ultrasound action on strength properties of polycrystalline metals

THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI
FASCICLE VIII, 2006 (XII), ISSN 1221-4590
TRIBOLOGY
73
ULTRASOUND ACTION ON STRENGTH PROPERTIES
OF POLYCRYSTALLINE METALS
Luminiţa MORARU
Physics Department, Faculty of Sciences, University of Galati, Galati, Romania.
Luminita.Moraru@ugal.ro
ABSTRACT
The mechanical properties of pure aluminum were investigated by tensile test on
the sample with average grain size between 60 and 1000 µm. The stress strain
functions given by the models are fitted to the measured points. The grain size
dependence on the uniform yield stress was studied. In order to put in evidence the
effect of ultrasonic field on grain size, the measurements were carried out for
samples solidified under similar conditions both with and without sonication. For
pure aluminium, the author quantitatively evaluated the validity of Hall-Petch
model.
KEYWORDS: Yield stress, grain size, ultrasonic field, acoustic streaming,
cavitation.
1. INTRODUCTION
One of the underlying principles in materials
science is that properties can be deduced from knowledge
of the microstructure. By microstructure we
mean the crystalline structure and all imperfections,
including their size, shape, orientation, composition,
spatial distribution etc. In the cast of polycrystalline
metals the deformation process becomes more
complicated even in pure metals because of the grain
boundaries. The effect of the grain size on the
strength properties of polycrystalline metals has been
studied and the Hall-Petch (HP) equation describes
the relationship between the yield stress σ and the
grain size d as [2, 6-8]:
−12
σ = σ0 + Ky
⋅ d
(1)
where σ 0 and K y are temperature dependence constants.
The grain size dependence of the stress parameters
characterizing the tensile tests was determinate.
During solidification of the metals the most
common growth morphology is the dendritic
formation. In most of the castings manufactured by
different methods, the dendritic growth will occur.
When the solidifying process takes place in presence
of ultrasonic field, the equiaxed aluminum grains
appeared. The average grain size of aluminum grain
is smaller. The presence of ultrasonic field in molten
is equivalent to mechanical stirring of the melted
metal. The advantages of this method are fine
microstructure, reduced micro porosity and cracking;
primarily all the aluminum dendrites will be
transformed to spheroids/ellipsoids grains. This
technique produces “nondendritic rheocast structure”.
The room temperature plastic deformation behaviour
of different samples (samples solidified in presence
and in absence of ultrasonic field) of aluminum has
been studied. By ultrasonic treatments, a wide range
of grain sizes varying from 60 to 1,000 µ m was
obtained in this study. The Hall–Petch behaviour of
the samples showed distinctly linear regions, one in
the fine grain size range ( 60 µ m〈 d 〈 160µ
m ) in the
ultrasonic solidification conditions and the other in
the coarse grain size range ( 170 µ m〈 d 〈 1000µ
m ) in
natural solidification conditions. The Hall–Petch
parameter K y was significantly higher in the fine grain
regime than coarse grain regime. Hardness
measurements were also performed.
2. EXPERIMENTAL SETUP
The measurements of the stress-strain curves
were carried out on 99.97% purity aluminum of
composition given in table 1.
The tensile tests were carried out at room
temperature. We used samples solidified in presence
and in absence of ultrasonic field. The ultrasonic field
was created with a magnetostrictive transducer and an
<strong>ultrasound</strong> generator, to generate continuous
longitudinal waves into the liquid sample. We
preferred to introduce the <strong>ultrasound</strong> waves through
the bottom of the crucible. In this arrangement, there
is no barrier between the <strong>ultrasound</strong> source and the
melted metal. The stepped stainless steel horn was
used to transmit the <strong>ultrasound</strong> to the molten and it is
completely resistant to ultrasonic erosion. Typical
operating parameters were frequency of 20.338 kHz

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THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI
FASCICLE VIII, 2006 (XII), ISSN 1221-4590
TRIBOLOGY
and the nominal input power of 600 W. The acoustic
power dissipated by ultrasonic probe in 1000mldeionized
water at ambient temperature and pressure
as a function of electrical input power was determined
by calorimetry. These data were used to allow
selection of the appropriate input power to give
constant transmitted power. After the completion of
measurements, the ultrasonic horn was examined
microscopically. No attack of the stainless steel by
liquid metal samples was observed in either case, so
there was no evidence of contamination of the liquid
metal by alloying.
Table 1. Chemical composition of sample (ppm).
Fe Si Cu Zn
60 60 30 30
The grain size of specimens was determined
using the linear intercept method in conjunction with
a logarithmic Gauss distribution.
