Maximum Strain Rate Sensitivity Research Articles

Factors influencing ductility in the superplastic Zn-22 Pct Al eutectoid

The maximum attainable ductility in the superplastic Zn-22 pct Al eutectoid depends critically on the imposed strain rate, the testing temperature, and the initial grain size. High ductilities are observed at intermediate strain rates, and there is a decrease at both higher and lower rates of strain. It is shown that i) the maximum ductility occurs at higher strain rates as the temperature is increased and/or the initial grain size is decreased, and ii) the maximum attainable ductility increases with increasing temperature and/or decreasing initial grain size. For specimens tested at different temperatures, similar macroscopic fracture characteristics are observed in specimens exhibiting a similar maximum flow stress. The experimental trends are qualitatively explained by relating maximum ductility to the maximum strain rate sensitivity and examining the influence of cavitation on the time to rupture.

Relative motion of grains during superplastic flow in an Al-9Zn-1 wt.%Mg alloy

The topological feature of the relative motion of grains during superplastic flow of an Al-9.3Zn-1.03Mg-0.22 wt.%Zr alloy has been examined by the translation of reference markers in the surface and interior of specimens. Microstructural evidence was obtained for grain-switching events based on the relative motion of a group of four grains. The translation of the grains seemed to occur by sliding and rotation. On the basis of the results of a texture study, accommodation strains required in this process are attributed predominantly to slip. Hence, the role played by slip in deformation at maximum strain-rate sensitivity is particularly significant for this alloy.

The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 μm) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures ≥500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.

The deformation behaviour of a dispersion strengthened superplastic zinc alloy

The deformation behaviour of a new dispersion strengthened superplastic zinc alloy was investigated. A significant long range internal stress was observed at all strain-rates. The activation volume of deformation decreased very rapidly with a decrease in the true effective stress. The maximum strain-rate sensitivity corresponds to a region of change from this high stress dependence of the activation volume to a much lower stress dependence. The observation of a metallographic halo effect shows that apart from dislocation movement, diffusive creep plays a role during superplastic deformation. It is stipulated that both these processes aid boundary sliding which accounts for the largest part of the strain.

Some observations of diffusional flow in a superplastic alloy

A metallographic study has been made of the denuded zones from diffusional flow in a hydrided alloy of magnesium with 6 wt pct Zn and 0.5 wt pct Zr (ZK 60). At the deformation temperature of 450°C, with an initial grain size of 18 um, the alloy was superplastic. Its maximum strain-rate sensitivity,m = (d log σ)/(d log ∈), was ⊥0.6 at fe –4 10~4 sec–1. A diffusional strain, ed = ΔLd/Ld, was calculated from measurements of zone thickness, ΔLd, and interzone distance,L d. This was a reasonably constant fraction of the total strain,e, up to the limiting strain of ⊥220 pct. Over a strain-rate range of 3.3 x 10–3 sec–1 to 3.3 x 10⊥5 sec–1,e d /e increased from about 0.05 to 0.30.