Zirconium-Based Bulk Glassy Alloys

Zirconium-based alloys were among the first non-noble metal-based alloys found to solidify in the amorphous state upon cooling from the melt at corrparatively low rates R, such as 10 Ks [1.69,70]. The term bulk glassy alloys refers to the fact that this solidification behavior permits us to obtain bulky parts with an amorphous structure by conventional casting procedures, e.g., in rod form, up to 30 mm. Data are given in [1.71,72]. 

The tendency to solidification in the amorphous state is due to a low rate of nucleation of the equilibrium phase(s) at a particular alloy composition. An empirical method to determine the ease of glass formation has proved to be an evaluation of the glass-transition tern- 

ASTM/UNS desi ation R60802 R60804 R 60901 R60904 

It is impossible to date to derive from first principles which alloy compositions are prone to easy glass formation. It has been found empirically that multi-component alloys with components of significantly different ionic radii are suitable candidates in principle. Examples of Zr-based systems that show bulk glassy solidification behavior are listed in Table 3.1-38. 

Empirical findings have been subjected to a systematic treatment involving a mismatch entropy term Sa and the enthalpy of mixing AH [1.73]. Such re- 

Common designation ASTM/UNS designation Zircaloy-2 R60802 Zircaloy-4 R60804 Zr-Nb R60901 Zr-Nb R60904 

Impurity elements, maximum permissible content (wtppm)  

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More recently magnesium-base, iron-base, and zirconium-titanium-base alloys have been developed that do not require such rapid cooling. In 1992, W. L. Johnson and co-workers developed the first commercial alloy available in bulk form Vitreloy 1, which contains 41.2 a/o Zr, 13.8 a/o Ti, 12.5 a/o Cu, 10 a/o Ni, and 22.5 a/o Be. The critical cooling rate for this alloy is about 1 K/s so glassy parts can be made with dimensions of several centimeters. Its properties are given in Table 15.3. 

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