Evidência da ocorrência de escorregamento de contorno de grão na deformação de uma liga AZ31 com grãos ultrafinos

Abstract

Magnesium is the lowest density metal capable of withstanding tensile and compressive loads. However, two mechanical characteristics prevent its wide application in transport and portable appliances to reduce weight: its low mechanical strength and ductility. An alternative to improving these two properties is to plastically deform through metal forming to achieve grain refinement. The Hall-Petch equation quantifies the dependence between grain size and mechanical strength in traditional mechanical metallurgy. It establishes that the smaller the grain size, the greater the resistance. However, the literature reports that grain refinement in pure magnesium can decrease mechanical strength when compared to coarser grains. However, this phenomenon still needs to be clarified in commercial alloys. Therefore, a detailed study on the influence of grain size, temperature and strain rate on mechanical strength was planned to elucidate the impact of these parameters on the mechanical strength of AZ31 alloy, which is one of the most commercially used magnesium alloys and is single-phase. The metal was processed by High-Pressure Torsion to achieve grains smaller than 1 µm. The results showed that the smallest grain size does not present the highest resistance at room temperature at different strain rates. Furthermore, the strain rate and temperature influence the mechanical resistance. The lower the strain rate and the higher the temperature, the easier it is to detect the loss of strength with grain refinement. These results validate the Grain Boundary Slip model for low temperatures as a mathematical relationship that best describes the relationship between grain size and mechanical strength for the AZ31 alloy. From this deformation model, it is possible to create a new processing window as temperature, strain rate and grain size must be considered to determine the yield strength.