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
FORMATION AND MORPHOLOGY OF TWINNING IN TITANIUM
UNDER HIGH STRAIN RATE DEFORMATION
B. Herrmann1, A. Venkert1, G. Kimmel1, A. Landau1, D. Shvarts1'2, E. Zaretsky2
/
2
Nuclear Research Center - Negev, P.O.Box 9001 Beer-Sheva 84106, Israel
Mechanical Engineering Dept, Ben Gurion University, P.O. Box 653, Beer-Sheva 84105, Israel
Abstract The dynamic behavior of pure titanium was studied in planar impact experiments using a
25-mm pneumatic gun, at impact velocities of 150-550 rn/sec. The sample free surface velocity was
monitored by VISAR. Softly recovered samples were characterized utilizing XRD, and optical, SEM
and TEM microscopy, Metallographic examination of the cross section areas of the impacted samples
revealed impact-oriented twins with significantly lower concentration in the direct vicinity of the spall
plane. TEM studies revealed dense dislocation areas and twins in the samples, with profound secondary
twins and ordered dislocations cells near the spall plane. The results allow estimating the relative
contribution of twinning mechanisms in shock-induced plastic deformation of titanium. The differences
in the morphology of the area near the spall plane, in contrast to the rest of the sample, implies on
alternative deformation mechanisms depending on the loading history.
INTRODUCTION
MATERIAL AND EXPERIMENTAL
Plastic deformation can take place by a number of
mechanisms depending on temperature, stress and
rate of deformation.
Mechanisms based on dislocations (formation,
motion and interaction with various structural
features) play a main role in plastic deformation.
Mechanisms based on twins become more
influential as temperatures are decreased and strain
rates increased. The relative influence depends on
metallurgical properties, such as crystallographic
structure of the material.
In h.c.p. metals, however, deformation twinning
plays an important role in plastic flow at all
temperatures and can effectively strengthen or
weaken the material, depending on the
circumstances.
Alpha-titanium (a-Ti) was chosen for this study
because it has an h.c.p. structure (below 1156 K)
and it exhibits high sensitivity to twinning under
plastic deformation.
The symmetrical (a-Ti/a-Ti) planar impact
experiments were performed using a 25-mm
pneumatic gun. The free surface velocity of the
sample was continuously monitored by VISAR [1].
The impactor/sample misalignment (tilt) and the
impact velocity were determined by charged pins. In
all experiments the tilt did not exceed 0.5 mrad.
The iinpactors and samples were cut from asreceived plates of commercial titanium (0.016 C, 0.1
Fe, 0.15 O and Al, V, Zr, Mn, Cu, Cr, Ni < 0.02
w/o). The plates were 2 and 5-mm nominally thick.
The surface planes of the impactors and samples
were parallel to the original surfaces of the plates.
The diameters of the impactors and samples were
24.5 mm and their measured thickness was 1.99 mm
and 4.88 mm, respectively.
The unloaded material was characterized
metallographically in the plane parallel to the
surface and in two orthogonal planes normal to the
surface, exhibiting equiaxed grains of 22-^m
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diameters. X-ray diffraction (XRD) revealed an
h.c.p. structure of a-Ti, with preferred orientation of
basal planes parallel to the plate surface. The
samples, being softly recovered after the impacts,
were subjected to optical (OM), electron (SEM and
TEM) microscopy and XRD analysis. Samples with
three degrees of plastic deformation and spall
fracture (initiation, partial and complete) were
examined. All the samples were cut normal to the
spall plane so that the examined area contained the
spall features. The cut surfaces were polished and
etched for optical microscopy, SEM and XRD
analysis. Specimens for TEM examination were
prepared by polishing and ion milling.
the results of the metallographic study, the
deviatoric stress, s = <J - p, and the true strain,
£, produced by the impacts, were calculated. The
simple wave approximation was used to estimate the
compressive stress, tJ, and strain, €. The particle
velocity was obtained by using the factor-of-two
approximation. The linear Hugoniot of titanium was
used to calculate the pressure, p. The corresponding
deviator/strain curves are shown in Fig. 1 b.
