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
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.