Materials Science and Engineering A244 (1998) 138 – 144
The vacuum hot pressing behavior of silicon carbide fibers coated
with nanocrystalline Ti–6Al–4V
Joseph M. Kunze *, Haydn N.G. Wadley
Intelligent Processing of Materials Laboratory, Department of Materials Science and Engineering, School of Engineering and Applied Science,
Uni6ersity of Virginia, Charlottes6ille, VA 22903, USA
Abstract
The vacuum hot pressing (VHP) of silicon carbide monofilaments coated with nanocrystalline Ti – 6Al – 4V has been studied.
During consolidation, very high densification rates were observed, even at temperatures and pressures well below those normally
used for processing conventional Ti–6Al–4V. From the cross-sections of partially consolidated specimens, the evolution of coated
fiber–fiber contacts and pore shapes were determined. The pores were found to be cusp-shaped throughout the consolidation
process. Columns of coated fibers were observed to form along the loading direction and resulted in regions of locally high fiber
volume fraction. Simulations of the VHP experiments were performed using a model which incorporated time and temperature
dependent microstructure relationships. In the model, the initial densification was based upon a recent micromechanical contact
analysis for a metal coated fiber. Final stage densification was analyzed by modifying the Qian et al. strain rate potential for a
power law creeping body containing isolated cusp-shaped pores. Overall, the simulations compared well with the experimental
density data, although the load supported by the regions of locally high fiber volume fraction resulted in the model slightly
overestimating the observed densification time response. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Vacuum hot pressing; Silicon carbide fibers; Densification
1. Introduction
Silicon carbide (SiC) fiber reinforced titanium or
nickel composites have exceptionally high specific stiffness, strength, and creep resistance and continue to
attract much interest [2]. A relatively new approach for
producing high quality metal matrix composites involves depositing the matrix material directly onto the
fibers via physical vapor deposition (PVD) processes
(e.g. sputtering or e-beam deposition) [2 – 6] and then
consolidating them to near-net-shape using either hot
isostatic pressing (HIP), vacuum hot pressing (VHP) or
roll bonding. The consolidation process must densify
the composite with minimal damage to the fiber or its
interfacial coating. Failure to do so will result in a loss
of composite strength, creep rupture life, fracture
toughness and fatigue resistance [7,8].
The PVD process, performed at low temperature,
produces a nanocrystalline matrix material with a grain
* Corresponding author. Present address: Triton Systems, 200
Turnpike Road, Chelmsford, MA 01824, USA. Tel.: + 1 978
2449500; fax: + 1 978 2449501.
0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 8 3 6 - 8
size of : 30–100nm [6]. Recent experimental studies
with low temperature sputtered Ti–6Al–4V have
shown that this fine grain size leads to an enhanced
superplastic effect when the temperature is within the
a+ b phase region (i.e. : 760–900°C) [6]. However,
the phenomenon is complicated by the additional observation of initially rapid grain growth during the
consolidation process which could result in a matrix
mechanical behavior that is a potentially strong function of the process path. Nevertheless, the opportunity
exists to consolidate these coated fibers into metal
matrix composites at temperatures and pressures well
below those conventionally used in processing these
materials. The ability to do so would reduce the risk of
damaging the fiber and its interface.
In this study, we experimentally investigate the consolidation behavior of metal coated fibers under constrained uniaxial compression. Several of the tests were
deliberately designed not to reach full density in order
to determine the effects of the stress state upon the
internal geometry of the specimen. The densification
responses of the experiments were recorded and compared with the predictions of a densification model.
J.M. Kunze, H.N.G. Wadley / Materials Science and Engineering A244 (1998) 138–144
2. Experimental procedure
2.1. Sample preparation
Tungsten cored s 1240 fibers with a nominal diameter of 100 mm and a carbon – TiB2 duplex coating [9]
were used for the consolidation study. A Ti –6Al–4V
matrix was deposited on the fibers via a sputtering
process at 3M’s Metal Matrix Composite Center (Mendota Heights, MN) using similar conditions to those
reported by Warren et al [6] ( :400°C processing temperature). The sputtering process produced an initial
grain size on the order of 30 – 100 nm [6]. The combination of the near line-of-sight sputtering process producing an asymmetric coating and the thermal expansion
mismatch between the fiber and matrix caused a large
number of the coated fibers to bend upon cooling. The
resulting curvature made it difficult to produce a close
packed fiber array and resulted in a random packing of
coated fibers.
