Pancake Grain Research Articles

Experimental and digital twinning in ZnAlMg coatings

Twinning is a major deformation mechanism in various materials, especially when few dislocation slip systems are operative. It is the case of zinc-rich coatings in galvanised steel sheets, made of pancake grains on a substrate and where the slip systems with a non-vanishing component along the c-axis present high critical resolved shear stress values. In addition, the abrupt lattice orientation change associated to twinning, the stress relaxation during its propagation and the localised nature of its early stages make it difficult to reproduce this deformation mechanism by using classical crystal plasticity models conceived for dislocation slip. In this sense, this work proposes a hierarchy of three twinning models in combination with a dislocation slip crystal plasticity model, for the case of a ZnAlMg coating. These three models are: a relaxed-Taylor model applied to individual crystal orientations of the coating, a “pseudo-slip” model for twinning and a localised twinning model. The latter incorporates a linear softening in the material law accounting for the unstable twinning initiation and enforces twinning lattice reorientation. A microstructure portion extracted from an in-situ SEM tensile experiment on galvanised steel is used to perform 3D full-field finite element simulations within a finite strain formulation. SEM observations and EBSD acquisitions are used to compare simulation and experimental results during the different steps of the in-situ SEM test, regarding the deformation and damage modes of the zinc-rich coating. The focus is set on twinning evolution inside some individual grains, and the pros and the cons of the three models are finally discussed.

Effects of hot rolling and coiling temperatures on microstructure, texture and magnetic properties of 1.6 wt% Si non-oriented silicon steel

Bigger grains prior to cold rolling are favorable to the magnetic properties of non-oriented electrical steels. However, it is hard to coarsen grains in hot rolled sheets of low silicon steels due to γ→α transformation. In the work, the influences of different hot rolling temperatures (820 °C, 840 °C and 875 °C) and coiling temperatures (680 °C and 750 °C) on the microstructural and textural evolution of 1.6 wt% Si steel were comparatively investigated in detail. The mechanism of grain growth during coiling was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. The main finding was that pancake grains with an aspect ratio of 1.5–3.5 formed after hot rolling at 840 °C and further significantly grew after coiling at higher temperature 750 °C through a strain induced boundary migration mechanism. However, the significant grain growth did not occur after coiling at low temperature of 680 °C. By contrast, elongated deformed grains and equiaxed grains were produced after finish rolling at 820 °C and 875 °C, respectively, in which pronounced grain growth was not observed even after coiling at 750 °C. The significant grain growth promoted the formation of shear bands during cold rolling and led to weakened γ-fiber (<111 >//ND) texture, enhanced λ-fiber (<100 >//ND) and big grains in the annealed sheet. As a result, both magnetic induction and iron loss were significantly improved. This work provides a promising method to coarsen grains before cold rolling by design hot rolling process for low silicon non-oriented silicon steels.

Evolution of microstructure and texture in grain-oriented 6.5%Si steel processed by strip-casting

Grain-oriented 6.5%Si steel was produced by strip-casting, followed hot rolling, cold rolling, primary annealing and secondary annealing. Microstructural and textural evolution was studied with particular focus on the effect of solidified microstructure on the ultimate microstructure and texture. The study indicated that solidified microstructure of the as-cast strip was determined by melt superheat. The relatively low melt superheat resulted in fine equiaxed grains with random texture, whereas large melt superheat led to coarse grains with moderate λ-fiber texture. After cold rolling, the fine-grained steel showed narrow pancake grain structure in comparison to the coarse-grained steel, which was similar to the microstructure of the initial as-cast strip. After primary annealing, fine equiaxed microstructure with favorable {111}⟨112⟩ texture was obtained in the fine-grained steel. While inhomogeneous microstructure with strong α-fiber and moderate γ-fiber texture was obtained in the coarse-grained steel. The desirable microstructure and texture in the fine-grained steel led to abnormal grain growth with B8=1.65T after secondary annealing. By comparison, normal grain growth occurred in the coarse-grained sample with B8=1.37T. Another observation was high frequency core losses of grain-oriented 6.5%Si steel sheet with adequate secondary grains was significantly lower than non-oriented 6.5%Si steel and grain-oriented 3.0%Si steel. The study suggests that the strip-casting is a suitable route to fabricate grain-oriented 6.5%Si steel, and the initial fine grain size with random texture is preferred to obtain adequate abnormal grain growth.

Compact forced simple-shear sample for studying shear localization in materials

A new specimen geometry, the compact forced-simple-shear specimen (CFSS), has been developed as a means to achieve simple shear testing of materials over a range of temperatures and strain rates. The stress and strain state in the gage section is designed to produce essentially “pure” simple shear, mode II in-plane shear, in a compact-sample geometry. The 2-D plane of shear can be directly aligned along specified directional aspects of a material's microstructure of interest; i.e., systematic shear loading parallel, at 45°, and orthogonal to anisotropic microstructural features in a material such as the pancake-shaped grains typical in many rolled structural metals, or to specified directions in fiber-reinforced composites. The shear-stress shear-strain response and the damage evolution parallel and orthogonal to the pancake grain morphology in 7039-Al are shown to vary significantly as a function of orientation to the microstructure.

The role of crack branching in stress corrosion cracking of aluminium alloys

Abstract Stress corrosion cracks of all types are characterised by extensive crack branching, and this is frequently used as the key failure analysis characteristic to identify this type of cracking. For aluminium alloys, stress corrosion cracking (SCC) is almost exclusively an intergranular failure mechanism. For plate and extruded components, this had led to the development of test procedures using double cantilever beam and compact tension precracked specimens that rely on the pancake grain shape to constrain cracking, so that fracture mechanics can be applied to the analysis of stress intensity and crack velocity and the evolution of a characteristic performance curve. We have used X-ray computed tomography to examine in detail SCC in aluminium alloys in three dimensions for the first time. We have found that crack branching limits the stress intensity at the crack tip as the applied stress is shared amongst a number of cracks that are held together by uncracked ligaments. We propose that the plateau region observed in the v-K curve is an artefact due to crack branching, and at the crack tips of the many crack branches, cracking essentially occurs at constant K almost irrespective of the crack length. We have amplified the crack branching effect by examining a sample where the long axis of the pancake grains was inclined to the applied stressing direction. Our results have profound implications for the future use of precracked specimens for SCC susceptibility testing and the interpretation of results from these tests.

Effect of the grain shape on the q-value evolution of steel sheets

Two polycrystalline plasticity models, namely the ALAMEL model and the viscoplastic self-consistent (VPSC) model were used to study the effect of the grain shape on the evolution of plastic anisotropy. The latter was characterized by the q-value in this paper. The two models both predict the evolution of grain shape and texture and then can predict the effect of the initial grain shape on the texture. The final anisotropy of the materials was also assessed with the help of these models. Data of tensile tests in three directions for three low carbon steel sheets with either equiaxed or a pancake grain shape were used to evaluate the model predictions. Both models give the same trend of the grain shape effect on the evolution of the q-values. Results demonstrate that the effect of the grain shape can be clearly reflected in the x-value distribution as defined and discussed in this paper. But it is not so obvious in the predicted textures for the investigated three materials according to both models.

Friction stir weld of 2060 Al–Cu–Li alloy: Microstructure and mechanical properties

This paper investigated the microstructural and mechanical properties of 2060 Al–Cu–Li alloy and its sound friction stir joints. The base materials present large pan-cake grains whereas the nugget zone of friction stir weld exhibits fine grain structure. Dispersions are aligned along rolling direction in the base material. They were re-distributed and uniformly decorated within the weld nugget. The base material is mainly strengthened by T1 precipitate with small amounts of θ′ and S′. The various strengthening precipitates were dissolved in the nugget zone where dense dislocations and nano-sized co-clusters were found. Additionally, the hardness, tensile strength and fatigue life limit of the joint are measured.

Analysis of deformation behavior and workability of advanced 9Cr–Nb–V ferritic heat resistant steels

Hot compression tests were carried out on 9Cr–Nb–V heat resistant steels in the temperature range of 600–1200°C and the strain rate range of 10−2–100s−1 to study their deformation characteristics. The full recrystallization temperature and the carbon-free bainite phase transformation temperature were determined by the slope-change points in the curve of mean flow stress versus the inverse of temperature. The parameters of the constitutive equation for the experimental steels were calculated, including the stress exponent and the activation energy. The lower carbon content in steel would increase the fraction of precipitates by increasing the volume of dynamic strain-induced (DSIT) ferrite during deformation. The ln(εc) versus ln(Z) and the ln(σc) versus ln(Z) plots for both steels have similar trends. The efficiency of power dissipation maps with instability maps merged together show excellent workability from the strain of 0.05 to 0.6. The microstructure of the experimental steels were fully recrystallized upon deformation at low Z value owing to the dynamic recrystallization (DRX), and exhibited a necklace structure under the condition of 1050°C/0.1s−1 due to the suppression of the secondary flow of DRX. However, there were barely any DRX grains but elongated pancake grains under the condition of 1000°C/1s−1 because of the suppression of the metadynamic recrystallization (MDRX).

