Cluster-based Composition Criteria Of Ternary Quasicrystals And Their Application In Ti-Zr-TM Alloy Systems

This work is dedicated to quantitative composition criteria for quasicrystals and to their application in designing new Ti-Zr based quasicrystalline alloys.First, by analyzing the cluster structures of binary approximants in typical ternary quasicrystalline systems and the distribution features of the clusters in the phase diagrams, we point out that the starting point of an e/a-variant line essentially corresponds to a particular binary cluster. The e/a-variant line can thus be defined exactly by linking the cluster with the third constituent, from which quantitative composition criteria can be established: a ternary quasicrystal lies at the intersection of two e/a-variant lines. The new criteria resolve the problem of the exact location of the e/a-variant lines.The criteria are then applied to Ti-Zr-TM(TM=Ni,Co,Cu,Si) quasicrystalline alloy systems. Samples are prepared by Cu-mold suction casting. From XRD, DSC and TEM experimental analysis the following results are obtained:· The Ti-Zr-Ni alloy system has a large quasicrystal-forming area, with 10~70at.%Ti, 10~70at.%Zr and 10~30at.%Ni. A single quasicrystalline phase forms in the intersection of the Ti9Ni4-Zr and Zr9Ni4-Ti e/a-variant lines, which shows that the formation of the Ti-Zr-Ni quasicrystal is related to the Ti9Ni4 and Zr9Ni4 clusters.· Co plays a negative role for quasicrystal formation. Substitution of Ni by Co deteriorates the Ti-Zr-Ni quasicrystal forming ability. In the alloys along the Ti9(Ni,Co)4-Zr line, as-cast quasicrystals form only from Ti42Zr40(Ni,Co)18 to Ti48Zr30(Ni,Co)22, the former lying at the intersection between the Ti9(Ni,Co)4-Zr and the Zr9(Ni,Co)4-Ti lines, and the latter, which is the ideal composition, lying at the intersection between the Ti9(Ni,Co)4-Zr and the Zr7(Ni,Co)6-Ti lines.· Minor alloying element Si benefits the quasicrystal formation in Ti-Zr-TM(TM=Ni, Co). The volume fraction of the icosahedral quasicrystal is increased in the Ti40Zr42Ni4.5Co13.5 alloy by adding 1~4at.% Si. In the Ti-Zr-Co ternary system, quasicrystals are formed in Ti40Zr42Co18 alloyed with ~2at.% Si, but not in Ti53Zr27Co20. This indicates that the Ti40Zr42Co18 alloy is superior to the Ti53Zr27Co20 alloy in quasicrystal forming ability.· Using the constant e/a and constant average atomic size criteria, a series of Ti-Zr-Ni-Cu alloys are designed based on the Ti40Zr40Ni20 alloy. In these alloys, a group of e/a-constant phases, including quasicrystalline, amorphous and tI-Zr2Cu, are produced. The quasicrystal and amorphous phases are comprised of icosahedrons, while the Zr2Cu phase consists of truncated octahedrons, which are a variant of the icosahedron. This shows that the quasicrystal, amorphous and tI-Zr2Cu phases have structural relationships, which is also confirmed by the fact that the crystallization of the Ti12Zr55Ni13Cu20 bulk metallic glass produces first quasicrystal and then the tI-Zr2Cu phase.