Showing papers on "Superplasticity published in 2009"

Seventy-five years of superplasticity: historic developments and new opportunities

Abstract: On this seventy-fifth anniversary of the first scientific report of true superplastic flow, it is appropriate both to look back and examine the major developments that established the present understanding of superplasticity and to look to the future to the new opportunities that are made possible by new processing techniques, based on the application of severe plastic deformation, that permit the production of fully dense bulk materials with submicrometer or nanometer grain sizes. This review proposes a minor modification to the present definition of superplasticity, it provides an overview of the current understanding of the flow of superplastic metals and ceramics and then it examines, and gives examples of, the new possibilities that are now available for achieving exceptional superplastic behavior.

Principles of grain refinement and superplastic flow in magnesium alloys processed by ECAP

Abstract: The superplastic properties of metallic materials are associated with the length scale and with the thermal stability of their grain structure. Whereas equal-channel angular pressing (ECAP) may be used to produce ultrafine-grained structures in f.c.c. metals through the homogeneous subdivision of the grains, research on two magnesium alloys reveals a different and heterogeneous process of grain refinement which is dependent upon the initial grain structure in the alloys. Experiments demonstrate that different structural features may be achieved using different processing routes and this leads to the development of a processing strategy for achieving an optimum microstructure. It is shown by mechanical testing that the optimum superplastic properties also depend on the processing route and, depending on the structural characteristics, the maximum elongations to failure may occur either in the early stages of processing by ECAP or after processing through large strains.

A Huge Effect of Weak dc Electrical Fields on Grain Growth in Zirconia

Abstract: We show that the application of a modest dc electrical field, about 4 V/cm, can significantly reduce grain growth in yttria-stabilized polycrystalline zirconia. These measurements were made by annealing samples, for 10 h at 1300°C, with and without an electrical field. The finding adds a new dimension to the role of applied electrical fields in sintering and superplasticity, phenomena that are critical to the net-shape processing of ceramics. Grain-growth retardation will considerably enhance the rates of sintering and superplasticity, leading to significant energy efficiencies in the processing of ceramics.

Strength of nanostructures

Abstract: A state-of-the-art review is presented of research into the strength, the ductility, the superplasticity, and other mechanical properties of metal, alloy, intermetallic, semiconductor, and high-melting-point compound-based nanomaterials. Various theoretical approaches are described. Size effects, interfaces, and other structural factors are examined from the standpoint of their influence on the strength and plasticity of nanostructured materials. Some problems yet to be solved are pointed out.

Superplastic behavior of micro-regions in two-pass friction stir processed 7075Al alloy

Abstract: Effect of overlapping passes on the microstructure and superplastic behavior of friction stir processed (FSP) 7075Al was subjected to a detailed investigation. Overlapping passes exerted no obvious effect on the size of recrystallized grains. Both single and two-pass FSP 7075Al exhibited similar grain sizes of 5.4-5.7 mu m. Compared to single-pass FSP, two-pass FSP resulted in an enhancement in superplastic elongation and a change in superplastic response. A shift to higher optimum temperature was observed in the two-pass FSP 7075Al. Furthermore, overlapping passes led to a shift to higher optimum strain rate in the center region of second pass in the two-pass FSP 7075Al. Maximum superplastic elongation of 1220% was achieved at 480 degrees C and an initial strain rate of 1 x 10(-2) s(-1) in the center region of second pass in the two-pass FSP 7075Al. Analyses of superplastic data indicated that grain boundary sliding is the main superplastic deformation mechanism for both single and two-pass FSP 7075Al. (C) 2008 Elsevier B.V. All rights reserved.

Superplastic Behavior of a Fine-grained ZK60 Magnesium Alloy Processed by High-Ratio Differential Speed Rolling

Abstract: Grain size of the ZK60 alloy was effectively reduced to 1∼2 μm through high-ratio differential speed rolling (HRDSR) for a thickness reduction of 70% in a single pass. Due to the strengthening effects of grain boundaries and particles, the HRDSR processed ZK60 exhibited a high tensile strength of 340 MPa. Low temperature superplasticity was attained at 473–493 K at low strain rates (5 × 10 −4 s −1 ) and high strain rate superplasticity was attained at 523–553 K at high strain rates (10 −2 s −1 ). The optimum superplastic temperature was found to be 553 K where a maximum tensile elongation of ∼1000% was obtained at 1 × 10 −3 s −1 . The deformation behavior of the HRDSR processed ZK60 at elevated temperatures could be depicted by considering contribution of grain boundary sliding and slip creep to total plastic flow. Difference in superplastic deformation behavior between the HRDSR processed and equal channel angular press processed ZK60 alloys was examined and discussed.

