Chill-cast in situ composites in the pseudo-ternary Mg-(Cu,Ni)-Y glass-forming system: Microstructure and compressive properties

Starting with a bulk metallic glass-forming alloy Mg₆₅ Cu₁₈Ni₆Y₁₁, we prepared in situ composites by increasing the Mg content in a series of alloys, Mg_x(Cu_0.51Ni_0.17 Y_0.32)_100-x (65 ≤ x ≤ 90), via copper mold casting of rods 4 mm in diameter. The fully glassy alloy at x = 65 showed a compressive fracture strength of 755 MPa but no observable macroscopic plasticity prior to failure. Metallic glass-based composites were formed when the Mg content was increased. For x > 80, the glassy phase no longer existed in the as-cast rods. In the composition range of 80 ≤ x ≤ 85, needle-shaped Mg solution with a 14H-type long period stacking (LPS) structure appeared as the primary phase in the as-cast microstructure. On further increase of the Mg content up to x = 90, the solidified primary phase became 2H-Mg, coexisting with the remaining eutectic structure. The best combination of mechanical properties was obtained for the alloy at x = 81.5, which showed a fracture strength of 665 MPa and a compressive plastic strain of 11.6%. The specific strength of this alloy was 2.8 × 10⁵ N m kg^-1, much higher than conventional cast magnesium alloys. The mechanical properties are discussed in light of the phase selection and microstructural features uncovered in microscopy examinations.