloys ever produced. The work demonstrates that, if designed properly, bulk nanocrystalline alloys can withstand the prolonged high-temperature exposure required for high-temperature applications, while grain growth is stagnated or halted.

Stability of nanocrystalline microstructural features allows structural materials to be synthesized and tested in ways that have heretofore been pursued only on a limited basis. Here, we demonstrate using quasi-static compression and three point bend tests that, in a stabilized nanocrystalline metal with tailored solute concentrations, i.e., NC-Cu-3 at.%Ta, extraordinary properties such as ultrahigh hardness along with anomalus modulus of resilience and springback effects can be manifested. Such effects influence a wide range of materials response including elastic energy absorption, damping, fatigue and wear. The present study, therefore, represents a pathway for designing highly resilient materials for everyday applications.

Basic concepts, related to the crack tip driving forces in relation to the material resistance, are analyzed for the elastic and elastic-plastic crack growth condition. This defines the crack initiation and growth conditions, as well as for crack arrest. Environment provides an additional driving force, thereby reducing the mechanical driving force required for the crack to grow. It is shown that (a) crack initiation and its growth are inseparable and (b) the magnitude of the applied and/or internal stresses; their gradients are also important for initiation and continuous growth of a crack. Elastic-plastic crack growth is also analyzed using the discrete dislocation models. The results show that its behavior is similar to that of an elastic crack. These concepts are valid for all subcritical crack growth. Mechanical and mechanical equivalent of chemical forces are defined for estimating the life prediction of a component in service. Failure diagrams are defined based on the extension of classical Kitagawa-Takahashi diagram that bridges the behavior of smooth and fracture mechanics specimens. Connections between crack initiation, growth, arrest, and overload fracture are established via these failure diagrams. Application of these diagrams for engineering components in service is outlined for diagnostic and prognostic purposes.

In this article, a numerical investigation examining the influence of various mechanical loading scenarios on the corrosion behavior of a galvanic joint formed between a magnesium alloy and mild steel was carried out. A wide spectrum of mechanical conditions were examined to facilitate the development of a comprehensive understanding on the effect of mechanical deformation during galvanic corrosion. In general, we found that the onset of nonlinear deformation drastically accelerates the localized corrosion. Furthermore, subjecting the galvanic joint to alternating corrosion and fatigue loading was found to increase the observed localized corrosion drastically. Finally, the simulation results are discussed with respect to the identification and description of the detrimental physical phenomena that undermine the structural integrity of joints subjected to various mechanical loading scenarios.

The physics of embrittlement is dictated by the various interactions between the impurities/defects and the local structure in polycrystalline material systems. In this study, a physically motivated model that describes the degree of interaction of hydrogen (H) defects on the segregation behavior to α-Fe grain boundaries (GBs) is developed. Molecular statics simulations were performed to quantify the segregation behavior of 1–2 H atoms at various interstitial sites around the , , , and symmetric tilt GBs. The results provide insights into the concentration profile of hydrogen defects along different GBs. Furthermore, the model accurately links the intrinsic GB character by quantifying the segregation length scale for the individual GBs based on the segregation behavior of defects. Finally, the metrics provided in this work are essential to comprehensively understanding the effect of hydrogen on the macroscopic behavior of α-Fe.

The degradation of corrosion preventative coatings contributes to the high cost and time requirements for maintaining structures in harsh environments. However, the development of new hydrophobic coatings holds the promise of extending the usable life of structures in marine environments. In this work, we quantify the barrier properties and corrosion resistance of two novel highly hydrophobic polysiloxane formulations and the legacy silicone alkyd topcoat used on the topside of Navy’s ships, all with haze gray pigmentation. Based on FTIR-ATR and EIS measurements of the pristine coatings, both the single- (1K) and the two-component (2K) polysiloxane provide significantly improved barrier characteristics (lower water diffusion coefficient and capacitance) than the silicone alkyd. These results were confirmed through a 3-month-long immersion corrosion test, which also showed that the 1K and 2K polysiloxane coatings had comparable degradation characteristic and remained highly hydrophobic.

