Fraction Of Special Grain Boundaries Research Articles

Twin-Related Grain Boundary Engineering and Its Influence on Mechanical Properties of Face-Centered Cubic Metals: A Review

On the basis of reiterating the concept of grain boundary engineering (GBE), the recent progress in the theoretical models and mechanisms of twin-related GBE optimization and its effect on the mechanical properties is systematically summarized in this review. First, several important GBE-quantifying parameters are introduced, e.g., the fraction of special grain boundaries (GBs), the distribution of triple-junctions, and the ratio of twin-related domain size to grain size. Subsequently, some theoretical models for the GBE optimization in face-centered cubic (FCC) metals are sketched, with a focus on the model of “twin cluster growth” by summarizing the in-situ and quasi-in-situ observations on the evolution of grain boundary character distribution during the thermal-mechanical process. Finally, some case studies are presented on the applications of twin-related GBE in improving the various mechanical properties of FCC metals, involving room-temperature tensile ductility, high-temperature strength-ductility match, creep resistance, and fatigue properties. It has been well recognized that the mechanical properties of FCC materials could be obviously improved by a GBE treatment, especially at high temperatures or under high cyclic loads; under these circumstances, the materials are prone to intergranular cracking. In short, GBE has tremendous potential for improving the mechanical properties of FCC metallic materials, and it is a feasible method for designing high-performance metallic materials.

Pitting behavior of friction stir repair-welded 304L stainless steel in 3.5% NaCl solution at room temperature: role of grain and defect structures

Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to < 10% in the SZ. During friction stirring, the oxide layer of artificial crack was broken and aligned to form a spiral defect called lazy-S structure. All these microstructural changes affected the corrosion behavior of the FSW specimens when tested in 3.5% NaCl at room temperature using cyclic polarization, chronoamperometry, and electrochemical impedance spectroscopy. The FSW specimens showed lower polarization resistance and lower transpassive potential than those of the base metal specimens. However, the pitting protection potentials of the FSW specimens were higher than that of the base metal. The pitting behavior of the FSW specimens was influenced more by the preferential attack on the lazy-S region than by passive film breakdown. The flat band potentials of the passive film formed on the FSW specimens were more positive than that of base metal. The charge carrier density of passive film formed on the FSW specimens was higher than that of the base metal. The higher fraction of low-angle grain boundaries present in the FSW specimens could supply more number of misorientation dislocations at the metal/film interface which could form anion vacancies by a climb process leading to formation of oxide at these locations without stressing the substrate. Therefore, low-angle boundaries are considered helpful for formation of stress-free passive films that will be highly stable and enhance both pitting and stress corrosion cracking resistance.

Characterization of Grain Boundary‐Engineered Aluminum–Magnesium Alloys

Herein, the effect of sputtering rate on the fraction of special grain boundaries in 5xxx series Al–Mg alloys is explored. Samples are synthesized via interrupted direct current (DC) magnetron sputtering with varying deposition rates, and the grain size and grain boundary character are evaluated with electron backscatter diffraction (EBSD). The highest sputtering rate (7 nm s−1) leads to an increase in the total number of special grain boundaries, ≈1.5× greater than that of the lower rates. Increased thermal energy and enhanced stress relaxation during film growth promote the formation of Σ3 and Σ7 boundaries.

High-temperature tensile and high cycle fatigue properties of inconel 625 alloy manufactured by laser powder bed fusion

High-temperature tensile and high cycle fatigue properties of hot isostatic pressed LPBF Inconel 625 (laser powder bed fusion, HIPed LPBF 625) were investigated and then compared to conventional wrought Inconel 625. HIPed LPBF and wrought alloys show similar microstructural characteristics in average grain size, defects, fraction of special grain boundaries, and grain morphologies. However, Al2O3 and TiN phase was found (without carbides) in HIPed LPBF 625, and various stoichiometry of carbides (without Al2O3) were identified in wrought 625. The yield and tensile strengths and elongation at fracture of both alloys have similar values at room temperature. At 650 °C, elongation at fracture of HIPed LPBF 625 significantly decreases compared to that of wrought 625. HIPed LPBF 625 also has a lower high-cycle fatigue limit of 500 MPa (at 107 cycles without fracture) and shorter life in low-stress conditions compared to wrought 625. The relatively higher sulfur content in HIPed LPBF 625 (33 ppm) is suggested as the reason for lower fatigue endurance at 650 °C. The LPBF (or additive manufacturing), which uses powder feedstock, is vulnerable to impurity concentration increases, elemental contamination control is critical in the initial powder and LPBF process.

