Grain Boundary Connectivity Research Articles

Optimization of Grain Boundary Character Distribution of Fe-19Cr-10Mn-1Ni-0.53N High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steel (HNASS) has outstanding mechanical properties, but in its hot working, welding, and high-temperature use, there will be a precipitation phase, especially nitride precipitation that reduces its intergranular corrosion, stress corrosion, corrosion fatigue, and other grain boundary-related properties. At present, the traditional methods of suppressing precipitation phase precipitation, such as solution treatment and alloying, have drawbacks. Grain boundary character distribution (GBCD) optimization of Fe-19Cr-10Mn-1Ni-0.53N HNASS was characterized by electron backscatter diffraction (EBSD) in this work. This study reveals that after grain boundary engineering (GBE) treatment, the percentage of low Σ coincidence site lattice (CSL) boundaries of the solid solution treated sample rises from 53.94% to 82.41%, because the sample was cold-rolled (CR) by 5%, followed by annealing at 1423 K for 10 min, and the twin related domains (TRD) size increases from 23.99 μm to 169.82 μm and ν (the ratio of TRD size to grain size) raises from 1.74 to 7.52. The connectivity of the random high-angle grain boundary (RHAGB) web is interrupted and the GBCD of the experimental steel is remarkably optimized.

Microevolution of grain boundary character distribution in Hastelloy C-276 during the annealing process

Grain boundary character distribution and disconnectivity of random boundaries were observed “in-situ” in the Hastelloy C-276 alloy sample through thermomechanical processing. The role of Σ3 boundaries in microstructural evolution of grain boundary character distribution was analyzed by electron back-scattered diffraction (EBSD) and the mechanisms of grain boundary engineering (GBE) during annealing were elucidated. The results showed that the fraction of Σ3n boundaries (including Σ3 boundaries) consistently increased in number with annealing time; however, the Σ3 boundaries had different effects on the disconnectivity of random boundaries because of their variants, i.e., coherent and incoherent Σ3 boundaries. At the initial annealing stage, new incoherent Σ3 boundaries were generated, according to the “Σ3 regeneration mechanism” model; Σ9 and Σ27 boundaries appeared and increased in number with increasing Σ3 boundary interactions. These incoherent Σ3 boundaries could disrupt the connectivity of the random grain boundaries and form large clusters. However, at the annealing stage, some incoherent Σ3 boundaries were annihilated, and new coherent ones emerged. As a result, the random grain boundary network evolved by obtaining significant connectivity with larger clusters, attributed to the reduction of the incoherent Σ3 boundaries. This implies that the incoherent Σ3 boundaries play an important role in the optimization of grain boundary engineering.

Strain-induced microstructural evolution and its implication on high-temperature hot corrosion (HTHC) phenomena in Alloy 617

The influence of various extents of strain on the microstructure and the high-temperature hot corrosion (HTHC) behavior in Alloy 617 is examined in this study. Electron backscatter diffraction characterization revealed an increase in the spread in the local misorientation zones from the vicinity of boundaries to the grain interiors with the extent of strain. Moreover, the increase in the degree of deformation also increased the connectivity of the random high angle grain boundaries (HAGBs), as corroborated by the fractal analysis, since most of the twin boundaries changed their character from ‘special’ to ‘random’ on the application of strain. The HTHC behavior of the as-received and the strained specimens is evaluated by exposing them to a salt mixture (Na 2 SO 4 + NaCl+V 2 O 5 ) at 1273 K for 24 h. The hot corrosion damage is assessed based on oxide scale characteristics (morphology, thickness, and composition), internal percolation depth, Micro-CT (porosity), and Mott-Schottky (passive film defects) analyses. In-depth characterization of the corroded specimens unveils an improved resistance to HTHC in the highly strained specimens due to the enhanced Cr diffusion through the defects and connected random HAGBs which facilitated the formation of a continuous, and compact Cr-rich oxide film. Importantly, the occurrence of recrystallization and the evolution of the Cr-rich carbides in the strained specimens in the course of the HTHC test increased its likelihood of resisting the aggressive environment for a prolonged duration. • Local misorientations, TTBs, and random HAGBs connectivity increased on straining. • Oxide layer formed on strained specimens is continuous, dense and protective. • Higher HTHC resistance in strained specimens is attributed to greater Cr diffusion. • The enhanced Cr diffusion is due to increased defects and random HAGBs connectivity. • Straining even led to superior resistance against damage during prolonged HTHC test.