Predicting grain growth in nanocrystalline materials requires modeling approaches that incorporate grain boundary thermodynamics and kinetics. In this work, the thermokinetic model of Chen et al. (2012) for grain growth was applied to experimental X-ray diffraction measurements from a binary nanocrystalline alloy in an effort (1) to understand the influence of thermodynamic, kinetic, and material parameters in the model; and (2) to extend the thermokinetic model by incorporating temperature dependence. The model performs well for the grain boundary saturated case in the binary nanocrystalline alloy, where it is assumed that solute segregates to the grain boundaries and thermodynamically/kinetically reduces the driving force for grain growth. In this work, a sensitivity analysis of parameters (Monte Carlo global sensitivity analysis) identifies the important thermodynamic/kinetic parameters and their correlation with one another for the present model. This model was then extended to include the change in these independent thermodynamic/kinetic parameters as a function of temperature and to model the effect of initial grain size distribution. This research shows that the thermodynamic and kinetic contributions can describe grain growth in nanocrystalline materials and this extended model can be parameterized for grain size evolution and stabilization with temperature for nanocrystalline systems.

The nucleation of helium (He) clusters and their growth in a monovacancy (V) eventually leads to the formation of He bubbles on grain boundaries and within the lattice for α-Fe, which can cause the macroscopic deterioration of material properties. The research objective herein is to model the nucleation and growth of small He clusters by calculating the formation/binding energetics of a single He atom to a HenV cluster (n = {1–7}) and by capturing how the grain boundary affects this behavior in bcc α-Fe. The formation energies for 1–8 He atoms in a monovacancy are calculated at all potential grain boundary sites within 15 A of ten select high angle grain boundaries. These results are combined with previously calculated vacancy formation energies and interstitial He formation energies to quantify how the local grain boundary structure impacts the binding of an interstitial He atom to a HenV cluster. We find that, despite the large range of different local environments within the grain boundaries, it is nearly always energetically favorable for a nearby interstitial He atom to combine with either a monovacancy or a HenV cluster to form a larger HenV cluster, with a binding energy that can be much greater (as much as 100% greater) than in the bulk crystal. Furthermore, a model is presented that captures the formation and binding energies of the various He–V clusters while capturing the subsequent binding energies of different clusters/defects in the presence of grain boundaries – both of which are important when accounting for the total energetics pertaining to He–V cluster growth in the presence of the high angle grain boundaries.

Nanocrystalline materials possess excellent room temperature properties, such as high strength, wear resistance, and toughness as compared to their coarse-grained counterparts. However, due to excess free energy, nanocrystalline microstructures are unstable at higher temperatures. Significant grain growth is observed already at moderately low temperatures, limiting their broader applicability. Here, we present a design approach that leads to a significant improvement in the high temperature tensile creep resistance (up to 0.64 of the melting temperature Tm) of a nanocrystalline Cu-Ta alloy. The design approach involves alloying of pure elements for engineering nanometer sized solute clusters within the solvent grains as well as along the grain boundaries. Using a chemically optimized nanocrystalline Cu-3at.%Ta alloy as a model material system, we demonstrate that the addition of Ta nanoclusters inhibits the migration of the planar defects at higher temperatures and reduces the dislocation motion, leading to extraordinary high temperature properties. For instance, the NC Cu-3Ta alloy tested under tensile creep conditions up to the temperature of 873 K (0.64Tm) displays highly unusual behavior, including the absence of any appreciable steady-state creep deformation which is normally observed in almost all materials. This approach can be readily scaled-up for bulk manufacturing of creep resistant nanocrystalline parts. Moreover, this design strategy can be transferred to other multicomponent systems such as Ni-based alloys for making nanocrystalline materials with tailored properties.