Polycrystal plasticity models based on crystallographic and morphologic texture: Evaluation of predictions of plastic anisotropy and deformation texture

Two micro–macro scale transition models, namely the ALAMEL and visco-plastic self-consistent (VPSC) models (both predict the evolution of grain shape and texture), were used to study the effect of the initial grain shape. The evolution of plastic anisotropy was characterized by the q-value and was simulated in three directions for three low carbon steels assuming either equiaxed (hypothetical one) or a pancake (measured one) grain shape. In addition, texture evolution during rolling was investigated for one of the materials. Comparisons between the model predictions and the measured values demonstrate that the grain shape effect can be predicted well by both models. In most cases, the initial q-value can be nicely estimated by both models. VPSC is better than ALAMEL on q-value evolution prediction for simulations which consider the measured aspect ratios. For texture evolution of rolling, the effect of initial grain shape is only significant at large deformation.

Effects of Annealing Condition on Texture and Deep Drawing Properties of Cold-Rolling Low Carbon Sheet Steel Based on CSP

With the help of electron backscatter diffraction (EBSD ) and orientation distribution function (ODF) analysis, experiments of the different batch annealing were conducted to give an study of recrystallization and texture of Cold-rolling Low Carbon sheet steel based on CSP , and the relationship between initial isothermal temperature and drawing properties has been investigated. The recrystallization soon finished the grains during annealing produce relatively strong {111} and {111} texture，at this temperature region as the initial isothermal temperature, the double isothermal cover annealing process design, enabling access to a strong favorable texture and uniform pancake grains, and the proper temperature is the key control factor level.

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Evaluation of Annealing Texture in IF and EDD Steel Sheets

The textural and microstructural studies of cold-rolled-batch annealed (CRBA) interstitial free-Ti (IF-Ti) stabilized steel and extra deep drawing steel (EDD) produced at Tata Steel have been investigated. In the IF steels, α-fiber has been found to be strong even after batch annealing, which is not desirable for deep drawing applications. The presence of a stronger recrystallization texture in the surface region rather than in the central region accounts for the deformation and orientation path history of cold-rolling texture components and the texture inherited from hot band. The hot band of IF steel has been observed to have mixture of grain structure along with inhomogeneous texture throughout the thickness. While the inhomogeneity in EDD steel, pancake grain structure in the through-thickness direction, milder pancake grains in the transverse section, and polygonal grain structure in the longitudinal section have led to the inference that the undesirable α -fiber is present. This observation has been explained by the early precipitation of AlN during coiling. The results of all the steels have been explained with the help of the orientation distribution functions, volume fraction of alpha and gamma fiber along with their components, volume fraction ratio (γ /α), and the microstructure. The analysis of the present data suggests that the deep drawability of both steels can be improved by modifying the hot-rolling and annealing parameters.

Refinement of grain boundary carbides in a Si–Cr spring steel by thermomechanical treatment

The microstructure and mechanical properties of conventionally heat treated or thermomechanically treated spring steel Fe–0.56C–1.50Si–0.67Cr–0.59Mn (mass%) are presented after tempering. At tempering temperatures between 623 and 673 K continuous cementite films form at the prior austenite grain boundaries when the austenite is quenched without any preceding deformation. Two austenite modifications, a recrystallized variant and an elongated pan-cake grain structure were produced by deformation prior to quenching. The first variant resulted in refined grain boundary carbides after tempering, while for the latter no grain boundary carbides were observed. Both modifications of austenite produced by thermomechanical treatment improve the ductility significantly.

Fabrication of Nano Grain Tungsten Compact by Mechanical Milling Process and Its High Temperature Properties

Mechanical milling (MM), which is one of SPD process, is applied to W powder and W-Remixed powder. MM processed W and W-Re powders easily form nano grain structure even though they have high melting temperature. The nano grain formation mechanism in these powders is as follows: multi axial deformation of the powders by milling, at the temperature of 333K at most, produces pan-cake grain structure at first. Extremely dense dislocations result in grain sub-division, and finally nano grain structure with high angle boundary forms. Nano grains with approximately 10 nm in diameter are obtained. The MM powders are sintered using Spark Plasma Sintering process. Sintering and high temperature deformation behaviors of the MM powders are also investigated. The MM treatment enables W powder to be able to sinter at 1273 K while the powder without MM could never sintered at the same temperature. Re addition prevents grain growth during sintering and thus increases hardness of the compacts. A large deformation of W-10mass%Re sintered compact, whose grain size is approximately 450nm, is observed at elevated temperatures.

