The role of processing temperature for achieving superplastic properties in an Al-3Mg-0.2Sc alloy processed by high-pressure torsion.

Abstract

Experiments were conducted to systematically assess the superplastic properties and the microstructural changes of an Al-3Mg-0.2Sc alloy processed by 10 HPT revolutions at room temperature (RT ≈ 300 K) or at 450 K after subsequent tensile testing at temperatures from 473 to 723 K over a wide range of strain rates. The HPT processing at RT led to the development of elongated grains with an average size of ~140 nm whereas the grain structures were equiaxed and slightly larger (~150 nm) after HPT at 450 K. After HPT processing at RT, the Al-Mg-Sc alloy exhibited true superplastic flow at low homologous temperatures and attained a maximum elongation of ~850 % at 523 K. Nevertheless, the elongations decreased at temperatures T ≥ 623 K and an elongation higher than 400 % was only achieved at 673 K for a strain rate of ε ̇ = 4.5 x 10-3 s-1. The material processed by HPT at 450 K displayed superior microstructural stability and substantially higher superplastic ductilities. Elongations of > 1100 % were attained at 673 K for strain rates from 3.3 x 10-4 to 1.0 x 10-1 s-1 and a record elongation of ~1880 % for an HPT-processed metal was achieved at 1.5 x 10-2 s-1 at 673 K. High strain rate superplasticity was also reached for an extended range of temperatures and strain rates. Analysis of the data confirms a stress exponent of ~2 which is consistent with superplastic flow by grain boundary sliding accommodated by dislocation glide and climb.