Density Of High-angle Boundaries Research Articles

Influence of cooling conditions on the microstructure and impact strength of welded joints made by automatic submerged multi-arc welding

The aim of the work was to study the effect of cooling rate on the formation of the structure and impact strength of a welded joint of 0,07C-1,6Mn-0,2Cr-Nb pipe steel. The cooling rate w8/5 was varied from 0.9 to 12.4 °/s in the temperature range of 800–500 °/s by changing the heat input of welding, the groove width, and the temperature of 22 mm thick plates welded on a laboratory welding stand. The conducted studies confirm the previously obtained results on the weldability of pipe steels, that a decrease in the heat input of welding and, as a result, an increase in the cooling rate of the w8/5 welded joint is accompanied by an increase in impact toughness. For the studied steel, a significant increase in the impact toughness of KCV-40 (more than 2.5 times) occurs in the range of cooling rates w8/5 from 0.9 to 7.3 °/s and is typical not only for tests along the fusion line, but also in the center of the seam weld. It is shown that with an increase in the cooling rate of the welded joint in the structure of the weld metal, the maximum length and width decrease, and, at the same time, the proportion of grain boundary ferrite crystallites. At high cooling rates, the matrix of the weld structure consists entirely of acicular ferrite, which is favorable in terms of increasing the density of high-angle grain boundaries, toughness, and cold resistance. In the heat-affected zone, with an increase in the cooling rate, the maximum and average grain sizes decrease; grain-boundary and Widmanstatt ferrite is eliminated; a completely bainite structure is formed and the proportion of lath bainite increases, which also leads to an increase in the density of high-angle boundaries and contributes to an increase in impact strength. With an increase in the cooling rate, the proportion of the MA-constituents in the structure of the seam and the HAZ are increases. It is assumed that an increase in the cooling rate, simultaneously with a favorable effect on the structure of the weld and the weld pool, reduces the time for the processes occurring in the weld pool, which can be an unfavorable factor in terms of the toughness of the weld.

Insight into the impact of microstructure on crack initiation/propagation behavior in carbide-free bainitic steel during tensile deformation

The impact of microstructural characteristics on the crack initiation/propagation behavior of carbide-free bainitic steel was investigated during austempering at 320 and 280 °C. The primary mechanism for causing damage to the material was via of the formation of micro-voids and cracks. The results demonstrated that an uneven distribution of carbon content in the blocky martensite/retained austenite (M/RA) led to decreased carbon content at grain interiors. This caused splitting and decohesion of both blocky M/RA and M/RA-BF phase boundaries. During tests at 280 °C, the blocky M/RA content was decreased, and the microstructure refined. The selection and arrangement of bainite variants also affected the deformation compatibility of bainite ferrite (BF), M/RA and the density of high-angle boundaries (HAGBs). As the covariant boundary fraction of the CP (CP1) variant (V1/V2 – V6) in bainite was much higher than that measured in other covariant boundaries, the ability of covariant deformation between the two phases was greater, and the delayed initiation of micropores was observed. The effect of packet boundaries and block boundaries for hindering crack propagation was different, the most effective barrier for crack propagation was high-angle misorientation boundaries between packet boundaries.

Microstructure and annealing behavior of three rolled pure tungsten plates with different thickness reduction

The annealing behaviors of three rolled pure tungsten plates with different thicknesses reduction ratios were investigated by hardness degradation during isothermal annealing at 1250 °C. The microstructures and textures of three rolled pure tungsten plates in the as-received and fully recrystallized states were characterized by Electron Backscatter Diffraction (EBSD). The stored energies of the plates in the as-received state were calculated through the specific surface density of high angle boundaries and low angle boundaries. All three plates undergo recrystallization and grain growth without an obvious recovery stage during the annealing process at 1250 °C. The recrystallization rate increases with the increased rolling thickness reduction ratios, which can be reasonably explained as the initial microstructure of the tungsten plate with the large rolling thickness reduction ratio has more dynamic recrystallization nuclei/grains and higher stored energy. The rolled and the recrystallized texture were both strengthened with the increased thickness reduction ratios, and the rolled texture is more random than the recrystallized one.

