Effect of Mg microalloying on elevated-temperature creep resistance of Al–Cu 224 cast alloys

The influence of microalloying with Mg (0–0.36 wt%) on the elevated-temperature strength and creep resistance of Al–Cu 224 cast alloys was investigated. The Mg-containing alloys yielded finer and denser θ′ precipitates after high-temperature stabilization (300 °C/100 h) than the Mg-free alloy, which improved the yield strength at both room temperature and 300 °C. Compressive creep tests were conducted at 300 °C over a wide range of the applied stresses (45–100 MPa). The results showed that the creep resistance increased with increasing Mg content at any fixed applied stress, and the alloy with 0.36 wt% Mg exhibited the best creep resistance among the four alloys studied. During creep deformation, the θ′ precipitates continued to coarsen. However, Mg microalloying greatly stabilized the θ′ precipitates in the matrix by slowing down the coarsening process. An obvious break was observed in the creep rate curves with different stress exponents (n). Below the break in the low-stress regime (LSR), stress exponent (n) was ∼3, whereas it increased to 9–12 above the break in the high-stress regime (HSR). In the LSR, the creep was controlled by the dislocation climbing, and it transformed into the Orowan looping in the HSR.