Grain Size Boundary Research Articles

A potential mechanism for abnormal grain growth in Ni thin films on c-sapphire

Normal grain growth (NGG) of a (111) textured Ni film on c-sapphire and abnormal grain growth (AGG) of (100) grains at the expense of this (111) texture has been studied as a function of temperature with and without a capping layer. The grain boundaries (GBs) in the Ni film are controlled by the preferred orientation relationships (ORs) adopted by the Ni grains on the sapphire substrate. The 2 variants of a single OR, Ni(111)<11¯0>//Al2O3(0001)<11¯00>, form a (111) mazed bicrystal with Σ3 GBs. The (100) grains have a single OR, Ni(100)<010>//Al2O3(0001)<11¯00> with 3 variants; their GBs within the (111) grains have the (111)<11¯0>//(100)<010> misorientation.(100) AGG within the (111) mazed bicrystal of the 100 nm Ni film takes place above 1023 K. The orientation transition is driven by the biaxial elastic modulus anisotropy which favors the growth of (100) grains over (111) grains, as this reduces the elastic strain energy induced by the thermal mismatch between Ni and sapphire. (100) AGG is suppressed and the NGG of the (111) texture is slowed down when the film is covered by a 10 nm amorphous alumina layer aimed at inhibiting surface diffusion. Thus, it is proposed that as long as the surface can act as a sink for the point defects diffusing along the GBs, the movement of the GBs is correlated to the diffusivity of atoms and vacancies, which is a function of their misorientation and crystallographic GB structure.

Development and morphological analysis of the zone refining process for high purity germanium

A highly efficient purification process has been developed to enhance the purity of germanium (Ge) up to 13 N from an initial 4N purity. Hall effect measurements demonstrated that part of the zone refined Ge (ZRGe) bars exhibited p-type conductivity and high purity with a net charge carrier concentration of around 4–9 × 1010 cm−3. Noteworthy observations include a rise in carrier concentration (1011 to 1012 cm−3) at a very low zone refining (ZR) velocity of 1 mm/min, which is attributed to the diffusion of impurities from the container into the Ge melt. The purification efficiency was enhanced by optimizing the ZR velocity to around 4–5 mm/min. Purification was carried out at different process steps using graphite and quartz containers. Subsequent microstructural examinations, conducted in conjunction with wet-chemical etching of the sample unveiled the presence of various grains, grain boundaries (GBs), and low & high-density of dislocation clusters: These observations were made in samples that had undergone zone refining under various gas atmospheres including argon, nitrogen and hydrogen. Electron back scattered diffraction (EBSD) analysis reveals the characteristics of multiple grains and grain boundaries (GBs) of Ge refined in argon and nitrogen atmosphere, including low-angle GBs, high-angle GBs with coincide site lattice (CSL) Σ3, Σ9, Σ27a, and random angle GBs. In contrast, the EBSD results of Ge refined in hydrogen atmosphere shows larger grains with only a few low-angle grain boundaries. The materials refined in argon and nitrogen show a lower carrier lifetime (10–30 μs) owing to the presence of various high angle grains boundaries as well as low purity. Conversely, zone refining in hydrogen resulted in the carrier lifetime of around 50 μs in polycrystalline Ge wafers with larger grains.

Disruptive atomic jumps induce grain boundary stagnation

Grain growth in polycrystalline materials can be impeded by grain boundary (GB) stagnation. Using atomistic simulations, we observed that during GB migration, the disruptive jumps of a few GB atoms can disturb the original ordered collective movement of GB atoms, leading to the stagnation of the entire GB. These disruptive atomic jumps can be activated by both high driving forces and high temperatures, with even jumps of a few atoms capable of causing the stagnation of an entire GB. This mechanism also explains the non-Arrhenius behavior observed in some GBs. Additionally, a large model size could increase the rate of disruptive atomic jumps, and a clear transition in thermal behavior is observed with the increase of the GB size in GBs exhibiting clear thermally activated stagnation. Our further investigation shows that the disruptive atoms involved in these jumps do not differ from other GB atoms in terms of atomic energy, volume, density, local entropy, or Voronoi tessellation, and no “jam transition” was observed in the energy barrier spectra. This fact makes those disruptive jumps challenging to detect. To address this issue, we propose a displacement vector analysis method that effectively identifies these subtle disruptive jumps.