The effect of mechanical loading, particularly at dynamic strain rates, on nanocrystalline (NC) materials has eluded researchers owing to the inherent instability of the NC structure. However, a recently developed NC Cu-10 at.%Ta alloy has exhibited an ability to maintain a NC structure at temperatures up to 873 K. Here, NC Cu-10 at.%Ta is tested under compressive strain rates ranging from 10−3 s−1 up to 105 s−1 and at temperatures from 298 K up to 1073 K. Typical materials show a sharp increase in flow stress occurring around 103 s−1 as deformation mechanisms shift away from thermal activation mechanisms; however, at 298 K, NC Cu-10 at.%Ta observes only a limited increase in flow stress indicating that typical thermally activated mechanisms still apply up to strain rates of 105 s−1. Post deformation analyses indicate a shift from nucleation of full dislocations to increased nucleation of partial dislocations at 298 K. However, as temperature increases, thermal activation mechanisms give way to viscous effects and the high density of nucleated full dislocations leads to a dramatic increase in flow stress.

An immiscible nanocrystalline (NC) copper-tantalum (Cu-Ta) alloy is shown to exhibit a stable microstructure under thermo-mechanical loading conditions with exceptional mechanical strength (i.e., 1200 MPa strength at 298 K) indicating anomalous deformation mechanisms as compared to microstructurally unstable nanocrystalline materials. Therefore, in this work, various aspects of strength partitioning in such NC Cu-Ta alloys are discussed and the role of tantalum nanoclusters on the dominant deformation mechanism is presented as a function of temperature. Toward this, initially, the mechanical responses of NC Cu-Ta alloy were measured under uniaxial compression experiments at various temperatures. Later, atomistic simulations were performed along with the high-resolution electron microscopy to identify and validate the rate limiting mechanism behind the plastic deformation in NC Cu-Ta alloys. In general, the observed trend through experiments and simulations identify a transition from a dislocation – nanocluster interaction mediated deformation mechanism to one controlled by grain boundary strengthening as the temperature increases. The former mechanism is shown here to have a crucial role in the observed strengthening behavior of microstructurally stable NC materials. Overall, the paper demonstrates that through effective nano-engineering techniques, it is expected to extend the scope of nanocrystalline materials to a number of engineering design applications. Keywords: Nanocrystalline, Deformation, Transmission electron microscopy, Atomistic

The evolution of the overall microstructure and contribution of constituent second phase particles during the accelerated corrosion of AA 7075-T651 warrants further scrutiny. A correlative microscopy-based approach was employed to study corrosion damage evolution in the microstructure of AA 7075-T651 subjected to potentiodynamic polarization. X-ray Computed Tomography (XCT) and scanning electron microscopy were used to track corrosion damage initiation and propagation globally and at specific regions of the microstructure starting from the first breakdown potential. Additionally, Focused Ion Beam and Electron Backscattered Diffraction were used to investigate local corrosion processes such as pitting and intergranular corrosion. Interesting concentric corrosion patterns, Fe-rich intermetallic particle dissolution and exfoliation corrosion were observed and mechanisms for the same have been proposed.

Microstructural instability in traditional nanocrystalline metals limits the understanding of the fundamental effect of grain size on mechanical behavior under extreme environmental conditions such as high temperature and loading rates. In this work, the interplay between Ta concentrations and processing temperature on the resulting microstructure of a powder processed, fully dense Cu-Ta alloy along with their tensile and compressive behavior at different strain rates are investigated to probe the possibility of manipulating or tuning microstructurally dependent parameters to control the flow-stress upturn phenomenon. Consequently, the results reveal that there is a crucial length scale, i.e., small grain size and appropriate cluster spacing, below which such upturn is damped out. The observation of changes in flow stress upturn behavior is consistent with the observed changes in measured high-rate plasticity, which enhances below the critical length scale. Furthermore, tension-compression asymmetry also tends to be suppressed in nanocrystalline Cu-Ta alloys, while it becomes evident as the grain size increases to an ultrafine regime. Overall, this work presents a systematic approach to control or engineer reduced high strain rate flow stress upturn behavior in metallic alloys, for high-rate applications.

