Showing papers on "Superplasticity published in 1997"

Superplasticity in metals and ceramics

Abstract: Preface 1. Introduction 2. Key historical contributions 3. Types of superplasticity 4. Mechanisms of high-temperature deformation and phenomenological relations for fine-structure superplasticity 5. Fine-structure superplastic metals 6. Fine-structure superplastic ceramics 7. Fine-structure superplastic intermetallics 8. Fine-structure superplastic composites and laminates 9. High-strain-rate superplasticity 10. Ductility and fracture in superplastic materials 11. Internal-stress superplasticity (ISS) 12. Other possible superplasticity mechanisms 13. Enhanced powder consolidation through superplastic flow 14. Superplastic forming and diffusion bonding 15. Commercial examples of superplastic products Index.

Structure and mechanical properties of ultrafine-grained metals

Abstract: This paper describes the fabrication of several pure metals and alloys with nano-and submicrometer grain sizes by severe plastic deformation techniques, the results of their structural characterization, and provides data on their mechanical properties at ambient temperature and superplastic conditions. Special attention is paid to the relationship between the defect structures of grain boundaries and the mechanical behaviour of produced ultrafine-grained metallic materials.

The thermomechanical processing of alpha/beta titanium alloys

Abstract: In this article, the thermomechanical processing of ingot-metallurgy alpha/beta titanium alloys is summarized, with special emphasis on microstructure evolution and workability considerations. Primary hot working dealing with the conversion of ingot structures to fine-equiaxed wrought structures is addressed. In this regard, the breakdown of lamellar microstructures, the occurrence of cavitation/wedge cracking, and the development of crystallographic texture are described. Secondary processes such as sheet rolling, superplastic forming, and closed-die forging are also reviewed.

Experimental investigation of superplastic behaviour in magnesium alloys

Abstract: Constant strain rate tensile tests have been conductedfor fine grained Mg-Al-Zn (AZ91) and Mg-Zn-Zr (ZK60 and ZK61) alloys processed by powder metallurgy (PM) and ingot metallurgy (IM) routes. The experimental results revealed that the strain rate was inversely proportional to the cube of the grain size and that the activation energy for superplastic flow was higher than that for grain boundary diffusion. The PM Mg alloys showed superplastic behaviour at higher strain rates than the IM Mg alloys. This is because of smaller grain sizes of the PM Mg alloys. The origin of the high strain rate superplasticity for the PM Mg alloys is unlikely to be associated with the presence of a liquid phase.

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

Abstract: The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure.

Structure and properties of ultra-fine grained aluminium alloys produced by severe plastic deformation

Abstract: Methods of imparting ultra-fine grained (UFG) structure to conventional aluminium alloys by thermomechanical treatment, including severe plastic deformation, are analysed. The effect of initial structure parameters, such as grain size and precipitates of secondary phases, on UFG structure formation is discussed in terms of physical mechanisms. Ambient temperature mechanical behavior of UFG alloys produced by several methods is reported. The effect of grain refinement on static strength and toughness is more pronounced in non-heat treatable alloys. Evolution of UFG structure during post-deformation heat treatment and superplastic behavior of UFG alloys is shown.

Stabilization of Supercooled Liquid and Opening-up of Bulk Glassy Alloys.

Abstract: Bulk glassy alloys with thicknesses up to 75mm and a wide supercooled liquid region reaching 127K before crystallization were found to be fabricated in a number of multicomponent systems which satisfy the three empirical rules for the achievement of large glass-forming ability, i.e., (1) multicomponent alloy systems consisting of more than three constituent elements, (2) significantly different atomic size ratios above 12%, and (3) negative heats of mixing. The scientific significance of the rules has been proved based on a number of experimental data as well as on the kinetic theories of the nucleation and growth of a crystalline phase. By choosing appropriate compositions which satisfy the empirical rules, bulk glassy alloys in Mg-, lanthanide metal-, Zr-, Pd-, Fe- and Co-based systems were produced in cylindrical and sheet forms by various solidification processes. The bulk glassy alloys exhibit high tensile strength, good ductility, high elastic energy, high impact fracture energy and high corrosion resistance for Zr-based system and good soft magnetic properties for Fe-based system. Furthermore, their glassy alloys heated in the supercooled liquid region can be deformed into various shapes through viscous flow. The ideal Newtonian flow has been achieved in the supercooled liquid. The utilization of the ideal superplasticity enabled the achievement of an extremely large elongation exceeding 15000%. These excellent data allow us to expect that the bulk glassy alloys develop as a new type of engineering material.

