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

摘要

The composition ranges of Zr-Cu-B and Zr-Cu-Ga amorphous alloys and their supercooled liquid region (triangle open _T_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, triangle open _T_X and T_g/T_m decrease in the order Al>Ga>B. However, the addition of a small amount (5 at percent) of B or Ga for the Zr-TM (TM = Co, Ni, Cu) base alloys is effective for the increase in triangle open _T_X and the largest triangle open _T_X value is 65 K for Zr_(65)Ni_(10)Cu_(20)B_5 and 73 K for Zr_(65)Co_5Cu_(25)Ga_5 which are larger than that (60 K) 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 bet 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 triangle open _T_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 triangle open _T_X 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 triangle open _T_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.