Effect of minor Gd addition on the microstructure and creep behavior of a cast Al-15Mg2Si in situ composite

The influence of 0.5 wt% Gd addition on the microstructure and creep behavior of a cast Al-15Mg2Si composite was studied by means of compression tests under different constant stresses in the range of 50-250 MPa, corresponding to the modulus-compensated stress levels of 0.002 ≤ σ/G ≤ 0.011, in the temperature range of 473-563 K. Analysis of the data showed that for all loads and temperatures, the Al-15Mg2Si-0.5Gd composite had lower creep rates, and therefore, higher creep resistances than the base composite. This is ascribed to the morphological modification of the primary Mg2Si particles, refinement of the eutectic structure, significant reduction in the size and volume fraction of the primary Mg2Si particles, and the formation of the fine thermally stable MgGd intermetallic phase at the grain boundary areas. Based on the obtained stress exponents of 4.6-5.9 and stress-dependent activation energies, which are close to that of the lattice self-diffusion in pure aluminum, it can be proposed that the dominant creep mechanism in the present composites is stress-assisted dislocation climb controlled by lattice self-diffusion.