The unloaded material was found almost free of
twins in the optical and SEM examinations. An
increasing number of twins are found in the
impacted samples as the impact strength is
increased. The number of twins increases due to the
higher twin density in each grain. The inter-twin
spacing changes from about 10 fim at the weakest
impact to about 2-3 fim at the strongest impact. The
twin planes are preferably parallel to the sample
surface while increasing the impact strength results
in the appearance of more twins at an angle of 2530° with respect to the impact plane. The twin length
is limited by the grain boundaries while the width is
about 500 nm. Only few twins were found in the
vicinity of the spall plane.
The XRD pattern of an un-shocked sample is
typical for h.c.p. material, with preferred orientation
of the basal planes parallel to the sample surface.
The impacted samples exhibit a decrease in the
preferred orientation as the impact velocity
increases. The pattern obtained after the strongest
impact corresponds to an almost random orientation
RESULTS AND DISCUSSION
The velocity profiles, recorded during three
impacts with a-Ti, are shown in Fig, la. It is
apparent that spall fracture takes place at the
stronger impacts, 486 and 327 m/sec. The velocity
pullbacks in the strongest and intermediate shots, are
very close, 253 and 244 m/sec, respectively. At the
strongest impact the spall fracture is very fast and
the metallographic investigation reveals a complete
separation of the spall surfaces. A partial spall is
found after the intermediate impact. The weakest
shot seems to be very close to the spall threshold and
only a single broken line of a spall micro-crack, 0.6
mm long and 0.01 mm wide, has been observed.
In order to relate the free surface measurements to
500
0.04
0.05
1
1,5
2
0
0,01
0,02
0.03
time after impact, ju sec
strain
Figure 1. Free surface velocity profiles of a-Ti samples obtained in the planar impact experiments (a) and the stress deviators calculated
from the corresponding velocity profiles (b). The impact velocities are given in m/sec near the curves.
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Figure 2. TEM micrographs of secondary twins. First type, within a twin, decorated by stacking faults (a), and second type, creating a
system with a 2000 nm period (b).
of the basal planes of the different grains.
TEM examination was used to reveal dislocation
and twin structure. The un-shocked material is
almost dislocation-free, contains a small number of
twins and TiC inclusions. The twins were of the
above-mentioned kind, about 500 nm wide. Much
greater densities of dislocations and twins, that even
conceal the inclusions, were found in the impacted
samples. In all studied specimens, the dislocation
density is much lower in grains that do contain
twins.
In addition to the primary twins, observed also by
OM and SEM, two types of secondary twins were
revealed by the TEM examination. The first type
was observed in the sample recovered after the
weakest impact, Fig, 2a. The TEM specimen was
taken from a region far from the plane containing
the single spall micro-crack. A sub-twin, about 200
nm wide, inclined at about 30 deg. to the primary
twin, was revealed within a twin having the typical
width of about 600 nm and strictly parallel
boundaries. The boundaries of both the sub-twin
and the twin, at the region close to the sub-twin, are
decorated by stacking faults. It is plausible to
assume that in this case the secondary twinning
occurs by the successive glide of partial dislocations
in the subsequent planes and not by a simultaneous
glide of the dislocation stack. At the spall crack
region of this sample only the primary twins were
found. The second type of the secondary twins was
found in the TEM specimens taken from the sample
recovered after the intermediate impact. Unlike in
the previous case the secondary twins were not
found within the primary twins. They produced
their own periodic system of thinner twins, almost
perpendicular to the primary twins. The period of
this secondary twin system is about 600 nm in the
region far from the spall. In the spall vicinity, the
period increases up to about 2,000 nm and the twins
start and terminate at the primary twin boundaries,
Fig. 2b, In the case of the strongest impact, it was
impossible to take TEM specimens from the spall
region. The specimens taken far from the spall
reveal only primary twins.
