Boundary Facets Research Articles

Twin networks in hexagonal close-packed metals: morphology, connectivity, and incompatibilities

The mechanical response of hexagonal close-packed metals is mediated by the formation of three-dimensional twin domain networks. To assess the morphology of these networks and statistically characterize their fingerprint, a three-dimensional twin network in cryogenically compressed high-purity Ti was reconstructed using serial sectioning electron backscatter diffraction (EBSD). Adjoining intergranular twin pairs, high order twin intersections (e.g. triple junctions of twins), and twin network hubs with over 15 separate intergranular and intragranular contacts are found to be salient morphological features of these networks. Using kinematics, the interfacial incompatibilities arising from these twin contact at grain boundaries are studied. The analysis reveals that numerous misaligned paired twin configurations generate high incompatibilities, likely resulting from the contact between two initially distinct twin chains. These configurations generally induce higher incompatibilities than the ones of the individual twins, equivalent to introducing a dense wall of dislocations at twin-grain boundary facets with a dislocation spacing on the order of 1.3 nm or less. In contrast, the shear incompatibilities resulting from the formation of aligned twin pairs are generally reduced compared to non-transmitting twins. Further, twin triple junctions and branches – consisting of intergranular contact of three co-located twins – induce higher incompatibilities on average compared to adjoining twin pairs and represent sources of high shear localization in the domain network.

Transitioning of deformation mode in bicrystalline Al-Mg alloy with Σ3 [1 1 1] 60° {11 8 5} faceted grain boundary

In this letter, the authors have elucidated the effect of alloying biocompatible and lighter magnesium (Mg) solute atoms in a bicrystalline aluminium (Al) solvent incorporated with a faceted coherent-incoherent grain boundary (GB) to analyze the tensile deformation mechanisms. Cryogenic temperature and high strain rate conditions were imposed through molecular dynamics simulations to report the detrimental effect of Mg addition on the tensile strength of Al-Mg alloys. Evidence of transitioning in deformation modes for pure Al and Al-Mg alloys was seen. On elaborating, for pure Al, GBs acted as the dislocation nucleation site; whereas for Al-Mg alloy, it was both the GBs and grains. It was observed that stacking faults (SFs) originated from GBs and propagated toward grains in Al. In contrast, in Al-Mg alloys, SFs were generated simultaneously from both GBs and grains. This simultaneous nucleation of dislocations and SFs from both GBs and grains renders bicrystalline alloys lower the elastic limit than pure metal. Conversely, beyond this elastic limit, the alloys exhibit higher flow stress than the pure metal due to the Mg-dislocation interactions. These findings offer valuable insights for tailoring the mechanical properties of Al-Mg alloys to meet aerospace, automotive and marine engineering-related application requirements.

Microstructural evolution in polycrystals with faceted boundaries

Various types of grain growth behavior have been observed in polycrystals with faceted boundaries. The grain growth behavior observed in many different ceramic as well as metallic systems has been well explained by the mixed mechanism principle of grain growth and microstructural evolution. However, relatively few studies have considered repetitive grain growth in materials. To examine the generality of the microstructural evolution principle, two cases of repetitive grain growth during solid-state sintering are considered, one metallic (Ni) and one ceramic system (BaTiO3). The observed repetitive grain growth behavior with an increased sintering temperature (in the case of Ni) and sintering time (in the case of BaTiO3) supports well the generality of the mixed mechanism principle.

The Role of Extrusions and Intrusions in the Initiation and Intergranular Growth of Fatigue Cracks

The role of extrusions and intrusions in the initiation and growth of intergranular fatigue cracks is studied by performing strain‐controlled fatigue experiments on polycrystalline copper and by analyzing the development of intergranular crack initiation and resulting fracture surfaces. Fatigue cracks initiate either in the persistent slipbands within a grain or on the grain boundaries. The grain boundary initiation mechanism is due to the production of extrusions and intrusions on the grain boundaries that decrease the grain boundary cohesion forming submicroscopic crack nuclei. The alternating extrusions and intrusions are found also on the grain boundary facets of the fracture surface of a growing fatigue crack. Up to three slip systems are identified on the facets of cracked grain boundaries. Their form and height are found using scanning electron microscope observations and focused ion beam sectioning. A mechanism similar to intergranular grain boundary crack initiation is considered to explain the formation of grain boundary facets. A novel mechanism of intergranular fatigue crack growth is proposed based on the damaging effect of the extrusions and intrusions produced in the cyclic plastic zone of the growing fatigue crack.

Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters.

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying a high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like domains with deformed QD layers, which originate at the first AlN buffer/superlattice interface and propagate vertically. The cones originate at a 30°-faceted shallow pit in the AlN, which appears to be associated with a threading dislocation that produces strong shear strain. The cone-like structures present Ga enrichment at the boundaring facets and larger QDs within the conic domain. The bimodality of the luminescence is attributed to the differing dot size and composition within the cones and at the faceted boundaries, which is confirmed by the correlation of microscopy results and Schrödinger-Poisson calculations.

Synergistic effects of temperature and strain rate on tensile properties of simulated Ni-6Cu alloy with Σ3 non-Arrhenius grain boundary

ABSTRACT Comprehending the mechanical response of materials on an atomic level is pivotal in the optimisation of advanced materials with superior mechanical properties. This research article utilises the atomistic-scale based molecular dynamics simulations to report the uniaxial tensile behaviour of bicrystalline Ni-6Cu (Nickel – 94% and Copper – 6%) alloy incorporated with pre-existing faceted Σ3 [111] 60° {11 8 5} grain boundaries. The primary aim of this investigation is to comprehend the performance of bicrystalline Ni-6Cu alloy under varying thermodynamic conditions and to assess the influence of pre-existing faceted grain boundaries on its tensile behaviour. This work encompasses a range of strain rates (108 to 1010 1/s) and temperatures (spanning from 100 to 900 K) for the uniaxial tensile deformation simulations. The outcomes unveil that the Young’s modulus of Ni-6Cu alloy (with pre-existing faceted grain boundaries embedded in its domain) was inversely proportional to temperature and constant with respect to strain rate. For the same configuration, yield stress was inversely and directly proportional to temperature and strain rate, respectively. Interestingly, incipient plasticity in the tensile stress–strain response was observed at lower temperature and lower strain rate. From the microstructural point of view, at lower temperatures, the incoherent twin boundary served as a source for the nucleation of stacking faults; however, as the temperature increased, both the incoherent twin boundary and the tips of coherent twin boundary function as the source for stacking faults formation. Our simulations also verified the GB’s anti-thermal (or non-Arrhenius) migration behaviour even under tensile load.

Deformation and boundary motion analysis of a faceted twin grain boundary

In this article, molecular dynamics simulations are used to understand how a nickel bicrystal with faceted incoherent Σ3 grain boundaries responds to uniaxial tensile loading. The deformation response is studied over a wide range of temperatures (100 – 900 K) and strain rates (107 – 1010 s−1). The dislocation extraction algorithm and common neighbor analysis are employed to identify the deformation mechanisms. Our results reveal that the yield stress decreases with temperature and increases with strain rate; whereas the elastic modulus decreases with temperature and is independent of strain rate. Furthermore, incipient plasticity is detected ahead of the yield point at lower temperatures and lower strain rates. Interestingly, the incoherent twin grain boundaries are quite mobile under the uniaxial tensile loading at lower temperatures and lower strain rates. But this mobility decreased at higher temperatures and higher strain rates, thereby, confirming this faceted grain boundary's non-Arrhenius (anti-thermal) migration behavior even under mechanical loading. From a deformation perspective, the incoherent twin facet of the grain boundary served as the major source for stacking fault formation at lower temperatures and higher strain rates. However, with the increase in temperature, the stacking faults became shorter and originated from both the incoherent twin facet and the tips of coherent twin facet. These results are in qualitative agreement with the experimental results documented in the literature.