Exploring defect behavior in helium-irradiated single-crystal and nanocrystalline 3C-SiC at 800°C: A synergy of experimental and simulation techniques

In this study, single crystal (sc) and nanocrystalline (nc) 3C-SiC samples were subjected to 30 keV He ion irradiation across various doses while maintaining a temperature of 800 °C. Employing techniques including Raman spectroscopy, transmission electron microscopy (TEM), and nanoindentation, the alterations in microstructure and hardness resulting from He irradiation with various fluences were examined.In sc-SiC, irradiation prompted the formation of He platelets, resulting in a hardness increase of 7 GPa. In contrast, nc-SiC, characterized by a higher stacking fault density, exhibited the formation of bubbles, primarily at grain boundaries (GBs), with fewer occurrences within the grain interior, leading to a hardness increase of 1 GPa. Notably, in both sc- and nc-SiC, hardness reached saturation and subsequently stabilized or declined with increasing He fluence.Through molecular dynamics (MD) cascade simulations, we discerned that various planar defects do not uniformly contribute to enhancing radiation resistance. For example, intrinsic stacking faults (ISF) and twins in SiC played a substantial role in altering defect density and configurations, thereby facilitating point defect annihilation. Conversely, extrinsic stacking faults (ESF) and Σ3 GBs had a limited impact on defect production during a cascade.Furthermore, calculations of cluster diffusivity revealed an accelerated movement of He-vacancy towards GBs compared to bulk material and other planar defects. Moreover, the scarcity of point defects and constrained mobility of He atoms towards stacking faults in nc-SiC elucidated the marked tendency of He to form platelets in sc-SiC.Additionally, our findings established a correlation between the calculated indentation hardness and the geometry of He defects, consistent with experimental results from nanoindentation. These results significantly contribute to ongoing efforts to design SiC materials with heightened radiation tolerance.

Effects of mechanical stress, chemical potential, and coverage on hydrogen solubility during hydrogen-enhanced decohesion of ferritic steel grain boundaries: A first-principles study

Hydrogen-enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen embrittlement, a phenomenon that severely impacts structural materials such as iron and iron alloys. Grain boundaries (GBs) play a critical role in this mechanism, where they can provide trapping sites or act as hydrogen diffusion pathways. The interaction of H with GBs and other crystallographic defects, and thus the solubility and distribution of H in the microstructure, depends on the concentration, chemical potential, and local stress. Therefore, for a quantitative assessment of HEDE, a generalized solution energy in conjunction with the cohesive strength as a function of hydrogen coverage is needed. In this paper, we carry out density functional theory calculations to investigate the influence of H on the decohesion of the Σ5(310)[001] and Σ3(112)[11¯0] symmetrical tilt GBs in bcc Fe, as examples for open and close-packed GB structures. A method to identify the segregation sites at the GB plane is proposed. The results indicate that at higher local concentrations, H leads to a significant reduction of the cohesive strength of the GB planes, significantly more pronounced at the Σ5 than at the Σ3 GB. Interestingly, at finite stress, the Σ3 GB becomes more favorable for H solution, as opposed to the case of zero stress, where the Σ5 GB is more attractive. This suggests that, under certain conditions, stresses in the microstructure can lead to a redistribution of H to the stronger grain boundary, which opens a path to designing H-resistant microstructures. To round up our study, we investigate the effects of typical alloying elements in ferritic steel, C, V, Cr, and Mn, on the solubility of H and the strength of the GBs. Published by the American Physical Society 2024

Characterisation of the interplay between microstructure and opto-electronic properties of Cu(In,Ga)S2 solar cells by using correlative CL-EBSD measurements

Cathodoluminescence and electron backscatter diffraction have been applied to exactly the same grain boundaries (GBs) in a Cu(In,Ga)S2 solar absorber in order to investigate the influence of microstructure on the radiative recombination behaviour at the GBs. Two different types of GB with different microstructure were analysed in detail: random high angle grain boundaries (RHAGBs) and Σ3 GBs. We found that the radiative recombination at all RHAGBs was inhibited to some extent, whereas at Σ3 GBs three different observations were made: unchanged, hindered, or promoted radiative recombination. These distinct behaviours may be linked to atomic-scale grain boundary structural differences. The majority of GBs also exhibited a small spectral shift of about ±10 meV relative to the local grain interior (GI) and a few of them showed spectral shifts of up to ±40 meV. Red and blue shifts were observed with roughly equal frequency.

