Effect of Sc, Hf, and Yb Additions on Superplasticity of a Fine-Grained Al-0.4%Zr Alloy

This research was undertaken to study the way deformation behaves in ultrafine-grained (UFG)-conducting Al-Zr alloys doped with Sc, Hf, and Yb. All in all, eight alloys were studied with zirconium partially replaced by Sc, Hf, and/or Yb. Doping elements (X = Zr, Sc, Hf, Yb) in the alloys totaled 0.4 wt.%. The choice of doping elements was conditioned by the possible precipitation of Al3X particles with L1(2) structure in the course of annealing these alloys. Such particles provide higher thermal stability of a nonequilibrium UFG microstructure. Initial coarse-grained samples were obtained by induction casting. A UFG microstructure in the alloys was formed by equal-channel angular pressing (ECAP) at 225 degrees C. Superplasticity tests were carried out at temperatures ranging from 300 to 500 degrees C and strain rates varying between 3.3 x 10(-4) and 3.3 x 10(-1) s(-1). The highest values of elongation to failure are observed in Sc-doped alloys. A UFG Al-0.2%Zr-0.1%Sc-0.1%Hf alloy has maximum ductility: at 450 degrees C and a strain rate of 3.3 x 10(-3) s(-1), relative elongation to failure reaches 765%. At the onset of superplasticity, stress (sigma)-strain (epsilon) curves are characterized by a stage of homogeneous (uniform) strain and a long stage of localized plastic flow. The dependence of homogeneous (uniform) strain (epsilon(eq)) on test temperature in UFG Sc-doped alloys is increasing uniformly, which is not the case for other UFG alloys, with epsilon(eq)(T) dependence peaking at 350-400 degrees C. The strain rate sensitivity coefficient of flow stress m is small and does not exceed 0.26-0.3 at 400-500 degrees C. In UFG alloys containing no Sc, the m coefficient is observed to go down to 0.12-0.18 at 500 degrees C. It has been suggested that lower m values are driven by intensive grain growth and pore formation in large Al3X particles, which develop specifically at an ingot crystallization stage.