Grain Boundary Cohesion Research Articles

Effects of Zn addition on the mechanical properties and texture of extruded Mg-Zn-Ca-Ce magnesium alloy sheets

Mg-x Zn-0.2Ca-0.2Ce (x = 0.5, 1.0, 1.5 and 2.0, wt%) alloys were subject to hot extrusion, for the purpose of evaluating the effects of Zn addition on the mechanical properties and texture characteristics in this current paper. A spread of basal poles in extrusion direction (ED) was found in all four extruded alloys, beyond that, the rare earth texture component existed in the alloys. It is interesting that both the ED-split texture and rare earth texture components reduced with the increasing Zn addition; furthermore, the increasing Zn addition leaded to a broader spread of basal poles toward transverse direction (TD). Both the strength and plasticity were improved with the increasing Zn addition, by solid solution hardening, solid solution softening, and the enhanced cohesion of grain boundaries; while further Zn addition deteriorated the plasticity, because of the dominant solid solution hardening. Mg-Zn-Ca-Ce magnesium alloys received the best comprehensive mechanical properties when Zn addition reached 1.5wt%, with tension yield strength (TYS) and tension fracture elongation (TFE) about 131.0MPa and 42.4%, respectively, in ED.

Compositional and microstructural optimization and mechanical-property enhancement of cast Ti alloys based on Ti-6Al-4V alloy

The effects of alloying additions on the microstructure and mechanical properties of cast Ti alloys based on Ti-6Al-4V were systematically investigated through a combination of thermodynamic calculations and experimental verifications. New cast titanium alloys with significantly improved properties were developed through combining thermodynamic calculations and physical metallurgy principles, which achieve an optimum combination of high strength and high ductility at both room and elevated temperatures. Our analysis indicates that the improved strength is due mainly to the α colony and lath size refinement induced by both macro- and micro-alloying of Cr, Fe, B and C elements, whereas the mechanisms for such property optimization include: (1) an increased volume fraction of ductile β phase in the cast state, (2) a refined α colony and lath size by the Cr, Fe, B and C alloying additions, and (3) an enhanced grain-boundary cohesion and refined grain size of prior-β grains induced by proper amount of B additions. The newly developed cast Ti alloys with substantially enhanced mechanical properties for engineering applications.

Segregation effects of Y, Ti, Cr and Si on the intergranular fracture of niobium

Abstract The intergranular fracture of materials is sensitive to the grain-boundary segregation of solutes, which is not yet clear for niobium alloys, a vital promising high-temperature material in the aerospace industry. In this study, first-principles calculations are performed to study the segregation effects of four mostly-used solutes (Y, Ti, Cr and Si) on the cohesion of two distinct grain boundaries (GBs) of Nb. Based on Rice-Wang Model, it is found that Y embrittles the GB; while Ti and Si strengthen it; the effect of Cr on the GB cohesion is structure-dependent. The strengthening/embrittling of the GBs induced by the segregation are attributed to the local structural distortion and charge redistributions around GBs. The underlying physical origin of different alloying effects on the intergranular fracture are identified to be the variation of bonding characters between the host Nb Nb atoms across the GB regions.

Influence of Impurities on Grain Boundaries Cohesion in <i>B</i>2-Ti<i>Me</i>

The effect of interstitial and substitutional impurities on grain boundary (GB) cohesion in the series of B2-TiMe alloys is studied from first principles using pseudopotential approach. It is shown that the TiMe Σ5(310) symmetrical tilt GB cohesion is reduced by the segregation of hydrogen while it is increased due to boron or carbon segregation. We analyze also the combined and accumulation effect of interstitial B (C) and H impurities on the change of the Griffith work.

Microstructural and creep properties of boron- and zirconium-containing cobalt-based superalloys

The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ′(L12) strengthened Co-9.5Al-7.5W-X at% alloys (X=0-Ternary, 0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at%) are studied. Creep tests performed at 850°C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions display no benefit. Atom-probe tomography (APT) is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57±1.04 atoms nm−2 was found for B at a GB in the 0.01B alloy and 2.88±0.81 and 2.40±0.84 atoms nm−2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is absent in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6%, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850°C) over the baseline Co-9.5Al-7.5W alloy.

