Kinds Of Grain Boundaries Research Articles

Making high-strength bimodal Ti–5Al–5V–5Mo–3Cr alloy more ductile by grain-boundary shearing

The conflict between strength and ductility of bimodal Ti–5Al–5V–5Mo–3Cr (Ti-5553) becomes more evident as yield strength attains 1400 MPa, which limits the further application of the alloy in the ultra-high-strength condition. In this paper, we found that the conventional bimodal Ti-5553 alloy with 1400 MPa yield strength processes four typical kinds of grain boundary microstructures. However, two kinds of grain boundaries with the short α side-plate are almost un-deformable in tension, contributing to the strength-versus-ductility conflict. Fortunately, short α side-plate might transform to α laminate that is parallel to the grain boundary of prior β grain after the dual-ageing consisting of the high-temperature ageing and the low-temperature ageing. It is found that α laminate is capable of shearing, which is beneficial to the ductility of bimodal Ti-5553 alloy. Base on the dual-ageing strategy, a novel bimodal Ti-5553 alloy with excellent combination of yield strength (1391 MPa), ultimate tensile strength (1484 MPa) and tensile elongation (12%) has been fabricated. The strength versus ductility data of the novel alloy might extend beyond the benchmark range established by the strongest and most ductile Ti-alloys known. The result of the present work is instructive for designing the grain-boundary microstructure of high-strength bimodal near-β titanium alloys.

Microscopic characteristics of slip process in a bainitic steel

Micro slip behaviors in a bainitic steel were studied both at room temperature and 650 °C recurring to a quasi in–situ approach, related ‘pencil glide’ process and the influence of different kinds of grain boundaries on the slip transfer process were discussed based on the slip traces on surface.

Fabrication and characterization of few-layer graphene oxide reinforced magnesium matrix composites

Abstract In this study, few-layer graphene oxide (FLGO) reinforced Mg matrix composite was successfully fabricated by the hetero-agglomeration method and spark plasma sintering (SPS) process. The microstructure, mechanical properties, and strengthening behavior of these composites were comprehensively investigated. A unique amorphous MgO/ultrafine MgO + reduced FLGO/amorphous MgO sandwich-like grain boundary of the α-Mg matrix grain was confirmed in FLGO/Mg matrix composites. This kind of grain boundary indicated the strongly interfacial bonding between the reduced FLGO and Mg matrix. The low mismatch between the d{2 0 0}MgO and d{1 0 0}graphene and the instantaneous elevated temperature during the SPS process could contribute to the formation of these unique grain boundaries. The mechanical strength and Vickers hardness were apparently increased with increasing FLGO concentration. The 0.5 vol%FLGO/Mg composite exhibits the maximum increase in the mechanical properties. The uniaxial compressive strength (UCS) of 373.2 MPa, yield strength (YS) of 140.7 MPa, failure strain of 14.9% and Vickers hardness of 57.6 was achieved in this composite. Quantitative analysis of the strengthening behavior in these composite was performed, and a high strengthening efficiency was achieved in the FLGO/Mg matrix composites. Therefore, FLGO could be considered as a promising candidate as reinforcement for the Mg matrix.

The effect of grain boundary structure on chromium carbide precipitation in alloy 600

The aim of the present work is to study the effect of grain boundary structure on the precipitation of chromium carbides in Alloy 600. The material was processed by thermo-mechanical treatment consisting of hot rolling (at 1000 °C) and a short-time annealing treatment (for 10 min) to generate a high fraction of twins and disrupted random boundaries network. These samples were then given sensitization heat treatment at 704 °C for various times in the range 0.5–100 h, in order to introduce varying degrees of carbide precipitation at the interface regions, depending on the structure of grain boundaries and triple junctions. The characteristics of chromium carbides (identified as Cr23C6) precipitation on different kinds of grain boundaries and triple junctions were investigated using field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) based orientation imaging microscopy (OIM) techniques. The propensity for chromium carbides precipitation increased with increasing aging time. However, it was largely dependent on the structure of grain boundaries. Low angle grain boundaries (LAGBs) and low-Σ coincidence site lattice (CSL) boundaries were observed to retard the precipitation of carbides. In contrast, on random high angle grain boundaries (HAGBs), coarse and discrete carbides were observed. Amongst the low-Σ CSL boundaries, the deviation in misorientation was observed to have a profound effect on the precipitation of chromium carbides. Low-deviation twin (Σ3) boundaries (having misorientation deviation within 1°) were found to be strongly resistant against carbide precipitation. However, on high deviation twin and Σ9 boundaries, needle-like carbides evolved. Based on the results, it can be concluded that among the special (i.e., low-Σ CSL) boundaries, only low-deviation twin boundaries have sufficiently low interfacial free energy to completely subdue the precipitation of chromium carbides.