Evolution of microstructure and grain boundaries during annealing of high-purity tantalum materials

The microstructure and texture evolution of tantalum (Ta) during annealing were studied in this work, in addition to simulating and analyzing of the grain boundary motions and their interactions using molecular dynamics simulations (MD). The results showed that the evolution of the recrystallized texture in the surface region of Ta is different from that in the core region. Coincident site lattice (CSL) is significantly related to the grain orientation and thus directly affects the recrystallization texture. MD results showed that the grain boundary mobility rate is higher at low angle grain boundaries (LAGB), which implied that the mobility rate is influenced by the tilt angle. Lower mobility rate of high angle grain boundary (HAGB) possibly caused by a large number of stabilized Σ3 grain boundaries (GBs) between them. It is shown that the inhomogeneous distribution of the recrystallization texture during the annealing process can be possibly attributed to the presence of a large number of Σ3 GBs.

Novel role of thermomechanical grain boundary engineering in the microstructure evolution of austenitic stainless steel

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Impact of edge dislocation and grain boundaries on mechanical properties in CoCrCuFeNi high entropy alloy

This study uses molecular dynamics simulations to explore the mechanical behavior of a CoCrCuFeNi high-entropy alloy (HEA) with Σ5 and Σ13 grain boundaries (GBs) as well as without GBs and dislocation. The analysis focused on understanding the influence mechanisms of these grain boundaries on the mechanical behavior of the HEA. Our findings reveal that the atomic size disparity among the constituent elements induces lattice distortion, leading to deformation in HEAs. The determined elastic constants met Born stability requirements, ensuring mechanical stability across both the examined GBs. Higher elastic moduli were associated with increased strength and stiffness, particularly evident in HEAs with Σ5 GB, surpassing those of non-GB structures. Notably, GB Σ5 demonstrated enhanced strength and hardness, indicated by larger elastic moduli compared with those of non-GB structures. Conversely, GB Σ13 exhibited increased Cauchy pressure and Poisson and Pugh's ratios. The ductility of face-centered cubic HEAs was found to be significantly influenced by the GBs, affecting mechanical properties. The Kleinman parameter highlighted a bending-type bonding with reduced strength at the GBs. Machinability indices indicated high machinability of the CoCrCuFeNi alloy, further enhanced by the presence of the GBs. Direction-dependent parameters underscored the anisotropic nature of the HEA, mitigated by the GBs. Overall, this study elucidates the nuanced influence of different GBs on the mechanical properties of HEAs, offering valuable insights for materials design and applications. The results of this investigation shed light on HEAs with improved mechanical properties via GB engineering.

Effect of fission defects on tensile strength of U3Si2 Σ5(210) grain boundary from first-principles calculations.

U3Si2 is regarded as a promising accident tolerant fuel (ATF) to replace the commercial fuel UO2; however, grain boundary (GB) embrittlement of U3Si2 caused by irradiation-induced defect segregation remains to be clarified. In this work, the U3Si2 Σ5(210) symmetrically tilted GB is taken as a representative to elucidate the individual effect of xenon (Xe) and vacancy on the tensile strength and failure of GBs using first-principles calculations. Compared with the predicted segregation energies of defects at the most energetically favourable positions of GBs, Si vacancy (VSi) has a much stronger preference to segregate to GBs than that of Xe substitution on the Si sublattice (XeSi). Moreover, the strengthening/embrittlement potency of GBs with single vacancy/Xe is evaluated using the first-principles-based uniaxial tensile test. Although both VSi and XeSi yield a weakening effect on the strength of the U3Si2 Σ5(210) GB, such defective GBs exhibit significantly stronger interface strengths compared to the corresponding defects segregated to the UO2 Σ3(111) GB. The underlying mechanism of strength change of U3Si2 GBs is discussed in terms of charge analysis. Our results can provide a fundamental understanding of the mechanical behavior of irradiated GBs from an atomic perspective.

