Vicinity Of Grain Boundaries Research Articles

A Bi-Pronged Attempt at Normalizing DL-EPRT Vis-à-Vis Grain-Boundary/σ-Phase Locales in Thermally Sensitized UNS S32205 Duplex Stainless Steel

In a quest to vet UNS S32205 as a potential structural material to serve moderate-to-high temperature operations of NPP auxiliary components, the DL-EPR test was exploited. A bifronted scheme comprised of 650 and 850 °C discrete treatments intended to explore progressive eutectoid decomposition and degree-of-sensitization (DoS) scenarios was adopted. The nuance witnessed with yet another dual approach—the Cihal- and image processing (IP)-normalized signal landscape—was rationalized through its attribution to culprit microstructures. This was sought, inter alia, in the vicinity of grain boundaries and σ-phase inclusions by virtue of postmortem FESEM, STEM-EDX, HRTEM SAED and XRD ascertainment. Discernable reactivation-kinetics resurgence was believed to mark the onset of deleterious σ-phase dissolution. This only came into fruition with longer ageing times (8–17 h) at 650 °C and succumbed to prematurely (1 h), and at DC biases more cathodic than −0.25 VAg/AgCl with the 850 °C counterpart. Opportune corroboration was offered in ir/ia breakaway for the respective conditions, which was unveiled to be particularly pre-emptive (5 h) with IP- vs. Cihal-normalized peers (8 h) related to the 650 °C condition. Meanwhile, the 850 °C condition endured a similar surge after as little as 1 h of ageing across the board, which hints at concomitant sigma-phase culpability.

Phase-field simulation of the effect of grain boundary on fission gas migration in UO2 fuel

Grain boundaries are widely recognized as line defect sinks. During reactor operation, vacancies and fission gas atoms within UO2 fuel grains migrate to the grain boundaries, forming bubbles at these locations. In order to better understand the effect of grain boundaries on the migration of fission gas atoms, this study employed a phase-field model to simulate the nucleation and growth process of fission gas bubbles within the UO2 fuel grains and in the vicinity of grain boundaries. This study also investigated the degree to which grain boundaries affect fission gas atoms under different temperature conditions. The results indicated that in models containing grain boundaries, the nucleation of fission gas bubbles occurred earlier, as compared to models without grain boundaries. A noticeable bubble denuded zone was also observed adjacent to grain boundary interfaces. Furthermore, with increasing temperature, the bubble denuded zone becomes thicker. The effects of irradiation, vacancy diffusion, Ostwald ripening, as well as grain boundary trapping were discussed.

Controllable precipitation behavior near grain boundaries enabled by artificial strain concentration in age-hardening aluminum alloys

Regulating the precipitation behavior in the vicinity of grain boundary (GB) is crucial for improving the mechanical property and corrosion resistance of age-hardening aluminum alloy. In this work, we innovatively propose a strain-concentration controlled strategy to realize the controllable fabrication of the thinner precipitate-free zone (PFZ), associated with the finer grain boundary precipitates in Al–Zn–Mg–Cu alloy. Specifically, the primary PFZ with certain solute atoms is introduced by short pre-aging treatment, followed by the subsequent cyclic deformation at room temperature to induce the localized plastic deformation in the primary PFZ. Compared with the traditional aging, the Al–Zn–Mg–Cu alloy treated by the current process manifests superior combination of impact toughness and corrosion properties, as well as the slightly improved unidirectional tensile properties. Furthermore, the atomic-scale mechanism of the unique precipitation behavior near GB is revealed using the molecular dynamics simulation. A large number of dislocations are generated in PFZ, which acts as the favorable precipitation sites and contribute to the secondary precipitates in the primary PFZ. The present work provides a novel design route and an atomic-scale understanding for controlling the precipitation behavior in the vicinity of GB, and it has the potential to direct the optimization of mechanical performance and corrosion resistance in a board spectrum of aging-hardening alloys.

