Promote Strain Localization Research Articles

Boudinage and the rheology of syntectonic migmatites in the high-strain Taili deformation zone, NE China

Abstract This paper presents a detailed field characterization of boudinage in a high-strain zone several kilometers wide in Northern China to establish relationships between boudin types and rheological contrasts between different parts of migmatites during the migmatization process. This zone contains nearly all types of boudins: foliation boudins, blocktorn boudins, pinch- and- swell structures, tapering boudins, object boudins, and modified boudins. These boudinage structures record the different stages of melt-involved and solid-state deformation. The boudinage of migmatites is significantly controlled by the evolving rheological contrasts between the leucosome and melanosome. During the melting stage, the deformation and boudinage of rocks are controlled by the melt fraction. Migmatite strength progressively decreases with increasing melt fraction. The occurrence of melt-filled foliation boudins and melanosome block boudins suggests that the residuum and melanosome are more competent than the leucosome. During solid-state deformation after crystallization, the existence of recrystallized solid-state leucosomes and the intrusion of pegmatites cause the migmatite strength to increase. The relationship is reversed: the leucosome is much more competent than the melanosome. The type and geometry of boudins and pinch- and- swell structures are correlated to the fraction of leucosome in the migmatites. The mechanical strength and strain localization of migmatites after crystallization depend on the presence and volume fraction of the different mineral phases, as well as the mineral grain size. The type and geometry of boudins suggest that the effective viscosity of migmatite can be ranked, from high to low, as: quartz veins; coarse-grained, thick pegmatite; coarse-grained, diatexite migmatite; medium-grained leucosome; and fine-grained melanosome. While experiencing partial melting and migmatization, a rheologically homogeneous protolith is turned into two dominant lithologic domains: a competent diatexite migmatite domain and an incompetent melanosome migmatite domain. Spatially, with the increasing leucosome fraction in migmatites, the migmatite rheology of rock changes from homogeneous to heterogeneous and anisotropic, and then back to homogeneous. The strain distribution likewise changes from uniform to partitioned, and then back to uniform. This evolutionary process of strength and rheological properties of rocks during migmatization may promote strain localization at mid-crustal conditions.

On fracture anisotropy in textured aluminium alloys

The yielding, plastic flow and fracture of textured aluminium alloys depend on their microstructure formed during the thermo-mechanical processing. In this work, we investigate two extruded aluminium alloys with different yield strength, work hardening, grain structure, crystallographic texture and tensile ductility. To study the fracture anisotropy of these alloys, i.e., the variation in the failure and fracture strain with loading direction, finite element simulations of an axisymmetric smooth tensile specimen are compared to experimental tests in five in-plane directions with respect to the extrusion direction. A newly proposed coupled damage and single crystal plasticity model is used in three-dimensional finite element analyses of the tensile tests. The tensile tests are simulated in Abaqus/Explicit, where each grain is explicitly modelled. This modelling framework is able to capture the effects of the heterogeneous yielding and plastic flow on ductile fracture, caused by the differences in the crystallographic orientation between grains, such as shear bands which may promote strain localization and ductile fracture. The overall agreement between the experimental and numerical results with respect to the plastic anisotropy, i.e., the anisotropy in yielding, plastic flow and work hardening, highlights the important role played by the crystallographic texture. Plastic anisotropy is found to have a marked influence on the tensile ductility and to induce fracture anisotropy. By particularly accounting for the crystallographic texture, the finite element simulations are able to capture the fracture anisotropy observed in the experimental tests.

Stress‐Driven Failure of Cylindrical Volcanic Conduits

AbstractMany explosive volcanic eruption sequences are characterized by exceedingly unsteady behavior, often punctuated by multiple explosive phases, separated by hours to days. What causes this unsteady behavior is not fully understood. Here, we explore one possible mechanism: stress‐driven failure of the volcanic conduit wall rock. We present a multiphase conduit flow model that predicts the shear and normal tractions exerted on the conduit wall, and couple (one way) these tractions with a numerical model of host rock failure. While analytical estimations of host rock stress serve as useful guides, analytical approximations which ignore shear tractions are insufficient to adequately predict conduit instability. Failure generally initiates near the fragmentation depth, where shear tractions are highest and magma pressure is low relative to the lithostatic stress. Shear tractions on the conduit wall, caused by viscous drag, promote strain localization and failure. The in situ stress state not only determines the range of conduit pressures which promotes failure, but also influences the style of failure. The mechanism that is most likely to lead to the disruption of flow, where wall rock moves radially into the conduit along normal faults, occurs in extensional stress regimes when large shear tractions at the fragmentation depth enhance normal fault formation. The results of this study indicate that conduit stability strongly depends on the ambient stress field and the nature of conduit flow at the fragmentation depth.