Hydrogen Video Imaging System for Visualizing Microstructure-Dependent Hydrogen Diffusion Behaviors in Polycrystalline Metals

Hydrogen diffusion in polycrystalline metals is known to depend on microstructure. The hydrogen-microstructure interactions have been extensively studied last decades, but the microstructure-dependent hydrogen diffusion behavior is still unclear. To clarify the hydrogen diffusion behavior, it is necessary to visualize the time variations of the microscopic hydrogen distribution in polycrystalline metals. Thus, we developed a hydrogen video imaging system (HVIS) in this study to analyze the hydrogen distribution in polycrystalline metals with sub-micrometer spatial and video-rate time resolutions. The inexpensive and atmospheric-control-free system is expected to be widely used as a versatile approach for analyzing the hydrogen-microstructure interaction in detail.Pure polycrystalline Ni (thickness: 125 µm) was used as a specimen. The specimen was electrochemically polished at a constant voltage of 50 V in a mixed solution of perchloric acid and acetic acid. The thickness was approximately 110 µm after polishing. Subsequently, a polyaniline layer (PANI) was polymerized on one side of pure Ni. The specimen was set to the acrylic cell, which is used to preserve a 0.1 M NaOH aqueous solution. So, only the bare Ni side of the specimen was exposed to the 0.1 M NaOH solution. Electrochemical hydrogen charging was conducted at the bare Ni side of the specimen at a constant current density of -10 A / m2. An inverted optical microscope was used to observe the color distribution of the PANI side. In our previous studies,1-3) it has been clarified that PANI changes its color due to the reaction with hydrogen in metals. Thus, the distribution of hydrogen permeated through pure polycrystalline Ni can be observed as a color distribution of PANI during hydrogen charging.The color of PANI was purple before the hydrogen charging. After 20 h of hydrogen charging, the color of PANI partially turned white. If PANI reacts with hydrogen, the color changes from purple or dark blue to yellow or transparent.1,2) Therefore, the local whitening of PANI is thought to be caused by the reaction with atomic state hydrogen permeated through pure polycrystalline Ni. The number of color changed areas increased with time during hydrogen charging. After 120 h of hydrogen charging, the specimen was immersed in a 1 M NaOH aqueous solution to remove PANI. Subsequently, the microstructure of pure polycrystalline Ni was analyzed using FE-SEM and EBSD. It was found that the color changed area of PANI corresponded to the grain boundaries (GBs) of pure Ni. This suggests that the GBs in pure Ni are preferential hydrogen diffusion paths. Furthermore, the clear color change of PANI was observed at the random GBs, while the whitening scarcely occurred at the Σ3 GBs, indicating that the preferential hydrogen diffusion mainly occurred at the random GBs. The results showed that HVIS can clarify the microstructure-dependent hydrogen diffusion in polycrystalline metals.

Grain boundary- and triple junction-induced martensitic transformations: A phase-field study of effects of grain boundary width and energy

An original thermodynamically consistent large strains-based general multiphase phase-field (PF) approach of Ginzburg–Landau type is developed for studying the grain boundary (GB)- and triple junction (TJ)-induced martensitic transformations (MTs) in polycrystalline materials at the nanoscale considering the structural stresses within the interfaces. In this general PF approach, N+1 order parameters are used for describing the austenite (A)↔martensite (M) transformations and N(>1) martensitic variants and other M order parameters are used to describe M(>1) grains in the polycrystalline samples. Neglecting the kinetics of the GBs and TJs and assuming a uniform temperature of the sample, the system of coupled mechanics and Ginzburg–Landau equations for the order parameters related to MTs is derived. Distinct energies for the GBs in austenitic and martensitic phases due to their structural rearrangement are considered by using variable energy for the GB regions as a function of the order parameter related to the A↔M transformation and misorientations. The strong effects of the austenitic GB width (size-effect), the difference of GB energies of two phases, and applied strains on heterogeneous nucleation of the phases and their subsequent growth are explored in bicrystals with a symmetric planar tilt GB during the forward and reverse transformations. A rich plot for the temperatures of transformations between A, premartensite (the intermediate states between A and M), and M in a bicrystal is plotted for varying austenitic GB width exhibiting a strong GB size effect. The TJ energy and the energy and width of the adjacent GBs are also shown to significantly influence the nucleation and microstructures using the tricrystals having three symmetric planar tilt GBs meeting at 120° dihedral angles. The elastic and structural stresses across the interfaces are plotted, which is essential for understanding the role of GBs and TJs in material failure. The plausible reasons for the nonintuitive behaviour of the GBs on martensite nucleation observed in experiments and atomistic studies are explained.

