MECHANOCHEMICAL MIXING IN IMMISCIBLE METALLIC
SYSTEMS
N. Lyakhov, T. Grigorieva
Institute of Solid State Chemistry, 630128 Novosibirsk, Russia
Abstract
It was shown that supersaturated solid solutions were obtained in
immiscible systems such as Fe/Bi, Fe/In, Cu/Bi under intensive
mechanical treatment.
It is revealed that formation of nanocomposites always precedes
solid solutions formation. The mechanochemically obtained
nanocomposites represent nanosized particles of iron (or copper) coated
with the layer of low melting metals with a few atoms thikness.
It is suggested that the important role in formation of such
nanocomposites plays wetting and spreading of low melting metals in
the course of severe plastic deformation during mechanical activation
and the driving force for diffusion on a nano scale level when
mechanocomposites are formed, probably connected with a non uniform
(gradient) composition of nano grains coated by a liquid (plastic) metal.
Introduction
A formation of supersaturated solid solutions between solids is a
difficult problem. The only known and recognized mechanism for
reactive mixing in solid state is diffusion. But evidently the diffusion
itself needs even higher concentration gaps on the interface than in a
bulk of solid in order to provide sufficient gradients and to make the
diffusion process observable in a reasonable time. In experimental
conditions this means that supersaturated solid solutions even for binary
systems with negative mixing enthalpy can be obtained only through
quenching procedure when superheated but equilibrium solid solution is
cooled very quickly just to avoid a diffusion decomposition of solid
solution. This way is shown schematically in the Fig.1.
For the solid-solid systems with positive enthalpy of mixing, that
is for immiscible in principle systems the situation looks even more
complicated. According to a literature at least among metals one can
find the phase diagrams that demonstrate practically total immiscibility
of the components (metals) neither in solid nor in liquid state, for
1
example, Fe-Bi system (1). It is difficult to imagine the formation of
solid solutions ever takes place for such metal pairs. An independent
confirmation of this conclusion has been recently obtained by molecular
dynamic (MD) simulation. The authors of (2) have followed the
behavior of an “artificial” (model) solid solution of different
concentrations for the immiscible system Co/Ag. Notwithstanding on
the very short time of observation (2 ns) they have found that solutions
with concentration higher than 18 % showed a remarkable trace of
decomposition which are in good agreement with classical
thermodynamics. It should be noted that some changes for lower
concentrations (less than 18 %) simply need more time to make them
visible in the atomic segregation of one component (Ag) within the other
one (Co). The results of MD calculations are reproduced in Fig. 2.
Fig. 1. A possible way to form supersaturated solid solutions. ∆C is over
equilibrium concentration after rapid quenching.
In our earlier experiments (3-5) we observed the formation of
supersaturated solid solutions for typical intermetallic systems with
negative enthalpies as a result of mechanical alloying. As a rule, solid
solutions were detected on the last stage of any phase transformations,
including formation of intermetallides, during mechanical activation. In
this study we describe the formation of solid solutions in immiscible
2
systems such as Fe/Bi, Fe/In and Cu/Bi under the intensive mechanical
treatment.
Fig. 2. Molecular Dynamic calculations of Co-Ag solid solutions time
evolution. A segregation is clearly visible after 2 ns (reproduced from
[2]). At the left - 12 (a) and 13 (b) at. % Co, on the right - 17 (a) and 18
(b) at. % Co, triangles - Co, circles - Ag
Experimental
An AGO-2 ball planetary mill (6) was used in the laboratory
experiments. In order to avoid oxidation, all experiments were
performed in argon medium.
The X-ray phase analysis was performed by URD-63
diffractometer equipped with graphite monochromator, using CuK
radiation.
Mössbauer spectra were recorded using YaGRS-4 spectrometer
with 57Со.
X-ray photoelectron spectra (XPE) were obtained using ES-2401
spectrophotometer with Mg anode. The vacuum in the chamber of
3
analyzer was 10-6 Pa. The spectrometer was calibrated using the Au4f7/2
line with 84,0 eV. The accuracy of determination of line positions was
0,10,2 eV, relative error of line intensity determination as not more
than 10 %. The profiles of concentrations of the lines over the depth
were obtained by means of etching with argon ions. Spraying rate was 1
nm/min. The O1s, Bi4f7/2, Fe3p spectral peaks were analyzed.
JSM-T20 scanning electron microscope, high-resolution electron
microscopes JEM-2010 and JEM-400 were used for electron
microscopic studies.
Results and Discussion
In order to understand properly the phenomenon a typical phase
diagram for Fe/Bi system is shown in Fig. 3. As we mentioned there is
no detectable regions where the solutions may appear. That means we
deal with practically immiscible systems in all selected pairs of metals.
