Boundary Misorientation Research Articles

Understanding the correlation of aging treatments and stress corrosion crack growth of a secondary hardening ultra-high strength steel

The correlation of aging treatments and stress corrosion crack growth of a secondary hardening ultra-high strength steel was investigated using microstructural characterization, electrochemical hydrogen penetration experiment, thermal desorption spectroscopy analysis, and stress corrosion crack growth test. The results showed that increase of aging duration led to slight increase in the fraction of reversed austenite but decrease in dislocation density while no notable variation in grain boundary misorientation distribution and the corrosion behavior was observed. The shrinkage and fracture toughness continued to increase with aging duration while the yield strength reaching a maximum value after 5 hours of aging. Longer aging durations led to the decrease in effective hydrogen diffusion coefficient but improvement of stress corrosion cracking resistance, mainly due to the coarsening and nucleation of M2C carbides, as well as the higher fraction of reversed austenite. This work provided the theoretical and technical basis for improving the stress corrosion cracking resistance of this steel.

Imaging and Segmenting Grains and Subgrains Using Backscattered Electron Techniques.

We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations.

Correlative electron microscopy of discontinuous precipitation

This paper presents a comprehensive study on discontinuous precipitation (DP) in Al-22 at.% Zn alloy, leveraging the correlative and synergic effects of advanced microscopy and modelling techniques. The study is structured into three pivotal steps: in-situ scanning electron microscopy combined with an energy backscattered diffraction analysis for real-time monitoring and an analysis of DP reaction kinetics and grain boundary misorientations; transmission electron microscopy and an energy X-ray dispersive microanalysis for detailed observation of solute-depleted α and solute-rich β lamellae, offering insights into the kinetics of the reaction front (RF) and solute concentration profiles; and a sophisticated modelling approach addressing the limitations of direct observation methods by simulating solute concentration changes within αphase lamellae during RF go-and-stop cycles. This multifaceted approach elucidates the nuances of DP mechanisms, from the nucleation and growth of precipitates to the estimation of instantaneous RF velocity and the impact of grain boundary misorientations. The integration of these methodologies enhances our understanding of DP kinetics and morphology at both meso- and nanoscales, highlighting the significant role of correlative microscopy in metallurgical research. The findings not only deepen our grasp of the fundamental mechanisms driving DP reactions but also provide a valuable comparison framework for experimental data, potentially guiding the development and optimization of metallurgical processes.

Study on Strain Hardening and Cryogenic Toughness of Marine 10Ni5CrMoV Steel during Tempering

The tempering process is applied in marine 10Ni5CrMoV steel to study microstructure, and mechanical properties by multi‐scale characterizations, strain hardening behavior, and cryogenic toughening mechanism are further investigated. As the tempering temperature increases from 590 to 630 °C, the dislocation density decreases by up to 25% and the austenite volume fraction decreases by up to 35%. Additionally, the morphology of austenite changes from strip to bulk, which weakened the pinning effect on the laths, leading to a coarsened martensite and an increase in the equivalent grain size. Based on the modified Crussard–Jaoul analysis, the specimens at different tempering temperatures exhibit a single‐stage strain hardening behavior during plastic deformation. The increase in strength is attributed to the continuous transformation‐induced plasticity effect and the increasing dislocation density. Furthermore, high austenite volume fraction and the proportion of grain boundary misorientation above 45° promote the release of stress concentration and hinder the propagation of cracks. This results in an increase in the ductile–brittle transition temperature (DBTT) of the specimens from −105 to −135 °C. At a tempering temperature of 610 °C, the specimen demonstrates an outstanding balance between yield strength (868 MPa) and cryogenic toughness (DBTT of −135 °C).

