Zr-Cu-Ni-Al exhibits one of the best glass forming alloys known. In a narrow concentration range, however, annealing leads to the formation of a metastable icosahedral phase. The aim of this paper is to investigate in detail the influence of alloying with Hf or Si, Sn, Y, Cr and Mo on the quasicrystal formation in Zr69.5Cu12Ni11Al7.5 glasses.
Whereas Y addition leads to loss in thermal stability and pronounced formation of quasicrystals, the addition of Si, Sn or Mo does not deteriorate thermal stability. Sn and Si were observed even to improve thermal stability significantly, but also to change phase selection during the early stages of crystallization. Si addition results in the formation of the tetragonal Zr2Ni phase and Mo as well as Cr to the formation of the fcc, so-called big-cube phase.
Nucleation rates for the quasicrystal formation were measured by means of crystallization statistics. By modeling the obtained nucleation rates in the framework of diffusion controlled classical nucleation the interface energies (between quasicrystals and amorphous matrix) as well as metastable melting temperatures for the quasicrystalline phase could be derived. In all cases nucleation could be described best with a homogeneous model with transient rates.
In order to understand the formation of the icosahedral structure also data on the influence of oxygen and hydrogen contamination as well as recent results of other authors on the influence of noble metals such as Ag, Pd, Au and Pt will be discussed in detail. There is some evidence that the quasicrystals are a hybrid of two structural elements, i.e. the tetragonal Zr2Ni (Al2Cu) and the tetragonal Zr2Cu (MoSi2) structure. It is further assumed that an icosahedral short-range order of the melt is stabilized by the addition of elements like Hf, probably Y or by contamination with very small amounts of oxygen, thus leading to a reduced interfacial energy between the quasicrystals and the amorphous matrix and therefore to a lower nucleation barrier.