Effect Of Grain Boundary Structure Research Articles

The effects of grain boundary structure and bulk Cr content on grain boundary degradation behavior of Ni-Cr binary alloys in high temperature CO2

The effects of grain boundary (GB) structure and bulk Cr content (from 15 wt.% to 30 wt.%) on GB oxidation behavior of Ni-Cr binary alloys were investigated in 650°C/1atm CO2 gas. The degradation behaviors of GBs, which varies greatly (including preferential intergranular oxidation, general intergranular oxidation and GB passivation), is dictated by the available Cr source which depends on the bulk Cr content and the Cr diffusion flux to the reaction front. The Cr diffusivity along GB is dependent on GB structure and decreases in the order of random high angle GB (RHAB), low angle GB (LAGB), incoherent twin boundary (ITB) and coherent twin boundary (CTB). Interestingly, other coincident site lattice boundary (CSLB (Σ>3)) shows a Cr diffusivity between ITB and CTB. More importantly, the threshold bulk Cr content for achieving GB passivation is also closely related to GB structure. To be specific, LAGB completely passivates when the bulk Cr content is between 20 wt.% and 25 wt.%, while ITB completely passivates at a bulk Cr content of 25 wt.% and CSLB (Σ>3) completely passivates when the bulk Cr content is between 25 wt.% and 30 wt.%. In contrast, CTB is subject to general intergranular oxidation while RHAB shows passivation regardless of the bulk Cr content.

Effects of grain boundary structure and shape of the solid–liquid interface on the growth direction of the grain boundaries in multicrystalline silicon

The unique samples fabricated using seed crystals with artificial grain boundaries (GBs) allowed a systematic investigation of the effect of the macroscopic GB structure (orientation, asymmetric angle, Σ values) on the GB growth direction.

Effect of Solid Solution Treatment on Corrosion Resistance of Al-Cu-Mg-Ag Alloy

In this experiment, Al-Cu-Mg-Ag alloy was used as material and solution temperature was used as variable to investigate its effect on the corrosion properties of the alloy and Hardness test, metallographic observation, electrochemical test, intergranular corrosion and exfoliation corrosion test were carried out on three groups of samples. The results show that the intergranular corrosion resistance of the alloy decreases with the increase of solution treatment temperature, and the sample treated at 505 °C has the best performance. This is mainly because grain boundary structure plays a role in increasing PFZ and expanding corrosion channels. The exfoliation corrosion resistance of Al-Cu-Mg-Ag alloy increases first and then decreases, and the sample treated at 515 °C has the best performance. This is due to the dual effects of grain boundary structure and grain morphology. On the one hand, the solution treatment temperature increases, which widens the precipitation-free zone and reduces the electrochemical corrosion resistance of the alloy. On the other hand, the increase of recrystallized grains decreases the cohesion of corrosion products and enhances the electrochemical corrosion resistance of the alloy.

Effect of Grain Boundary Structure on Yielding and Plastic Deformation of Magnesium

Effect of Grain Boundary Structure on Yielding and Plastic Deformation of Magnesium

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.

The effect of grain boundary structure on the intergranular degradation behavior of solution annealed alloy 690 in high temperature, hydrogenated water

The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360°C hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a continuous surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.

Theoretical analyses of nanocrack nucleation near the main crack tip in nano and micro crystalline materials

Nano or micro crack nucleation in the process of metallic fracture is a common phenomenon and it affects fracture behaviors of the materials. The mechanisms of nanocrack nucleation are different for metals with nanoscale and microscale grain sizes. In this paper, according to the deformation mechanisms of materials with different grain sizes, theoretical models are established to investigate nanocrack nucleation near a main crack tip under mode I load. The theoretical solution is presented based on the distributed dislocation technique. For nanocrystalline metals, the process of nanocrack nucleation due to grain boundary sliding is analyzed and the effect of grain boundary structure on nanocrack nucleation is studied. The results show that nanocracks may be preferentially nucleated at the triple junctions which are located on the upper or lower sides of the main crack plane, instead of the front of the crack tip. A larger nanocrack may be nucleated at triple junctions with a larger characteristic abutting angle of the triple junctions. For microcrystalline metals, nanocrack nucleation due to lattice dislocation pileup is considered. The problem of inclusion cracking due to lattice dislocation pileup is analyzed. The results show that lattice dislocation pileup causes the entire soft inclusion to crack easily, but a relatively small nanocrack can be only nucleated in the hard inclusion.

The Effect of Grain Boundary Structure on the Intergranular Degradation Behavior of Solution Annealed Alloy 690 in Simulated Pressurized Water Reactor Primary Environment

The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360 oC hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a compact surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.