Microstructural characterization of copper coatings in development for application to used nuclear fuel containers

In the design of used nuclear fuel containers for deep geological repositories, copper is considered to be a suitable and long-lived barrier for corrosion resistance. The microstructures of state-of-the-art copper materials used in this application produced through extrusion, a grain boundary engineered electrodeposition technique and cold spraying were studied via electron backscattered diffraction. Desirable microstructural characteristics for localized corrosion resistance of pure copper were compiled from the literature considering grain size, grain boundary character distribution, and crystallographic texture. The subject copper materials were found to have favourable microstructures for localized corrosion resistance, in particular, a high fraction of special grain boundaries, especially Σ3 twins, rendering them suitable for the given application.

Influence of heat treatment on corrosion resistance of explosive welding 316L stainless steel composite plate

The Electron Backscattered Diffraction (EBSD) was used to study the distribution of low and high angle boundaries and coincidence site lattice (CSL) grain boundaries of 316L cladding, which were in four different conditions (explosive condition, 580 °C–3 h, 720 °C–1.5 h and 900 °C–0.5 h). Polarization curves and intergranular corrosion tests were used to test the corrosion resistance of 316L stainless steel composite plate in four conditions. The results showed that there were substructures in explosive samples, and the internal substructures gradually disappear after heat treatment. The fraction of low-angle grain boundaries of the explosive condition, 580 °C, 720 °C and 900 °C samples were 15.76%, 3.77%, 2.32% and 5.52%, respectively. The corrosion potential of 316L treated by 900 °C–0.5 h was the highest (−0.196 V). The result of intergranular corrosion test shows that the sum of two stages of 900 °C–0.5 h heat treatment was the lowest (274.048 g.(m2.h)−1). The quantity and the fraction of special grain boundaries of different conditions can influence the corrosion resistance of 316L composite plate. The fraction of low ΣCSL (CSL ≤ 29) grain boundaries of sample treated by 900 °C–0.5 h is the highest (67%), meanwhile, its corrosion resistance is the most excellent.

In-situ Study of the Effect of Hydrogen on Fatigue Crack Initiation in Polycrystalline Nickel

Correlating hydrogen embrittlement phenomenon with the metallic microstructural features holds the key for developing metals resistant to hydrogen-based failures. In case of fatigue failure of hydrogen charged metals, in addition to the hydrogen-based failure mechanisms associated with monotonic loading such as HELP, HEDE etc., microstructural features such as grain size, type of grain boundary (special/random), fraction of special grain boundaries; their network and triple junctions can play a complex role. The probable sites for fatigue crack initiation in such metals can be identified as the sites of highest hydrogen concentration or accumulated plastic strain. To this end, we have developed an experimental framework based on in-situ fatigue crack initiation and propagation studies under scanning electron microscope (SEM) to identify the weakest link in the metallic microstructure leading to failure. In-situ fatigue experiments are performed on carefully designed polycrystalline nickel (99.95% pure) specimens (miniaturised, shallow-notched & electro-polished) using a 10 kN fatigue stage inside the SEM. Electron Back Scattering Diffraction (EBSD) map of the notched region surface helps identify the distribution of special/random grain boundaries, triple junctions and grain orientation. The specimen surface in the shallow notched region for both the hydrogen charged and un-charged specimens are then carefully studied to correlate the microstructural feature associated with fatigue crack initiation sites. Such correlation of the fatigue crack initiation site and microstructural feature is further corroborated with the knowledge of hydrogen trapping and grain’s elastic anisotropicity to be either the site of high hydrogen concentration, accumulated plastic slip or both.

