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
, G=7.8K/mm. L=40~33mm, in the state of primary phase was determined by
the lamellae orientation seen as (Figure 2). When the 44 at. pct Al the b upon 6~42 orientation were confirmed, which was the same as described in
Chapter III 3. For 48 and 52Al the b may be known according to
angle of 80 for 48 at. pct Al a nd 85 & 58 for 52 at. pct Al. According to
v that a had been observed for the mostly state in the scope of below 100?m/s.
That might be upon the [0001] PGD of primary phase which caused the near 90?and
45?partly distribution respectively. The trend would be raised in the speed of
below 40?m/s and reduced after 80?m/s that may influence the alignment of
lamella orientation a certain. Here the curve for Ti-43Al-3Si was for
reference. The positive line is the tendency. Upon Al increased the angle will
negatively tend to occur after 40 micm/s, that may been caused by Al amount. A
concept of distribution has given a set of measurements of n being angles of
TiAl-3Si and Ti-44~52Al here. For measuring change quantitatively analysis has
been taken as reference. The high of ? means the high of the deviation. By the
analysis the value has better credit than the 7.0. That means the orientation
between lamella and growth had more approached angles to near 45~70?. It was
included that no many arrange with orientation of 0~45°should be happen basally
in those alloys. It probably should be caused by ? solidification more than ?,
which was thought. The g solidification may be the
one reason for Al rich alloys.
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.