In Fig. 4 the successive in time diffraction patterns are presented which
clearly indicate that the presence of Bi phase is observed only on the
first stage of the process of mechanical treatment.
Fig. 3. The equilibrium phase diagram for the Fe-Bi immiscible system.
After some time (depending on conditions) one cannot find any
traces of low melting phase though the chemical analysis confirms the
4
I, a.u.
250
200
150
Fe
Fe
40 min
Bi
100
20 min
Bi
5 min
50
40 s
0
24
34
44
54
64
74
84
2 theta, degree
Fig. 4. X-ray diffraction changes in the Fe + 10 wt.% Bi with activation time.
concentration of Bi in the mixture is close to the initial one. Moreover,
no peaks diffraction shifted with respect to the “mother” phase. The only
visible result of mechanical activation on this stage is the remarkable
broadening of some diffraction peaks in their “normal” positions.
This is a strange state. We have lost one metallic phase, we
have no evidence of solid solutions formed (the ratio of atomic
radii Bi / Fe = 1.4). If we examine the alloy on this stage with
HRTEM we shall find a finely divided state, which we called
mechanocomposite (7). It is a fact that the rate of comminuting is
much higher in the presence of a low melted phase. The nanosized
state may be reached after less than 10 minutes of mechanical
activation (Fig. 5).
The first 10 minutes of mechanical activation the unit cell
parameter of iron remains unchanged, as well as the degree of micro
strains. Only the grain size of α-Fe goes down to 10 nm.
The mechanism of this initial step of mechanical alloying is not
clear but general for all systems (with negative or positive enthalpy of
mixing independently) if only one metallic phase has higher plasticity
(lower melting temperature) than the other one. Taking into account the
severe conditions of plastic deformation in the mill AGO-2 (8, 9)
assume a liquid like properties at least of one component in the near
5
interphase zone. This may lead to a and to a fast spreading of plastic
(liquid?) metal over the newly formed surface of a more hard metal. The
specific surface area estimated from TEM photographs is on the level of
40 – 60 m2/g. This assumption explains probably why we do not find Bi
in the X-ray diffraction. When taken in an amount of 10 % or less this
metal may be uniformly distributed on the grain boundaries forming a
layer with a thickness of a few atoms only.
Fig. 5. Evolution of the system Fe + 10 wt.% Bi in the course of
mechanical alloying.
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This assumption may be confirmed by the changes in XPES
intensities of Fe and Bi spectral lines in the course of a surface etching
by Ar ions bombardment. The profile of Bi concentration with time of
etching is shown in Fig. 6. The concentration of Bi falls down up to a
constant value within 3 minutes. Taking into account the rate of etching
around 1 nm/min the thickness of the Bi enriched near surface layer may
be estimated as 2 or 3 nm.
Fig. 6. Changing of XPES intensities with time of Ar etching of the Fe/Bi
mechanocomposite. The estimated rate of etching was near 1 nm/min.
The Mössbauer spectra also indicate clearly that during these first
10 minutes of mechanical activation there are no Bi in the first
coordination sphere of Fe (see Fig.7).
If we continue the mechanical treatment for a longer time we will
observe more profound structural changes which may be interpreted as
formation of classical solid solution (Fig. 5). This is to be underlined
that the increase of crystal lattice parameters starts only from the
moment when the low melting phase totally disappears from X-ray
diffraction. This fact indicates the dramatic changes in thermodynamic
state of one or both components when they reach the nano sized contact
conditions.
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Fig. 7. Mössbauer spectra for the Fe+10 wt.% Bi with time of mechanical alloying.
Further mechanical activation leads to the formation of a solid
solution which is positively detectable by X-ray diffraction after 40 min
of activation. At this stage we observe the formation of nano structure of
α-Fe (grain size l < 10 nm). Penetration of Bi into the Fe structure is
followed by a remarkable increase of microstrains (from near 0 value up
to 0.4%, Fig. 5). The concentration of Bi dissolved in Fe may be
8
estimated from Mössbauer spectra. The respective values are shown in
Tabl. 1 upon the time of mechanical activation.
Table 1
Parameters of Mössbauer spectra and concentration of Bi in the bcc solid
solution obtained by mechanical alloying of Fe/Bi mixture
, min
10
20
40
120
Н0, kE
330
329
330
333
Н1, kE
306
306
306
Р0
1.00
0.97
0.90
0.78
Р1
0.00
0.03
0.10
0.22
СBi, at.%
0
0.5
1.5
3.5
The TEM also confirms nano sized structure of solid solutions
obtained (see Fig. 8).
