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Results for “Grain boundary plasticity”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Extended Rice–Thomson analysis and atomistic simulations revealing grain boundary effects on fracture in refractory high-entropy alloys

Significance This work serves to extend the fundamental ductile vs. brittle fracture theory, specifically the Rice–Thomson criterion, by introducing a grain boundary ahead of an initiating crack which propagates at an oblique angle to impinge the boundary. Atomistic fracture simulations on two refractory complex concentrated alloys, the brittle NbMoTaW and the ductile Nb 45 Ta 25 Ti 15 Hf 15 , demonstrate qualitative correspondence with the extended Rice–Thomson criterion and experimental observations. Abstract Understanding how grain boundaries mediate fracture remains a critical challenge in designing ductile, high-performance refractory alloys. Here, we extend the Rice–Thomson criterion to account for the angle between cracks and the impinging grain boundaries (GBs), capturing the competition between intergranular fracture and dislocation-mediated plasticity. Using machine learning interatomic potentials, we performed molecular statics simulations to probe fracture mechanisms in nanocrystalline NbMoTaW and Nb 45 Ta 25 Ti 15 Hf 15 , each with two different grain sizes, revealing trends consistent with experimental observations and the extended Rice model. Comparison with averaged R-curves for bulk samples demonstrates that GBs enhance ductility in Nb 45 Ta 25 Ti 15 Hf 15 in both grain sizes investigated. In contrast, GBs only locally improve fracture resistance in NbMoTaW when cracks are temporarily pinned at GBs inclined at high angles from the crack, but generally promote brittle intergranular fracture. These contrasting behaviors are attributed to differences in GB cohesion, reflecting clear alloying trends that align with ab-initio calculations and trends observed experimentally. Our results bridge classical fracture theory, atomistic simulations, and experimental observations, providing a comprehensive understanding of the fracture mechanisms in nanocrystalline refractory complex concentrated alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Effect of grain size on iron-boride nanoglasses

Metallic nanoglasses are made of amorphous grains that are separated by lower-density amorphous boundaries, which have been proposed to enhance plasticity through the deflection of cracks and shear bands at interfaces. It has been difficult to experimentally control grain size and interfacial structure to understand their roles in plastic deformation. Here, we fabricate bulk nanoglasses via compaction and sintering of colloidally synthesized amorphous iron-boride nanoparticles. These nanoglasses have amorphous grains with diameters from 116nm to 576nm and were tested using nanoindentation and micropillar compressions. The nanoglass with a grain size of 576 nm shows the highest elastic modulus and hardness of 101 GPa and 7.4GPa, respectively. Transmission electron microscopy reveals that nanocrystals form within the nanoglasses during compaction. Higher nanocrystal density correlates with higher nanoparticle crystallization enthalpy, an increase in plasticity, and a decrease in yield strength. Plastic strain of 5.0%, yield strength of 3.8GPa, and ultimate compressive strength of 2.7–3.8GPa were achieved. Here we show that the compaction of colloidal metallic glass nanoparticles results in robust bulk samples, with mechanical properties similar to that of other iron-based bulk metallic glasses.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Role of grain boundary crystallography on void growth in FCC metals

The growth of an isolated spherical void at and near a grain boundary or a triple junction is studied by means of full-field crystal plasticity simulations with explicit representation of the void. To examine the fundamental aspects of void/grain boundary interactions in these cases, a three-dimensional large-strain elasto-viscoplastic fast-Fourier transform (LS-EVP-FFT) model with axisymmetric tensile periodic boundary conditions was developed. We employ this model to reveal the role of crystallography (grain orientations, grain boundary misorientation and inclination angle with respect to the largest component of the applied stress) and type of loading on void growth. It was found that grain orientation was the primary crystallographic feature governing void growth in strain-rate controlled simulations. For strains small enough to not significantly alter the local geometry, the overall growth of the void in bicrystal boundaries was not strongly affected by grain boundary inclination angle. The relative amount of void growth experienced within each grain in a bicrystal or tri-crystal was strongly dependent on the grain boundary inclination angle. We found a subset of orientations that exhibit faster void growth in bicrystal boundaries than inside the constituent single crystals. Last, we show that when placed primarily in one grain but near the grain boundary, void growth was similar to void growth when placed at the grain boundary. Furthermore, the growing void remained in the original grain and did not grow into the other grain, regardless of crystallographic character.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The effect of microstructure on the relationship between grain boundary sliding and slip transmission in high purity aluminum