Anomalous Subduction Initiation: Young Under Old Oceanic Lithosphere

AbstractWhen subduction initiates along a transform fault, it is often interpreted that the older and therefore denser oceanic lithosphere should evolve into the downgoing plate. However, in the western Pacific, several geological cases are probably characterized by young oceanic plates underthrusting older and denser plates. Using numerical simulations of a transform fault under compression, we propose that subduction polarity depends mainly on the age offset between the two adjacent plates, the strength of the transform fault, and the plasticity of the plates. A small age offset across the plate interface favors the subduction initiation of the younger plate. Furthermore, weak transform faults promote strain localization along the plate boundary and trigger the underthrusting of the older and denser plate. Plastic deformation is prone to localize within the thinner and weaker plates in a location far from the transition zone of the two plates if the rate of strain weakening of plates is fast enough. When plates have a large age offset the younger plate may still underthrust if plasticity plays a role. Our numerical models explore factors controlling the induced subduction polarities and the potential mechanism of young plate subduction and explain certain related subduction geological settings.

Effects of crystallography on hot-spot formation in porous RDX single crystals

The thermomechanical behavior of porous RDX single crystals is studied under dynamic loading using an anisotropic dislocation-based crystal plasticity model that accounts for deformation-induced heating. A micromechanics-based framework is proposed to account for micro-inertially confined dynamic collapse of pores. A suite of finite element calculations are carried out to systematically elucidate the role of crystallographic orientation on hot-spot formation. Under particular loading orientations, the porous RDX crystals are found to be quite sensitive to deviatoric and volumetric strain localizations, even for loading situations that do not particularly promote strain localization in isotropic materials. Due to the anisotropy-induced strain localizations, fairly severe hot-spots are predicted even under moderate applied strains. The susceptibility of porous RDX to strain localize and form severe hot-spots under particular loading orientations has important implications for accidental detonation of these energetic materials.

Evidence for a Deep Hydrologic Cycle on Oceanic Transform Faults

AbstractOceanic transform faults represent abundant yet relatively unexplored components of the hydrologic cycle in the mantle lithosphere. Current models limit fluid circulation to 600 °C, the thermal limit of earthquakes recorded by teleseismic surveys. However, recent ocean‐bottom seismic surveys have located earthquakes at depths corresponding to >1000 °C in modeled thermal structure. To constrain the depth extent of brittle deformation and fluid infiltration, we analyzed peridotite mylonites dredged from the Shaka Transform Fault, Southwest Indian Ridge. Samples range from high strain mylonites that preserve ductile microstructures to lower strain mylonites that are fractured and overprinted by hydrothermal alteration. Microstructural analysis of the high strain samples reveals brittle deformation of pyroxene concomitant with ductile deformation of olivine and growth of amphibole. Porphyroclasts preserve healed fractures filled with fluid inclusions, implying repeated episodes of fracture, fluid infiltration, and healing. The association of hydration features with brittle structures points to seawater, rather than melt, as the fluid source. Textural analysis indicates that strain localization was initiated by grain boundary pinning and that olivine grain size was reduced to ~1 μm in the presence of amphibole. Comparing the amphibole stability field to thermometry estimates for the limit of recrystallization suggests that fluid flow extended to ~650–850 °C. Our results indicate that the hydrologic cycle extends past the brittle‐ductile transition and promotes strain localization via hydrolytic weakening and hydration reactions. We propose that seawater infiltration on oceanic transform faults is driven by the seismic cycle and represents a first order control on the rheology of the oceanic lithosphere.