Effect of Dislocation Density on the Dynamic Strength of Aluminium

AbstractThe effects of cold work on dynamic flow strength is studied through measurements of dislocation density and elastic precursor attenuation in shocked aluminium. High-purity aluminium and Al 6082 alloy samples are subjected to up to three passes of rotary swaging, a severe plastic deformation (SPD) technique, in order to produce large variations in starting material conditions. Detailed material characterisation is performed using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) to determine the initial dislocation density, texture, grain boundary density and misorientation distribution. Variations in dynamic strength are studied through the evolution of the elastic precursor amplitude with propagation distance in specimens with thicknesses between 0.2 and 1 mm, shock loaded to impact stresses of about 4.6 GPa. It is shown that dynamic strength decreases with increasing initial dislocation density in both materials, demonstrating rare, quantified measurements of a reversal in the effect of dislocation density on yield strength at strain rates around 10$$^{4}$$ 4 s$$^{-1}$$ - 1 and above.

Continuous Dynamic Recrystallization and Deformation Behavior of an AA1050 Aluminum Alloy during High-Temperature Compression

Continuous dynamic recrystallization (CDRX) forms a new recrystallized microstructure through the progressive increase in low-angle boundary misorientations (LAGBs) during the hot forming of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. The present work investigates the effect of deformation parameters on the evolution of the dynamic recrystallization microstructures of an AA1050 aluminum alloy during compression at elevated temperatures. The alloy microstructure is investigated at deformation temperatures and strain rates in the range of 300 °C to 500 °C and 0.001 to 0.8 s−1. A well-defined substructure and subsequent DRX grains provide indication that recrystallization can proceed with continued strain under high-temperature compression. At a strain rate of 0.1 s−1, the DRX fraction is observed to be 0.25 at a temperature of 300 °C. This fraction increases to 0.32 as the temperature rises to 400 °C. The recrystallization mechanism is identified by analyzing the flow stress, the evolution of the subgrain misorientation angle, and the distribution of recrystallized grains. The observations of discontinuous dynamic recrystallization (DDRX) and CDRX under various deformation parameters are discussed. Moreover, the main substructure evolution laws observed from the high-temperature compression of an AA1050 Al alloy are summarized.

Dynamic response of equiatomic and non-equiatomic CrMnFeCoNi high-entropy alloys under plate impact

A typical Fe-rich non-equiatomic CrMnFeCoNi high-entropy alloy (HEA), Cr10Mn10Fe60Co10Ni10, and the equiatomic Cr20Mn20Fe20Co20Ni20 HEA, are investigated via plate impact experiments along with in situ free surface velocity measurements. Both the initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. These two HEAs contain a single face-centered cubic phase, are texture-free, and have similar grain sizes. Dynamic yield stress and spall strength of the non-equiatomic CrMnFeCoNi are lower by ∼55 % and 15 % than those of its equiatomic counterpart, respectively. After shock compression, dislocation slips, stacking faults, Lomer-Cottrell locks and nanotwins are found in the postmortem samples of both HEAs. Due to the reduced stacking fault energy in the non-equiatomic HEA, more deformation twins and newly formed hexagonal close-packed phases are found. For spallation, intergranular voids are larger in size but smaller in quantity than intragranular voids for both HEAs. Both intragranular or intergranular voids show strong dependence on grain boundary misorientation.

Microstructural analysis of ex-service neutron irradiated stainless steel nuclear fuel cladding by high-speed AFM

Fuel cladding in advanced gas-cooled nuclear reactors is made of an austenitic stainless steel referred to as 20Cr/25NiNb. This material can undergo microstructural changes in these extreme environments through a combination of thermal effects and radiation damage. In this work, the microstructures present in an ex-service irradiated 20Cr/25Ni-Nb sample are studied using correlated high-speed atomic force microscopy (HS-AFM) and electron microscopy techniques for the first time. Correlated topographic, crystallographic, and chemical information from the sample surface enabled identification of secondary phase precipitates (SPPs) including M23C6, sigma phase, NbC, and G phase. These SPPs can have adverse effects on the material properties. HS-AFM analysis showed surface textures unique to each SPP. Voids formed due to irradiation were also identified across the surface by HS-AFM. These voids were found to be larger in size and depth along grain boundaries. Further analysis identified a relationship between void size and frequency and grain boundary misorientation and SPP presence.

Stressful crystal histories recorded around melt inclusions in volcanic quartz

Magma ascent and eruption are driven by a set of internally and externally generated stresses that act upon the magma. We present microstructural maps around melt inclusions in quartz crystals from six large rhyolitic eruptions using synchrotron Laue X-ray microdiffraction to quantify elastic residual strain and stress. We measure plastic strain using average diffraction peak width and lattice misorientation, highlighting dislocations and subgrain boundaries. Quartz crystals across studied magma systems preserve similar and relatively small magnitudes of elastic residual stress (mean 53–135 MPa, median 46–116 MPa) in comparison to the strength of quartz (~ 10 GPa). However, the distribution of strain in the lattice around inclusions varies between samples. We hypothesize that dislocation and twin systems may be established during compaction of crystal-rich magma, which affects the magnitude and distribution of preserved elastic strains. Given the lack of stress-free haloes around faceted inclusions, we conclude that most residual strain and stress was imparted after inclusion faceting. Fragmentation may be one of the final strain events that superimposes stresses of ~ 100 MPa across all studied crystals. Overall, volcanic quartz crystals preserve complex, overprinted deformation textures indicating that quartz crystals have prolonged deformation histories throughout storage, fragmentation, and eruption.

Intrinsic mechanism of grain size effect and grain boundary misorientation angle effect on crack propagation in martensitic steels

Grain size and grain boundary misorientation angle as two important parameters in the polycrystalline system have a significant influence on the mechanical properties of advanced martensitic steels. A phase field model is used to investigate the intrinsic mechanism of the effects of grain size and grain boundary misorientation angle on the crack propagation process which considers the coupling damage behavior between grains and grain boundaries in martensitic steels by means of the finite element finite method. The transgranular (through grain boundaries) and intergranular (along grain boundaries) failure can be simulated by this model through constructing a grain boundary energy function and using a specific order parameter to track crack propagation in the grain boundary area. This model can directly display the crack propagation path inside the grain (martensite and austenite phases) and grain boundary area with the stress distribution. The preferential cleavage planes in austenite and martensite phases are 111γ and 100α in grains with different orientations, respectively. Moreover, the simulation results show that the crack mainly bifurcates near the trifurcated grain boundary. The crack tip tends to induce transgranular fracture in low-angle grain boundaries and intergranular fracture in high-angle grain boundaries when it reaches the grain boundary area. Consequently, this model will be a powerful tool to investigate the influence of grain size and grain boundary misorientation angle on the mechanical properties of polycrystalline martensitic steels.

Substructure heterogeneity during hot deformation of ferritic stainless steels - Experimental characterization and discussion assisted by a mean-field model

The microstructure evolution of ferritic stainless steels during hot deformation is complex and needs to be finely described. Quantifying the evolution of the misorientation of low-angle boundaries depending on the thermomechanical path is a key point in controlling the recrystallization of such steels. This paper proposes an experimental methodology based on large-scale electron back-scattered diffraction (EBSD) mapping characterization using a Symmetry 2 camera to track the low-angle boundary evolution. Different thermomechanical paths, from 900 to 1100 °C with strain from 0.3 to 0.9, were studied using uniaxial compression tests. Flow stress follows a usual hardening stage, followed by a slight softening regime, mostly attributed to continuous dynamic recrystallization. Distributions of low-angle boundary populations (density vs sub-grain size) are quantified by two populations either near grain boundaries or inside grain bulk; the proportion of sub-grains at grain boundaries increases with strain. This bimodal distribution is made with a probabilistic Gaussian mixture model. These results are discussed using a modified mean-field model of Gourdet-Montheillet. This model is insufficient to capture the transient stage of continuous dynamic recrystallization unless its set of parameters is adjusted to accommodate the saturation of the density of low-angle boundaries near grain boundaries. This saturation is a result of the low-angle boundary density gradients in the grains.