Fracture mode transition from intergranular to transgranular in (TiZrNbTaCr)C: The grain boundary purification effect of Cr carbide

(TiZrNbTaCr)C high-entropy carbide ceramics with different Cr content are fabricated by hot-pressing and the mechanical properties are tested. With increasing Cr content, the fracture mode changed from intergranular to transgranular. Nanoindentation tests indicated that this transition is ascribed to the enhancement of grain boundary strength and atom probe tomography (APT) analysis confirmed indeed different grain boundary impurity concentration ascribed to a purification effect of Cr carbide. Besides, the mismatch degree between the atoms at and in the vicinity of grain boundaries as well as the grain size may also impact grain boundary strength. The fracture toughness presents a decline tendency with increasing Cr content, and its relationship with fracture mode transition is discussed.

Development of Graphitic 2024 Al Alloy by Mechanical Alloying

In this study, the 2024 Al powder with different weight fractions of graphite is mechanically milled using a high-energy ball mill for 3 hours each in the nitrogen environment. The milled powder is compacted at an elevated temperature. X-ray diffraction is used to phase analysis of milled powder as well as compacted specimens. Optical microscopy is used for microstructural analysis and hardness measurements are done for the evaluation of mechanical properties. The hot compacted specimens are also tested for their wear properties. Results show that there is no new phase formed during mechanical milling. But, after hot compaction of the milled powder, Al2Cu formed due to precipitation. No reaction is observed between the aluminum and the carbon (graphite) after milling as well as hot compaction. Microstructures of all hot compacted specimens are not showing pores, which, signifies full density after compaction. The formation of Al4C3 is not observed at any stage of processing. Therefore, graphite is uniformly distributed in all specimens, and the same is observed at grain boundaries of α-Al grains in the microstructures. Hardness increases with the addition of 1 wt.% graphite but it decreases with a further increase in graphite. The wear resistance of 2024 Al with 1 wt% graphite is the highest among all the compositions. The high hardness and wear resistance of 2024Al with 1 wt% graphite is the consequence of precipitation of Al2Cu during hot compaction and the presence of graphite which creates hindrances in the metal matrix. The presence of free graphite in the vicinity of grain boundaries acts as a solid lubricant which improves wear resistance of 2024 Al.

The Casting and Hot Forging of Low-Carbon Copper-Bearing Steel and Its Substructural Characterization

The casting of metal alloys followed by hot forging is a widely used manufacturing technology to produce a homogeneous microstructure. The combination of mechanical and thermal energy envisages the microstructural properties of metal alloys. In the present investigation, a metal alloy of composition 0.05C-1.52Cu-1.51Mn (in weight %) was cast in an induction furnace using a zirconia crucible. The melt pool was monitored using optical emission spectroscopy (OES) to maintain the desired composition. The as-cast block was then subjected to forging under a pneumatic hammer of 0.5 t capacity so that any casting defects were eliminated. The as-cast block was reheated to a temperature of 1050 °C and held at that temperature for 6 h to homogenize, followed by hammering with a 50% strain using a pneumatic hammer. The microhardness was calculated using a Vickers microhardness testing apparatus. The microstructure characterization of the processed alloy was carried out using an optical microscope, electron backscattered diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDXA), and a transmission electron microscope (TEM). The sample for optical microscopy was cut using a diamond cutter grinding machine and surface polishing was carried out using emery paper. Further, mechanical polishing was performed to prepare the samples for EBSD using a TEGRAPOL polishing machine. The EBSD apparatus was operated at a 20 kV accelerating voltage, 25 mm from the gun, and with a 60 µ aperture size. HKL Technology Channel 5 Software was used for the post-processing of EBSD maps. The procedure of standard polishing for OES and TEM sample preparation was followed. Recrystallization envisages equiaxed grain formation in hot forging; hence, the strain-free grains were observed in the strained matrix. The lower distribution of recrystallized grains indicated that the driving force for recrystallization was not abundant enough to generate a fully recrystallized microstructure. The fractional distribution of the misorientation angle between 15 and 60° confirms the formation of grain boundaries (having a misorientation angle greater than 15°) and dislocations/subgrain/substructures (having a misorientation angle less than 15°). The fraction of misorientation angle distribution was higher between the angles 0.5 and 6.5°; afterwards, it decreased for higher angles. The substructure was observed in the vicinity of grain boundaries. The softening process released certain strains, but still, the dislocation was observed to be deposited mostly in the vicinity of grain boundaries and at the grain interior. The fine precipitates of the microalloying element copper were observed in the range of size in nanometers. However, the densities of these precipitates were limited and most of these precipitates were deposited at the grain interior. The microhardness of 210.8 Hv and mean subgrain size of 1.61 µ were observed the enhanced microhardness was due to the limited recrystallized grains and accumulation of dislocations/subgrain/substructures.

