Boundary Cohesion Research Articles

First-principles determination of the effect of boron on aluminum grain boundary cohesion

Despite boron being a common alloying element in aluminum, its segregation into the aluminum grain boundary and its effect on the grain boundary strength have not been studied. Here, the electronic structures of the boron-doped $\ensuremath{\Sigma}5(012)[100]$ symmetrical tilt grain boundary and (012) free surface systems for aluminum are investigated by means of first-principles calculations using the full-potential linearized augmented plane-wave method with the generalized gradient approximation, within the framework of the Rice-Wang thermodynamic model and the theoretical tensile test approach. We establish that boron has a large driving force to segregate from Al bulk to the symmetrical grain boundary hollow site, and its segregation significantly enhances the grain boundary strength. Through precise calculations on both the grain boundary and free surface environments, it is found that boron is a strong cohesion enhancer in aluminum with a potency of \ensuremath{-}0.19 eV/atom. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-boron bond has mixed covalent and metallic character and is strong in both grain boundary and free surface environments. The strengthening effect of boron is due to creation of additional B--Al bonds across the grain boundary, which are as strong as existing Al--Al transgranular bonds and thus significantly increase grain boundary adhesion and its resistance to tensile stress and cracking.

The effect of alloying elements on grain boundary and bulk cohesion in aluminum alloys: An ab initio study

The effect of B, Si, P, Cr, Ni, Zr and Mg impurities on cohesive properties of Al and its special grain boundary (GB) Σ5 (2 1 0) [1 0 0], as well as their segregation behavior at the GB and (2 1 0) surface are studied from first principles. Our analysis determines Ni to be the best and P the worst alloying elements in regard to the overall resistance to decohesion of Al alloys.

Enhanced wear resistance in AlMgB14–TiB2 composites

► Composites of AlMgB 14 –TiB 2 comprised of mechanically alloyed or commercial TiB 2 were successfully consolidated by hot pressing. ► Microstructure, erosion, and abrasive wear behavior of various composites were compared and contrasted as a function of the percent and type of TiB 2 addition. ► Refined microstructure associated with mechanically alloyed TiB 2 was shown to blunt cracks at grain boundaries. ► Maximum resistance to erosive wear was found at the 80 vol.% TiB 2 composition. Studies of bulk AlMgB 14 and TiB 2 composites have shown that these materials exhibit exceptional resistance to erosive and abrasive wear. Multi-hour ASTM erosion tests with Al 2 O 3 abrasive against composite samples comprised of AlMgB 14 (40 vol.%) and TiB 2 (60 vol.%) resulted in erosion rates of 0.5 mm 3 /kg of erodent, compared with 10.5 mm 3 /kg for wear-resistant grades of WC–6% Co. Increasing the TiB 2 fraction to 80 vol.% further reduced erosion rates to as low as 0.26 mm 3 /kg. Fracture nucleation in the TiB 2 grains was identified by SEM analysis as a primary damage mechanism. Additionally, diamond abrasion testing revealed a slightly different trend between composition and wear than that observed in erosion testing. Results of wear tests are discussed in terms of microstructure, hardness and indentation toughness of each phase, and grain boundary cohesion. Analysis suggests several energy dissipative mechanisms including fracture termination at grain boundaries and conversion of mechanical energy to thermal energy act to improve wear resistance in the fine-grained boride composites.

Effect of rare earth element yttrium addition on microstructures and properties of a 21Cr–11Ni austenitic heat-resistant stainless steel

In this comparative study, the microstructure and the mechanical properties of a 21Cr–11Ni austenitic heat-resistant stainless steel with and without addition of rare earth (RE) element yttrium have been investigated. The results show that a number of fine spherical yttrium-rich oxide particles are not uniformly distributed in the matrix of steel with yttrium; instead, they are aligned along the rolling direction. The grains surrounding the alignment are nearly one order of magnitude smaller than those farther away from the alignment. The approximate calculation results indirectly show that the grain refinement may be mainly attributed to the stimulation for nucleation of recrystallization rather than to pinning by particles. Furthermore, the alignment has resulted in significant loss in transverse impact toughness and tensile elongation at room temperature. There is a trough in the hot ductility–temperature curve, which is located between 973 and 1173K. The ductility trough of steel with yttrium becomes shallow within a certain temperature range, especially around 1073K, indicating that improvement on hot ductility is achieved by yttrium addition. The results may be attributed to the increase of grain boundary cohesion indicated by the effective improvement on intergranular failure tendency, and the inhibitory effect of yttrium on sulfur segregation to grain boundaries is believed to be an important cause.

