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a Qi c
, Tingkun Gu d
a The Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Qianfoshan Campus, Jinan 250061, China b College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
c School of Physics, Shandong University, Qianfoshan Campus, Jinan 250061, China
d School of Electrical Engineering, Shandong University, Qianfoshan Campus, Jinan 250061, China
article info abstract
Article history:
Received 26 July 2015
Received in revised form 8 November 2015
Accepted 26 November 2015
Available online xxxx
The structure and mixing enthalpy of liquid Al-Si alloys has been investigated by ab initio molecular dynamics
simulation. It is found that the three most frequent Voronoi polyhedron around Si atoms are nearly unchanged
as the Si composition decreases from pure Si to about 4 at.% Si. The robustness of local structure of Si atoms permits us to establish a model to describe the dissolution behavior of Si particles into liquid Al, in which the structure of interface between liquid Al and Si particle might be like that of liquid pure Si, and hence the dissolution of
Si particles should be interfacial reaction controlled. Some previous observations related to Al-Si alloys may be
well understood based on this model. The calculated mixing enthalpy has a maximum of about −7 kJ/mol,
which is much larger than what is used in the present phase diagram calculations.
© 2015 Published by Elsevier B.V.
Keywords:
Liquid Al-Si alloys
Structure of liquid alloy
Mixing enthalpy
Dissolution model of Si particles
1. Introduction
Al-Si alloys are very important to the modern society. In order to improve the mechanical properties of some Al-Si alloys, physical or chemical methods are often employed upon the liquid alloys to control the
solidified microstructure. On one hand, overheating of the melts as
one of the physical methods, often influence the microstructure very
much [1,2]; on the other hand, as one of the chemical methods the addition of a small amount of master alloy can modify the microstructure
greatly [3,4,5]. Researchers suggested that treatment on the liquid alloy
might influence the liquid structure and then the nucleation.
Many researchers believed that structure transition of liquid alloy
should manifest itself through variation of physical properties such as
density. Popel et al. [6] observed the anomalous temperature dependences of density during heating of liquid eutectic Al-Si alloy, while
during cooling only linear behavior was observed. More recently,
Schmitz et al. [7] reached similarly linear variation of density of Alrich liquid Al-Si alloys against temperature by contactless measurement, in which measurements were started at high temperatures and
data were acquired after subsequent cooling steps. In order to explain
the abnormal behavior of density vs temperature, it was conjectured
that the dissolution of some large clusters started at a temperature Td
and became homogeneous at a branching temperature Tb well above
the liquidus. During heating process, the structure of liquid alloy was
thought to be changed to a more homogeneous state above Tb [8-10].
In addition, Ref. [7] gave a nearly linear curve for the density of liquid
Al-Si alloys along composition, while Ref. [8] showed that the density
curve experienced an abrupt decrease at the eutectic composition.
To get insight into the inhomogeneous phenomenon, the microstructure of Al-Si alloys around the eutectic (Al87.8Si12.2) compositions
was investigated by small angle neutron scattering (SANS) above
liquidus temperature in a heating-cooling loop [8,10]. The SANS results
unambiguously showed that for the eutectic alloy the microstructure
changes with increasing temperature in a partly reversible way while
for the hypereutectic (20 at.% Si) alloy the change was almost completely irreversible. For all compositions studied, the melt at the maximum
temperature after heating was more homogeneous, and the eutectic
one was the most homogeneous. The results showed that "particles"
of different sizes exist in Al-Si melts of different compositions and
that their distribution change during thermal cycles above the melting
point until a true equilibrium solution state is reached. The "particles"
size ranges from 10 nm to 100 nm. We estimated that these "particles"
contain about 70 thousand to 70 million atoms, hence such information
on "particles" is nearly on the macro scale, and the cluster structure in
the micro scale is still not sufficient.
Investigation on the structure of liquid eutectic Al-Si alloy conducted by both X-ray diffraction and neutron diffraction in the time period
from 1979 to 2011 has been reviewed in Ref. [9] at 2013, and then
will be not fully cited again in this paper. Results on eutectic liquid Al-
Si alloy with respect of the first peak position and height were compared
[9]. It was shown that the parameters are rather scattered in spite of the
Journal of Non-Crystalline Solids 433 (2016) 31-37
⁎ Corresponding author.
E-mail address: [email protected] (J. Qin).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.11.032
0022-3093/© 2015 Published by Elsevier B.V.
Contents lists available at ScienceDirect
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Це лише тому, що ще ядерка не почалась. А як почнеться, це буде дуже дорогоцінна техніка, ви її не купите навіть за 40 пачок сірників чи 5 банок тушенки.