Metastable phases in Zr-based bulk glass-forming alloys detected using a synchrotron beam in transmission

Nucleation and crystallisation in Zr-based bulk metallic glasses has been directly detected using high intensity high energy monochromatic synchrotron beam diffraction in transmission during in-situ heat treatment. Previously, it was established that crystallisation of the amorphous phase occurs via initial nucleation and growth of metastable phases that transform to equilibrium tetragonal Zr2Cu before melting. However, no trace of nucleation of the metastable phase was obtained during cooling of the liquid alloy from above liquidus temperature Tl and Zr2Cu forms directly from the liquid. The metastable crystalline nanostructure obtained during rapid heating was found to depend sensitively on the alloy composition. Ti addition suppresses the previously detected metastable state and leads to a new phase with structure close to that of tetragonal Zr2Ni phase but with a highly stressed nanostructure. On the other hand, increasing the Zr content to nearly 70 at.% allows easy preparation of the metastable state during rapid heating at rates of the order of 1 K/s as used in most DSC experiments.