Graduate Thesis Or Dissertation
 

On the fragility and equilibrium phases of metallic glass forming alloys

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  • The heating rate dependence of the glass transition temperature of the Ni₆₅Nb₃₅, Ni₆₀Nb₃₅Sn₅, Ni₅₉.₃₅Nb₃₄.₄₅Sn62, Ni₆₀(Nb₄₀Ta₆₀)₃₄Sn₆, and Ni₅₇Fe₃Nb₃₅Sn₅ metallic glass forming alloys was investigated with a differential scanning calorimeter (DSC). The inverse heating rate for each DSC experiment was plotted versus inverse temperature and a Vogel-Fulcher- Tamman (VFT) type relation was fit to the data in the plot. The fragilities of the alloys were characterized with the steepness index, m, the fragility parameter D*, and the VFT temperature, T₀ which are the fit parameters from the VFT relation. It was found that for the binary alloy D* = 6.2, for the ternary alloys D* = 11.0, and that for both the quaternary alloys D* was about 16. For these alloys, D* increases linearly as the number of components in the alloy increases. The fragilities of Zr₄₅.₃₈Ti₉.₆₂Cu₈.₇₅Ni₁₀Be₂₆.₂₅, Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅, Zr₄₂.₆₃Ti₁₂.₃₇Cu₁₁.₂₅Ni₁₀Be₂₃.₇₅, Zr₄₁.₂Ti₁₃.₈Cu₁₂.₅Ni₁₀Be₂₂.₅, Zr₃₉.₈₈Ti₁₅.₁₂Cu₁₃.₇₇Ni₉.₉₈Be₂₁.₂₅, Zr₃₈.₅Ti₁₆.₅Cu₁₅.₂₅Ni₉.₇₅Be₂₀, and Zr₄₆.₇₅Ti₈.₂₅Cu₇.₅Ni₁₀Be₂₇.₅ were also characterized by the same method. It was found that D* increases with increasing concentration of Ti and Cu and decreasing concentration of Zr and Be. The crystalline equilibrium phases of the Zr₅₇Cu₁₅.₄Ni₁₂.₆AI₁₀Nb₅ (Vit 106) bulk metallic glass-forming alloy were determined with electron microprobe analysis (EMPA) and X-ray diffraction (XRD). Five equilibrium phases were found. XRD, EMPA and cluster analysis show that those phases are Zr₂Ni, Zr₂Cu, ZrCu, Zr₃Al₂, and Zr₄AI₃. All the equilibrium phases, derived from the binary intermetallic compounds, contain the other elements as solutes. In Zr₂Ni and Zr₂Cu, Ni substitutes Cu and vice versa. In ZrCu, Ni and Al substitute the Cu. In Zr₃Al₂ and Zr₄AI₃, Nb substitutes Zr, and Al is substituted by Cu and Ni. This study shows that the additions of 10% Al and 5% Nb are optimum concentrations in the composition space to retard the nucleation and growth of these equilibrium phases when the melt is undercooled.
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