Effects of Strain Rate on the GND Characteristics of Deformed Polycrystalline Pure Copper

Geometrically necessary dislocations (GNDs) play a pivotal role in polycrystalline plastic deformation, with their characteristics notably affected by strain rate and other factors, but the underlying mechanisms are not well understood yet. We investigate GND characteristics in pure copper polycrystals subjected to tensile deformation at varying strain rates (0.001 s−1, 800 s−1, 1500 s−1, 2500 s−1). EBSD analysis reveals a non-linear increase in global GND density with the strain rate rising, and a similar trend is also observed for local GND densities near the grain boundaries and that in the grain interiors. Furthermore, GND density decreases from the grain boundaries towards the grain interiors and this decline slows down at high strain rates. The origin of these trends is revealed by the connections between the GND characteristics and the behaviors of relevant microstructural components. The increase in grain boundary misorientations at higher strain rates promotes the increase of GND density near the grain boundaries. The denser distribution of dislocation cells, observed previously at high strain rates, is presumed to increase the GND density in the grain interiors and may also contribute to the slower decline in GND density near the grain boundaries. Additionally, grain refinement by higher strain rates also promotes the increase in total GND density. Further, the non-linear variation with respect to the strain rate, as well as the saturation at high strain rates, for grain boundary misorientations and grain sizes align well with the non-linear trend of GND density, consolidating the intimate connections between the characteristics of GNDs and the behaviors of these microstructure components.

Coordinated deformation mechanisms of high formability Al–Mg–Si–Cu–Zn–Fe alloys via coupling control of thermomechanical processes

Both the high formability and strengths are greatly important for the automotive application of Al–Mg–Si–Cu alloys. In the present work, four novel thermomechanical processing routes were designed to improve the comprehensive properties of Al–Mg–Si–Cu–Zn–Fe alloys. The results show that, the interspace between Fe-rich particles and Fe/Mn-containing dispersoids can be significantly reduced by an optimized thermomechanical processing route. This enhanced the interaction between them during subsequent cold rolling process, resulting in increased number density and uniform distribution level of multi-scale Fe-rich particles, which further substantially improve both the recrystallization microstructure and mechanical properties, i.e., higher bendability, average r value of 0.72 (0.59 for traditional sheet) and tensile strength of 302 MPa (283 MPa for traditional sheet). The microstructure evolution of alloy sheets was schematically illustrated, and in-depth analysis on the mechanism of high formability was proceeded from the aspects of grain boundary misorientations and recrystallization texture.

How are microstructural defect interactions linked to simultaneous intergranular and transgranular fracture modes in polycrystalline systems?

The major objective of this investigation is to fundamentally understand and predict how intergranular (IG) and transgranular (TG) fracture modes nucleate and propagate in f.c.c. polycrystalline systems due to defects, such as total and partial dislocation densities and grain boundary (GB) structures and misorientations. A dislocation density crystalline plasticity (DCP) formulation based on the evolution and interaction of total and partial dislocation densities was integrated with a recently developed fracture approach to investigate the fracture nucleation and propagation of simultaneous multiple fracture events, including both IG and TG fracture events. The aggregate grains and GB orientations and morphologies are based on EBSD measurements that are representative of polycrystalline copper aggregates with a broad range of random high angle and low angle GBs. The predictions indicate that dislocation density pileups induce IG fracture due to interrelated stress, slip, and total and partial dislocation density accumulations and interactions for both high angle GBs (HAGBs) and low angle GBs (LAGBs). TG fracture nucleation and propagation occurred due to normal stress accumulations, which exceeds the fracture stress, along cleavage planes. Furthermore, it is shown how IG cracks transition from the GB plane to the cleavage planes as cracks nucleate and propagate. Accumulated plastic zones due to different slip system activities can impede and blunt crack propagation and fronts. These plastic zones form due to high Lomer and Shockley partial dislocation densities. These predictions, which are consistent with experimental observations, provide a fundamental understanding of how simultaneous failure modes initiate and propagate for physically representative microstructures.

