Superplastic Zn-22 Research Articles

超塑性Zn-22%Al合金にCuとMgを添加した場合の強化機構

The effects of the addition of 0.5%Cu and 0.01%Mg into the superplastic Zn-22%Al alloy (SPZ·CM) on the mechanical properties and the deformation behaviour were investigated.The results obtained were as follows:(1) The temperature of the nose of TTT diagram for the SPZ·CM was atout 50°C higher than that of the binary eutectoid alloy (SPZ), but the starting time of transformation was nearly equal. The dissolution into two phases at room temperature started 20 min after solution treatment and water quenching and lasted about 2 hr. The aging phenomenon that grains grew with aging time after the dissolution was found similarly in the case of SPZ.(2) This alloy was hardened and strengthened with aging time at room temperature, and this alloy did not show softening as in the case of SPZ.(3) Superplastic elongation more than 200% for the SPZ·CM was obtained above 200°C, but such a large elongation was not observed at room temperature independently of the change of microstructure or grain size.(4) Cu and Mg added were mainly dissolved in the β-phase and the β-phase was strengthened. The β-phase was precipitated continuously at the boundary of the mother phase called α′. Therefore, the strength of SPZ·CM was larger than SPZ.(5) The α′-boundary was destroyed by the instantaneous cold-rolling after the solution treatment. The precipitation of the β-phase was dis-continuous so that the flow stress was reduced and the ductility was improved.

The influence of strain rate on ductility in the superplastic Zn–22% Al eutectoid

Abstract There is an optimum range of strain rate for maximum ductility in the superplastic Zn–22% Al eutectoid, and the elongation at fracture decreases at both low and high strain rates. For a testing temperature of 473 K and a grain size of 2·5 μm, tensile specimens deform to strains in excess of 2000% over almost two orders of magnitude of strain rate.

Creep at low stress levels in the superplastic Zn-22% Al eutectoid

A sigmoidal relationship between strain rate and stress was observed in a superplastic Zn-22% Al eutectoid alloy with grain sizes in the range of 2.1–7.5 μm. The relationship was independent both of the testing technique employed (whether constant stress or constant strain rate) and of the mode of deformation selected (whether tensile or shear).In the superplastic region (strain rates of ~10−5-10−2 sec−1), the stress exponent was ~2.25, the exponent of the inverse grain size was ~2.3, and the activation energy was close to that for grain boundary diffusion. These results are in good agreement with the predictions of a model based on grain boundary sliding accommodated by the climb of dislocations into boundaries. At very low strain rates ≲10−5 sec−1), the stress exponent was ~4.1, the exponent of the inverse grain size was ~2.4, and the activation energy was close to that for volume self-diffusion. These results are not consistent with any of the existing deformation mechanisms, but suggest that the sigmoidal relationship may arise through the sequential operation of two different processes

Microstructure of superplastic alloys

Abstract Studies are described which involve the routes to the equiaxed fine structure needed for superplastic behaviour in two-phase Zn–Al alloys. These indicate that the development of a spinodal structure is not an essential step towards achieving a superplastic structure in this alloy. Some microstructural aspects and mechanical behaviour characteristics of superplastic Zn–Al alloys are presented which suggest that the major mechanism of deformation in these alloys is crystallographic slip accompanied by grain boundary migration or recrystallization.