A microstructural examination of the flow behaviour of a superplastic copper alloy

There are important differences in the microstructures of specimens of a superplastic copper alloy deformed in the three regions of flow associated with superplasticity. There is very extensive dislocation activity at high strain rates in Region III, whereas at intermediate and low strain rates in Regions II and I the dislocation density is low and many of the grains appear to be dislocation-free. Measurements show that grain-boundary sliding is important in Region II but decreases in magnitude in the less superplastic Regions I and III.     

Effect of strain,strain rate and temperature on cavity size distribution in a superplastic copper-base alloy

The distribution of cavity sizes in a microduplex / copper-zinc-nickel-manganese alloy (a nickel-silver) subjected to superplastic tensile straining has been examined as a function of strain, temperature and strain rate using quantitative optical metallography. The number of cavities that became optically visible increased throughout straining, but the rate at which they became visible decreased at higher strains. The distributions of cavity sizes in specimens deformed to the same strain at different temperatures or strain rates were essentially identical. The size distribution data were fully consistent with the observations that for a given strain the overall volume of cavities formed in the alloy was independent of temperature and strain rate. Growth for all cavity sizes is dominated by matrix plastic flow. The insensitivity of void volume and cavity size distribution to strain rate and temperature, and hence stress, reflects the resolution of the density and metallographic techniques. While higher stresses will lead to a wider range of initial cavity sizes by lowering the critical nucleus size, cavities at the lower end of the size range will not grow sufficiently to become optically resolvable or to produce a density differential compared with a specimen deformed to the same extent at a lower stress.     

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.     

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.     

High strain rate superplastic behaviour of powder-metallurgy processed 7475Al+0.7Zr alloy

Superplasticity was investigated in powder-metallurgy (PM) processed 7475Al+0.7Zr alloy. Strain-rate-change (SRC) tests were carried out at various temperatures to examine the relationship between strain rate and flow stress. After the compensation by threshold stress, the superplastic flow was found to be well correlated with lattice diffusivity in aluminium, like that in the ingot-metallurgy (IM) processed 7475Al alloy having a coarser grain size. Large tensile elongations of up to 1000% could be obtained at a very high strain rate near 10⁻¹ s⁻¹ and at 515°C. Short fibre formation was observed after the superplastic deformation. This formation seemed to be related to liquid formation on the grain boundaries and similar evidences were found over a wide range of temperature, not necessarily near the incipient melting point.     

LC4高强铝合金的慢应变速率拉伸试验

采用慢应变速率拉伸 (SSRT)技术测试了LC4铝合金在空气和质量分数为 3.5 %的NaCl溶液中的应力腐蚀断裂 (SCC)行为 .研究了应变速率对铝合金SCC行为的影响和氢在LC4高强铝合金应力腐蚀断裂过程中的作用 .试验结果表明 ,LC4合金具有SCC敏感性 ,在潮湿空气中发生应力腐蚀断裂 ,而在干燥空气中不发生应力腐蚀断裂 .对于长横取向的LC4铝合金试样 ,在应变速率为 1.331× 10 6s 1时 ,其SCC敏感性比应变速率为 6 .6 5 5× 10 6s 1时的敏感性大 .在潮湿空气和阳极极化条件下 ,铝合金的应力腐蚀断裂机理是以阳极溶解为主 ,氢几乎不起作用 .在预渗氢或阴极极化条件下 ,氢脆起主要作用 ,预渗氢时间延长可加速LC4合金的应力腐蚀断裂 .     

Flow stress,strain rate,strain relationships in superplastic deformation

The superplastic deformation of materials has commonly been characterized by the strain rate sensitivity, m, determined from two-dimensional, flow stress against strain rate plots. For material in which flow stress varies with strain or because of microstructural changes due to time at a high temperature, superplastic deformation must be characterized with respect to a three dimensional plot in which strain or time is the third axis. Techniques for generating the necessary plots are indicated and a simple experimental method for determining the suitability of a given material to a particular forming operation is described.     

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     

The stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy

The stress-strain rate behaviour of a superplastic Zn-22% Al alloys was investigated by the differential strain rate and constant load cycling tests on an Instron machine. A region with a rate sensitivity approaching unity was observed at low strain rates. On increasing the strain rate, a transition to the superplastic region occurred through an intermediate region of lower m (0.35). These observations are interpreted in terms of a transition from diffusional creep to superplastic flow with a threshold stress for superplastic deformation.     

Cavitation in alloy steels during superplastic deformation

A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.     

