Microstructural evidence for convection in high-silica granite

High-silica (>70 wt% SiO2) granites (HSGs) are critical carriers of tin, copper, and other melt-incompatible elements, yet much remains unknown about the mechanisms responsible for their formation. One of the key issues is the apparent lack of evidence for crystal-melt segregation (e.g., modal layering), without which little can be inferred about the dynamics (or lack thereof) of crystallizing HSGs. We examined the crystallographic orientation relationships of clustered quartz crystals from the 300-m-thick Bobbejaankop sill, Bushveld Complex, South Africa. We report an inward increase in the number density and size of quartz clusters toward the central horizon of the sill, coinciding with a significant increase in concentrations of tin, copper, and tungsten. The majority of crystal pairs within each cluster exhibit coincident-site lattice orientation relationships, representing low grain-boundary energy configurations. These clusters must have formed by synneusis in a magmatic environment where crystals could have moved freely, rotating into low-energy orientations on contact. We argue that this not only demonstrates that 100-m-scale crystal-poor and liquid-rich regions can be present in bodies of HSG, but also that such bodies can undergo long-lived convection during crystallization, driven by downwards movement of crystal-rich plumes at the roof, without significant crystal-melt segregation. This dynamic behavior provides a mechanism to homogenize major-element distribution across HSGs and to concentrate highly incompatible and economic elements into central mineralized horizons.

Interplay of Microstructural Parameters On β-Phase Distribution in Al 5xxx Alloys

Al-Mg alloys undergo sensitization when exposed to elevated temperatures, making them susceptible to intergranular corrosion and stress corrosion cracking. Most of the existing research on microstructure effects on sensitization is centered on the effect of intrinsic grain boundary characteristics such as misorientation angle and coincident site lattice (CSL) values. Very few studies have systematically investigated the influence of extrinsic characteristics such as dislocation density. In this paper, the influence of local microstructure characteristics on the sensitization susceptibility of AA5456 was investigated using in situ optical microscopy corrosion experiments and electron back scattering diffraction (EBSD) analysis. The results show a clear trend between the local geometrically necessary dislocation (GND) density and β phase precipitation, with higher GND densities correlating with higher rates sensitized boundaries. This trend held true even for low angle grain boundaries. These results demonstrate the importance of considering factors beyond grain boundary characteristics in determining susceptibility to sensitization.

Complete Goss Secondary Recrystallization by Control of the Grain Size and Texture of Primary Recrystallization in Grain-Oriented Silicon Steel

Matrix microstructure and texture controlling is an important way to optimize Goss ({110}<001>) abnormal grain growth (AGG) in high magnetic induction grain-oriented silicon (Hi-B) steel during primary recrystallization. In the present work, a matrix with homogeneous grain size and favorable texture components was obtained through two-stage normalized annealing followed by primary recrystallization. Furthermore, secondary recrystallization was performed for sharp Goss orientation by slow heating and purified annealing. It was found that plenty of island grains, which occurred and disappeared gradually, accompanied the process of AGG. Through analyzing the evolution of microstructure and texture, we realized that the formation of island grains was related to the large-size grains in matrix, and the elimination of that was attributed to the special grain boundaries which satisfied both coincident site lattice (CSL) and high-energy (HE) models. It was essential to control grain size and favorable orientations in matrix comprehensively for the high-efficient abnormal growing of sharp Goss orientation, through which excellent magnetic properties could be obtained simultaneously.

Crystallographic Peculiarities of β-α Transformation in Brass Induced by Hot Extrusion

Structure-texture states in brass rods after hot extrusion and air-cooling have been investigated with the orientation microscopy (EBSD). In the examined samples, a significant concentration of β-phase with the lattice, close to bcc and fcc α-phase, has been detected. The β-phase texture consisted of the main components: two close to {110}<110> and {001}<110>. The α-phase texture consisted of the main components: close to {001}<100> and two close {110}<111>. The analysis of crystallographic relationship of the texture components of β-and α-phases demonstrates that they may all be obtained, in accordance with the orientation relations, which are intermediate between the Kurdjumov-Sachs and Nishiyama-Wasserman types It is assumed that β-α transformation began in β-phase at coincident site lattice Σ3 and Σ33a boundaries.