Under intense heating and/or deformation, pure nanocrystalline (NC) metals exhibit significant grain coarsening, thus preventing the study of length scale effects on their physical response under such conditions. Hence, in this study, we use in-situ TEM heating experiments, atomistic modeling along with elevated temperature compression tests on a thermally stabilized nanostructured Cu–10 at.% Ta alloy to assess the microstructural manifestations caused by changes in temperature. Results reveal the thermal stability attained in NC Cu-10 at.% Ta diverges from those observed for conventional coarse-grained metals and other NC metals. Macroscopically, the microstructure, such as Cu grain and Ta based cluster size resists evolving with temperature. However, local structural changes at the interface between the Ta based clusters and the Cu matrix have a profound effect on thermo-mechanical properties. The lattice misfit between the Ta clusters and the matrix tends to decrease at high temperatures, promoting better coherency. In other words, the misfit strain was found to decrease monotonically from 12.9% to 4.0% with increase in temperature, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Overall, the evolution of such fine structures is critical for developing NC alloys with exceptional thermo-mechanical properties. Keywords: In situ TEM, Nanocrystalline, Atomistic, Misfit strain

Nanocrystalline (NC) metals (mean grain sizes d ≤ 100 nm) have enhanced mechanical strength as compared to coarse-grained metals (d ≥ 1 μm), and thus, are a promising alternative as structural materials for future high energy nuclear reactors. However, during extreme conditions, the NC microstructure has been found to be thermodynamically unstable, thereby limiting its applicability. Further, for materials with average grain size

The preferential slip systems in β-tin were investigated using density functional theory (DFT). The gamma surface entering dislocation modeling was calculated using DFT for three different nonequivalent slip systems in β-tin. The generalized stacking fault energies (GSFE) of different slip systems led to the conclusion that the {100)

The interplay between residual strain and corrosive medium can accelerate the material failure either by stress corrosion cracking or corrosion fatigue. Thus, in this work, the effect of deformation history on the electrochemical behavior of Al–Mg (AA5083) alloy in a 0.6 M NaCl solution was investigated. Initially, the Al5083 samples were deformed using forward Taylor anvil experiments and digital image correlation measurement was performed to characterize the accumulated plastic strain in the samples as well as the effect of higher defect density and fragmentation of coarse cathodic intermetallic particles. Potentiodynamic and cyclic polarization experiments were performed and results showed an increase in corrosion current density and a negative shift in protection potential in impacted samples. This is further confirmed using post corrosion microstructure, which showed a higher probability of trenching around fragmented cathodic particles near the impacted end. Hence, prior deformation history found to alter the electrochemical property to a large extent.

Solute addition is an effective way to enhance mechanical properties, especially in magnesium based alloys due to the limited number of slip systems available for deformation at the room temperature. Hence, the effects of various alloying elements on ideal shear resistance (ISR) across different slip systems of Mg were investigated using first-principles calculations. The addition of a Ce, Y, or Zr solute atom was found to decrease ISR, whereas the substitution of a Sn, Li, Al, or Zn atom increased the ISR of Mg. The most active slip system in Mg changed from the basal partial (0001)\( \left[ {10\bar{1}0} \right] \) to prismatic \( (10\bar{1}0)[11\bar{2}0] \) upon substitution of a Ce, Y, or Zr solute atom, whereas the addition of Sn, Li, Al, or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer provides a quantum insight into the underlying factors influencing the observed softening/strengthening behavior. For instance, the electronic density of states calculation shows that the contribution from d states of Ce, Y, and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. Theoretical analyses were also performed, and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys.

In wrought Mg alloys, such as AZ31 alloy, a strong basal texture is developed during the rolling process, which leads to a high tension–compression anisotropic behavior at room temperature and irrespective of deformation rate. This classic anisotropy mainly arises due to activation of different deformation modes such as slip versus twin.

We present the first results of high-rate tension followed by compressive loading on a textured AZ31 alloy using a novel split-Hopkinson-pressure-bar (SHPB). The traditional-SHPB was modified in order to apply tension followed by compression along the rolling direction within few millisecond differences and a strain-rate of 103 s−1. The initial and post-deformed microstructure was examined by electron backscatter diffraction. The results reveal a reduced tension–compression asymmetry along with a shallower than the traditional sigmoidal-curve, indicating a significant influence of forward tensile loading on the subsequent compression behavior, manifested by the role of adiabatic-heating along with some unique deformation behavior.