Effect of Annealing Conditions on the Texture and Normal Anisotropy of Al‐Killed Steels in Simulated Batch Annealing

Three commercial cold rolled Al‐killed steels were investigated to study the effect of annealing conditions on the crystallographic texture and the resulting plastic anisotropy. The heating rates, maximum temperatures and isothermal holding times were varied in 18 laboratory scale thermal cycles applied to all steels. The annealed and skin‐passed samples were tensile tested to determine mechanical properties and anisotropy parameters. Grain structures and average grain sizes were examined by an optical microscope and the textures were measured by X‐ray diffraction.It was found that the chemical composition of the steel had a significant effect on the texture and normal anisotropy obtained at certain heating rates. The differences between the current steels were discussed in the light of the competition between the kinetics of AIN precipitation and recrystallization. Excess heating rates led to a low normal anisotropy, which was not improved by extending the isothermal time of annealing. Pancake grain structure and the grain size larger than 20 mm were found to be indicators of the properly chosen heating rate and resulting in high normal anisotropy. At the proper heating rate high normal anisotropy was attained at low annealing temperatures and in short isothermal times, but a longer isothermal annealing was required to obtain a low yield strength.

Local structure in CoFe/Al 2O 3 multilayers determined by nuclear magnetic resonance

We studied NMR spectra at 1.5 K in a series of [Co 80Fe 20( t)/Al 2O 3(3 nm)] 10 multilayers. The spectra are interpreted by mixed contributions from Co atoms inside CoFe granules and at their interface with Al 2O 3. They display three characteristic shapes when t varies from 0.8 to 2 nm. These shapes have been interpreted as resulting from granular CoFe layers for the thinnest ones ( t<1.3 nm), and continuous CoFe layers for the thickest ones ( t>1.6 nm), with an intermediate regime—pancake grains of different sizes—in between. These structural data correlate with three magnetic states deduced from the FMR and SQUID measurements in this system.

Plastic anisotropy of textured ODS nickel-base alloy PM 1000

Strongly textured ODS nickel-base alloy PM 1000 with a pancake grain structure has been deformed in axisymmetric and plane strain compression at 1000°C and a true strain rate of 10 −4 s −1. The texture has been measured by neutron diffraction. Based on the texture, Taylor factors have been calculated for the different deformation modes. The linear dependence of the strength on the Taylor factor indicates that the plastic anisotropy observed is due to the texture and not to the anisotropy of the grain structure.

Pancake Grain Structure and Recrystallization Texture Formation in Ultra-Low Carbon Al-Killed Sheet Steels

Pancake Grain Structure and Recrystallization Texture Formation in Ultra-Low Carbon Al-Killed Sheet Steels

High-temperature creep of the particlehardened commercial Al-Li-Cu-Mg alloy 8090

Creep of the particle-hardened commercial Al-Li 8090 alloy has been studied at temperatures of 425 and 445 K. The measured stress sensitivity of the minimum creep rates changes abruptly at a given applied stress with stress exponents being around 4–6 at low stresses and 30–40 at high stresses. Creep activation enthalpies were determined by both temperature cycling and by comparing creep rates at two temperatures at a given applied stress, the results from both gave the same unrealistically high values. The internal stresses, σi, developed during creep were determined using the strain-transient dip test. These increased linearly with the applied stress, σa, at low stresses and were effectively constant at high stresses. The minimum creep rate was found to be a simple function of the effective stress, σa-σi, with a stress exponent of between 5 and 6, at all applied stresses. The dislocation and precipitate structure of the alloy was examined before and after creep using thin-film electron microscopy. The initial structure consisted of pancake grains with a well-developed {1 1 0}〈1 1 2〉 type texture. The grains contained well-developed sub-cells and δ′ and S precipitates. The structure developed during creep consisted of dislocation pairs, single dislocations and dislocations loops. There was evidence to suggest that slip took place on both {1 0 0} and {1 1 1} planes. The dislocation loops were most likely to have been Orowan in character and around the rodlike S precipitate, with the coherent δ′ precipitate being sheared by pairs of dislocations. The measured internal stresses result from inhomogeneity of plastic deformation. These stresses increase continuously with applied stress up to the observed macroscopic yield stress, and then become constant. The internal stresses are likely to have arisen from the Orowan loops around S and the behaviour of sub-grain boundaries. The increases in internal stress may have resulted from an increased loop density with increasing applied stress. This rate of increase is likely to slow down if S particles are sheared or fractured at high applied stresses.

Acoustoelastic determination of the higher order orientation distribution function coefficients up tol = 12 and their use for the on-line prediction ofr-value

The higher order orientation distribution function (ODF) coefficients up tol = 12 in cold-rolled and annealed sheet steels were evaluated and calculatednondestructively from the anisotropy of the ultrasonic velocities of the lowest order symmetrical Lamb (S0) and shear horizontal (SH0) waves propagating in the rolling plane. The elastic energy method was employed, together with a decomposition of the texture into the principal preferred orientations, following a procedure originally developed for Young’s modulus data obtained destructively. Plastic strain ratios are estimated using the series expansion method, in conjunction with a relaxed constraint (pancake) grain interaction model. Ther- value predictions based on the ODF coefficients obtained in this study are compared with tensile data and with empirical predictions made using the Modul-r approach.