Bundles of the coated fibers were cut to a length of
: 57 mm. The bundle was positioned within a graphite
channel die with a b21S titanium alloy foil next to the
coated fibers and a graphite foil adjacent to the die as
shown in Fig. 1. The graphite foil prevented the specimen from reacting and adhering to the die while the
b21S foils kept the softer graphite foils from penetrating into the specimen. The titanium alloy foils also
facilitated in measuring the composites height after
consolidation by providing a clear boundary with the
Ti –6Al–4V matrix.
After loading the coated fibers and foils in the die,
the die was positioned in the VHP. The VHP was
equipped with a single zone molybdenum furnace. Two
type-K thermocouples were used to monitor and con-
Fig. 1. Experimental set-up of graphite channel die.
139
trol the furnace. An additional type-K thermocouple
was placed in a 1/2 in. bore hole in the die and used to
monitor the sample temperature. A strain gauge was
attached externally to the ram and used to record the
ram displacement. A PC monitored and stored the
pressure, furnace temperature, sample temperature and
ram displacement.
There were three contributions to the displacement of
the ram; the thermal expansion of the ram and die, DhT,
the elastic compliance of the ram and die, DhC, and the
sample densification, DhD. The overall displacement
may thus be written as;
Dh =DhD + DhT + DhC
(1)
In order to determine the displacement due to densification, the effects of thermal expansion and the elastic
compliance were determined by performing calibration
experiments in which the die, containing two graphite
and b21S foils, was placed in the VHP and ramped to
the temperature setpoint. The thermal displacement was
recorded as a function of temperature. Although the
thermal expansion of the composite was not measured,
it was determined that its thermal expansion was B1%
of the displacement due to densification and effected
the initial relative density measurement by B0.25%. At
the temperature setpoint, the pressure was slowly increased and the elastic displacement as a function of
pressure was recorded. With this data, it was possible
to determine the displacement due to densification.
2.2. Process cycle
Specimens were consolidated using various combinations of temperature and pressure [10]. Three experiments conducted at a temperature 840°C and pressures
of 10 and 17 MPa will be examined here. Before
consolidation, the vacuum chamber was purged by
pulling a vacuum and back-filling twice with argon.
After the purge, a final vacuum on the order of 10 − 6
Torr was pulled. Due to atmospheric pressure acting on
the ram, a pressure of : 0.25 MPa was applied to the
sample when the chamber was evacuated. In all of the
experiments, the temperature was raised at 50°C min −
1. The setpoint was maintained to within 9 5°C. Before
pressure was applied, thermal equilibrium was attained
by holding the temperature for 15 min before the
pressure was applied. The thermocouple attached to the
die verified thermal equilibrium had been attained.
Pressurization occurred very rapidly with the target
pressure being attained within 30 s of application. The
pressure setpoints were maintained to within 90.25
MPa of the target pressure. After holding for the
programmed amount of time (6 or 8 h), the furnace was
allowed to cool. The cooling rate was : 50°C min − 1
for temperatures above 400°C.
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J.M. Kunze, H.N.G. Wadley / Materials Science and Engineering A244 (1998) 138–144
Table 1
VHP process conditions and metallography results
2.3. Post consolidation characterization
After consolidation, the specimens were cross-sectioned, mounted in epoxy and polished. Using an optical microscope, measurements of the composites height,
hf, were made across the width of the specimen and the
average computed. The specimens were then examined
in a scanning electron microscope (SEM). Using an
image analysis software package, the volume fraction of
fiber and the final density of each specimen were determined.
One may calculate the density, D, of the preform at
any time from the final metallographically determined
density, Df, the final height of the composite, hf and the
measurements of the displacement made by the strain
gauge (calibrated by Eq. (1)) using;
D= Df/[(1+ Dh)/hf]
(2)
From the total displacement, the initial density may
be determined. The uncertainty in the computed values
for the initial density arises from the variability in the
values of the final density and the final height. The
difference in the starting densities may be attributed to
the random nature of the bundle caused by the curvature of the coated fibers and to the uncertainty of the
values used in Eq. (2).
