Self healing aluminium

In recent positron annihilation spectroscopy studies we have monitored the time-dependent precipitation process responsible for self healing in underaged Al-Cu-Mg alloys in-situ during deformation and annealing [1-4]. Positrons are a uniquely suited for these in-situ studies as they have a strong affinity for open volume defects when they are implanted in a material. During their life time of 100-500 ps they are generally trapped at defect sites until they annihilate with an electron generating two 511 keV gamma particles. Simultaneous measurements of the positron life time, characteristic for the defect concentration, and the Doppler broadening, reflecting the momentum distribution of the electrons involved in the annihilation, gives a detailed insight in the concentration and chemical composition of the defect sites. The Doppler broadening gives a finger print of the chemical environment at the annihilation site. In its simplest form, two parameters S and W are derived from the Doppler broadened 511 keV gamma s, where parameter S reflects the spectral weight of the low-momentum electrons and W for the high-momentum electrons. 

In Fig. 1 the Doppler-broadening data for the low- and high-electron momenta are shown in a (S,W) plot. The positron data measured in the underaged (UA) alloy indicate the existence of positron-trapping sites with high local Cu content. Immediately after plastic deformation the concentration of open-volume defects is high in both the fully precipitated and UA material. During ageing the character of the positron traps is observed to change depending on the thermal pretreatment of the sample. In contrast to the fully precipitated T3 material, in the UA alloy the rapid initial diffusion of retained solute atoms to the deformation-induced open-volume defects yields final positron parameters close to those of the undeformed alloys. This shows that by interrupting the conventional heat treatment a significant number of solute atoms remains in solution and that these can be driven to and associate with the deformation-induced open-volume defects, a requirement for the successful implementation of self-healing in age-hardenable alloys.

Figure 1: High-momentum parameter W as a function of the low-momentum

Parameter S for the undeformed underaged (open circles), deformed underaged (open triangles) and deformed T3 (solid circles) aluminum alloy samples obtained from positron annihilation spectroscopy experiments. The arrows next to the data show the direction of the development of the positron parameters during ageing at room temperature of the underaged alloy. (Open squares) S, W points for defect-free Al, and an Al vacancy. The location (outside the range of the figure) of the S, W points for Cu and Mg is indicated [1].

Publications on self healing in Al alloys:

Review on self healing materials: