Medium Angle Boundaries Research Articles

Effects of different cooling rates on the microstructure, crystallographic features, and hydrogen induced cracking of API X80 pipeline steel

Hydrogen-Induced Cracking (HIC) is a primary failure mechanism of pipeline-welded joints in the absence of external loading in the oil and gas exploration industries. Three different cooling rates after austenitization were used to simulate in the laboratory different regions of the heat-affected zone (HAZ) formed when welding an API X80 pipeline steel specially designed to enhance the HIC resistance. The samples were characterized with regard to microstructure and crystallography as well as HIC resistance. The HIC resistance test used NACE TM0284-2011 methodology. The microstructure and its homogeneity varied as a function of cooling rates. Samples containing inclusions and segregation zone from the segregation bands of specimens showed reduced HIC resistance, while specimens containing only acicular ferrite and granular bainite coupled with the absence of segregation zone showed significant improvement in HIC resistance. The best HIC resistance results came from samples presenting fine acicular ferrite consisting of fine interlocking plates, with divergent crystallographic orientations, preventing the formation of localized strain distribution inside the grain and at grain boundaries. It was also found that a large proportion of medium-angle boundaries prevent microcrack initiation and the transgranular mode of crack propagation.

Microstructure Evolution and Kinetics of Static Recrystallization of Medium Mn Steel in the Two‐Hit Isothermal Compression

The thermal deformation behavior and microstructure evolution of medium Mn steel (0.15C–7Mn) are investigated by the two‐hit compression tests. The results reveal that different static softening dynamics are the functions of deformation temperature, interval time, and strain rate. The short static softening plateaus are exhibited in the static softening curves during post‐deformation holding at 950 °C and 0.1, 1 s−1. Based on the electron backscatter diffraction (EBSD) analysis, the progresses of static recovery (SRV) and static recrystallization (SRX) are enhanced by the increment of deformation temperature. The frequencies of medium angle boundaries (MAGBs) and high angle boundaries (HAGBs) increase gradually with an increased interval time at 1000 °C and 0.1 s−1, which suggests that both SRV and SRX contribute to static softening, but the predominant softening mechanism is SRV. In addition, the influence of temperature, interval time, and strain rate on the distribution and average magnitude of grain size is investigated. The deformation temperature and strain rate have a significant influence not only on average grain dimension, but also on the distribution of grain size. Interval time has great influence on average grain size, but has slight effect on distribution of grain size.

Preparation of ultrafine-grained copper alloy by annealing treatment after multi-directional compression

Ultrafine grained(UFG) metallic materials arouse a great interest due to their excellent mechanical properties.The UFG materials,produced by severe plastic deformation(SPD) technique such as channel angular pressing(ECAP) and high-pressure torsion(HPT),exhibit obvious improvement in strength but a large decrease in their thermal stability and ductility.In this article,an UFG QBel.7 copper alloy with good comprehensive properties was prepared by multi-directional compression (MDC) at room temperature and subsequent annealing at high temperature.The deformed and subsequent annealed microstructures were investigated by OM.TEM and SEM/EBSD techniques.The integrated flow curves plotted over a number of compression passes increase to a maximum at moderate strains of 1 to 2 followed by steady-state-like flow at high cumulative strains.Fine grains were not observed even at a higher cumulative stain of￡￡=4.8,although there were many sub-grains when the samples were deformed to￡s=2.4.This indicates that the dynamic recrystallization or recovery was completely inhibited by fine precipitates.Static recrystallization(SRX) of the MDCed structure at 973 K was also investigated.With the increment of cumulative strains,the effect of grain refinement became more obvious,but the thermal stability was getting worse.At a medium strain of∑e=2.4. the minimal grain size of 0.8μm could be developed with an excellent combination of properties.The formation of ultrafine grains is attributed to the development from low-angle boundaries to medium angle boundaries to large angle boundaries.The change of the average grain size with annealing time could be divided into three stages:a recovery period for grain refinement,rapid grain refinement and normal grain growth.

