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

Abstract A series of alloys designed along a Ti 9 (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 Ti 55 Zr 20 (Ni,Co) 25 to Ti 34.6 Zr 50 (Ni,Co) 15.4 with Ni/Co ratio equal to 3/1, and in a sharply diminished region from Ti 48 Zr 30 (Ni,Co) 22 to Ti 42 Zr 40 (Ni,Co) 18 with decreased Ni/Co ratio equal to 2/2 or 1/3. Ti 48 Zr 30 Ni 16.5 Co 5.5 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 Ti 2 Ni, fcc-Zr 2 Ni, 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.