Effects of Ag content on the micro-yield strength and dimensional stability of Al-Cu-Li-Sc-(Ag) alloys

To improve the dimensional stability of Al-Cu-Li-Sc-(Ag) alloys and identify the underlying micro-yield mechanism, the effects of silver (Ag) content (0, 0.2, 0.4, 0.6 and 0.8 wt%) on the microstructure, micro-yield behaviour and dimensional stability of Al-Cu-Li-Sc-(Ag) alloys were investigated by transmission electron microscopy (TEM), cyclic load-unload test and thermal cycle test. The results showed that increasing the Ag content promoted the precipitation of the T-1 and delta' phases in the alpha-Al matrix and decreased their diameters, which resulted in the increase of the micro-yield strength and strain hardening rate of the Al-Cu-Li-Sc-(Ag) alloys. Because T-1 and delta' precipitates can effectively pin the mobile dislocations and impede their movement, the critical resolved shear stress (CRSS) resulting from the pinning of mobile dislocations by T-1 and delta' precipitates played a critical role in the micro-yield behavior. There existed a relation between sigma(mys) and tau(CRSS) similar as that of macro-yield strength, sigma(mys) = alpha center dot tau(CRSS) + sigma(0)'. However, the low alpha value in the micro-yield stage compared with the macro-yield stage indicated that fewer activated slip systems would be enough to produce a residual strain of 1 x 10(-6) at the beginning of the micro-yield stage. The thermal cycle decreased the residual relative length (RRL) of the alloys and the stable RRL after the thermal cycle could serve as an indicator of the thermal dimensional stability of the alloys. Increasing the Ag content decreased the stable RRL of the Al-Cu-Li-Sc-(Ag) alloys and hence improved their thermal dimensional stability.