Relationship between mechanical properties and age-hardening behavior around grain boundaries of Al-Mg-Si alloy

Nanoindentation tests were conducted near the grain boundary (GB) of the Al-Mg-Si alloy, and the influence of GB character on the aging precipitation behavior and the mechanical properties was confirmed. After obtaining the GB characters by electron back scattered diffraction (EBSD) analysis, nanoindentation tests were carried out on under-aged, peak-aged, and over-aged samples. And then, the indentation areas were observed by back scattered electrons imaging (BSE) in order to identify indentation positions with respect to the GB. In this study, for the GB character, focusing on the rotation angle, the high-angle GB (HAGB) and the low-angle GB (LAGB) were selected. In addition, coincident site lattice GBs (CSL) were selected as the special GB. In the 180°C under-aged samples, the nano-hardness near GB is higher than that far from GB, while 180°C peak-aged samples, the nano-hardness is lower than that far from GB. Then the range near the GB where the hardness changes was larger at HAGB than at LAGB and CSL3. This suggests that the GB character affects the aging precipitation behavior and mechanical properties.

First-Principles Study of the Impact of Grain Boundary Formation in the Cathode Material LiFePO4

Motivated by the need to understand the role of internal interfaces in Li migration occurring in lithium-ion batteries, a first-principles study of a coincident site lattice grain boundary in LiFePO4 cathode material and in its delithiated counterpart FPO4 is performed. The structure of the investigated grain boundary is obtained, and the corresponding interface energy is calculated. Other properties, such as ionic charges, magnetic moments, excess free volume, and the lifetime of positrons trapped at the interfaces are determined and discussed. The results show that while the grain boundary in LiFePO4 has desired structural and bonding characteristics, the analogous boundary in FePO4 needs to be yet optimized to allow for an efficient Li diffusion study.

Hydrogen Environment Assisted Cracking of a Modern Ultra-High Strength Martensitic Stainless Steel

A modern martensitic stainless steel (Ferrium® PH48S™) resists hydrogen environment assisted cracking (HEAC) in aqueous NaCl at ultra-high yield strengths (1,400 MPa to 1,600 MPa). HEAC is transgranular, because of increased steel purity and La addition, compared to severe intergranular HEAC in Custom® 465-H900 without rare earth elements. Minimum threshold for HEAC (KTH) is low (8 MPa√m to 17 MPa√m) for each steel under substantial cathodic polarization. Transgranular HEAC occurs along martensite packet and {110}α′-block interfaces in PH48S, likely a result of H decohesion enabled by localized plasticity. Martensite transformation produces a large area of coincident site lattice interfaces in the refined microstructure of PH48S. However, a susceptible network of random packet/block interfaces is connected in 3D to limit interface engineering. Nanoscale strengthening precipitates in PH48S reduce effective H diffusivity to the mid-10−10 cm2/s range, because of reversible H trapping with a binding energy of 12 kJ/mol. This diffusivity reduces the Stage II growth rate by 1 to 3 orders of magnitude compared to C465 and carbide strengthened ultra-high strength steels. PH48S and C465 are nearly immune to HEAC when cathodically polarized by 50 mV to 500 mV, attributed to a minimum in occluded-crack tip overpotential for H production. The breadth of this protective-potential window increases with decreasing steel strength. Increased Cr does not degrade HEAC resistance, suggesting that crack passivity dominates cation acidification to reduce H production and/or uptake. A quantitative decohesion model effectively predicts the potential dependence of da/dtII using crack tip H solubility reverse calculated from a KTH model.

A Predictive Framework for Dislocation-Density Pile-Ups in Crystalline Systems With Coincident Site Lattice and Random Grain Boundaries

Evolving dislocation-density pile-ups at grain-boundaries (GBs) spanning a wide range of coincident site lattice (CSL) and random GB misorientations in face-centered cubic (fcc) bicrystals and polycrystalline aggregates has been investigated. A dislocation-density GB interaction scheme coupled to a dislocation-density-based crystalline plasticity formulation was used in a nonlinear finite element (FE) framework to understand how different GB orientations and GB-dislocation-density interactions affect local and overall behavior. An effective Burger's vector of residual dislocations was obtained for fcc bicrystals and compared with molecular dynamics (MDs) predictions of static GB energy, as well as dislocation-density transmission at GB interfaces. Dislocation-density pile-ups and accumulations of residual dislocations at GBs and triple junctions (TJs) were analyzed for a polycrystalline copper aggregate with Σ1, Σ3, Σ7, Σ13, and Σ21 CSLs and random high-angle GBs to understand and predict the effects of GB misorientation on pile-up formation and evolution. The predictions indicate that dislocation-density pile-ups occur at GBs with significantly misoriented slip systems and large residual Burger's vectors, such as Σ7, Σ13, and Σ21 CSLs and random high-angle GBs, and this resulted in heterogeneous inelastic deformations across the GB and local stress accumulations. GBs with low misorientations of slip systems had high transmission, no dislocation-density pile-ups, and lower stresses than the high-angle GBs. This investigation provides a fundamental understanding of how different representative GB orientations affect GB behavior, slip transmission, and dislocation-density pile-ups at a relevant microstructural scale.