Ordering and Segregation in Liquid Alloys
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Faculty of Physics
Abstract
The design and fabrication of new alloys is usually approached by a method that
combines theoretical analysis and experimental observation (composition−structure−
property), but in many cases, due to the experimental difficulties related to high
temperatures, the theoretically predicted property values are of key importance. This is
particularly relevant for all industrial processes, such as casting, joining, crystal growth,
etc. that involves the presence of the liquid phase, i.e. liquid metals and alloys. Alloying
processes have been evolved as one of robust tool to achieve desired materials with
required characteristics. The thermal treatments of materials, chemical compositions and
operating parameters (temperature, pressure and working atmosphere) overrule the
microstructure of an alloy. The in−depth knowledge of thermodynamics, kinetics and
thus, the energetic of the alloy prototypic process has great significance in metallurgical
science and engineering.
In view of the aforementioned, we have studied and explained the mixing behaviour of
two Al−based (Al−Fe at 1873 K and Al−Mg at 1073 K) as well as two Bi−based (Bi−Tl at
750 K and In−Bi at 900 K) liquid binary alloys with the help of theoretical modeling. The
thermodynamic properties, such as free energy of mixing (G
), activity (a), enthalpy of
mixing (H
) and entropy of mixing (S
vii
), structural properties, such as concentration
fluctuation in long wave length limit (S
(0)), chemical short range order parameter (α
)
and ratio of diffusion coefficients (D
) for above mentioned liquid alloys at chosen
temperatures have been analyzed in the fame work of regular associated solution model.
For this purpose, we have determined the model parameters which are the interaction
energy parameters (ω
,
, ω
,
and ω
/D
,
), equilibrium constant (k), mole fractions of
complex (x
) and free monomers (x
and x
). The compositional contribution from
the heat associated with the formation of the complex and the heat of mixing of species to
the net enthalpy change has been studied for each system. The comparative studies of the
thermodynamic properties of these systems reveal that the Al−Fe being the most
interacting system followed by the Bi−Tl, the Al−Mg and the In−Bi, in which the
viii
ABSTRACT
interactions are the weakest. Theoretical investigations of structural properties show that
all the preferred liquid alloys show complete ordering nature at least close to their
respective melting temperatures. The interaction energy parameters are found to be
temperature dependent. The surface properties of the chosen liquid alloys have been
explained with the help of Renovated Butler model. The trends of surface segregations in
these liquid binary alloys have been studied by computing surface tension (σ) and surface
concentrations (x
and x
). Theoretical investigations confirm the segregation of the
alloy component having a lower surface tension, i.e. the extent of segregation of
Bi−atoms at the surface layer, which is more pronounced in In−Bi melts with respect to
that of liquid Bi−Tl alloys. Moreover, in case of two Al−based melts, the Al−atoms
segregate on the Al−Fe surface phase whereas they remain in the bulk region of Al−Mg.
Additionally, the regular associated solution model has been extended to study and
predict the thermodynamic and structural properties of concerned liquid alloys at different
temperatures. For this purpose, ω
,
, ω
,
and ω
for each of the system have been
computed at different temperatures keeping x
,
, x
, x
and k invariant. The modeling
equations obtained by the polynomial fitting of different orders along with the values of
parameters to forecast these properties have also been included in this work. Theoretical
computations indicate that the excess Gibbs free energy of mixing (G
) of the alloys
gradually decreases with an increase in temperature above melting temperatures.
Accordingly, at higher temperatures, the ordering or the compound formation tendencies
of these alloy systems gradually decrease and sometimes show segregating nature. These
findings are further supported by decrease in deviations between the computed and
observed values of S
(0) at higher temperatures. The liquid alloys thus show the
maximum tendency towards complex formation at their respective melting temperatures,
however, these tendencies significantly decreases at elevated temperatures.
Thermodynamic properties have been then correlated with the Renovated Butler model to
explain the surface properties at different temperatures. The computed values for the
surface concentrations of the segregating components of the liquid alloys approach
respective ideal values at higher temperatures. Similarly, the surface tension of metallic
melts metals and alloys, decrease at elevated temperatures.
Description
Keywords
Liquid alloys, Prediction equations