Carbon microalloying effect of base material on variant selection in coarse grained heat affected zone of X80 pipeline steel

The carbon is one of the basic alloying solutes in steels. In the present study, the newly carbon microalloyed X80 steel was designed for the next generation of pipeline and the carbon microalloying effect of base material on the crystallographic structure of coarse grained heat affected zone (CGHAZ) of X80 girth welded joints was systematically investigated using electron back-scattering diffraction (EBSD), considering the fact that crystallography is the most intrinsic aspect in evaluating the microstructure. The results indicated a remarkable variation in the density of high angle grain boundaries and their dispersion by increasing the carbon content of 0.03wt%. It was attributed to the differential shearing mechanism, which was primarily sheared by variant pairing of V1/V2 at 0.06 wt% carbon, whereas at lower 0.03wt% carbon, V1/V4 pairing mechanism dominated the transformation. The preferential variant pairing transformation mechanism at differential carbon content therefore generated dissimilarly selected variants. Thus, as the carbon content increased, the variant selection effect upon phase transformation was attenuated by generating more variants from different Bain groups. The variant pair V1/V2 belonging to different Bain groups neighboring in CGHAZ of high carbon steel increased the density of high angle boundary (Σ3 boundary). Moreover, the variation in CTOD (crack-tip opening displacement) property of HAZ suggested that it should be primarily correlated to the block size (or density of block boundaries) of CGHAZ.

Structure and mechanical properties of a high-carbon steel subjected to severe deformation

The structure and mechanical properties of a high-carbon eutectic steel subjected to the cold plastic deformation by hydrostatic extrusion in a wide range of true strain have been studied. Using scanning and transmission electron microscopy, it has been shown that the formation of cellular, fragmented, and submicrocrystalline structures occurs in the ferritic constituent of the pearlite structure of the steel upon extrusion. This is a consequence of the occurrence of dynamic recovery and continuous dynamic and post-dynamic recrystallization, which cause a decrease in the density of free dislocations at the true strain of more than 1.62. The partial dissolution of the carbide phase is also observed. It has been found that, at a true strain of up to 0.81, the strength properties of the investigated steel are determined mainly by subgrain, dislocation, and precipitation mechanisms of the strengthening; in the deformation range of 0.81–1.62, the role of the grainboundary strengthening increases. At strains above 1.62, grain-boundary strengthening is a prevailing mechanism in the formation of the level of strength properties of the extruded U8A steel. The ultimate tensile strength and yield stress over the entire strain range only uniquely correlate with the density of highangle boundaries; the dependences of the strength characteristics on other structural parameters are not monotonic.

Effect of initial microstructure on Ti–6Al–4V joint by friction stir welding

Ti–6Al–4V alloy plates with three types of initial microstructures, which have different densities of the high angle boundaries, were joined by friction stir welding (FSW), and the microstructure evolution in the stir zone by the FSW was clarified. Concerning the morphology of the microstructure, all three joints show similar characteristics; the top surface of the SZs exhibited a mixture of the lamellar structure with equiaxed α grains and the area fraction of the equiaxed α grain gradually increased with the increasing distance from the top surface due to the temperature gradient. On the other hand, the mean α grain size of the fully equiaxed α structure area decreased with the increasing initial density of high angle boundaries. This accelerated grain refinement was discussed based on the effect of the inhomogeneous microstructure on the recrystallization.

Ultrafine Grained Austenite in a Low Carbon Vanadium Microalloyed Steel

Ultrafine austenite grains with average size of 2 μm were successfully obtained by combining thermo-mechanical control process followed by reheating in a vanadium microalloyed steel. The mixed microstructure transformed from pancaked austenite formed during controlled rolling has a higher density of high angle boundaries, compared to that transformed from equiaxial austenite. It contributes to increasing nucleation density of austenite grain during the reheating process. A certain volume fraction of undissolved nano-sized (Ti, V)C particles, which are formed during the controlled rolling process and/or the reheating process, effectively inhibit austenite grain growth and consequently refine austenite grain size significantly. The critical grain size of austenite calculated by Gladman model agrees well with the experimental result.

Investigation on the microstructure and toughness of coarse grained heat affected zone in X-100 multi-phase pipeline steel with high Nb content

Effect of increasing heat input on microstructure evolution and impact toughness of coarse grained heat affected zone (CGHAZ) in high Nb X-100 multi-phase pipeline steel was investigated by means of Gleeble simulator, optical microscope (OM), scanning electron microscope (SEM) and electron backscattering diffraction (EBSD). Charpy impact test confirmed the optimum toughness of CGHAZ was achieved at heat input of 20kJ/cm, equivalent to the excellent toughness of the base plate. Observations performed by OM, SEM and EBSD show that the microstructure of CGHAZ varies dramatically with heat input without a noticeable changing in prior austenite grain size, and the optimum toughness achieved at the heat input of 20kJ/cm is related to the cumulative contribution of its well-refined martensite/austenite (M/A) constituent and the highest density of high angle boundaries. Analysis on crystallography shows that high angle boundaries are mainly the boundaries between the products from different Bain groups produced from the fcc to bcc coherent transformation within prior austenite grain, and the density of high angle boundary is controlled by the configuration of Bain groups within the crystallographic packet in each austenite grain. With the ideal configuration, the density of high angle boundary can be optimized to be beneficial to keep high toughness in CGHAZ, together with well-refined M/A constituent. This indicates that in addition to M/A refinement, the characteristic in crystallography of the crystallographic packet (the configuration of Bain groups within it) is related to the mechanical properties of CGHAZ and should be controlled to be optimum.