Grain boundary engineering parameters for ultrafine grained microstructures: Proof of principles by a systematic composition variation in the Cu-Ni system

The main principles of grain boundary engineering of an ultrafine grained microstructure are studied via a systematic variation of the stacking fault energy (γSFE), solid solution effects (SSEs) and the homologous deformation/annealing temperatures choosing the Cu-Ni system as a model case for non-segregating alloys. Cu and Ni are completely miscible and the γSFE varies strongly with the alloy composition. Ultrafine grained microstructures are produced by high-pressure torsion and their evolution upon annealing is investigated. The thermal stability and the saturation grain sizes after deformation are determined by SSEs. The fraction of deformation twins varies in accordance with the γSFE. The increase of the length fraction of Σ3 grain boundaries versus the grain size is found to be governed by the SSEs, whereas the increase of the length fraction of Σ9 grain boundaries versus the grain size depends on the γSFE. The results indicate that grain boundary engineering may lead to an optimum ultrafine grained structure, i.e. with a high length fraction of Σ3 and Σ9 grain boundaries of approximately 40%. This is a significantly large fraction in comparison to a not-engineered microstructure and achieved for grain sizes below 1000 nm in alloys close to the equiatomic composition having a low γSFE and a high melting temperature. This high fraction of Σ3n grain boundaries (including their conjunctions) proved itself most effective for microstructure stabilization in this virtually non-segregating system. Thus, for Cu50Ni50 and Cu65Ni35, a narrow grain size distribution of small grains, a high hardness and a high fraction of special grain boundaries can be adjusted.

Dynamic evolution of grain structure and micro-texture along a welding path of aluminum alloy profiles extruded by porthole dies

The solid state welding process during porthole die extrusion of hollow structure aluminum alloy profiles is very important since it directly determines the microstructure and mechanical properties of welding seams in extruded profiles. In this study, the grain structure, special grain boundary and micro-texture along the welding path of an aluminum alloy profile were characterized, and the relation between the microstructure and mechanical properties of the welding seam was revealed. It was found that, along the welding path, the fine equiaxed grains around the welding line are elongated along extrusion direction under the combined action of shearing and compression, then, the elongated grains grow into the strip-shaped coarse grains by means of grain boundary migration, and finally, some strip-shaped coarse grains evolve into the fine equiaxed grains by means of continuous dynamic recrystallization and geometric dynamic recrystallization. Along the welding path, the fraction of special grain boundaries firstly increases and then decreases, while the fraction of low angle grain boundaries increases continually. The strong {111} fiber is formed in the zone near the surface of bridge, and it transforms into the very strong shear-type texture consisting of A*1 and A*2 components at the beginning stage of extrusion welding. Then, these shear-type texture components transform into the strong goss and copper components. Finally, the goss and copper components weaken and the weak cube texture appears in the welding zone of the extruded profile. It was also found that the increase of the low angle grain boundaries and the decrease of the average grain size in welding zone contribute to the improvement of harness, strength and fracture strain, while the existence of the strip-shaped coarse grains and different types of textures in welding zone is adverse for the improvement of welding quality.

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.

Study on cryorolled Al–Cu alloy using X-ray diffraction line profile analysis and evaluation of strengthening mechanisms

Al–Cu alloy sheets were rolled at room temperature and at liquid nitrogen temperature to true strain values of 0.69 and 1.38. The microstructural changes and variation in mechanical properties due to the effect of rolling temperature was evaluated. X-ray diffraction line profile analysis was performed to calculate crystallite size, microstrain and dislocation density. Irrespective of the amount of reduction, cryorolled samples exhibited reduced crystallite size along with enhanced lattice strain and dislocation density than room temperature rolled samples. At both the reductions, cryorolled samples exhibited better hardness and strength compared to the room temperature rolled samples and the enhanced properties are attributed to the higher dislocation density of the cryorolled samples. The electron backscatter diffraction results indicated that the grains are submicron in size in rolled conditions. It also revealed that majority of the fraction of special grain boundaries are Σ1 boundaries with decreasing tendency of Σ3 boundaries. Quantitative contributions of different strengthening mechanisms were performed on the rolled samples and it was observed that significant contribution comes from grain boundary strengthening.

