The physical significance of the glass transition observed by differential scanning calorimetry (DSC) in the metallic glasses is explained in a study of measurements of the heating-rate(β) dependence of the glass transition temperature Tg and the crystallization temperature Tx in the Zr70Cu3O and Zr70Ni30 amorphous alloys and their structures in as-quenched and crystallized states. Zr70Cu30 exhibits the glass transition before crystallization in the normal DSC measurement and Zr70Ni30 immediately crystallizes. The heating rate β_c at the intersection of the two linear curves of Tg and Tx against logβ provides us a significant measure to determine the GFA and thermal stability of the metallic glasses. By heating at β larger than β_c, the crystallization is suppressed and the glass transition is observed even in Zr_<70>Ni_<30>. The thermal stability of the Zr_2Cu amorphous alloy is caused by retardation of crystallization due to the amorphous structure different from the Zr_2Cu crystalline phase. By contrast, the thermal instability of Zr_<70>Ni_<30> is attributed to the structural similarity to Zr_2Ni. Consequently, suppressing the crystallization is a key to gain the thermal stability of the present amorphous alloys. In the present paper, I will also introduce an example to evaluate a thermally stable cluster size near glass transition in Pd-based bulk metallic glasses.

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