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 d irectly 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 equil ibrium tetragonal Zr2Cu before melting. However. no trace of nucleation of the metastable phase was obtained during cooling of the liquid alloy from a bove liquidus temperature T-1 and Zr2Cu forms directly from the liquid. The metastable crystalline nanostructure obtained during rapid heating was fou nd to depend sensitively on the alloy composition. Ti addition suppresses t he previously detected metastable state and leads to a new phase with struc ture close to that of tetragonal Zr2Ni phase hut with a highly stressed nan ostructure. Oil the other hand, increasing the Zr content to nearly 70 at.% allows easy preparation of the metastable state during rapid heating tit r ates of the order of I K/s as used in most DSC experiments. (C) 2001 Elsevi er Science B.V. All rights reserved.