Solute Segregation at Σ11(113)[110] Grain Boundary and Effect of the Segregation on Grain Boundary Cohesion in Aluminum from First Principles

We investigate the energy of segregation of solute Ca at symmetric tilt grain boundary in aluminum from the first-principles calculations. As energy of segregation of Ca is negative, Ca atoms tend to segregate at the grain boundary. Furthermore, on basis of the Rice-Wang model, we study the effect of the segregation of Ca on the grain boundary embrittlement of aluminum. Our first-principles calculations of energies of segregation at grain boundary and free surface show that Ca behaves as embrittler.

Influence of Phosphorous and Boron on the Recrystallization, Grain Growth and Mechanical Properties of 3% Si Steel

A new approach to obtain high strength of non-oriented electrical steel by addition of phosphorus is proposed. The method includes B-additions which suppress grain boundary P segregation, strengthen the grain boundary cohesion and enhance the P solid solution hardening. Two 3% Si steels, a B-free 0.1%P steel and a 20 ppm B-added 0.1%P steel were analyzed. The microstructures were studied by EBSD. The B-addition resulted in a pronounced rotated cube component, {100}<011>, after a hot-band annealing treatment. A -fiber texture was developed in the B-free steel. The B-addition caused a retardation of the recrystallization, allowing for the growth of grains with a lower stored energy, such as rotated cube oriented grains. The steels were further cold rolled and recrystallization annealed to observe a similar effect after large cold reductions. The present contribution focuses on the potential of this concept to obtain high strength 3% Si steels with low core losses.

Toughening and hardening in double-walled carbon nanotube/nanostructured magnesia composites

Dense double-walled carbon nanotube (DWCNT)/nanostructured MgO composites were prepared using an in situ route obviating any milling step for the synthesis of powders and consolidation by spark-plasma-sintering. An unambiguous increase in both toughness and microhardness is reported. The mechanisms of crack-bridging on an unprecedented scale, crack-deflection and DWCNT pullout have been evidenced. The very long DWCNTs, which appear to be mostly undamaged, are very homogeneously dispersed at the grain boundaries of the matrix, greatly inhibiting the grain growth during sintering. These results arise because the unique microstructure (low content of long DWCNTs, nanometric matrix grains and grain boundary cohesion) provides the appropriate scale of the reinforcement to make the material tough.

Effects of Microalloying on the Mobility and Mechanical Response of Interfaces in Nanocrystalline Cu

We utilize a novel computational approach to model the problem of impurity segregation at grain boundaries in nanophase materials. It is based on a parallel MonteCarlo algorithm that places the impurities according to the local chemical potential for the species, following the thermodynamic driving force for segregation. This technique is combined with molecular dynamics techniques to study the role played by Fe impurities in the properties of nanocrystalline Cu grain boundary properties. The impurities were found to improve microstructural stability as studied by high temperature annealing simulations, and grain boundary cohesion as studied via spall resistance high stresses produced by simulated laser irradiation. Virtual tensile tests of samples with and without impurities revealed that the impurities did not affect the high flow stress typical of nanostructured material. We interpret these results in terms of impurity dragging and grain boundary sliding.

Positron lifetime and Doppler broadening study of defects in hydrogen charged Al–Mg alloys

The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime τav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion.

Effect of impurities on grain boundary cohesion in bcc iron

The effect of B, N and O impurities placed in interstitial and substitutional positions at symmetric Σ5(210) Fe grain boundary is studied by means of first principles calculations. Full relaxation of supercell shape and volume is applied which results in stable asymmetric grain boundaries, depending on the impurity and its position. In all cases the big shifts (0.47–2.33Å) of grains with respect to each other are observed. The equilibrium distance between the grains is decreased for impurities in substitutional positions, and increased for atoms in interstitial sites, compared to the relaxed clean GB. We found that nitrogen both in interstitial and substitutional positions, and boron in substitutional position enhance cohesion while oxygen in both positions, and interstitial boron, are embrittlers. The magnetic moments of Fe atoms at clean grain boundary are remarkably increased compared to the bulk. They tend to the bulk value in the middle of the grain in an oscillatory way.

Hot flow behavior of boron microalloyed steels

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hot-compression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 °C) and constant true strain rates (10 −3, 10 −2 and 10 −1 s −1). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure.