The relationship between grain boundary sliding (GBS) and slip transmission is investigated experimentally at grain boundaries in 99.99% aluminum with a through-thickness, coarse grained microstructure deformed in tension at 190 °C. Using scanning electron microscope enabled digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD), high resolution strain fields and microstructural information were measured to examine the influence of microstructural neighborhoods on interactions between GBS and slip transmission and strain localization. Several distinct cases are presented that highlight important microstructural factors that govern deformation near grain boundaries. The findings include (1) direct transmission and GBS were anti-compatible and facilitated by opposing boundary types (low misorientation and high energy grain boundaries respectively); (2) increased GBS activity was correlated with decreased indirect transmission behavior; (3) GBS accommodation at triple junctions was enabled by intragranular plasticity; and (4) the local intragranular plastic strain discontinuity between grains determined the magnitude of GBS gradients. Finally, this work provides insight into the nature of these mechanisms and can be used to identify strain transfer criteria that can lead to improved GBS-sensitive crystal plasticity models.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Crystal plasticity modeling and analysis for the transition from intergranular to transgranular failure in nickel-based alloy Inconel 740H at elevated temperature

The precipitation-strengthened Nickel alloy Inconel® 740H® (IN740H) exhibits increased ductility at higher applied strain rates during quasi-static tensile tests at an elevated temperature of 760°C. The examination of fracture surfaces in this context reveals a noteworthy transition of underlying fracture mechanisms from transgranular to intergranular fracture as the applied strain rate decreases from 1×10 -3 / s to 0.83×10 -4 / s . To thoroughly understand the mechanical response of IN740H under these conditions, this study develops a crystal plasticity finite element (CPFE) model. Further, this model incorporates various deformation mechanisms including dislocation slips, climb, and grain boundary sliding, which are relevant to the test conditions. The model is calibrated using data from both tensile tests at different strain rates and creep tests across a broad stress range at 760°C, enabling the accurate determination of model parameters for each mechanism. Simulation results well captured the experimental observations of different failure modes. At higher strain rates, the model shows a dominance of dislocation slip leading to heterogeneous plastic deformation and formation of transgranular shear bands causing the failure, while at lower strain rates, an increased activity of grain boundary sliding causes grain boundaries crack leading to intergranular failure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Modeling Microstructural Effects on Heterogeneous Temperature Fields within Polycrystalline Explosives

The paper addresses the role of crystal anisotropy on the evolution of heterogeneous temperature fields in plastic-bonded explosives (PBXs) under conditions of weak shock. The modeling approach is based on simplified idealizations of PBX microstructure including RDX grains and estane binder regions subjected to velocity boundary conditions representative of impact conditions in situ. The constitutive description of the microstructure constituents includes a dislocation-based, anisotropic, single crystal plasticity model for the explosive grains and a linear viscoelastic model to represent the estane binder. Large suites of simulations were used to systematically study the correlation in heating of the grains with local wave dynamics, crystal orientation, and the microstructural neighborhood. These correlations were identified through the selection of characteristics of crystal anisotropy including oriented wave speeds and Taylor factor. A key observation is that the wave dispersion within a certain distance from the impact interface plays a dominant role in the temperature field. Beyond this distance, individual crystal orientations play a more dominant role, but cannot entirely account for the observed heterogeneity without consideration of the local microstructure neighborhood.

