Monotonic and Cyclic Plastic Deformation of High-Strength Aluminium Alloys

Doctoral thesis, 2008

Dynamic strain ageing (DSA) and dynamic precipitation (DP) are two, closely interrelated, phenomena arising from the dynamic interaction between mobile solute atoms and dislocations. The occurrence of DSA and DP is confined to specific ranges of strain rate and temperature, and aluminium alloys are generally prone to experience DSA and DP already at ambient temperatures and conventional strain rates. Several other materials have been shown to exhibit DSA effects also under cyclic plastic deformation but aluminium alloys are relatively unexplored in this sense, as is the effects of DP during cyclic deformation. The present work aims to provide an insight into the relationships between microstructure, test conditions and DSA/DP in a high-strength AA7030 alloy (Al-5.4Zn-1.2Mg-0.3Cu) in low-cycle fatigue (LCF) testing. The effects are also investigated under monotonic tensile deformation to facilitate a comparison of the DSA/DP manifestations under different types of loading. The results show that AA7030 in the peak-aged condition is relatively insensitive to DSA/DP in the temperature range −20oC to 60oC, both under monotonic and cyclic deformation, although some trace effects can be seen. The initial cyclic hardening and subsequent softening is controlled by dislocation accumulation and strain localisation due to repeated shearing of precipitates. In the naturally (room temperature) aged temper on, the other hand, both DSA and DP have pronounced effects. DSA is responsible for the observed serrated yielding and inverse strain rate and temperature sensitivity of the yield stress and ductility during monotonic deformation, whereas the effects of DP are seen primarily through the anomalous strain hardening behaviour. Under LCF conditions, DP either caused continuous cyclic hardening until the on-set of crack growth, in spite of strain localisation, or initial hardening followed by a stress amplitude drop and subsequent saturation or softening, depending on the temperature. Some additional aspects of the plastic deformation of AA7030 was also investigated, such as comparison with an Al-Mg-Si alloy and the micro-plastic relaxation during unloading from peak stress in the LCF tests