Inhibiting segregation enabled outstanding combination of mechanical and corrosion properties in precipitation-strengthened aluminum alloys

Segregation of solutes to grain boundary (GB) is the dominant restriction on enhancement of mechanical properties and corrosion resistance during aging in precipitation-strengthened aluminum alloys. Here, we innovatively introduce the cyclic deformation by the vibration during aging to eliminate the GB segregation, resulting in the formation of the narrow precipitate-free zone (PFZ) widths near the GB, as well as the fine and discontinuous grain boundary precipitations (GBPs). Compared with the traditional peak-aging, the 2014 aluminum alloy treated by thermal cooperative vibration aging (TCVA) exhibits a superb combination of impact toughness and corrosion resistance, and retains the strength and ductility. In addition, the atomic simulations show that TCVA generates numerous vacancies near the GB, but does not change the dislocation density with the increase of cycle time. These results indicate that the vacancy significantly promotes the nucleation and growth of precipitates in the vicinity of GB, resulting in the narrow PFZs and fine discontinuous GBPs. The present work provides the fundamental knowledge and method to inhibit the equilibrium segregation of solutes to GB during aging, and further realizes the precipitation-strengthened aluminum alloys with excellent mechanical and corrosion properties.

Room temperature deformation mechanisms of a Fe–Mn–Al–C steel

This study aims to investigate the deformation mechanisms of an austenite-based duplex Fe–Mn–Al–C steel at room-temperature through uniaxial tensile tests, strain rate sensitivity and stress relaxation analysis. Through numerical analysis and microstructural studies, this investigation reveals that increasing strain leads to the formation of low-angle boundaries (LABs) at the vicinity of grain boundaries. Consequently, these LABs play a facilitating role in the stress-assisted diffusion of carbon atoms, thereby triggering the dynamic strain aging (DSA) phenomenon. The evidence for this is substantiated by the strain rate sensitivity coefficient and stress relaxation results obtained from numerical analysis during uniaxial tensile tests. As deformation progresses, the misorientation of LABs increases, influencing the strain hardening behavior. The intensified DSA contributes to enhanced work hardening, while substructure development results in softening, creating a trade-off between these contrasting effects.

Temperature damping capacity and microstructure evolution of Mg–Al–Zn–Sn alloy

The temperature damping capacities of Mg–3Al–1Zn-xSn (x=3, 6, 9) alloys were investigated using a Dynamic Mechanical Thermal Analyzer (DMA) under varied loading frequencies and Sn concentrations. The addition of Sn resulted in a leftward shift of the P1 dislocation damping peak around 80 °C and a rightward shift of the P3 peak around 220 °C, which were attributed to the second phases in the vicinity of grain boundaries. By combining the Granato-Lücke model and the Peguin model, the damping curves were analyzed, and the temperature damping mechanism of Mg–3Al–1Zn-xSn alloy was summarized. Before 220 °C, the addition of Sn significantly enhanced the damping capacity at different frequencies, which was attributed to the increased of point defects and precipitates. The damping mechanism was dominated by microplastic internal friction in the later part of the test. There is a negative correlation between frequency and damping. The observation of microstructures suggested that the dynamic recrystallization, the twining, and the non-basal slipping systems were the sources of the microplastic damping capacity. The dynamic recrystallization mechanism owing to cyclic loading was identified.

Molecular dynamic study of oxygen ion diffusion and grain boundary in SrSc0.1Co0.9O3-δ perovskite solid oxide membrane

• The effects of grain boundaries on the diffusivity of oxygen ions in the SrSc 0.1 Co 0.9 O 3-δ membrane were studied by molecular dynamics simulation . • The reduction of the diffusivity of oxygen ions due to grain boundary was found to be present both perpendicular and parallel to the grain boundary. • Oxygen ions tended to remain in the vicinity of the grain boundary, instead of in the membrane as is the case when there are no grain boundaries. • The diffusivity of oxygen ions in the vicinity of grain boundary in SrSc 0.1 Co 0.9 O 3-δ is almost the same as that in yttria-stabilized zirconia.