Twinning contributes significantly to plastic
deformation in a-Ti[2]. The results of the present
study confirm this statement. In addition to the
direct OM and SEM observations of the increase,
with the impact strength, in the number of twins, the
XRD yields quantitative correlation between the
amount of the plastic strain, introduced by the
impact loading, and the contribution of twins to the
XRD pattern of the deformed oe-Ti. The changes in
the relative integral intensity, Ihkj /lrpc, for different
crystal planes (hk.l) of a-Ti are shown in Fig. 3. The
normalization factor, IrpC9 corresponds to a
randomly-oriented polycrystalline sample while the
values of the shock-induced plastic strain are taken
from Fig.lb. The twinned volumes are rotated in
relation to the un-twinned parts of the grains. The
latter are preferably oriented in the un-shocked
material. For example, the basal planes (00.2) are
initially parallel to the impact plane, and thus are
normal to the surface of the XRD samples. As a
result, the (00.2) reflection is absent in the unshocked XRD specra. The angle between this plane
and the main twinning plane of a-Ti (10.2), is 42.5
deg., thus the process of twinning brings the rotated
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0.01
0.02
0.03
compressive strain
Figure 3. XRD peak intensities versus compressive strain.
0.04
Figure 4, TEM micrograph of a crack in an intermidiate impact.
(00.2) planes into position almost normal to the
impact plane. This results in the increase of the
100.2 ft pic up to about 0.4, in the strongest impact.
Note, that while the (00.2) plane is parallel to the
impact plane, the (10.2) plane almost coincides with
the plane of maximum shear stress. The angle
between the (00.2) plane and the secondary twin
plane (11.2) is about 57.8 deg. The impact loading
seems to actuate both the main and the secondary
twin systems, and up to the intermediate impact
strength the rate of the relative intensities change of
both the (10.2) and (11.2) peaks coincides with that
of (00.2). This is confirmed by the TEM findings
from the weak impact, where secondary twins
crossing the primary ones are observed. The XRD
pattern, obtained after the strongest impact, shows
the surplus increase of (10.2) reflection intensity
with respect to that of (11.2) and (00.2). This may
be explained by possible actuation of new twinning
planes of {10.2} family. Since this surplus of (10.2)
peak intensity is not accompanied by additional
increase in the intensity of (00.2) peak, this
twinning mode should result in a preferable
production of basal planes parallel to the sample
surface. This condition could be met if a secondary
(10.2) twinning will be started inside a primary
(10.2) twin. The new twin planes are almost normal
(95 deg.) to those of the primary twin. Possibly, the
periodic secondary twins of the intermediate impact
are the mentioned (10.2) twins.
The path of a micro-crack propagation in the
sample recovered after the intermidiate impact was
observed. It can be seen in Fig. 4 that the crack path
follows a specific direction before crossing the
boundary and changes the direction during the
passage through the twin. After exiting the twin, the
crack restores the initial propagation direction. The
angle between the crack propagation and the twin
plane is very close to 45 deg.. Note, that the crack
cuts the twin belonging to the above-mentioned
twin system with the 2000 nm period. Assuming
that the twin plane belongs to the {10.2} family,
yields that the crack plane should lie in (10.0) plane,
inclined at 47.5 deg. to the twin plane. Since the
{10.0} planes are glide planes in oe-Ti, the crack
presented in Fig. 4 is a shear crack. When the crack
stops at the twin boundary, tensile stresses are
produced at the crack tip and the twin fails by
cleavage. After leaving the twin the crack resumes
its previous direction and shear mode.
CONCLUSIONS
The twin patterns in the shocked oc-Ti are studied
by optical and electron microscopy and X-ray
diffraction. The combined analysis of the microstructural and hydrodynamic data allows revealing
the different twin systems, operating in oe-Ti
undergoing high strain-rate deformation, and their
relation to the quantity of the plastic deformation
produced by the shock. A combined shear/cleavage
mechanism of the crack propagation is proposed on
the basis of TEM observations of the spalled
samples.
REFERENCES
1. Barker, L. ML, and Hollenbach, R. E., J.Appl.Phys.,
43,4669(1972).
2. Yoo M. H., Metall Tram. 12A, 409-418 (1981).
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