Time resolved studies of metallic phase formation
Simmons, LM 2011, Time resolved studies of metallic phase formation , PhD thesis, Salford : University of Salford.
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The processes of phase formation and transformation in crystallised amorphous metallic materials can be investigated using a wide variety of techniques. Time and temperature resolved neutron and x-ray scattering offers an insight into these processes in-situ often highlighting the formation of metastable and unreported phases. In this work, time- and temperature-resolved synchrotron x-ray diffraction and small angle neutron scattering studies are complemented with differential scanning calorimetry and transmission electron microscopy to gain a complete insight into the crystallisation and subsequent phase formation and transformation in two Fe-based metallic glasses: Cogo-xFe x B2o, x = 20, 40 of current technological importance, and the Y-Fe system of academic interest. In Co4oFe4oB2o time resolved synchrotron x-ray diffraction shows that a a-(Co,Fe) solid solution is formed in isolation irrespective of isothermal temperature treatment. In contrast, temperature resolved synchrotron x-ray diffraction indicates a two-stage crystallisation process; primary cc-(Co,Fe) phase formation is followed by polymorphic crystallisation of (Co,Fe)2B. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for isothermal phase transformation kinetics indicates one-dimensional growth with a decreasing nucleation rate for the isolated a-(Co,Fe) solid solution. In a direct comparison, Co6oFe2oB2 o, is shown to undergo multi-phase crystallisation during both isothermal and continuously heated temperature profiles. The JMAK model is in agreement to those for Co4oFe4oB2o, i.e. one dimensional growth with a decreasing nucleation rate. Secondly the Rare-Earth Transition Metal YeyFess is shown to crystallise from elemental Y to the YFe2 Laves phase via a novel 'YFe' phase with suggested structure P6/mmm, a = 12.72A and c = 8.00A. Small angle neutron scattering data modelled to a Lorentzian with variable power is in agreement with previously published kinetic neutron diffraction and small angle scattering data highlighting the temperature and time dependence of a critical scattering event evident at the point where the whole sample crystallises. xiv
|Item Type:||Thesis (PhD)|
|Depositing User:||Institutional Repository|
|Date Deposited:||03 Oct 2012 14:34|
|Last Modified:||18 Feb 2014 15:52|
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