CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
TiC BY SHS AND DYNAMIC COMPACTION
E.P. Carton, M. Stuivinga, A. Boluijt
TNO Prins Maurits Laboratory, P.O. Box 45, 2280 AA, Rijswijk, The Netherlands
Abstract. By ball-milling the Ti/C powder mixture before their Self-sustained High-temperature
Synthesis (SHS) to TiC, the propagation velocity of the SHS process has been increased from 7 to
22 mm/s. The reaction of the milled powder mixture was accompanied by the release of a large
volume of gas.
The micro-strain and the crystallite size of the milled reactants have been determined using x-ray
diffraction line-broadening analysis. The graphite did not change during the milling process, but a
large line-broadening effect was measured in the x-ray diffraction peaks of the milled titanium
particles. The TiC fabricated by hot shock compaction of the porous SHS-product (SHS/DC)
contained cracks and an axial hole in the center. It is believed that the large gas release rather than a
Mach-stem was responsible for the formation of the axial hole.
INTRODUCTION
the SHS process (7 mm/s). The detonation can only
be initiated when the SHS has been completed. For
a 10 cm long SHS-tube it takes 14 seconds to
complete the SHS process. In this time heat is
leaking away to the surroundings; the SHS-tube, the
flyer-tube, and the explosive layer. In order to
decrease the SHS reaction time, and therefore
reduce the heat loss, the propagation velocity of the
SHS should be higher. In this work efforts to
increase the SHS propagation velocity by ballmilling the reactants and its use in the SHS/DC
combination are reported.
Dynamic or explosive compaction of ceramic
powders to a high final density is difficult, since
micro and macro cracks occur due to the brittle
fracture behavior of these materials at room
temperature. Above the Ductile-Brittle Transition
Temperature (DBTT) materials show a remarkable
increase in fracture energy due to a more ductile
fracture behavior. The hot explosive compaction of
TiC by combining the Self-sustained Hightemperature Synthesis (SHS) and dynamic
compaction (DC) processes was demonstrated in [1,
2,3].
The combination of both processes enables one
to compact TiC above its DBTT without the need of
expensive equipment. The use of the indirect
cylindrical configuration has the benefit of having
an intrinsic thermal insulation layer between the
metal tube in which the SHS occurs and the
explosive layer surrounding the flyer tube, see
Figure 1. However, the length of the SHS-tube is
restricted by the rather slow propagation velocity of
THEORY
A simple energy balance for a one-dimensional
interpretation of the SHS process was used by
Merzhanov [4] to determine its propagation
velocity, VSHS:
V2SHs=A(Tad)exp(-Ea/RTad)
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(1)
with a diameter of 5 mm. After the milling process,
the liquid was removed by decantation and further
evaporated under vacuum at 40 °C. Then the x-ray
line-broadening analysis was performed. The SHS
propagation velocity was measured by timing the
response of thermocouples placed in the reactants
(with a relative density of 65 %TMD) at known
axial positions. For comparison, also the SHS
propagation velocity of a Ti/C mixture, with only
the Ti-particles had been ball-milled was measured.
Finally, the ball-milled powder was used in
SHS/DC experiments and the cross-section of the
hot shock compacted TiC was analyzed by light
microscopy.
For comparison, an at room
temperature shock compacted Ti/C powder mixture
was analyzed. The experimental parameters for the
SHS/DC experiments was the same as described
earlier [1]. The set-up is schematically shown in
Figure 1.
Here, A is a constant, R the gas constant, Tad is the
adiabatic temperature, and Ea is the activation
energy. Apart from the temperature, as the most
important rate determining parameter, the
propagation velocity can be influenced by the
activation energy of the reaction (EJ. The lower this
energy, the higher the propagation velocity.
Lee [5] introduces the work of Benderskii et al.
