Doctor of Philosophy
Department of Metallurgy and Materials Engineering
Gibson, Mark Antony, Crystallisation of metallic glasses, Doctor of Philosophy thesis, Department of Metallurgy and Materials Engineering, University of Wollongong, 1988. http://ro.uow.edu.au/theses/1608
The crystallisation behaviour of a range of Fe-based metallic glasses, in particular Fe-Si-B alloys, has been investigated by differential scanning calorimetry, X-ray diffraction, optical and transmission electron microscopy.
A series of hypoeutectic Fe-Si-B metallic glasses produced by single roller melt-spinning were found to have substantial wheel-side crystallinity in the as-cast condition. These crystals were shown to be associated with areas on the ribbon surface which were in good contact with the wheel rather than in the depressions formed by gas entrainment during casting, as has been previously suggested for similar effects in other alloys. Transmission electron microscopy revealed that the crystals have a fine cellular substructure with the intercellular regions freezing as glass and that the substructure becomes progressively finer during growth.
It was found that the conventional procedure of applying the Johnson-Mehl-Avrami analysis to determine reaction mechanisms, i.e. calculating a mean value of the Avrami exponent over a range of volume fraction transformed, can be misleading. The technique suggested by Calka and Radlinksi , when used in conjunction with detailed metallography, is shown to be a more sensitive and reliable indicator of reaction mechanisms.
The transformation kinetics of certain hypoeutectic Fe-Si-B alloys, which crystallise via two sequential reactions, have been evaluated for each reaction individually. The first transformation is associated with the nucleation and growth of Fe-rich dendrites with an Avrami exponent of 2.5 and the second reaction is associated with the nuclear and growth of a eutectic product with an Avrami exponent of 4.
Samples of an Fe75Si10B15 metallic glass have been isothermally annealed at 723 K and 793 K. At the lower temperature, the crystallisation products consisted of Fe3Si dendrites and a fine eutectic product, (Fe3Si + Fe2B), while at the higher temperature, an additional phase crystallised from the matrix after approximately 50% transformation. This phase has an orthorhombic structure and has not previously been reported in the Fe-Si-B system. Such variation in crystallisation products may be a feature of many metallic glass systems but a significant range of annealing temperatures, beyond that normally used for kinetic studies, may be necessary to detect them.
The orthorhombic phase crystallises as penetration twins, in the form of a "cross", in Fe68Si10B22 and Fe67Si13B20, a structure not previously reported in metallic glasses.
A comparison has been made between the microstructures of conventionally solidified alloys and crystallised glassy ribbons in the Fe-B and Fe-P systems. It is shown that the types of transformation products found in hypereutectic metallic glasses are consistent with growth within a coupled zone in either the stable or metastable phase diagram. Similar comparisons in other alloys suggest that this may be a common feature of crystallisation of metallic glasses.