Fabrication and Mechanical Properties of Mg-Zn-La Amorphous Alloys Containing Nanoscale hcp-Mg Particles

An amorphous single phase and coexistent amorphous and hcp-Mg phases in the Mg-Zn-La system were found to form in the composition ranges of 20 to 40%Zn and 0 to 12%La and 12 to 20%Zn and 0 to 4%La, respectively. The hcp phase has an ellipsoidal morphology and the particle size and interparticle distance are in the range of 5 to 10 nm and 3 to 10 nm, respectively. The mixed phase alloys exhibit high mechanical strength combined with good ductility and the tensile strength (σ_B) and fracture elongation (ε_f) at 288 K are 675 MPa and 4.3% for Mg₈₅Zn₁₂La₃. The yield strength (σ_y) decreases from 570 to 150 MPa with increasing temperature from 288 to 368 K, accompanying an increase of ε_f to 50%. The maximum ε_f value is obtained near the temperature (≅365 K) at which the hcp Mg phase begins to precipitate. The largest values of σ_B and ε_f are considerably larger than those (600 MPa and 2.0%) for amorphous Mg-Zn-La alloys. The increase in σ_B by the formation of the mixed structure is presumably due to a dispersion hardening of the hcp supersaturated solution which has the hardness higher than that of the amorphous phase with the same composition.