Heterogeneity of grain boundary properties in Cu2ZnSnS4: A first-principles study

Using first-principles density functional calculations, we investigate the structure and properties of previously unstudied grain boundaries (GBs) in the solar absorber material copper-zinc-tin-sulfide (CZTS). We identify four stable low-Σ value symmetric tilt GBs with low formation energies: Σ3 (111) and Σ5 (201), each with two different GB terminations. Compared to CdTe and CuInSe, GBs in the quaternary semiconductor CZTS exhibit a wider variety of electronic states due to the more complex chemical environment near the GB, including under-coordinated atoms and dangling bonds. Further analysis confirms that strong dangling bonds introduce deep gap states in all GBs studied. We also investigate segregation and electronic properties of intrinsic point defects to GBs and find that one of the Σ3 (111) GBs exhibits an abnormal defect segregation behavior that favors Cu-poor (Zn-rich) GB composition, which is beneficial for its overall performance.

First-principle studies of oxidation effects on grain boundary strength in nickel

Nickel-base alloys are susceptible to stress corrosion cracking (SCC), in which the fracture mainly propagates along oxidized grain boundaries (GBs). To understand how oxidation degrades GB strength, first-principle-based density functional theory (DFT) calculations are conducted to study three types of interfaces: partially oxidized GBs, fully oxidized GBs, and metal-oxide interface, using Ni as a model system. For partially oxidized GBs, both substitutional and interstitial oxygen atoms of different coverages are inserted at three Ni GBs: Σ3(111) coherent twin, Σ3(112) incoherent twin, and Σ5(210) high-angle GB. The results show that the GB strength decreases almost monotonically with the increasing oxygen coverage at all GBs. Typically, substitutional oxygen causes a stronger embrittlement effect than interstitial oxygen, except at the Σ3(111). In addition, the oxygen-induced mechanical distortion has a smaller contribution to the embrittlement than its chemical effect except for high-coverage oxygen interstitials at the Σ3(111). For the fully oxidized GBs, three NiO GBs of the same types as in Ni are studied. Although the strengths of Σ3(112) and Σ5(210) NiO GBs are much weaker than the Ni counterparts, the Σ3(111) NiO GB has a higher strength than that in Ni. Finally, the strength of a Ni/NiO metal-oxide interface is calculated, which is found to be the weakest one among all interfaces studied in this work, suggesting that metal-oxide interfaces could be favorable crack initiation sites.

Initial oxidation on 12 grain boundaries in pure TiN coating and on (Al,Si,Cr) doped Σ3 (111) grain boundaries: A first-principles calculations

In this work, using first-principles calculation method, the effect of twelve <110> symmetric tilt grain boundaries (GBs) on the initial oxidation behavior of TiN was studied. Firstly, the calculated GB energies show that for the TiN (1–10) surface with GBs from Σ3 to Σ27, Σ3 (111) GB possesses an extremely small GB energy (0.404 J/m2), while the other GBs possess a high energy (2.681∼4.834 J/m2). The electronic structure results show that the intrinsic characteristic of GB with high energy is well associated with the extra DOS peak existing in high energy region. Secondly, the oxygen adsorption results show that for TiN surfaces without GB, the order of adsorption energy of O is (100) > (110) > (111). When TiN (1–10) surface with different GBs, the sites at GBs are chemically more favorable sites for oxygen. The degree of weakening of oxidation resistance varies depending on the type of GBs, due to the apparent difference in oxygen adsorbability. Moreover, an interesting discovery is that Σ3 (111) GB provides a possibility to enhance the oxidation resistance of TiN, attributed to the easy introduction of Σ3 (111) GBs and suppressed inward progression of oxidation. Finally, the doping investigation in Σ3 (111) GB shows that among the three doping elements (Al, Si, and Cr), Si doping can further improve the oxidation resistance of (1–10) surface with Σ3 (111) GB in TiN, owing to the easy introduction of Si and formation of diffusion barrier layer of silicon oxides.

Atomic-scale observation of dynamic grain boundary structural transformation during shear-mediated migration.

Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of [Formula: see text]-type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of [Formula: see text]-type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.

