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
ON THE SHOCK RESPONSE OF
THE SHAPE MEMORY ALLOY, NiTi
J.C.F. Millett, N.K. Bourne, G.T. Gray III*, G.S. Stevens
Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
*MST8, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USA.
Abstract. There has been recent interest in the behaviour of the shape-memory alloy NiTi since
it undergoes a stress-induced phase change at a low stress value. It has been additionally noted
that the NiTi does not appear to exhibit a Hugoniot elastic limit (HEL) in the way normally
associated with other metals. In order to investigate the possible mechanisms operating to give
rise to these effects, a series of plate impact experiments have been conducted in order to probe
the material's response to shock. In particular attention has been paid to determination of the
material Hugoniot in order to ascertain whether the observed features of the response may be
explained. A series of other shots where shaped waves are applied are described in order to probe
the lower rate response.
induced formation from powders (see for example
the works of Matsumoto et al. (4), Han et al (5)
and Usherenko et al. (6)) and microstructural
examination of the monolithic material. Thakur et
al. (7, 8) investigated two alloy compositions, a
high nickel material (54.7 %), and a lower nickel
content alloy (50.2%) with a small amount of iron.
In particular, they were interested in changes in
microstructure due to release wave interactions. In
the first material, tensile stress levels above 2 GPa
formed the martensitic phase in an acicular
morphology, with significant amounts of internal
microtwins. The second alloy, when shocked to the
same stress, did not display martensite formation.
Above 4.1 GPa however, the specimens spalled,
with martensitic needles around the failed regions.
Finally, Escobar et al. (9), using pressure shear,
loading, observed that the limiting shear stress at
which the transformation occurs is 46 MPa. They
also suggested that the process involved a diffuse
boundary, with a gradual increase in shear stress
through the specimen.
NiTi additionally appears to display anomolous
plastic-wave propagation behaviour. Such affects
INTRODUCTION
The intermetallic compound NiTi is
characterised by a stress induced, martensitic phase
transformation whereby the material transforms
from the ordered body centred cubic B2 structure to
a lower density monoclinic unit cell. This results in
the material displaying shape memory behaviour.
As such, NiTi has seen use in applications such as
heart valves. Knowles and Smith (1) have shown
that the twinning shear direction [Oil] m is derived
from the cell edge [001] in the parent B2 phase.
More recently, it has also attracted interest from the
high-strain-rate community for the seismic
protection of buildings (2). However, the existing
literature on the high-strain-rate and shock loading
behaviour of this (and similar) materials is not
extensive. Miller et al. (3) have used the tensile
Hopkinson bar to investigate NiTi. Their results
indicate that specimens fail in a mixed mode with
ductile void growth followed by transgranular
failure.
The behaviour of NiTi under shock loading has
been concentrated in two areas, those being shock
579
were observed during Taylor cylinder impact
testing of the alloy U-6%Nb which is also a shapememory alloy. In U-6%Nb (10), instead of the
traditional 'mushrooming' behaviour of the rod
material directly adjacent to the impact end, plastic
deformation appeared to propagate along almost the
full length of the Taylor cylinder for modest
impacts. The same authors also noted that U-6%Nb
does not appear to display a Hugoniot Elastic Limit
(HEL). Understanding the shock Hugoniot of NiTi,
as an example of a shape-memory alloy, is therefore
of interest.
Gauge placement and specimen configurations are
shown in Fig 1.
y
EXPERIMENTAL
Shots were performed on a 50 mm bore, 5 m
single stage gas gun. 2 mm thick flyer plates of
dural (aluminium alloy 6062-T6) and copper were
mounted on acetal sabots with a recessed front
surface such that the rear of the flyers was
unconfmed. Impact velocities were in the range 200
to 875 m s"1. 5 mm plates of NiTi were ground such
that they were flat and parallel to 5 optical fringes
over 50 mm. Target assemblies were constructed
such that they consisted of a 1 mm coverplate of the
same material as the flyer plate, the NiTi plate itself
and a 12 mm backing plate of polymethylmethacrylate (PMMA). Between the coverplate and
the NiTi (the embedded configuration), and the
NiTi and the PMMA (the back surface
configuration), manganin stress gauges
(MicroMeasurements type LM-SS-125CH-048)
were embedded and insulated from the metallic
sections with 25 jam of mylar. The gauge between
the coverplate and the NiTi was placed such that it
would experience the full Hugoniot stress as
defined by the impact conditions. This method has
been used successfully to measure the Hugoniot of
other materials, including the intermetallic alloy Ti48Al-2Cr-2Nb-lB (11). The gauge between the
NiTi and the PMMA, due to the close impedance
matching of the PMMA, mylar, the epoxy adhesive
and the gauge backing would have a faster response
time than the embedded gauge (20 ns compared to
ca. 100 ns), and thus would show details in the
shock response of NiTi (such as the Hugoniot
Elastic Limit - HEL) that would be missed in the
embedded configuration. Gauge calibrations were
according to the methods of Rosenberg et al (12).