Removing mantle lithosphere under orogens: delamination versus convective thinning

SUMMARY Thinning of the lithosphere under continental collisional orogens is often attributed to delamination or convective thinning. Both processes remove part or all of the mantle lithosphere that has become denser and gravitationally unstable. Previous studies mostly focused on the different thermomagmatic consequences of these two processes; the dynamic links between them, and the critical conditions for one or the other process to dominate lithosphere thinning, remain uncertain. Here, we used high-resolution thermomechanical models with various rheology (linear viscous, power-law viscous and/or the extended Drucker–Prager plasticity) to systematically investigate the dynamics of delamination and convective thinning under collisional orogens. Our results show that convective thinning is favoured in models of linear (Newtonian) viscous rheology and low viscosity $({10^{19}}\!-\! {10^{20}}\,\,{\rm{Pa}} \, {\rm{s}})$. Power-law viscous rheology promotes strain localization, which reduces the effective viscosity and may lead to localized rising of the asthenosphere to the crustal base, thus triggering delamination. Further strain localization and stronger delamination are predicted with inclusion of plastic rheology in the model. These results indicate that convective thinning and delamination are dynamically linked and can occur in the same orogeny. Their relative dominance during orogenesis may be distinguished by the resulting spatiotemporal evolutions of thermal perturbation, magmatism and elevation changes. We applied the models to show that the evolution of the Central Anatolian Plateau is consistent with the dominance of convective thinning, whereas delamination played a major role in thinning the mantle lithosphere under central-northern Tibetan Plateau.

Strain localization by shear heating and the development of lithospheric shear zones

Analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization. Such domains commonly occur in the lithosphere through variations in composition or mineral fabric. Here we use a 2D plane-stress, non-Newtonian, viscous model to explore how strain localization develops from an initial isolated weak inclusion. We use a temperature-dependent rheological law for which the material weakens as a result of work done by shear converted to heat. The progress of strain localization follows a power-law growth that is strongly non-linear and may be regarded as an instability. Although this localization mechanism is ultimately limited by thermal diffusion, this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere. Shear zones in the lower crust are typically depicted as the downward continuation of faults. We argue that the depth-extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2D numerical experiments. When applied to the rheological laws for common lithospheric minerals, the combination of temperature and stress-dependence provides a direct means of predicting the depth below which the localization instability does not occur. For an olivine based rheology, the maximum depth at which rapid localization is expected is in the range of ~20 to 60 km, depending on heat flow, strain-rate and water fugacity. We apply our calculations to two major continental strike-slip zones, the San Andreas Fault and North Anatolian Fault, and compare our predicted maximum localization depths with published seismological images. Strain localization in the lower crust requires a dry rheology comparable to plagioclase. Observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine.

Cyclic plastic deformation behaviour of a directionally solidified nickel base superalloy at 850 °C: Damage micromechanisms

Dislocation based deformation micromechanisms during low cycle fatigue deformation of nickel base superalloy CM 247 DS LC at 850 °C was investigated by conducting fatigue tests employing constant strain amplitudes for strain ratio (R) values of 0, −1 and carrying out extensive SEM and TEM examinations. Cyclic life of the alloy reduces for all fatigue tests conducted employing R = 0 in comparison with R = −1 owing to sustained mean stress developed during fatigue at R = 0. TEM examinations confirmed that sustained mean stress developed during low strain amplitude fatigue test (Δε/2 = 0.5%) using R = 0 condition prevented slip transfer from γ-channels to γ′-precipitates and resulted in the formation of dislocation substructures such as networks, nodes etc. and also promoted dislocation looping around γ′-precipitates. Lower fatigue life at R = 0 is mainly attributed to the development of these types of substructures, which promotes strain localization in both intra as well intergranular regions. Whereas, in specimen fatigue tested (Δε/2 = 0.5%) using R = −1 condition, shearing of γ′-precipitates by APB coupled dislocation and formation of stacking faults were observed. The formation and nature of stacking faults were analysed using weak beam imaging technique. Stacking fault formed during fatigue tests using R = −1 condition matured to micro-twins when Δε/2 value was increased to 0.8%. The mechanism of formation of these microtwins is discussed in detail. SEM based microstructural and fractographic examinations revealed that mean stress induced creep effect resulted in intergranular crack initiation and grain boundary cavitation during fatigue tests under R = 0 condition and therefore verified the facts revealed in TEM studies.

