which has Lc = 3 mm. In the later chapters, we focus only on highly processable BMGs based in the Zr-Ti-Be system with Lc > 1.5 cm.
The confusion principle, which states that the more elements in a liquid the more difficult it is to select a viable crystal structure [15], is an empirical rule with many exceptions. Clearly, most good glass formers are comprised of 4–5 elements and yet addition of further elements does not necessarily improve GFA. For example, Vitreloy 1, comprised of Zr-Ti-Cu-Ni-Be is an optimal composition, based on other criteria for GFA. Further additions of elements actually reduce GFA, which contradicts the confusion principle. In fact, the discoveries of binary bulk glasses in the Cu-Zr system prove that good glass forming can be achieved without numerous elements. In the Pd-Si binary system, 6 mm glasses can be obtained, representing larger GFA than many systems with five or more elements.
We have observed that the two most important criteria for bulk glass formation are deep eutectics and atomic size mismatch. Good glass-forming systems can be found by first looking at binary phase diagrams for deep eutectics. The deeper the eutectic, the easier it is to cool a liquid below Tg without allowing enough time for crystal nucleation and growth. Figure 1.1 is a binary phase diagram of Cu-Zr, which exhibits several deep eutectics. These eutectics are so prominent that three compositions, Zr65Cu35, Cu46Zr54, and Cu50Zr50, lead to the formation of bulk glasses.
All bulk metallic glasses are based in systems that have deep binary eutectics between two constituents. Examples of deep eutectics are Pd-P, Pt-P, Au-Si, Pd-As, Pt-As, Zr- Be, Ti-Be, Cu-Ti, Cu-Zr, Ni-Ti, Ni-Zr, Pd-Si, Cu-P, and Fe-P. Figure 1.2 and Figure 1.3 show binary phase diagrams from Zr-Be and Ti-Be, which are the basis for the glass forming systems used in this work. In many cases, by combining several alloys that all exhibit binary eutectics, even lower melting temperatures can be achieved. For
instance, despite deep eutectics in Zr-Be and Ti-Be, bulk glasses cannot be created in those systems. However, by combining the three elements, Ti-Zr-Be glassy alloys can be created up to 6 mm thick [17]. With additions of Cu and Ni, glass forming increases to 2.5 cm and melting temperatures can dip as low as ~ 900 K. Practical metallic glasses (those that don’t use precious or expensive metals) are typically comprised of elements such as Zr, Ti, Cu and Ni. To design highly processable BMG composites, this work deals solely with the Ti-Zr-Cu-Be system as its base.
Atomic size mismatch is the last constraint used to determine GFA. Elements that exhibit large differences in atomic size produce lattice stresses that increase the energy of the crystalline phase. Like the other strategies, there are many exceptions to this rule. Boron, for instance, is a very small metalloid that appears to be an excellent addition to some Zr and Ti-based alloys and yet it never seems to improve GFA. The most important contribution of atomic size mismatch is that a highly processable BMG made from practical elements cannot be made without mismatch or deep eutectics. The optimal combination of both is Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1) [10]. In this alloy, Cu, Ni, and Be all exhibit extremely deep eutectics with Zr and Ti.
Additionally, Be is a small metalloid which allows for pronounced atomic size mismatch between the other elements. As a result, GFA is on the order of 2.5 cm. The only practically based highly processable glasses that do not contain Be are Zr57Nb5Cu15.4Ni12.6Al10 and Zr52.5Ti5Cu17.9Ni14.6Al10, Vitreloy 106 and 105, respectively (see X. Lin, Caltech thesis, 1997). In these non-beryllium alloys, the deep eutectic obtained from Zr-Be and Ti-Be is absent, raising the melting temperature by
~ 200 K over Vitreloy 1. However, these alloys contain the small metalloid Al, which
increases GFA substantially, even though it raises the melting point of Ti. Vitreloy 105 and 106 utilize the strategy of atomic size mismatch but don’t utilize the deep eutectic found in Be-containing alloys. Thus, GFA is reduced to ~ 1.5 cm.
Additionally, without the oxygen-gettering affects of Be, these alloys are extremely prone to crystallization [18]. GFA is severely reduced when the oxygen content exceeds ~ 500 parts per million. Vitreloy 1, 105, and 106 are all considered to be highly processable, since they can be produced in ingot form. As we discuss in Chapter 6, small additions of Al dramatically increase the shear modulus of BMG composites, which eliminates ductility. To create highly processable BMG composites we require bulk glasses that don’t contain a substantial amount of Al. Thus, our current composites are limited to Be-containing alloys. No highly processable BMGs made from practical elements currently exist which do not contain Be or Al.
Figure 1.1 – Binary Cu-Zr phase diagram (adapted from [16]). The Cu-Zr eutectic temperatures are so deep that three binary bulk glasses form near 38.2%, 44%, and 54.3% zirconium. The best glass former, Cu46Zr54, can be cast up to 2 mm in diameter.
Figure 1.2 – Binary Zr-Be phase diagram (adapted from [16]).
Figure 1.3 – Binary Ti-Be phase diagram (adapted from [16]).