Effect of Co substitution for Ni on the Ti–Zr–(Ni, Co) pseudo-ternary quasicrystal formation

A series of alloys designed along a Ti9(Ni,Co)4–Zr e/a-variant line in the Ti–Zr–(Ni, Co) pseudo-ternary alloy system, are investigated by XRD and TEM. Experimental results show that bulk icosahedral quasicrystals (IQC) prepared by copper mold suction casting, can be obtained in a large region from Ti55Zr20(Ni,Co)25 to Ti34.6Zr50(Ni,Co)15.4 with Ni/Co ratio equal to 3/1, and in a sharply diminished region from Ti48Zr30(Ni,Co)22 to Ti42Zr40(Ni,Co)18 with decreased Ni/Co ratio equal to 2/2 or 1/3. Ti48Zr30Ni16.5Co5.5 is the optimal quasicrystal-forming composition, where a nearly pure bulk IQC phase is achieved. The stepwise substitutions of Co for Ni in this pseudo-ternary system contribute to the formation of Ti2Ni, fcc-Zr2Ni, C14-type Laves and β-(Ti, Zr) solid solution phases. However, these substitutions greatly reduce the forming ability of the IQC phase. It is suggested that Co is essentially unfavorable to Ti/Zr-based quasicrystal formation.