Dynamics of Ga penetration in textured Al polycrystal revealed through multimodal three-dimensional analysis

We investigate in situ the penetration of liquid Ga in a polycrystalline Al rod at a temperature of 31.9°C. Our study is made possible with the aid of a novel, multimodal laboratory-based X-ray imaging platform, which enables us to characterize the three-dimensional (3D) grain structure, Ga wetting behavior, and correlations of the two. Our sample shows a strong texture along [001] and a higher fraction of low-angle grain boundaries (LAGBs) compared to a random distribution, signifying a poor connectivity of high-angle grain boundaries (HAGBs). We detect channels of Ga along HAGBs after 1-hour of exposure to the liquid metal, but such channels gradually disappear upon prolonged annealing. We interpret this phenomenon in the lens of geometric percolation theory and diffusion theory, which point to the synergistic effects of a limited HAGB connectivity and the enhanced Ga leakage from HAGBs into the bulk through dislocation-pipe diffusion. This work provides detailed insight into the influences of texture and grain boundary connectivity on the dynamics of liquid metal embrittlement, with relevance to grain boundary engineering of embrittlement-resistant materials with high stacking fault energy.

Dynamic graphitization of ultra-nano-crystalline diamond and its effects on material resistivity

Detailed structural and electrical properties of ultra-nano-crystalline diamond (UNCD) films grown in H2/CH4/N2 plasma were systematically studied as a function of deposition temperature (Td) and nitrogen content (% N2) to thoroughly evaluate their effects on resistivity. It was found that even the films grown with no nitrogen in the synthetic gas mixture could be made as conductive as 10−2 Ω cm. The overall resistivity of all the films was tunable over 4 orders of magnitude through varying growth parameters. On a set of 27 samples, Raman spectroscopy and scanning electron microscopy show a progressive and highly reproducible material phase transformation from ultra-nano-crystalline diamond to nano-crystalline graphite as deposition temperature increases. The rate of this transformation is heavily dependent on the N2 content estimated by secondary ion mass spectroscopy. The addition of nitrogen greatly increases the amount of sp2 bonded carbon in the films, thus enhancing the physical connectivity in the grain boundary (GB) network that has high electronic density of states. However, the addition of nitrogen greatly slows down crystallization of the sp2 phase in the GBs compromising electron transport. Therefore, the proper balance between GB connectivity and crystallinity is the key in resistivity engineering of UNCD.

Effect of hot rolling reduction and annealing treatment on grain boundary character distribution in austenitic low-density Fe-Mn-Al-C-Cr steel

Grain boundary engineering in the austenitic low-density Fe-Mn-Al-C-Cr steel was implemented by simple annealing treatment at 1100 °C for 30 min after hot rolling with 66% and 80% reduction. The fractions of low-Σ coincidence site lattice (CSL) boundaries have exceeded 60% after hot rolling, and reached 77% after annealing treatment. Whether after hot rolling or annealing, the fractions of Σ9 and Σ27 in samples with 80% reduction were obviously higher than those with 66% reduction. It indicated that hot rolling reduction was the key to realize grain boundary character distribution optimization. The results showed that the connectivity of random high angle grain boundaries was broken through the formation of large grain clusters only occurred in the sample with 80% reduction, which inhibited the grain growth during annealing. It provided a convenient method to carry out grain boundary engineering.

New perspectives on twinning events during strain-induced grain boundary migration (SIBM) in iteratively processed 316L stainless steel

Characterization of microstructure and grain boundary character distribution (GBCD) during iterative thermo-mechanical processing (TMP) of 316L stainless steel were done using electron backscatter diffraction (EBSD). Results from EBSD scans were analyzed in terms of the fraction of special boundaries, their deviation from ideal misorientation, connectivity of random high-angle grain boundaries, and grain size. In order to understand the efficacy of the iterative processing route leading to a well-connected twin boundary network, additional analysis was done in terms of twin-related domain statistics and triple junction distribution. Results from these analyses show that initial iteration results in a microstructure with insufficient twin boundary density, and a poor twin boundary network. Although the next iteration leads to a small increase in twin statistics, it also does not significantly improve the statistics of favorable grain boundaries and their network. However, the fourth iteration of the processing results in a large improvement in both the population and distribution of favorable grain boundaries. All the subsequent iterations were found to result in microstructural deterioration in terms of both population and network of these grain boundaries. Based on the analysis, role of underlying mechanisms such as strain-induced grain boundary migration and static recrystallization in the effective optimization of GBCD was analyzed. The present research gives important insights into these mechanisms and their control during TMP in order to achieve an engineered microstructure.

Structural and electrical properties of La0.67(Ca0.3Sr0.03)MnO3 composites prepared with added Ag

In this account, La0.67(Ca0.3Sr0.03)MnO3:mol%Agx (LCSMO:Agx, 0 ≤ x ≤ 0.5) high-density composites were successfully prepared by the conventional sol-gel method using deionized water as solvent. The structures, surface morphologies and electrical transport properties of obtained composites were analyzed by various analytical methods. The X-ray diffraction (XRD) profiles of LCSMO:Agx exhibited single perovskite structure with pure orthorhombic crystal structure (Pnma space group) and without presence of traces secondary phases. Field emission scanning electron microscopy (FESEM), XRD and X-ray photoemission spectroscopy (XPS) depicted good grain surface connectivity of the specimens. In addition, grain size increased with Ag doping. The influences of Ag2O doping on materials electrical properties were also explored. The results indicated the presence of silver as Ag+ ions substituting A-site ions. Furthermore, small amounts of metallic silver gathered at the grain boundaries (GBs) were detected. This increased both the MnO bond length and MnOMn bond angle, and slightly expanded both cell volume (V) and lattice constant (a, b, c). The maximum value of temperature coefficient of resistivity (TCR) reached 11.2% K−1 with peak TCR temperature (Tk) of 281.5 K. The improved electrical transport performances of LCSMO:Agx materials were attributed to Ag doping, which enhanced Mn4+ ion amounts and GBs connectivity in each specimen. In addition, excess replacement of A-site ions by Ag+ expanded the metal-insulator transition characteristics and reduced the temperature coefficient of resistivity. Finally, a double exchange based mechanism was proposed to explain the above results.

Computer-aided analysis of grain-boundary connectivity of B10 copper-nickel alloys

ABSTRACTA numerical calculation method based on the angle in a triple junction composed of a random grain boundary is proposed to predict the connectivity and stability of a grain boundary in a B10 copper-nickel alloy. The grain-boundary connectivity and its effect on corrosion resistance are studied combining computer-aided analysis with electrochemical impedance testing. The results show that the prediction of corrosion resistance using a grain-boundary connectivity numerical method is consistent with immersion experimental results. The B10 alloy exhibited the best corrosion resistance after cold rolling with a 9% reduction rate. The relationship between the grain-boundary characteristics and corrosion resistance is well established using the numerical method to quantify the grain-boundary connectivity. A higher connective frequency and a low proportion of grain-boundary angles between 60° and 180° in the triple junction is detrimental to corrosion resistance of the B10 alloy.

Quantification of grain boundary connectivity for predicting intergranular corrosion resistance in BFe10-1-1 copper-nickel alloy

Abstract

Helium bubble nucleation at grain boundaries and its influence on intergranular fracture

ABSTRACTAn in-depth understanding of the formation of intergranular helium bubbles and its relation to embrittlement is an important issue in the nuclear industry. In this paper, a thermodynamic model is developed to analyze the nucleation of intergranular helium bubbles. Microstructural observation using scanning electron microscopy and electron backscatter diffraction gives a detailed description for the relation between the bubble formation and the grain-boundary (GB) misorientation in helium-implanted nickel and Inconel X750. The theoretical and the experimental results confirm that the nucleation of intergranular helium bubbles is GB structure-dependent, the helium-to-vacancy ratio plays an important role in the bubble precipitation, and the interfacial tension of bubbles cannot be approximated to be the interfacial energy. The bubble-induced intergranular embrittlement in a polycrystal is modelled. The GB misorientation distribution, the intergranular bubble nucleation and growth and the GB connectivity are the key factors affecting the GB fracture toughness. The hoop ductility of the cladding tubes containing helium is analyzed. The hoop stress-induced increase in the GB energy promotes the precipitation of bubbles at the radial GBs and lead to the loss of tube ductility. Based on this work, the complicated correlation among the intergranular helium bubbles, the GB structure, the helium concentration, the applied stress and the helium embrittlement is clarified.

The effect of cold rolling on grain boundary structure and stress corrosion cracking susceptibility of twins in alloy 690 in simulated PWR primary water environment

The effects of cold rolling on the structure and stress corrosion cracking (SCC) susceptibility of twin boundaries in alloy 690 were studied. Most of the twin boundaries were transformed into random high angle boundaries (RHABs) following cold rolling. The transformed twin boundaries (TTBs) showed increased susceptibility to carbide precipitation during subsequent aging at 475°C. The SCC initiation test in 360°C hydrogenated water indicated that the prior RHABs exhibited increased SCC susceptibility after cold rolling. Moreover, TTBs become susceptible to SCC due to the promoted outward diffusion of Cr and can enhance the connectivity of susceptible grain boundaries.

Preferential growth of boron layer in magnesium diboride (MgB2) by Mg diffusion method

Growth mechanism and grain boundary (GB) contact of polycrystalline MgB2 fabricated by Mg diffusion method are studied by STEM and EELS analyses. In contrast to the previous reports based on the computational calculation, preferential growth of (001) boron (B) layer and the B-B contact at MgB2 GBs are confirmed by annular dark field (ADF) –STEM image and the combined EELS analyses. The effect of B-B contact at the GB on the superconductivity is further evaluated using First principles calculation. Superior GB linkage of the supercurrent flow via GB B-B contact is expected from the calculated density of states at Fermi level. B-terminated growth mechanism in Mg diffusion method and the effect of GB connectivity via B-B and Mg-Mg contacts are discussed. Finally, we suggest a model of GB linkage of supercurrent flow via B-B contact in polycrystalline MgB2.

Grain boundary engineering: fatigue fracture

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.

Effects of Ag addition on the structural and electrical properties of La0.67Sr0.33MnO3 ceramics

ABSTRACTLa0.67Sr0.33MnO3:mol-%Agx (LSMO:Agx, x = 0, 0.1, 0.2, 0.3 and 0.4) polycrystalline ceramics were prepared by chemical co-precipitation methods. All of the LSMO:Agx samples showed rhombohedral perovskite structure with a space group of . The diffraction peaks of metallic Ag did not appear in the XRD graph, due to the Ag substitution at A sites (La3+ or Sr2+ ions) and the evaporation loss of the element Ag in the process of high-temperature sintering, or the residual amount of Ag beyond the sensitivity of the XRD. With the increase of Ag addition, unit cell volume (V), metal−insulator transition temperature (Tp) and conductivity at Tp (ρTp) of the LSMO:Agx samples increased significantly. At the same time, the temperature coefficient of resistance (TCR%), temperature peak of the TCR% (Tk) and the figure of merit (Z) reached the optimal values of 3.3% K−1, 367 K and 0.049 at x = 0.3. Ag substitution at A sites (La3+ or Sr2+ ions) and gathering at grain boundaries (GBs) were proposed to explain the existence of Ag addition. All results suggested that the enhancement of electrical properties was due to silver-doping and it increased the bond angles of Mn−O−Mn (θMn−O−Mn) and the bond distance of Mn−O (dMn−O), improving Mn4+ concentration, grain sizes and GBs connectivity in LSMO:Agx ceramics.

