Tensile Creep Behavior and Microstructure Evolution of As-Cast Mg–9.82Gd–0.38Zr Alloy at 250 °C

Herein the tensile creep behavior and microstructure evolution of an as-cast Mg–9.82Gd–0.38Zr alloy under different stresses at 250 °C are investigated. The results of creep test show that the creep strain and steady creep rate increase with the creep stress at 250 °C. The fitted stress exponent n value (4.4) and transmission electron microscopy analysis suggest that the steady creep is dominated by the dislocation climb and glide mechanisms together. At the primary stage, due to the segregation of the Gd content, a fine-strengthening β′ phase precipitates near the α-Mg grain boundary. Then the transformation of β′ precipitates to a needle-like β phase, coarsening, and growth of the β phase toward the intragranular direction gradually deteriorate the creep resistance of the experimental alloy. Moreover, two kinds of precipitate-free zones (PFZs) appear at the end of the secondary creep stage and the PFZs nearly perpendicular to the applied stress direction widen gradually because of the directional diffusion. The nucleation and propagation of creep crack along the cracked eutectic phase and PFZs lead to the greatly rising creep rate at the end of the tertiary creep stage, which eventually results in the intergranular creep fracture of the experimental alloy.