Fraction Of Special Boundaries Research Articles

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.

Effect of strains in grain boundary modification on plasticity and plasticizing mechanism of commercial-purity nickel

The influence of grain boundary (GB) modification on the plasticity and plasticizing mechanism of commercial pure nickel was studied in this paper. Thermo-mechanical process including compressing (with strain of 5%–30%) and annealing was carried out on cylindrical nickel samples to regulate the microstructure of GBs. Hardness measurements were conducted using Vickers hardness tester. The effects of different strains on the special boundary fraction, grain boundary character distribution (GBCD), the connectivity of the boundary network and Schmid factor were analyzed by electron backscatter diffraction (EBSD). And then, the relationship between GB characters and plasticity which was represented by Vickers hardness and Schmid factor was obtained. It was found from experiment results that the plasticity of nickel samples increased firstly and then decreased with the rising of strain from 5% to 25% after compressing and annealing. The optimal plasticity of nickel was achieved when the strain was 10%, in which the special boundaries fraction (73.48%) was twice that of the as-received sample (37.51%). In this case, the Vickers hardness reduced by 25% than that of the as-received one and the average Schmid factor reached the highest. The representation results of plasticity employing Vickers hardness obtained by physical method was consistent with the average Schmid factor acquired by microscopic EBSD technique. It was also found that the size of grain clusters and the length fraction of Σ3ic to Σ3c twin boundaries both played an important part in the plasticizing by grain boundary modification. The large cluster size and high fraction of Σ3ic to Σ3c boundaries led to the fine plasticity. The effect of grain boundary modification on plasticity and the plasticizing mechanism through thermo-mechanical processing in pure nickel were illustrated in this paper based on above microstructure characterization researches. It was of great significance to plasticize the pure nickel and to promote the application of grain boundary modification on plasticization in metal billets.

Effect of grain boundary character distribution on hydrogen embrittlement in Fe–Ni based alloy

The feasibility of using grain boundary engineering (GBE) to reduce the susceptibility of hydrogen embrittlement in precipitation strengthened Fe–Ni based austenitic alloy is investigated. After GBE treatment, the fraction of special boundaries (SBs) increased from 46% to 78% while the hydrogen-induced loss of elongation is decreased from 21.8% to 8.6%. This improvement of hydrogen resistance can be attributed to not only the high fraction of SBs but also the disruption of random grain boundaries (RGBs) network. As the former has high resistance to hydrogen-induced crack initiation and the later can suppress the propagation of cracks.

Retarding the precipitation of η phase in Fe-Ni based alloy through grain boundary engineering

It is important to inhibit the precipitation of η phases in precipitation strengthened Fe-Ni based alloys, as they will deteriorate not only the mechanical property but also the hydrogen resistance. The present investigation shows that grain boundary engineering (GBE) can retard the formation and growth of η phase in J75 alloy. After GBE treatment with 5% cold rolling followed by annealing at 1000 °C for 1 h, the fraction of special boundaries (SBs) increases from 38.4% in conventional alloy to 77.2% and the fraction of special triple junctions increases from 10% to 74%. During 800 °C aging treatment, quite amount of cellular η phases adjacent to random grain boundary (RGB) will be found in conventional alloy, and only a few small η phases have been observed in GBE treatment alloy subjected to the same aging treatment for long time. The reason for GBE in inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks. As nucleation and growth of η phases on SBs are difficult due to their lower Ti concentration and diffusion rate, and the disruption of RGB networks reduces supply of Ti atoms to the η phases significantly, which impedes their growth at RGB.

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.

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

Abstract

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.

Development of Grain Boundary Character Distribution in Medium-Strained 316L Stainless Steel During Annealing

A medium-strained (13% cold-drawing) 316L austenitic stainless steel rod was subjected to annealing between 750 and 1100 °C. Electron backscatter diffraction and electron channeling contrast imaging techniques were jointly employed to investigate the development of microstructures and grain boundary character distribution during the annealing treatments. Results show that annealing at temperatures below 900 °C does not evidently change microstructures of the as-drawn specimen. Full recrystallization is observed after the temperature reaches 1000 °C and the deformation microstructures are replaced by fine and uniform recrystallized grains. A large number of annealing twins (with Σ3 misorientation) are produced by the recrystallization, leading to markedly increased fractions of special boundaries (fSB). The maximum fSB is found to be 67.3% in the present study. In addition, for the recrystallized grains, rapid growth is noticed at relatively high temperatures due to easy migration of grain boundaries.

A novel method to enhance CSL fraction, tensile properties and work hardening in complex concentrated alloys ― Lattice distortion effect

A framework has been developed to reduce the stacking fault energy of complex concentrated alloys (CCAs) or high entropy alloys based on increasing lattice distortion by choosing principal elements with large differences in moduli and atomic size. The framework and selection criteria have resulted in the inclusion of five CCA compositions for validation of the lattice distortion and three Fe-containing CCAs. Orientation imaging microscopy (OIM) was used to examine the fraction of special boundaries in all of the CCAs. Alloys with large lattice distortion contained high density of annealing twins. Both tensile properties and work hardening rate characteristics were evaluated. CCAs that were designed based on lattice distortion demonstrated improved ultimate tensile strength. Work hardening rate curves revealed the underlying variation in the tensile properties of various CCAs. For selected alloys, post-deformation OIM analysis of the lateral surface was carried out for establishing defect density variation.