Enhanced strength and ductility in sand-cast Al-Li-Cu-Mg-Zr alloy via synergistic microalloying with Sc and Ti

Cast Al-Li alloys show attractive potential for application in the field of manufacturing large-size and complex parts as a result of profound weight reduction and good fluidity. However, the ductility and strength of sand-cast Al-Li alloys are relatively poor due to the low cooling rate of sand casting. In this study, Sc and Ti were added to sand-cast Al-Li-Cu-Mg-Zr casting alloy to investigate the microstructure evolution and tailor the final performance for the first time. The results showed that grain decreased from 189 µm to 56 µm and the needle Cu-rich phase increased in the vicinity of grain boundaries as the total element content of Sc and Ti increased. After aging at 175 °C, the enrichment of the T1 and S′ phases, the finer δ′, the higher number density of core-shell composites, and the narrowed δ′-precipitation free zones (δ′-PFZs) were stimulated by Sc and Ti, leading to a significant increase in strength of the peak-aged alloy. The SZT alloy exhibits the best yield strength of 359 MPa and ultimate tensile strength of 452 MPa with an adequate elongation of 4.6 %. In addition, a Li-rich zone was observed and intersected with T1 phase, which provided a new precipitation path for Li atom diffusion and nucleation of T1 precipitates. The present study suggests that microalloying with Sc and Ti is expected to be an effective strategy to simultaneously improve the strength and ductility of sand-cast Al-Li alloys.

Machine learning-enabled identification of micromechanical stress and strain hotspots predicted via dislocation density-based crystal plasticity simulations

The present work uses a full-field crystal plasticity model with a first principles-informed dislocation density (DD) hardening law to identify the key microstructural features correlated with micromechanical fields localization, or hotspots, in polycrystalline Ni. An ensemble learning approach to machine learning interpreted with Shapley additive explanation was implemented to predict nonlinear correlations between microstructural features and micromechanical stress and strain hotspots. Results reveal that regions within the microstructure in the vicinity of grain boundaries, higher Schmid factors, low slip transmissions and high intergranular misorientations, are more prone to being micromechanical hotspots. Additionally, under combined loading and large plastic deformations, slip transmissions take precedence over intergranular misorientations in formation of both strain and stress hotspots. The present work demonstrates a successful integration of physics-based crystal plasticity with DD-based hardening into machine learning models to reveal the microscale features responsible for the formation of local stress and strain hotspots within the grains and near the grain boundaries, as function of applied deformation states, grain morphology/size distribution, and microstructural texture, providing insights into micromechanical damage initiation zones in polycrystalline metals.

Unsupervised clustering of nanoindentation data for microstructural reconstruction: Challenges in phase discrimination

Automated clustering of input nanoindentation data using a K-means algorithm was performed on dual phase (DP) and high strength low alloy (HSLA) steels with the aim of accurate microstructural classification of the phases present. Electron microscopy and diffraction techniques were used to correlate the clustering result with the microstructure, and for the DP steel an accurate separation of polygonal ferrite, martensite and anomalous indents (i.e. located on grain boundaries or adjacent to martensite islands) was achieved. In direct contrast, K-means did not reveal an accurate separation of the HSLA constituent phases. After comparing the hardness level of selected HSLA grains and the minimum distinguishable hardness level of the DP steel, it is found that K-means input variables should differ from each other by at least 10% in order to achieve reasonable separation of clustering results. Further targeted nanoindentation experiments on polygonal ferrite grain pairs were then performed to investigate the impact of indentation near grain boundaries between two adjacent grains with similar mechanical properties on the K-means clustering analysis. It is shown that the optimal number of clusters could not converge, and consequently, for the HSLA steel and its complex microstructure the limiting factors to obtain a phase classification by using K-means clustering based on nanoindentation input variables are: (i) the influence on the measured hardness in the vicinity of grain boundaries; (ii) the similar mechanical properties of both phases; (iii) the inhomogeneous substructure of granular bainite.

The Role of Strain in Proton Conduction in Multi-Oriented BaZr0.9Y0.1O3-δ Thin Film.