ULCF assessment of X52 piping steel by means of cyclic bending tests

Pipelines and piping components may experience large cyclic deformations during a reduced number of cycles (Ni=1–100cycles), when subjected to extreme cyclic loading events (e.g. hurricanes, support settlements, earthquakes). In accordance with these loading scenarios, a lateral movement can be applied to the pipeline inducing bending stresses that gradually promotes strain localization, due to progressive plastic instabilities (buckling), damage evolution and final failure. This work aims at characterizing ultra-low cycle fatigue (ULCF) behaviour of the X52 piping steel under bending and local buckling state. An experimental program was carried out to derive ULCF data for smooth, notched and flat-grooved specimen geometries under cyclic bending. Furthermore, the small-scale tests were simulated in ABAQUS® with the objective of computing the parameters governing the fatigue damage models. The classical Coffin-Manson strain-life relation commonly used in the low cycle fatigue (LCF) regime is proposed to model the fatigue lives. In addition, the Xue model, particularly dependent of the monotonic fracture strain was also used for the prediction of the number of cycles until the crack initiation. The numerical data obtained with these models are compared, being achieved similar fatigue lives predictions for notched plane specimens. For the case of flat-grooved specimens which provides plain strain conditions, an overestimation from the Coffin-Manson relation was observed while the Xue model reproduces very good results for both specimens' series.

Measurement of Strain Localization Resulting from Monotonic and Cyclic Loading at 650 ∘C in Nickel Base Superalloys

A methodology is presented for the use of the oxide scale that develops in polycrystalline Ni-base superalloys at service temperature, as a speckle pattern for μm-scale resolution strain measurements. Quantitative assessment of the heterogeneous strain field at the grain scale is performed by high-resolution SEM digital image correlation under monotonic and cyclic loading in polycrystalline Ni-base superalloys up to 650 ∘C. In the Rene 88DT superalloy, strain localization is observed near twin boundaries during low cycle fatigue (LCF) at intermediate temperatures, correlating with activation of {111} 〈110〉 and {111} 〈112〉 slip systems. A strong correlation between the microstructural configuration that promotes strain localization during monotonic loading and crack initiation at 650 ∘C in low cycle fatigue was observed.

Rheological weakening of high-grade mylonites during low-temperature retrogression: The exhumed continental Ailao Shan-Red River fault zone, SE Asia

We present a detailed case study of an exhumed continental strike-slip fault zone, the Ailao Shan-Red River (ASRR) strike-slip fault zone, to investigate how deformation promotes strain localization, and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation. Along the ASRR strike-slip fault zone, in the Diancang Shan (DCS) metamorphic massif, high-temperature ductile deformation (D1) pervasively occurred during shearing and exhumation since late Oligocene. The high-temperature microstructures and textures are in part or entirely altered by subsequent low-temperature shearing (D2) since late Miocene, which is under the conditions of frictional-viscous transition of K-feldspar (ca. 450°C) during further exhumation to the upper crustal levels. The formation of D2 microstructures and shear bands overprinted high-temperature intracrystalline plasticity phases (D1) in mylonitic rocks. Depending on the main rock-forming minerals, the results also demonstrate that the brittle–ductile transition involves a combination of different deformation mechanisms and possible rheological paths. In quartz-rich rocks, quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature, which lasted until retrogression related to exhumation. As a result, grain-size reduction associated with fluids circulating within the ASRR strike-slip fault zone at brittle–ductile transition leads to reaction and texture weakening. Rheological weakening is the consequence of the syntectonic deformation, fluid flow, reaction softening, reaction creep and textural softening. The hydrous fluids resulted in hydration of silicates. Decompression occurred during shearing and as a result of tectonic exhumation. All these results demonstrate that the exhumation through the ductile to ductile-brittle transition involves a combination of different deformation mechanisms, rheological transition features and feedbacks between deformation, decreasing temperature and fluids in the ASRR fault zone.