Engineering the grain boundary network of thin films via ion-irradiation: Towards improved electromigration resistance

Controlling the grain boundary network of small-scale materials—such as coatings and thin films—is an ambitious goal that would ultimately enable the production of components with tailored properties and improved reliability. In this work we present a new technique—which we term ion-induced grain boundary engineering (iGBE)—to engineer the character distribution and connectivity of grain boundaries in gold films in situ, as they are being deposited. iGBE consists of a repeated sequence of ion-induced material removal and material deposition which results in the selection and growth of crystal grains in twinned relationship. This phenomenon yields a substantial increase in the density and connectivity of Σ3 twin boundaries, which have renowned beneficial effects on the material's resistance to intergranular degradation. We confirm the improved reliability of iGBE microstructures by assessing their resistance to electromigration—one of the most common causes of failure in integrated circuit interconnects. We find that, through iGBE, interconnect lifetime increases by three orders of magnitude at standard operating conditions. Since iGBE is material-insensitive and compatible with standard microfabrication technology, we expect it to have significant impact on microelectronics industry.

Iterative thermomechanical processing of alloy 600 for improved resistance to corrosion and stress corrosion cracking

The effects of thermomechanical processing (TMP) with iterative cycles of 10% cold work and strain annealing, on corrosion and stress corrosion cracking (SCC) behavior of alloy 600 was studied. The associated microstructural and cracking mechanisms were elucidated using transmission (TEM) and scanning electron microscopy (SEM), coupled with precession electron diffraction (PED) and electron back scatter diffraction (EBSD) mapping. TMP resulted in increased fraction of low coincident site lattice (CSL) grain boundaries whilst decreasing the connectivity of random high angle grain boundaries. This disrupted random grain boundary network and large fraction of low CSL boundaries reduced the propensity to sensitization, i.e. carbide precipitation and Cr depletion. After TMP, alloy 600 (GBE) also showed higher intergranular corrosion resistance. Slow strain rate tests in 0.01 M Na2S4O6 solution at room temperature show TMP lowered susceptibility to intergranular SCC. To better understand the improvements in corrosion and SCC resistance, orientation maps of regions around cracks were used to analyze the interactions between cracks and various types of grain boundaries and triple junctions (TJs). Detailed analysis showed that cracks were arrested at J1 (1-CSL) and J2 (2-CSL) type of TJs. The probability for crack arrest at special boundaries and TJs, calculated using percolative models, was found to have increased after TMP, which also explains the increase in resistance to corrosion and SCC in GBE alloy 600. A clear correlation and mechanistic understanding relating grain boundary character, sensitization, carbide precipitation and susceptibility to corrosion and stress corrosion cracking was established.

Surface grain boundary engineering of Alloy 600 for improved resistance to stress corrosion cracking

In this paper, we demonstrate a novel method for grain boundary engineering in Alloy 600 using iterative cycles of ultrasonic nanocrystal surface modification (UNSM) and strain annealing to modify the near surface microstructure (~250µm) for improved stress corrosion cracking (SCC) resistance. These iterative cycles resulted in increased fraction of special grain boundaries whilst decreasing the connectivity of random grain boundaries in the altered near surface region. A disrupted random grain boundary network and a large fraction of low CSL boundaries (Σ3–Σ27) reduced the propensity to sensitization. Slow strain rate tests in tetrathionate solutions at room temperature show that surface GBE lowered susceptibility to intergranular SCC. Detailed analysis of cracks using Electron Back-scattered Diffraction showed cracks arrested at J1(1-CSL) and J2 (2-CSL) type of triple junctions. The probability for crack arrest, calculated using percolative models, was increased after surface GBE and explains the increase in resistance to SCC.

Characterisation of grain boundary cluster compactness in austenitic stainless steel

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>Σ29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of Σ3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.

Three-dimensional morphology of grain boundary Widmanstätten ferrite in a low carbon low alloy steel

The three-dimensional morphology of austenite grain boundary Widmanstätten ferrite in a low carbon low alloy steel was revealed by means of serial sectioning in conjunction with computer-aided three-dimensional reconstruction techniques. Primary and secondary Widmanstätten sideplates are better described to be wedges instead of plates. Primary Widmanstätten sawteeth appear to be spikes or triangular cones whereas secondary Widmanstätten sawteeth have the shape of elongated spikes or triangular cones. These results indicate that the three-dimensional shape and connectivity of grain boundary Widmanstätten ferrite are different from those deduced on random planar sections by two-dimensional microscopy.