Cohesive Energy Of Grain Boundary Research Articles

Discontinuous model combined with an atomic mechanism simulates the precipitated η′ phase effect in intergranular cracking of 7-series aluminum alloys

A first-principles calculation reveals that the phase that is precipitated in a grain boundary weakens the cohesiveness of the grain boundary. The hydrogen atom is likely to be trapped in the Al/MgZn2 boundary when it diffuses through the grain boundaries, which further reduces the cohesive energy of grain boundaries including Al-H-MgZn2. A discontinuous peridynamic model is proposed to compensate for a deficiency of the finite element method. The peridynamic simulation results show that the cracks grow obviously along the grain boundaries containing Al-H-MgZn2.

Hydrogen Embrittlement Susceptibility of Fe-Mn Binary Alloys with High Mn Content: Effects of Stable and Metastable ε-Martensite, and Mn Concentration

To obtain a basic understanding of hydrogen embrittlement associated with e-martensite, we investigated the tensile behavior of binary Fe-Mn alloys with high Mn content under cathodic hydrogen charging. We used Fe-20Mn, Fe-28Mn, Fe-32Mn, and Fe-40Mn alloys. The correlation between the microstructure and crack morphology was clarified through electron backscatter diffraction measurements and electron channeling contrast imaging. e-martensite in the Fe-20Mn alloy critically deteriorated the resistance to hydrogen embrittlement owing to transformation to α′-martensite. However, when e-martensite is stable, hydrogen embrittlement susceptibility became low, particularly in the Fe-32Mn alloys, even though the formation of e-martensite plates assisted boundary cracking. The Fe-40Mn alloys, in which no martensite forms even after fracture, showed higher hydrogen embrittlement susceptibility compared to the Fe-32Mn alloy. Namely, in Fe-Mn binary alloys, the Mn content has an optimal value for hydrogen embrittlement susceptibility because of the following two reasons: (1) The formation of stable e-martensite seems to have a positive effect in suppressing hydrogen-enhanced localized plasticity, but causes boundary cracking, and (2) an increase in Mn content stabilizes austenite, suppressing martensite-related cracking, but probably decreases the cohesive energy of grain boundaries, causing intergranular cracking. As a consequence, the optimal Mn content was 32 wt pct in the present alloys.

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

Abstract A multiscale analysis has been conducted on hydrogen-induced intergranular cracking at ambient temperature in medium strength (840 MPa) Ni-Cr steel with antimony, tin, and phosphorous segregation. Combining first-principles calculations and fracture mechanics experiments, a multiscale relationship between threshold stress intensity factor (K th) and cohesive energy of grain boundary (the ideal work of interfacial separation, 2γ int) was revealed. The K th was found to decrease rapidly under a certain threshold of 2γ int, where the 2γ int decreases mainly by mobile hydrogen segregation on fracture surfaces. This segregation is considered to arise during formation of the fracture surfaces under thermodynamic equilibrium in slow crack growth. The resulting strong decohesion probably makes it difficult to emit dislocations at the microcrack tip region, leading to a large reduction in stress intensity factor. Our analysis based on this mobile hydrogen decohesion demonstrates that the K th decreases dramatically within a low and narrow range of hydrogen content in iron lattice in high-strength steels.

Atomistic modeling of He embrittlement at grain boundaries of α-Fe: a common feature over different grain boundaries

He atoms introduced into materials may lead them to fracture intergranularly, and understanding such an effect is a key issue in the design of future fusion reactors. In the current study, we investigated the decrease of grain boundary (GB) strength caused by He segregation at several kinds of α-Fe GBs by exploiting first principles calculations and a set of empirical potentials. We found enough evidence to support the notion that the GB cohesive energy, a critical measure of GB strength, approximately scales with the He concentration at the GB surface, regardless of the GB type.

Mobile effect of hydrogen on intergranular decohesion of iron: first-principles calculations

Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Σ3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLean's formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70–80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10−9 atomic fraction during slow cracking. This is in contrast to only 10–20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.

First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part I. Iron (Fe)-Solute (B, C, P, and S) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by solute segregation has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy (=2·surface energy − GB energy) of bcc Fe Σ3(111) symmetrical tilt grain boundary (STGB) is reduced by the segregation of sulfur (S) and phosphorous (P) while it is increased by the segregation of boron (B) and carbon (C). The rate of the decrease/increase in the cohesive energy is proportional to the experimental shift in the DBTT of high-purity iron with increasing segregation. This indicates that the change in the cohesive energy of GB plays a key role in the GB embrittlement of metals.