"X
X
FIGURE 1. Specimen configuration and gauge placement.
The material's data for NiTi were; longitudinal
sound speed (CL) 5.36±0.01 mm us"1, shear wave
speed (cs) 1.80 ±0.01 mm j^s"1, and density 6.42 g
cm"3. The Poisson's ratio (v) is 0.436. The quasistatic yield stress in compression was 500 MPa.
RESULTS AND DISCUSSION.
In Fig. 2, we present typical embedded and back
surface gauge traces for a 3 mm copper flyer
impacted at 562 m s"1.
12.0
3.00 o
eg
2.50 I'
Embedded Gauge
£ 10.0
O
¥ a°
2.00 |
* 6.0
150 1
15
1.00 *
S
0.50
4.0
§ 2.0
Back Surface Gauge
0.0
0
0.5
1
1.5
2
2.5
3
0.00 13
3.5
Time (us)
FIGURE 2. Representative gauge traces for NiTi. The impact
conditions were 3 mm copper flyer at an impact velocity of 562
ms" ! .
Here it can be seen that the embedded gauge
traces rises to ca. 10 GPa over about 100 ns. This
580
that these are genuine materials effects and not due
to an interaction involving the coverplate. The onedimensional stress yield stress, 7, can be determined
from the HEL through the relation,
rise time is rather long due to the fact that the stress
has to equilibrate in the insulating layer between the
coverplate and the NiTi specimen. As such, the
stress will take time to reach its final value, and thus
some temporal resolution will be lost. The situation
with the back surface gauge is more revealing.
Observe that there is now some structure present
within the rising part of the gauge trace. There is
now a clear change in slope at ca. 0.6 GPa. The
traditional interpretation of such features in metallic
systems is that this is the HEL. If so, it is therefore
possible to calculate the genuine material value
(0Nm), through,
-Or
(2)
1-v
With a Poisson's ratio of 0.436 calculated from
the measured longitudinal and shear wave speeds,
and an HEL of 3.5 GPa, this gives 7=794 MPa.
Whilst this is higher than the quasi-static yield
stress of 500 MPa, other materials with a B2
structure such as NiAl (13) have been shown to
display a significant degree of strain-rate sensitivity.
Therefore, it is feasible that the break in slope
measured at 3.5 GPa is indeed an HEL.
From Fig. 2, the temporal spacing between the
embedded and back surface gauges was used to
determine the shock velocity (£/s), using the known
thicknesses of the NiTi plates. The height of the
embedded gauge traces, in combination with the
particle velocity (wp), determined from impedance
matching techniques, was used to determine the
Hugoniot of NiTi in both Us-up and crx- up space.
(1)
2Z
where <7P is the stress measured by the back surface
gauge and the values of Z are the elastic impedances
of each material. However, when one does so, using
elastic impedances 34.4 and 3.2 for NiTi and
PMMA respectively, this yields an 'HEL' of 3.5
GPa. This value seems anomolously high for an
HEL, in particular since the quasi-static
compressive yield strength of NiTi is ~ 500 MPa.
The concern that this may be an artefact of the
coverplate on the front surface of the NiTi plate
prompted an extra experiment to be carried out
under the same conditions, but with a back surface
gauge only. The results are presented below in Fig.
3.
CO
E
I
o
CO
0
0.1
0.2
0.3
0.4
Particle Velocity (mm |js"1)
0.5
FIGURE 4. Us-Up for NiTi. A simple polynomial has been fitted
to aid interpretation.
0.0
0.2 0.4 0.6 0.8
1
In Fig. 4, shock velocity can be seen to decrease
slightly with increasing particle velocity, before
increasing rapidly. This is contrast to other metallic
systems, where a simple straight line fit according
to US=CQ+S up is generally found to be sufficient.
Such behaviour has been seen in other materials
such as borosilicate glass (14), where it has been
correlated with a leading ramp (15) on the rising
part of the shock pulse. Therefore, it is possible that
1.2 1.4 1.6 1.8
Time (ys)
FIGURE 3. Back surface gauge traces forNiTi.
