Grain Boundary Complexion Research Articles

Discovery of Ni activated sintering of MoNbTaW predicted by a computed grain boundary diagram

Activated sintering of refractory metals represents a classical phenomenon in powder metallurgy. This study discovers that Ni addition can enhance sintering of MoNbTaW both above and below the bulk solidus composition at 1800 °C, thereby demonstrating the first example of activated sintering of a high-entropy alloy. To probe the underlying mechanism, experiments reveal complete grain boundary (GB) wetting above the bulk solidus composition, which logically infers the stabilization of a liquid-like interfacial phase (GB complexion) in a prewetting region below the bulk solidus composition. Furthermore, bulk CALPHAD (calculation of phase diagram) methods have been extended to model GBs in Ni-doped MoNbTaW to compute a GB “phase” diagram to forecast high-temperature GB disordering in the prewetting region and rationalize the observed Ni-activated sintering of MoNbTaW. Only weak GB segregation of Ni is found in furnace-cooled specimens, which suggests GB “drying” during cooling, consistent with the observed nanoscale precipitates along GBs.

Genetic algorithm assisted multiscale modeling of grain boundary segregation of Al in ZnO and its correlation with nominal dopant concentration

Grain boundary (GB) segregation of Al in ZnO plays an important role in lowering its thermal conductivity for thermoelectric applications. However, the effect of Al concentration on the GB complexions and their transition is not well understood. Herein, a genetic algorithm assisted multiscale modelling framework was used to study the role of GB concentration on the GB segregation of Al on five special twin GBs of ZnO. A critical concentration of 5–6 atoms/nm2 was determined for the complexion transition from single layer to multilayer. Calculated segregation energies were used in a phenomenological model to link GB concentration with the nominal concentration of dopants. The model was used to calculate the nominal solubility of Al in ZnO as a function of grain size, which was validated with the experimental data from the literature. The proposed framework provides a path for establishing GB-structure – property correlation and thereby, predictive dopant engineering of ceramics.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.

A novel electromagnetic shock treatment to selectively modify grain boundary and improve the corrosion resistance of aluminium alloy

A novel electromagnetic shock treatment (EST) is put forward to selectively modify grain boundary (GB) microstructures and further improve corrosion resistance of T73-Al7075 alloys. Electrochemical tests and GB microstructure characterization by TEM (HRTEM) are conducted. The results show that EST with appropriate electromagnetic pulse energy being utilized can modify GB microstructure and enhance the corrosion resistance of T73-Al7075 alloy. In EST sample, GB precipitates (GBPs) dissolve and coarsen simultaneously, and stripy/wavy GBs with nanometer scale are more commonly observed which indicates that GBs relax within nanometer scale and even promote nonlinear grain bridging. The transformation of GBPs as well as the transition of GB complexions improve the corrosion resistance of EST sample. This provides a promising method to selectively modify GB microstructure instantaneously and improve related properties of solid alloys.

Grain boundary relaxation induced ultrastrong-and-ductile bulk pure Ni

Both strength and ductility are essential for high-performance engineering structural materials, thus great endeavors have been invested to solve the strength-ductility trade-off of them during recent two decades. Here, we utilized grain boundary (GB) relaxation to circumvent the trade-off in bulk pure Ni through optimizing grain size when literatures tells that GB relaxation can improve strength but ductility. Both tensile strength and uniform elongation of Ni were elevated from 1450 MPa to 2013 MPa and from 2.33% to 5.17%, respectively. The combination of strength and ductility extends beyond the range established by the strongest bulk pure Ni known. Our investigation unraveled that the increased strength was produced by GB relaxation which remarkably mitigated GB softening produced by GB sliding, and that partial dislocation emission from GBs became the dominant plastic deformation mechanism. The dislocation activities inside the grains increased the strain hardening rate (θ). At the same time, enhanced probability of interaction between dislocations and GBs improved the strain rate sensitivity (m). It was the GB relaxation induced dislocation activities that improved θ and m, which stabilized the plastic deformation to enhance ductility. The present work offers a foundation for the ongoing of more advanced engineering structural materials with the advisable design of GB complexion.

Relating the kinetics of grain-boundary complexion transitions and abnormal grain growth: A Monte Carlo time-temperature-transformation approach

Grain boundaries undergo thermally-activated, first-order transitions that result in discontinuous changes of interfacial properties. Importantly, grain boundary transitions lead to changes in bulk material properties (e.g., embrittlement) and/or behavior (e.g., abnormal grain growth). Numerous studies have been completed on the equilibrium states of grain boundaries and their transitions (i.e., complexion transitions), but there have been far fewer investigations of complexion transition kinetics; complexion transitions occur on the atomic-scale and are therefore challenging to detect experimentally. In this work, a 3D Potts grain growth model with stochastic complexion transitions was employed to investigate complexion transition kinetics. A Johnson-Mehl-Avrami-Kolmogorov (i.e., JMAK) approach was used to extract nucleation and growth rates (i.e., transformation rates), while point process analyses and correlation functions were used to infer complex interrelated nucleation and growth events. Time-temperature-transformation (TTT) diagrams, in particular grain-boundary complexion, transformed grain, and abnormal grain TTT diagrams, were constructed to summarize the progress of complexion-related transformations. Such diagrams relate complexion-induced grain growth to the underlying complexion transitions and, in the case of abnormal grain growth (AGG), permit one to assess the role of AGG as a temperature-dependent, time-displaced indicator of complexion transitions. Overall, this work details a theoretical framework that can be used to better understand complexion transition kinetics as well as to develop tools for the design of bulk microstructures.