Within the emerging field of proton-conducting fuel cells, BaZr0.9Y0.1O3-δ (BZY10) is an attractive material due to its high conductivity and stability. The fundamentals of conduction in sintered pellets and thin films heterostructures have been explored in several studies; however, the role of crystallographic orientation, grains, and grain boundaries is poorly understood for proton conduction. This article reports proton conduction in a self-assembled multi-oriented BZY10 thin film grown on top of a (110) NdGaO3 substrate. The multiple orientations are composed of different lattices, which provide a platform to study the lattice-dependent conductivity through different orientations in the vicinity of grain boundary between them and the substrate. The crystalline stacking of each orientation is confirmed by X-ray diffraction analysis and scanning transmission electron microscopy. The transport measurements are carried out under different gas atmospheres. The highest conductivity of 3.08 × 10-3 S cm-1 at 400 °C is found under a wet H2 environment together with an increased lattice parameter of 4.208 Å, while under O2 and Ar environments, the film shows lower conductivity and lattice parameter. Our findings not only demonstrate the role of crystal lattice for conduction properties but also illustrate the importance of self-assembled strategies to achieve high proton conduction in BZY10 thin films.

Anisotropy in cyclic behavior and fatigue crack growth of IN718 processed by laser powder bed fusion

This study aims at characterizing the role of strong anisotropy in fatigue crack growth for a superalloy processed by additive manufacturing. The choice has been made to investigate two specimens exhibiting significantly different crystallographic textures, representative of possible variations of microstructures within a real part.Both cyclic mechanical behavior and fatigue crack growth have been investigated at high temperature. As a consequence of the oriented microstructure, high levels of anisotropy of elastic and viscoplastic behaviors were observed. These behaviors have been modeled by the combination of orthotropic elasticity, a Hill’s criterion and linear and non-linear kinematic hardening laws. To assess fatigue crack growth driving forces, in the context of complex crack shape, finite element analysis was proceeded taking care of 3D crack shape description. J was derived from G-θ, this method being convenient for anisotropic material.In most cases, fatigue cracks were found to grow preferentially at the vicinity of grain boundaries, regardless of the crystallographic orientation. An intrinsic fatigue crack growth rate related to this intergranular mechanism has been determined. On the other hand, mostly transgranular crack growth were observed when the loading was applied in the processing direction. In this case, the crystallographic texture appears to be the main factor governing the fatigue crack growth rate.

Nanohardness and indentation fracture resistance of dual-phase high-entropy ceramic

Fine-grained (Ti-Zr-Nb-Ta-Hf)C/(Ti-Zr-Nb-Ta-Hf)B2 dual-phase high-entropy boride/carbide ceramic (HEC/HEB) was prepared from powders synthesized via a boro-carbothermal reduction and sintered by spark plasma sintering. The developed ceramic has a very high density and relatively homogeneous chemical composition of HEC and HEB grains with a size of HEC grains from 0.1 μm to 1.5 μm and HEB grains from 0.1 μm to 5 μm. The nanohardness of the HEC and HEB grains are very high with values of 37.4 ± 2.3 GPa and 43.3 ± 2.9 GPa, respectively. The nanohardness in the vicinity of grain boundaries is 30.0 ± 5.2 GPa. The Young's modulus of HEC with a mean value of 536.5 ± 34.2 GPa is significantly lower in comparison to Young's modulus for HEB grain with a mean value of 766 ± 45.7 GPa. Vickers hardness HV1 of the developed HEC/HEB ceramic is very high with a value of 29.4 ± 2.0 GPa which is the highest between the up-to-now reported dual-phase high-entropy ceramics. The developed system shows good indentation fracture resistance with a value of 3.9 ± 0.62 MPa m1/2. The most significant toughening mechanism is crack branching in larger HEC grains with sizes from 1.0 μm to 1.5 μm.

Diffusion and redistribution of hydrogen atoms in the vicinity of localized deformation zones

The presence of hydrogen atoms in the lattice of polycrystalline metals can lead to hydrogen embrittlement. The diffusion of hydrogen atoms and formation of hydrides depend on several parameters such as the microstructure of the metal alloy, the presence of stress risers, and the applied loading condition. In this study, a non-local crystal plasticity finite element model is coupled with hydrogen diffusion equations to study the effects of such parameters on the stress-assisted diffusion of hydrogen atoms in zirconium polycrystals. The concentrations of hydrogen atoms in the lattice and dislocation trap sites under different scenarios are studied to deconvolute the effects of texture, microstructure, and applied strain on hydrogen diffusion. It is shown that the concentration of hydrogen in the lattice peaks in the vicinity of grain boundaries and most importantly, at the triple-junction points. It is further shown that in localized deformation zones, such as twin or notch tips where dislocations concentrate, the effects of trapped hydrogen content can be significant.

