Cavitation and cavity-induced fracture during superplastic deformation

The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.     

Cavitation characteristics of a superplastic 5083 Al alloy during gas blow forming

Cavitation behavior of a superplastic 5083 Al alloy during gas blow forming has been investigated by deforming the sheet into a die with a rectangular cavity. Cavitation characteristics could be separated into two stages. In stage I, the sheet deformed freely as part of a hemi-cylindrical shape, cavity volume increased exponentially with deformation. The evolution of cavity volume was due to both nucleation and growth of cavities. In the second stage the surface friction would restrict thinning of the sheet, and the cavity volume first increased and then decreased with forming time for all test-forming rates. Decrease in cavity volume in the later stage could be related to the cavity shrinkage rising from sintering effect. A higher strain rate utilizing a higher imposed pressure during blow forming led to a greater average cavity shrinkage rate.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

Grain growth behaviour of the Al-Cu eutectic alloy during superplastic deformation

Grain growth behaviour of the Al-Cu eutectic alloy was investigated as a function of strain (ε), strain rate and deformation temperature (T) over = 10⁻² s⁻¹ and T=400 to 540°C. The grain size increases with increase in strain and temperature. Upon deformation to a fixed strain, the grain growth is generally seen to be more at lower strain rates. The rates of overall grain growth and due to deformation alone , however, increase with increasing strain rate according to and , respectively. The increase in the grain growth rate with strain rate is attributed primarily to the shorter time involved at higher strain rate for reaching a fixed strain. The activation energy for grain growth under superplastic conditions is estimated to be 79 kJ mol⁻¹.     

Internal variable approach to microstructural change in 7475 aluminium alloy during superplastic deformation

Abstract

Flow behaviour and microstructural changes in fine grained 7475 Al during superplastic deformation have been investigated. A series of mechanical tests and transmission electron micrography has been conducted at various temperatures ranging from 430 to 516°C. Quantitative constitutive parameters have been determined from load relaxation tests by applying the internal variable theory of structural superplasticity proposed recently. The validity of the mechanical analysis is proved by the evolution of the microstructure (precipitate free zones PFZs) during the accommodation process. Additional information about the mechanism of PFZ formation was determined by microchemical analysis.     

Grain rearrangements during superplastic deformation

Current models for obtaining large superplastic flow without change of grain size are two-dimensional; they therefore involve rearrangement of grains without increasing the surface area of the specimen as it deforms. A new model is proposed in which grainboundary sliding (GBS) in a group of grains is accommodated by a grain emerging from the next layer of grains, giving the correct increase in surface area. This also produces curved grain boundaries and there is some rotation of grains involving plastic flow in a zone along grain boundaries (the mantle) of predictable width. Grains do not have to be uniform and regular for the process. Characteristic configurations of marker lines are produced by the deformation. All these features are shown to have been observed in the literature. The model does not predict a threshold stress. It can be linked with a previous constitutive equation based on the climb and glide of dislocations in the grain mantles.     

Evolution of surface and bulk textures during superplastic deformation in a zinc alloy

Textured Zn-1.1%Al alloy sheet, free of through-thickness texture gradient, has been subjected to superplastic deformation. The texture of the samples are characterized by two 0002 components. During superplastic deformation, their intensity remains quite stable at the surface and decreases significantly in the mid-layer. Such texture changes are explained by the different sliding-rotation behavior of grains at the surface and in the bulk.     

Texture evolution during deformation of an Al-6%Cu-0.4%Zr superplastic alloy

Texture measurements of the as-received rolled Al-6%Cu-0.4%Zr alloy revealed that a copper component, {311}<233>, is developed at the surface and a S component, {631}<113>, is formed at the middle of the sheet. During early stages of superplastic deformation at 480 °C/5×10⁻⁴ s⁻¹ the intensity of the Cu component increases slightly whereas the S component changes toward the brass component Bs, {110}<112>, by a slip process. For larger strains, both components decrease by a grain boundary sliding mechanism.     

Cavitation and fracture in the superplastic Al-33% Cu eutectic alloy

Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on the- interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys     

On flow stress anisotropy in Ti-6Al-4V alloy sheet during superplastic deformation

The anisotropy of flow stress in a cold rolled sheet of Ti-6Al-4V alloy has been observed during superplastic deformation at 850 °C. At this temperature, the alloy has duplex microstructure with almost equiaxed grains of the alpha and beta phases. The maximum value of flow stress has been established for the rolling direction and minimum—for the transverse one. Also, the anisotropy of crystallographic texture weakening in the alpha phase has been observed. However, it has been demonstrated that texture in the alpha phase cannot be responsible for the observed anisotropic behavior. Texture in the beta phase is the suggested reason for the flow-stress anisotropy during superplastic deformation.     

Processes of microstructural evolution during superplastic deformation

Microstructural changes occurring at the surface and in the bulk of superplastically deformed materials have been considered. Surface studies showed formation of macroscopic surface steps and fibers, which determine surface roughness and can affect corrosion properties, respectively. Bulk microstructural changes include morphological and crystallographic changes, as well as defect accumulation. Phase/grain growth, phase spatial distribution, and phase/grain shape changes control the morphology of the phase constituents. Weakening of preexisting texture and an increase in the ratio of high angle grain boundaries determine crystallographic changes. Defect accumulation is related to cavity formation; density of lattice dislocation is superplastically deformed materials is low. Various explanations proposed for these processes of microstructural evolution in superplastic materials are considered. It is shown that these processes are closely related to cooperative grain boundary sliding—that is, the sliding of grain groups.     

Microstructural changes occurring during superplastic deformation of Ti-6AI-6V-2Sn alloy

Abstract

Microstructural changes occurring during superplastic deformation of Ti-6Al-6V-2Sn alloy with an initial microstructure consisting of mixed fine lamellar and equiaxed α grains were investigated. Uniaxial tensile tests with constant strain rate were conducted at temperatures ranging from 775 to 925°C and at strain rates rangingfrom 7 × 10^-5 to 1 × 10^-3 S^-l. To investigate the microstructural changes occurring during deformation, some of the tests were terminated at preprogrammed true strains of 0.5, 0.9, and 1.5 for subsequent metallographic investigation. The effects of high temperature exposure on the microstructural changes and on the superplastic deformation behaviour were also evaluated. It was found that both static and dynamic recrystallisation were initiated under certain test conditions and could be related to the flow stress behaviour during the superplastic deformation tests. For tests at low temperature and high strain rate, the flow stress increased quickly at the very beginning of the deformation without significant microstructural change. After the flow stress reached its maximum value, dynamic recrystallisation occurred at a lamellae accompanied by a decrease of the flow stress, known as strain softening. Raising the test temperature or decreasing the deformation strain rate provided the opportunity for thermal energy to initiate static or semidynamic recrystallisation. Thereafter, the flow stress behaviour at the beginning of the test changed to a slow strain hardening type. There also existed a transition temperature; soaking before tensile testing above this temperature would result in static recrystallisation, and the superplastic deformation characteristics would be affected.     

Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract

Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.     

The deformation behaviour of a dispersion strengthened superplastic zinc alloy

The deformation behaviour of a new dispersion strengthened superplastic zinc alloy was investigated. A significant long range internal stress was observed at all strain-rates. The activation volume of deformation decreased very rapidly with a decrease in the true effective stress. The maximum strain-rate sensitivity corresponds to a region of change from this high stress dependence of the activation volume to a much lower stress dependence. The observation of a metallographic halo effect shows that apart from dislocation movement, diffusive creep plays a role during superplastic deformation. It is stipulated that both these processes aid boundary sliding which accounts for the largest part of the strain.