scholarly journals Full crystallographic orientation (<i>c</i> and <i>a</i> axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

2021 ◽  
Vol 15 (1) ◽  
pp. 303-324 ◽  
Author(s):  
Morgan E. Monz ◽  
Peter J. Hudleston ◽  
David J. Prior ◽  
Zachary Michels ◽  
Sheng Fan ◽  
...  
Keyword(s):  
Dynamic Recrystallization ◽  
Boundary Migration ◽  
Shear Plane ◽  
Sampling Bias ◽  
Optical Measurements ◽  
Coarse Grained ◽  
Shear Direction ◽  
Preparation Technique ◽  
Thin Sections ◽  
Crystallographic Preferred Orientation

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T>-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

scholarly journals Electron backscatter diffraction (EBSD) based determination of crystallographic preferred orientation (CPO) in warm, coarse-grained ice: a case study, Storglaciären, Sweden

2020 ◽  
Author(s):  
Morgan E. Monz ◽  
Peter J. Hudleston ◽  
David J. Prior ◽  
Zachary Michels ◽  
Sheng Fan ◽  
...  
Keyword(s):  
Electron Backscatter Diffraction ◽  
Shear Plane ◽  
Preferred Orientation ◽  
Coarse Grained ◽  
Strain History ◽  
Shear Direction ◽  
Preparation Technique ◽  
Crystallographic Preferred Orientation ◽  
Electron Backscatter ◽  
Backscatter Diffraction

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice dynamically recrystallizes, with the dominance of intracrystalline glide on the basal plane. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in "warm" (T > −10 ºC), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We have developed a new sample preparation technique, by constructing composite sections, to allow us to use electron backscatter diffraction (EBSD) to obtain a representative, bulk CPO on coarse-grained ice. We suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin section may result in the typical multiple maxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c-axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction, and a concomitant girdle of a-axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by sub-sampling data sets from experimentally sheared ice at high homologous temperatures up to strains of ~ 1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO has been continuously reset and represents a smaller, final fraction of the shear history, rather than the entire finite strain history.

scholarly journals Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

2021 ◽  
Author(s):  
Sheng Fan ◽  
David Prior ◽  
Travis Hager ◽  
Andrew Cross ◽  
David Goldsby ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Dynamic Recrystallization ◽  
Boundary Migration ◽  
High Stress ◽  
Kink Band ◽  
Coarse Grained ◽  
Compression Axis ◽  
Crystallographic Preferred Orientation ◽  
Kink Bands ◽  
Subgrain Rotation

Kinking can accommodate significant amounts of strain during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of cold planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained ice (~1300 µm) deformed under uniaxial compression at -30°C. Microstructural data are generated using cryogenic electron backscattered diffraction (cryo-EBSD). Deformed samples have bimodal grain size distributions, with thin and elongated (aspect ratio ≥ 4) kink domains that develop within, or at the tips of, remnant original grains (≥ 300 µm, aspect ratio < 4). Small, equiaxed subgrains also develop along margins of remnant grains. Moreover, many remnant grains are surrounded by fine-grained mantles of small, recrystallized grains (< 300 µm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization (via subgrain rotation). Low- (< 10°) and high-angle (mostly > 10°, many > 20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. Moreover, previous studies suggest the kinematics of kinking and subgrain rotation should be fundamentally the same. Therefore, progressive kinking and subgrain rotation should be crystallographically controlled, with rotation around fixed misorientation axes. Furthermore, the c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating a tight correlation between kinking and the development of crystallographic preferred orientation. Kink band densities are the highest within remnant grains that have basal planes sub-parallel to the compression axis (i.e., c-axes perpendicular to the compression axis)—these data are inconsistent with models suggesting that, if kinking is the only strain-accommodating process, there should be higher kink band densities within grains that have basal planes oblique to the compression axis (for low kink-host misorientation angles, e.g., ≤ 20°, as in this study). One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of grains. For grains with basal planes sub-parallel to the compression axis, strain-induced GBM is inhibited, and large intragranular strain incompatibilities can be relaxed via kinking, when other processes such as subgrain rotation recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced grain boundary migration (GBM) might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

scholarly journals Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

2019 ◽  
Vol 13 (5) ◽  
pp. 1495-1511 ◽  
Author(s):  
Baptiste Journaux ◽  
Thomas Chauve ◽  
Maurine Montagnat ◽  
Andrea Tommasi ◽  
Fabrice Barou ◽  
...  
Keyword(s):  
High Temperature ◽  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Simple Shear ◽  
Shear Plane ◽  
Preferred Orientation ◽  
Dislocation Slip ◽  
Crystallographic Preferred Orientation ◽  
Polycrystalline Ice

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.

Shear Deformation and Texture Evolution in Al Alloys Processed for One Pass by ECAP

2008 ◽  
Vol 584-586 ◽  
pp. 679-684 ◽  
Author(s):  
Marco J. Starink ◽  
Shun Cai Wang ◽  
Xiao Guang Qiao ◽  
Nong Gao ◽  
Hans Jørgen Roven ◽  
...  
Keyword(s):  
Shear Deformation ◽  
Texture Evolution ◽  
Shear Plane ◽  
Al Alloys ◽  
Coarse Grained ◽  
Al Alloy ◽  
Shear Direction ◽  
Crystallographic Slip ◽  
Slip Planes ◽  
Low Strength

The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions <uvw> parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

scholarly journals Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

2018 ◽  
Author(s):  
Chao Qi ◽  
David J. Prior ◽  
Lisa Craw ◽  
Sheng Fan ◽  
Maria-Gema Llorens ◽  
...  
Keyword(s):  
Shear Strain ◽  
Electron Backscatter Diffraction ◽  
Numerical Models ◽  
Boundary Migration ◽  
Shear Plane ◽  
Equivalent Strain ◽  
Lattice Rotation ◽  
Dislocation Creep ◽  
Shear Direction ◽  
Primary Cluster

Abstract. We sheared synthetic polycrystalline ice at temperatures of −5, −20 and −30 °C, to different shear strains, up to γ = 2.6 (equivalent strain of 1.5). Cryo-electron backscatter diffraction (EBSD) shows that basal intra-crystalline slip planes become preferentially oriented parallel to the shear plane, in all experiments. This is visualized as a primary cluster of crystal c-axes (the c-axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 °C, a secondary cluster of c-axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 °C experiments, the angle between the two clusters reduces with increasing strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of and irregularity of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Recrystallized grains and subgrains are similar in size and are both smaller than the original grains. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

scholarly journals Dynamic Recrystallization in a Naturally Deformed Albite

1983 ◽  
Vol 5 (4) ◽  
pp. 219-237 ◽  
Author(s):  
J. D. Fitz Gerald ◽  
M. A. Etheridge ◽  
R. H. Vernon
Keyword(s):  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Boundary Migration ◽  
Experimental Studies ◽  
Shear Zones ◽  
Optical Microscope ◽  
Coarse Grained ◽  
Transmission Electron ◽  
Subgrain Growth ◽  
Diffraction Patterns

Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, California. In some grains, deformation and recrystallization are concentrated in narrow shear zones less than 50 μm wide. We have examined the substructural progression across these zones by transmission electron microscopy (TEM), in an attempt to determine the details of the dynamic recrystallization mechanism. The misorientation across subgrain and recrystallized grain boundaries has been determined by analysis of electron diffraction patterns.Dynamic recrystallization apparently proceeded by the following stages: 1) the formation of a well-ordered substructure from a more tangled, cell-like array, 2) increasing misorientation between subgrains, 3) rapid growth of subgrains at a misorientation between 3° and 5° to produce new “grains” with straighter grain boundaries and lower internal dislocation densities and 4) continued deformation and rotation of the recrystallized grains with local grain-boundary migration to maintain relatively equiaxed shapes. The ultimate recrystallized structure in the narrow deformation zones consists of grains misoriented by between 5° and at least 30°, most of them containing a well-developed substructure.The combination of subgrain growth and rotation explains a number of features common to dynamically recrystallized minerals. The smaller subgrains present prior to growth and also within recrystallized grains form a population distinct from the larger subgrains and recrystallized grains of approximately equal size, which are those observed in an optical microscope. The smaller subgrains are visible only in TEM. Individual recrystallized grains may remain through substantial straining, rotating in response to dislocation and sub-boundary motion within them, thus preserving and even enhancing the crystallographic fabric (texture). The retention of an initial recrystallized grain population throughout significant continuing deformation may explain the absence of strain softening in some recent experimental studies.

scholarly journals Microstructure and texture evolution in polycrystalline ice during hot torsion. Impact of intragranular strain and recrystallization processes

2018 ◽  
Author(s):  
Baptiste Journaux ◽  
Thomas Chauve ◽  
Maurine Montagnat ◽  
Andrea Tommasi ◽  
Fabrice Barou ◽  
...  
Keyword(s):  
High Temperature ◽  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Simple Shear ◽  
Texture Evolution ◽  
Boundary Migration ◽  
Shear Plane ◽  
Dislocation Slip ◽  
Polycrystalline Ice

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 ⋅ Tm) to reproduce simple shear conditions close to those encountered in ice streams and at the base of fast flowing glaciers. As well documented more than 30 years ago (Hudleston, 1977; Bouchez and Duval, 1982), under simple shear ice develops a two-maxima c-axis texture, which evolves rapidly into a single cluster texture with c-axis perpendicular to the shear plane. This evolution still lacks a physical explanation. Current viscoplastic modeling approaches on ice involving dislocation slip on multiple slip systems (basal pyramidal, and prismatic) fail to reproduce it. Dynamic recrystallization mechanisms that occur in both laboratory conditions and in natural setups are likely candidates to explain the texture evolution observed. In this study, we use Electron BackScattering Diffraction (EBSD) and Automatic Ice Texture Analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed by dislocations with [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, even at high temperature and high finite shear strain. Based on these observations, we propose that that nucleation by bulging, assisted by sub-grain boundary formation, is a very likely candidate to explain the progressive disappearance of the texture cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new models limiting dislocation slip on non-basal slip system and allowing for efficient polygonization by an association of bulging and formation of sub-grain boundaries with a significant [c]-component.