Atomistic Simulation Study of Cohesive Energy of Grain Boundaries in Alpha Iron under Gaseous Hydrogen Environment

Highly accurate prediction of material strength under a practical hydrogen environment and development of materials with minimal hydrogen effect are essential for safe use of hydrogen energy. Here, we estimated the relationship between grain boundary (GB) properties and GB cohesive energy under gaseous hydrogen environment for <110> symmetrical tilt GBs and <001> twist GBs in alpha iron using atomistic simulations. We employed the embedded-atom-method (EAM) potential developed by Wen et al. and conjugate gradient (CG) method for structure relaxation. First, we estimated the relationship between GB energy and GB free volume for various misorientation angles. After the estimation of GB properties, we incorporated a hydrogen atom into various occupation sites and obtained the distribution of hydrogen trap energy around the GBs. We found a good correlation among GB energy, GB free volume, and the number of trapped hydrogen atoms: high energy GBs have large gaps, and many hydrogen atoms are trapped in those spaces. Finally, we estimated the GB cohesive energy and found that there is negligible hydrogen influence on the low energy GBs. The reduction of the GB cohesive energy of Σ3{112} boundary was estimated to be only 6.61% under high-pressure hydrogen environment (T = 300K, p = 70MPa).

1013 水素ガス環境下のα鉄における粒界凝集エネルギーに関する電子・原子シミュレーション(OS10.電子・原子・マルチシミュレーションに基づく材料特性評価(3:粒界),オーガナイズドセッション)

1013 水素ガス環境下のα鉄における粒界凝集エネルギーに関する電子・原子シミュレーション(OS10.電子・原子・マルチシミュレーションに基づく材料特性評価(3:粒界),オーガナイズドセッション)

Chemistry, bonding and fracture of grain boundaries in Ni 3Si

The chemistry and electronic structure at grain boundaries in B-free and B-doped Ni23 at.% Si alloys were examined, with energy dispersive X-ray spectroscopy (EDX) providing information on the former and electron energy loss spectroscopy (EELS) on the latter. Ni-enrichment was seen at large angle boundaries, both in the absence and presence of B. EELS of the Ni L 3 edge showed that the bonding at Ni-rich grain boundaries was similar in both undoped and doped alloys. Comparison of the Ni L 3 edge recorded at the grain boundary and in the bulk suggests that reduced hybridization and weaker bonding occurs at Ni-rich grain boundaries in both doped and undoped alloys. These changes in bonding are interpreted in terms of changes in the cohesive energy of the boundaries. The relationship between the cohesive energy of grain boundaries and the fracture mode of Ni 3Si is discussed.

The Segregation of Boron and Its Effect on the Fracture of An Ni3Si Based Alloy

AbstractIt is now well established that microalloying additions of B to Ni3Al drastically reduce low temperature grain boundary fracture and consequently increase the ductility of this intermetallic compound. One possible explanation for such effects involves the relationship between boron segregation to grain boundaries and free surfaces, and the resulting effect of such segregation on the cohesive energy of the grain boundaries. This study involves the extension of these concepts to an alloy based on Ni3Si. Auger spectroscopy has been carried out on fractured grain boundaries, grain interiors, and free surfaces to determine how B segregates in Ni3(Si,Ti). The consequences of the segregation of B on the cohesive energy of grain boundaries in Ni3Si based alloys are discussed.

Calculation of the Defect and Interface Properties of Ni3Al

AbstractThe structure and energetics of point defects, surfaces and grain boundaries in Ni3A1 are investigated using the Embedded Atom Method. The approach is shown to reproduce the experimental phase diagram of the Ni-Al system and the elastic properties of Ni3AL. The vacancy and anti-site defect energies are calculated and used to predict the vacancy concentration as a function of bulk composition. The preferred geometries and energies of the low index surfaces are also computed. The equilibrium structure of certain ideal grain boundaries are computed by Monte Carlo computer simulations as a function of bulk composition. It is found that the boundaries act as a sink for anti-sitedefects and the degree of ordering at the boundaries is strongly affected by the bulk composition. The cohesive energy of grain boundaries in Ni3A1 is computed and is found to be comparable to that for pure Ni.