EBSD characterization of deformed lath martensite in IF steel

Rolling deformation results in the transformation of a lath martensite structure to a lamellar structure characteristic to that of IF steel cold-rolled to medium and high strains. The structural transition takes place from low to medium strain, and electron backscatter diffraction analysis shows that the frequency of medium angle boundaries with misorientation angles of 5-10° decreases with increasing strain, while the frequencies of boundaries with angles in the ranges of 1-5° and 10-25° increase, resulting in the evolution of a bimodal misorientation angle distribution. The microstructural evolution and the strength are characterized for lath martensite rolled to a thickness reduction of 30%, showing that large changes in the misorientation take place, while the strain hardening rate is low.

Microstructural evolution and grain subdivision mechanisms during severe plastic deformation of aluminum particles by ball milling

Electron backscattered diffraction technique was used to investigate the microstructure of aluminum particles deformed by high-energy ball milling. The lengths of different types of boundaries per area were calculated for different samples. The results show that the deformation mechanism and the rate of grain subdivision changed considerably as milling time increased. At the beginning of the milling, deformation banding subdivided grains and dynamic recovery formed a cellular structure of low angle boundaries. After further milling, particles were flattened; an increase in the aspect ratio of the original grains together with cold welding of the particles contributed to the formation of high angle grain boundaries (HAGBs). Lattice rotation progressively increased the misorientation of low and medium angle boundaries and transformed them to HAGBs, which resulted in formation of new small equiaxed grains by continuous dynamic recrystallization. This research shows subgrain rotation was the main mechanism for formation of new HAGBs.

Computer simulation of the formation of fragments with medium-angle boundaries in shear bands

The formation and transformation of low-angle dislocation boundaries into medium-angle grain boundaries are analyzed using computer simulation in the particular case of plastic deformation localized in shear bands. The conditions of appearance of fragments with medium-angle boundaries in shear bands are determined. It is shown that the evolution of low-angle boundaries into medium-angle boundaries in a shear band proceeds due to the suppression of active plastic deformation inside a subgrain by the elastic fields of the disclinations that appear in the subgrain junctions as a result of the misfit of plastic rotations along the subgrain boundaries. When these conditions are met, a subgrain behaves like an undeformable inclusion in a surrounding matrix during continuing deformation and undergoes crystallographic rotation. In this case, the subgrain misorientation increases continuously, which leads to the transformation of initial low-angle dislocation boundaries into medium-angle and, in the limit, high-angle boundaries.

Formation of fragments with medium-angle boundaries in shear bands: Computer simulation

The conditions for the formation of fragments with medium-angle boundaries in shear bands are analyzed using computer simulation. It is shown that the main condition for the transformation of weakly disoriented dislocation structures into a fragmented structure is the suppression of active plastic deformation in a subgrain by the elastic fields of disclinations appearing at subgrain boundary junctions as a result of mismatch of plastic rotations in individual subgrain boundaries. Under these conditions, during continued straining in the surrounding matrix, such a subgrain behaves as an undeformed inclusion and experiences a crystallographic rotation. The disorientation of the subgrain continuously increases, thereby transforming initial small-angle dislocation boundaries into medium-angle and (in the limit) large-angle boundaries.

Mechanisms of grain refinement in Mg–6Al–1Zn alloy during hot deformation

Abstract Mechanisms of grain refinement were investigated in an extruded Mg–6Al–1Zn alloy during hot compression at 623 K and a true strain rate of 3 × 10−3 s−1. With increasing strain, grain fragmentation gradually took place; the dominant grain refinement mechanism, however, depended on the strain. At strains up to ɛ = 0.1, mechanical twinning contributed notably to grain refinement and to the formation of high angle boundaries. At medium to high strains, continuous dynamic recrystallization took place. Kinking contributed to grain fragmentation at all strains. The latter two mechanisms also increased the fraction of low to medium angle boundaries.

