The synergistic mechanism of mechanical properties and degradation rate of as-extruded Mg-Bi-Sc alloy by controlling the evolution of BiY phase by Y content

Yttrium (Y) is an effective alloying element for tuning the overall performance of magnesium (Mg) alloys. However, the optimal content of Y in low-alloyed magnesium alloys and the synergistic mechanism between degradation rate and mechanical properties is still unclear. The current study investigates the effects of Y addition on the microstructure, mechanical properties and degradation rate of as-extruded Mg-1Bi-0.5Sc alloy in 3.5 wt.% NaCl solution. Studies have shown that the addition of Y promotes the formation of BiY with a semi-coherent relationship with the matrix, and changes from strips to bulky blocks. Tensile evaluations reveal that the strength and plasticity of the alloy increase synchronously with the increase of Y content to 0.8 wt.%, which is attributed to the activation of non-basal slip. However, at a Y content of 1.0 wt.%, the coarse bulk BiY phase acts as a stress concentration source, resulting in a significant decrease in plasticity. The corrosion mechanism analysis shows that an appropriate amount of Y (0.6 wt.%) promotes uniform corrosion and helps to form a dense corrosion product film rich in protective oxides such as Y 2 O 3 , which reduces the degradation rate of the alloy. However, excessive Y (0.8 - 1 wt.%) can form a large number of coarse cathodic BiY phase, which aggravates the micro-galvanic corrosion and damages the integrity of the protective film thereby accelerating the degradation rate of the alloy.