Precipitation strengthening at ambient and elevated temperatures of heat-treatable Al(Sc) alloys

Yield strength at ambient temperature and creep resistance between 225 and 300°C were investigated in dilute Al(Sc) alloys containing coherent Al₃Sc precipitates, which were grown by heat-treatments to radii in the range 1.4-9.6 nm. The dependence of the ambient-temperature yield stress on precipitate size is explained using classical precipitation strengthening theory, which predicts a transition from precipitate shearing to Orowan dislocation looping mechanisms at a precipitate radius of 2.1 nm, in good agreement with experimental data. At 300°C creep threshold stresses are observed and found to be much lower than the yield stresses, indicative of a climb-controlled bypass mechanism. The threshold stress increases with increasing precipitate radius, in qualitative agreement with a climb model taking into account stiffness and lattice mismatches between matrix and precipitates.