Mechanical behaviour and microstructural evolution in superplastic Al-Li-Mg-Cu-Zr AA8090
TL;DR: In this article, the effect of tensile deformation at 530°C at a constant strain rate of 5 × 10−4 −1 on the microstructure, texture and mechanical characteristics of the central layer of commercially processed superplastic Al-Li alloy (AA8090) sheet has been investigated.
Abstract: The effect of tensile deformation at 530 °C at a constant strain rate of 5 × 10−4 s−1 on the microstructure, texture and mechanical characteristics of the central layer of commercially processed superplastic Al–Li alloy (AA8090) sheet has been investigated. The strain rate sensitivity remained essentially constant during straining with m ∼ 0.53, despite a progressively changing microstructure. The initial sheet had a dominant cross-rolled “brass” texture centred on { 0 1 1 } 〈 1 1 2 〉 , which became weaker during deformation, but it was also possible to identify similar orientations which remained spatially aligned in the rolling direction for strains up to unity. The changes in microstructure and texture were not compatible with relative grain translation, i.e. grain boundary sliding. Modelling studies incorporating relative grain translation together with grain growth and orientation changes reinforced that conclusion, and lead to the view that rate sensitive slip is the primary deformation mechanism.
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TL;DR: In this paper, the development history and applications of Al-Li alloys over the last few years are reviewed, and the main issue of anisotropic behavior of all the alloys is discussed.
Abstract: Al-Li alloys are attractive for military and aerospace applications because their properties are superior to those of conventional Al alloys. Their exceptional properties are attributed to the addition of Li into the Al matrix, and the technical reasons for adding Li to the Al matrix are presented. The developmental history and applications of Al-Li alloys over the last few years are reviewed. The main issue of Al-Li alloys is anisotropic behavior, and the main reasons for the anisotropic tensile properties and practical methods to reduce it are also introduced. Additionally, the strengthening mechanisms and deformation behavior of Al-Li alloys are surveyed with reference to the composition, processing, and microstructure interactions. Additionally, the methods for improving the formability, strength, and fracture toughness of Al-Li alloys are investigated. These practical methods have significantly reduced the anisotropic tensile properties and improved the formability, strength, and fracture toughness of Al-Li alloys. However, additional endeavours are required to further enhance the crystallographic texture, control the anisotropic behavior, and improve the formability and damage tolerance of Al-Li alloys.
360 citations
TL;DR: In this article, surface observations were used to elucidate the deformation mechanisms responsible for the superplastic effect in Ti-6Al-4V. The authors used a scanning electron microscope to perform high-temperature in-situ tests for tensile and shear deformation modes.
Abstract: Surface observations are used to elucidate the deformation mechanisms responsible for the superplastic effect in Ti–6Al–4V. High-temperature in-situ tests for tensile and shear deformation modes are performed in the scanning electron microscope at temperatures in excess of 700∘ C. Grain boundary sliding is predominant; the micro-mechanics of accommodation are consistent with the dislocation-based Rachinger theory. The volume fraction of β plays a crucial role. For temperatures greater than 850 °C, the α grains remain unaffected; cavitation is minimal and slip bands needed for dislocation-based accommodation are detected in the β phase but are absent in α. At this temperature, grain neighbour switching is observed directly under shear deformation. At a temperature lower than 850∘ C, the β volume fraction is lower and a different mechanism is observed: slip bands in α and cavitation are found to accommodate grain boundary sliding. In addition, an increase in the α phase intragranular dislocation activity triggers the formation of subgrains and dynamic recrystallisation, consistent with the Rachinger dislocation creep effect. For temperatures lower than 700∘ C, superplasticity is absent; classical creep behaviour controlled by dislocation climb persists. A numerical treatment is presented which accounts for the Rachinger effect. The computational results are used to deconvolute the contributions of each of the competing mechanisms to the total strain accumulated.
162 citations
TL;DR: In this article, the plastic deformation of two classes of fine-grained aluminium alloys at elevated temperatures and slow strain rates have been investigated, and the results from surface grid measurement in the Al-Mg-Mn alloys give results which indicate that the superplasticity is primarily a result of diffusion creep.
Abstract: The plastic deformation of two classes of fine-grained aluminium alloys at elevated temperatures and slow strain rates have been investigated One class of material, Al–Cu–Zr, was processed to develop banded microstructures; the other class, based on Al–(Mg)–Mn, had near-equiaxed microstructures In both classes, superplastic behaviour was found in the variants with the higher solute content The evolution of the banded microstructures and the results from surface grid measurement in the Al–(Mg)–Mn alloys give results which indicate that the superplasticity is primarily a result of diffusion creep, and the effect of solute is proposed to be via an enhancement of solvent self-diffusion
105 citations
TL;DR: In this paper, the authors applied friction stir processing to extruded Al-Mg-Sc alloy to produce fine-grained microstructure with a grain size of 2.2 mu m.