The effect of grain boundary structure on sensitization behavior in a nickel-based superalloy

The present work discusses the evolution of grain boundary structure during thermomechanical processing and its effect on sensitization behavior in a nickel-based superalloy. Alloy 600 was deformed to varied degrees of strain (4–25%) using hot rolling followed by annealing at 1000 °C for 10 min followed by water quenching. Structure of the grain boundary was analyzed with reference to various parameters, such as grain boundary character distribution, twin-related domains, misorientation, and triple junction distribution. Each thermomechanically processed sample was heat-treated at 650 °C for 24 h before studying its sensitization behavior. The effect of structure of the grain boundary on sensitization was assessed through double loop electrochemical potentiokinetic reactivation test and measured in terms of degree of sensitization (DOS). DOS was found to be in a direct relation with the fraction of random high-angle grain boundaries and their connectivity, while it was inversely related to the fraction of low-Σ coincidence site lattice boundaries and special triple junctions. Residual strain and the fraction of low-angle grain boundaries were found to be weakly related to DOS. We show that a simple parameter can be used to predict the combined effect of all these factors on sensitization behavior.

Electrochemical Scanning Tunnelling Microscopy Study of Early Stages of Intergranular Corrosion of Copper

Electrochemical scanning tunnelling microscopy (EC-STM) was used to investigate in situ and locally the topographical alterations of copper surfaces under electrochemical control. The aim of this work was to better understand the early stages of intergranular corrosion and the relationship between local structure of grain boundaries emerging at the solid/liquid interface and corrosion properties. A prerequisite for this approach is the use of a high purity material, in order to exclude the segregation of impurities, such as sulphur, at grain boundaries, which would strongly affect the intergranular corrosion behaviour and prevent us from studying the intrinsic effect of grain boundary structure This was achieved in this work by using high purity cast electrolytic tough pitch (ETP-) Cu, cryogenic rolled and post-annealed at relatively low temperature (200°C). This process also produces a grain size compatible with the limited field of view of STM, as confirmed by electron back scatter diffraction (EBSD) analysis. EC-STM data obtained in situ after a surface preparation producing oxide-free surfaces with emerging grain boundaries show that performing cycles of anodic dissolution in the absence of any surface oxide reveals a grain boundary structure-dependent corrosion behaviour. Localized attack is initiated at grain boundaries that can be assigned to either random-type boundaries or non-coherent coincidence site lattice (CSL) grain boundaries whereas, under the same conditions, coherent twin grain boundaries are not attacked. Direct coupling of EC-STM and EBSD, using a re-positioning procedure allowing the analysis of the same local surface area by both techniques, show that even for the corrosion resistant coherent twin grain boundaries, the intergranular corrosion behavior measured at the emergence of the grain boundaries at the solid/liquid interface is dependent on the local surface structure. Further combined EC-STM and EBSD analysis will be discussed to link grain boundary type, local surface structure and susceptibility to intergranular corrosion.

Effects of grain boundary structure controlled by artificially designed seeds on dislocation generation

We investigated the effects of grain boundary (GB) misorientation and the change in GB energy on the generation of dislocations during the directional growth of a crystalline silicon ingot. For this purpose, two ingots were grown using artificially designed seeds to introduce plural GBs with and without misorientation from an ideal coincident site lattice boundary. We revealed that dislocations are frequently generated from Σ3 GB with misorientation. On the other hand, dislocations are hardly observed around naturally formed perfect Σ3 GBs. In addition, we found that Σ5 and Σ25 GBs with higher GB energies are not sensitive to misorientation, and few dislocations are found around Σ25 GBs.

Effects of grain boundary structure on lithium transport in graphite

Molecular dynamics simulations are used to investigate lithium (Li) transport rates in symmetric tilt graphite grain boundaries (GBs). Experiments have quantified highly varied diffusion rates of Li in graphite and recent computational work exposed similar differences in Li intercalation rates into GBs from a free surface. This work extends findings of intercalation studies to provide explanations for various rates and uses said differences to predict bulk GB transport behavior. Various structural properties are presented for graphite GBs, including bond angles, bond order, and free volume analysis which is rarely presented in covalent systems. The importance of free volume connectivity is also discussed as is Li structure in GBs.

Effect of Grain Boundary Structure on Defect Absorption and Denuded Zone Formation in Irradiated Nanocrystalline Iron

Journal Article Effect of Grain Boundary Structure on Defect Absorption and Denuded Zone Formation in Irradiated Nanocrystalline Iron Get access O El-Atwani, O El-Atwani Drexel University, Department of Materials Science & Engineering, Philadelphia, PA USA Search for other works by this author on: Oxford Academic Google Scholar k Hattar, k Hattar Department of Radiation Solid Interactions, Sandia National Laboratories, NM, United States Search for other works by this author on: Oxford Academic Google Scholar G Vetterick, G Vetterick Drexel University, Department of Materials Science & Engineering, Philadelphia, PA USA Search for other works by this author on: Oxford Academic Google Scholar ML Taheri ML Taheri Drexel University, Department of Materials Science & Engineering, Philadelphia, PA USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 439–440, https://doi.org/10.1017/S1431927615002998 Published: 23 September 2015

