Theory of ordering transformations in metals and minerals Page: 4 of 118

                                
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Theory of Ordering in Metals and Minerals
by
Timothy Francis Lindsey
ABSTACT
This dissertation presents an investigation of ordering in FCC based
systems using the pair potential approximation in the ground state and
mean field limits. The theoretical approach is used to explain the occur-
rence of observed equilibrium phases and characteristics of thermodynamic
instabilities, in particular, spinodal ordering and decomposition.
It is shown that the stability of non-integer domain sizes in long period
superstructures such as Al3Ti and Ag3Mg may result from the tendency
of a system to reduce the number of non-dominant ordering waves, thus
producing domain sizes that have the rational fr action form n/m. This con-
clusion is used to explain the domain size stability with respect to variations
in temperature and electron concentration.
The cation ordering in the precipitate phases in calcite and dolomite
is analyzed by analogy with ordering in FCC based metals. The ordered
phases in calcite and dolomite are shown to be consistent with pair potential
minima at {100} and { a , z , z} positions in reciprocal space respectively. The
results of the thermodynamic analysis show that the ordered phases may
have resulted from an ordering instability of the disordered matrix. This
suggests that the ordered precipitates form spontaneously on the surface of