Superplasticity in Zn-based alloys

• Main Author: Naziri, Hurmuze
• Other Authors: Pearce, Roger
• Published: Cranfield University 1972
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.466898

This thesis is concerned with two-basic zinc-based superplastic alloys, the virtually. single-phase Zn - 0.4 wt % Al alloy and the two-phase Zn - Al eutectoid alloy. The first investigation is concerned with superplasticity in the Zn - 0.4 % wt Al alloy, while the second investigation is devoted to the effect of copper additions on the behaviour of the Zn - Al eutectoid superplastic alloy and the possible mechanisms operating during deformation. A zinc - 0.4 Al alloy was developed, which showed a remarkable degree of superplasticity at room temperature, and elongations of greater than 500% could be obtained at the relatively fast crosshead velocity of 0.1 inch/min. The strain-rate sensitivity (m) was found to increase with strain up to 300% on elongation (.35 →.5) parallel to the rolling direction. and it was also found to be anisotropic in the plane of the sheet. The strain ratio r was also strain dependent and varied in the plane of the sheet. At 90° to the rolling direction, the strain ratio increased from .35 to .75 after an elongation of 350%, that is, tending to unity (isotropy). Texture determination after straining showed a marked change and agreed qualitatively with the change in strain ratio. Grain growth occurred at room temperature, but was observed to be inhibited on superplastic deformation. Surface observations after deformation revealed that grainboundary sliding was taking place; fracture behaviour, though characteristically ductile in nature, varied with strain-rate. This alloy obeyed the Hall-Petch relationship above a certain critical grain size, below which it was inapplicable, due to the occurrence of superplasticity. Thin-foil transmission electron microscopy showed the importance of a dislocation recovery mechanism in the interpretation. of this deformation behaviour, and calculations based on the current theories of volume and grainboundary diffusion, grainboundary sliding and dislocation climb/recovery, showed that a grainboundary sliding/dislocation climb recovery model could reasonably predict the observed strain rate sensitivity and strain-rate. Biaxial tests confirmed the anisotropic properties observed in uniaxial tests and also in punch-stretching the effect of friction on cup height was contrary to that observed with workhardening metals. In the second investigation, measurements of the flow stress (σf) and strain-rate sensitivity (m) over a wide range of strain-rates (ε̇) and temperatures between -75° to 300°C have been made on a range of superplastic alloys, based on. the Zn - Al eutectoid, but with additions of up to 1% copper. These additions do not significantly affect σf or m above about 150°C. The peak in the m - ε̇ curve is not displaced by these copper additions at temperatures above 150°C. Increasing the grain-size (L) increases σf for temperatures above about 60°C but decreases σf below this temperature. With increasing grain-size, the peak in m-value moves towards lower ε̇. The dependence of σf vs. La showed that the exponent a was strain-rate dependent and varied, from 0.6 to 1 at 250°C. The exponent b in the relation ε̇ vs. 1/Lb was independent of stress and varied from 1.8 at 250°C to 3 at 20°C. The activation energy was grain-size and stress-dependent, and for the smallest grain size (.55 μm), a value of about 14 k cal/mole was obtained in the superplastic region, while a value of near 20 k cal/mole was obtained at the low ε̇ region. Creep-rate measurements at room-temperature showed that the secondary creep-rate decreased with increasing copper content by a factor of about 120 between 0 and 1.0% copper. Increasing the grain size from 0.55 μm to 1.75 μm decreased the creep-rate further by a factor varying from 10-50 times and thus an overall gain in creep resistance of 1200 times can be obtained. Uniaxial tension, torsion and camplastometer tests were used to cover strain-rates between 10⁻³ to 10⁴ min⁻¹ in order experimentally to determine the multistage σf - ε̇ curve. Hot-torsion stress-strain curves were typical of those shown in previous published work on hot working, and showed that a steady-state stress was obtained during superplasticity. In uniaxial tension, elongations of greater than 1000% were obtained at the very high initial strain-rate of 1.6 x 10¹ min⁻¹. Grain coarsening occurred during deformation and was found to be strain-rate and strain dependent. Thin-foil transmission electron microscopy showed the extensive activity of dislocations and diffusional processes. Dislocation-free structures and rounded interphase boundaries were observed under superplastic conditions. Also, though many of the grains appeared equiaxed after deformation, a variety of odd-shaped grains were observed, particularly in hot torsion, where there was evidence of accelerated spherodization. Fracture. behaviour was dependent on grain size, temperature and strain-rate sensitivity. Calculated σf - ε̇ curves, from current theories, suggested that a dislocation-climb-recovery/grainboundary sliding model, based on grainboundary diffusivity was the operating mechanism during superplasticity.