First combined electron backscatter diffraction and transmission electron microscopy study of grain boundary structure of deformed quartzite

The structures of boundaries in a deformed and dynamically recovered and recrystallized quartz polycrystal (mylonite) were characterized by transmission electron microscopy, after the misorientation angles across the same grain boundaries had been analysed using electron backscatter diffraction in a scanning electron microscope. In this new approach, a specific sample area is mapped with electron backscatter diffraction, and the mapped area is then attached to a foil, and by the ion beam thinned for transmission electron microscopy analysis. Dislocations in grain boundaries were recognized as periodic and parallel fringes. The fringes associated with dislocations are observed in boundaries with misorientations less than 9°, whereas such fringes cannot be seen in the boundaries with misorientations larger than 17°. Some boundaries with misorientations between 9° and 17° generally have no structures associated with dislocation. One segment of a boundary with a misorientation of 13.5° has structures associated with dislocations. It is likely that the transition from low‐angle to high‐angle boundaries occurs at misorientations ranging from approximately 9° to 14°. Change in the grain boundary structure presumably influences the mobility of the boundaries. In the studied deformed quartz vein, a relative dearth of boundaries between misorientation angles of θ = 2° and θ = 15° has previously been reported, and high‐angle boundaries form cusps where they intersect low‐angle boundaries, suggesting substantial mobility of high‐angle boundaries.     

Low-angle subgrain misorientations in deformed NaCl

The development of subgrain boundary misorientations with strain in NaCl polycrystals has been investigated. At low strains, a power law relationship exists between strain and average misorientations. The accuracy of this relationship is assessed in terms of material and electron backscattered diffraction (EBSD) processing parameters and is found to hold for a material of constant grain size deformed in compression, providing EBSD mapping and processing conditions were similar. Average misorientations are strongly influenced by grain orientation, suggesting that the misorientation–strain relationship may also be texture dependent in materials with high plastic anisotropy. A slight grain size dependency of the average misorientations was observed.     

EBSD characterization of carbide–carbide boundaries in WC–Co composites

A sample of WC‐6wt%Co was investigated for grain boundary character distribution and occurrence of coincidence site lattice (CSL) boundaries on a statistical basis. For this purpose orientation measurements of the grains were carried out using electron back‐scattered diffraction (EBSD). The dominant misorientation relationships were determined by complementary EBSD data representation tools such as orientation maps, misorientation angle distribution histograms and the sectioned three‐dimensional misorientation space. It was found that the grain boundary character distribution of the material is nearly random and the CSL boundaries are not present in statistically significant amounts. It was also found that the amount of binder phase does not play a role in the formation of special boundaries. The paper focuses on the methodology of characterizing grain boundaries in a hexagonal material using EBSD.     

The preferred CSL misorientation distribution in polycrystalline SrTiO3

Electron backscattered diffraction is used to investigate the preferred CSL (coincidence site lattice) distribution of polycrystalline SrTiO₃ as a function of annealing times (1 h and 16 h). Comparison of the CSL misorientations suggests that the CSL boundary energy plays a role in the preferred grain growth.     

Misorientation distributions in hot deformed NaCl using electron backscattered diffraction

EBSD orientation mapping has been used to derive subgrain boundary misorientation distributions in a series of hot deformed and etched NaCl samples. The main objective of this study has been to examine the influence of data processing, noise caused by angular resolution limits and step size on the subgrain misorientation distributions in hot deformed NaCl. Processing of non‐indexed EBSD patterns increased the average misorientations in etched NaCl. Noise contributed significantly to low angle misorientation peaks for step sizes less than the minimum subgrain size. Orientation data collected using a step size larger than the average subgrain size cumulated misorientations across individual subgrains and effectively measured an orientation gradient between steps. Orientation gradient distributions were not influenced by noise. Average misorientation values calculated from large step data correlated well with average misorientation from small step size data. Average misorientations showed a power law relationship with strain. Three types of substructures were identified using scanning electron microscopy and EBSD mapping, equiaxed subgrains, long subgrain boundaries and a core‐mantle subgrain arrangement.     

