Formation Of Subgrain Boundaries Research Articles

Localized plastic deformation and hardening in laser shock peened Inconel alloy 718SPF

Localized plastic deformation and hardening was studied in laser shock peened (LSP) Inconel 718 SPF (Super plastic forming), a fine grained Ni-Base superalloy, using electron backscatter diffraction (EBSD), nanoindentation, and micro pillar compression testing. 718SPF samples were LSP-treated with two different laser power densities. After LSP, the plastic deformation is manifested as increase in the fraction of low angle misorientations and the formation of sub-grain boundaries. EBSD was used to study plastic deformation as a function of distance from the peened surface for different laser intensities. Different approaches like image quality mapping and local misorientations analysis were used to study the in-depth distribution of the plastic strain. Results show that EBSD is effective in the qualitative characterization of localized plastic strain of the materials subjected to LSP.

Transient creep behavior of a novel tempered martensite ferritic steel G115

The transient creep deformation behavior and dislocation evolution in G115 steels were investigated in the temperature range 625–675 °C for applied stresses of 120–220 MPa. The transient creep curves showed that creep rate decreased with increases in time and creep strain. In this study, a novel method was proposed to determine the transient creep strain and the transient creep time. The strain decreased with increase in applied stress between 625 °C and 675 °C. A phenomenological constitutive equation was used to characterize the transient creep curves of G115 steels. The results showed that the constitutive equation for the G115 steel had a high precision. An internal stress was introduced to study the interactions of dislocations and precipitates, and a mechanism-based equation for transient creep in G115 steels was derived. The dependences of normalized strain and transient creep time on applied stress and temperature were analyzed in detail to better understand the transient creep deformation mechanism. It was concluded that dislocation annihilation at the grain boundaries was the dominant rate-controlling mechanism in the transient creep deformation of G115 steels. It was observed that the dislocation structures became more complex and no obvious textural features occurred after the transient creep deformation. The calculated dislocation density decreased at 650 °C relative to the initial value, which was mainly attributed to the annihilation processes occurring at the grain boundaries and the subsequent formation of subgrain boundaries.

Enhanced strength and corrosion resistance of Mg-Gd-Y-Zr alloy with ultrafine grains

Simultaneous improvements in strength and corrosion resistance of Mg-Gd-Y-Zr alloy were realized through forging at room temperature with formation of ultrafine grained (UFG) structure. Compared with its coarse grained (CG) counterpart, the UFG sample exhibits an enhancement of 211 MPa (103%) in yield strength and a reduction of 0.644 × 10−5 A/cm2 (25%) in corrosion current density. Effect of microstructure on the mechanical property was quantitatively investigated, revealing a good coincidence of the experimental result with the calculated one based on Bailey-Hirsch equation. The improvement in corrosion resistance is attributed to the formation of sub-grain boundaries.

Relaxation of the elastic field of a crystal during the subgrain boundary formation induced by plastic deformation

The change in the energy of a crystal induced by subgrain boundary formation during plastic deformation is studied by computer simulation. The crystal is shown to react to the changes caused by the appearance of plastic misfits: it changes its defect structure to minimize an increase in its elastic energy.

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

Abstract. Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = 〈a〉, while glide on non-basal planes can involve dislocations with b = 〈a〉, b = [c], and b = 〈c + a〉. During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth). EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal 〈a〉 slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or 〈c + a〉 slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism 〈a〉 slip or slip of 〈a〉 screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of non-basal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.

Grain‐ to multiple‐grain‐scale deformation processes during diffusion creep of forsterite + diopside aggregate: 1. Direct observations

AbstractWe uniaxially deformed fine‐grained (~ 1 μm) forsterite + diopside (5 and 20 vol %) aggregates in the diffusion creep regime. Prior to deformation, line markers were milled on a lateral surface of a cylindrical sample to detect single‐ to multiple‐grain‐scale deformation. We performed deformation experiments and observations of the marker‐etched surface after sample cooling multiple times on the same specimens. The strain measured at the scale of several tens of grains from macroscopic shortening of the markers parallel to the compression axis is consistent with the total strain of the sample. However, microscopically, the markers are intensely segmented and rotated at the grain scale increasing with the sample strain. Meanwhile, essentially, no deformation is observed within the grains in most of the samples. The surface microstructures, including the deformation of the markers, reveal the serial operations of grain boundary migration, grain boundary sliding, rigid body grain rotation, and grain neighbor switching, which correspond well to processes expected in diffusion‐controlled superplasticity. This sequence is commonly observed in both samples consisting of forsterite grains of tabular and equiaxed grain shapes, which have been shown to develop notable crystallographic preferred orientation (CPO) and random (or weak) CPO, respectively, during diffusion creep. Intragranular regions of relatively larger forsterite grains in the specimens deformed at stresses near the transition between deformation mechanisms from diffusion creep to dislocation creep reveal marker deformation and formation of surface creases and subgrain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.