42 ENGINEERING↗
DOE OSTI2021

Investigating the dislocation reactions on Σ3{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel

Abstract Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Modeling the evolution of slip localization: Realization of link to material strength

Slip localization formation is the chief mechanism underlying the deformation of almost all metals, from pure elements to high-performance superalloys. The intensity of individual slip localizations is often related to the ultimate strain level for failure but not to the strength of the metal. Here we show that across 15 distinct metals, the intensity of slip in individual slip localizations and slip localization spacings are strongly related to material yield strength. Using a three-dimensional crystal plasticity-based micromechanical model that explicitly simulates the growth of discrete slip localizations, we reveal that the stronger the metal, the faster and earlier slip localizations intensify. The relationship is attributed to the formation of a zone that surrounds the slip localization where the driving force for slip is absent. We find that the zone size is controlled by the strength of the neighboring crystal. Consequently, as strength increases, slip becomes increasingly preferred within the slip localization itself and formation of other slip localizations becomes more likely further away.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A finite element formulation for deformation twinning induced strain localization in polycrystal magnesium alloys

Deformation twinning induces shear strain localization in hexagonal close-packed crystals and is critical for the material’s ductility and failure. Cracks often occur at twin-twin or twin-grain boundary intersections and propagate along twin bands. However, most crystal plasticity models for deformation twinning are based on a “pseudo-slip” approach and do not capture the localized deformation associated with the formation of each discrete twin band. The few exceptions are discrete twin models that involve very complex numerical algorithms and are often compromised in accuracy due to the numerical convergence. These factors make the discrete twin models hard to adopt. This paper proposes a modification to the conventional finite element weak form, to fully incorporate a twin-induced heterogeneous deformation that does not depend on the “pseudo-slip” assumption. The model starts by splitting the deformation gradient into elastic-slip-twinning components. The twin-induced deformation gradient component is computed separately by solving a microstructural evolution problem and then implemented into finite element weak form by constructing a global “twin-force” vector. The constitutive update (e.g., in the user-defined material subroutine, or UMAT, for ABAQUS) therefore avoids dealing with the twinning and recovers to the form of a regular slip-based crystal plasticity model. The results presented here indicate that the twin-induced strain localization and the associated stress-reversal phenomena near the twin band were naturally captured in the model, which was validated against an in-situ synchrotron X-ray micro-diffraction experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Size effect in stainless steel thin wires under tension

Miniaturization of modern devices drives the exploration into how materials behave in small dimensions. However, the mechanical behaviors of micro-scale specimens under varied loading conditions are still challenging to characterize and predict. In this work, specimen size-dependent weakening effect was experimentally revealed in 316L stainless steel thin wires under tension with wire diameter decreasing from 200 μm down to 61 μm. The underlying mechanisms were primarily associated with surface grain softening and strain localization, which were largely dependent on the number of grains across wire diameter. Through an analog composite model with surface and interior grains that follows the Hall-Petch relation, the extent of surface grain softening was found to coincide with the flow stress reduction due to lack of boundary strengthening. Further using elasto-plastic self-consistent polycrystal modeling, the stress-strain behaviors of interior and surface grains were well described in analogy to those of bulk materials with similar-size grains and extremely large grains, respectively. Subsequently, the analog composite model well simulated the specimen size-dependent weakening effect under tension. This article provided more insights into the size effect exploration and proposed a practical model for describing size-dependent mechanical behaviors of small-scale materials to be applied to miniaturized devices.

316L stainless steel↗
DOE OSTI2023

Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

Creep mechanisms are studied in θ'-Al 2 Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ' precipitates within grains via the Orowan mechanism, as evidenced by (i) very high stress exponent (n~20-25) and (ii) α-Al and θ' lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ' precipitates. At intermediate stresses, both alloys exhibit a n~3 regime, where α-Al and θ' lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ' is now inactive within the grains. Rather, dislocation motion occurs solely in θ'-precipitate-free zones (θ'-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ'-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n~3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ'-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al 2 Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ'-PFZ. So, high creep resistance in the n~3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ'-PFZ, which effectively share load with the grains.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Stabilized Nanocrystalline Alloys: The Intersection of Grain Boundary Segregation with Processing Science

Processing science for nanocrystalline metals has largely focused on far-from-equilibrium methods that can generate many grain boundaries with excess defect energy. Conversely, the science of stabilizing nanocrystalline alloys has largely focused on the lowering of that excess defect energy through grain boundary segregation, bringing nanocrystalline structures closer to equilibrium. With increasing technological adoption of stabilized nanocrystalline alloys, there is a substantial need for research at the intersection of these two fields. This review lays out the basic thermodynamic issues of the two subfields and surveys the literature on the most common processing methods, including severe plastic deformation, ball milling, physical vapor deposition, and electrodeposition. We provide an overview of studies that have examined grain boundary segregation through each of these methods and identify general themes. We conclude that there is substantial scope for more systematic work at the intersection of these fields to understand how nonequilibrium processing affects grain boundary segregation.

