Local structure and glass transition in Zr-based binary amorphous alloys

The physical significance of the glass transition observed by differential scanning calorimetry (DSC) in the metallic glasses was considered through the measurements of the heating-rate, β, dependence of the glass transition temperature, T_g, and the crystallization temperature, T_x, in the Zr₇₀Cu₃₀ and Zr₇₀Ni₃₀ amorphous alloys and X-ray study of their structures in as-quenched and crystallized states. Zr₇₀Cu₃₀ exhibits the glass transition before crystallization, but Zr₇₀Ni₃₀ is immediately crystallized at heating rates of conventional time scale in the DSC measurement. The heating rate β_c at the intersection of the two linear curves of T_g and T_x against log β provides us with a significant measure to determine the glass-forming ability or thermal stability of the metallic glasses. By heating at β larger than β _c, the crystallization is suppressed and the glass transition is clearly observed even in Zr₇₀Ni₃₀. The thermal stability of the Zr₇₀Cu₃₀ amorphous alloy is caused by retardation of crystallization due to the amorphous structure that is different from the Zr₂Cu crystalline phase. In contrast, the thermal instability of Zr₇₀Ni₃₀ is attributed to the structural similarity to the Zr₂Ni crystalline phase. Thus, suppressing the crystallization is shown to be a key to enhance the thermal stability of the present amorphous alloys.