Secondary recrystallization, crystallographic texture and magnetostriction in rolled Fe–Ga based alloys

The surface-energy-induced secondary recrystallization growth with a specific surface plane can be governed in polycrystalline Fe–Ga alloy doped with sulfur by controlling the segregation of sulfur through conventional rolling and texture annealing. Results that demonstrate the recrystallization, magnetostriction, and selective development of {100}⟨001⟩ and/or {110}⟨001⟩ preferred texture in Fe81.3Ga18.7 plus B (0.5–1.0at.%) and S (∼0.05at.%) alloys are presented in this work. The addition of boron was used to improve ductility by enhancing grain boundary cohesion during rolling. The as-rolled (Fe81.3Ga18.7)+0.5at.% B+0.005at.% S sheet had {100}⟨011⟩ and {111}⟨011⟩ rolling-texture grains. A near {100}⟨001⟩ texture was formed in fully (∼99%) secondary-recrystallized sheet that was annealed at 1200°C for 2h [(3∕2)λS=198ppm], while the texture of mostly (∼80%) secondary-recrystallized sheet annealed at 1100°C for 4h [(3∕2)λS=165ppm] was closer to {110}⟨001⟩. The maximum observed magnetostriction in samples with 0.005at.% S was 220ppm and was obtained as a result of a double annealing, with the resultant texture closer to the ideal {100}⟨001⟩ than the less desirable {110}⟨001⟩. The maximum magnetostriction of 150ppm observed in the (Fe81.3Ga18.7)+1.0at.% B+0.05at.% S sheet resulted when the sheet was annealed at 1200°C for 2h. This sheet had a major texture of {110}⟨112⟩ and minor textures of {110}⟨001⟩∕{100}⟨011⟩.

Influence of solidification conditions on grain boundary cohesion in the mushy zone during directional solidification of nickel base superalloy

The effect of solidification rates on grain boundary (GB) cohesion in mushy zone during directional solidification was explored. The mushy zone structures were frozen by tin bath quenching. It was found that high solidification rate leads to an extended mushy zone thus vulnerable region, while there are larger areas where dendrites are bridged at higher solidification rate, i.e. there is a high fraction of bridging areas in the mushy zone as solidification rate is increased. The smaller hot tearing susceptibility at higher solidification rate cannot be explained by the greater mushy zone or vulnerable region, but it can be explained by stronger GB cohesion. The results suggest more attention should be paid to the GB cohesion rather than the range of mushy zone or vulnerable region in order to avoid hot tearing at least in some alloys.

Hot ductility behavior of boron microalloyed steels

The current study analyses the influence of boron contents (between 29 and 105 ppm) on the hot ductility of boron microalloyed steels. For this purpose, hot tensile tests were carried out at different temperatures (700, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1. In general, results revealed an improvement of the hot ductility of steels at increasing boron content. At 700, 900 and 1000 °C the ductility is higher than at 800 °C, where boron microalloyed steels exhibit a region of ductility loss (trough region). Likewise, dynamic recrystallization only occurred at 900 and 1000 °C. The fracture surfaces of the tested steels at temperatures giving the high temperature ductility regime show that the fracture mode is a result of ductile failure, whereas it is ductile–brittle failure in the trough region. Results are discussed in terms of dynamic recrystallization and boron segregation towards austenite grain boundaries, which may retard the formation of pro-eutectoid ferrite and increase grain boundary cohesion.

Effect of dendrite arm spacing on castability of a directionally solidified nickel alloy

The effect of dendrite arm spacing on hot tearing susceptibility during directional solidification was explored by casting bi-crystal samples with the same grain boundary misorientation but different dendrite arm spacing. It was found that smaller dendrite arm spacing does reduce the hot tearing tendency. The eutectic melt is more finely dispersed and increasingly discontinuous as the dendrite arm spacing is reduced. The better castability is attributed to the stronger grain boundary cohesion.

The effect of rare earth dopants on grain boundary cohesion in alumina

The effect of rare earth (RE) grain boundary segregation on mode of fracture in alumina has been investigated. In order to isolate the effects of microstructure (i.e. grain size and residual porosity) from those due to grain boundary chemistry, the fracture behaviour of virtually pore-free (i.e. nearly transparent) undoped alumina has also been studied. This showed that mode of fracture becomes increasingly transgranular as grain size is reduced, a trend which has been explained by thermal expansion anisotropy effects. The addition of RE (i.e. Yb, Gd or La) dopants to alumina resulted in a substantial increase in the proportion of intergranular fracture relative to the undoped material of similar grain size. This can be explained by the significant reduction in the free surface energy that results from RE segregation at grain boundaries, which reduces the work of fracture for intergranular failure. This is expected to lead to a reduction in strength compared to undoped aluminas with equivalent microstructures, although this is often more than offset by the improved microstructures that using a RE dopant can provide.

