Article Type : Research Article
Authors : Xu R, Anh HJ and Kim K
Keywords : Solidification; Gradient; Velocity; DS; Structure; Undercooling; Concentration of Al; TiAl
The
effects of the applied thermal gradient and pulling velocity, the spacing and
nucleation cooling are investigated in the present. High value would be found
when G was7.8K/mm in contrast to that 0.12~0.06mm was observed when high G was
10K/mm. That would be caused upon high v. The value of measured and literature
has agreed with the curve of 10K/mm. The effects of Al content and ternary
additions such as Mo, V and Si in as cast ? based alloys made by plasma arc
melting on solidification structures and mechanical properties were studied. ?
/?2 Columnar lamellar structures in Al-lean alloys because primary
solidification had higher room temperature fracture strength and strain than ?
phase structures through the reaction of L+?=? in Al-rich alloys. The fraction
of ?2 phase was found to decrease with increasing Al content in binary alloys.
?1 is about 140~40 ?m, which might be caused by low velocity approximately. The
value ?Tn with 10K is a little smaller than that of 1K. So the well choice is
10K for a bit high v. The ?Tt are in same relation to v in proportional. The
reason for the big varies is analysed as nucleated temperature vary. The G with
10K/mm fit to large v, and the value of both G and ?Tt is smaller than 1K/mm.
That might be explained on changed v that means big v has large value. The
temperature of reactions were concluded and the angle between lamellar and
growth direction were investigated in the equiaxed and columnar grains
meanwhile the concentration of Al and the fraction of ?/? and ?/L were studied
in this paper. The direction between lamellar and growth direction is 0 and 90°
upon primary phase ? and ? respectively. The structures of alloy are
equilibrium phases such as ?2+? & ? taken columnar and equiaxed according
to Al content. The primary phase and equilibrium phase may be confirmed
according to phase diagram for Ti-44?56at. %Al alloys. As for Ti-44at. %Al used
?????+ ? to measure Al concentration, while for above Ti-46at. %Al used ???.
TiAl alloys have had high strength and high
temperature strength compared with other high temperature (HT) alloys,
anti-oxidized and better creep properties, was dominant as very promising
material to substitute for Ti and Ni base. In particular DS TiAl alloys with an
aligned lamellar microstructure (MS) have a very good combination of strength
and ductility over a wide temperature range that columnar dendrite structures
are desired. As for the mechanical mechanism, Hunt developed the first
analytical model to predict transition on the basis of equiaxed grains
nucleated in the constitutional undercooling region ahead of a columnar front
blocking the advance of the front if they occupy a sufficient volume fraction
[1]. It was estimated that the preferred growth directions of b
dendrite grown at HT near melting point was in the [001] b
direction at a growth rate of 30mm/h and in the [111] direction at a growth
rate of 90mm/h. The mechanical properties controlled by microstructures had
inverse relationship in them, which had been reported [2-3]. A b
solidified directional solidification (DS) method has used seed crystals, while
the initial solidification must be crossed into full transformation [4].
However in binary TiAl, Al contents with the full b
transus were low at RT so that their mechanical properties were brittle. Adding
elements will move to b
stabilization of Al rich was confirmed [5-12]. In the full b
transus the thermal gradients was low with Bridgman method and high with
Floating zone method (FZM) to be used. Using b
solidified method, lamellar orientation of dendrites must be aligned to grow in
the direction [001]. An alternative approach for balancing mechanical
properties is gained by DS techniques. The calculation results indicated that
the columnar branch spacing depends not only on the thermal gradient and the
pulling velocity, but also on number. A spacing adjustment can occur to develop
to new columnar grains. As for the effect of them on the thermal gradient and
velocity, qualitatively agrees well with the literature. By analysis it was
evaluated that the preferred growth directions of primary ? dendrite near the
melting point had been in the ? and ? primary phase at 10~180mm/h. On the other
side g-TiAl
with high ratio strength had been promised in the high temperature structures
in the future. The TiAl alloys developed by now had excellent casting property
so that they could be used for the engine of craft and automobile with good
evaluation. For the needs of high temperature, light quality & speed, new
advanced materials would be searched: 1) high melting point; 2) low density; 3)
elastic modulus; 4) good structure stability & excellent oxidized
resistance [13]. In the high temperature application, such as engines.
Intermetallic Compounds of Ti2AlNb could compete with lately developed TiAl and
the HT titanium base alloys & nickel base materials [14]. Al content which
was one of the important factor to affect process and mechanical property was
chosen by controlling microstructure in TiAl base alloys. According to Al
content the casting structure was obtained with different primary phase. In
general, equiaxed grains had been trend to form when primary phase was b, columnar’s had been
trend to form when primary phase was a.
