Formation of Zr 70 Ni 23 Ti 7 Glassy Alloy and Phase Transformation upon Annealing
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Liqun Ma and Akihisa Inoue Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan (Received 13 June 2001; accepted 9 January 2002)
Zr70Ni23Ti7 alloy contains a single amorphous phase when it is melt-spun at a wheel surface velocity over 20 m/s. The crystallization of these amorphous ribbons takes place through two exothermic reactions and shows a significant supercooled liquid region of about 30 K, indicating that the Zr70Ni23Ti7 alloy has a good glass-forming ability. The crystallization products of the first exothermic reaction for the ribbon prepared at a wheel surface velocity of 40 m/s are mainly an icosahedral quasicrystalline phase (I-phase) and some Zr2Ni phases. Further heating to a higher temperature will lead to the transformation of the metastable I-phase to Zr2Ni. Some icosahedral atomic clusters with a structure similar to those in face-centered-cubic Zr2Ni may exist in the alloy after rapid quenching, and most of them may act as nuclei of I-phase. The formation of I-phase in this alloy without any noble metals may be due to the proper atomic ratios in the system.
I. INTRODUCTION
The precipitation of icosahedral quasicrystalline phases (I-phase) from Zr-based amorphous alloys was reported initially by Köster 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) systems2–4 and Zr–TM–M systems5–7 for the formation of I-phases from amorphous phases by crystallization. The typical alloy compositions for the Zr–Al–TM–M system are Zr65Al7.5Ni10Cu12.5M5. In these alloys, noble metals (Pd, Pt, Au, and Ag) are added to the well-known good metallic glass forming Zr65Al7.5Ni10Cu17.58 to acquire and stabilize I-phases. For the Zr–TM–M system, the noble metals also play an important role in the formation of I-phases. For example, no icosahedral phase was observed in the Zr70(Ni,Cu)30 amorphous alloys; addition of Pd was found to be effective in promoting the formation of an I-phase.7 It has also been noticed that most of the above amorphous alloys exhibit a significant supercooled liquid region, ⌬Tx ⳱ Tx − Tg, where Tx and Tg are the crystallization and glass transition temperature, respectively. The existence of a supercooled region usually corresponds to a good glass-forming ability of an amor-
phous alloy. There are few resports on formation of an I-phase from Zr-based amorphous alloys with high Zr content and without any noble metals.
II. EXPERIMENTAL
The alloy ingots of Zr70Ni23Ti7 were prepared by arc melting a mixture of pure metals in an argon atmosphere. From these alloy ingots, ribbons were prepared by a single roller melt-spinning method at different wheel surface velocities in an argon atmosphere. The thickness of the ribbons was about 20–45 m; the width of the crosssection of the ribbons was about 1.0–1.3 mm. The structure was examined by x-ray diffraction (XRD) and the thermal stability was evaluated by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s. The microstructure was examined by transmis
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