Formation of nano-scale icosahedral quasicrystalline phase in amorphous Zr60Ni25Ti15 alloy

Abstract

The precipitation of icosahedral quasicrystalline phases (I-phase) from Zr-based amorphous alloys was reported initially by Koster et al. [1]. Recently, much attention has been paid to amorphous Zr–Al–TM– M (TM = Co, Ni, Cu, M = Ag, Pd, Au, or Pt) system [2–4] and Zr–TM–M system [5–7] for the formation of I-phases from amorphous phases by crystallization. A typical alloy composition for the Zr–Al–TM–M system is Zr65Al7.5Ni10Cu12.5M5. In these alloys, noble metals (Pd, Pt, Au and Ag) are added to the wellknown good metallic glass former Zr65Al7.5Ni10Cu17.5 [8] in order to stabilize I-phases. For the Zr–TM– M system, the noble metals also play an important role in the formation of I-phases. It is noticed that most of the above amorphous alloys exhibit a large supercooled liquid region, Tx(=Tx − Tg), where Tx and Tg are the crystallization and glass transition temperature, respectively. Such a large supercooled region means good glass-forming ability for an amorphous alloy. It also seems that the addition of noble metals is essential for the formation of I-phases in these Zr-based amorphous alloys. For example, no icosahedral phase was observed in the Zr70(Ni, Cu)30 amorphous alloy, while I-phases could be acquired in the quaternary, ternary, and even binary Zr70(Ni, Cu, Pd)30 amorphous alloys [7]. In order to determine the possibility of getting an I-phase from a Zr-based amorphous alloy without a significant supercooled liquid region and noble metals, a study was undertaken on the formation of a nano-scale I-phase in amorphous Zr60Ni25Ti15 alloy. The Zr60Ni25Ti15 alloy ingot was prepared by arc melting a mixture of pure Zr, Ni and Ti. From the alloy ingot, a ribbon with a cross section of 0.03 × 1 mm2 was produced by a single roller melt-spinning method in an argon atmosphere. Annealing of the specimen was performed in a differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s. The microstructure was examined by a transmission electron microscope (TEM). The sample for TEM observation was prepared by the ion milling technique. The XRD pattern of the melt-spun Zr60Ni25Ti15 ribbon is shown in Fig. 1. The existence of a broad peak indicates the formation of an amorphous state. It is very clear that the as-quenched ribbon only contains a single amorphous phase. The DSC curve of the melt-spun Zr60Ni25Ti15 ribbon is shown in Fig. 2. We can see from this figure that the crystallization proceeds through two Figure 1 XRD patterns of the Zr60Ni25Ti15 ribbon: (a) melt-spun, (b) annealed to 685 K, and (c) annealed to 900 K at a heating rate of 0.67 K/s and then cooled rapidly in the DSC cell.