Analysis of Large Strain Hot Torsion Textures Associated With “Continuous” Dynamic Recrystallization

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
S. M. Lim ◽  
C. Desrayaud ◽  
F. Montheillet
Keyword(s):  
Grain Boundary ◽  
Dynamic Recrystallization ◽  
Large Strain ◽  
Texture Evolution ◽  
Boundary Migration ◽  
Grain Boundary Migration ◽  
Shear Direction ◽  
Continuous Dynamic Recrystallization ◽  
Hot Torsion ◽  
Continuous Dynamic

The development of ideal orientations within the steady-state region of hot torsion flow curves of fcc and bcc metals undergoing “continuous” dynamic recrystallization is analyzed. It is well known that in fcc metals, e.g., Al deformed at 400°C and above, the experimentally observed end texture consists of the twin-symmetric B(112¯)[11¯0]/B¯(1¯1¯2)[1¯10] component, whereby the (hkl)[uvw] indices correspond to the shear plane z and the shear direction θ, respectively. In bcc iron however, only one of the self-symmetric D1(112¯)[111] and D2(1¯1¯2)[111] components dominates (the former in the case of positive shear or clockwise rotation about the r-axis, and the latter during negative shear). The tendency toward a single end orientation imposes certain limitations on grain refinement, as this would ultimately imply the coalescence of subgrains of or close to this orientation, and therefore the disappearance of existing high angle boundaries (≥15 deg). It is believed that the preference of D1 over D2, or vice versa, could be related to phenomena other than glide-induced rotations, e.g., grain boundary migration resulting from differences in work hardening rates. In this paper, the standard Taylor model is first used to predict the texture evolution in simple shear under the full-constraint rate-sensitive scheme. This is then coupled with an approach that takes into account grain boundary migration resulting from differences in dislocation densities within grains of varying orientations. The preliminary results are in agreement with experimental findings, i.e., grains with initial orientations close to D2 grow at the expense of neighboring grains during negative shear and vice versa.

scholarly journals Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

2019 ◽  
Vol 13 (1) ◽  
pp. 351-371 ◽  
Author(s):  
Chao Qi ◽  
David J. Prior ◽  
Lisa Craw ◽  
Sheng Fan ◽  
Maria-Gema Llorens ◽  
...  
Keyword(s):  
Shear Strain ◽  
Electron Backscatter Diffraction ◽  
Numerical Models ◽  
Boundary Migration ◽  
Shear Plane ◽  
Line Length ◽  
Lattice Rotation ◽  
Dislocation Creep ◽  
Shear Direction ◽  
Primary Cluster

Abstract. Synthetic polycrystalline ice was sheared at temperatures of −5, −20 and −30 ∘C, to different shear strains, up to γ=2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 ∘C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 ∘C experiments, the angle between the two clusters reduces with strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

Effect of Niobium and Titanium on the Dynamic Recrystallization during Hot Deformation of Stabilized Ferritic Stainless Steels

2004 ◽  
Vol 467-470 ◽  
pp. 1229-1236 ◽  
Author(s):  
Tarcisio R. Oliveira ◽  
Frank Montheillet
Keyword(s):  
Dynamic Recrystallization ◽  
Stainless Steels ◽  
Hot Deformation ◽  
Boundary Migration ◽  
Solute Drag ◽  
Shear Direction ◽  
Ferritic Stainless Steels ◽  
Solid Solution Strengthening ◽  
Continuous Dynamic Recrystallization ◽  
Continuous Dynamic

The study was carried out to understand the mechanisms occurring during dynamic recrystallization of hot deformed 11% chromium stabilized ferritic stainless steels and to compare the behaviour induced by various types of stabilization. It was observed that continuous dynamic recrystallization (CDRX) operates in all materials starting at the onset of straining. Niobium has a more pronounced influence on hardening than titanium during hot deformation, which is due to solid solution strengthening and also to the reduction or stopping of grain boundary migration by solute drag effect. The D2 component, { 2 1 1 }<111>, was found as the major texture component at the steady state for the torsion tests carried along the negative shear direction. It was likely to be formed by the combination of straining and growth of the grains exhibiting both low stored energy and low rotation rate of the crystallographic axes.

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