The development of superplastic ductilities and microstructural homogeneity in a magnesium ZK60 alloy processed by ECAP

An extruded ZK60 magnesium alloy was processed by equal-channel angular pressing (ECAP) and then tested in tension at elevated temperatures. The results show the alloy is superplastic at a testing temperature of 473 K with an optimum ductility of 1310% when using an initial strain rate of 2.0 × 10⁻⁴ s⁻¹. The results demonstrate that optimum superplasticity is achieved at intermediate strain rates and there are decreases in the elongations to failure at both faster and slower strain rates. Microhardness measurements were taken both on the cross-sectional plane and along the axial direction after processing by ECAP. These measurements show the alloy is essentially homogeneous in the as-pressed condition.     

Predicting the effects of microstructural features on strain localization of a two-phase titanium alloy

Strain localization influenced by microstructural features has an important effect on mechanical properties of α + β titanium alloy. To address the effect, a microstructure-based finite element model is established. In this model, regions for primary α (α_p) and transformed β matrix (β_t) are generated from real microstructures of a two-phase titanium alloy (TA15 alloy); the plastic flow behaviors of these two features are determined directly rather than from single phase alloy. A constitutive equation of α_p is developed with consideration of dislocation–obstacle interaction, whereas the constitutive equation of β_t is determined by nanoindentation tests. Finally, the calculated stress–strain responses of the alloy are verified by experiments. The simulated results show that strain localization bands (SLBs) have two morphologies: short and long-continuous SLBs. Lots of short SLBs appear mainly in β_t and α_p when the volume fraction of α_p is small and moderate, respectively. Long-continuous SLBs appear mainly in α_p when the volume fraction of α_p is large. With the increase of α_p in SLBs, the strength of the alloy decreases while the ductility increases. By decreasing the disparity of strength between α_p and β_t, the strain gradient in SLBs reduces and the ductility of the alloy increases.     

In situ TEM investigation of microstructural behavior of superplastic Al–Mg–Sc alloy

Dynamic changes in microstructure of the superplastic ultrafine-grained Al–3Mg–0.2Sc (wt.%) alloy refined by equal-channel angular pressing (ECAP). were observed by in situ transmission electron microscopy at temperatures up to 300 °C (annealing and tensile deformation) in order to simulate the initial stages of superplastic testing. It was found that the microstructure changes significantly during the preheating before the superplastic deformation, which was accompanied by decreased microhardness. During the deformation at 300 °C, high dislocation activity as well as motion of low-angle grain boundaries was observed while high-angle grain boundaries did not move due to the presence of scandium in the alloy.     

Effect of strain rate sensitivity and cavity growth rate on failure of superplastic material

Abstract

Necking development and fracture strain of superplastic material under tensile load are analysed by introducing a model of cavity growth into the long wavelength approximation analysis which can describe the external neck development of specimens during deformation. The results show that both strain rate sensitivity m and cavity growth rate η have an important influence on the fracture strain of superplastic material. According to these results, a fracture diagram is presented in m–η coordinates, which is divided into three: a region in which material fails by macroscopic external necking, a region where cavity growth is predominant leading to fracture without pronounced external necking, and an intermediate region where both fracture modes occur. The prediction of fracture strain for various superplastic alloys exhibiting cavity growth during deformation is in good agreement with experimental results. The present analysis thus enables quantitative prediction of the effects of both strain rate sensitivity and cavity growth on superplastic fracture under uniaxial tension.

MST/491     

Mesoscopic grain boundary sliding as the rate controlling process for high strain rate superplastic deformation

Important features observed during high strain rate superplastic deformation are enumerated. Starting from the premise that the phenomenon of structural superplasticity in different classes of materials results when grain boundary sliding that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) controls the rate of flow, the particular case of high strain rate superplasticity is explained. The rate equation developed is validated using experimental results concerning 5 alloy systems in which an ultra-fine grain size is developed by thermomechanical processing and retained in a similar condition during superplastic deformation by fine, grain boundary pinning particles and 3 alloy composites in which the volume fraction of the reinforcing constituent is significant (15–25%). It is demonstrated that the analysis results in estimates for the externally measured strain rates that are within a factor of two, in addition to providing a physically meaningful free energy of activation for the rate controlling process. This approach explains superplastic flow in different classes of materials in terms of a single rate controlling mechanism of deformation, viz., mesoscopic grain boundary sliding, with the help of a few constants that have the same values for all systems. The system-dependent variables of threshold stress needed for the onset of mesoscopic boundary sliding and free energy of activation are obtained directly from superplasticity stress–strain rate data, without external inputs.