Desensitization of AA5083 alloy via pulsed laser irradiation and its microstructural mechanism

In the present work, a laser surface desensitization (LSD) method was carried out on sensitized AA5083 alloy with the underlying microstructural evolution examined. The enhanced corrosion resistance after LSD process was confirmed by nitric acid mass loss test (NAMLT), immersion test and electrochemical measurements, which is mainly ascribed to the modification of grain boundary chemistry. Accompanied with the dissolution of grain boundary β (Al3Mg2) phase during LSD process, the grain boundaries could be characterized by Mg segregation or nano-sized Mg-rich clusters, which may also be absent from chemical uniformity depending on grain boundary misorientation.

In-situ study of the effect of grain boundary misorientation on plastic deformation of Inconel 718 at high temperature

In-situ study of the effect of grain boundary misorientation on plastic deformation of Inconel 718 at high temperature

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

Microstructure and property evolution during tempering of a medium carbon bainitic steel

Tempering a medium carbon carbide-free bainite steel led to two distinct microstructures and properties below and above the bainitic start (Bs) temperature. Austenite decomposition during tempering, depended on carbon content, tempering temperatures and time, determining retained austenite (RA) stability. Up to 400 °C, most RA remained intact, but a significant reduction occurred above the Bs temperature. Film austenite primarily turned into carbides within the bainitic–ferrite matrix and form lines of carbide particles, while blocky austenite decomposed into troostite at 500 °C and sorbite at 600 °C. During tempering, on the other hand, bainitic ferrite lost tetragonality losing its carbon and resulting in the growth of bainitic–ferrite plates through grain boundary migration and misorientation annihilation. Above Bs, drastic microstructural changes caused significant alterations in tensile properties.

The strength and ductility mechanism of gradient fine grains prepared by surface severe plastic deformation

The gradient fine grains on low-carbon steel surface were produced via ultrasonic impact treatment, which led to excellent combination of high strength and ductility. Results showed that the grain size had a gradient increasing from 120 nm at top surface to 5–8μm at the matrix. In the gradient fine grain region, the grain boundary misorientation is in the range of 2–15°. Compared with original microstructure, the surface gradient fine grains can achieve a higher work hardening rate. A digital image correlation(DIC) technology was employed to analyze variations in strain during the tensile deformation. It is demonstrated that the surface gradient distribution of grain size can reduce the stress concentration, resulting in higher ability for uniform deformation. Such a capability is attributed to the decreased dislocation density and Schimid factor due to gradient fine grains distribution. Compared with the original microstructure, a number of dislocation walls (or cells, or tangles) can hinder the movement of dislocations. In addition, the surface gradient fine grains have faster recovery and proliferation of mobile dislocation density, which can lead to higher work-hardening ability. As a result, the surface gradient fine grains can attain a combination of excellent strength and ductility.

A correlative approach to evaluating the links between local microstructural parameters and creep initiated cavities

The study and modelling of material degradation processes, such as the initiation and growth of creep cavities in high-temperature applications, require a correlative and comprehensive knowledge of the microstructure. However, individual microscopy is limited to a small region and specific microstructural information of the specimen. This work demonstrates a novel correlative microscopy approach for characterising creep cavitation and establishing correlations with local microstructural parameters in a statistical manner. This approach combines datasets from stitched higher-resolution backscattered electron (BSE) images, XeF2 Focused Ion Beam (FIB) images, and backscattered electron diffraction (EBSD) maps with advanced image correlation techniques. Deep-learning image segmentation techniques and statistical analysis are applied to find relations between creep cavitation and local microstructural environment. This approach is demonstrated in a cyclic creep-tested 316H stainless steel specimen with extensive creep cavities. The results show that in this material, strain localization, grain boundary misorientation, and substantial precipitation dominate the nucleation of cavities, whereas other microstructural properties such as grain size and Schmid factor play smaller roles. This study presents the use of the correlative microscopy approach to provide new insights into creep cavitation behaviour and its implications for establishing creep cavitation damage models.