Investigation on the crystallography of reverted structure and its effect on the properties of low carbon steel

Abstract A new type of reverted structure was found to form in upper bainite steel with 0.06% C, 1.5 wt% Cu and 2.0 wt% Ni through annealing at the temperature between A1 and A3 (two-phase region heat treatment) with slow cooling. Tensile and charpy tests showed that the sample with reverted structure exhibited high improvement in −20 °C impact toughness and slight decrease in yield strength. Microstructure observed in detail by TEM suggests that Cu precipitates are helpful to keep the yield strength level. By using SEM and EBSD techniques, two conclusions were gained from the analysis focusing on morphology and crystallography. (I) Together with toughness improvement, reverted structure is formed and dispersed along the prior bainite lath boundaries, apparently increases the density of high angle boundary, especially the boundary above 45° which can effectively stop the propagation of micro-cracks and hence inhibit the initiation of brittle fracture. Thus increasing the density of high angle boundary by generating this new type of reverted structure could be considered as another available option for improving low temperature impact toughness in addition to generating high amount of retained austenite film. (II) Reverted structure and its nearby parallel un-reverted matrix compose “reverted packet”, within which orientations from all the reverted structure and the matrix closely follow the orientations’ distribution of the crystallographic packet described in coherent transformation. Due to this particular orientations’ distribution, high angle boundaries forming in this “reverted packet” are effective in refining the effective grain size for trans-granular cleavage fracture but relatively ineffective in increasing yield strength, thereby theoretically beneficial to decrease the DBTT.

Microstructure and properties of 13Cr5Ni1Mo0.025Nb0.09V0.06N super martensitic stainless steel

Abstract The morphological microstructure, the density and dispersion of high angle boundaries, morphology and micro chemical composition of precipitates and the volume fraction of retained austenite of a commercial super martensitic stainless steel (SMSS) normalized and tempered at various temperatures were characterized by optical microscope, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and X-ray diffraction (XRD) in the light of equilibrium phase diagram of the alloy calculated using Thermo-Calc software. The mechanical properties and pitting corrosion resistance were determined to correlate with microstructures. Two kinds of morphology of precipitate were observed in tempered commercial super martensitic stainless. Besides the globular Nb and V rich carbo-nitride precipitates, rod-like Cr rich nitrides were formed due to excess N content. While high density of high angle boundaries and precipitates contribute to strength properties, the dislocation softening of the matrix and retained austenite from tempering restore the ductility and impact toughness properties. The poor resistance to pitting corrosion is attributed to the occurrence of Cr rich precipitates. It is demonstrated that by lowering the nitrogen content and adding niobium, the Cr rich precipitates can be suppressed and the mechanical properties and resistance to pitting corrosion can be significantly improved.

INFLUENCE OF AUSTENITIZING TEMPERATURE ON THE MICROSTRUCTURE AND IMPACT TOUGHNESS OF A HIGH STRENGTH LOW ALLOY HSLA100 STEEL

The effect of austenitizing temperature on the microstructures and −20 impact toughness of HSLA100 steel was investigated by Gleeble-3500 thermal simulator. Its microstructures were observed by SEM and EBSD, and the relevant transformation kinetics was also analyzed by means of dilatometer. The results show that the microstructure of HSLA100 steel changes gradually from granular to lath bainite with increasing austenitizing temperature. The highest impact toughness of samples was achieved at austenitizing temperature of 1000 , in which martensite-austenite (M/A) islands are finer and dispersed and the density of high angle boundaries is maximum. M/A islands, however, become coarser and this density lowers below 1000 , beyond 1000 , these islands are refined, being accompanied by a dramatic decrease of this density of high angle boundaries. Kinetics analysis indicates that with increasing austenitizing temperature, the transformation start temperature decreases but the transformation rate increases. Both lower start temperature and faster rate would facilitate M/A islands refining. All the transformation occurring in samples might be divided into two stages: bainite and martensite stages. The highest transformed fraction of bainite is achieved in the bainite stage at about 1000 , resulting in the best impact toughness of HSLA100 steel. The

Evolution of grain and subgrain structure during cold rolling of commercial-purity titanium

The evolution of microstructure in commercial-purity titanium during cold rolling to a thickness strain of 2.6 was quantified using electron backscatter diffraction. The measurements were analyzed in terms of the mean grain size and the density of boundaries (the ratio of total boundary length to the scanned area). The density of high-angle boundaries as a function of thickness strain had three distinct stages, each of which was associated with a different mechanism of microstructure formation, i.e., (i) twinning, (ii) an increase in dislocation density and the formation of substructure, and (iii) the formation of deformation-induced high-angle boundaries. The influence of twinning on the kinetics of microstructure evolution was also interpreted.

