Tension-compression asymmetry of stress-relaxation ageing behavior of AA2219 alloy over a wide range of stress levels

Abstract

The asymmetric stress-relaxation ageing behavior of AA2219 alloy has been experimentally investigated under both tension and compression loading conditions, over a wide range of stress levels at 165 °C for 11 h, using the stress-relaxation ageing tests, room-temperature tensile tests and transmission electron microscopy observations. The relaxed stresses in tension are much larger than those in compression and this deviation mostly originates from the initial rapid reduction stage of stress relaxation. With increasing initial stress, the yield strength in tension increases monotonously, while it remains basically unchanged with only a slight fluctuation of 8 MPa in compression. The mechanisms for asymmetric deformation and strengthening are unveiled on the basis of microstructural analysis. At low initial stress region, the yield strength under compression is higher as compared with that under tension, due to the GP zones and θ'' phases with larger size and weaker stress orientation effect, leading to more stronger hindering effects on the dislocation movement. At high initial stress region, massive dislocations caused by loading and main strengthening θ′ promoted by dislocations result in the larger yield strength under tension, exhibiting an unique reversed strength asymmetry in contrast to low stress level; meanwhile, these dislocations considerably accelerate the tensile stress relaxation process. In addition, the effect of tensile and compressive stresses on precipitate variants is quantitatively discussed based on the Eshelby inclusion theory. These findings benefit the deep understanding of tension-compression asymmetry and offer the theoretical basis for stress-relaxation ageing prediction models.