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
THE SHEAR STRENGTH OF POTASSIUM CHLORIDE ABOVE
THE B1-B2 PHASE TRANSITION DURING SHOCK LOADING
J.C.F. Millett, N.K. Bourne
Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
Abstract. Previous work on the shock-induced phase transformation of potassium chloride has
been unable to assess the deviatoric (shear) stresses in either the mixed phase (Bl and B2)
region or the high pressure phase (B2). This work has attempted to balance this deficiency
through the measurement of lateral stresses in one-dimensional shock loading of this material.
Through knowledge of both longitudinal and lateral stresses, the shear strength can be deduced.
Measurements have been taken both above and below the dynamic phase transformation in
pressed, polycrystalline potassium chloride, and the results discussed.
INTRODUCTION
presence of a non-equilibrium rhombohedral phase,
also using x-ray diffraction. More recently, Millett
et al. (7) used strain gauges to follow the phase
transformation in KC1. At lower stresses above the
phase transformation, these measured strains agreed
well with the calculated values from both Hayes (1)
and Al'tshuler et al. (4). However, as stress
increased, measured strains were observed to be
noticeably lower than the calculated values. If true,
it would seem to agree with the observations of
Egorov et al. (6). Finally, Bourne and Townsend (8)
measured the conductivity of shock-loaded
potassium chloride, and related its variation with
the known mechanical thresholds induced by shock
loading. Their results showed that there were no
electrical changes associated with either the HEL or
the phase change. Further, they also showed that
single crystalline material behaved no differently to
its pressed, polycrystalline counterpart.
In his investigation of potassium chloride,
Hayes (1) made the assumption that it could not
support deviatoric (shear) stresses above the phase
transformation, if only to simplify his analysis. We
have chosen to investigate this assumption by using
manganin stress gauges placed in such an
orientation that renders then sensitive to the lateral
component of stress (cry). In combination with the
The shock induced phase transformation in
potassium chloride (KC1), whereby the material
transforms from the Bl NaCl structure to the B2
CsCl structure has been a source of interest for
many years. Given its relatively low magnitude (at
around 2 GPa) it is readily accessible and thus is an
obvious choice for the investigation of dynamic
phase transformations. Much of the work has
concentrated on single crystal material. Hayes (1)
has placed the transformation at a stress of 2.12
GPa, whilst Rosenberg (2) identified it at a slightly
higher stress of 2.34 GPa. In between these values,
Galbraith et al. (3), in investigating a pressed,
polycrystalline material, placed the transformation
stress at 2.2 GPa. Al'tshuler et al. (4), in contrast,
showed in their own investigations that the
transformation occurred at 1.97 GPa. Aside from
stress measurements, others have used dynamic Xray crystallography to monitor the changes in KC1
during shock loading. D'Almeida and Gupta (5)
showed that below the phase transformation, and
when shocked along the [001] direction, interplanar
spacing measurements indicated isotropic
compression of the unit cell. Above the phase
transformation, Egorov et al. (6) reported the
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shear wave speed (cs) was 2.10±0.01 mm jis"1. The
grain size was 150 to 500
longitudinal stress (<7X), the shear stress (T) can be
determined using the well-known relation,
(1)
In this article, we present lateral stress data
taken both above and below the phase transition
stress of ca. 2 GPa, and using equation 1, determine
the variation of shear strength of potassium
chloride.
Flyer
Plate
EXPERIMENTAL
Commercially produced potassium chloride powder
was pressed into 100 mm diameter by 15 mm thick
disks, using a 100 ton hydraulic press. These in turn
were cut into sections 50 mm by 25 mm by 15 mm
thick. Shock targets were made by introducing a
manganin stress gauge (MicroMeasurements type
J2M-SS-580SF-025) between two of the cut
sections such that it was 6 mm from the impact
face. Extra insulation and protection was supplied
in the form of 25 Jim of mylar sheet on either side
of the gauge. Targets were reassembled using a low
viscosity epoxy adhesive (curing time
approximately 12 hours) and held in a special jig.