Grain boundary engineering approach to improve hydrogen embrittlement resistance in Fe[sbnd]Mn[sbnd]C TWIP steel

This investigation aimed to improve hydrogen embrittlement (HE) resistance of Fe17Mn−0.8C (wt %) TWIP steels by use of grain boundary engineering approach. The degree of enhancement in HE resistance by grain boundary engineering was compared with those of Al-added TWIP steels (Fe17Mn−0.8C1Al and Fe17Mn−0.8C2Al). By applying grain boundary engineering, the special boundaries that shared a high fraction of lattice points between neighboring grains were increased from 48% to 59%, and the morphology of grain boundaries was changed to ragged type. After hydrogen charging, fracture strength and reduction of area were clearly degraded in all four samples (Base, GBE, 1Al, and 2Al), revealing a transition of fracture mode from dimple fracture to intergranular fracture. However, the alloy with higher fraction of special boundaries had higher resistance to hydrogen-induced intergranular embrittlement. The HE resistance of GBE was obviously improved to a level similar to that of Al-added TWIP steels. Important implication of this work is that HE resistance can be greatly enhanced by controlling the distributions of grain boundary characters (GBCs) without adding Al. This was attributed to not only the higher fraction of special boundaries, but also the loss of continuity of random boundaries.

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.

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.

The role of microstructure and grain orientations on intergranular cracking susceptibility of UNS 17400 martensitic stainless steel

Abstract The aim of this work was to investigate the role of crystallographic orientations and grain boundary characteristics as a new approach to controlling the intergranular cracking (IGC) susceptibility. In this respect, hydrogen cathodic charging under two different current densities was carried out on 17–4 pH steel (H900 condition) in order to search for correlations between intergranular corrosion and microstructure, inclusions, and grain orientations. Then, to emphasize the importance of crystallographic orientation in IGC, hydrogen cathodic charging was carried out on 17–4 pH steel in solution condition, in which grains oriented with {110} and {111} parallel to the normal direction (ND) are dominant. The results showed that a reduction in the fraction of undesired {001}//ND grains and development of {110}, {112}, and {111}//ND texture components improved the IGC resistance. In addition, a higher fraction of low angle and special boundaries with low internal energy due to better compatibility between adjacent grains was observed in the uncracked sample and led to increased IGC resistance.

Communication—Hydrogen Permeation Resistance of Nickel Coating Enhanced by Adding the Rare Earth Element Ce

Due to the concentration of high density of trapping sites such as dislocations and vacancies, special grain boundaries (GBs) can be considered as the potential zone for trapping hydrogen, resulting into the slowdown of hydrogen diffusion in materials. A new approach was proposed that the rare earth element Ce was added into the electrodeposition of Ni to enhance the resistance of coating against hydrogen diffusion, which could be attributed to the decrease of grain size and the increased fraction of special boundaries in deposits.

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.

Grain Boundary Assemblies in Dynamically-Recrystallized Austenitic Stainless Steel

The grain boundary misorientation distributions associated with the development of dynamic recrystallization were studied in a high-nitrogen austenitic stainless steel subjected to hot working. Under conditions of discontinuous dynamic recrystallization, the relationships between the grain or subgrain sizes and flow stresses can be expressed by power law functions with different grain/subgrain size exponents of about −0.76 (for grain size) or −1.0 (for subgrain size). Therefore, the mean grain size being much larger than the subgrain size under conditions of low flow stress gradually approaches the size of the subgrains with an increase in the flow stress. These dependencies lead to the fraction of high-angle boundaries being a function of the flow stress. Namely, the fraction of ordinary high-angle boundaries in dynamically-recrystallized structures decreases with a decrease in the flow stress. On the other hand, the fraction of special boundaries, which are associated with annealing twins, progressively increases with a decrease of the flow stress.

Plasticity-induced restructuring of a nanocrystalline grain boundary network

The grain boundary-mediated mechanisms that control plastic deformation of nanocrystalline metals should cause evolution of the grain boundary network, since they directly alter misorientation relationships between crystals. Unfortunately, current experimental techniques are unable to track such evolution, due to limits on both spatial and temporal resolution. In this work, molecular dynamics simulations are used to study grain boundary restructuring in nanocrystalline Al during both monotonic tension and cyclic loading. This task is enabled by the creation of new analysis tools for atomistic datasets that allow for a complete characterization and tracking of microstructural descriptors of the grain boundary network. Quantitative measurements of grain boundary character distribution, triple junction type, grain boundary plane normal, and other interfacial network characteristics are extracted and analyzed. The results presented here show that nanocrystalline plasticity leads to an increase in special boundary fraction and disruption of two-dimensional boundary connectivity, with the most dramatic evolution occurring in the smallest grain sizes.

On Plastic Deformation Behavior of Cryorolled AA8090 Alloy

AA8090 alloy was rolled up to 50 and 75 % reductions at both liquid nitrogen (LNR) and room temperatures (RTR). Both hardness and tensile behavior were evaluated on rolled samples. Optical microscope, TEM and EBSD were used for detailed microstructural examination of rolled samples. Williamson-Hall peak broadening analysis on X-ray diffraction data was made to evaluate crystallite size, lattice strain and dislocation density. An enhanced tensile strength was evidenced in LNR samples when compared to RTR samples without sacrificing ductility, which was ascribed to the higher density of dislocations in LNR samples than RTR samples. A large number of dislocation tangled regions along with ultrafine grain structure were evidenced through TEM and EBSD. Significant fraction of special boundaries in combination with increased fraction of texture components like S, Brass, Cu and Goss would be another reason for enhanced properties in LNR conditions than that of RTR. These components were observed to be strengthened with increased rolling reduction. Work hardening behavior clearly evidenced the variation in amount of work hardening and recovery phenomenon. It showed large variation in recovery in the case of 50 % reduction than that of 75 % reduction, which was attributed to significantly higher density of dislocations in 75 % rolled samples in RTR and LNR.

Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L

Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability.