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 = ΔL_d/L_d, was calculated from measurements of zone thickness, ΔL_d, 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.