Complexions and stoichiometry of the 60.8°//[100](011) symmetrical tilt grain boundary in Mg2SiO4 forsterite: A combined empirical potential and first-principles study

AbstractIn this work we explore the low-energy complexions of the symmetrical tilt grain boundary (GB) 60.8°//[100](011) in forsterite through molecular dynamics and first principles calculations. Using a conservative sampling, we find six stoichiometric complexions with energies ranging from 0.66 to 1.25 J/m2. We investigate the segregation of MgO vacancy pairs, and find that in most cases it is more favorable for the vacancies to lie within the GBs than in the surrounding crystals, leading to new atomic structures. From these results we infer that at finite temperature when vacancies are present in the system, GBs are likely to absorb them and to be non-stoichiometric. We find many GB complexions containing a free oxygen ion, which may have profound implications for geological processes.

Survey of Grain Boundary Energies in Tungsten and Beta-Titanium at High Temperature.

Heat treatment is a necessary means to obtain desired properties for most of the materials. Thus, the grain boundary (GB) phenomena observed in experiments actually reflect the GB behaviors at relatively high temperature to some extent. In this work, 405 different GBs were systematically constructed for body-centered cubic (BCC) metals and the grain boundary energies (GBEs) of these GBs were calculated with molecular dynamics for W at 2400 K and β-Ti at 1300 K and by means of molecular statics for Mo and W at 0 K. It was found that high temperature may result in the GB complexion transitions for some GBs, such as the Σ11{332}{332} of W. Moreover, the relationships between GBEs and sin(θ) can be described by the functions of the same type for different GB sets having the same misorientation axis, where θ is the angle between the misorientation axis and the GB plane. Generally, the GBs tend to have lower GBE when sin(θ) is equal to 0. However, the GB sets with the <110> misorientation axis have the lowest GBE when sin(θ) is close to 1. Another discovery is that the local hexagonal-close packed α phase is more likely to form at the GBs with the lattice misorientations of 38.9°/<110>, 50.5°/<110>, 59.0°/<110> and 60.0°/<111> for β-Ti at 1300 K.

Grain boundary segregation transitions and critical phenomena in binary regular solutions: A systematics of complexion diagrams with universal characters

A systematics of grain boundary (GB) segregation transitions and critical phenomena has been derived to expand the classical GB segregation theory. Using twist GBs as an example, this study uncovers when GB layering vs. prewetting transitions should occur and how they are related to one another. Moreover, a novel descriptor, normalized segregation strength (ϕseg), is introduced. It can represent several factors that control GB segregation, including strain and bond energies, as well as misorientation for small-angle GBs (in a mean-field approximation), which had to be treated separately in prior models. In a strong segregation system with a large ϕseg, first-order layering transitions occur at low temperatures and become continuous above GB roughing temperatures. With reducing ϕseg, the layering transitions gradually merge and finally lump into prewetting transitions without quantized layer numbers, akin to Cahn's critical-point wetting model. Furthermore, GB complexion diagrams with universal characters are constructed as the GB counterpart to the classical exemplar of Pelton-Thompson regular-solution binary bulk phase diagrams.

Demonstration of ultrahigh thermoelectric efficiency of ∼7.3% in Mg3Sb2/MgAgSb module for low-temperature energy harvesting

<h2>Summary</h2> Thermoelectric harvesting of low-temperature waste heat offers great opportunities for sustainable energy production. However, the investigations of related thermoelectric materials and modules remain sluggish. Here, we reported a great advance in the n-type Mg₃Sb_1.5Bi_0.5 system by minor Cu additions. Some Cu atoms preferentially occupy interstitial sites within the Mg₃Sb₂ lattice and significantly modified phonon modes via filling in the phonon gap and increased anharmonic phonon scattering, thereby leading to the anomalously low thermal conductivity. Simultaneously, the detrimental behavior of thermally activated electrical conductivity was completely eliminated through grain-boundary complexion engineering. These two critical roles contributed to the remarkable improvement of <i>zT</i>. Based on this developed high-performance material coupled with p-type α-MgAgSb-based material, a fabricated thermoelectric module rivaling long-time champion Bi₂Te₃, demonstrated a record-high conversion efficiency ∼7.3% at the hot-side temperature of 593 K. These results pave the way for low-temperature thermoelectric harvesting.