Double grain boundary configurations on graphite surfaces

We investigated the atomic structure of different kinds of grain boundaries on highly oriented pyrolytic graphite (HOPG) by scanning tunneling microscopy. We categorized several grain boundary configurations as a function of the misorientation angle between the adjacent grains, highlighting the occurrence of double grain boundaries (i.e., systems of two grain boundaries separated by a nanometric-scale inner region with specific atomic arrangement) for misorientation angles in the range 22°–32°. By using Molecular Dynamics simulations, we analyzed the structure and energy stability of single and double grain boundaries according to the misorientation angle. The experimental evidence is corroborated by Molecular Dynamics results and total energy calculations, which found a comparable stability between single and double grain boundaries for the same misorientation angle range. Our combination of experimental measurements and theoretical calculation extends the understanding of the structural configuration of large angle grain boundaries beyond the range of misorientation angles reported to date.

Resistive switching behaviors mediated by grain boundaries in one longitudinal Al/MoS2&PVP/ITO device

Nonvolatile memory devices based on 2D layered transition metal dichalcogenides have inspired tremendous amounts of research interests in recent years. In this work, resistive switching (RS) devices based on hybrid molybdenum disulphide and Polyvinylpyrrolidone (MoS2&PVP) is explored. One longitudinal Al/MoS2&PVP/ITO device exhibits three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and threshold switching (TS), which can be explained by the formation and rupture of conducting filaments (CFs) consisting of sulfur vacancy filaments in MoS2 nanoflakes and sulfur ions filaments in PVP. These CFs form from three different kinds of grain boundaries (GBs), intersecting-GB, bisecting-GB, and bridge-GB in which GBs attach one electrode, parallel to the two electrodes, and attach both electrodes, respectively, corresponding to the three modes of I-V characteristics. These three kinds of GBs coexist in one device and the CF forms randomly from one kind of them during sweeping loops. The mechanism was experimentally proved by our results of the increasing probability of typical bipolar RS in device with one layer of PVP thin film inserted into the interface of MoS2&PVP and ITO and other researchers’ results in all-surface MoS2 single-layer MoS2 devices.

Effect of grain boundaries on shock-induced phase transformation in iron bicrystals

Non-equilibrium molecular-dynamic simulations with a modified analytic embedded-atom model potential have been performed to investigate the effect of three kinds of grain boundaries (GBs) on the martensitic transformation in iron bicrystals with three different GBs under shock loadings. Our results show that the phase transition was influenced by the GBs. All three GBs provide a nucleation site for the α → ε transformation in samples shock-loaded with up = 0.5 km/s, and in particular, the elastic wave can induce the phase transformation at Σ3 ⟨110⟩ twist GB, which indicates that the phase transformation can occur at Σ3 ⟨110⟩ twist GB with a much lower pressure. The effect of GBs on the stress assisted transformation (SAT) mechanisms is discussed. All variants nucleating at the vicinity of these GBs meet the maximum strain work (MSW) criterion. Moreover, all of the variants with the MSW nucleate at Σ5 ⟨001⟩ twist GB and Σ3 ⟨110⟩ tilt GB, but only part of them nucleate at Σ3 ⟨110⟩ twist GB. This is because the coincident planes between both sides of the GB would affect the slip process, which is the second stage of the martensitic transformation and influences the selection of variant. We also find that the martensitic transformation at the front end of the bicrystals would give rise to stress attenuation in samples shock-loaded with up = 0.6 km/s, which makes the GBs seem to be unfavorable to the martensitic transformation. Our findings have the potential to affect the interface engineering and material design under high pressure conditions.