Effects of Eu2O3 addition on microstructure, grain-boundary cohesion and wear resistance of high-alumina ceramics

The effects of europium oxide addition on microstructure, bulk density, grain-boundary cohesion and wear resistance of high-alumina ceramics (>97 wt% Al2O3) composed of CaO-MgO-SiO2-Al2O3 have been investigated by normal pressure sintering. The content of Eu2O3 varies from 0 to 0.8 wt%. Test results show that doping with a small amount of Eu2O3 decreases the bulk density and wear resistance of high-alumina ceramics. The phase compositions and microstructure of samples were measured by XRD and FESEM, respectively. XRD result shows that Eu can be dissolved into CaAl12O19 crystallites to form a solid solution. SEM result shows that adding Eu2O3 is easier to generate the plate-like grain and decrease grain size compared with undoped-Eu2O3 of alumina ceramics. The wear resistance mechanism was discussed. The break form of cross-section happened from transgranular fracture to intergranular fracture, which causing by weaker grain-boundary cohesion, leads to the reduction of wear resistance.

Ab initio search for cohesion-enhancing impurity elements at grain boundaries in molybdenum and tungsten

We report high throughput density functional theory (DFT) calculations to simulate segregation of s- and p-elements in Mo and W. First, the preference of solutes for interstitial or substitutional positions in the bulk is evaluated and then the segregation energies for the solutes to interstitial and different substitutional sites at a grain boundary (GB) and a free surface (FS) are computed. We show that several solutes change their site preference from substitutional to interstitial position upon segregation to the GB. With the segregation energies to GB and FS, the changes in cohesion can be calculated and GB cohesion enhancing solutes can be identified. The results show striking similarity for both W and Mo. In addition, we collected the available literature data from experimental and theoretical side, which we consequently compare to our results. From our results and the comparison to literature, we identify B, C and Be as potential alloying additions for an increased GB cohesion in Mo and W.

Effect of transition metal impurities on the strength of grain boundaries in vanadium

Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

First-principles determination of grain boundary strengthening in tungsten: Dependence on grain boundary structure and metallic radius of solute

The control of segregation-induced grain boundary (GB) strengthening is a strategy for improving the ductility and intergranular fracture of metals. In this work, by first-principles calculations we study the strengthening effect of transition metals on a series of tungsten GBs to uncover its dependence on the GB structures and the radius of solute itself. The results show that the GB strengthening depends strongly on the GB structures. The solutes tend to enhance cohesion for GBs with larger GB energies, while they decrease cohesion for those with smaller GB energies. In addition, it is generally found that for all GBs the strengthening energies of elements correlate positively with their metallic radii, implying that the size effect plays a major role in the GB strengthening in tungsten. By analyzing the strengthening effect of solutes at different segregated sites, we find that the oversized solutes (Zr, Nb, Hf, Ta) act as cohesion enhancers when segregated in the GB plane, while the undersized elements (Ru, Rh, Re, Os, Ir) strengthen the GBs at the nearest neighbor positions. Experimentally, we show that the presence of Zr/Ta/Re impurity, which leads to an increase of intergranular fracture, increases the theoretical tensile strength and the GB cohesion. These results suggest that the improved fracture resistance by Re, Ta and Zr originates mainly from the GB strengthening due to their segregation.

First-principles study of the effect of boron on grain boundary in NiAl

The grain boundary (GB) is critical to the mechanical properties of NiAl. The addition of B in NiAl can lead to the increase of yield strength due to its segregation in GB. To elucidate the B-induced enhancement of GB, we investigate the segregation state and its strengthening mechanism of the additive B in the typical ∑5(310)[001] GB of NiAl at atomic scale using the first-principles method. Our calculations shown that the additive B atom prefers to segregate at the interstitial I1 site of GB, in which case it is surrounded by four first-nearest-neighboring (1NN) and two second-nearest-neighboring (2NN) Ni atoms. It is found the segregated B prefers to bond with its neighboring Ni instead of Al atoms. Our calculations further shown not only the intergranular cohesion is improved by the covalent bonding between the segregated B and its four 1NN Ni atoms, but also the intra-planar cohesion of GB is enhanced by the covalent bonding between the segregated B and its two 2NN Ni atoms. The covalent bonding between B and Ni atoms mainly stem from the contribution of B-p and Ni-d states, partly from the weak interactions of B-s and Ni-s states. Our first-principles tensile test shown that the yield stress and ductility of NiAl are indeed improved due to the segregation of B in GB.

