Microstructural analysis of aluminium alloy 2096 as a function of heat treatment.

A detailed microstructural investigation into possible metallurgical culprits of stress corrosion cracking (SCC) of Al-Li-Cu alloy AA2096 was carried out as a function of isothermal aging time at 160°C. Two tempers were analysed for this study: under-aged at 4 hours (UA4) and under-aged at 12 hours (UA12). Transmission electron microscopy (TEM) was used to document T1 (Al2CuLi) precipitation near to and on low and high angle misorientation grain boundaries. The low-angle grain boundaries of the UA4 temper exhibited a fine, near continuous decoration of T1 precipitates and a narrow, but distinct T1 precipitate free zone (PFZ) adjacent to the boundary. The high angle boundaries of the UA4 temper either showed no grain boundary T1 precipitation, or contained a single, large T1 precipitate that grew along the boundary causing it to pucker. The low-angle boundaries of the UA12 temper exhibited no such PFZ, with no greater T1 precipitation than observed in the grain matrix. An energy dispersive X-ray (EDX) study was conducted to investigate the possible occurrence of copper solute depletion across grain boundaries. In the UA4 temper, the low-angle grain boundaries exhibited a distinct copped depleted zone that corresponded with the PFZ observed