Effect of boron on fatigue crack growth behavior in superalloy IN 718 at RT and 650 °C

The effect of boron on the fatigue crack growth rate (FCGR) in Inconel 718 (IN 718) was studied at room temperature (RT) and 650 °C. The results showed that the addition of B improved the fatigue crack propagation resistance of IN 718. The higher the B concentration, higher was the fatigue threshold at 650 °C. While the FCGRs increased as the test temperature increased from RT to 650 °C in the Paris regime, a rapid drop in the FCGRs in the near-threshold regime and a higher fatigue threshold at 650° were observed due to the oxide-induced crack closure. The fracture surfaces were observed to exhibit transgranular cracking with fatigue striations in specimens tested at RT, and a mixture of transgranular and intergranular cracking at 650 °C. The fracture mode changed from intergranular cracking to transgranular cracking and plastic deformation marks increased with increasing B concentration at 650 °C. The micromechanism of improvement in the fatigue crack growth resistance due to B addition was further studied via observations of crack growth path, fractography and TEM examination of the plastic zone ahead of the crack tip. The plastic deformation mode within the crack tip plastic zone was planar slip, along with twinning, on {1 1 1} planes. A crystallographic cracking model was, thus, proposed on the basis of restricted slip or twinning. The improvement in the fatigue crack growth resistance in IN 718 due to B addition was mainly attributed to the increase in the grain boundary cohesion via minimizing the deteriorative effect of oxygen and the increase in the resistance to the dislocation movement at the crack tip.

Effects of Co and Cr on bcc Fe grain boundaries cohesion from first-principles study

The segregation effects of Co and Cr on the bcc Fe ∑ 3 [ 1 1 ¯ 0 ] ( 1 1 1 ) grain boundary cohesion are investigated based on the Rice–Wang thermodynamics model by the first-principles DMol method within the framework of density functional theory. The electronic properties are studied for Co/Fe and Cr/Fe systems. The calculated segregation energy difference between the grain boundary and the corresponding free surface is 0.25 eV for solute Co and −0.43 eV for solute Cr, which indicates that Co could weaken and Cr enhance the grain boundary cohesion in bcc Fe. In these systems the chemical effect induced by Co and Cr play crucial effects, but the geometry effect can be neglected. In addition, an anti-parallel spin island is formed around the central of GB.

Effect of element Re on the grain boundary cohesion of α-Fe

The effect of Re segregation on the α-Fe Σ5 [001] (010) grain boundary (GB) is investigated by using a software called DMol and discrete variational method (DVM). Based on the Rice–Wang model, the calculated segregation energy and defect formation energy show that Re is a strong cohesive enhancer. We also calculated the interatomic energy (IE) and bond order (BO) of several atomic pairs to investigate the mechanism of the cohesive effect of Re microscopically and locally. The results show that IEs of atomic pairs formed by those atoms which cross the plane of GB are strengthened due to the segregation of Re, while the BOs of the corresponding pairs are slightly decreased. This discrepancy demonstrates that IE which contains the Hamiltonian of interaction between atoms is a good quantity to describe the bonding strength. The analysis suggests that the electronic effect between atomic pair which comes directly from Hamiltonian is the key factor. The charge density is also presented and the result indicates that the bonding strength between the Fe atoms on the GB is enhanced due to the segregation of Re, which is consistent with the analysis of IE.

Effects of Ca on grain boundary cohesion in Au ballbonding wire

Measurements of the tensile properties of Ca doped Au ballbonding wires show that Ca significantly increases the strength of Au and improves ductility. The strengthening mechanism is poorly understood but the limited solubility of Ca in Au and the large atomic misfit factor precludes solid solution strengthening and there is insufficient Ca present for conventional precipitation hardening to be effective. However, Ca segregation would be a possible mechanism. The effects of Ca segregation are investigated by calculating the cohesive strength of a Ca-containing Au grain boundary relative to a free surface using a first-principles calculation within the framework of density functional theory. The simulation results support the theory that the tensile strength and ductility increase with Ca doping may be due to enhanced Au grain boundary cohesion.