No abstract

A set of unified constitutive equations is presented that predict the asymmetric tension and compression creep behaviour and recently observed double primary creep of pre - stretched/naturally aged aluminium - cooper - lithium alloy AA2050 - T34. The evolution of the primary micro - and macro - variables related to the precipitation hardening and creep deformation of the alloy during creep age forming (CAF) are analysed and modelled. E quations for the yield strength evolution of the alloy, including an initial reversi on and subsequent strengthening, ar e proposed based on a theory of concurrent dissolution, re - nucleation and growth of precipitates during artificial ageing . We present new observations of so - called double primary creep during the CAF process . This phenomenon is then predicted by introducing effects of interact ing microstructures , including evolving precipitates, diffusing solutes and dislocations , into the sinh - law creep model. In addition, concepts of threshold creep stress ??? ??? ??? and a microstructu re - dependant creep variable H , which behave differently under different external stress directions, are proposed and incorporat ed into the creep model . This enables predict ion of the asymmetric tension and compression creep - ageing behaviour of the alloy. Q uanti tative transmission electron microscopy (TEM) and related small - angle X - ray scattering (SAXS) analysis ha ve been carried out for selected creep - aged samples to assist the development and calibration of the constitutive model. A good agreement has been achieved between the experimental results and the model. The model has the potential to be applied to creep age form ing of other heat - treatable aluminium alloys

No abstract

The present work is concerned with the study of the relationship between microstructure and ductility of nanostructured bainite. The tensile behavior of two steels treated at the same temperature during different times has been analyzed. Special attention has been paid to the role that the retained austenite mechanical stability plays in enhancing the ductility through its contribution to the work-hardening and the damage resistance of these materials. The results have shown that the relative mechanical properties of the phases present affect both the martensitic transformation behavior and the total elongation.

The effect of Mo additions on the development of bainitic ferrite in hot rolled low carbon (0?05 wt-%C) Nb containing steel strips has been studied. The steel strips were fabricated by a combined process of controlled rolling and accelerated cooling. Microstructural characterisation and mechanical testing for the corresponding strips were investigated. The results indicated that a small amount of Mo addition (0?1-0?3 wt-%) causes the production of a high volume fraction of bainite, which undergoes significant secondary hardening after tempering treatment at 600uC for 1 h. It is noticeable that the secondary hardening effect provides an additional way to significantly increase the strength of low carbon Nb-Mo containing bainitic steels.