Comportamento mecânico, evolução microestrutural e estabilidade térmica da liga Al-3%Mg processada por torção sob alta pressão em diferentes temperaturas

Abstract

The Severe Plastic Deformation (SPD) processes are used in polycrystalline materials with the objective of promoting the refining of grains until they reach average sizes in the submicrometric order (<1 µm). Consequently, SPD techniques are responsible for changing physical and mechanical properties of these materials such as increasing mechanical strength and diffusivity at low temperatures. Among the SPD techniques is High-Pressure Torsion (HPT), through which compressive and torsional stresses are imposed on specimens in the form of disks, positioned between the two dies of the equipment. Recent investigations have revealed that metals processed by HPT at room temperature exhibit poor thermal stability due to the occurrence of recrystallization during annealing at relatively low temperatures. Experiments were conducted to examine the effect of processing temperature on the hardness evolution, microstructure, and the flow properties of an annealed Al-3Mg alloy processed by high-pressure torsion (HPT) at either 300 or 450 K. The results demonstrate that HPT processing at room temperature (RT) leads to higher microhardness values with a more uniform hardness distribution in the Al alloy compared with processing at 450 K. After 10 HPT turns at RT, the microstructure displayed a more intense grain refinement, a higher dislocation density and smaller Al3Mg2 precipitates than after HPT at 450 K. Accordingly, the metal processed at RT showed enhanced strength at RT and consistently exhibited superplastic flow during deformation at 523 K, with tensile elongations of ~530-650 % for strain rates from 10-3 to 10-2 s -1 . Conversely, a maximum elongation of ~110 % was recorded at 523 K in the alloy processed by HPT at 450 K. This behaviour is attributed to an enhanced thermal stability in the metal processed at RT compared with HPT at 450 K where deformation at 523 K led to the onset of abnormal grain coarsening due to a more disperse distribution of grain sizes and the partial dissolution and coalescence of Al3Mg2 precipitates after HPT processing.