Time resolved studies of metallic phase formation

Simmons, LM

Abstract

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

Citation

Simmons, L. Time resolved studies of metallic phase formation. (Thesis). Salford : University of Salford

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Award Date: Jan 1, 2011