Exploring the Local Optoelectronic Properties of ALD Grown TiO2 Photoelectrode-Coatings with Atomic Force Microscopy Methods

Photoelectrochemical cells (PECs) hold great promise as an environmentally friendly method of converting sunlight into valuable fuels. However, the semiconductor components of PEC photoelectrodes are susceptible to corrosion under photoelectrochemical conditions.In order to protect the surface of these semiconductors from degradation, a thin overlayer of Titanium dioxide (TiO2) is often applied by atomic layer deposition (ALD). In addition to their protecting function, the physicochemical properties of these thin films significantly affect the overall performance of the device. The local nano-scale optoelectronic characteristics, which considerably influence their macroscopic properties, have rarely been reported in the literature.AFM methods constitute a powerful tool to investigate photo-induced charge separation, charge transport and recombination on photoelectrode surfaces with nanometer resolution.In this context, we have employed Tunneling Conductive AFM (TUNA) and Kelvin Probe Force Microscopy (KPFM) in the dark and under illumination to investigate the surface of ALD-grown TiO2. The synthesis conditions determine the morphology, which has a strong impact on the local optoelectronic properties. TUNA measurements reveal higher photogenerated currents on the grain and facet boundaries, showing that these regions can be favorable conduction pathways. Based on KPFM experiments we can quantify the more efficient photo-induced charge separation on the crystalline TiO2 inclusions, which generate a significant surface photopotential (~450 mV) upon UV-illumination. Categorization of different regions of surface topography based on morphology allows us to analyze the evolution of surface potential over time and extract localized time constants for carrier dynamic processes on the films. This spatially resolved information provides valuable insight to guide the development of macroscopically stable and efficient photoelectrodes through the engineering of microstructure at the nano scale.

Unravelling the atomistic-scale insights into tensile response of equiatomic cupronickel alloy with pre-existing faceted grain boundary interface

The aim of this research work is to investigate the effect of strain rates and temperatures on the mechanical properties of an equiatomic copper-nickel (Cu–Ni) alloy with a pre-existing faceted Σ3 [111] 60° {11 8 5} grain boundary. Molecular dynamics simulations conducted in this scrutiny focuses on understanding the tensile behaviour of bicrystalline equiatomic Cu–Ni alloys (denoted by Cu50Ni50) under various thermodynamic conditions. The uniaxial tensile deformation simulation was carried out at various strain rates (108, 109 and 1010 1/s) in conjunction with different conditions of cryogenic temperature (100 K), room temperature (300 K) and high temperature (500 K). The authors found that the yield stress and Young's modulus of the bicrystalline Cu50Ni50 alloy were highest at the cryogenic temperature, and decreased as the temperature increased. To add on, the yield stress of the bicrystalline Cu50Ni50 alloy increased with an increase in the strain rate value. From the microstructural and dislocation analysis point of view, it was observed that the formation of dislocations was lowest at the cryogenic temperature. Interestingly, at the yield point, the coherent twin boundary tip act as the nucleation site for the dislocations for cryogenic temperature condition. These findings shed light on the relationship between the temperatures, strain rates, and the mechanical properties of Cu–Ni alloys, which will aid in reporting the optimized conditions for their desired engineering applications. It is also proposed that tailoring of the tensile properties is achievable by exposing Cu–Ni alloys to various environmental conditions (cryogenic, ambient, and elevated temperatures with different applied strain rates).

Insights on grain boundary effects on crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates: a molecular dynamics simulation study