Once cured, the impact face was ground such that it
was flat to 5 optical fringes over 50 mm. Shock
stresses in the range 1 to 4 GPa were induced by
firing flat and parallel disks of 10 mm thick dural
(aluminium alloy 6082-T6) or copper at velocities
from 200 to 776 m s"1. The targets were aligned to
better than 1 mradian using an adjustable specimen
mount, and impact velocities were measured to an
accuracy of 0.1% using sequentially mounted pairs
of pins. The voltage data from the gauges was
converted to stress using the methods of Rosenberg
and Partom (9), using a modified analysis that does
not require prior knowledge of the impact stress
(10). Specimen configurations and gauge
placements are presented in Fig. 1.
Stress Gauge
FIGURE 1. Specimen configuration and gauge placement.
RESULTS AND DISCUSSION
In Fig. 2, the Hugoniot of KC1 is presented in
stress-particle velocity space. It has been included
so that it might serve as a reference for the
following deductions.
Is
•
n
o
•
Hayes - Equiibrium (1)
Hayes <100>(1)
Hayes <111>(1)
Galbraith-Polyxtal(11)
55
0.1
0.2
0.3
0.4
0.5
0.6
Particle Velocity (mm ps"1)
0.7
FIGURE 2. The Hugoniot of pressed potassium chloride.
Note that the results of Galbraith (11), who
investigated pressed, polycrystalline material is
effectively identical to the equilibrium values of
Hayes (1), and thus we have confidence that the
quoted values of longitudinal stress for the lateral
stress measurements are correct.
Lateral gauge traces are shown in Fig. 3.
MATERIALS DATA
The density of the pressed potassium chloride was
1.96 g cm"3. This compares to an average single
crystal density of 1.99 g cm"3. The longitudinal
sound speed (CL) was 3.81±0.01 mm jis"1 and the
214
1
2
3
4
5
duration than the two below the phase transition as
10 mm flyer plates were used as compared to 10
mm dural flyer plates (with its higher wave speed)
below the phase transition.
The measured lateral stresses from Fig. 3, in
combination with the longitudinal stresses from the
known impact conditions, have been used, through
equation 1, to calculate the shear strength of
potassium chloride. The results are shown in Fig. 4.
Simple linear regressions have been fitted to
indicate trends. Below the phase transition, there is
an overall increase in shear strength, although there
is quite a large degree of scatter. Above the phase
transition, the trend in shear strength with
increasing impact stress is again increasing. Note,
however, that the shear strength at 2.49 GPa
appears anomalously high, but from Fig. 3, it can be
seen that the arrival of the phase transformation
occurs very late, and is possibly caught by release
from the rear of the flyer before it has had a chance
to reach completion. This would result in a low
lateral stress and consequently a higher shear
strength.
6
Time (MS)
FIGURE 3. Lateral gauge traces in potassium chloride.
Below the quoted phase transformation of 2.2
GPa (the traces marked 0.94 and 1.70 GPa), lateral
stress rises to a near constant value before lateral
releases enter the gauge location ca. 2 us
afterwards. Note that the rise times of the gauge
traces are quite long, in contrast to those above the
phase transformation. It is possible that this is due
to residual porosity in the microstructure ramping
the wave. Stresses at, and above, the phase
transformation may be sufficiently high to 'overdrive' the porosity, thus resulting in a faster rising
signal. Above the phase transformation, lateral
stress behaves initially in the same way, rising to
ca. 1.70 GPa. This corresponds to the lateral stress
at the B1/B2 phase transition, which of course be a
constant value like it longitudinal counterpart.