The lamellar structure was still brittle in spite of good toughness. Moreover
the course lamellar with no crystallizing was produced with continuous and slow
growth so that structure was not easily controlled by heat treatment. However
the full lamellar would fit for promoting high temperature strength because the
lamellar was possible for use in high temperature. Using b solidified method,
lamellar orientation of dendrites must be aligned to grow in the direction
[001] [15,16]. An alternative approach for balancing mechanical properties is
obtained by DS techniques. The purpose of this study is to examine
solidification mechanism such as the effects of the temperature of reactions,
and the angle between lamellar and growth direction and the Al concentration on
TiAl alloys.
The thermal dynamic super alpha cooling has been to
avoid or eliminate heterogeneous nucleation role, promote Gcr, hold back
homogenous nucleates making alloys or metal difficult to arrive cooling on the
general status. Super cooling method had changed thermal dynamic to obtain high
cooling. Herlach had demonstrated super cooling melt and rapid cool, liquid
alloys or metal had same mechanism being rapid solidification. The solute at
the S/Liquid interface is distributed, at the local of the secondary dendrite
arm spacing by diffusion or convection. It is to show the effect of coarsening
can be accounted for in a conventional segregation model by a back-diffusion
term. That results in a net diffusion process. The solidified condition is for
homogeneous nucleation, here DG
is change of system free energy and r is radius of nuclear crystal. The primary
dendrite arms space generally decreases with increasing cooling rate, and it is
crucial to take that effect into account. The relatively simple relationship
between given in as was found to be applicable to a wide range of DS. It is
thought to be ideal directional solidification. It is specified by the average
temperature gradient G, and a speed v, so the mean cooling rate is described
as. The extent of convection in the procedure is the relation used to calculate
the local permeability of the mushy zone as a function of the liquid volume
fraction and primary dendrite arm space l1.
It implies a lower space leads to lower permeability and a higher resistance to
flow in the mush zone. A best fit of calculated data was for parallel and
perpendicular to l1. The value of l1
generally decreases with increasing cooling rate. It was found to apply to a
range of DS alloys in spite of preciser’ done no bad. The procedure to solve
the conservation equations. A phase equilibrium in this zone offers a way to
calculate the solid volume fraction. Some modifications necessary to the use of
equilibrium instead of a relation between lquidus temperature and
concentration. In the evolution of the morphology of solid and liquid, growth
velocities have made important and complicated roles. In the low velocity zone,
with the growth v increased, make plane interface unstable. However, in the
high velocity zone, with the increasing of v it promotes interface to develop
absolutely stability. It has increased the effect of composition undercooling
and curvature. With raising growth rate v mushy zone length shrinks shorten to
a certain of mushy length. That is a factor of Dendritic-cellular change.
The results measured with EDS had been shown. The
deviation with 2at% was found. In Fig equilibrium solid was complexed and
sensitive to concentration Al. According to the phase diagram above 55at% the
primary was been solidified. That means that in the case of Ti-44~48at%Al was
transformed with L??, there are the solidified course as follow. The lamellar
structures were thought to be upon ? ? (?+?) L ? (?+?) L to obtain the plate
nucleation. It was found that the Ti-44Al was formed to the equiaxed in center
and slight columnar in edge with the binary structure. The grain growth was
formed from outer to center in the 48Al. On the contrary the fine grain was
formed except the growth in the 52at%Al. In 44Al full lamellar structure was
shown that was thought to be primary ?. In 48Al fine lamellae was formed with
the 80~90 ?direction with growth most. Other boundaries among grain growth were
found. Upon Y. that was formed to the primary ? from liquid. So it was thought
that those rate was fast the primary ?. Meanwhile those columnar grains was due
to necleate from solidification. The 48Al in the grain obvious dendrite was
formed so that coarse lamellae ?/? were grown with different directions in
varied grains. It was found that was due to the random growth directions to
view in general. That was through the reaction of L+??? to form the grains had
surround the lamellae. On the other hand those fine structures was upon
segregation and heterogeneous. That was the deep Al seperation and non-stead
phases was to form the primary ? forms the second phase to form the
non-equilibrium structures within the boundaries with the peritectic reaction.
Moreover it was thought that peritectic transformation ?+? ?? was not to be
proceeded upon confining the solid diffusion with kinematics. The separated ?
inter dendrites was shown in the 52~54Al obviously so that the following
reaction was obtained. L?L+?? L+?+?segregation ??+? segregation. Structure of
the A alloy consists of coarse grains, some of which contain the widely spaced lamellar.