Materials Science↗
DOE OSTI2021

In-situ TEM observation of shear induced microstructure evolution in Cu-Nb alloy

Phase boundaries in multiphase alloys govern defect interaction and chemical intermixing across different phases during plastic deformation. Dynamic interaction of defects with phase boundaries in multiphase alloys, especially for immiscible alloys, has been topic of significant research interest in recent years. In this work, we describe a novel approach for carrying out in-situ TEM shear deformation to directly observe interfacial microstructural evolution of a Cu-Nb alloy. A unique double shear specimen geometry is microfabricated by a focused ion beam technique to apply shear deformation upon push loading inside the TEM. From the real-time observation, we discover that the phase boundary with a zigzag morphology effectively blocks stacking faults nucleated in a Cu grain from slipping into a Nb grain. Meanwhile, the Cu phase bears the most plastic deformation through slip or twinning mechanisms. This work sheds light on understanding the shear deformation and the behavior of phase boundaries in multiphase alloys during shear deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Quantitative characterization of gradient microstructures: A study on friction stir spot processing of pure cobalt

Heterogeneous microstructures in polycrystalline metals can enhance the strength and ductility, outperforming homogeneous structures of similar composition. This study investigates deformed cobalt via friction stir spot processing with varying dwell times to uncover the effects of plastic deformation and heat generation on the formation of morphological, phase, and grain boundary character gradients. A new approach to quantify the morphological gradients in materials, which describes grain morphology in terms of density followed by parametric regression, enables direct quantification of processing depth and gradient sharpness. Results show that longer processing times increase the steepness of morphological gradients and reduce the deformation depth for friction stir spot processing with low plunge depths and high tool rotational speeds. The amount of retained FCC is increased in the shorter processing conditions, primarily due to refined grain size, increased defect content, and reduced heat generation. Crystallographic texture analysis of the HCP phase indicated a dominant B-fiber described by (0001) ∥ shear plane normal in the extreme processing conditions and the formation of a P-fiber, shear direction ∥ ⟨11$\bar2$0⟩ for intermediate dwell times. The texture of the FCC phase for low processing times was a C texture {100}⟨011⟩ where longer processing times were dominated by a [001] fiber texture with a main {110}⟨100⟩ orientation and emergence of a slight [111] fiber in the longest processing condition. The approaches outlined in this work give insight into quantifying gradients and improve the understanding of highly deformed cobalt.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Creep ductility limiting mechanisms in an additively manufactured Al-Ce-Ni-Mn-Zr alloy

Tensile creep response and cavitation damage evolution in an additively manufactured Al-7.5Ce-4.5Ni-0.4Mn-0.7Zr (wt%) alloy with peak-aging and overaging treatments were investigated in the 300–400 ºC range. Microstructural heterogeneity and its response to heat treatment and subsequent creep deformation were studied to understand the interplay between cavity formation, creep lifetime and ductility. Increasing the applied stress activated the nucleation of more cavities, an experimental observation that is well described using the vacancy accumulation model. Cavities nucleated prematurely due to localized plasticity in the denuded zones that formed at/near melt-pool or grain boundaries. Microstructure/deformation heterogeneity with consequent evolution of stress triaxiality, especially at lower stresses, causes accelerated cavitation, thus producing low creep ductility (∼ 0.2–2.4 %), compared to (∼12–21 %) ductility of the alloy measured by regular tensile tests at equivalent temperatures. A constrained diffusional cavity growth mechanism with continuous cavity nucleation during creep is established as the dominant mechanism, implying that cavitation involves vacancy diffusion, yet its growth rate is dictated by the minimum creep rate. In conclusion, the ductility-limiting creep and cavitation mechanisms discussed here provide new insight into the creep behavior of 3D-printed metallic alloys.

Additive manufacturing↗