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(CALculation of PHAse
Diagrams)
Phase diagrams
provide the graphical presentation of the equilibrium
state of a material as a function of temperature, pressure,
and composition of the components. This is why they are
frequently used as roadmaps for alloy design or a better
understanding of the processing of materials. The thermodynamic
properties of materials, such as the heat of solidification
or the chemical activities of components, are also frequently
used to understand, for example, metallurgical reactions
of materials. These two aspects, phase diagrams and thermodynamic
properties, have been treated separately for a very long
time despite the fact that their fundamental interrelations
had been established more than a century ago by J. W.
Gibbs.
Eventually, the mathematical calculation of phase diagrams
arose, and in 1970, Larry Kaufman et al initiated the
Calphad Method with a detailed description of procedures
together with a listing of computer software. The subsequent
meetings of the ever growing group, organized by Larry
Kaufman, soon reached the level of annual international
conferences, and the international Calphad journal was
established as well.
The modern Calphad approach is characterised by the following
points:
-
A predictive capability allows the extrapolation of
thermodynamic descriptions and phase equilibrium calculation
from assessed binary systems to ternary, quaternary
and higher order systems.
-
Identification of key experiments drastically reduces
the necessary experimental effort in multicomponent
systems.
-
Stable and metastable phase equilibria can be calculated.
-
The driving forces for all phase transformations are
available.
-
Local phase equilibria can be calculated, providing
a numerical input to materials processing software,
for example in solidification simulation or reactor
modelling.
-
The reading of multicomponent phase diagrams is drastically
simplified by the fact that all the interesting two-dimensional
sections in temperature, composition or even chemical
potential con be readily calculated, plotted and read
directly.
This
powerful tool in materials research goes beyond a mere
“calculation of phase diagrams”. This is why
the term “computational thermochemistry” is
frequently used to describe the current state of the Calphad
approach.
Reference:
“Focused Development of Magnesium Alloys using the
Calphad Approach”, Rainer Schmid-Fetzer and Joachim
Gröbner, Advanced Engineering Materials, vol 3 No
12, pp. 947-961, 2001
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