Abstract
Bao-chen Lu (Material Physics and Chemistry)
Supervised by Prof. R. Yang and Prof. Jian Xu
Ti-based metallic glasses attracted more and more attention due to their significant advantages such as high specific strength, high elastic limit and high corrosion-resistance. It is of great interest to develop Ti-based metallic glasses with high glass-forming ability and large size by melt casting. Most of bulk metallic glass alloys can be reduced to pseudo-binary or ternary systems. Binary & ternary alloys with certain glass forming ability are the bases for developing high order systems with high glass forming ability.
The effects of the topological & chemical factors on the glass−forming ability (GFA) for Ti−Ni−M (M = Al, Sn and Si) ternary alloys were investigated, so does for the relationship between glass forming ability and the types of eutectic reaction. An model predicting for alloy compositions with best (or high) GFA and high concentration was advanced. The following conclusions are drawn:
1. The composition dependence of glass-forming ability was investigated for the TiNiAl ternary alloys. The optimal glass-forming composition was located at Ti54Ni32Al14, with a critical thickness of about 90 m for complete glass formation. With respect to the TiNi binary alloy, Al addition in the system only marginally improves the glass-forming ability. It is attributed to a small atomic size difference between Ti and Al. The optimal glass-forming composition, Ti54Ni32Al14, is near the univariant eutectic groove of [LTi(Al)2Ni+-Ti(Al)] in the phase diagram.
2. The glass-forming ability (GFA) of TiNiSn ternary alloys was investigated. Applying recent models based on atomic size ratio and efficient packing, the composition favoring the glass formation is predicted. Our experiments indicate that the optimal glass-forming composition is Ti56Ni38Sn6, with the critical thickness of complete glass formation approaching 100 m in the case of melt-spun ribbons. We demonstrate that the glass formation of the Ti56Ni38Sn6 alloy correlates with the (LTiNi+Ti3Sn) pseudo binary eutectic reaction. Compared with the Ti-rich TiNi binary alloys, the GFA is enhanced for the TiNiSn ternary alloys, but the improvement is limited possibly due to changes in the competing crystalline compounds.
3. Within the composition region bordered by three compounds, TiNi, Ti2Ni and Ti5Si3, in the TiNiSi ternary system, the optimal glass-forming composition is located at Ti57Ni35Si8. For this alloy, the critical thickness of complete glass formation for the melt-spun ribbon is around 200 m. Compared with the TiNiAl and TiNiSn ternary alloys, the TiNiSi ternary alloy exhibits a higher glass-forming ability. It is attributed to the formation of the invariant ternary eutectic reaction （LTi2Ni+TiNi+Ti5Si3）resulting from the compatibility of the chemical interaction between components and the uniform distribution of the atomic sizes. The glass formation of the Ti57Ni35Si8 alloy correlates mainly with the ternary eutectic reaction (LTi2Ni+TiNi+Ti5Si3).
4. In view of the case that solute-solute bonds in binary metallic glass alloys with high solute concentration is inevitable, the quasi-chemical model is adopted for the packing mode of the nearest atoms, i.e. short range order in metallic glasses, upon which the quasi-sc or -fcc with the ordered form was adopted to construct the medium range order. The atomic packing mode in the present model is a result of the chemical compatibility and atomic size effect (topology). The calculated 20 metallic glass compositions including ZrCu systems etc., agree with the experimental results very well. Under the condition of the volume-equivalent approximation treatment for high order systems, the calculated 43 high solute concentration compositions also correspond very well to the experimental results with best (or high) glass forming ability.
Keywords: amorphous alloys; metallic glasses; Titanium; eutectic reaction; atomic cluster.
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