Effect of Grain Boundary Structure on Misorientation Change of Pure Copper Bicrystals Pressed by One-Pass Equal-Channel Angular Pressing

Following a previous paper, (Materials Transactions, Vol. 53 (2012), p. 1858), which reports on the dynamic misorientation change absorbed by the grain boundaries themselves during one-pass equal-channel angular pressing (ECAP), the effect of grain boundary structure on the misorientation change during ECAP was examined as the next step. Two bicrystals with a ­3{111} grain boundary and a random one were subjected to ECAP for one pass, and compared. Both the bicrystals were designed to have a similar slip pattern where slip activity was concentrated on one slip plane. Direct evidence of misorientation changes absorbed by grain boundary themselves was obtained again, and its marked dependency on grain boundary structure became evident. Namely, in ­3 grain boundary, misorientation was partially carried by the grain interior in its vicinity, which stems from the dislocation accumulation. On the other hand, in the random grain boundary, most of the misorientation was carried by the boundary itself. These differences are discussed in terms of the capability as a sink of lattice dislocations during severe plastic deformation. [doi:10.2320/matertrans.M2013076]

Effect of grain boundary structure on plastic deformation during shock compression using molecular dynamics

Grain boundaries (GBs) can play an important role in governing the mechanical behavior and damage evolution of a material during both quasistatic and dynamic loading. However, the general consensus of the shock physics community has been that minute details about the GB structure should not affect the response of a material to dynamic loading. In this paper, we present results of molecular-dynamics simulations investigating whether or not small changes in boundary structure are ‘recognized’ by the shock wave and can in turn affect the spall strength of a material. As a test case, we study a Σ11 asymmetric tilt GB in copper with an ordered and a disordered structure. The results are also compared with face-centered-cubic single crystals which correspond to the crystal orientations of the two grains in the bi-crystal. These results show that ordered and disordered boundaries undergo dissimilar amounts of plastic deformation during shock loading, which leads to spall strengths that vary by 12%, likely due to differences in the GB structures.

Effect of grain boundary structure on the intercrystalline damaging of austenic steel during brazing

During brazing of austenitic stainless steel with copper based brazing material a common failure occurs, namely that the brazing material solutes along grain boundaries, which looks like cracks. This unfortunate effect occurred when AISI 304 and 310 steels are brazing. To avoid this unwanted effect - since the cracks propagate mainly on high angle grain boundaries - our goal was to enhance the number of special coincident site lattice type grain boundaries with thermomechanical treatment. Experiments were performed for 1, 48 and 72 hour heat treatments on materials cold rolled at different levels. After the thermomechanical treatment significant decrease in the crack size was found in depth and width, respectively.

Part II. Effects of grain-boundary structure on the path of cracking in polycrystals

Four triple junctions along an intergranular crack caused by oxygen-induced dynamic embrittlement of the nickel-base superalloy IN718 have been modeled with bicrystals having the same kinds of boundaries as the boundaries at the triple junctions that did and did not crack. It was found that in all cases the cracking susceptibilities of the bicrystals matched those found at the triple junctions. A low-angle boundary and a Σ3 boundary were found to be much more resistant to cracking than four random boundaries and a Σ19 and a Σ31 boundary. This supports the position that thermo-mechanical processes that increase cracking resistance do so mainly because they increase the content of Σ3 boundaries.

Effect of Grain Boundary Structure on Diffusion‐Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion‐induced grain boundary migration (DIGM) has been investigated in BaTiO3. SrTiO3 particles were scattered on the polished surfaces of two kinds of BaTiO3 samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H2 for those with rough boundaries. In the BaTiO3 samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain‐boundary migration hardly occurred in the BaTiO3 samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.

The effects of grain boundary structure on binding of He in Fe

Computer simulations were performed to explore the effect of grain boundary (GB) structure and properties on the binding of He to boundaries in Fe. Symmetrical tilt GBs spanning a range of GB energies and excess volumes were examined. Molecular statics was used to map the He trapping efficiency at numerous substitutional and interstitial sites in and near each GB. The results showed that both substitutional and interstitial He atoms were trapped at GBs. Interstitial He was more strongly bound ( E gb i ∼0.5–2.7 eV) to the GB core than substitutional He ( E gb s ∼0.2–0.8 eV). The He binding energy was found to increase linearly with GB excess volume. The He capture radius varied from ∼0.3 to 0.7 nm and also depended on GB properties. Finally, the He binding energy varied significantly within the GB core and approximately corresponded to the variation in atomic excess volume normal to the GB plane.

Effects of Grain Boundary Structure and Chemistry on Electrical Activity in Polycrystalline Silicon

Effects of Grain Boundary Structure and Chemistry on Electrical Activity in Polycrystalline Silicon