Effects of Microalloying on Precipitation in Age Hardenable Al Alloys

A. TOLLEY

Cartagena

Congreso; XII Interamerican Congress of Microscopy CIASEM 2013; 2013

In age hardenable Al alloys, the mechanical behaviour depends on the distribution of second phase precipitates within the Al matrix. Microalloying additions have been found to alter the precipitation distribution significantly, thus influencing mechanical properties. In Al-Cu-Mg alloys, minor Ag additions have been found to strongly modify the precipitation processes (1,2). In this presentation, examples will be shown to illustrate the effects on the precipitation processes of minor additions of Ag in Al-Mg-Cu and of Si and Ge in Al-Cu-Mg and Al-Cu. Microstructural characterization using transmission electron microscopy was used to follow the precipitation process. Figure 1 compares the microstructure of Al-4%Mg-1,5%Cu and Al-4%Mg-1,5%Cu-0.5%Ag (wt.%). Whereas S phase precipitates are found together with small equiaxed precipitates in the former, only the small type of precipitates are formed in the latter. In Al-4%Cu-1,5%Mg (wt%) alloys, the main precipitation reaction involves the S phase. However, by adding minor additions of Si and Ge the microstructure corresponding to peak ageing at 190°C consists of ´ phase precipitates together with a high density of small, needle shaped precipitates (figure 2). In Al-Cu, the addition of Si-Ge is known to enhance the hardening response by stimulating the precipitation of the ´ phase (3). In this system, microalloying in combination with plastic deformation prior to ageing produces a faster hardening response that is not achievable if both processes are applied separately (figure 3). The microstructural characterization indicates that deformation enhances the formation of Si-Ge precipitates that in turn stimulate the precipitation of the theta´ phase that causes hardening. Summarizing, minor alloying additions may significantly alter the precipitate distribution in Al based age hardening alloys, offering interesting possibilities to tailor their mechanical properties. (1) J. H. Auld et al. Nature 209 (1966) 703. (2) H. D. Chopra et al. Phil.Mag.Lett. 71 (1995) 319. (3) D. Mitlin et al. Met. Trans. 34A (2003) 735.