Comparison of Single-Step Recovery Anneal Processing and a Single-Step Recrystallization Processing on the Generation of Special Boundaries in Commercially Pure Nickel

The strain recovery route to generate a significant increase in special boundaries nominally involves a strain level of less than about 10%, plus a recovery anneal at a temperature–time combination sufficient to cause grain boundary migration, but insufficient cold work/time/temperature combinations to cause recrystallization of the material being processed. In the present communication using a simple material to avoid complications from industrial alloys, commercially pure (C.P.) Ni was processed using a single low strain value of 6% and a temperature of 800–1000 °C for time up to 38 min. The fraction of special grain boundaries, Fsp, based on a Σ3–29 as evaluated by Orientation Image Microscopy was then compared with the as-received C.P. Ni and Ni strained 25% and annealed at 800 °C to give a recrystallized structure. For the single low strain route, a 6% deformation at a temperature of 800 °C and a time of 8–10 min resulted in an Fsp value of close to 80%, as well as a grain size of about 25–35 μm compared to the as-received value of about 25 μm. Thus, while grain boundary migration occurred under these processing conditions, the increase in grain size was not excessive and would not result in a large reduction in tensile properties. The single 25% rolled and 800 °C recrystallized test samples however resulted in no major increase in Fsp relative to the as-received material, both being in the range 35–40% over a range of times. Therefore, compared to the optimized Fsp low strain recovery values of close to 80%, the fully recrystallized Fsp values were approximately half. It should be noted however that multiple cycles of higher strain values rather than a single strain value, coupled with a recrystallized anneal, have been shown in the literature to result in increase in the Fsp levels.

Evolution of a grain-boundary ensemble of submicrocrystalline molybdenum annealed under nickel diffusion along grain boundaries

The effect of annealing, including that performed under impurity diffusion from an external medium (coating), on an evolution of the structure and grain-boundary ensemble of submicrocrystalline molybdenum produced by severe plastic deformation is investigated. The distribution of grain boundaries over misorientations is shown to depend on the degree of strain experienced by the material during severe plastic deformation. Grainboundary diffusion fluxes of nickel atoms from the surface of the specimen are found to increase the fraction of special grain boundaries in the grain-boundary ensemble of submicrocrystalline molybdenum. Presumably, this is related to nucleation of new grains induced by the grain-boundary diffusion fluxes of nickel.

Effect of partial recrystallization on the grain size and grain boundary structure of austenitic steel

Abstract Cyclic thermomechanical treatment combined with caliber rolling was applied in order to obtain very fine grain structure with high fraction of special grain boundaries in austenitic stainless steel. Partial recrystallization was observed. Recrystallized fraction was assessed from misorientation data measured by electron back scattering diffraction. Due to the partial recrystallization, elastic energy was stored in the deformed parts, and helped grain boundary movement. As a consequence, very fine grained material with high fraction of special boundaries was formed.

Oxidation behavior of grain boundary engineered alloy 690 in supercritical water environment

Nickel-base alloy is an important structural material that is known for its exceptional high temperature oxidation resistance. Oxidation in this alloy at high temperatures occurs to a greater extent along the grain boundaries. Grain boundary engineering (GBE) was applied to modify the grain boundary characteristics of this alloy to affect its oxidation resistance. Specimens with both low level and high level cold works showed a high fraction of special grain boundaries, and were tested for supercritical water oxidation resistance at 500°C and 24MPa. Both GBE and as-received samples exhibited mass gain followed by mass loss during 10weeks of exposure, but the normalized mass change was small and less than 0.12mg/cm2. GBE samples showed better oxide layer retention compared to the as-received sample. XRD results indicate that nickel oxide, chromium oxide, and spinel oxide were the three main types of oxides that form on as-received and GBE alloy 690. Three distinct regions were identified on the oxidized surface: a flat region with oxide flakes aligning relatively parallel to the surface, a rough region with polygon-type oxide particles randomly distributed on the surface, and a region with aggregated oxide flakes perpendicular to the surface. The flat region of oxidation consisted of (111) orientated oxide spinel flakes formed on (111) oriented alloy 690 grains. The flat oxide region was thinner and showed better oxide adhesion compared to the rough region. Chromium oxidation was found only at random grain boundaries, leading to formation of thick Cr2O3 layer on the surface and chromium depletion underneath. None of this oxidation was found at low angle or special boundaries. The chromium oxidation was attributed to fast chromium diffusion through random boundaries and mechanically deformed regions such as scratches left after polishing. It is envisioned that the oxidation behavior of alloy 690 in supercritical water can be tailored by microstructure engineering that involves changes in grain orientation and grain boundary character distribution.

Production of very fine grained AISI 304 steel with high special grain boundary density by grain boundary engineering

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries.

Modeling of grain boundary character reconstruction and predicting intergranular fracture susceptibility of textured and random polycrystalline materials

A grain boundary character reconstruction model has been developed that can reproduce the experimentally determined grain boundary character distribution (GBCD) from the simple texture and overall GBCD descriptions. It is based on defining the texture intensity by Gaussian half-width, orientation exchange of the grains, and Monte Carlo simulations. Subsequently, this model has been integrated with a Voronoi microstructure based Markov Chain–Monte Carlo algorithm to evaluate the intergranular fracture susceptibility of polycrystalline materials. The character dependent individual grain boundary fracture strength and the projected local stress on the grain boundary plane were considered in the crack-propagation decision strategy. The predicted threshold fracture stress has been compared with the experimental fracture stress data of both fiber and random textured polycrystals available in the literature, and good agreement was observed. Besides, it has been shown that the overall GBCD could be a misleading parameter in predicting the relative fracture strengths of different types of specimens and how a texture-based grain boundary character reconstruction model can help overcome this difficulty by assessing the true fraction of special grain boundaries in a given microstructure. The effect of grain shape on relative change of fracture stress has been also quantitatively presented and discussed.

Grain-boundary engineering markedly reduces susceptibility to intergranular hydrogen embrittlement in metallic materials

The feasibility of using “grain-boundary engineering” techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of “special” grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J c fracture toughness values were some 20–30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced.

IGSCC of grain boundary engineered 316L and 690 in supercritical water

This study evaluated the influence of a high fraction of special grain boundaries on the intergranular stress corrosion cracking susceptibility of 316L stainless steel and nickel base alloy 690 in supercritical water. By thermomechanically processing the alloys to create specimens with largely different special boundary fractions, it was possible to isolate the effects of the grain boundary structure on the intergranular stress corrosion cracking behavior. Constant extension rate tensile experiments were performed in 500 °C deaerated supercritical water, and SEM analysis of the cracking behavior was performed on the gage surfaces of the specimens. Results indicate that the fraction of cracked grain boundary length in the specimens with higher fractions of special boundaries is reduced for 316L and 690 by factors of 9 and 5 at 15% strain, and 3 and 2 at 25% strain, respectively. This reduction is due to the special boundaries, which at 25% strain have a frequency of cracking that is 9–18 times lower than that for a random high angle boundary.

The effect of rejuvenation heat treatments on the repair weldability of wrought Alloy 718

Extensive precipitation of needle- and plate-shaped δ-phase in the γ-nickel matrix of wrought Alloy 718 is the major microstructural change resulting from multiple weld repair/post weld heat treatment (PWHT) cycles. Isothermal heat treatments at 954 °C for times up to 100 h were used to simulate the multiple PWHTs in laboratory samples. Grain size did not change appreciably during these heat treatments owing to δ-phase pinning of the grain boundaries (GBs). The susceptibility of Alloy 718 to heat-affected-zone (HAZ) liquation cracking degraded as a result of these heat treatments. This degradation is due to the short time, high temperature GB liquation caused by the combined effects of δ-phase dissolution, boron carbide constitutional liquation, and GB segregation. A rejuvenation treatment (1010 °C/2 h) effectively restored the degraded weldability by removing the adverse influence of δ-phase through dissolution above the δ-phase solvus. This heat treatment also promoted a spontaneous grain refinement and increase in the fraction of special GBs owing to the elimination of δ-phase pinning of GBs. The combined effects of δ-phase, grain size and fraction of special GBs are discussed in the context of HAZ liquation cracking that occurs during repair welding of Alloy 718.