It shows the general features of the grains. Analysis of the lamellar grain
shows that it consists of g,
twin-related g
and ? two phases as has been observed by other investigators. It should be
noted that while the increase in Ti/Al ration refines the grain size, it has an
opposite, and small effect on the inter lamellar spacing. Results above clearly
show the beneficial effect of adding on the structure, refinement of grain size
and spacing and decrease in volume fraction increase with addition. However the
ration greatly refines the size and further lows the tetragonality and unit
cell size. There was the gradient-solidified velocity and phase transformation
studies at the interface of solid/liquid From the Fig. 1 the decreased trend
will be observed the total value is about 140-40?m/s which might be caused by
low velocity and a certain cooling rate of 0.5K/s. The low value of 0.3K/s is
in low rate approximately. As shown in reference about 20K would be fitting one
for 48Al in terms of phase diagram. As seen in Fig. 1 GD is into right as
arrow, v is into left. The coarse grain will be gained in low v as a (Figure
1). This is a thermal flow, growth grain has been right part. Left takes role
of seed effect, the better state is 10micm/s. usually first v was demanded
lowly make sure to be grown with morphology of plane and cell. Heating
temperature was 1492?C taken on Ttip. According to
Mean m =58.6
Variance
s2 =1/n S (orie.i-m) 2 =40
Standard
deviation ? =6.3
The trend for r and rate is shown that a line was observed to be negative proportional as seen in (Figure 3). That may be explained upon the raising v. That of rate is 10~250?m/s. Here r is the space of dendrite that was influenced by rate. As the rate was low below 20?m/s the trend maintained the near a certain value. The lowest curve would happen to under 10?m/s with least rate. That may be due to the r limiting. Where much nucleation will occur with the minimum velocity. The trend for K and rate is shown that a line was observed to be negative proportional as seen in (Figure 4). That may be explained upon the raising vG. That of rate is 10~1250?m/s. Here ?1 is the space of dendrite that was regulated by K. Where K is 1.75. ?1/K could be high as vG was low. That was to be caused with low cooling rate as predicted. The nucleation growth would occur much. In the meantime the quantity will decrease. The growth directions are shown in (Figure 5) with the lamellar orientation. The certain angels had been arranged along the GD in terms of the primary phases, they may be formed on parallel rule. The lamellar directions are thought to be like as Table 1 which specified two types. One is 0 and the other is 90° upon primary phase (Table 1). The detail investigation is concluded as the same results, being in accordance with primary phase shown in (Figure 5). The structures of alloy are equilibrium phases such as ?2+? & ? taken columnar and equiaxed according to Al content. The DS course will be stated follow, as for 44~46 at. %Al (I) the growing direction is (110) [001] ?, and for 48 at. % (II), 50~54at. %Al (III) that is (0001) [1120] ?, for 56at. % Al (III) is (111) [110] ?. The peritectic temperature is 1490°C & 1470°C in terms of ? & ? and eutectoid reactive with1120°C. As shown in Table 2 detailed reactive results demonstrated the phase transformation states (Table 2). Q was 1.98KJ/mol and 1.04KJ/mol according to peritectic and eutectoid reaction respectively for Ti-48 at. %Al. They reacted as below sequence of I, II & III. The equilibrium phase is the final two phases or singles as shown above. In this study, it is assumed that there is a constant positive liquid thermal gradient.
This simulated
material is a Ti-44~56 at. % Al binary alloy, and its properties and the model
of parameters used in the simulations are given in reference. Characteristic
for phase transformation in binary ? may describe as follow. PGD (110)[001] ?
is for 44 at.% Al, (0001)[1120] ? is for 50 at.% Al, and (111)[110] ?(111)[110]
? is for 56 at.% Al. The primary phase may be ? and ? (or ?) for 44 and 48 at.
% Al respectively. The equilibrium phase
will be ?2+ ?, (?2+ ? or) ? for 50 at. % Al. That may be
primary ? and equilibrium ? for 56 at. % Al. The may be transition phase for 52
at. % Al. The equilibrium phase is ?2+ ? and ? for 44~48 at. % Al
and 52~54 at. % Al. The used procedure is similar to the one described above
having an explicit relation between liquidus temperature and concentrations.
During directional solidification, the change in liquid solute concentration
affects the undercooling, and in turn, results in the nucleation and growth
processes of the equiaxed grains. In this section, the calculated solute
concentration profile ahead of the growth fronts is provided and the solute
interaction ahead of growth front is discussed. In solid region, there is a
slight increase in solid composition. At the columnar front, there is an
exponential drop in solute concentration at the Solid/Liquid interface, quickly
decaying to bulk liquid composition along the growth direction in liquid
region. It shows the solute variation in the inter-dendritic region between
primary columnar dendrites at solidification distance or different time. It can
be thought that a gradient in solute concentration is observed in the liquid
region, giving a slope of a certain about 1.2 at. %/mm, which agrees well with the predicted values. Comparing the results,
because of the low concentration gradient, the greatest undercooling area
occurs at the dendrite groove region rather than at the region ahead of the
columnar tips, making it a favorite location for the nucleation of the equiaxed
grains. It can be seen from the cooling that increasing the thermal gradient
decreases the maximum cooling in the liquid along the dendrite axis.
This work was supported in part by the Korea of
Science and Engineering Fund, under the Specified Base program Granted as
96-0300-11-01-3.