2.4. Grain size
The importance of the grain size on the superplastic
nanocrystalline matrix and the resulting densification
kinetics has been noted elsewhere ([6,10 – 12], unpublished data). In order to verify the grain growth models
for the extended periods of time used in the VHP
experiments, the final grain size of the consolidated
specimens were measured. Cross sections of specimens
from each temperature were etched using a Kroll’s
reagent and the grain size determined using Hilliard’s
circle intercept method [13].
3. Experimental results and discussion
3.1. Metallography results
The results of the metallographic analysis performed
on each of the experiments are summarized in Table 1.
Metallographic analysis of each composite revealed
that the fiber volume fraction of experiments A and B
was very consistent at :46%. For these experiments,
the effect of the fiber on the consolidation process
should be the same. In order to investigate the effect
the volume fraction of fiber has on the densification
process, the coated fibers used in experiment C were
prepared differently. As the volume fraction of fiber is
dependent upon how much matrix is deposited during
Temperature (°C)
Pressure hold (MPa)
Time at pressure (h)
Initial density, Di
Pressurization density, Dp
Final density, Df
Fiber volume fraction, 6f
A
B
C
840
17
8
0.586
0.627
0.972
0.461
840
10
8
0.549
0.584
0.928
0.461
840
17
6
0.533
0.598
0.999
0.324
the sputtering process, a longer sputtering time was
used to deposit more matrix onto the fibers used in
experiment C resulting in a fiber volume fraction of
: 32%.
3.2. Internal geometry e6olution
Examination of the cross-sections of the polished
specimens revealed the formation of columns of coated
fibers in the direction of the applied pressure. The
formation tended to occur with the relatively flat sides
of the coated fibers making contact with each other.
Fig. 2(a) shows the initial formation of these columns.
It was noted that there were relatively few lateral
contacts at this stage indicating that the restraining
effect of the die wall had not yet been fully realized.
As the density increased, the number and size of
lateral contacts also increased. However, contacts
within the columns of coated fibers grew even faster as
they were perpendicular to the loading direction and
apparently under the greatest stress. Voids located on
either side of a column remained large. Further densification increased the size of the lateral contacts. The
pores on the sides of the columns also decreased.
However, since these types of voids were mostly filled in
with matrix flowing from between the fibers within a
column, the distance between the fibers decreased in the
direction of the applied load. Approaching full density,
the columns were still evident as seen in Fig. 2(b). Note
that the two remaining voids shown in this micrograph
are on either side of a column of fibers. The distance
between the fibers is significantly less in the direction of
the applied load compared with the lateral distance
between fibers.
3.3. Grain growth kinetics
The grain growth kinetics of nanocrystalline Ti–
6Al–4V have been investigated by Warren et al [6]. At
840°C, the grain growth relationship was of the form:
d= 0.20+ 0.23t 0.20
(3)
where d is the average grain size in mm and t is the time
in s.
J.M. Kunze, H.N.G. Wadley / Materials Science and Engineering A244 (1998) 138–144
141
In order to verify the empirical grain growth relationship of Warren et al. for these extended exposure
times, measurements of the matrix grain sizes were
made on the consolidated specimens. The measured
grain size of the matrix in the VHP experiments was
found to be 1.9490.12 mm which compared very well
with the predicted grain size of 2.00 mm. Conventionally processed Ti – 6Al – 4V typically has a grain size
of 3–7 mm [14]. Although not experimentally
validated for even greater lengths of time, Eq. (3)
predicts that at 840°C it would take : 74 h for this
material to reach a conventionally processed Ti–6Al–
4V grain size of 3 mm. These results therefore indicate
that the enhanced superplastic nature of this matrix
might be realized for extended periods of time at temperature.
Fig. 3. Time dependent behavior for a typical VHP consolidation
cycle (experiment B). (a) Densification response and (b) temperature
and pressure histories.