Superplasticity in alloys

Abstract: In recent years superplasticity research has been mainly developed through two major streams: the discovery of superplasticity in various advanced materials such as intermetallics and high-performance ceramics; and studies on the high strain rate superplasticity in Al-base composites and mechanically alloyed materials. As the deformation of superplastic flow takes place primarily by grain boundary sliding in these materials a deepening of our understanding of atomic bonding in grain boundaries and its role in grain boundary sliding is important for future studies of the superplasticity in various materials.

Consolidation of nanostructured metal powders by rapid forging: Processing, modeling, and subsequent mechanical behavior

Abstract: Fe-10Cu powders containing 20-nm grains were produced by attritor milling of elemental powders in argon. A rapid powder forging technique was developed to consolidate the powders into fully dense compacts while maintaining nanoscale grain sizes. Grain growth during the consolidation was controlled by reducing the time of exposure at elevated temperature to a few minutes or less, a technique which is applicable to all materials and does not necessitate the addition of dispersoids. This was achieved by heating green compacts quickly using an induction heater, and then forging and rapidly cooling them back to room temperature. Forging was conducted in a protective argon atmosphere to limit contamination. Fully dense compacts were produced at relatively low temperatures, mainly due to the accelerated creep rates exhibited by the nanostructures. Transmission electron microscopy and X-ray diffraction analysis found an average grain size of 45 nm in the fully dense samples forged at 530 °C. Indications are that finer grain sizes should be attainable by using slightly lower temperatures and higher pressures. The success of the technique (compared to hot-isostatic pressing (“hipping”)) is due to both reducing time at elevated temperatures and applying relatively high pressures. Microhardness tests revealed a significant strengthening effect due to grain size refinement, following a Hall-Petch relation. Compression testing at room temperature showed no strain hardening during plastic deformation, which occurred by shear banding. High strengths, up to 1800 MPa, were obtained at room temperature. Compression testing at 575 °C revealed a significant strain rate dependence of mechanical behavior and also the possibility of superplastic behavior. Power-law creep was observed at 575 °C, with very high steady-state creep rates on the order of 50 pct/s at 230 MPa. The consolidation process was successfully modeled by slightly modifying and applying the Arzt, Ashby, and Easterling (AAE) hot-isostatic press (HIP) model. The experiments and modeling indicated that creep was the dominant densification mechanism in these materials, even at relatively low temperatures and high loading rates. The results of this investigation suggest the possibility of a commercially viable nanostructured metal, which is easily processed to large strains at moderate temperatures, yet maintains high strength at room temperature without the necessity of heat treatment or mechanical working.

Dynamic evolution of fine grained structure and superplasticity of 7075 aluminum alloy

Abstract: Hot deformation of an unrecrystallized 7075 aluminum alloy was studied by means of 2-step tensile testing and metallographic observations. Typical superplasticity took place accompanied with the evolution of new fine grains in the medium region of strain rate. Grain boundary sliding (GBS) operated just after yielding even in the unrecrystallized coarse grained structure. It is concluded from the mechanical and metallographic results that GBS can play a key role in the evolution of fine grains as well as the appearance of superplasticity.

Superplastic forming of an α-phase rich silicon nitride

Abstract: The deformation behavior of fine-grained α-phase rich silicon nitride materials has been studied between 1550°C and 1615°C, both in compression and in tension. First, it is shown that higher the α-phase content, better the superplastic forming ability. A large tension-compression flow asymmetry was evidenced. For instance, shear-thickening flow shows up in compression whereas shear-thinning is observed in tension. Furthermore, much higher flow stresses and hardening rates are reported in compression than in tension. Elongation of more than 80% were achieved for strain rates between 2.5 and 5×10−5S−1. In the light of our results and of the abundant literature dealing with the high temperature deformation in silicon nitride, a sketch of the different deformation stages is proposed, which emphasizes the tension-compression flow asymmetry. Starting from the promising results obtained at the laboratory scale, the feasibility for net-shaping of a real part was demonstrated by hot-forging of a parabolic shell.

Determination of superplastic constitutive equations and strain rate sensitivities for aerospace alloys

Abstract: Constitutive equations for the superplastic behaviour of titanium and aluminium aerospace alloys have been presented and computational procedures developed for the determination of the material parameters arising in the equations. The equations include a description of grain growth kinetics with deformation enabling the influence of microstructure on superplastic deformation behaviour to be modelled.Three predictive methods have been developed for the determination of strain rate sensitivity. The models represent the dependence of the strain rate sensitivity on strain, strain rate, and grain growth, and therefore, provide useful information for process design and optimization. The models are validated by comparison of predictions with experimental data reported in the literature and obtained in this work. Finally, two conventional experimental testing methods for the determination of strain rate sensitivity, namely the constant strain rate and strain rate jump methods are appraised using the methods devel...