Low angle boundary migration of shot‐peened pure nickel investigated by electron channeling contrast imaging and electron backscatter diffraction

Study on recrystallization of deformed metal is important for practical industrial applications. Most of studies about recrystallization behavior focused on the migration of the high‐angle grain boundaries, resulting in lack of information of the kinetics of the low angle grain boundary migration. In this study, we focused on the migration of the low angle grain boundaries during recrystallization process. Pure nickel deformed by shot peening which induced plastic deformation at the surface was investigated. The surface of the specimen was prepared by mechanical polishing using diamond slurry and colloidal silica down to 0.02 μm. Sequential heat treatment under a moderate annealing temperature facilitates to observe the migration of low angle grain boundaries. The threshold energy for low angle boundary migration during recrystallization as a function of misorientation angle was evaluated using scanning electron microscopy techniques. A combination of electron channeling contrast imaging and electron backscatter diffraction was used to measure the average dislocation density and a quantitative estimation of the stored energy near the boundary. It was observed that the migration of the low angle grain boundaries during recrystallization was strongly affected by both the stored energy of the deformed matrix and the misorientation angle of the boundary. Through the combination of electron channeling contrast imaging and electron backscatter diffraction, the threshold stored energy for the migration of the low angle grain boundaries was estimated as a function of the boundary misorientation.     

Process optimization and microstructural development during superplastic-like forming of AA5083

An advanced sheet forming process was utilized by combining hot drawing and blow forming to establish a fast forming technology. As a continuation of the development in superplastic-like forming, this study dealt with the process optimization and evaluation of post-forming properties. Aluminum alloy 5083 (AA5083) parts with near-net shape were successfully fabricated at 400 °C. Thickness uniformity has been improved by optimizing the mechanical preforming (hot drawing) and adopting a strain-rate-control gas forming (blow forming). Fairly uniform microstructure can be achieved with this forming process. To investigate the microstructural information, the annealed and hot deformed samples were characterized using electron backscatter diffraction technique. Fine grains with high-angle grain boundaries occurred near the elongated grains during hot drawing stage as a result of dynamic recrystallization. Subgrain structure was also examined by characterizing the distribution of grain boundary misorientation angles. Grain growth and subgrain boundary migration were two main microstructural features observed during the gas forming stage.     

Electron backscattered diffraction as a tool to quantify subgrains in deformed calcite

In this work, we investigated processing methods to obtain subgrain sizes from electron backscattered diffraction data using samples of experimentally deformed calcite (CaCO₃) polycrystals. The domain boundary hierarchy method, based on area measurements of domains enclosed by boundaries larger than a given misorientation angle, was applied to these calcite samples and was found to be limited by: (i) topological problems; (ii) undersampling of large grains; and (iii) artefacts caused by nonindexing. We tested two alternative methods that may reduce the problems: (i) the measured linear intercept hierarchy method, based on measurements of linear intercept between boundaries having larger misorientations than a given minimum angle; and (ii) the calculated linear intercept hierarchy method, based on the total length of boundaries having misorientations larger than a given minimum angle. The measured linear intercept hierarchy method was found to produce results more representative for the microstructure than the calculated linear intercept hierarchy method, because the calculated linear intercept hierarchy method has a significant uncertainty related to the grid‐based nature of the measurements. Preliminary results on calcite suggest that the measured linear intercept hierarchy method is related, in a complex way, to deformation conditions such as stress, strain and temperature as well as to the characteristics of subgrain rotation and grain boundary migration processes.     

An analysis of microband orientation in a commercial purity aluminium alloy subjected to forward and reverse torsion using Electron Backscatter Diffraction (EBSD)

High‐resolution electron backscatter diffraction has been used to study the effects of strain reversal on the evolution of microbands in commercial purity aluminium alloy AA1200. Deformation was carried out using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300 °C and strain rate of 1 s^?1 to a total equivalent tensile strain of 0.5. In both cases, microbands were found in the majority of grains examined with many having microband walls with more than one orientation. For the forward/forward condition, the microband clusters were centred around ?20° and +45° to the equivalent tensile stress axis, whereas for material subjected to a strain reversal, the clusters were at ?65° and ?45°. There was no evidence of microbands that were formed in the forward deformation step in the reversed material, which would suggest that a strain of 0.25 is sufficient to dissolve any microstructure history generated by the first step. Furthermore, the microbands within the strain‐reversed material had a reduction in misorientation compared with the lineally strained material, suggesting that these microbands only formed at the onset of the second deformation step. This confirms that microband formation is complex and sensitive to strain path; however, it is still unclear to what extent microband formation is dependent on strain path history compared with the instantaneous deformation mode.     

EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre‐existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non‐cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub‐boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub‐boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.     

Time evolution of sigma 3 annealing twins in secondary recrystallized nickel

Samples of commercially pure nickel have been annealed in air at 0.68T_m (900 °C) for 1, 2 and 3 h in order to study the relationship between the grain growth characteristics and grain boundary misorientation, particularly annealing twins (Σ 3). Orientation mapping by electron backscatter diffraction was used to obtain the experimental data. Anomalous grain growth was observed in commercially pure nickel after each of the anneals. The main findings are as follows. The texture was mainly {100}<001> and {112}<111> and it was more pronounced in coarse-grained areas than in fine-grained areas. The length fraction of Σ 3s (annealing twins) increased with annealing time and therefore with the level of anomalous grain growth. Two to three twins per grain were sited in coarse-grained regions whereas less than one twin per grain was sited in fine-grained regions. It is suggested that the nucleation and growth of twinning is mechanistically linked to anomalous grain growth.     

Effect of prior deformation on microstructural development and Laves phase precipitation in high‐chromium stainless steel

This study investigated the influence of deformation on precipitation behaviour and microstructure change during annealing. Here, the prior deformation of high‐chromium stainless steel was tensile deformation of 3%, 6% and 10%, and the specimens were then annealed at 700?C for 10 h. The specimens were subsequently analyzed using backscattered electron image and electron backscattering diffraction measurements with SEM. Compared with the deformation microstructure, the grains revealed no preferred orientation. The precipitates of TiN and NbC were formed homogenously in the grain interior and at grain boundaries after annealing. Fine Laves phase precipitates were observed in grains and along subgrain boundaries as the deformation increased. Furthermore, the volume fraction of Laves phase increased, but the average particle diameter of precipitate was reduced as the deformation increased.     

Investigation of the microstructure and mesotexture formed during thermomechanical controlled rolling in microalloyed steels

Thermomechanical controlled rolling (TMCR) has been widely used as an effective method to improve the properties of steels. In the present paper, two commercial TMCR microalloyed steels were investigated. The grain size distributions and grain boundary misorientation angles were measured using electron backscattered diffraction patterns obtained using orientation imaging microscopy (OIM). The equivalent grain diameters were also measured using optical microscopy. Mixed coarse‐ and fine‐grained regions were observed and the microhardness values were measured for both areas. Grain boundary misorientation angle distributions showed that the magnitude of mesotexture developed in the steel is dependent upon the rolling passes, the reduction ratio and the rolling temperatures including the finish rolling temperature. The surface layer, up to 2 mm depth, in Com‐A steel had about 55% of grain boundaries with a misorientation angle below 12°, much higher than in the central area (～30%), whereas Com‐B steel showed similar distributions from the rolling surface to the centre. Misorientation results obtained from laboratory rolled steel plates with various TMCR procedures are also discussed as are OIM results from Charpy impact brittle fracture surfaces examining the effect of misorientation on crack propagation.     

Characterization of crystallographic properties of SMC poly Si using electron backscattered diffraction

Crystallographic properties of silicide mediated crystallization (SMC) polycrystalline silicon (poly Si) and excimer laser annealing (ELA) poly Si were studied by electron backscattered diffraction. Large‐grain sized poly Si with a large fraction of low‐angle grain boundaries was acquired by SMC, and small‐grain sized poly Si with high‐angle grain boundaries especially around 60° was acquired by ELA. The thin film transistor (TFT) device characteristics were investigated in view of short‐range crystallinity (pattern quality) and long‐range crystallinity (misorientation distribution) of the specimens. Short‐range crystallinity did not significantly affect the TFT device characteristics, and long‐range crystallinity considering the low energy level of special boundaries could be better related to the TFT device characteristics of poly Si.     