As can be seen, the breaks in slope on each trace
are at the identical stress level, and gives confidence
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the break in slope observed in Fig 3. is not in fact an
HEL, but rather a feature caused by the shape
memory effect, i.e. stress-induced phase
transformation, in this material. Note that in the
uranium alloy, U-6%Nb, (10) where similar
behaviour in Taylor cylinders has been reported, no
clear elastic behaviour was observed in VISAR
traces.
Finally, in Fig. 5, the Hugoniot of NiTi in stressparticle velocity space is presented. Here, it could
be argued that there is a point of inflexion in the
curve, although more points would clarify this
issue. As with the shock velocity-particle velocity
curve, such behaviour has been observed in
materials with a ramped shock front such as
borosilicate glass (15).
shock velocity and stress-particle velocity space
show anomalous behaviour when compared to other
metals and alloys. However, such behaviour has
been observed in materials such as borosilicate
glass, where this behaviour has been correlated with
a leading ramp on the shock front.
Further work on this material, including soft
recovery, shear strength measurements and
measurements of the 'HEL' at different thicknesses
during shock loading should reveal more about the
shock behaviour of this unusual material.
REFERENCES.
1. Knowles, K.M. and Smith, D.A. Acta Met. 29 (1981)
101-110.
2. Tirelli, D. and Mascelloni, S. C J. Phys. IV10 (2000)
665-670.
3. Miller, D.A., Thissell, W.R. and Macdougall, D.A.S. J.
Phys, /F10 (2000) 341-346.
4. Matsumoto, H., Kondo, K.-L, Dohi, S. and Sawaoka,
A. J. Mater. Sci. 22 (1987) 581-586.
5. Man, X., Zou, W., Wang, R., Jin, S., Zhang, Z., Li, T.
and Yang, D. J. Mater. ScL 32 (1997) 4723.
6. Usherenko, S.M., Louchenok, A.R. and Zvorykin, L.O.
J. Phys. IV10 (2000) 143-146.
7. Thakur, A.M., Thadhani, N.N. and Schwarz, R.B.,, in
Shock Compression of Condensed Matter - 1989, S.C.
Schmidt, J.N. Johnson, andL.W. Davison, Editors. 1990,
Elsevier Science Publishers B.V.: New York. p. 139-142.
8. Thakur, A.M., Thadhani, N.N. and Schwarz, R.B. Met.
Mat. Trans. A 28 (1997) 1445-1455.
9. Escobar, J.C., Clifton, R.J. and Yang, S.-Y., in Shock
Compression of Condensed Matter - 1999, M.D. Furnish,
L.C. Chhabildas, andR.S. Hixson, Editors. 2000,
American Institute of Physics: Woodbury, NY. p. 267270.
10. Zurek, A.K., Hixson, R.S., Anderson, W.W.,
Vorthman, I.E., Gray III, G.T. and Tonks, D.L. J. Phys.
IV10 (2000) 677-682.
11. Millett, J.C.F., Bourne, N.K. and Jones, I.P. J. Appl.
Phys. 89(2001)2566-2570.
12. Rosenberg, Z., Yaziv, D. and Partom, Y. J. Appl.
Phys. 51(1980)3702-3705.
13. Gray III, G.T., in Deformation and Fracture of
Ordered intermetallic Materials III, W.O. Soboyejo, T.S.
Srivatsan, and H.L. Fraser, Editors. 1996, The Minerals,
Metals and Materials Society: p. 57-73.
14. Marsh, S.P., LASL Shock Hugoniot data. 1980, Los
Angeles: University of California Press.
15. Bourne, N.K., Rosenberg, Z. and Ginsberg, Proc. R.
Soc. Lond. A 452 (1996) 1491-1496.
20
15
Typical Error
£
35
0.1
0.2
0.3
0.4
Particle Velocity (mm |js~1)
0.5
FIGURE 5. The Hugoniot of NiTi in stress-particle velocity
space.
CONCLUSIONS.
Plate impact experiments have been performed
on the shape memory alloy, NiTi, where stress,
shock velocity and particle velocity have been
determined. From the back surface gauges, a break
in slope on the rising part of the shock at ca. 3.5
GPa has been identified. However, this would seem
to be very high value for the HEL in a metal, and as
such it is believed that this is a reflection of the
shape memory effect, i.e. stress-induced phase
transformation, in this material instead. However,
conversion to a one-dimensional stress yield stress
of 794 MPa would seem reasonable given that other
B2 materials have been shown to display high
strain-rate sensitivities. Plots of the Hugoniot in
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