Effects of alloying on the behavior of B and S at Σ5 (2 1 0) grain boundary in γ-Fe

To get the atomic scale understanding of impurities induced intergranular embrittlement or enhancement and the control of these effects on austenitic stainless steels, behavior of impurities B and S at γ-Fe grain boundary (GB) and effects of alloying elements (Cr, Ni, Mn, Co, Mo) additions on the cohesive properties of the γ-Fe GB doped by impurities are investigated by performing density functional theory (DFT) calculations. The results showed that, as the GB enhancer and embrittler respectively, impurities B and S tend to stay at the interstitial sites of γ-Fe GB, while alloying elements also can segregate at the GB studied. The presence of all the alloying elements considered can inhibit B GB segregation to some extent due to the higher segregation energies and can impair the enhancement effect of B on GB cohesion. However, these elements almost have no impact on GB segregation and embrittlement effect of S, except for Ni, Mn and Mo:Ni and Mo inhibit and improve S GB segregation, respectively; Mn and Mo strongly enhance and reduce S-induced GB embrittlement, respectively. Moreover, the remedial effect of (Cr+Ni), (Cr+Mn) on S-induced GB embrittlement is also discussed. Additional atomic structure and total valence charge density analysis demonstrated and explained our conclusions.

Atomic investigation of alloying Cr, Ti, Y additions in a grain boundary of vanadium

The effect of alloying additions (Cr, Ti and Y) in a vanadium (V) ∑3 (111) grain boundary (GB) is investigated by first-principles calculations. To determine site preference and segregation properties of Cr, Ti and Y in the GB and bulk, we calculate the formation energies and segregation energies for different interstitial and substitutional sites. Cr/Ti/Y atom prefers to segregate to the substitutional sites of the GB from bulk environment, whereas Cr segregation to GB is very weak. Based on the Rice and Wang's model, Cr acts as the GB cohesion, while Ti and Y are strong embrittlers. The analysis of atomic and electronic structures provides a reasonable expansion for the embrittlement behavior. Moreover, the effect of Cr, Ti and Y in the GB on solution of interstitial impurities C, N, O, H, and He are determined. The results show that Cr restrains solution of these impurities in the GB, while Ti tends to form Ti–N complex by absorbing N impurities and Y can absorbs O and He impurities. The present calculations are helpful for understanding the behavior of alloying Cr, Ti, Y additions at the grain boundary of vanadium.

Effect of Ti and B microadditions on the hot ductility behavior of a High-Mn austenitic Fe–23Mn–1.5Al–1.3Si–0.5C TWIP steel

This research work studies the effect of combined Ti and B microadditions and the solidification route on the hot ductility behavior of a high-Mn austenitic Twinning Induced Plasticity (TWIP) steel. For this purpose, uniaxial hot tensile tests were carried out at different temperatures between 700 and 1100°C under a constant strain rate of 10−3s−1. The hot ductility was determined by measuring the reduction of transverse area (%RA) after specimen rupture. Characterization was performed by SEM-EBSD and TEM techniques in order to identify the relationship between microstructural features and cracking phenomena. Results indicate that the early occurrence of dynamic recrystallization (DRX) at the intermediate temperature range (800–900°C) is the favorable mechanism that enhances the ductility, achieving RA values up to 82%. These high RA values are discussed in terms of the boron effect on the improvement of the grain-boundaries cohesion through non-equilibrium segregation, and Ti(C,N) precipitation, which reduces the formation of harmful precipitates such as BN and AlN. Additionally, the Fe23(B,C)6 and B4C compounds were identified, which are less detrimental to hot ductility than boron-nitride compounds. Finally, the fracture surfaces of the present TWIP steels in the temperature range of the highest ductility indicate that the failure mode is of the ductile type as evidenced by the presence of many dimples.

