Microstructural Refinement and Property Enhancement of Cast ML5 Magnesium Alloy by Ti, Zr, and Hf Microalloying

This study investigates the effect of Ti, Zr, and Hf microalloying on the microstructure, microhardness, tensile properties, and high-temperature performance of cast ML5 magnesium alloy. The alloy was modified with 0.05, 0.10, and 1.0 wt.% Ti, Zr, or Hf, and the resulting alloys were characterized in the as-cast and heat-treated conditions. The effects of microalloying on grain size, secondary dendrite arm spacing, intermetallic phase morphology and volume fraction, matrix microhardness, tensile strength, elongation, and time to failure under prolonged loading at 150 °C and 80 MPa were evaluated. Ti, Zr, and Hf refined the cast structure and reduced the size of the intermetallic constituents. At 0.05–0.10 wt.%, the additions increased the absolute volume fraction of fine spherical intragranular intermetallic particles; however, the relative balance between spherical and plate-like constituents depended on the alloying element and concentration. At 1.0 wt.%, the plate-like fraction increased more strongly. All three elements increased matrix microhardness and tensile strength, but their relative effects were property-dependent: the increase in matrix microhardness ranked Ti < Zr < Hf, whereas the tensile-strength response after heat treatment ranked Ti < Hf < Zr at the investigated concentrations. For all three elements, elongation increased up to 0.10 wt.% and decreased at 1.0 wt.%. After heat treatment, ML5 containing 0.10 wt.% Hf exhibited a tensile strength of 260.0 MPa, an elongation of 4.9%, and a time to failure of 288.3 h at 150 °C and 80 MPa, compared with 231.8 MPa, 2.9%, and 141.4 h, respectively, for the reference alloy. The results provide a controlled comparison of the concentration-dependent effects of Ti, Zr, and Hf on the grain structure, room-temperature properties, and prolonged-loading performance of cast ML5, while the provenance of overlapping and newly obtained values is specified separately.