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
INVESTIGATION OF SHOCK-INDUCED CHEMICAL REACTIONS IN
Ni-Ti POWDER MIXTURES USING INSTRUMENTED EXPERIMENTS
Xiao Xu and Naresh N. Thadhani
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245
Abstract. Instrumented experiments using PVDF stress gauges were employed to investigate the
occurrence of shock-induced chemical reactions in -50% dense Ni+Ti powder mixtures. At low input
stresses (~1 GPa), the as-blended powder mixture showed characteristics of powder densification and
dispersed propagated-wave stress profiles with rise-time of 36 nanoseconds. At input pressure as high as
3.22 GPa, the as-blended mixture showed a sharp rise-time (< 15 ns) of the propagated-wave profile and an
expanded state of products revealing evidence of shock-induced chemical reaction. Experiments performed
on the ball-milled Ni+Ti powder mixtures showed that while the powder ball-milled to the state of
becoming fully alloyed, mechanically amorphized, remained inert and showed no expansion, those powder
mixtures ball-milled for intermittent times underwent shock-induced reaction. The expansion due to the
resulting shock-induced reaction increased with decrease in ball-milling time. These results support
previous studies on other intermetallic forming systems that show similar volume expansion.
INTRODUCTION
Shock compression of elemental powder
mixtures produces a unique configuration of densepacked highly activated state of material that can
result in "shock-induced" chemical reaction (due to
the effects of high pressure in the microsecondduration time scale of pressure equilibration), or
"shock-assisted" reaction (due to residual postshock thermal effects in the time scale of thermal
equilibration) [1-4]. Nickel-titanium represents an
exothermic intermetallic forming system, which can
undergo reaction forming intermetallics by shock
compression of the elemental powder mixtures
under certain conditions [5,6]. Without the help of
in-situ measurements of shock states, post-mortem
analysis of the product microstructures may not
ascertain whether the observed reaction products
formed via "shock-induced" reactions or post-shock
"shock-assisted" process [7]. In the present work,
instrumented experiments using PVDF piezoelectric
stress gauges were used to study the reaction
behavior during shock compression of as-blended
and ball-milled Ni+Ti powder mixtures.
powders of -325 mesh (<40 Jim) were mixed in an
equiatomic ratio, either using a slow speed Vblender or by ball-milling. Ball-milling was
performed using Spex 8000 Mixer/Mill for different
time periods in Ar atmosphere. The powder
mixtures were then pressed into a copper capsule to
form a ~ 3 mm thick disk of 50.8 mm in diameter at
a density of -50% of the theoretical maximum
density (TMD). The setup for instrumented
experiments was similar to that used in prior work
[7,8]. Two PVDF stress gauges were placed in
intimate contact with front and back powdercapsule planar surfaces, respectively, to monitor the
input-shock and propagated-wave characteristics as
well as the transit time of shock wave through the
powder layer. Tektronix TDS 7 84 A digital
oscilloscope was used to capture the current signals
generated from PVDF gauges. OFHC-copper flyer
plates were used in all experiments. The projectile
velocity was measured using three in-line shorting
pins, and standoff pins were used to trigger the
digital oscilloscopes. The ball-milled mixtures were
also reacted in the Perkin-Elmer DTA 7, to
determine the reaction heat evolved, as a function of
ball-milling time.
EXPERIMENTAL PROCEDURE
Elemental Ni (Cerac) and Ti (Alfa Aesar)
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TABLE 1. Summary of Experimental Results
Expt.
No.
0109
0104
0105
9923
0101
0108
Packing
Density
(%TMD)
50
47
49
52
53
46
Projectile
Velocity
(m/s)
522
930
1046
918
930
940
Input
Stress
(GPa)
1.12
2.71
3.22
3.67
3.82
3.75
Input Risetime (ns)
(10%-90%)
26.5
4.5
4
6.5
6.5
7
Wave Speed
Equilibrium Propagated
Relative Volume
Propagated Rise-time (ns) (mm/jis) (toe-toe- (toe-toe-10%, ¥2
Stress (GPa) (10%-90%)
max)
10%, ¥2 max)
1.84
1.15,1.14
36
0.92, 0.92
3.39
8.5
1.27, 1.27
1.50,1.50
3.88
14.5
1.77,1.77
1.36, 1.35
0.40
33
1.24, 1.23
1.79,1.77
0.22
26
1.11,1.10
1.68,1.67
0.44
40.5
1.58, 1.57
1.03, 1.01
model provides a method to determine a calculated
Hugoniot of the reaction products formed via
"shock-induced" chemical reaction in a powder
mixture.