[6], who advanced the hypothesis that the global
activation energy of a reaction (EJ is a combination
of a thermal energy (E0) and an elastic compression
energy (TI):
E =E 0 -k.7i
(2)
Here, k is a constant of the order of unity. The
elastic compression energy can be increased by
elastic lattice deformation of the reactants. The
elastic lattice deformations can, for example, be
introduced by ball milling the SHS reactants. Ball
milling the powder mixture for several hours results
in an intense cold plastic deformation of the
particles. Ball-milling also distributes the reactants
more homogeneously and brings the reactants into a
more intimate contact with each other. This will
reduce the amount of mass transport during the
reaction.
The average elastic strain in the lattice can be
determined by analysis of line broadening in X-ray
diffraction peaks, for example using the HallWilliamson plot [7]. In these figures the 26-position
and the width of all x-ray diffraction peaks of a
crystalline material are plotted in a graph with
special axis. When a straight line is drawn through
these points, the average micro-strain is determined
by the slope, while the intercept gives the crystallite
size (average size of the coherently diffracting
regions in the material).
Detonator
Figure 1: Experimental set-up for SHS/DC in the indirect
cylindrical configuration.
Contramass
Container with
powder and balls
EXPERIMENTS
Rotation axis
Elastic lattice deformations have been generated
by ball milling the reactants, an equiatomic mixture
of Ti (<45 micron) and C (<50 micron) powder, in a
planetary ball mill, see Figure 2. The powder
mixture (100 gram) was milled for 24 hours in
ethanol as a liquid coolant using 150 alumina balls
Figure 2. Planetary ball-mill.
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The graphite probably acts a storage material for
ethanol during and after ball-milling, preventing a
complete drying of the reactants in vacuum at 40 °C.
The SHS propagation velocity was not higher then
normal, when the Ti/C powder mixture was used
with only the Ti-particles being ball milled. Also the
gas release was at its "normal" volume.
Ball-milling did not lead to broadening in the xray diffraction peaks of graphite. However, the
milling broadened the x-ray diffraction peaks in Ti
considerably. Figure 4 shows Hall-Williamson plots
for three Ti-powder samples. Sample (a) is the
starting Ti-powder (as atomized), it has a low
micro-strain value of 0.09% and an average
crystallite size of 657 A. Sample (c) is the Ti after
24 h ball-milling as a Ti/C mixture. The average
crystallite size of the Ti had decreased to 283 A,
while the micro strain had increased to 0.49%.
Sample (b) is the starting Ti/C mixture after shock
compaction at room temperature. During this only
micro-seconds lasting powder treatment the microstrain in the Ti-particles has increased to 0.42%, and
the crystallite size was reduced to 380 A.
Morosin and Graham in [8] have done research on
the differences and similarities between the defects
formed ?n TiC powders that were treated by shock
wave and ball-milling, respectively. Although both
processes can lead to the same amount of microstrain and crystallite size, the anisotropy in residual
strain is large in ball-milled powder and more
homogeneous in shock-modified powder. This can
also be seen in Figure 4, since the scatter around
line c (ball-milled) is larger than that for the line b
(shock-modified).
Combined SHS/DC experiments with the ballmilled powder have been performed using the same
parameters as described in [1]. The compacts
showed an increase in plastic deformability of the
hot TiC, due to the absence of spiral cracks that
typically occur at the shock compaction of brittle
material at low starting density.
The density of the TiC after SHS is only 47
%TMD, but increased to 98 %TMD due to the hot
shock compaction process. However, an axial hole
did form at the center of the compact as well as
some cracks. The hole did not form due to a Machstem, since no loss of mass was detected in a
cylindrical segment of the sample.
Figure 3: Particle after 24 hours ball-milling in a Ti/C powder
mixture.
RESULTS AND DISCUSSION
Figure 3 shows a SEM image of a 24 hours
ball-milled Ti/C powder particle. Graphite plates
(the black spots) have been crushed into the Tiparticle, clearly indicating a closer contact between
the two reactants. XRD analysis further indicated
that no TiC had formed during the ball-milling
process.
