Achieving superplastic elongations in an AZ80 magnesium alloy processed by high-pressure torsion.

Abstract

Processing magnesium alloy by High-Pressure Torsion (HPT) is a technique used successfully to refine the grains of an alloy to the submicrometer and nanometer scale to produce an ultrafine grained microstructure. Grain refinement can improve the mechanical properties of magnesium alloys as well as enhancing its ductility and providing a potential for exhibiting superplastic behaviour at elevated temperature. Research was conducted to process the AZ80 magnesium alloy by HPT at room temperature for different numbers of turns with the microstructures before and after HPT critically investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers microhardness (Hv) tests. Subsequently, tensile specimens were cut from the processed disks and pulled in tension to failure at temperatures of 473, 523 and 573 K and at strain rates in the range from 1.4 × 10 4 to 1.4 × 10-¬1 s-1. The introduction of superplasticity in the AZ80 magnesium alloy when processing by HPT was demonstrated for the first time with a maximum elongation of 645% at a testing temperature of 573 K. There was also evidence for low temperature superplasticity with an elongation of 423% at 473 K. The dominant mechanism for superplastic flow was grain boundary sliding with a strain rate sensitivity of m ≈ 0.5 and an activation energy of Q ≈ 73 kJ mol-1.