Structural and calorimetric evolutions of mechanically-induced solid-state devitrificated Zr60Ni25Al15 glassy alloy powder

A single phase of glassy Zr60Ni25Al15 alloy powder was synthesized by mechanically induced solid-state reaction (MISSR) technique. The MISSR was performed in a room-temperature high-energy ball mill, using a mechanical alloying method. Whereas the glass transition temperature of the obtained glassy alloy is 681 K, the melting and liquidus temperatures are 1179 K and 1256 K, respectively. The mechanically alloyed Zr60Ni25Al15 glassy powders maintain its unique disordered structure through a large supercooled liquid region (99 K). It, however, transforms into a mixture of Zr5Ni4Al and Zr6NiAl2 crystalline phases at 780 K with a large enthalpy change of crystallization of -81.8 J/g. The possibility of devitrification of the synthetic glassy phase upon increasing the ball milling time was investigated. The results have shown that the glassy powder obtained after 173 ks of milling is subject to heavy lattice imperfections and tends to transform into a metastable-big cube phase after further ball milling times (216-259 ks). After 446 ks of milling, a complete glassy-metastable phase transformation is achieved and the end-product of this stage of milling is nanocrystalline big-cube powders which have a lattice constant of 1.2282 nm. The big-cube Zr(60)Ni(25)A(15) phase transforms into the same crystalline mixture of Zr5Ni4Al and Zr6NiAl2 phases at 799 K with an enthalpy change of transformation of -73.7 J/g. As the milling time increases (720 ks), the obtained big-cube phase can no longer withstand against the shear and impact stresses that are generated by the milling media and surprisingly transformed into a new metastable phase of nanocrystalline fcc- Zr60Ni25Al15. The lattice constant of this metastable phase was calculated and found to be 0.45449 nm. The fcc-metastable phase transforms into a mixture of Zr5Ni4Al and Zr6NiAl2 crystalline phases at rather a high temperature, as high as 901 K with a heat change of transformation of -24.4 J/g. The reported metastable phase here is new and has never been, so far as we know, reported for ternary Zr-Ni-Al system, or its binary phase relations. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.