Effect of interstitial impurities on grain boundary cohesive strength in vanadium

We studied the embrittling and strengthening effects of non-metallic (C, N, O, S, Si, and B) and metallic (Fe and Mo) interstitial impurities at a ∑3 (111) [11¯0] grain boundary (GB) in vanadium (V) using first-principles calculations. All impurities incorporated into the GB are exothermic reaction except for Mo along with outward expansion of the GB space. Upon optimization, the small C, N, and O impurities at the GB prefer the same interstitial site, while the S, Si, B, Fe and Mo impurities prefer another interstitial site. According to strengthening energy calculations, C, N, B, and Fe act as the GB cohesion, while O and S are strong embrittlers as well Si and Mo are weak embrittlers. The analysis of atomic and electronic structures indicate that the embrittling and strengthening behavior of the impurities mainly depends on the bonding behavior of the impurity with surrounding vanadium atoms, the impurity species and the atomic size.

Failure of metals III: Fracture and fatigue of nanostructured metallic materials

Pushing the internal or external dimensions of metallic alloys down to the nanometer scale gives rise to strong materials, though most often at the expense of a low ductility and a low resistance to cracking, with negative impact on the transfer to engineering applications. These characteristics are observed, with some exceptions, in bulk ultra-fine grained and nanocrystalline metals, nano-twinned metals, thin metallic coatings on substrates and freestanding thin metallic films and nanowires. This overview encompasses all these systems to reveal commonalities in the origins of the lack of ductility and fracture resistance, in factors governing fatigue resistance, and in ways to improve properties. After surveying the various processing methods and key deformation mechanisms, we systematically address the current state of the art in terms of plastic localization, damage, static and fatigue cracking, for three classes of systems: (1) bulk ultra-fine grained and nanocrystalline metals, (2) thin metallic films on substrates, and (3) 1D and 2D freestanding micro and nanoscale systems. In doing so, we aim to favour cross-fertilization between progress made in the fields of mechanics of thin films, nanomechanics, fundamental researches in bulk nanocrystalline metals and metallurgy to impart enhanced resistance to fracture and fatigue in high-strength nanostructured systems. This involves exploiting intrinsic mechanisms, e.g. to enhance hardening and rate-sensitivity so as to delay necking, or improve grain-boundary cohesion to resist intergranular cracks or voids. Extrinsic methods can also be utilized such as by hybridizing the metal with another material to delocalize the deformation - as practiced in stretchable electronics. Fatigue crack initiation is in principle improved by a fine structure, but at the expense of larger fatigue crack growth rates. Extrinsic toughening through hybridization allows arresting or bridging cracks. The content and discussions are based on experimental, theoretical and simulation results from the recent literature, and focus is laid on linking microstructure and physical mechanisms to the overall mechanical behavior.

Ab initio description of segregation and cohesion of grain boundaries in W–25 at.% Re alloys

We investigate grain boundaries (GBs) in W–25at.% Re alloys with special focus on the segregation of Re to the GBs and the changes in their cohesive properties that arise therefrom. Our simulations rely on density functional theory and a mean-field approximation, the virtual crystal approximation, to model the highly alloyed system. Based on a gamma-surface approach, the geometry of a wide range of GBs, including tilt, twist and mixed GBs, is obtained. Segregation energies are found to vary strongly between different segregation sites, with the strongest segregation energy amounting to −0.75 eVs. We show that bond-order parameters are able to identify strong segregation sites based on purely geometric information only. With a thermodynamic model based on a Bragg–Williams approach, the concentration of Re atoms is calculated as a function of GB character at 1913K and compared with atom probe experiments. The segregation levels are similar, with a trend towards higher segregation levels observed in the calculations for certain misorientation angles. Finally, Re alloying is found to lead to a general increase in the work of separation of the GBs, suggesting a reduction in the tendency for intergranular fracture.

