Bulk Glass Formation and Crystallization of Zr-Ti Based Alloys

Citation

Lin, Xianghong (1997) Bulk Glass Formation and Crystallization of Zr-Ti Based Alloys. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dwm0-yq34. https://resolver.caltech.edu/CaltechETD:etd-06042004-160010

Abstract

The first metallic glass was discovered by Pol Duwez and his colleagues at Caltech in 1960. Shortly after that, Turnbull pointed out that a high ratio of melting temperature over glass transition temperature favors glass formation based on classical nucleation theory. Using Turnbull's theory as a guideline, many alloy systems were found forming glasses. As a new state of metallic alloys, metallic glass is of scientific interest by itself as well as attractive for practical applications due to unique properties. However, since the critical cooling rates for glass formation of conventional alloys are relatively high, at least one dimension of the metallic glass is limited to a few tenths of milimeter. This severely limited the application of metallic glasses.

Recently, several multicomponent alloy systems were discovered that form bulk metallic glasses with the smallest dimension of several mm up to several cm. Two of these alloy systems are the Ti-Zr-Cu-Ni and the Zr-Ti-(Nb)-Cu-Ni-Al alloys, which were discovered during this thesis research.

In this thesis homogeneous and heterogeneous nucleation theories and their influence on glass formation are reviewed. Experimental methods for metallic glass processing are introduced. The formation of Ti-Zr-Cu-Ni and Zr-Ti-(Nb)-Cu-Ni-Al bulk metallic alloys are reported. The glass transition, crystallization behavior, and mechanical properties of these glassy alloys are presented. The optimized combinations of four or five elements in these alloys leads to very deep eutetic multicomponent alloys, in which nucleation of crystals upon undercooling becomes increasingly improbable. Thermodynamic, kinetic and topological factors are discussed that contribute the exceptional glass forming ability of these alloys.

The very high stability of the undercooled liquids of bulk glass forming alloys makes it possible to perform studies on undercooled liquids in a time and temperature window that was not accessible thus far. In this thesis the time temperature transformation diagrams were measured for the crystallization of undercooled Zr-Ti-(Nb)-Cu-Ni-Al liquids. It has been discovered that the crystallization of the undercooled liquid is caused by heterogeneous nucleation induced by oxide particles, even when the oxygen concentration is as low as 250 atomic ppm. Overheating the melts above the liquidus temperature of the oxide particle results in maximum undercooling. Strong oxygen concentration dependence of the overheating, undercooling, and TTT diagrams was found. These results suggest that impurities play a key role in the crystallization of undercooled liquid. This first systematic study of impurity effect on crystallization of undercooled liquid will have many important implications on making bulk glasses and any study of undercooled liquids.

To summarize this thesis, important factors which affect the crystallization are revealed and discussed. Strategies on how to find new bulk metallic glasses are suggested.

Thesis Files