To make individual hardness measurements for
aluminium, Brinell hardness tests are widely used.
3. RESULTS AND DISCUSSIONS
Figures 1 and 2 show the yield stress as a
function of grain size for samples solidified under
similar conditions both with and without sonication.
−1 2
A linear relationship between σ and d can be
established and it shows that HP equation is valid.
The scatter of experimental values is high and it can
be explained by the experimental difficulties in the
determination of the yield stress in this very soft
material. Substituting the constants obtained for the
fitted straight line into eq. (1), we obtained:
−12
σ = 9.095 + 0.714 ⋅ d without <strong>ultrasound</strong>,
−12
σ = 9.620 + 1.166 ⋅ d with <strong>ultrasound</strong>.
The slope of the Hall-Petch plots (K y values) is
higher for samples solidified in ultrasonic field
presence. This higher value (see figures 1 and 2) it is
expected to be to twinning. However, this remark will
be followed up by further investigation.
The samples solidified in presence and in
absence of the ultrasonic field present a different
grain size due to various solidification conditions.
The samples solidified without <strong>ultrasound</strong>s presence
present the grain size significantly larger (fig. 3).
When the solidifying process takes place in presence
of ultrasonic field, the equiaxed fine-grained
aluminum appeared.
The average size of aluminum grain is smaller.
The grain size was measured on transversal sections
with areas of 40 mm 2 after mechanical and
electrochemical polishing in optical microscope
equipped with image analyzer. Investigations in our
laboratory on aluminum revealed superior mechanical
properties for samples solidified in ultrasonic field.
The mechanical properties obtained for the aluminum
are given in table 2. The ultrasonic field presence
caused hardness increased unto 28%.
σ ( MPa )
11.5
11.0
10.5
10.0
9.5
9.0
without us
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
d - 1/2 ( mm - 1/2 )
Fig. 1. The yield stress as a function of grain size
(without <strong>ultrasound</strong>).
σ ( MPa )
14.5
14.0
13.5
13.0
12.5
12.0
with us
2.5 3.0 3.5 4.0
d - 1/2 ( mm - 1/2 )
Fig. 2. The yield stress as a function of grain size
(with <strong>ultrasound</strong>).
a) b)
Fig. 3. Microstructure of 99.97%Al;
(a) without <strong>ultrasound</strong> and (b) with <strong>ultrasound</strong>.

THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI
FASCICLE VIII, 2006 (XII), ISSN 1221-4590
TRIBOLOGY
75
Without
<strong>ultrasound</strong>
With
<strong>ultrasound</strong>
Table 2. Hardness test values (HB)
1 2 3 4 5
21.8 21.8 22.0 22.2 22.3
28.0 27.9 28.2 28.2 28.0
It is known that grain boundaries could act as
barriers to dislocation motion, but we also viewed a
grain boundary as a sort of "film". If the properties of
the grain boundary were known then a more complete
analysis of their strengthening effect could be made.
Grain boundaries are simply planar defects between
adjacent grains, each grain having different
crystallographic orientations. Low angle grain
boundaries occur when the miss-orientation of the
lattices of adjacent grains is less than 15°. These
boundaries can be modeled as an array of edge
dislocations. The structure of high angle grain
boundaries is more complex. The other characteristics
of grain boundaries are that they tend to attract
impurities. In a sense, this might be thought of as a
film which can either strengthen or weaken a
material. Grain boundary segregation of impurities,
however, does not always embrittle a material [9].
Grain boundaries are lattice imperfections and
as such increase the free energy of the material. There
is a tendency to minimize the contribution from grain
boundaries and this involves reducing the grain
boundary area to grain volume ratio, something
which happens as the grains grow larger. While other
factors can cause grains to grow, this is essentially the
process involved in normal grain growth (also called
ideal grain growth). Non-ideal grain growth occurs
when grain growth is inhibited by the presence of a
second phase or is restricted by the edges of the
specimen. Ultrasonic <strong>action</strong>, cold work, pressure,
magnetic fields, etc. also cause grain growth to
deviate from the ideal case.
The effect of grain size on the hardness
properties of polycrystalline metals consists in
increasing of hardness for the sample with finegrained
structure. It is suggested that this morphology
of ingot solidified in ultrasonic filed results from
increased growth rate and smaller crystallite size. So,
reducing the grain size of a polycrystalline material is
an effective way of increasing its strength [4].
Under ultrasonic conditions, the acoustic flow
takes place in the liquid metal. It is clearly
demonstrated that the acoustic flows are associated
with the <strong>ultrasound</strong> absorption, whatever its nature.