The flow stress evolution and grain refinement mechanisms during hot deformation of Al-Mg alloy

Abstract In this study, the hot deformation behavior of an Al-1% Mg alloy with very coarse initial grain size was investigated in terms of flow stress evolution and grain refinement mechanism. The large grain size was employed to study the traditional continuous dynamic recrystallization (CDRX) behavior below the critical strain to reach classical geometric dynamic recrystallization (GDRX), i.e., the thickness between HAGBs decreases and impingement of serrated HAGBs finally leads to the formation of new grains. The microstructure evolution during hot deformation with different deformation temperatures and strain rates on Gleeble 3800 machine was examined systematically by EBSD. It is concluded that, among the investigated conditions, the stress increases with strain and could reach steady state at small strain of ~0.01 at higher deformation temperature, while it keeps increasing at lower deformation temperature. The grain refinement mechanism clearly depends on the hot deformation condition, (micro)shear band assisted grain refinement was observed at lower deformation temperature and CDRX dominated at high deformation temperatures. In the conditions favoring CDRX, the grain refinement process is strongly grain orientation dependent, there exist some stable orientations inside which the formation of subgrain boundaries are difficult. The connection between the stress-strain behavior and microstructural evolution during hot deformation is further discussed. As compared to the traditional CDRX mechanism, the results presented in this paper provide a clearer and more complete picture of the grain refinement mechanism during hot deformation of Al-Mg alloys.

Texture/Microstructural Evolution and Mechanical Properties of a Hot and Cold Rolled Al-Mg-Si-Cu Alloy

The evolutions of texture, microstructure as well as mechanical properties of an Al-Mg-Si-Cu alloy during hot and cold rolling were investigated by TEM, EBSD and X-ray diffraction in this study. The results revealed that formation of sub-grain boundaries and //RD fiber texture after hot rolling is beneficial to the increase of the elongation of the alloy. The shear texture of //RD fiber and γ-fiber occupies a dominant role during the hot rolling process, while the main texture during the cold rolling process is the rolling texture of β-fiber. β-fiber and τ-fiber texture components are identified as two main types of texture components during cold rolling, and the Brass component {011} is transformed into Goss component {011} with the increase of the deformation.

Microstructural evolution in the welding zone of laser shock peened 316L stainless steel tube

Typical microstructure morphologies in the near-surface layer of the welding zone of gas tungsten arc welded 316L stainless steel tube with and without laser shock peening (LSP) were characterized by optical microscopy and transmission electron microscopy observations. Experimental results and analysis indicated that massive LSP treatment has an important influence on microstructures in the welding zone and heat-affected zone of the welded stainless steel tube. Special attention has been paid to the grain refinement mechanism induced by massive LSP treatment is proposed in the near-surface layer of the welding zone, consisting of the formation of dislocation tangles and dislocation walls due to the pile up of dislocation lines, the development and formation of subgrain boundaries, and the evolution from subgrain boundaries to new grain boundaries. Meanwhile, under laser shock wave, the coarse grain in the near-surface layer in heat-affected zone of the welded specimen was also divided by dislocation movement.

Continuous dynamic recrystallization in low density steel

The high temperature deformation behavior of a duplex low density steel (Fe-17.5Mn-8.3Al-0.74C-0.14Si, wt. %) was investigated. This was performed through utilizing the hot tensile deformation scheme with a particular emphasis on the austenite grain refinement mechanisms. The extended dynamic recovery under the lowest applied strain rate (0.0001s−1) was considered as the main restoration mechanism. In the latter, the misorientation was largely positioned below 15° and this was attributed to the progressive formation of subgrain boundaries. Therefore, a positive effect on the material ductility behavior was provided through stress relieving phenomena. The occurrence of partial recrystallization was also traced in the microstructure during tensile deformation in the low strain rate regime. Increasing the applied strain rate to 0.1s−1 provided an appropriate condition for the occurrence of continuous dynamic recrystallization, where the number of subgrain boundaries was decreased and the continuous transformation of subgrains into the recrystallized grain was occurred. The recrystallized grains generally acquired a different microtexture with respect to the parent austenite grains. It was found that the presence of fine grains associated with continuous recrystallization could favour the grain boundary sliding thereby resulting in wedge-type cracking which would deteriorate the ductility behavior of the material.