3.4. Densification kinetics
Fig. 2. Initial VHP densification of nanocrystalline Ti – 6Al – 4V
coated s 1240 fibers illustrating the formation of columns with few
lateral contacts (750°C and 3 MPa for 8 h). (b) Final VHP densification illustrating the decrease in fiber spacing in the direction of the
applied load and the resulting non-uniform fiber distribution near full
density (900°C and 17 MPa for 8 h). Pressure was applied in the
vertical direction.
The densification kinetics, along with the pressure
and temperature profiles, of a typical experiment are
shown in Fig. 3. Note that as the temperature was
increased, the atmospheric pressure acting on the ram
caused an increase in the density. The reason for this
behavior is 2-fold. First, the coated fiber contacts were
initially very small resulting in a high contact stress for
even a very low applied ram pressure. Second, as the
temperature increased, the matrix became superplastic.
It has already been shown that the nanocrystalline
microstructure of the matrix leads to an enhanced
superplastic effect. Hence, the 0.25 MPa resulting from
the atmospheric pressure acting on the ram caused a
high enough contact stress for the matrix to flow and
led to densification. Similar behavior was observed in
HIPping experiments reported by Kunze and Wadley
[12] where a pressure of 2 MPa caused an increase in
density as the temperature in the HIP was increased.
After thermal equilibrium had been achieved, the
pressure was applied. We denote the density at this time
as Dp (see Table 1). As seen in Fig. 3, the pressurization
rate was very rapid and may be interpreted as a step
function. The densification response to the application
of pressure was likewise very fast. After the initial rapid
increase in density, the densification rate decreased
(note the decreasing slope of the densification profile in
Fig. 3).
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J.M. Kunze, H.N.G. Wadley / Materials Science and Engineering A244 (1998) 138–144
As each experiment underwent a similar process cycle
involving a single temperature and pressure hold, it is
convenient to compare the densification response of
each sample from the pressurization density, Dp. As
seen in Fig. 4, the initial densification response of
experiment A, conducted at 17 MPa, was much greater
than the response of experiment B, done at 10 MPa. In
both cases the densification rate, illustrated by the slope
of the profile, dropped off rapidly after this initial
densification. There are several contributing factors
leading to this. First, the stress acting on each contact
decreased with increasing density due to the growth of
the contact areas and the number of coated fiber–fiber
contacts. Second, the effect of the grain growth on the
densification rate of this material has already been
demonstrated to cause a decrease in the densification
rate in proportion to 1/d 2 ([10 – 12], unpublished data).
In addition to these factors, as the matrix flowed out
from between the fibers comprising a column, the local
fiber volume fraction increased. Previous work ([15],
unpublished data) has shown that the flow resistance of
the matrix increases as the fiber volume fraction is
increased. This factor contributing to the decrease in
the densification rate, is due to the deformation characteristics of constrained uniaxial compression. Finally,
any frictional effects within the die would consume a
larger proportion of the applied pressure at the lower
load.
Fig. 4 also shows the effect of the volume fraction of
fiber. Both experiments A and C were conducted at 17
MPa but had different fiber volume fractions; 0.461 for
experiment A and 0.324 for experiment C. This clearly
had a significant effect on the densification process.
Experiment C reached a much higher density as there
was more matrix to flow and less constraint from the
fibers. The overall fiber volume fraction was much
lower and so the local volume fraction of fiber did not
become as great as in experiment A.
Fig. 4. VHP experiments conducted at 840°C; experiments A (17
MPa, 0.461 fiber volume fraction), B (10 MPa, 0.461 fiber volume
fraction) and C (17 MPa, 0.324 fiber volume fraction).
4. VHP process simulations
It is possible to simulate the VHP process using a
model developed from a densification potential for
general loading and evaluating it for constrained uniaxial compression as experienced in a VHP channel
die. Details of the model are described elsewhere
([10], unpublished data). Briefly, early stage densification is predicted using a micromechanical analysis of
contact deformations combined with the experimentally determined number of coated fiber–fiber contacts as a function of density. The Qian et al. [1]
strain rate potential for cusped-shaped voids, modified
to incorporate the presence of the fiber, provides the
required means of modeling the specimen as it approaches full density. Although the coated fiber–fiber
contact evolution was determined from the VHP experiments, the model assumes the distribution of contacts to be isotropic. The effect of the non-uniform
distribution of the number of contacts has a second
order effect on the densification behavior of the composite. The model addresses the effect of a rigid
monofilament upon contact deformation of the matrix
and the somewhat unusual matrix mechanical properties resulting from the nanocrystalline matrix produced by the PVD process (e.g. the initial rapid grain
growth). The densification mechanisms of plasticity,
power law creep (PLC) and diffusion accommodated
grain sliding (DAGS) are incorporated within the
model. As has been done successfully elsewhere [6,10],
the two phase system of Ti–6Al–4V is taken into
account by imposing an isostrain/isostrain-rate
condition. Along with the process conditions, inputs
into the model include material properties for each
phase of Ti–6Al–4V and the appropriate fiber volume fraction and initial densities reported in
Table 1.
The simulations were performed at constant temperature as thermal equilibrium had been reached in
each VHP experiment before the application of pressure. Upon pressurization, represented by a step function, the plastic densification was determined. The
response of the time dependent mechanisms, PLC and
DAGS, was then started at this density.
The results of the simulations are plotted in Fig. 5,
along with the experimental density data for comparison. The model predicts the overall trend of the densification fairly well. At 17 MPa, experiment A (Fig.
5(a)), the model only slightly overestimates the densification response. When the pressure is decreased,
experiment B (Fig. 5(b)), the overestimation increases.
Comparison of the simulations of experiments A and
C (Fig. 5(a) and (c)) reveal that the overestimation of
the model decreases with reduced fiber volume fractions.
J.M. Kunze, H.N.G. Wadley / Materials Science and Engineering A244 (1998) 138–144
143
the applied load. Based upon the finite element contact
mechanic research used in developing the models
[15,16], this was noted to cause a decrease in the
densification rate due to the increased flow resistance of
the matrix with the locally increasing volume fraction
of fiber.
Further examination of experiments A and C which
demonstrate the effect of the fiber volume fraction,
provide additional insight into this effect. As noted
earlier, the correlation between the model and experiment improves as the fiber volume fraction is reduced.
The lower fiber volume fraction in experiment C means
that there is more matrix available between the fibers
and so the local fiber volume fraction cannot become as
great as in experiment A. Hence the agreement is better
for experiment C.
6. Summary
Fig. 5. VHP model simulation and experimental density data for
experiments (a) A (17 MPa, 840°C), (b) B (10 MPa, 840°C) and (c) C
(17 MPa, 840°C).
5. Discussion
In general, the model overestimates the densification
response of the samples. From the experiments and the
way in which the process was modeled, a possible
factor which would affect the densification behavior is
the local variation in the fiber volume fraction observed
earlier. In the model, the fiber volume fraction was
assumed to be constant and uniform throughout the
composite. The formation of columns observed previously demonstrated a ‘squeezing out’ of matrix from
between the fibers comprising the columns during densification. The voids on either side of the columns were
filled in by this movement of the matrix. Hence, the
distance between the fibers decreased in the direction of
The VHP experiments have identified several important factors in the densification of nanocrystalline metal
coated fibers under constrained uniaxial compression.
(1) An initial very high densification rate was observed in the experiments. The two contributing factors
to this enhanced densification were the small contact
area resulting in relatively high contact stresses and the
very small grain size causing an enhanced superplastic
effect.
(2) During densification, the formation of columns
of coated fibers were observed. The formation of the
columns and lack of lateral contacts at low densities
were interpreted to mean that the die wall reaction
force was initially quite low.
(3) A local variation in the volume fraction of fiber
was observed as a result of the formation and subsequent deformation of the coated fiber columns. It was
noted that voids on either side of the coated fiber
columns were difficult to densify as material had to
flow from between the fibers.
(4) A densification model was used to simulate the
VHP process. Overall, the densification model compared well with the experimental results although the
model simulations tended to overestimate the densification behavior. The reasons for the overestimation were
due to a local increase in the volume fraction of fiber.
Acknowledgements
This work was supported by the Advanced Research
Projects Agency (A. Tsao, Program Manager) and the
National Aeronautics and Space Administration (D.
Brewer, Program Monitor) through grant NAGW-1692
and through the ARPA/ONR funded URI program at
UCSB.
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144
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