Figure 3 shows the pressure-volume relationship
calculated based on the measurements of the three
PVDF gauge experiments (#0109, #0104 and
#0105) performed on the as-blended powder
mixture. Also shown in the P-V plot is the P-a densification behavior of as-blended powder mixture
RESULTS AND DISCUSSION
A summary of the experimental results obtained
from the aforementioned instrumented experimental
measurements is listed in Table 1. The parameters
include, the input stress and input pulse rise-time
(from 10% to 90% of peak) measured by the input
shock gauge; the equilibrated propagated-stress and
propagated-pulse rise-time recorded by the
propagated-stress gauge; wave speed determined
using both the toe-to-toe (10%) and half-max values
of input- and propagated-wave profiles; the relative
volume calculated using the values of initial powder
density, measured input stress, shock-wave speed
(both toe-to-toe and half-max), and shock jump
conditions for conservation of mass and
momentum.
Input Stress Profiles
A. Reactions in As-Blended Powder Mixture
Figure 1 shows the measured input stress
profiles from experiments performed on the asblended powder mixture. The propagated-stress
traces for those three experiments are shown in
Figure 2. With increasing impact velocities, the
amplitude of the corresponding input stress
increases, while the rise-time of the input stress
pulse decreases. In addition, at lower stress level,
propagated-stress profile shows longer rise-time
revealing characteristics dominated by powder
densification instead of reaction. In all three cases,
the equilibrium propagated-stress is slightly higher
than the input stress.
Densification of the powder mixtures from an
initial porous to final solid density is considered
using the P-a pore collapse model [9]. A
thermodynamic consideration is also used to
calculate the pressure-volume (Hugoniot) curve of a
fully reacted NiTi product, based on Bennett and
Horie's model [10], which implements a constant
pressure adjustment of the reference state. This
time
FIGURE 1. Combined plot of input stress profiles from PVDF
gauge experiments on as-blended powder mixture.
Propagated Stress Profiles
time
FIGURE 2. Combined plot of propagated-stress profiles from
PVDF gauge experiments on as-blended powder mixture.
1124
drawn to pass through the datum for Expt. #0109,
which illustrates crush strength of about 1.6 GPa
for the as-blended Ni+Ti mixture. The calculated
solid product NiTi Hugoniot, the porous Hugoniot
considering densification of inert powder mixture
from V/V0=2 to V/V0=1 with zero crush strength,
and the calculated compressibility curve (obtained
based on Bennett and Horie's model [10]) of the
fully reacted NiTi product formed from Ni+Ti
reactants at -50% TMD, are also indicated.
It can be seen that if the data point at 1.12 GPa is
considered to fall on the curve representing the P-a
densification behavior, then the two data points
above 2.5 GPa show significant volume expansion,
and approach the calculated compressibility curve
of the fully reacted NiTi product formed from
Ni+Ti powder reactants. Hence, it can be reasoned
that the 3.22 GPa data point represents almost
complete shock-induced reaction in the powder
mixture, and 2.71 GPa data point indicates an
incomplete but appreciable shock-induced reaction.
P-V Plot of Ni+Ti As-Blended Powder
B
I
——
-
InertPowder
...o-<»p -Alpha
•
0
1
#01 05|
'
•
1/2 moc
RecctedPovicter
*1
8
#0104
*x
I
0.8
#0109
'•-a
"v"x----o...
1.2
1.4
1.6
Initial Volume
^•-,o^ Voo
1.8
2.2
Relative Volume (V/Vo)
FIGURE 3. Plots of measured input stress versus calculated
relative volume (based on half-maximum values), isothermal
compressibility curves of dense NiTi, Ni+Ti inert mixture with
zero crush strength, P-a densification curve and Hugoniot of
reacted powder forming product.
B. Effect of Ball-Milling
The effect of ball-milling on the reaction
behavior of Ni+Ti powder mixtures was also
investigated using the PVDF gauge experiments.
Ball-milling was used to change the configuration
of powder mixtures. The obvious effect of ballmilling is the improved intimate mixing of the
powder reactants. In present work, Ni+Ti powder
mixtures were ball-milled for 4, 8 and 18 hours
(designated as BM 4hr, 8hr and 18hr, respectively).
The BM 4hr and 8hr powders were prepared in a
sealed steel miller vial filled under Ar gas, with
dripping liquid nitrogen as coolant outside the vial.
