Positron Annihilation Studies of Hydrostatically Extruded AA1050 Aluminum.

Abstract

AA1050 aluminum was hydrostatically extruded at room temperature to true strains of 0.9 and 3.2, and at cryogenic temperature to a true strain of 0.9. As a result of the extrusion process, the yield strength (YS) increased by 130-160% to 120-130 MPa, and the ultimate tensile strength (UTS) rose by 64-81% to 125-140 MPa. The hardness reached 46-49 HV. YS and UTS values correspond to mechanical properties typical of the H6 or H8 temper designations, with unusually high elongation at break ranging from 15% to 16.4%. Differences in lattice parameters, crystallite size, and lattice strain between samples deformed under various conditions-as well as those annealed after deformation-were within the margin of measurement uncertainty. This indicated that differences in defect density between the samples were relatively small, due to dynamic recovery occurring during extrusion. However, positron annihilation spectroscopy demonstrated that the cryo-cooled material extruded at a true strain of 0.9, as well as the one extruded at RT at a true strain of 3.2, exhibited significantly higher mean lattice defect concentrations compared to the sample extruded at RT at a true strain of 0.9. The predominant defects detected were vacancies associated with dislocations. The extrusion parameters also significantly affected the crystallographic texture. In particular, they altered the relative proportions of the <111> and <100> components in the axial texture, with the <100> component becoming dominant in cryogenically extruded samples. This trend was further intensified during recrystallization, which enhanced the <100> component even more. Recrystallization of the deformed materials occurred in the temperature range of 520-570 K. The activation energy for grain boundary migration during recrystallization was estimated to be approximately 1.5 eV.