However, in the trace labelled 2.49 GPa, the lateral
stress at B1/B2 is slightly lower than the
corresponding values for the two higher stresses,
which from 1, corresponds to a slightly higher shear
stress. The reasons for this are unclear, but the fact
that this shot was performed only just over the
phase transformation stress may have resulted in
non-equilibrium conditions. In all three traces above
the phase transition, a secondary rise in lateral stress
can be observed. This is due to the arrival of the
phase transformation wave. Note that as impact
stress increases, the phase transformation velocity
increases to the point where at an impact stress of
4.05 GPa, it has almost caught up with the first
plastic shock. These three traces are of longer
1.2
Below Phase Transition
Above Phase Transition
1.0
CS
0.4
0.2
0.0
0
1
2
3
4
Longitudinal Stress (GPa)
5
FIGURE 4. The shear strength in shock loaded potassium
chloride
Overall, the shear strength, both above and
below the phase transition, is increasing. Perhaps
the most obvious reason for this would be
dislocation generation, but unfortunately, there
appears to be little, if any information in the
literature on the shock deformation mechanisms
(with the exception of the phase transition)
215
concerning potassium chloride. However, as a
window material used in interfermetric velocity
experiments, lithium fluoride has been studied
extensively, including post-shock microstructural
evaluation. Gupta et al. (12) report that dislocation
densities in the shock front (as determined from the
plastic strain rate) can be up to three orders of
magnitude higher than those in regions of rapid
stress decay. Vorthman and Duvall (13) examined
the dislocation density in shocked and soft
recovered specimens by etch pitting, and comparing
the results to undeformed samples. Their conclusion
was that dislocation density did not change
significantly, but rather deformation occurred by the
movement of dislocations already present within the
microstructure. Therefore, whilst there would
appear to be a difference of opinion about
dislocation generation and/or motion in shocked
lithium fluoride, dislocation mechanism can be used
to explain the shock induced deformation in simple
ionic solids. Thus is would seem likely that similar
processes occur in potassium chloride.
Above the phase transition, the material will be
in a mixed B1/B2 phase field. Meyers and Murr
(14) have shown that in two phase materials, if the
elastic moduli of both phases are different, the
passage of a shock can generate considerable
dislocation debris, which could manifest itself as an
increase in shear strength.
2. Rosenberg, Z. J. Appl Phys. 53 (1982) 1474-1476.
3. Galbraith, S.D.,, in Shock Compression of Condensed
Matter 1995, S.C. Schmidt and W.C. Tao, Editors. 1996,
American Institute of Physics: Woodbury, New York. p.
219-222.
4. Altshuler, L.V., Pavlovskii, M.N. and Komissarov,
V.V. J. Exper. Theor. Phys. 79 (1994) 616-621.
5. d'Almeida, T. and Gupta, Y.M., in Shock Compression
of Condensed Matter 1999, M.D. Furnish, L.C.
Chhabildas, and R.S. Hixson, Editors. 2000, American
Institute of Physics: Melville, New York. p. 113-116.
6. Egorov, L.A., Barenboi'm, A.I., Makeev, N.G.,
Mokhova, V.V. and Rumyantsev, V.G. J. Exper. Theor.
Phys. 76(1993)73-81.
7. Millett, J.C.F., Bourne, N.K., Galbraith, S.D. and
Rosenberg, Z. J. Appl Phys. 87 (2000) 2765-2768.
8. Bourne, N.K. and Townsend, D., in Shock
Compression of Condensed Matter 1999, M.D. Furnish,
L.C. Chhabildas, and R.S. Hixson, Editors. 2000,
American Institute of Physics: Melville, New York. p.
109-112.
9. Rosenberg, Z. and Partom., Y. J, Appl Phys. 58 (1985)
3072-3076.
10. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z. J.
Phys. D. Applied Physics 29 (1996) 2466-2472.
11. Galbraith, S.D., The response of potassium chloride,
ammonium nitrate solutions, and emulsion explosives to
plate impact loading. Ph.D Thesis, University of
Cambridge. 1998.
12. Gupta, Y.M., Duvall, G.E. and Fowles, G.R. J. Appl
Phys. 46(1975)532-546.
13. Vorthman, J.E. and Duvall, G.E. /. Appl. Phys. 53
(1982)307-3615.
14. Meyers, M.A. and Murr, L.E., in Shock Waves and
High Strain Rate Phenomena in Metals, M.A. Meyers and
L.E. Murr, Editors. 1981, Plenum: New York. p. 487-530.
CONCLUSIONS
Lateral stress measurements have been made in
pressed, polycrystalline potassium chloride, above
and below the phase transition. A secondary
increase in lateral stress has been observed in the
three higher stress traces, corresponding to the
phase transition itself. In combination with known
longitudinal stresses, the shear strength has been
determined. Both below and above the phase
transition, the trend has been increasing with
increasing longitudinal stress. The movement and
generation of dislocations by passage of the shock
front have been suggested as possible mechanisms
for this observed increase in strength.
REFERENCES
1. Hayes, D.B. J. Appl. Phys. 45 (1974) 1208-1217.
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