Superplastic Deformation Mechanisms in High Magnesium Contenting Aluminum Alloy

The evolution of surface, grains and dislocation structures during superplastic deformation was studied in Al–6.8%Mg–0.6%Mn–0.25%Cr alloy by SEM, EBSD, TEM techniques. The effective activation energy of superplastic deformation was calculated. Contribution of grain boundary sliding was defined during superplastic deformation. Low value of grain boundary sliding, significant dynamic grain growth in stress direction, high dislocations activity and permanent continuous formation of sub-grain boundaries during superplastic deformation were found.

Strain field evolution during dynamic recrystallization nucleation; A case study on ice

Nucleation mechanisms occurring during discontinuous dynamic recrystallization (DDRX) is investigated by Digital Image Correlation (DIC) during creep experiment on polycrystalline columnar ice. Thanks to the columnar microstructure, discrimination of the nucleus can be done without ambiguity comparing pre- and post- deformation texture. In-situ DIC analyses are performed around a triple junction were nucleation occurred to follow strain field evolution. Strain field evolution appears strongly linked to nucleation mechanisms, local grain boundary migration and sub-grain boundary formation such as tilt sub-grain boundaries and kink bands. Nucleation processes are correlated with strong strain heterogeneities well characterized by the principal strains evaluated by DIC. It was possible to follow nucleus growth through the evolution of strain localization along the new grain boundaries. Kink bands act as a buffer zone close to the triple junction and accommodate shear parallel to the c-axis. The local strain field appears to be efficiently redistributed by recrystallization processes which create a new microstructure more compatible with the local stresses.

Effect of ausforming temperature and strain on the bainitic transformation kinetics of a low carbon boron steel

The effect of ausforming temperature and strain on the bainitic transformation kinetics was investigated in a low carbon boron steel. A new mechanism, which is based on the competition between the increase in nucleation rate and the decrease in average volume of bainite sheaf after deformation, is proposed. The increase in nucleation rate is due to the decrease in boron concentration at the grain boundaries after small deformation and the formation of sub-grain boundaries at the grain interior after large deformation. The decrease in average volume of bainite sheaf is ascribed to the frequent impingement of bainite sub-units after deformation. The increase in nucleation rate after deformation results in the decrease in incubation time, which is confirmed from the experiment. The increase in nucleation rate overcomes the decrease in average volume of bainite sheaf, resulting in the increase in transformation velocity and volume fraction after small deformation. On the contrary, the decrease in the average volume of bainite sheaf overcomes the increase in nucleation rate after large deformation, leading to the decrease in transformation velocity and volume fraction of bainite.

Superplastic deformation mechanisms in fine-grained Al–Mg based alloys

The superplastic deformation behaviour at elevated temperatures and constant strain rates of two fine-grained AA5083 type aluminium alloys was investigated. The first alloy with chromium contains Al6(Mn,Cr) particles and the second alloy without chromium has Al6Mn particles. The effective activation energy of superplastic deformation and the activation parameters for the grain boundary relaxation was calculated. The microstructure evolution and contributions of grain boundary sliding, intragranular deformation and diffusion creep to total superplastic deformation were studied by SEM, EBSD, FIB, TEM techniques. Low values of grain boundary sliding and permanent continuous formation of sub-grain boundaries were found in both alloys. Significant dynamic grain growth during superplastic deformation and large value of intragranular deformation were found in the alloy without chromium. Intragranular deformation is not significant and the superplasticity is primarily a result of diffusion creep in the chromium containing alloy with Al6(Mn,Cr) particles.

Finite lattice distortion patterns in plastically deformed zircon grains

Abstract. This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea–Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed. The investigated zircon grains ranging from 10 to 50 μm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6–1.8° per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25° within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress. Investigated microstructures demonstrate that misorientation axes are usually parallel to the < 001 > and < 100 > crystallographic directions; dominant slips are < 010 > {001}, < 010 > {100} and < 001 > {010}, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase.