Mechanical behavior of Nb3Sn coatings is inherently correlated with the anti-interference ability of the superconducting cavity system and the amplitude and phase stability of the cavity field. We report on atomic-scale simulation and analysis of grain boundary effects on the crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates. For tensile-deformed single crystal Nb3Sn, accompanied by the occurrence of slips, micro voids propagate along the () and (210) planes (stretching the crystal along the [100] direction), and subsequently coalesce, leading to the cracks formation. For tensile-deformed single-crystal Nb3Sn coating on Nb substrate, it fails by crack propagation in the coating before showing significant yielding plateaus. In the Nb3Sn coating, the slip bands on the (201) and () planes meet and merge, inducing the breaking of atomic bonds and the formation of voids at the slip band intersections, and the subsequent micro-crack initiation. The misfit dislocations of Nb3Sn/Nb heterostructure interface and the dislocations nucleated on slip systems control the plastic deformation of the Nb substrate, causing the BCC-to-FCC and FCC-to-HCP phase transformation in the substrate, for which, ductile fracture occurs after necking. For polycrystalline Nb3Sn coating on Nb substrate, the simulations demonstrate that cracks propagate entirely along grain boundaries, exhibiting the intergranular fracture characteristics. The composite exhibits a significant strain rate effect. At high strain rates, crack propagation across grain boundaries in Nb3Sn is hindered, dislocations are emitted into the both adjacent grains from the grain boundary facets, they glide across the grain, and voids are nucleated at the intersections of dislocations inside the grains, which leads to the formation of voids and ultimately the occurrence of trans-crystalline fracture. The findings provide important information about the crack evolution in Nb3Sn coatings, which are critical to the fabrication and performance analysis of Nb3Sn superconducting radio frequency cavities.

Σ3 grain boundary dynamics studied by atomistic spherical bicrystal modeling

In this study, we propose an atomistic spherical bicrystal model that employs the synthetic driving force method to investigate the dynamics of curved interfaces. The model allows tracking the migration and faceting of spherical grain boundaries. As a demonstration case, a simulation for a grain boundary with Σ3 misorientation is performed. A subsequent analysis of interface energy and stiffness is performed, whereby it is found that the dynamics of [111] and segments in the Σ3 boundary plane fundamental zone follow energy and stiffness trends, while others, e.g., the segment, do not.

Investigation of the Abnormally Large Grains of a Super304H Heat-Resistant Steel Tube over Long-Term, High-Temperature Service

Abstract Abnormally large grains (ALGs) were detected in the outer walls of Super304H superheater steel tubes of an ultra-supercritical unit in long-term service. In this study, the microstructure and mechanical properties of steel tube samples were investigated to reveal the formation mechanism of the ALGs. The results showed that the room-temperature and high-temperature strength and plasticity of the coarse-grain zone were much lower than those of the fine-grain zone. Inhomogeneous residual strain distribution in the outer wall led to high distortion energy differences in the as-supplied steel tube. The low-distortion austenite grains annex the neighboring high-distortion grains via the strain-induced grain boundary migration mechanism. Moreover, the two-dimensional step mechanism of the faceted grain boundaries also causes the ALGs to form.

Weakly imposed Dirichlet boundary conditions for 2D and 3D Virtual Elements

In the framework of virtual element discretizations, we address the problem of imposing non homogeneous Dirichlet boundary conditions in a weak form, both on polygonal/polyhedral domains and on two/three dimensional domains with curved boundaries. We consider a Nitsche’s type method (Nitsche et al. 1970; Juntunen et al. 2009) and the stabilized formulation of the Lagrange multiplier method proposed by Barbosa and Hughes in Barbosa and Hughes (1991) . We prove that also for the virtual element method (VEM), provided the stabilization parameter is suitably chosen (large enough for Nitsche’s method and small enough for the Barbosa–Hughes Lagrange multiplier method), the resulting discrete problem is well posed, and yields convergence with optimal order on polygonal/polyhedral domains. On smooth two/three dimensional domains, we combine both methods with a projection approach similar to the one of Bramble et al. (1972). We prove that, given a polygonal/polyhedral approximation Ωh of the domain Ω, an optimal convergence rate can be achieved by using a suitable correction depending on high order derivatives of the discrete solution along outward directions (not necessarily orthogonal) at the boundary facets of Ωh. Numerical experiments validate the theory.

Irradiation-induced grain boundary facet motion: In situ observations and atomic-scale mechanisms.