Ab initio modelling of solute segregation energies to a general grain boundary

We apply a quantum mechanical/molecular mechanical (QM/MM) multiscale approach to calculate the segregation energies of Mg and Pb to two kinds of grain boundaries in Al. The first boundary, a symmetric (310)[001] Σ5 tilt boundary, is also tractable using traditional QM calculations, and serves as a validation for the QM/MM method. The second boundary is a general, low-symmetry tilt boundary that is completely inaccessible to pure QM calculations. QM/MM results for both of these boundaries are used to evaluate the accuracy of empirical (EAM) potentials for the Al-Mg and Al-Pb alloy systems. Based on these results we develop a physical model for the segregation energy based on elastic interaction and bond breaking terms. Both MM calculations with the EAM potentials and the model work quantitatively well for describing Mg-GB interaction across a wide range of local environments. For Pb, MM performance is weaker and the model provides only qualitative insight, demonstrating the utility of a QM/MM approach.

Distinct fatigue cracking modes of grain boundaries with coplanar slip systems

Three groups of Cu bicrystals with their component grains sharing one common {111} slip plane were cyclically deformed. By careful design on the loading direction, coplanar slip systems operated in the two component grains. Three kinds of grain boundaries (GBs): a low angle grain boundary (LAGB), a high angle grain boundary (HAGB) and an incoherent twin boundary (ITB), were all impinged by coplanar slip bands (SBs) on both sides. Nonetheless, the three kinds of GBs showed different fatigue cracking behaviors: the fatigue cracks nucleated along the HAGB earlier than along the SBs which produced cracks earlier than both the LAGB and the ITB. It is found that the slip vectors of the two component grains deviate from each other to different degrees for the three kinds of GBs. Lattice dislocations tend to pile up at the HAGB due to the large difference between the slip vectors, while lattice dislocations tend to pass through the LAGB and the ITB ascribed to the small differences between the slip vectors. High ability of dislocation transmission can generate good strain compatibility and low stress concentration near the GBs so as to increase the intergranular fatigue cracking resistance. In addition, lattice dislocation transmission and boundary dislocation motion can induce migration of LAGB and ITB which could improve the intergranular fatigue cracking resistance by consuming part of plastic work done by the cyclic loading. Thus, fatigue cracking resistance of GBs with impingement of coplanar SBs can be improved by increasing the penetrability and mobility of the GBs.

Creep behavior and life assessment of anisotropic bicrystals with a void and without void in different kinds of grain boundaries

Based on crystallographic theory, a creep constitutive relationship and a life predictive model have been presented. The crystallographic creep constitutive relationship has been implemented as a user subroutine ’CRPLAW' to MACR. Bicrystal models containing a void in the grain boundary and bicrystal model without void have been studied by the finite element method. Different loading direction has been studied in order to show the influence of relative direction of loading to grain boundary on the creep behavior of the bicrystals. The numerical results of bicrystal models show that there are a high stress gradient and stress concentration near the void and grain boundary. The existing of the void has strong influence on creep durability life of the crystal. The stress distribution and creep strain characterization are dependent on the crystallographic orientations of the two crystals and the grain boundary direction as well as the existing of the void and loading directions. It is shown that the bicrystal model of the loading direction perpendicular to the grain boundary has the highest creep strain and creep damage, while that model of the of the loading direction parallel to the grain boundary has the minimum. This above conclusion is also same to the growth of void.

Stable line defects in silicene

Line defects in two-dimensional (2D) materials greatly modulate various properties of their pristine form. Using ab initio molecular dynamics (AIMD) simulations, we investigate the structural reconstructions of different kinds of grain boundaries in the silicene sheets. It is evident that depending upon the presence of silicon adatoms and edge shape of grain boundaries (i.e., armchair or zigzag), stable extended line defects (ELDs) can be introduced in a controlled way. Further studies show the stability of these line-defects in silicene, grown on Ag(111) surface at room-temperature. Importantly, unlike most of the 2D sheet materials such as graphene and hexagonal boron nitride, 5-5-8 line defects modify the nonmagnetic semimetallic pristine silicene sheet to spin-polarized metal. As ferromagnetically ordered magnetic moments remain strongly localized at the line defect, a one-dimensional spin channel gets created in silicene. Interestingly, these spin channels are quite stable because, unlike the edge of nanoribbons, structural reconstruction or contamination cannot destroy the ordering of magnetic moments here. Zigzag silicene nanoribbons with a 5-5-8 line defect also exhibit various interesting electronic and magnetic properties depending upon their width as well as the nature of the magnetic coupling between edge and defect spin states. Upon incorporation of other ELDs, such as 4-4-4 and 4-8 defects, 2D sheets and nanoribbons of silicene show a nonmagnetic metallic or semiconducting ground state. Highlighting the controlled formation of ELDs and consequent emergence of technologically important properties in silicene, we propose new routes to realize silicene-based nanoelectronic and spintronic devices.