The BM 18hr powder was prepared using hexane as
lubricant. The reaction behaviors of those ballmilled powders as well as the as-blended powder
mixture were also examined using a Perkin-Elmer
DTA 7 [6]. It was found that the reaction heat
evolved decreased with increasing ball-milling
time, indicating occurrence of partial reaction
during ball-milling, also known as mechanical
alloying. Reaction behavior of BM 18hr powder
was different from those of the as-blended and BM
4hr and 8hr powders in that the heat evolved was
dominantly due to the crystallization of the
amorphous phase.
Figure 4 compares the measured input stress
profiles from experiments performed on the three
ball-milled powders with the as-blended powder
mixture at high velocities. The corresponding
propagated-stress profiles are included in Figure 5.
Input Stress Profiles
time
FIGURE 4. Combined plot of input stress profiles from PVDF
gauge experiments at high velocity.
Propagated Stress Profiles
#0105
#9923
#0101
#0108
-J40ns
time
FIGURE 5. Combined plot of propagated-stress profiles from
PVDF gauge experiments at high velocity.
1125
powder mixture at input stress less than the crush
strength (-1.6 GPa), show characteristics of
densiflcation represented by the P-a behavior. The
measured propagated-wave stress profiles show
characteristics of wave dispersion with rise-time of
36 nanoseconds. In experiments at input stress
higher than 2.7 GPa, the powder mixtures show
evidence of shock-induced reaction, based on
propagated-wave profiles showing a sharp rise-time
(<~5 ns), and the data points of shock states
revealing expansion and approaching the pressurevolume compressibility curve of thermodynamically determined Hugoniot of reacted
powders. Experiments on Ni+Ti powder mixtures
ball-milled for 4, 8 and 18 hours at input stresses
greater than 3 GPa, show partial reaction or
complete lack of shock-induced reaction. Their
propagated-stress profiles show slower rise-time
and significantly reduced stress amplitude due to
shock attenuation. The recorded wave speeds in
these experiments for ball-milled powders,
performed at even higher pressures, are also
reduced. Finally, the data points falling on the
corresponding pressure-volume compressibility
curves for ball-milled powder mixtures show
decreasing evolution of reaction heat with
increasing ball-milling time.
Ni+Ti P-V Plot
ft-
50101
I
2-"
I
#010*
CO
9923
23-
55
(————————77-;—————!———| 5
did
j
—— — nert Powder
1
•I
oiosl
•*,
- -o- - -AlphaBM4hr powder
- -0- - -AlphaBM8hrpo*der
/2mo(BMOhrpcv«ter /2ma<BM4hr powder
12 mac BM8hr powder
12 macBMlShr powder MOhr react edpowder
M4rrrea=tedpowder
^°——— T* ****-=
v
• ---o... - T7—"Q^ XVoo
0 - ———
0.8
1
1.2
1.4
1.6
1.8
2
2
Relative Volume (V/Vo)
FIGURE 6. Plots of measured input stress versus calculated
relative volume (based on half-maximum values), isothermal
compressibility curves of dense NiTi, Ni+Ti inert mixture with
zero crush strength, P-a densiflcation curves and Hugoniots of
reacted powders forming product.
It can be seen that both the input stress and input
rise-time for the ball-milled powder mixture are
slightly higher than those of the as-blended powder.
The higher input stress can be attributed to the work
hardening of the ball-milled powders. However, as
shown in Figure 5, the equilibrium propagatedstress level of the ball-milled powders is one order
of magnitude less than that of as-blended powder
mixture, indicating that while the shock
compression of ball-milled powders is dominated
by shock attenuation, the behavior of the as-blended
powder is dominated by shock induced reaction.
Furthermore, the propagated-stress rise-time for the
ball-milled powders is also higher than that of asblended powder mixture.
Figure 6 plots the pressure-volume relationship,
showing results of the measurements obtained from
the above four PVDF gauge experiments at higher
velocity. Also shown in the P-V plot are P-a
densiflcation behaviors of each of the as-blended
and BM 4hr and 8hr powder mixtures, considering
different values of crush strengths. The calculated
compressibility curves corresponding to possible
NiTi reaction products formed in as-blended, BM
4hr, and BM 8hr Ni+Ti powder reactants, are also
illustrated. It can be seen that reactions in BM 4hr
and 8hr powder mixtures show less volume
expansion due to less exothermic heat (as revealed
by the DTA analysis).
ACKNOWLEDGEMENTS
Funding for this research was provided by the
Army Research Office under Grant DAAG55-97-10163. (Dr. W. Mullins, program monitor)
REFERENCES
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CONCLUSIONS
Instrumented experiments performed using
PVDF gauges on -50% dense as-blended Ni+Ti
1126