The measurement of the SHS propagation
velocity indicated an increase from 7 mm/s for the
starting Ti/C powder mixture, to 22 mm/s for the 24
hours ball milled powder mixture (all at a relative
density of 65 %TMD). During the reaction of the
latter, a much greater volume of gas escaped from
the SHS-tube compared to the starting powder
mixture. This indicates that the powder has not been
properly dried.
(sinGA,)2
Figure 4: Hall-Williamson plot of three Ti-powder samples.
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No increase in SHS propagation velocity was
measured if only the Ti-particles had been ballmilled. This indicates that the cause of the increase
in SHS velocity in the ball-milled Ti/C powder
mixture, is primarily the better distribution and
contact between the reactants.
ACKNOWLEDGEMENT
The authors thank E. Wilken for performing the
X-ray line-broadening analyses.
REFERENCES
[1] Carton, E.P., Stuivinga, M. and Verbeek,
HJ.,"Shock compaction of combustion synthesized
ceramics in the cylindrical configuration", in Shock
Compression of Condensed Matter-1999, edited by
M.D. Furnish et al., AIP Conference Proceedings
741, New York, 1999, pp. 549-552.
[2] Grebe, H.A., Advani, A., Thadhani, N.N., Kottke, T.,
"Combustion synthesis and subsequent explosive
densification of titanium carbide ceramics", Metall.
Trans. A, Vol 23A, pp. 2365-2372 (1992).
[3] Grebe, H.A. and Thadhani, N.N.,"High-rate
chemical reaction and high pressure processing of
bulk titanium-carbide ceramics", Processing and
fabrication of advanced materials for high
temperature applications, TMS Proceedings, eds.
Srivatsan T.S. and Ravi, V.A (1992).
[4] Merzhanov, A.G.,"Pyrotechnical aspects of SelfPropagating High-Temperature Synthesis", in XX
Intern. Pyrotechnics Seminar, Colorado Springs,
1994 NSWCCR/RDTN-94/004.
[5] Lee, J.H.S., Goroshin, S., et al., "Attempts to initiate
detonations in metal-sulphur mixtures", in Shock
Compression of Condensed Matter-1999, edited by
M.D. Furnish et al., AIP Conference Proceedings
741, New York, 1999, pp. 775-778.
[6] Benderskii, V.A., Fillipov, D.G., Ovchimikov,
"Ratio of thermal and deformation ignition in low
temperature solid phase reactions:, Doklady Akad.
Nauk. SSR, 308 (2), 401 (1989).
[7] Morosin, B. and Graham, R.A., "X-ray diffraction
line-broadening studies on Shock-modified Rutile
and Alumina", Materials Science and Engineering,
Vol. 66, pp. 73-87 (1984).
[8] Morosin, B. and Graham, R.A., "X-ray diffraction
line-broadening of shock modified titanium carbide",
in Materials Letters, Vol. 3(3), pp. 119-123 (1985).
Figure 5: Cross-section of ball-milled sample after SHS/DC.
In a Mach-stem material normally escapes from the
tube, due to its high kinetic energy and liquid or
even gaseous state. The hole formed during this
SHS/DC, probably formed by the accumulation of
gasses at the line of symmetry of the cylindrical
configuration.
CONCLUSIONS
Line broadening analysis of X-ray diffraction
peaks of the milled Ti/C powder mixtures indicated
no change for graphite, and an increase in microstrain, as well as a decrease in crystallite size for the
Ti-particles.
Ball-milling the reactants in a planetary ballmill did increase the SHS propagation velocity from
7 to 22 mm/s. This made it possible to reduce the
time for the SHS process to complete, and therefore
to shock compact the TiC at a higher temperature.
However, the large volume of gas escaping from the
powder during SHS of ball milled powder produced
a central hole in the compacts. This central (axial)
hole was not formed by a Mach-stem. Probably the
ethanol, that was used as a coolant during the
milling process, was adsorbed by the graphite.
Several hours of drying in vacuum, did not dry the
powder enough to prevent the large gas escape
during SHS.
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