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Key engineering materials
Trans Tech Publications Ltd.
Key engineering materials

Trans Tech Publications Ltd.

半月刊

1013-9826

Key engineering materials/Journal Key engineering materials
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    The Development of Superplastic Magnesium Alloy Sheet

    Richard DashwoodDavid KlaumunzerMartin JacksonZhongyun Fan...
    p.273-279页
    查看更多>>摘要：While magnesium alloys are routinely used in engineering applications in the form of net shape castings, applications for sheet product have been limited due to the poor cold formability of magnesium combined with the perceived expense of sheet. The issues associated with poor cold formability could largely be overcome if magnesium alloys were to be superplastically formed. Superplasticity in magnesium is well established with research papers on the subject dating back to the late 1960s. In recent years, interest in this area has grown to the point where a number of companies have successfully superplastically formed prototype automotive panels from magnesium alloy sheet. Concurrent to this the scientific community have demonstrated superplasticity in a wide range of magnesium alloys using processing techniques ranging from the exotic (severe plastic deformation) to the mundane (traditional warm rolling). Work by the current authors has shown, rather surprisingly, that superplasticity can be achieved in magnesium alloys in the as-cast condition. This has led to some initial exploratory work involving twin roll casting. The concept being that affordable superplastic magnesium sheet could be produced via twin roll casting with only limited rolling reduction to final gauge. This paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast AZ31 and AZ91. The experimental material has included strip cast AZ91 subjected to large shear strains immediately prior to casting. The material was tested in the as-cast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in the strip cast alloys. Furthermore, good superplastic capability was also demonstrated in sheet subsequently rolled from the cast metal and rolling strain did not significantly influence the ductilities obtained. The mechanism for achieving superplasticity in as-cast magnesium alloys will be considered and the contrasting deformation characteristics of AZ31 and AZ91 will be discussed in terms of m value analysis and microstructural characterisation.
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    Superplasticity for Lightweight Metal Foams

    Koichi KitazonoRyosuke Suzuki
    p.281-286页
    查看更多>>摘要：Potential applications of superplasticity for lightweight metal foams are reviewed in this paper. Metal foams have been used for lightweight structures, biomedical implants, filters, heat exchangers, sound absorbers and mechanical damping devices. Superplasticity has advantages on metal foaming process and mechanical properties of metal foams. Four examples of metal foams combined with superplastic forming are presented; (ⅰ) Enhanced foaming under internal stress superplastic condition, (ⅱ) Superplastic diffusion-bonding of metal foams, (ⅲ) Superplastic forming and foaming and (ⅳ) Superplastic Zn-22Al alloy foam for energy absorbing material.
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    Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

    Kenji SekidoTeppei SeoKoichi Kitazono
    p.287-290页
    查看更多>>摘要：Superplastic Zn-22Al alloy foams were manufactured through the melt foaming process using titanium hydride powder as a foaming agent. The compressive properties of the Zn-22Al foams were investigated under quasi-static and dynamic loading conditions. Experimental results show that the flow stress and the energy absorption of the Zn-22Al foam significantly increased with increasing the strain rate. At high strain rate region, the energy absorption of the Zn-22Al foams is also larger than that of conventional aluminum foams. These behaviors are due to superplastic deformation of cell walls.
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    Superplastic Behavior in Magnesium Alloy with Dispersion of Quasicrystal Phase Particle

    Hidetoshi SomekawaAlok SinghToshiji Mukai
    p.291-295页
    查看更多>>摘要：Superplastic behavior was investigated using an extruded Mg-Zn-Y alloy with the dispersion of the quasicrystal phase particle in fine-grained matrix. Tensile tests showed that the low temperature superplasticity was behaved at a temperature of 473 K and maximum elongation was 462 % at 573 K in 1 × 10~(-5) s~(-1). The deformed microstructure observation showed that the dominant deformation process was grain boundary sliding. The present alloy also demonstrated a high possibility for secondary forming, such as superplastic forge forming. Furthermore, the forged alloy had a homogeneous microstructures, no mechanical anisotropy and uniform micro-hardness properties in any portion of a forged product.
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    Grain Boundary Sliding Below Ambient Temperature in H.C.P. Metals

    Eiichi SatoTetsuya Matsunaga
    p.299-303页
    查看更多>>摘要：Hexagonal close-packed metals and alloys show significant creep behavior with extremely low activation energies at and below ambient temperature even below their 0.2% proof stresses. It is caused by straightly-aligned dislocation arrays in a single slip system without any dislocation cuttings. These dislocation arrays should, then, pile up at grain boundary (GB) because of violation of von Mises' condition in H.C.P. structure. The piled-up dislocations have to be accommodated by GB sliding. Electron back scatter diffraction (EBSD) analyses and atomic force microscope (AFM) observations were performed to reveal the mechanism of GB sliding below ambient temperature in H.C.P. metals as an accommodation mechanism of ambient temperature creep. EBSD analyses revealed that crystal lattice rotated near GB, which indicates the pile up of lattice dislocations at GB. AFM observation showed a step caused by GB sliding. GB sliding below ambient temperature in H.C.P. metals are considered to compensate the incompatibility between neighboring grains by dislocation slip, which is called slip induced GB sliding.
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    Analysis of grain-boundary sliding with rotating hexagonal particles

    Byung-Nam KimKeijiro HiragaKoji MoritaHidehiro Yoshida...
    p.305-310页
    查看更多>>摘要：When the sliding of the grain boundary containing hexagonal particles is accommodated by grain-boundary diffusion, we evaluate the sliding rate and the stress distribution on the boundary, by taking the particle rotation and the intrinsic boundary viscosity into account. The sliding rate was obtained by the energy-balance method, and the particle-rotation rate by a condition of minimum energy-dissipation. With increasing boundary viscosity, the rotation rate increases and then decreases after a maximum. The sliding rate is enhanced by the particle rotation, and decreases with the boundary viscosity.
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    Superplastic Forming Properties of TIMETAL~?54M (Ti-5%Al-4%V-0.6%Mo-0.4%Fe) Sheets

    Yoji KosakaPhani Gudipati
    p.311-317页
    查看更多>>摘要：Superplastic forming (SPF) properties of TIMETAL~?54M (Ti-5Al-4V-0.6Mo-0.4Fe, or Ti-54M) sheet were investigated. A total elongation of Ti-54M exceeds 500% at temperatures between 750℃ and 850℃ at a strain rate of 10~(-3)/S. Values of strain rate sensitivity (m-value) measured by jump strain rate tests are 0.45～0.6 in a temperature range of 730℃ to 900℃ at a strain arte of 5 × 10~(-4)/S or 1 × 10~(-4)/S. Flow stress of the alloy is 20 ～ 40% lower than that of Ti-6Al-4V (Ti-64) mill annealed sheet. The observation of microstructure after the tests revealed the indication of grain boundary sliding in a wide range of temperatures and strain rates.
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    Constitutive Relations for Superplastic Flow Modeling from Two Axial Loading Experiments

    A. AkhunovaS. DmitrievA. KruglovR. Safiullin...
    p.319-323页
    查看更多>>摘要：A critical analysis of approaches to find constitutive relations from the tests on uniaxial and two axial loading is presented. Then we report on the methodology of fitting parameters of constitutive relations for superplastic forming based on the results of cone cup and elongated wedge cup tests. Optimal cone and wedge angles are estimated analytically and also from the results of finite element simulations. Our methodology is based on comparison of the experimental results and the results of numerical simulations of the corresponding tests in frame of the commercial software such as ANSYS10.0 (ED). We carry out numerical simulations of the cone cup and the wedge cup tests for different sets of parameters of the chosen constitutive relation and different values of friction coefficient in order to reproduce the experimental results. The parameters corresponding to the best fit of the experimental data are then recommended for the use in computer simulations of the superplastic forming of commercial products.
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    The Effects of Specimen Geometry on the Accuracy of Tensile Testing of Metallic Superplastic Materials

    Fadi Abu-FarhaMohammed NazzalRichard Curtis
    p.325-331页
    查看更多>>摘要：This work investigates the sensitivity of a superplastic material's tensile test to the major geometrical parameters of the selected test specimen. This required generating a large number of specimens by systematically varying the gauge length, gauge width, grip length and width of a standard geometry. The specimens were prepared from a moderately superplastic AZ31B-H24 magnesium alloy sheet and were then stretched at a selected rate and temperature. Deformation in each specimen was tracked via an electrochemically-etched fine grid which was particularly used to quantify the amount of material flow from the grip into the gauge region. The consequences of the latter on the accuracy of measured stresses and strains were correlated back to the corresponding geometrical parameters. Ultimately, the results were utilized to set the guidelines for selecting the optimum parameters in a "proper" specimen, for testing the unique class of superplastic materials.
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    High-Strain-Rate Superplastic Flow Mechanism in ZrO_2-30vol% Spinel Two-Phase Composite

    Koji MoritaKiraga HiragaByung-Nam KimHidehiro Yoshida...
    p.333-338页
    查看更多>>摘要：High-strain-rate superplasticity (HSRS) can be attained in tetragonal ZrO_2-30vol% MgAl_2O_4 spinel composite. In order to examine the flow behavior of the two-phase composite, the standard rule of the mixture model was employed. The strain rate of the composite can be explained by the isostrain model that is predicted from the data set of Al_2O_3 doped ZrO_2 and spinel polycrystals. For the isostrain model, since the strain and strain rate are the same for ZrO_2 and spinel phases, the harder ZrQ_2 phase carries more of the stress in the composite. In order to preserve homogeneous deformation and material continuity, a concomitant accommodation process within the harder ZrO_2 grains is also necessary. For HSRS in the ZrO_2-spinel composite, therefore, the rate of deformation may be controlled by the slower dislocation recovery process limited by the lattice diffusion within harder Zr_O2 grains rather than within spinel grains.
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