The effects of quartz recrystallization and reaction on weak phase interconnection, strain localization and evolution of microstructure

We conducted axial compression and general shear experiments, at T = 900 °C and P = 1.5 GPa, on samples of banded iron formation (BIF) and synthetic aggregates of quartz, hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization, and the role of weak second phases (oxides) on the rheology and microstructural evolution of the aggregates. Experiments showed strain localization into oxide rich layers, and that the oxide content and oxide distribution are key factors for the strength of the aggregate. Only 2–10 wt.% hematite leads to pronounced weakening and increasing hematite content above ∼10% has only a minor additional effect. Where oxide grains are dispersed, the initial strength contrast with quartz induces stress concentrations at their tips, promoting high stress recrystallization-accommodated dislocation creep of quartz. Fine recrystallized quartz reacts with oxide, forming trails of fine reaction product (ferrosilite/fayalite) leading to the interconnection/percolation of a weaker matrix. The strength contrast between the quartz framework and these fine-grained trails promotes strain localization into micro-shear zones, inducing drastic strain weakening. Thus dynamic recrystallization of quartz promotes syn-deformational reactions leading to a microstructurally-controlled evolution of phase strength contrast. It results in a rheologic transition from load-bearing framework to a matrix-controlled rheology, with transition from S–C′ to S–C fabric with increasing strain.

Investigation and modeling of the anomalous yield point phenomenon in pure tantalum

The monotonic and cyclic behavior of commercially pure tantalum has been investigated at room temperature, in order to capture and understand the occurrence of the anomalous yield point phenomenon. Interrupted tests have been performed, with strain reversals (tensile or compressive loading) after an aging period. The stress drop is attributed to the interactions between dislocations and solute atoms (oxygen) and its macroscopic occurrence is not systematically observed. InfraRed Thermography (IRT) measurements supported by Scanning Electron Microscopy (SEM) pictures of the polished gauge length of a specimen during an interrupted tensile test reveal the nucleation and propagation of a strain localization band.The KEMC (Kubin–Estrin–McCormick) phenomenological model accounting for strain aging has been identified for several loadings and strain rates at room temperature. Simulations on full specimen using the KEMC model do not show strain localization, because of the competition between viscosity and strain localization. However, a slight misalignment of the sample can promote strain localization.

Influence of water on rheology and strain localization in the lower continental crust

We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnøya, Norway. Over a distance of ∼1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice‐preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (∼1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn‐deformational, water‐consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

Melt and shear interactions in the lithosphere: Theory and numerical analysis of pure shear extension

AbstractWe present a linear instability analysis and numerical simulations describing deformation and melt patterns in pure shear extension of a partly molten rock. Our models implement numerical techniques that enable strong strain localization and are applied to study melt‐strain interactions during continental rifting. Our results show that instabilities can initiate with either strain localization or melt localization, followed by a coupled evolution of melt and shear bands driven by a strong melt‐viscosity‐shear feedback. This indicates that a local increase in melt fraction due to segregation and/or local melting promotes strain localization and may lead to the formation of large shear bands. Melt‐shear interactions can therefore enable rifting where tectonic forces are not sufficient to induce melt‐free rifting, resulting in lubricated faults, but not necessarily observed volcanism. Finally, our simulations reveal significant asymmetry in melt segregation around localized shear bands, providing new insights into melt distribution across rift boundary faults and other extensional structures.

Effective elastic thickness and mechanical anisotropy of South China and surrounding regions

South China and surrounding regions extend from the eastern Tibetan plateau, through the tectonically stable Sichuan basin and the broad Mesozoic magmatic and fold belt, to the trench-arc-basin system in the western Pacific which provide an ideal place to study deformation of the continental lithosphere under long-term magmatism and oceanic subduction. We obtained the effective elastic thickness (Te) and its anisotropy of South China and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods. The Te values of the study area vary from 2 to 75km, with relatively low Te values (≤30km) along the tectonic boundaries, the North–South Gravity Lineament (NSGL) and seismic zones, and in the regions with high surface heat flow. The evenly low Te values in the Lower Yangtze region and the Cathaysia block can be attributed to the long-lived subduction of the Paleo-Tethys and the Paleo-Pacific ocean basins beneath the South China block (SCB). The NSGL in the SCB may separate the unmodified (high Te) and thermally weakened (low Te) continental lithosphere due to oceanic subduction. Despite different distances to the tectonic boundaries, earthquakes occur more frequently in regions with Te values of 10–30km, implying strain concentration in the low-Te regions. A positive correlation between seismic activity and the magnitude of Te anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere. The poor correlation between the weak axis of Te anisotropy and the dynamic indicators of the present tectonic regime (the shear-wave splitting direction, the maximum horizontal compressive stress direction) confirms that Te anisotropy mainly reflects tectonic inheritance of the continental lithosphere.