Fig. 8. TEM photograph of a Fe + 10 wt.% Bi sample taken after the very long
(120 min) mechanical activation.
A similar behavior we observed for Cu/Bi systems for which there
are neither solid solution no intermetallic phases on the equilibrium
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phase diagram nor any solubility of one metal in another. The mixing
enthalpy for this system calculated with the help of Miedema approach
(10), is about + 1 kJ/mole (positive).
X-ray diffraction study of the product obtained through
mechanical alloying for the initial mixture Cu + 10 wt.% Bi, revealed
the fast decreasing of Bi lines intensities up to zero within the first 10
min of mechanical activation. No shifts of a Cu lines positions were
observed on this stage though all reflections are remarkably broadened
(see Fig. 9). Nevertheless the chemical analysis after 15 minutes of
activation shows that the concentration of Bi is still 9.8 wt.%, that is
close to initial one 10 wt.%.
Fig. 9. X-ray diffraction patterns for the Cu + 10 wt.% Bi. Time of activation is
indicated on the curves.
TEM demonstrates the high defect concentration and nano sized
grain structure of a copper (Fig. 10).
The same changes were observed for the Fe + 10 wt.% In system
for which the grain size decreased up to 20 nm after 30 min of activation
with a simultaneous increase of micro strains concentration. The
changes in Mössbauer spectra were interpreted by the same way as in
the case of Fe/Bi system described earlier.
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Рис. 10. TEM photographs of a Cu + 10 wt.% Bi sample taken after 15 min of
mechanical activation.
In all these systems with so coherent response on the mechanical
activation the diffusion was not possible in a bulk state and became
available in the nanocomposite state obtained. Similar phenomena were
observed by Yasuda and Mori (11, 12) when they put together the nano
particles of different metals resulting in spontaneous alloying.
We cannot explain the formation of solid solutions in immiscible
systems by a classical way of local melting and subsequent quenching as
it is shown in Fig.1. Though melting is possible in our activation
conditions (8, 9) there is no mixing in a liquid state for the studied
systems. That means the melting itself, if any, serves as a forming factor
for the huge interface surface. In the nano state (nanocomposite) the
wetting seems to be very important because the heat of wetting is always
negative. This additive to the enthalpy of mixing plays a negligible role
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for the bulk systems and may be decisive for the nanocomposite state if
only a good wetting takes place during mechanical activation. In our
conditions (Ar atmosphere) we could avoid the fast oxidation of the
newly formed surfaces (due to severe plastic deformation, extrusion).
So, two metals could immediately and directly interact providing very
good wetting. To make this mechanism realistic we need spreading to be
a rather fast process. The high velocity of spreading in liquids
(especially in immiscible liquids) was confirmed experimentally many
times (13) but we could not find the accurate data for metals used in our
experiments.
Recently the possibility of spreading of one metal over the surface
of another one has been demonstrated by MD simulation for Ni/Al
system (14). Though this system is characterized by a negative enthalpy
of mixing the result of the computer experiment looks very convincing.
For the MD computations a nano crystal of Ni with 50 layers and 1250
atoms in each layer has been placed on the surface of larger size Al
crystal both oriented to each other by (100) plane. At the initial time
moment the 14 layers of Ni coherent to the surface of Al, were frozen
after what the system could freely relax to the state corresponding to the
temperature 500 K. A reverse picture has been modeled, too.
When nano particle of Ni is placed on the free surface of Al
crystal the wetting and spreading is remarkable after 0.1 ns, a very short
time of observation (Fig. 11)!
Fig. 11. The Molecular Dynamic simulation of two phases contact interface
evolution. Size of a small crystal is 5 nm approximately. (Ni is light gray;
Al is dark gray)
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In this pair of metals Al has the lower melting temperature and
evidently it is much more plastic than Ni. No diffusion is observed at
this time of contact. Our nearest purpose is to reproduce this computer
experiment for at least one immiscible pairs of metals described above in
this paper.
Concluding remarks
The main result of this paper is that formation of solid solutions in
thermodynamically immiscible metallic systems is possible. We
understand that this fact do not contradict with thermodynamic
limitations, and the probable explanation of this phenomenon leads to
the understanding of the important role of wetting and spreading of
metals in the course of severe plastic deformations during mechanical
activation.
Nevertheless, the kinetic aspect of this problem needs to be
understood, too. We suggest the driving force for diffusion on a nano
scale level when mechanocomposites are formed, probably connected
with a non-uniform (gradient) composition of nono grains coated by a
plastic (liquid) metal. But to study this process on such a deep level one
need a special techniques, which are not available in nanoscience till
now.
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