Helium bubble growth in tungsten nanotendrils

In order to explore the growth mechanism of helium-induced fuzz beyond the initial stage, molecular dynamics simulations have been used to systematically study the growth of helium bubbles in tungsten nanotendrils, with particular attention to the effects of grain boundaries (GBs). The results show that helium bubble growth under the influence of GBs in tendrils has distinct features compared to bubble growth in the bulk or planar surfaces without GBs. While tall adatom islands are observed in the model of Σ3 and Σ7 GB structures, short and wide adatom islands are formed in the model of Σ5 and Σ17 GB structures. The difference in the resulting surface morphology is mainly due to the type of the dislocation lines punched out by the bubbles in different models. The results can help to explain the formation of branch structures in the fuzz tendrils as observed in experiments.

Grain boundaries induce significant decrease in lattice thermal conductivity of CdTe

Semiconductors are promising in photoelectric and thermoelectric devices, for which the thermal transport properties are of particular interest. However, they have not been fully understood, especially when crystalline imperfections are present. Here, using cadmium telluride (CdTe) as an example, we illustrate how grain boundaries (GBs) affect the thermal transport properties of semiconductors. We develop a machine-learning force field from density functional theory calculations for predicting the lattice thermal conductivity (LTC) via equilibrium molecular dynamics simulations. The LTC of crystalline CdTe decreases with the relationship of κL∼1/T in the simulation temperature range of 300 – 900 K, in which the isotropic LTC decreases from 3.34 to 0.23 W/(m⋅K) due to the enhanced anharmonicity. More important, after introducing GBs, the LTC is suppressed in all directions, especially in the direction normal to the GB planes. More severe LTC suppression occurs in CdTe with Σ9 GB than that with Σ3 GB at 300 K, decreasing by 92.8% and 61.4% along the direction normal to the GB planes compared to the isotropic LTC of the crystalline CdTe, respectively. The decreased LTC is consistent with the weaker bonding near GB planes and lower shear modulus of the defective material. The analyses of the phonon dispersion curves, vibrational density of states, and phonon participation ratio indicate that the decreased LTC mainly arises from phonon scattering at GBs. Overall, our work highlights that GBs can greatly influence the LTC of semiconductors, thus providing a promising approach for thermal property design.

First-principles investigation on electronic structures and energetic characteristics of γ/γ tilt grain boundaries in γ-TiAl intermetallic

The energetic and electronic characteristics of γ/γ tilt grain boundaries (GBs) in γ-TiAl intermetallic were comprehensively analyzed using first-principles methods. The interfacial bonding energy, strength, and features of GBs were quantitatively and qualitatively described by the formation energy, excess free volume (EFV), electron localization function (ELF), bonding charge density (Δρ), and density of states (DOS). The formation energies and EFV of Σ3, Σ11, and Σ5 GBs are relatively low in sequence among different GBs. The electronic structure analyses manifested that the smaller the local lattice strain, the stronger the Ti-d-Al-p bonding, the weaker Al-p-Al-p bonding nearby the GB interface, and the lower the GB formation energy. Further, the interface structure of Σ11 GBs in Widmanstätten structure and that of Σ5 GBs in feathery structure were firstly characterized by TEM. Based on the analyses, the interfacial energetic mechanism of metastable transformations, and possible alloying effects were discussed. This work provides an insight into not only the atomic and electronic basis of GBs but also the understanding of the mechanism of metastable transformations and interface engineering in γ-TiAl alloy.

Disconnection-mediated Twin/Twin-junction migration in FCC metals

We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (CTBs) and a Σ9 GB in copper. As for all GBs, disconnection theory implies that multiple modes/local mechanisms for CTB migration are possible and that the mode selection is affected by the nature of the driving force for migration. While we observe (HRTEM and MD) CTB migration through the motion of pure steps driven by chemical potential jump, other experimental observations (and our simulations) show that stress-driven CTB migration occurs through the motion of disconnections with a non-zero Burgers vector; these are pure-step and twinning-partial CTB migration mechanisms. Our experimental observations and simulations demonstrate that the motion of a GB drags its delimiting TJ and may force the motion of the other GBs meeting at the TJ. Our experiments and simulations focus on two types of TJs composed of a pair of CTBs and a Σ9 GB; a 107° TJ readily migrates while a 70° TJ is immobile (experiment, simulation) in agreement with our disconnection theory even though the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We also demonstrate that disconnections may be formed at TJs (chemical potential jump/stress driven) and at GB/free surface junctions (stress-driven).