Large Strain Deformation and Annealing of Aluminium

TEM, Kikuchi diffraction analyses, EBSD, neutron diffraction and hardness measurements have been applied in a study of commercial purity aluminum (AA1200) cold rolled to strains 2 and 4 and afterwards recovered by a heat treatment for 2h at temperatures up to 220 °C. The deformation microstructure is a lamellar structure delineated by dislocation boundaries and high angle boundaries ( ) parallel to the rolling plane. The macrotexture is a typical rolling texture which is composed of individual texture components present as micrometer- and submicrometre-sized volumes. In the lamellar structure, correlations have been established between microstructural parameters and the local texture, showing for example that the density of high angle boundaries and the stored energy vary locally. The local variations affect the annealing behaviors in a way that some regions coarsen faster than others, leading to a recovered structure which is heterogeneous.

From geometry to dynamics of microstructure: using boundary lengths to quantify boundary misorientations and anisotropy

The microstructure of a quartzite experimentally deformed and partially recrystallised at 900 °C, 1.2 GPa confining pressure and strain rate 10 −6/s was investigated using orientation contrast and electron backscatter diffraction (EBSD). Boundaries between misoriented domains (grains or subgrains) were determined by image analysis of orientation contrast images. In each domain, EBSD measurements gave the complete quartz lattice orientation and enabled calculation of misorientation angles across every domain boundary. Results are analysed in terms of the boundary density, which for any range of misorientations is the boundary length for that range divided by image area. This allows a more direct comparison of misorientation statistics between different parts of a sample than does a treatment in terms of boundary number. The strain in the quartzite sample is heterogeneous. A 100×150 μm low-strain partially recrystallised subarea C was compared with a high-strain completely recrystallised subarea E. The density of high-angle (>10°) boundaries in E is roughly double that in C, reflecting the greater degree of recrystallisation. Low-angle boundaries in C and E are produced by subgrain rotation. In the low-angle range 0–10° boundary densities in both C and E show an exponential decrease with increasing misorientation. The densities scale with exp(− θ/ λ) where λ is approximately 2° in C and 1° in E; in other words, E has a comparative dearth of boundaries in the 8–10° range. We explain this dearth in terms of mobile high-angle boundaries sweeping through and consuming low-angle boundaries as the latter increase misorientation through time. In E, the density of high-angle boundaries is larger than in C, so this sweeping would have been more efficient and could explain the relative paucity of 8–10° boundaries. The boundary density can be generalised to a directional property that gives the degree of anisotropy of the boundary network and its preferred orientation. Despite the imposed strain, the analysed samples show that boundaries are not, on average, strongly aligned. This is a function of the strong sinuosity of high-angle boundaries, caused by grain boundary migration. Low-angle boundaries might be expected, on average, to be aligned in relation to imposed strain but this is not found. Boundary densities and their generalisation in terms of directional properties provide objective measures of microstructure. In this study the patterns they show are interpreted in terms of combined subgrain rotation and migration recrystallisation, but it may be that other microstructural processes give distinctive patterns when analysed in this fashion.

Structural and mechanical properties in AA 5083 processed by ECAE

Equal channel angular extrusion has been used to analyse refining of grains in an industrial 5083 aluminium alloy during severe plastic deformation. The influence of the total strain as well as of the processing route were studied by tensile tests and TEM. The room temperature behaviour and the high temperature properties suggest that large strains increase the density of high angle boundaries in the material. The optimal processing route to achieve grain refinement appears to be route B for the present investigation.

High-Resolution EBSD Analysis of Severely Deformed Submicron Grained Aluminum Alloys

AbstractThere is currently much interest in the use of severe plastic deformation techniques for the production of submicron grained Al-alloys. Because of their small grain sizes, previous investigations have relied on the TEM to study the grain structures after processing. However, the TEM is not that well suited to obtaining statistically significant data on the misorientations of the boundaries present in the deformed state. Recent improvements in the resolution of SEMEBSD systems means that this powerful technique can now be used to quantitatively analyse severely deformed structures more reliably than was previously possible. Results using this method have shown that the deformation structure is very sensitive to the strain path, material, and processing conditions. With a constant strain path fibrous grain structures tend to be formed and homogeneous submicron grain structures can only be produced at very high plastic strains, of greater than ten. Materials deformed with cyclic strain paths are more isotropic, but generally have bimodal grain structures and contain a lower density of high angle boundaries and only bands of submicron grains.