Focused ion beam and scanning electron microscope (FIB-SEM) instruments are extensively used to characterize nanoscale composition of composite materials, however, their application to analysis of organic corrosion barrier coatings has been limited. The primary concern that arises with use of FIB to mill organic materials is the possibility of severe thermal damage that occurs in close proximity to the ion beam impact. Recent research has shown that such localized artefacts can be mitigated for a number of polymers through cryogenic cooling of the sample as well as low current milling and intelligent ion beam control. Here we report unexpected nonlocalized artefacts that occur during FIB milling of composite organic coatings with pigment particles. Specifically, we show that FIB milling of pigmented polysiloxane coating can lead to formation of multiple microscopic voids within the substrate as far as 5 μm away from the ion beam impact. We use further experimentation and modelling to show that void formation occurs via ion beam heating of the pigment particles that leads to decomposition and vaporization of the surrounding polysiloxane. We also identify FIB milling conditions that mitigate this issue.

Magnesium (Mg) and its alloys offer great potential for reducing vehicular mass and energy consumption due to their inherently low densities. Historically, widespread applicability has been limited by low strength properties compared to other structural Al-, Ti- and Fe-based alloys. However, recent studies have demonstrated high-specific-strength in a number of nanocrystalline Mg-alloys. Even so, applications of these alloys would be restricted to low-temperature automotive components due to microstructural instability under high temperature creep loading. Hence, this work aims to gain a better understanding of creep and associated deformation behavior of columnar nanocrystalline Mg and Mg–yttrium (Y) (up to 3 at.% Y (10 wt.% Y)) with a grain size of 5 nm and 10 nm. Using molecular dynamics (MD) simulations, nanocrystalline magnesium with and without local concentrations of yttrium is subjected to constant-stress loading ranging from 0 to 500 MPa at different initial temperatures ranging from 473 to 723 K. In pure Mg, the analyses of the diffusion coefficient and energy barrier reveal that at lower temperatures (i.e., T ∼573 K) lattice diffusion dominates the overall creep behavior. Further, we observe a negligible change (within the fitting error) in the overall secondary creep rate with creep activation energy changing from 1.128 to 1.154 eV for 0 to 3 at.% Y, respectively, indicating that stage two creep activity is insensitive to Y for a given grain size. We also present novel results showing that the ( 1 0 1 ¯ 1 ) , ( 1 0 1 ¯ 2 ) , ( 1 0 1 ¯ 3 ) and ( 1 0 1 ¯ 6 ) boundary sliding energies are reduced with the addition of yttrium. This softening effect in the presence of yttrium suggests that the experimentally observed high temperature beneficial effects of yttrium addition is likely to be attributed to some combination of other reported creep strengthening mechanisms or phenomena such as formation of stable yttrium oxides at grain boundaries or increased forest dislocation-based hardening.

Cu6Sn5 intermetallic is an important compound formed during reaction between Sn-rich interconnects and copper metallization. The full elastic constants of Cu6Sn5 were quantified experimentally by Resonant Ultrasound Spectroscopy (RUS). The single crystal elastic properties were modeled by density functional theory. A mesoscale polycrystalline model, incorporating the single crystal constants was compared to the experimental results, yielding excellent agreement.

The immiscible Cu-Ta system has garnered recent interest due to observations of high strength and thermal stability attributed to the formation of Ta-enriched particles. This work investigated a metastable Cu-1 at.% Ta solid solution produced via mechanical alloying followed by subsequent consolidation into a bulk specimen using equal channel angular extrusion at 973 K (700°C). Microstructural characterization revealed a decreased number density of Ta clusters, but with an equivalent particle size compared to a previously studied Cu-10 at.% Ta alloy. Molecular dynamic stimulations were performed to understand the thermal evolution of the Ta clusters. The cluster size distributions generated from the simulations were in good agreement with the experimental microstructure.