The Plastic Flow Behaviour of Aluminium Alloys Exhibiting Dynamic Strain Ageing

Licentiate thesis, 2006

The phenomenon known as dynamic strain ageing, abbreviated DSA, has been known since the beginning of last century. It was first discovered as serrations in the load-elongation curve obtained upon deforming specimens of certain alloys in tension. This phenomenon became known as the Portevin-Le Chatelier (PLC) effect. In connection with this it was also discovered that these alloys exhibit a negative strain rate sensitivity and often reduced ductility in the strain rate and temperature ranges where the PLC effect could be observed. It has since been shown that the negative strain rate sensitivity is actually the mechanism behind the PLC effect. DSA is now well documented and (at least rather well) understood under monotonic loading, but the effects of DSA on fatigue properties, in particular the development of the stress-strain response and fatigue life is not as well investigated. As the beginning of a project which aims to clarify these effects, an AA7030 alloy (Al-5.4Zn-1.2Mg) is investigated for the temperature and strain rate effects on the DSA and its influence on the monotonic mechanical properties in naturally aged and peak aged material. This is done in order to obtain a solid theoretical and experimental base for further studies of the cyclic stress-strain behaviour. It is shown that DSA occurs under a variety of conditions concerning temperature, strain rate and heat treatment, resulting in inverse temperature and strain rate dependence of the flow stress and ductility as well as serrated yielding. Even the peak-aged temper, nominally DSA insensitive, show some of these signs, which is important since this (T6) is the temper used in industrial applications. Due to the clear signs of DSA in both tempers, the material is considered suitable for further studies of the cyclic deformation properties. Furthermore, a thermal activation based model for plastic deformation is used to qualitatively explain the observed effects, and possible extensions of the model to suit the present material are discussed.