Superplasticity of two-phased alloys: Qualitative and quantitative approach (original) (raw)

Microstructure changes in superplastically deformed ultrafine-grained Al-3Mg-0.2Sc alloy

Letters on Materials

Experiments were conducted on an ultrafine-grained Al-3wt.%Mg-0.2wt.%Sc alloy to characterize the microstructure changes occurring during large tensile deformation at elevated temperature within a transition between superplasticity and creep. The coarse-grained material was subjected to equal-channel angular pressing at room temperature using a die which had a 90° angle between the channels. The billets were rotated by 90° in the same sense between each pass in the processing route Bc. The billets were subsequently pressed by 2 and 8 passes. Microstructure was investigated by scanning electron microscope equipped with an electron back scatter diffraction unit. Microstructure analysis showed that the movement of dislocations and the growth of grains were restricted by Al3Sc coherent precipitates formed during stabilizing annealing at 623 K for 1h. It was found that deformation behaviour of investigated Al alloy was influenced by inhomogeneity of microstructure and deformation-induced coarsening of grains. The fracture behaviour was affected by cavity formation due to high local concentration of plastic deformation at the triple points and inefficient accommodation processes of grain boundary sliding. The results showed that total tensile deformation is a consequence of a various mechanisms occurring at different stages of tensile deformation. The superplastic tensile deformation was predominantly realized by grain boundary sliding and formation of mesoscopic shear bands. The role played by grain boundary sliding is discussed.

Microstructural evolution and superplastic deformation behavior of fine grain 5083Al

Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1996

The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 µm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.

Superplastic characteristics of fine-grained 7475 aluminum alloy

Journal of Materials Engineering and Performance, 2006

A 7475-aluminum alloy was subjected to a thermomechanical heat treatment that resulted in a final recrystallized grain size on the order of 10 µm. Tensile specimens of dimensions 10 × 4 × 2.3 mm were machined such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at high temperatures in the range of 773 to 803 K at different cross-head speeds corresponding to initial strain rates in the range of 10−4 to 10−2 s−1. Elongations of several hundred percent were observed at strain rates of −3 s−1. The correlation between flow stress and strain rate suggests that the strain rate sensitivity m is close to 0.5 at the lower strain rates. The value of m decreases to ≈0.2 at high strain rates. The decrease in m suggests a transition in the rate-controlling process from superplastic deformation (m ≈ 0.5) to dislocation creep (m ≈ 0.2) with increasing strain rate. The calculated activation energies in the two deformation regions are consistent with the suggested rate-controlling processes.

Recent Development of Superplasticity in Aluminum Alloys: A Review

Metals

Aluminum alloys can be used in the fabrication of intricate geometry and curved parts for a wide range of uses in aerospace and automotive sectors, where high stiffness and low weight are necessitated. This paper outlines a review of various research investigations on the superplastic behavior of aluminum alloys that have taken place mainly over the past two decades. The influencing factors on aluminum alloys superplasticity, such as initial grain size, deformation temperature, strain rate, microstructure refinement techniques, and addition of trace elements in aluminum alloys, are analyzed here. Since grain boundary sliding is one of the dominant features of aluminum alloys superplasticity, its deformation mechanism and the corresponding value of activation energy are included as a part of discussion. Dislocation motion, diffusion in grains, and near-grain boundary regions being major features of superplasticity, are discussed as important issues. Moreover, the paper also discusses...

Grain Growth During Superplastic Deformation

Interface Science, 2002

Significant grain growth occurring during superplastic deformation is related to the micro-mechanism of superplastic flow. Observations performed on the deformed surface of superplastically deformed tensile and shear Pb-62%Sn samples and bi-axially formed AA7475 samples directly indicate that cooperative grain boundary sliding, i.e. sliding of grain groups, is accompanied by cooperative grain boundary migration that can result in an enhanced grain growth. Such a long range correlation in migration of sliding grain boundaries is related to movement of grain boundary dislocations having a step associated with its core. Observed correlation between grain size and strain measured in different regions of a superplastically formed Ti-alloy part and alignment of grain boundaries along shear surfaces support coupling of grain boundary sliding and migration. A model of grain growth considering climb of cellular dislocations, topological defects in a grain array, has been expanded to incorporate gliding and mixed cellular dislocations.

Superplasticity in powder metallurgy aluminum alloys and composites

Acta Metallurgica et Materialia, 1995

Superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis. The stress strain curves can be put into four categories: a classical well-behaved type, continuous strain hardening type, continuous strain softening type and a complex type. The origin of these different types of stress strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been determined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the superplastic mechanism at high strain rates. The role of incipient melting has been analyzed. A stress exponent of 2, an activation energy equal to that for grain boundary diffusion and a grain size dependence of 2 generally describes superplastic flow in modified conventional aluminum alloys and mechanically alloyed alloys. The present results agree well with the predictions of grain boundary sliding models. This suggests that the mechanism of high strain rate superplasticity in the above-mentioned alloys is similar to conventional superplasticity. The shift of optimum superplastic strain rates to higher values is a consequence of microstructural refinement. The parametric dependencies for superplasticity in aluminum alloy matrix composites, however, is different. A true activation energy of 313kJmol ~ best describes the composites having SiC reinforcements. The role of shape of the reinforcement (particle or whisker) and processing history is addressed. The analysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding.

High-temperature deformation in a superplastic 7475 Al alloy with a relatively large grain size

Materials Science and Engineering: A, 1995

Tensile tests were conducted on a superplastic 7475 Al alloy having a grain size of 14 pm, which is relatively larger than that of conventional micrograin superplastic alloys, at temperatures in the range of 723-789 K. The objective of the present investigation was to examine the possible rate-controlling mechanisms that govern superplastic deformation of the alloy at both intermediate and low stresses. The values of the stress exponent and the activation energy, as well as microstructural observation, indicate that deformation of the alloy at intermediate stresses is rate-controlled by lattice diffusion. At low stresses, the stress exponent and the activation energy are higher than those at intermediate stresses. The mechanical behaviour at low stresses may be attributed to the existence of a threshold stress which strongly depends on temperature. The origin of the threshold stress is discussed in terms of the various dislocation-particle interaction models that have been proposed for the creep of dispersionstrengthened alloys. The present analysis reveals that the mixed climb model can qualitatively explain the very strong temperature dependence of the threshold stress, but not quantitatively. Therefore an alternative explanation is suggested for the very strong temperature dependence of the threshold stress on the basis of the role played by interface diffusion in recreating the dislocation core segment that is smeared out following attachment of dislocation to the dispersoids.

NIEH 1997 Superplasticity in metals and ceramics

This book describes advances in the field of superplasticity. This is the ability of certain materials to undergo very large tensile strains, a phenomenon that has increasing commercial applications, but also presents a fascinating scientific challenge in attempts to understand the physical mechanisms that underpin it. Breakthroughs include the development of superplasticity in metallic materials at very high strain rates that are of interest to the automobile industry. The authors emphasize the materials aspects of superplasticity. They begin with a brief history of the phenomenon. This is followed by a description of the two major types of superplasticity-fine-structure and internal-stress superplasticity-together with a discussion of their operative mechanisms. In addition, microstructural factors controlling the ductility and fracture in superplastic materials are presented. The observations of superplasticity in metals (including alloys of aluminium, magnesium, iron, titanium and nickel), ceramics (including monolithic alloys and composites), intermetallics (including iron, nickel, and titanium base), and laminates are thoroughly described. The technological and commercial applications of superplastic forming and diffusion bonding are presented and examples given. This book will be of interest to graduate students and researchers in materials science and engineering, especially those working in the aerospace and automobile companies.