Microstructure and texture evolution of Cu–Nb composite wires

Abstract The evolution of microstructure and texture in Cu–Nb composite wires fabricated by an accumulative drawing and bundling process was investigated by backscattered electron (BSE), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Results indicate the onset of severe curling and shape changing occurred at the size of Nb ~ 400 nm with a surface increase of about 6.91 μm 2 /μm 3 (the area per unit volume). Two kinds of grain boundaries in Nb are suggested: one is 20°–50° boundary with a rotate/tilt axis around parallel to drawing direction (DD), and another is > 50° boundary with the axis perpendicular to DD. The curling phenomenon occurred at the Cu–Nb interface and is related not only to the deformation mechanism of Nb but also to the presence of interface. This result is distinct from reported works showing that curling takes place when BCC metals are heavily drawn (Area reduction > 73%). The variation in microstructure and texture evolution between Cu and Nb filaments was discussed based on the differences in deformation mechanisms of these two metals.

An electroplating method for copper plane twin boundary manufacturing

A twin boundary is a special kind of grain boundary that plays an important role in the deformation process of nanocrystalline metals, as it may affect the migration of atoms and electrons in polycrystalline solids, resulting in different electrical and mechanical properties. In this study, plane twin structures were introduced into an electroplated copper film by an electroplating method that inserts an interlayer film with a very small current density (<3mA/cm2). It was found that the small-current interlayer formed a demarcation line for copper grain growth, and enhanced the twin boundaries by self-annealing at room temperature. Based on this, a method was developed to manufacture multi-plane twin boundaries to improve electron migration. Transmission electron microscopy, focused ion beam analysis, and secondary ion mass spectrometry were employed to examine this interesting phenomenon.

An atomistic modeling of He bubble stability at grain boundaries in alpha-Fe

Development of He bubbles at grain boundaries (GBs) may lead materials to serious embrittlement, but the nucleation mechanism of such bubbles is not well understood. In the present paper, we analyzed the stability of various He–vacancy clusters, which are precursors of He bubbles, at several kinds of GBs of α-Fe using a set of empirical potentials. The results clearly indicated the preferred nucleation of He bubbles at GBs. We also found that the dissociation energy of vacancies from He–vacancy clusters at GBs is generally less than that for the intragranular counterparts, and that the He-to-vacancy ratio in equilibrium for various GB cases becomes larger in comparison with the intragranular case. The results obtained here are invaluable in establishing a kinetic model of He bubble nucleation at GBs.

Effect of grain boundary on the buckling of graphene nanoribbons

The buckling of graphene nano-ribbons containing a grain boundary is studied using atomistic simulations where free and supported boundary conditions are invoked. We consider the buckling transition of two kinds of grain boundaries with special symmetry. When graphene contains a large angle grain boundary with θ = 21.8°, the buckling strains are larger than those of perfect graphene when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. This is opposite for the results of θ = 32.2°. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions, the presence of the particular used grain boundaries, and the direction of applied in-plane compressive tension.

Grain Boundary Analysis of HT9 Steel after Accelerated Creep Testing

AbstractFerritic/martensitic steels are attractive materials for use as components in nuclear reactors because of their high strength and good swelling resistance. Grain boundary specific phenomena (such as segregation, voiding, cracking, etc) are prevalent in these materials so grain boundary character is of primary importance. Certain types of boundaries are more susceptible to thermal creep damage whereas others tend to resist damage. If more damage resistant boundaries can be introduced into the structures, this will result in steel that is more resistant to the processes of degradation that prevail in high-temperature environments. We have characterized the grain boundary structure in HT9 steel by electron backscatter diffraction to identify boundaries that are resistant to degradation and those that are more susceptible to damage in extreme environments. It is found that intergranular damage is mitigated by a high fraction of low energy boundaries, and certain kinds of grain boundaries are more favored by intergranular cracks.