Effect of process conditions on superplastic forming behaviour in Ti–6Al–4V friction stir welds

Abstract: Friction stir welding of Ti–6Al–4V was performed on 5 mm thickness sheet. Wide ranges of processing conditions were tested, and an experimentally determined process window was established for this given material, thickness and tooling configuration. Welds made within this process window were also superplastically formed. It was found that the weld parameters influence joint quality in terms of microstructure, penetration and void formation in addition to process traits such as loads and tool wear. Furthermore, since the microstructure of the weld could be altered with the welding parameters, the degree of superplasticity in the joints could also be controlled. Finally, experimental observations were compared to a proposed analytic process model in an attempt to establish trends between the weld characteristics and the welding conditions used. Reasonable correlations were observed between the analytic model and experimental results.

Ultrafine-grained Mg–9Li–1Zn alloy sheets exhibiting low temperature superplasticity

Abstract: The microstructure and mechanical properties of the ultrafine grained Mg–9Li–1Zn (LZ91) alloy sheets prepared through HRDSR (high-speed-ratio differential speed rolling) process were examined. During HRDSR for a thickness reduction of 65% by a single pass at 398 K, the microstructure was significantly refined to grains with submicron size and the refined microstructure was homogeneous along thickness direction of a sheet. The HRDSRed alloy exhibited enhanced strength and ductility at room temperature, and excellent low temperature superplasticity in the temperature range 423–523 K (0.49–0.6 T m ). The superplastic elongations were comparable with those obtained by other severe-plastic-deformation techniques or even better. The superplastic deformation mechanism of the ultrafine grained LZ91 alloy in the temperature range between 423 and 523 K was found to be grain-boundary-diffusion controlled grain boundary sliding.

Effect of calcium content on the microstructural evolution and mechanical properties of wrought Mg–3Al–1Zn alloy

Abstract: The effect of Ca element on the hot workability and microstructure evolution of AZ31 magnesium alloy was investigated. Conventional AZ31, AZ31 + 0.7 wt.%Ca, and AZ31 + 2.0 wt.%Ca alloys were tested. Electron back scattered diffraction microscopy revealed that the alloys containing Ca exhibited much finer and more homogeneous microstructure than the conventional AZ31 alloy. Hot compression and tensile tests showed that the Ca element generally increased flow strength and decreased ductility at low temperature. High temperature elongation was considerably improved by the operation of the thermally activated process. TEM work suggested that the large volume fraction of fine (Mg, Al)2Ca particles played an important role in preventing significant grain growth of the AZ31 + 2.0Ca alloy.

Molecular dynamics simulation of tensile elongation of carbon nanotubes: Temperature and size effects

Abstract: We report molecular dynamics simulations of tensile elongation of carbon nanotubes (CNTs) over a wide temperature range. In particular, we examine temperature and size effects on tensile ductility of CNTs and compare our results with recent experimental observation on superplastic deformation of CNTs at high temperatures. Our simulations produce substantial tensile ductility in CNTs with large diameters at high temperatures and reveal that similar behavior can be realized over a surprisingly large temperature range between 500 and 2400 K that is yet to be fully explored by experiments. At lower temperatures, tensile deformation modes become brittle due to defect localization attributed to insufficient thermal energy for wide distribution of defect nucleation. For CNTs with smaller diameters, our simulations produce strong defect localization which leads to brittle behavior even at high temperatures. Sensitive dependence on the distribution of incipient defects on thermal energy results in a significant decrease in the elastic limit with increasing temperature. We propose an effective tensile ductility enhancement via temperature reduction beyond the elastic limit. The results offer insights for understanding intriguing temperature effects on tensile deformation modes of CNTs.

Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Abstract: Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10?2 s?1. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing