Browsing by Author "Naziri, H."

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    Anisotropic superplasticity
    (College of Aeronautics, 1969-06) Naziri, H.; Pearce, R.
    Anisotropy has already been mentioned in connexion with SP. Johnson et al (1) have shown that specimens of circular cross-section, machined from hot-rolled SP Zn/Al eutectic and eutectoid plate, become elliptical on straining in the rolling direction, while the fine-grain Zn/Al eutectoid produced by the quench-4 spinoidal decomposition method did not.
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    The effect of grain size on workhardening and superplasticity in Zn/0.4% Al Alloy
    (College of Aeronautics, 1969-06) Naziri, H.; Pearce, R.
    Superplasticity*requires, amongst other things, a metal with a grain-size in the range 0.5-5μ. Theories of SP invoking dynamic recovery require that the cell-Size of the substructure for the alloy in question is larger than the SP grain-size, so that gliding dislocations are always annihilated in the grain boundaries and workhardening cannot occur (1,2). .Thus the grain-size is critical, and for a given set of conditions, there must be a grain-size greater than which SP cannot be achieved.
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    The effect of plastic anisotropy on flange wrinkling behaviour during sheet metal forming
    (College of Aeronautics, 1967-10) Naziri, H.
    During the drawing of sheet metal between a die and a blankholder, compressive hoop-stresses are developed which attempt to thicken or wrinkle the flange. Previous work on this behaviour has ignored any effects due to normal or planar plastic anisotropy. In this paper it is shown that the blankholder pressure necessary to suppress wrinkling increases with decreasing normal anisotropy (r) and increases with increasing planar anisotropy (AO. The approximate plane strain conditions (daz = 0) operating in the flange can be simulated by an edge-notched tensile specimen and this simulation demonstrates the effect of texture hardening and softening upon flange wrinkling behaviour. The results obtained can be interpreted valitatively by the use of anisotropic plasticity theory. The speed of drawing also effects wrinkling, in general, the number of wrinkles decreases with increasing drawing speed.
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    Extended plasticity in commercial-purity zinc
    (College of Aeronautics, 1968-09) Williams, D. A. C.; Naziri, H.; Pearce, R.
    90% rolling-reduction of annealed commercial-purity zinc sheet (grain size 100 - 150 μ) results in the fragmentation of the large grains into, finally, stable micro-grains, 1 - 211 in diameter. The stability of the micro-grains is due to the presence of soluble and insoluble impurities which prevent recrystallization. This micro-grain material is strain-rate sensitive, and elongations of 200% have been obtained at room temperature. Although this as-rolled, 90% reduction zinc sheet is not super-plastic according to the current definition, its behaviour has led to the coining of the phase 'extended plasticity'. Evidence of grain-boundary sliding is found on examination of the surface by scanning electron microscopy, while the examination of thin foils and activation energy measurements support the dynamic softening (recovery) theory; thus, both these mechanisms must be operating, to a greater or less extent, to confer on this material the observe mechanical behaviour. It is finally concluded that it is dangerous to draw conclusions regarding the mechanism of plastic deformation from surface observations alone.