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