Metals subjected to irradiation environments undergo microstructural evolution and concomitant degradation, yet the nanoscale mechanisms for such evolution remain elusive. Here, we combine in situ heavy ion irradiation, atomic resolution microscopy, and atomistic simulation to elucidate how radiation damage and interfacial defects interplay to control grain boundary (GB) motion. While classical notions of boundary evolution under irradiation rest on simple ideas of curvature-driven motion, the reality is far more complex. Focusing on an ion-irradiated Pt Σ3 GB, we show how this boundary evolves by the motion of 120° facet junctions separating nanoscale {112} facets. Our analysis considers the short- and mid-range ion interactions, which roughen the facets and induce local motion, and longer-range interactions associated with interfacial disconnections, which accommodate the intergranular misorientation. We suggest how climb of these disconnections could drive coordinated facet junction motion. These findings emphasize that both local and longer-range, collective interactions are important to understanding irradiation-induced interfacial evolution.

Atomic-scale understanding of the reversible HCP↔FCC phase transition mechanisms at [formula omitted] twin tip in pure titanium

In this paper, by using high-resolution transmission electron microscopy (HRTEM) observations and molecular dynamics (MD) simulations, the mechanisms of hexagonal close-packed (HCP) structure to face-centered cubic (FCC) structure transition and its inverse transition at the {101¯1} twin tip in pure titanium were investigated. The HCP→FCC phase transition at the {101¯1} twin tip in cold-rolled pure Ti was experimentally observed for the first time. On the other hand, under electron beam irradiation, the FCC lamella shrank gradually and eventually recovered to the HCP structure. After the HCP↔FCC phase transformations, the interfacial structures of the twin tip changed from the (101¯0)T//(101¯3)M facets to a common Basal-Pyramidal (BPy) facets. MD simulations indicated that the reversible HCP↔FCC phase transitions were caused by successive slip of Shockley partial dislocations on {0001}HCPand{111}FCC planes, respectively. The nucleation of Shockley partial dislocations stemmed from the dissociation of twinning dislocation (TD) with the Burgers vector of b⇀4 = 4γ2−93+4γ2<101¯2¯> at the twin tip. The Shockley partial dislocations could be absorbed by twin tip boundaries through reacting with the residual dislocations to form new TDs b⇀4 during the FCC→HCP phase transition. The periodic serrated feature of the (101¯0)T//(101¯3)M facets of the twin tip boundaries in pure Ti was discovered via atomistic simulations. The interfacial structure evolution of the twin tip was caused by a series of dislocation reactions and generation of phase transformation at the twin tip. The transformation from the HCP structure to the FCC structures was due to the stress concentration caused by the plugging of the periodically distributed TDs b⇀4 at the twin tips in the case of external loading. The shrinkage of FCC lamella was caused by the relaxation of internal stress at the twin tip during the thermal relaxation by electron beam.

Effect of Nitrogen Partial Pressure on the Structural, Mechanical, and Electrical Properties of (CrHfNbTaTiVZr)N Coatings Deposited by Reactive Magnetron Sputtering

Multi-element (CrHfNbTaTiVZr)N coatings were prepared through the magnetron sputtering of an equimolar CrHfNbTaTiVZr alloy target. This study determined the influences of N2-to-total (N2 + Ar) ratios (RN) on the composition, structure, mechanical properties, and electrical performance of the coatings. Coating thickness decreased from 898 nm to 128 nm with increasing RN from 0% to 100%. The alloy coating has bundles of fibrous structures with remarkable void boundaries. The coating changed from amorphous phase to face-centered cubic (FCC) phase with (111) preferred orientation, then to FCC phase with (200) preferred orientation, and finally to near-amorphous phase as RN increased from 0% to 100%. The microstructure of the nitride coatings transformed from a columnar structure with rough faceted tops and void boundaries into a dense and small structure with smooth domed tops. The grain size of the nitride coatings also decreased with RN. Accordingly, the electrical performance at high RN was poor. The nitride coating deposited at RN = 60% had the highest hardness of 16.6 GPa and the lowest friction coefficient of 0.52, owing to structural densification and grain refinement.

