Revisit the stress-orienting effect of θ′ in Al-Cu single crystal during stress aging

Three-dimension precipitation distribution of 8'-precipitates was established to revisit the stress-orienting effect of theta'-phase in Al-Cu single crystal during stress aging, via determining the microstructure observed along two different < 001 >(Al) zone axes. The results show that the theta'-precipitates prefer to precipitate on the habit planes parallel to the loading orientation, rather than the habit planes perpendicular to the loading orientation proposed in current views. Two opposite models of the stress-orienting effect on theta'-phase are compared and discussed in detail. They show the same aligned theta'-precipitate structure while observed perpendicular to the loading direction (i.e. [100](Al) or [010](Al)). The crucial difference actually lies in the microstructure observed along the loading orientation (i.e. [001](Al)), which shows an increasing tendency in the number density of theta'-precipitates with increasing applied stress. This result further proves that the theta'-precipitates prefer to precipitate on the habit planes parallel to the loading orientation. In addition to the aligned theta'-precipitate structure, the fact that the number density of theta'-precipitates is increasing can be also treated as the other sign of the stress-orienting effect.