However, the absorption coefficient is quite small for
the liquid metals, so the increase in the temperature of
the melt caused by absorption process has been
eliminated. In these conditions, the reason for this
prominent change in solidification kinetic is assumed
to be large-scale acoustic streaming. Its effect is a
permanent stirring of the melt so the effects of
thermal and mass homogeneity of the melt are quite
obvious [3]. The increasing in the intensity of fluid
flow can give rise to grain multiplication, which can
be attributed to the increased effective nucleation rate
caused by the extremely uniform temperature and
composition fields in the bulk liquid at early stages of
solidification. Also, the forced convection increases
the growth rate. The solidification starts by
heterogeneous nucleation at the crucible wall through
the so- called “big-bang” mechanism. Only a fr<strong>action</strong>
of the nuclei formed at this stage contributed to the
formation of the chilled zone and the majority of the
nuclei are transferred into the hotter bulk liquid and
remelted. The final solidified microstructure depends
largely on the amount of nuclei surviving after the
big-bang nucleation. Under the <strong>ultrasound</strong> <strong>action</strong> both
the temperature and composition fields of the liquid
metal are extremely uniform. The nuclei formed will
survive due to the uniform temperature field,
resulting in an increased effective nucleation rate. In
addition, the intensive stirring may also disperse the
cluster of potential nucleation agents, giving rise to an
increased number of potential nucleation sites. Also,
under forced convection, the nucleation and the
growth at the chilled wall were suppressed, while the
nucleation and growth in the bulk liquid were
enhanced [1].
It has been suggested that the forced convector
fluid flow induced by <strong>ultrasound</strong> may be sufficient to
break small dendrite arms and distribute them
throughout the melt. If a high energy boundary is
formed in a metal in contact with its liquid then the
condition indicates that the grain boundary will be
wetted by the liquid phase, i.e. replaced by a thin
layer of liquid and thus the dendrites break appear.
Further these broken dendrites act as nucleants and
grow as globular nondendrite structures. The acoustic
streaming produced the change in possibility that
hydrodynamic force to cause breakage of dendritic
arms under the solidification conditions. In the same
time, due to supplementary energy contribution, the
ultrasonic field presence hinders the long-range
ordering processes of atoms. At this moment, they act
as nuclei for the growth of more particles and the
relatively small dendrite spacing are created.
The possibility that fluid flow could disrupt the
crystal bonding is also considered [5]. The shear
forces resulting from natural convection flow of the
melt are too weak to disrupt the crystal bonding
during solidification. However under ultrasonic field,
these forces are dramatically increasing. The accuracy
of sonic measurements is reasonable taking into
account the difficulties associated with getting the
<strong>ultrasound</strong> into the melted metal.
The ultrasonic field presence into a liquid
causes cavitation phenomenon [1]. This imposes a
sinusoidal variation in pressure on a steady state
ambient pressure. One new question of this study is
the problem of cavitation and its microstreaming
effect. The effect of <strong>ultrasound</strong> increases with
increasing power, but not indefinitely since there is an
optimum value beyond which the effect diminishes.

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THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI
FASCICLE VIII, 2006 (XII), ISSN 1221-4590
TRIBOLOGY
When 20.338 kHz high-intensity <strong>ultrasound</strong> was
applied to the molten system, a mixing of the melted
metal close to the solid-liquid interface and the
crucible wall due to cavitation was produced. Near
the solid surface, cumulative jets can be generated
and the diffusion layer is thinned due to enhanced
mass transport resulting from microstreaming. In our
experiment, these optimum conditions in cavitation
were studied in deionized water at ambient
temperature. The ultrasonic treatment of liquid metals
differed essentially from that of aqueous solutions
and organic liquids. This is due to the different nature
of cavitation nuclei and, hence different conditions
required for the initiation and development of
acoustic cavitation. Only fine solid particles (mainly
oxides, e.g. Al 2 O 3 in aluminum melt) can act as
cavitation nuclei in metallic melts. At the same time,
because the molten metals feature light opacity, the
cavitation cannot be studied directly.
4. CONCLUSION
In the cast of polycrystalline metals the deformation
process has been studied related with the grain
size in order to highlight the strength properties of
polycrystalline metals and to study the applicability
of the Hall-Petch (HP) equation.
A linear relationship between σ and d
−1 2
can
be established and it shows that HP equation is valid.
This study was making for samples solidified
under similar conditions both with and without high
power ultrasonic field presence. Our investigations on
aluminum revealed superior mechanical properties for
samples solidified in ultrasonic field.
Studies into the Hall-Petch relationship show
that the factor K is different in tension and
compression. No systematic studies have been done
on the effects of solutes on the K values and the
difference in tension and compression. In future this
represents another aims to investigate.
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