Time resolved studies of metallic phase formation

Doctoral thesis English OPEN
Simmons, LM

The processes of phase formation and transformation in crystallised amorphous metallic\ud materials can be investigated using a wide variety of techniques. Time and temperature\ud resolved neutron and x-ray scattering offers an insight into these processes in-situ often\ud highlighting the formation of metastable and unreported phases. In this work, time- and\ud temperature-resolved synchrotron x-ray diffraction and small angle neutron scattering\ud studies are complemented with differential scanning calorimetry and transmission\ud electron microscopy to gain a complete insight into the crystallisation and subsequent\ud phase formation and transformation in two Fe-based metallic glasses: Cogo-xFe x B2o, x =\ud 20, 40 of current technological importance, and the Y-Fe system of academic interest.\ud In Co4oFe4oB2o time resolved synchrotron x-ray diffraction shows that a a-(Co,Fe) solid\ud solution is formed in isolation irrespective of isothermal temperature treatment. In\ud contrast, temperature resolved synchrotron x-ray diffraction indicates a two-stage\ud crystallisation process; primary cc-(Co,Fe) phase formation is followed by polymorphic\ud crystallisation of (Co,Fe)2B. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model\ud for isothermal phase transformation kinetics indicates one-dimensional growth with a\ud decreasing nucleation rate for the isolated a-(Co,Fe) solid solution.\ud In a direct comparison, Co6oFe2oB2 o, is shown to undergo multi-phase crystallisation\ud during both isothermal and continuously heated temperature profiles. The JMAK\ud model is in agreement to those for Co4oFe4oB2o, i.e. one dimensional growth with a\ud decreasing nucleation rate.\ud Secondly the Rare-Earth Transition Metal YeyFess is shown to crystallise from\ud elemental Y to the YFe2 Laves phase via a novel 'YFe' phase with suggested structure\ud P6/mmm, a = 12.72A and c = 8.00A. Small angle neutron scattering data modelled to a\ud Lorentzian with variable power is in agreement with previously published kinetic\ud neutron diffraction and small angle scattering data highlighting the temperature and\ud time dependence of a critical scattering event evident at the point where the whole\ud sample crystallises.\ud xiv
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