Effects of boron on the fracture behavior and ductility of cast Ti–6Al–4V alloys

Minor amounts of boron additions have been found to greatly enhance the ductility of cast Ti–6Al–4V alloys, which was considered to be due to the grain-size refinement. In this paper, we report our interesting finding that the beneficial effect of boron on the ductility of the cast titanium alloys is due not only to the grain-size refinement but the enhancement of the prior- β grain-boundary cohesion by boron segregation at the grain boundaries, as evidenced by Auger electron microscopy.

Properties of He clustering in α-Fe grain boundaries

Classical molecular dynamics and density functional theory calculations are performed to study the impact of two distinct Fe grain boundaries (GBs) on the clustering properties of helium (He) and the possible He effect on GB decohesion. Several He concentrations are considered. Common properties of He clustering are found for the both GBs, which are visibly different from the bcc bulk. In particular, He clusters in the GBs are always elongated in the directions parallel to the interface and contracted in the direction normal to the GB plane, while they are isotropic in the bcc bulk. When the He number in the clusters is sufficiently large, the strong local pressure promotes the occurrence of loop punching, which is easier to trigger in the GBs than in the bulk, resulting in a lower He-to-vacancy ratio in the GB clusters. The emitted self-interstitial atoms (SIAs) can more easily dissociate from the clusters in the GBs than in the bulk, leading to relatively lower local pressures around the clusters in the GBs, and facilitating the clusters growth. He is found to decrease GB cohesion, and the embrittling effect of He increases with its concentration. But interestingly, this effect decreases with He clustering. The present findings are fully compatible with existing experimental evidence, for instance, for a stronger GB embrittlement due to He at rather low temperatures than at higher temperatures.

Effects of rare-earth on the cohesion of Ni Σ5 (0 1 2) grain boundary from first-principles calculations

A first-principles calculation is performed to investigate the effect of segregated rare-earth (RE) on a Σ5 (012) nickel grain boundary (GB). Energetic studies show that the GB substitutional site is the preferred segregation site for all the REs. According to the Rice-Wang model, the strengthening/embrittling energy is calculated to find that all the REs can impair the GB cohesion. Furthermore, the strengthening/embrittling energy is decomposed into the mechanical and chemical components. The radius and charge of the RE in GB, along with the charge density distribution, are analyzed to uncover the physical origins of these two components and their change tendencies. Our work presents a fundamental understanding of the effects of all the REs on the Ni GB cohesion systematically for the first time.

Distinct element method simulation of an analogue for a highly interlocked, non-persistently jointed rockmass

A grain-based distinct element model is used to reproduce the laboratory response of both intact and granulated Wombeyan marble. The term “granulated” refers to a heat treated marble where the cohesion of grain boundaries has been destroyed. The unconfined compressive strength of granulated marble is less than 50% of that of intact marble, while the strength of the granulated marble increases to about 80% of that of the intact marble at higher confining stresses. An iterative calibration approach is developed to match the unconfined and confined strengths of the models to those of intact and granulated marble. The simulation test results of the models of intact and granulated marble including the transition in the failure mode, stress–strain response, and the evolution of inter- and intra-grain micro-cracks with increasing confinement are discussed. The observed rapid strengthening effect, in terms of increasing confinement, is interpreted to be due to the high degree of grain assembly geometric interlock, which arises from the tight fit geometric shape of the grains as well as the roughness of the grain boundaries. It is suggested that the granulated marble can be considered to be an analogue for a highly interlocked, non-persistently jointed rockmass. It is shown that when the generalized Hoek–Brown failure criterion and the Geological Strength Index (GSI) are used to match the strength of the granulated marble at zero confinement, the confined strength of the granulated marble is underestimated by as much as a factor of two. Therefore, the confined strength of a highly interlocked, non-persistently jointed rockmass, with strong, brittle rock blocks, could be significantly higher than that predicted by commonly adopted empirical approaches. This has practical implications for the design of highly confined pillars and abutments, which is discussed in this paper.