Effect of second introduced phase on magnetotransport properties of La 2/3Sr 1/3MnO 3/0.33(CuO, ZnO, Al 2O 3) composites

The structure, magnetic and magnetotransport properties of La 2/3Sr 1/3MnO 3(LSMO)/0.33(CuO, ZnO, Al 2O 3) composites were investigated to explore the role of second introduced phase. The microstructural analysis shows two kinds of grain boundaries: LSMO/LSMO and LSMO/second phase/LSMO. Two maximal resistivities appear in LSMO/0.33CuO and LSMO/0.33ZnO composites while the resistivity of LSMO/0.33Al 2O 3 decreases monotonically with increasing the temperature from 200 K to 400 K. Moreover, the temperature dependence of magnetoresistance(MR) of LSMO/0.33Al 2O 3 that decreases monotonically with increasing the temperature is different from that of LSMO/0.33CuO and LSMO/0.33ZnO. A developed two-channel model consisting of scattering model and tunneling model was proposed to fit the resistivity—temperature curves of these composites. The role of second introduced phase and the magnetotransport mechanism of these composites were elucidated.

Influence of dual-Ru intermediate layers on magnetic properties and recording performance of CoCrPt–SiO2 perpendicular recording media

Co Cr Pt – Si O 2 perpendicular recording media containing dual-Ru intermediate layers have been studied. The bottom Ru layer deposited under a higher mobility condition, i.e., low Ar gas pressure and bias on substrate, helps to achieve a good crystalline texture, and the top Ru layer under a lower mobility condition, i.e., high Ar gas pressure, promotes the formation of a columnar microstructure. Three kinds of grain boundaries have been observed. The nucleation field and the coercivity as a function of the thickness and dc bias of the bottom Ru layer have been investigated. Medium noise reduction has been observed for dual-Ru intermediate layer media (dual-Ru media) in comparison with that of single intermediate layer media (single-Ru media). This is mainly attributed to the optimization of the physical grain segregation.

Comparison of fatigue cracking possibility along large- and low-angle grain boundaries

In this paper, intergranular fatigue cracking possibility along large- and low-angle grain boundaries (GBs) was compared by using a copper bicrystal with embedded grains. It was found that secondary slip lines were always activated near the large-angle GB segment owing to the plastic strain incompatibility and those slip bands couldn’t pass through the GB. However, the slip bands showed a good one-to-one correlation across the low-angle GB segment without the operation of secondary slip. If the cyclic number was high enough, fatigue cracks firstly initiated along the large-angle GB segment, but no fatigue crack could be observed along the low-angle GB segment. With the help of the electron channelling contrast (ECC) technique in SEM, the fatigued dislocation patterns near the GBs were observed. Those observations provided a direct evidence on the interactions of persistent slip bands (PSBs) with GBs. Based on the experimental results, the dependence of intergranular fatigue cracking on the interaction modes of PSBs with different kinds of GBs is discussed. A defect model (including dislocations and vacancies) leading to intergranular fatigue cracking is proposed. Because the defects carried by PSBs beside a large-angle GB are often piled-up, i.e. ‘piling-up of defects’, and make a contribution to fatigue cracking along the large-angle GB segment. On the contrary, the defects carried by PSBs can pass through the low-angle GBs and be moved into the adjacent grain. This process is defined as ‘passing-through of defects’ and cannot lead to the fatigue cracking along the low-angle GB.

Atomic and Electronic Structure of High Purity SiC Grain Boundary

ABSTRACTFully relaxed SiC grain boundaries were produced by the sublimation re-crystallization method. Several kinds of grain boundaries of bi- and tri-crystals were investigated applying an atomic resolution high-voltage transmission electron microscopy (ARHVTEM). The rotation axis and the rotation angle were common tofrequently observed grain boundaries, which were, respectively, 70.5 degrees and &lt;1120&gt;. But boundary planes differed from each other. It was shown that the atomic bonding direction was continuous across the stable grain boundary.