Characterization of Melting and Solidification Phenomena in Electromagnetic Punching of Aluminum Tubes

Electromagnetic punching of tubular products is considered to be a promising innovative perforating process. The required punching energy decreases when using high velocities. Also, less tools are required when compared to conventional mechanical punching. However, the increase in punching speed can involve new strain and fracture mechanisms which are characteristic of the dynamic loading. In high energy rate forming processes the effect of temperature versus time gradient on the material properties becomes important due to the heat accumulated from plastic deformation and friction. The deformation induced heating will promote strain localization in it, possibly degrade its formability and cause premature failure in the regions of high localized strain. The feasibility of the electromagnetic pulse forming process for punching holes in aluminum cylindrical specimens has been investigated on an experimental trial-and-error basis. Experiments were performed using a Pulsar system (model 50/25) with a maximum charging energy of 50 kJ and a discharge circuit frequency of 14 kHz. Microscopic and metallographic inspection of the punched workpieces, together with hardness measurements, was performed to critically evaluate the quality of the cuts. It was observed that damage occurred at part of the edge of the punched hole during some of the perforation experiments. It was evidenced that in most workpieces, especially those performed at higher charging energy levels, a considerably high temperature must have been reached in the regions near the punched hole. The aluminum in this region was assumed to have melted and resolidified. These assumptions were affirmed by the following observations. Microscopic-size precipitates present in the unaffected base metal microstructure, had completely dissolved in that region; shrinkage cavities and dendrite rich regions were clearly visible. Next to this region, a heat affected zone was present where the grain boundaries had partially melted and precipitates partially disappeared. Considerably high temperatures, in the order of 520 to 660 °C, were reached in the regions around the punched holes, leading to melting and resolidification of the material. The total width of the thermally affected regions appeared to be larger at higher energy levels. The combination of heat generated by ohmic heating and by plastic deformation in a very short time interval is the most probable cause of the high peak temperatures that have occurred during the electromagnetic punching process.

Microstructural manifestation of dynamic strain aging in AISI 316 stainless steel

Dynamic strain aging (DSA) affects a material’s mechanical behaviour. In the current study, the deformation microstructures of AISI 316 stainless steel specimens were examined after tensile testing at several temperatures both in and out of the DSA-regime (200, 288 and 400 °C), and the effect of straining on free nitrogen evolution at those temperatures was studied by Anelastic Mechanical Loss Spectrometry (Internal Friction). Analysis of the nitrogen-induced Snoek-like peak after straining indicated that DSA behaviour is associated with the formation of sub-micron scale, nitrogen-enriched zones in the austenitic matrix. Long-range planarity was observed in the dislocation structures at 400 °C and short-range planarity at 288 °C, but at 200 °C the microstructure exhibited cellular dislocation structure. Diffusion re-distribution of nitrogen in the DSA-regime affected the deformation behaviour of the material by restricting cross-slip, which in turn promotes strain localization, degrading the mechanical performance of the material.

A Study of the Fundamental Relationships between Deformation-Induced Surface Roughness and Strain Localization in AA5754

Three-dimensional, matrix-based statistical analysis methods were developed and integrated with high-resolution topographical imaging, to assess how microstructural changes influence the evolution of plastic deformation and strain localization in a commercial AA5754-O aluminum sheet in three in-plane strain modes. Analysis of the raw surface data revealed that the general composition of the surface roughness was highly sensitive to strain mode and strain level. The microstructural conditions that promote strain localization were assessed by extending a profile-based surface roughness parameter (Rt) to matrix form. Both analyses revealed that different strain modes produce characteristic dissimilarities in the deformation at the grain level. The localization data can be well characterized with a two-parameter Weibull distribution, suggesting that strain localization is a stochastic process that can be modeled reliably with Weibull statistics. This study clearly demonstrates that an accurate and straightforward probabilistic expression that captures the microstructural subtleties produced by plastic deformation can be developed from rigorous analyses of raw topographic data. Because variations in surface morphology profoundly influence the reliability of the numerical models used to predict strain localization, incorporating expressions of this type could greatly enhance the accuracy of these models.