Upper–greenschist facies intragrain deformation of albite in mylonitic meta–pegmatite and the influence of crystallographic anisotropy on microstructure formation

We studied the deformation behaviour of albite from Permian meta–pegmatite in Cretaceous upper–greenschist facies shear zones from the Austroalpine Matsch Unit in the Eastern Alps (Italy). Sodium–feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid–crustal conditions in grains with different angular relations between their (010) planes and the kinematic frame. The studied rocks were deformed at c. 500 °C in localized shear zones with well characterized top–W shear kinematics supposedly during the Cretaceous upper–greenschist facies tectonometamorphic event. Microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing, dissolution–precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction. Dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine–grained albite with some compositional variability (Ab96–98 and Ab89–91) in cracks. New albite precipitates form aggregates with straight segments of high angle grain boundaries, nearly 120° dihedral angles and only a poor or no orientation relation to the hosting clast. Intragranular kinking is related to continuous lattice rotation of up to 15° by a misorientation axis close to albite [100] and the formation of subgrain boundaries with maximum misorientations of 7°. Synthetic microshear zones supposedly nucleated on pre–existing cracks, and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone. New microstructural and textural data from mylonitic Permian meta–pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation.

Effects of Strain Rate on Dynamic Strain Aging of SA508-III Steel

The dynamic strain aging (DSA) behavior of SA508-III steel was evaluated through tensile tests with different strain rates from 10-4 to 10-1s-1 at 350°C. The OM, SEM and TEM were carried out to observe the microstructures and fracture morphologies of the steel. The results show that the serrated flows appear in the stress-strain curves when the strain rate is between 10-3~10-2s-1, indicating that DSA occurs. Under the strain rate range, the tensile strength increases and the elongation and the reduction of area decrease. However, the fracture surface of the steel after tensile tests is still ductile. DSA in SA508-III steel at the strain rates from10-3 to 10-2s-1 is mainly caused by the interaction between the internal solute atoms and dislocations, which leads to the dislocations multiplication and the formation of sub-grain boundaries and dislocation cell structure.

Flux pinning properties of Gd–Ba–Cu–O trapped field magnets grown by a modified top-seeded melt growth

We report a modified top-seeded melt-growth technology for processing high performance Gd–Ba–Cu–O trapped field magnets (TFMs) by providing an additional liquid source during the melt-textured growth. The enriched liquid source allows sufficient growth in the a−b plane with an increased growth rate, and depresses the accumulation of Gd2BaCuO5 (Gd211) particles, which largely suppresses the formation of sub-grain boundaries in the a-growth sectors. An average trapped magnet field of 1.2 T is achieved for the 32 mm Gd–Ba–Cu–O TFMs at 77 K. These Gd–Bd–Cu–O TFMs exhibit superior in-field flux trapping performance, thanks to the significant second peak effect of the critical current density (Jc). The additional liquid source intensifies the Gd–Ba substitution, and therefore increases the in-field Jc, especially at the temperature range of 30–70 K, which is meaningful for practical applications.

The subgrain structure in turbine blade roots of CMSX-4 superalloy

As-cast single-crystal turbine blade roots made of CMSX-4 were investigated. The Laue method, X-ray diffraction topography, novel X-ray diffractometer provided by EFG company and scanning electron microscopy were applied to study the subgrain structure. It was found that low angle boundaries of macro scale length were present in the roots of the blades. Additionally, linear distributions of angles describing inclination of [001]γ′ direction to the main blade axis and rotation of this direction were defined across the low angle boundaries by EFG diffractometer. With the use of this diffractometer, the lattice parameter linear distributions across the low angle boundaries were also defined. The dendrites arrangement were determined by scanning electron microscopy (SEM) macro images. The mechanism of the formation of subgrain boundaries and characteristics of the orientation interactions between dendrites inside subgrains were proposed.

Small-angle subgrain boundaries emanating from dislocation pile-ups in multicrystalline silicon studied with synchrotron white-beam X-ray topography

The formation of dislocation pile-ups and related small-angle subgrain boundaries in block-cast multicrystalline silicon for photovoltaic applications has been studied by means of white-beam X-ray topography (WB-XRT). For this purpose, samples sliced perpendicular and parallel to the growth direction have been investigated in reflection and transmission geometry, respectively. During the growth process of the silicon ingot, the dislocation density increases. WB-XRT measurements revealed the formation of small-angle subgrain boundaries. The subgrains have a slightly changed orientation related to a rotation of ∼0.07–0.80° around an axis parallel to the growth direction. This tilt results from the high number of dislocations forming dislocation pile-ups and walls. The spacings between dislocations in such subgrain boundaries were found to be between 297 and 28nm. A qualitative model for the formation of dislocation pile-ups is proposed.