Magnesium stearate surface coverage on tablets and drug crystals: Insights from SEM-EDS elemental mapping

Scanning electron microscopy-based energy dispersive X-ray spectroscopy (SEM-EDS) is proposed as a versatile tool for quantifying surface area coverage (SAC) by magnesium stearate (MgSt) on pharmaceutical tablets and particles. Our approach involved fast elemental mapping and subsequent SAC quantitation by image analysis. The study was conducted using a multi-component system, but the particle-level mapping was limited to active pharmaceutical ingredient (API) crystals. For both tablets and API particles, the calculated SAC against MgSt loading afforded a positive linear correlation over the range of MgSt levels examined in this work. On the tablet surface, MgSt was found to be preferentially concentrated at or in the close vicinity of grain boundaries, supporting the idea of compression-driven migration and relocation of MgSt within the tablet. On the particle surface, only discrete aggregates of MgSt were observed, as opposed to the widely accepted phenomenon of the formation of a thin lubricant film around host particles. The selection of proper SEM-EDS operating conditions and the challenges confronted in particle surface mapping are discussed in detail.

Effects of crystallinity on residual stresses via molecular dynamics simulations.

Mechanical properties of materials are highly dependent on microstructure. One characteristic example is tensile stresses at the grain boundaries, which is one of the most critical factors in crack nucleation. Although experimental techniques have significantly evolved during the past decades with respect to obtaining high-resolution snapshots of the microstructure with methods such as scanning electron microscopy, the quantitative estimation of continuum quantities, such as localized stresses, still remains a very challenging task. The molecular dynamics simulation method has been proven to be a quite effective simulation tool for providing insights in such challenges due to its high spatial and temporal resolution. In this study, molecular dynamics simulations have been performed to obtain a spatial resolution of the residual stresses in solidified aluminum. A best-effort realistic microstructure was obtained by starting from a pure aluminum block which was initially melted and subsequently quenched under various cooling rates, and finally relaxed. The obtained results suggest that residual stresses are higher in absolute terms at the vicinity of grain boundaries than at the grain interiors, and higher crystallinity has been found to be correlated to lower residual stresses. Moreover, it has been shown both qualitatively and quantitatively that grain boundaries undergo tensile loading, in contrast to the grain interiors which are compressed; this result comes to support the conclusions of quite recent experimental investigations, showing that the residual stress is tensile at the grain boundaries and gradually transits into compressive in the grain interiors, and highlights the potential of molecular dynamics simulation to capture nanoscale physical phenomena.

Grain refinement mechanism of SiC nanoparticles/Mg-9 wt. % Al composite investigated by sharp interface model at microscale and nanoscale

The grain refinement mechanism of Mg-9 Al (wt. %) alloy with the addition of SiC nanoparticles (NPs) is studied by a new strategy, a combination of experimental characterizations and sharp-interface model (SIM) simulations. Experiments show that the NPs near grain boundaries form a multilayer coating. Mean grain size is refined with the increase of NPs volume fraction. When the addition level of NPs is raised to 1.0 vol. %, the grain size decreases from 144 µm to 52 µm. The SIM simulations support that the primary interaction between NPs and the moving solid-liquid interface is NPs being pushed to the vicinity of grain boundaries. An equivalent solute-diffusion coefficient for the liquid phase is numerically determined based on iterative SIM simulations. Three possible refinement mechanisms are numerically examined at the microscale and nanoscale. It is the first SIM nanoscale simulation of the Gibbs-Thompson pinning effect. The growth restriction through blocking solute diffusion is identified as the dominant mechanism in the present experiment. The modification of thermal condition and the Gibbs-Thompson pinning effect is less critical. The Gibbs-Thompson pinning is the most promising mechanism for producing nanocrystalline metals with a strict dependence on the uniform dispersion of NPs. This work provides deep insights for controlling grain refinement of Mg matrix composites and shows a new synthetic way, the experiment-sharp interface model, to obtain equivalent solute-diffusion coefficients.