Abstract: Friction stir processing (FSP) was applied to extruded Al-Mg-Sc alloy to produce fine-grained microstructure with a grain size of 2.2 mu m. Electron backscatter diffraction (EBSD) result showed that the grain boundary misorientation distribution was very close to a random grain assembly for randomly oriented cubes. Superplastic investigations in the temperature range of 425-500 degrees C and strain rate range of 1x10(-2)-1x1(0) s(-1) showed that a maximum elongation of 1500% was achieved at 475 degrees C and a high strain rate of 1x10(-1) s(-1). The FSP Al-Mg-Sc exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. The origin for enhanced superplastic deformation kinetics in the FSP alloy can be attributed to its high fraction of high angle grain boundaries (HAGBs). The analyses of the superplastic data and scanning electron microscopy (SEM) examinations on the surfaces of deformed specimens indicated that grain boundary sliding is the main superplastic deformation mechanism for the FSP Al-Mg-Sc alloy.
57 citations
15 Jun 2012-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the microstructural evolution of unrecrystallized (extruded) and recrystallised (friction stir processed, FSP) Al-Mg-Sc alloys during superplastic straining was investigated using electron backscatter diffraction (EBSD).
Abstract: The microstructural evolution of unrecrystallized (extruded) and recrystallized (friction stir processed, FSP)Al-Mg-Sc alloys during superplastic straining was investigated using electron backscatter diffraction (EBSD). The unrecrystallized structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. This evolution was divided into three stages based on true stress-strain curves and EBSD maps, i.e. subgrain rotation and coalescence in the early stage, dynamic recrystallization in the middle stage, and grain boundary sliding (CBS) and dynamic grain growth in the final stage. By comparison, the recrystallized grains in the FSP Al-Mg-Sc maintained a random distribution during the whole deformation process, however the grain size increased significantly with increasing strain, indicating that the main deformation mechanism was always GBS and dynamic grain growth. A deformation model was proposed to explain the microstructural evolution during superplastic deformation. The microstructure with the random boundary misorientations reaches a dynamic balance because the transformation between high-angle grain boundaries and low-angle grain boundaries is equivalent. (C) 2012 Elsevier B.V. All rights reserved.
54 citations
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TL;DR: In this article, the authors investigated the nucleation of recrystallization at large second phase particles in deformed aluminium alloy single crystals and found that particle stimulated nucleation occurs above a critical particle size which increases with decreasing strain.
Abstract: Techniques of X-ray and electron diffraction, and in situ annealing in a High Voltage Electron Microscope have been used to investigate the nucleation of recrystallization at large second phase particles in deformed aluminium alloy single crystals. It is found that particle stimulated nucleation occurs above a critical particle size which increases with decreasing strain. Recrystallization originates within a zone of high dislocation density and large lattice misorientation at the particles, and proceeds by a rapid polygonization process, resulting in grains with orientations closely related to the as deformed structure. The deformation structure, mechanisms of nucleation and the resulting orientations are discussed in terms of a model of the inhomogeneous deformation which occurs in the vicinity of non deformable second phase particles.
679 citations
TL;DR: In this paper, an analysis of the normal tractions acting on grain boundaries in a solid with a perfectly regular hexagonal grain structure deforming via diffusional creep is made, where restrictions are placed on the allowable diffusion paths solely by requiring that the normal stresses on opposite sides of a grain boundary be identical.
Abstract: An analysis is made of the normal tractions acting on grain boundaries in a solid with a perfectly regular hexagonal grain structure deforming via diffusional creep. Restrictions are placed on the allowable diffusion paths solely by requiring that the normal stresses on opposite sides of a grain boundary be identical. It is shown that a model for grain switching recently proposed by Ashby and Verrall is inconsistent with this requirement. The problem of grain boundary diffusional flow is solved by treating grains as elastically rigid, and the solution, which agrees with earlier results in the limit of small strains, provides an explicit description of the equilibrium boundary traction distribution during steady state flow. This solution suggests that grain neighbor switching can occur in single phase materials only when grain boundary migration occurs. In two phase materials it is expected that diffusional creep will give rise to a grain switching process in which a grain of one phase wedges between and separates two grains of the other phase.
197 citations
130 citations
TL;DR: In this paper, the orientation coherence function (o.c.f) is defined to be the probability density for the joint occurrence of crystallite orientation g at a point p and orientation h at point h where p and h are independently located in a specified measurement volume, Vm, and are separated by a vector r.
Abstract: The preference in polycrystals for grains of specified orientation to reside near other grains of a particular orientation is quantified using a new microstructural measure called the orientation coherence function (o.c.f.). The o.c.f. is defined to be the probability density for the joint occurrence of crystallite orientation g at a point p and orientation ′g at point ′p where p and ′p are independently located in a specified measurement volume, Vm, and are separated by a vector r. The o.c.f. is shown to reduce to the orientation distribution function and the misorientation distribution function under suitable restrictions. It also contains morphological information such as crystallite size and shape as a function of orientation. In this paper the o.c.f. is defined, and represented in a series of orthogonal polynomials. An exemplary calculation of a conditional form of the o.c.f. is described for an extensive set of orientation measurements in alloy 304 stainless steel tubing. The results suggest a well defined coherence mesostructure for this material which can be explained, only in part, by twinning relationships.
74 citations
Metz1
TL;DR: In this paper, the authors studied the effect of strain paths on the formation of limit diagrams in mild steel sheets submitted to uniaxial and equibiaxial tension in the transmission electron microscope.
Abstract: Dislocation cells in cold rolled mild steel sheets submitted to uniaxial tension and equibiaxial tension have been studied in the transmission electron microscope. Their differences have been taken into account to explain how strain paths influence forming limit diagrams.
59 citations