Domain Engineering Enabled Giant Linear Electro‐Optic Effect and High Transparency in Ferroelectric KTa1−xNbxO3 Single Crystals

A giant linear electro‐optic (EO) effect and high transparency in ferroelectric potassium tantalate niobate [(), KTN] crystal is achieved via a thermally controlled domain engineering method. It involves a two‐step thermal annealing process: 1) a rapid cooling process that forms polar nano‐regions (PNRs), i.e., a cooling rate of from to where is the Curie temperature; and 2) a slow cooling process that facilitates abnormal domain growth (AGG) i.e., a cooling rate of from to . Since PNR can have a faceted boundary and high anisotropy, it can promote AGG within single crystals to realize solid‐state domain conversion macroscopically from a multi‐domain to single‐domain crystal within a slow cooling process. The resultant KTN crystal offers high transparency that is equivalent to its paraelectric phase; and a linear EO coefficient () as large as , which is five times the value of conventional KTN crystals with similar composition. This giant linear EO coefficient represents a major technical advance in EO materials and significantly reduces the driving voltage, power, and footprint of many types of EO devices.

Faceting of twin interfaces in rolled pure magnesium

ABSTRACT Recent studies proved that the faceting of twin boundaries is a common feature in hcp materials. It was demonstrated that the asymmetrical facets in the most frequent and twin interfaces are the consequence of the twin boundary migration mechanism. We demonstrate that a similar mechanism is also applicable to less common twins, observed in pure magnesium rolled at room temperature. The migration of twin boundary is mediated by disconnections, which have Burgers vector and step height equal to four interplanar distances. The mechanism of asymmetrical facets nucleation is also demonstrated by using atomistic simulations. The simulations are supported by transmission electron microscopy (TEM) of asymmetrical basal-pyramidal interfaces of twins.

DEVELOPMENT OF PLASTIC DEFORMATION IN TWO-DIMENSIONAL COPPER POLYCRYSTALS WITH DIFFERENT TWIN STRUCTURE

Experimental studies of regularities of occurrence and development of substructural and orientational heterogeneity of polycrystalline copper foils with different twin structure in the process of their deformation in conditions of active tension have been carried out. It is shown that by selecting the regime of thermal treatment of fine-grained copper foil three types of samples with different twin structure can be obtained. The first type is characterized by the presence of twins, which differ in size, shape and type of boundaries. Samples of the second type contain twins with faceted boundaries. Samples of the third type contain parallel twins with rectilinear boundaries. Such a variety of twin structures not only predetermines the difference of mechanical characteristics for samples of different types but also regularities of substructure and orientation changes accompanying the plastic deformation of samples. The use of a highly sensitive imaging method that allows in situ characterization of substructural and orientational heterogeneity during deformation and other methods (optical and raster microscopy, interferometry) allowed us to detect specific features of the course of relaxation processes in samples of different types.

Exploiting Knowledge Graph for Multi-faceted Conceptual Modelling using GCN

The relevant information obtained from multiple sources usually contributes to one intricate phenomenon in the industrial processes. Data fusion of different sources usually leads to more expressive and informative information than that of each single data source. Integrated information has been widely used to model a multi-faceted conceptual phenomenon, which provides a comprehensive and versatile view of understanding of the process. However, the conventional approaches concatenate feature vectors to integrate different facets, not considering the semantic gaps between them. Meanwhile, knowledge graph (KG) receives considerable attention in recent years as it comprises rich relational information among elements. Thus, KG provides a promising way to fuse multiple data sources by bridging the semantic gaps, which can be exploited in the modelling of a multi-faceted phenomenon. Inspired by the advancement of KG, we proposed an approach based on KG and a machine learning algorithm for multi-faceted modelling. Firstly, a domain-specified ontology was built to eliminate the varying distance metrics across facet boundaries, and KGs were generated by populating the data surrounding a multi-faceted phenomenon into this ontology. Secondly, the KGs were fed into a graph convolutional neural network (GCN) to learn the node features and the graph structure for graph embedding simultaneously with the shared parameters. Lastly, with the aim of multi-faceted conceptual modelling, the features obtained from the GCN model were used as inputs for machine learning algorithms to learn the hidden patterns of KGs. An experimental study using real-world data from the cold rolling process was conducted to demonstrate the feasibility of the proposed model.