Thermal Properties of Zr–TM–B and Zr–TM–Ga (TM=Co, Ni, Cu) Amorphous Alloys with Wide Range of Supercooling

Abstract

The composition ranges of Zr-Cu-B and Zr-Cu-Ga amorphous alloys and their supercooled liquid region (ΔT x ) and reduced glass transition temperature (T g /T m ) were examined in comparison with those for Zr-Cu-Al amorphous alloys. The glass formation range, ΔT x and T g /T m decrease in the order Al>Ga>B. However, the addition of a small amount (5 at%) of B or Ga for the Zr-TM (TM = Co, Ni, Cu) base alloys is effective for the increase in ΔT x and the largest AT value is 65K for Zr 65 Ni 10 Cu 20 B 5 and 73K for Zr 65 Co 5 Cu 25 Ga 5 which are larger than that (60K) for Zr 65 Ni 10 Cu 20 Al 5 . The crystallization of the B- or Ga-containing alloy takes place through a single stage due to the polymorphous precipitation of bct Zr 2 (Ni, Cu, B) or bct Zr 2 (Co, Cu, Ga) while the Al-containing alloy shows a two-stage process of Am→Am'+metastable cubic Zr 2 (Ni, Al)→bct Zr 2 (Ni, Al) + bct Zr 2 (Cu, Al). The addition of B, Ga or Al to the Zr 65 (Co, Ni, Cu) 30 Al 5 alloy causes the further increase in ΔT x to 84, 90 and 104 K respectively, and the crystallization process consists of the polymorphous-type single stage. The reason why the Zr-Cu-B ternary amorphous alloys have the lower glass-forming ability and smaller AT value among the three Zr-Cu-M (M=B, Ga or Al) ternary systems is presumably because Cu-B pair has a positive heat of mixing and the atomic size ratios between B and Zr or Cu are too large to construct a higher degree of dense random packed structure. This is consistent with the previous result that the dissolution of Al with large negative heats of mixing and different atomic size ratios causes the formation of a higher degree of dense random packed structure and the ΔT x value increases further for the Zr-Cu-Al amorphous alloys containing a larger amount of Al. These results support the previous concept that the simultaneous satisfaction of the significantly different atomic size ratios and large negative heats of mixing is necessary for the formation of an amorphous alloy with a wide supercooled liquid region before crystallization.