Influence of plastic deformation and prolonged ageing time on microstructure of a Haynes 242 alloy

The material used in this study was a commercial HAYNES® alloy 242? with a nominal composition of Ni‐25% Mo‐8% Cr (in wt.%). In the standard heat treatment, the 242 alloy is annealed at a temperature between 1065 and 1095 °C and then water quenched. The ageing treatment is carried out at 650 °C for 24 h in order to develop the long‐range‐order strengthening. The alloy in the conventionally aged condition was additionally cold rolled to 50% reduction in thickness and subsequently subjected to prolonged ageing at 650 °C for 4000 h. The enhanced diffusion resulted in the decomposition of the Ni₂(Mo,Cr) metastable phase into the stable Ni₃Mo‐based phase. The presence of the new stable phase increased the yield and tensile strengths but deteriorated the ductility of the alloy at both room and 650 °C temperatures.     

Experiment research on grind-hardening of AISI5140 steel based on thermal compensation

The grind-hardening process utilizes the heat generated to induce martensitic phase transformation. However, the maximum achievable harden layer depth is limited due to high grinding forces, and the tensile residual stress appears on the ground surface in the grind-hardening process. This paper proposes a new grind-hardening technology using thermal compensation. The workpiece of AISI5140 steel is preheated by electric resistance heating, and ground under the condition of the workpiece temperature 25°C, 120°C, 180°C and 240°C. The grinding force, harden layer depth and surface quality including residual stress on ground surface, surface roughness and micro-hardness are investigated. The experimental results show that a deep harden layer with a fine grain martensite can be obtained with the thermal compensation. The ground workpiece surface produces a certain compressive residual stress, and the residual compressive stress value increases with preheating temperature. As the preheating temperature increases, grinding force slightly decreases, while there is slightly increment of surface roughness. Compared with the conventional grind-hardening process, both the harden layer depth and residual stress distribution are significantly improved.

     

The role of misorientation and phosphorus content on grain growth and intergranular fracture in iron–carbon–phosphorus alloys

The relationship between the crystallography of intergranular fracture and phosphorus segregation has been investigated in a Fe?0.06wt%P?0.002wt%C alloy aged for 1 h at temperatures between 600 °C and 1000 °C. Two novel techniques were devised for the investigation: first, electron back‐scatter diffraction (EBSD) across the reconstructed fracture surface and, second, a combination of Auger electron spectroscopy, stereophotogrammetry and microscopy to measure phosphorus and carbon on fracture facets combined with EBSD measurements direct from the fracture surface. In total, 700 misorientations were measured from across the reconstructed fracture surface and in ‘control’ areas away from the fracture. It was found that Σ 3s were in general more resistant to brittle fracture than were random boundaries, and it was suggested that alloys of this type could be grain boundary engineered to improve fracture resistance by a short anneal in the austenite region to increase the final proportion of Σ 3s. Sixteen fracture facets yielded combined Auger/EBSD data. The combined Auger/EBSD methodology to acquire joint crystallographic and segregation information from facets was shown to be feasible, although laborious. There were significantly more {110} planes than any other type in the sample population of facets from which combined segregation/crystallography data had been collected. The data suggested that there was on average lower phosphorus segregation on fracture facets that were near {110} than on other intergranular fracture facets.     

The effect of plastic strain and the orthogonal bias field on the processes of magnetization of low-carbon steel

The field dependences of the differential magnetic permeability on the magnetization curves of annealed and plastically strained specimens from low-carbon 20 steel were investigated. The measurements were performed with the simultaneous action of the magnetizing field and perpendicular bias fields of different magnitudes. The results were obtained for non-strained and pre-strained specimens. It was revealed that application of an orthogonal bias field allowed the separation of the contributions of different domain boundaries (the 180° and 90° boundaries) into the processes of magnetization and remagnetization. The critical fields of 90° transitions will be determined not only by the induced magnetic anisotropy that is due to strain, but also by the magnitude of the applied bias field.