Transmission Electron Microscopy of Precipitate Free Zones in Aluminium Alloys Subjected to Uniaxial Compression

Efficient design of structures and prototypes made from age-hardenable aluminium alloys requires detailed knowledge about the various deformation mechanisms at play. Most well-known is the precipitate hardening mechanism in which metastable precipitates formed during annealing provide obstacles for dislocations. In Al-Mg-Si alloys the main hardening phase is β'' that appear as long needles along <100> Al. The formation of precipitates requires vacancies and solute. However, grain boundaries (GBs) deplete their surroundings of solute and vacancies, thus preventing precipitates to form. This leads to soft and narrow precipitate free zones (PFZs) [1] and the deformation mechanisms in these zones are less well known. It is thought that the combination of this weak zone and large GB precipitates promotes intergranular fracture through enhanced GB void nucleation and growth [2]. However, to understand ductile fracture better, more details regarding the mechanisms occurring in PFZs during deformation are required.

In this work, cylinders machined along the transverse direction of extruded plates of three different Al-Mg-Si alloys have been subjected to uniaxial compression and studied by transmission electron microscopy (TEM). TEM results from specimens with observation direction normal to the compression-extrusion plane will be presented and discussed.

[1] J.D. Embury and R.B. Nicholson, Acta Metallurgica 13, 403-417 (1965).

[2] A.K. Vasudévan and R.D. Doherty, Acta Metallurgica 35, 1193-1219 (1987).