The effect of interrupted ageing on the strength and elongation of precipitation hardened aluminium alloys

Yield strength and elongation are both important properties of engineering alloys, but they typically exhibit an inverse correlation and engineers are forced to design around this constraint in applications of the alloys. It was recently reported that precipitation hardenable Al alloys subjected to a new type of heat treatment called ‘interrupted ageing’ exhibited simultaneous increases in both yield strength and elongation, i.e. the inverse correlation was broken. This finding is of considerable technological significance but the physical origin of the effects is not currently understood. The objective of this study is to develop a quantitative explanation for the simultaneous enhancement of yield strength and uniform elongation in age hardenable Al alloys subjected to the interrupted ageing schedule. The investigation of the simultaneous improvement of yield strength (y) and uniform elongation (u) in a model Al-4Cu-0.05Sn (wt. %) alloy has been the main focus of this work and a quantitative model explaining the improvements has been developed. The experiments confirm the reports of Lumley et al. that simultaneous improvements in both y and u are possible through the interrupted ageing procedure. Low temperature ageing of the model Al-4Cu-0.05Sn alloy induces the precipitation of a fine distribution of shearable GP zones with no major change in the precipitate distribution that is formed in the elevated temperature step of the interrupted ageing schedule. Tensile tests demonstrate that the strain hardening behaviour of the samples with and without GP zones is approximately the same at low strains (<6%). However, at strains greater than 6%, the samples containing GP zones clearly exhibit enhanced strain hardening and this results in the enhanced elongations experimentally measured. Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) were used to demonstrate that strain-induced dissolution of GP zones occurs during deformation and this leads to a dynamic repartitioning of Cu into solid solution during straining. A quantitative model has been developed to describe the strain hardening behaviour for the Al-Cu alloy after interrupted ageing that includes the effects of strain-induced GP zone dissolution. It is shown that the effect of the dynamically repartitioned Cu in solid solution on the dislocation-dislocation junction strength is able to quantitatively explain the enhanced strain hardening observed experimentally and the resulting enhanced uniform elongation. The model predictions are also qualitatively consistent with experimental observations of the temperature and strain-rate dependence of the deformation behaviour of the alloys. This ‘strain-induced dissolution’ effect requires two key characteristics: a) strain-induced dissolution of shearable precipitate must occur in the strain range of interest and, b) the repartitioned solutes must affect the strain hardening of the matrix in a substantial way. In order to further test this proposed mechanism, the commercial 6061 alloy has also been investigated after interrupted ageing. Experimentally it is observed that whilst an increase in the yield strength results from the interrupted ageing schedule, no significant increase in uniform elongation is observed, in comparison to material subjected only to the primary ageing condition. DSC results suggest that strain-induced dissolution of GP zones does occur during deformation of interrupted aged 6061. Tensile results show that the effect of the dynamically repartitioned Si and Mg on the strain-hardening behaviour of the matrix is comparatively small compared with Cu. Therefore, even though strain-induced dissolution does accompany deformation, the small effect of Si and Mg on the strain hardening behaviour is probably the reason for the absence of the simultaneous improvement in both yield strength and uniform elongation as observed in the Al-4Cu-0.05Sn alloy. Further studies has been carried out to test the potential possibility of simultaneous improvements in y and u in other age hardenable Al alloys (e.g. 7xxx Al alloys) after interrupted ageing. Tensile results show that the effect of Zn (up to 5 wt. %) in solid solution probably does not influence significantly the strain hardening behaviour of Al, suggesting that simultaneous improvements in strength and elongation of 7xxx series alloys are unlikely. This appears consistent with literature reports of no simultaneous improvement in the yield strength and elongation for the 7050 alloy.