Effect of Shear Strain on the Microstructural Evolution of a Low Carbon Steel during Warm Deformation

The microstructural evolution of a low carbon steel was investigated under two different conditions with and without shear strain, using electron backscattered diffraction (EBSD) when a uniaxial compression of 67% was applied at a warm working temperature of 773 K. The equivalent strain and shear strain imposed by the compression test were calculated quantitatively by finite element analysis (FEA). As the equivalent strain was increased without any shear strain, the texture of the α and γ fibers was strongly developed, and the grain refinement was accelerated. However, as the shear strain was increased at the same equivalent strain, the thickness of the pancaked ferrite decreased and the texture was weakened. With increasing shear strain, a shear texture, such as (110) // TD and {110} // ND, was observed and a high angle boundary over 15° developed at the expense of low and medium angle boundaries below 15°. The shear strain accelerated the subdivision of the ferrite grain and randomized the texture.

Subgrain Rotation Recrystallization in Minerals

Subgrain rotation is a common mechanism of continuous dynamic recrystallization in minerals and some metals. The mechanism involves new grain boundary formation by progressive rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This paper reviews the status of our current knowledge of rotation recrystallization in minerals. In minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚. In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have formed. Current models assume that all subgrain boundaries increase in misorientation with strain. However, recent studies show that many different types of subgrain boundary develop in minerals. The formation of new high angle grain boundaries is only likely along some types of geometrically necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle network of subgrain boundaries that develop in many minerals may be incidental boundaries, as found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries formed by trapping of statistically stored dislocations. New and extended models for rotation recrystallization are needed that consider i) incidental subgrain boundaries as well as different types of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link between the development of subgrain misorientation and texture development.

Recognising the crystallographic signature of recrystallisation processes in deformed rocks: a study of experimentally deformed rocksalt

The microstructural development of synthetic rocksalt experimentally deformed at 100–200°C can be dominated either by grain boundary migration recrystallisation or by subgrain rotation recrystallisation, depending on water content. Samples taken from both regimes have been analysed using automated electron backscatter diffraction in order to collect crystallographic orientation and misorientation data. The frequency distribution of boundary misorientations, the boundary hierarchy characteristics and the nature of any crystallographic preferred orientation (CPO) have been used to determine the crystallographic signature of both recrystallisation processes. Dominant subgrain rotation recrystallisation results in many low to medium angle (4–20°) boundaries, a strong CPO and a continuous boundary hierarchy. Dominant grain boundary migration recrystallisation results in few low or medium angle boundaries, and a discrete boundary hierarchy. The causes of these differences and the potential application of crystallographic signatures to the study of naturally deformed rocks are discussed.

Microtexture, Grain Boundary and Grain Junction Evolution in Al-Li Sheet During Superplastic Forming

The microtexture of superplastically deformed 8090 Al-Li sheet has been measured using electron backscatter diffraction, for true strains of 0, 0.25, 0.75, 1.5 and 2.4. The data has been interpreted in terms of individual texture variants, grain boundary types (low angle or high angle) and grain junction types (I-lines or U-lines, as defined by an extension to the O-lattice theory). A key feature of the results is that local textures tend to be dominated by only one or perhaps two orientation variants, which leads directly to the presence of a high proportion of low or medium angle boundaries and hence a high proportion of I-line grain junction types. These effects decrease with increasing superplastic strain. Since it has been previously suggested that the presence of I-lines indicates dislocation-related phenomena, and the presence of U-line grain junction types denotes diffusion-related phenomena, the proportion of each of these provides as indication of the predominant accommodation mechanism.

Oriented growth and definition of medium angle semiconductor bicrystals

The physical and metallurgical procedure in the preparation of semiconductor bicrystals is described, methods of seed orientation are given and precision limits are discussed. It is shown that medium angle boundaries can be grown at a high precision if proper techniques are used. A number microphotographs of grown grain boundary structures is given.