First-order grain boundary transformations in Au-doped Si: Hybrid Monte Carlo and molecular dynamics simulations verified by first-principles calculations

Hybrid Monte Carlo and molecular dynamics simulations (hybrid MC/MD) reveal the occurrence of first-order phase-like adsorption transformations in Au-doped Si twist grain boundary (GB) at low temperatures, which become continuous at high temperatures above a GB critical point. The predicted first-order GB transformations from nominally “clean” GBs to bilayer adsorption of Au are supported by a prior experiment. The hexagonal Au adsorption pattern predicted by the hybrid MC/MD simulation is further verified by first-principles calculations. Differential charge density maps indicate that strong charge transfer at the Au-doped Si GB. A GB complexion (interfacial phase) diagram is computed.

Influence of Grain Boundary Complexion on Deformation Mechanism of High Temperature Bending Creep Process of Cu Bicrystal

Despite of substantial advancement, the effect of grain boundary (GB) complexions on high temperature creep deformation process has not been fully understood. In this paper, we have studied the high temperature bending creep deformation of copper bicrystal with various GB complexions under different loads using molecular dynamics simulation. It has been found that specimen with normal kite GB complexion have better creep resistance properties when subjected to comparatively lower applied load. In case of monolayer Zr segregation, a drastic decrease in creep strength as well as creep plasticity is observed due to inhibition of GB migration. On the other hand, deviation between creep properties for specimen with split-kite GB complexion and split-kite bilayer Zr segregation GB complexion is minimal. Enhanced creep plasticity is observed in case of split-kite bilayer Zr segregation GB complexion, which is due to formation of interpenetrating icosahedral clusters in the necking region. Fracture in specimen with monolayer Zr segregation GB complexion has occurred by means of slip phenomenon at lower deformation load whereas amorphization and necking is observed at higher deformation load. In case of specimen with bilayer Zr segregation GB complexion, it is found that fracture has occurred through amorphization and necking at all deformation loads due to higher GB thickness.

Effect of grain boundary complexions on the deformation behavior of Ni bicrystal during bending creep.

The dependence of creep deformation behavior of nickel bicrystal specimens on grain boundary (GB) complexion was investigated by performing a simulated bending creep test using molecular dynamics methods. Strain burst phenomena were observed during the low temperature [500K, i.e., <0.3 * melting point of nickel (Tm)] bending creep process. Atomic strain and dislocation analyses showed that the time of occurrence of strain burst depends on how easily GB migration happens in bicrystal specimens. Specimens with kite monolayer segregation GB complexion were found to be stable at low temperature (500K), whereas specimens with split-kite GB complexion were stable at a comparatively higher temperature (900K). In case of further elevated creep temperatures, e.g., 1100K and 1300K, split-kite GB complexion becomes unstable and leads to early failure of the specimen at those temperatures. Additionally, it was observed that split-kite bilayer segregation and normal kite GB complexions exhibit localized increases in elastic modulus during bending creep process, occurring at temperatures of 1100K and 1300K, respectively, due to the formation of interpenetrating icosahedral clusters. Graphical abstract Representative creep curves during bending creep deformation of various grain boundary complexions at 900 K.

First-Order Interfacial Transformations with a Critical Point: Breaking the Symmetry at a Symmetric Tilt Grain Boundary.

First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric ∑5 (210) tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid MonteCarlo and molecular dynamics simulations. Density functional theory calculations confirm this prediction. This first-order coupled structural and adsorption transformation, which produces two variants of asymmetric bilayers, vanishes at an interfacial critical point. A GB complexion (phase) diagram is constructed via semigrand canonical ensemble atomistic simulations for the first time.

Skutterudite with graphene-modified grain-boundary complexion enhances zT enabling high-efficiency thermoelectric device

Wrapping grain boundaries with rGO enhances zT by increasing thermal boundary resistance, Rκ, with minimal effect on the electronic transport.

Calculation and validation of a grain boundary complexion diagram for Bi-doped Ni

A grain boundary (GB) “phase” (complexion) diagram is computed via a lattice type statistical thermodynamic model for the average general GBs in Bi-doped Ni. The predictions are calibrated with previously-reported density functional theory calculations and further validated by experiments, including both new and old aberration-corrected scanning transmission electron microscopy characterization results as well as prior Auger electron spectroscopy measurements. This work supports a major scientific goal of developing GB complexion diagrams as an extension to bulk phase diagrams and a useful materials science tool.

Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.

A grain boundary phase transition in Si–Au

A grain boundary transition from a bilayer to an intrinsic (nominally clean) boundary is observed in Si–Au. An atomically abrupt transition between the two complexions (grain boundary stabilized phases) implies the occurrence of a first-order interfacial phase transition associated with a discontinuity in the interfacial excess. This observation supports a grain-boundary complexion theory with broad applications. This transition is atypical in that the monolayer complexion is absent. A model is proposed to explain the bilayer stabilization and the origin of this complexion transition.

Diffusion Controlled Abnormal Grain Growth in Ceramics

The grain growth kinetics of silica and calcia doped alumina at 1400oC and their grain boundary complexion is characterized. These data are compared to predictions of both diffusion controlled and nucleation limited interface controlled grain growth theory. It is deduced from the indicators that the mechanism for normal and abnormal grain growth in these aluminas is diffusion controlled.