B. Zhang

Bio: B. Zhang is an academic researcher from University of Manchester. The author has contributed to research in topics: Superplasticity & Strain rate. The author has an hindex of 5, co-authored 6 publications receiving 127 citations.

Microstructure and texture evolution in the tension of superplastic Al–6Cu–0.4Zr

Abstract: The effect of tension at 450 °C with a strain rate of 10 −3 s −1 on the microstructure and texture of a superplastic Al–6Cu–0.4Zr alloy has been investigated. The strain rate sensitivity index was essentially constant with strain, and therefore microstructure, at m ≈ 0.45. The texture was dominated by a component centred on {0 1 1}〈√2 1 1〉, which became weaker with deformation. The two variants of that texture allowed the identification of bands in the microstructure which acted as internal markers, and persisted to high strain. The texture and microstructure changes are not consistent with relative grain translation via boundary sliding, and a combination of rate-sensitive slip and dynamic grain growth is a better candidate to explain the behaviour of this material.

Mechanical behaviour and microstructural evolution in superplastic Al-Li-Mg-Cu-Zr AA8090

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.

Material modelling data for superplastic forming optimisation

Abstract: Optimisation of the superplastic forming process involves the determination of the minimum forming time within the constraints of product characteristics such as thickness distribution and cavitation level. However, superplastic forming is a non-linear system, with the material behaviour giving a significant contribution to that non-linearity. For example, dynamic grain growth gives useful strain hardening but can ultimately reduce strain rate sensitivity and, conversely, forming with a decreasing strain rate can enhance ductility. The results of tests on two commercial superplastic alloys, AA5083 and AA7475, have been used to provide data for material modelling. Simple power law descriptions of the mechanical behaviour were not adequate, and a basis for constitutive relationships for these materials is discussed.

Effect of strain rate path on cavitation in superplastic aluminium alloy

Abstract: The effect of a rapid pre-strain on the cavitation behaviour of commercially processed superplastic (SP) aluminium alloy 5083 sheet material deformed in uniaxial tension has been compared with that for deformation under constant strain rate conditions. It was observed for the two test conditions, that there was continuous nucleation of cavities. There were some differences in the distributions of cavity sizes for similar strains between the two testing conditions. However, these were not considered of significance because when the volume fraction of cavities was plotted against SP strain the two sets of data lay on, or close to, the same exponential curve. Metallographic examination showed that cavities were often associated with intermetallic particles. Analysis of the experimental data led to the conclusion that the growth of cavities was controlled primarily by plastic deformation, although diffusion may play a role in the early stages of growth. Overall, the levels of cavitation were lower than those often reported for other SP aluminium alloys. This is attributed to the low levels of iron and silicon impurities, and hence the absence of large constitutive particles considered to promote cavitation, and to the relatively low strains attainable in the alloy.

Optimisation of the superplastic forming of aluminium alloys

Abstract: Superplastic forming involves the shaping of metal sheets by gas pressure at elevated temperatures. It relies on the fact that fine-grained metals can exhibit high sensitivities of flow stress to strain rate, but this usually only occurs at quite slow strain rates. As a result, the process is much slower than conventional pressing operations and this is a major factor in process costs. Although there have been several attempts at optimising the process to minimise the forming time, most do not achieve a true optimum. In the present work, the non-linear system is optimised within the constraints of product thickness and damage (cavitation) level by searching a control space of enclosed volume history. The finite element method is used in that process, and good constitutive models and damage evolution models are required. These models are described, along with the optimisation strategy, and results are given for the forming of commercial aluminium alloys.

Strengthening mechanisms, deformation behavior, and anisotropic mechanical properties of Al-Li alloys: A review

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.

On the mechanisms of superplasticity in Ti–6Al–4V

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.

Diffusion creep and superplasticity in aluminium alloys

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

High Strain Rate Superplasticity in a Micro-grained Al{Mg{Sc Alloy with Predominant High Angle Grain Boundaries

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.

Evolution of the Brass texture in an Al-Cu-Mg alloy during hot rolling

Abstract: Texture and microstructure development of Al-4Cu-1.6Mg alloy during hot rolling was examined by using XRD, EBSD and TEM. The results showed that starting with a random texture during the early stages of rolling with reduction lower than 58.9% at blooming temperature of 430 °C, the materials developed a typical α-fiber texture in the center layer as deformation reduction reached 75.7%. And then the α-fiber textures in the center layer rotated into Brass component mainly through the activity of {111} slip system as the reduction reached 96.3%. Different from the center textures, the main texture in the surface layer was r-Cube with 96.3% reduction. The increase in rolling temperature was beneficial for the enhanced texture intensity of Brass component in the center layer. The analysis of substructure energy density indicated that Brass subgrain had a lower substructural energy density than other oriented subgrains, which together with increased slip rate at elevated temperature, contributed to the development of center Brass texture.