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Materials Science and Engineering A 398 (2005) 209219
Effect of deformation twinning on microstructure and textureevolution during cold rolling of CP-titanium
Y.B. Chun a, S.H. Yu a, S.L. Semiatin b, S.K. Hwang a,
a School of Materials Science and Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, South Koreab Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433, USA
Received 22 November 2004; received in revised form 10 March 2005; accepted 16 March 2005
Abstract
The evolution of microstructure and texture during cold rolling of commercial-purity titanium (CP-Ti) was studied with particular reference
to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain was
accommodated by slip and deformation twinning. In this stage, both compressive ({1 1 2 2}1 1 2 3) and tensile ({1 0 1 2}1 0 1 1) twins,
as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneous
microstructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shear
bands developed. The crystal texture of deformed specimens was weakened by twinning but was strengthened by slip, resulting in a split-basal
texture in heavily deformed specimens.
2005 Elsevier B.V. All rights reserved.
Keywords: Titanium; Cold rolling; Microstructure; Texture; Deformation twinning
1. Introduction
Plastic deformation of metals is usually governed by the
activation of slip or deformation twinning. The specific defor-
mation mechanisms in metals with a hexagonal close packed
(hcp) crystal structure are less well understood than those in
cubic metals which usually have a large number of indepen-
dent slip systems. In pure titanium, for example, slip occurs
most easily via the activation of dislocations with a type
Burgers vector primarily on prism planes, to some extent on
basal planes and least on pyramidal planes [1]. Because a
slip alone cannot provide five independent slip systems, as
required to accommodate an external strain imposed on thegrains of a polycrystalline aggregate, deformation by c + a
slip (on pyramidal planes) or by twinning usually must be ac-
tivated in addition to a slip [26]. In this respect, it has been
suggested on a theoretical basis that twinning can account
for a maximum strain of only 0.1 [3], or a value consider-
ably less than the ductility of pure titanium [1]. Despite such
Corresponding author. Tel.: +82 32 860 7537; fax: +82 32 862 5546.
E-mail address: [email protected] (S.K. Hwang).
assertions, there are reports that twinning plays an essentialrole in deformation and texture formation for titanium [79].
Other research has shown that heavy cold rolling of high-
purity titanium results in the development of a split-basal
texture Ti [1015], whereas a normal basal texture forms in
less pure Ti containing alloying element such as Al [7]. The
difference in texture development for the different types of
Ti has been attributed to the effect of composition on the
activation of deformation twinning [8]. Due to the tedious
nature of the determining deformation twins via transmis-
sion electron microscopy (TEM) in early work; however, a
quantitative explanation has not been developed to describe
which twin systems become active under specific modes ofdeformation, how twinning contributes to microstructure re-
finement or how twinning affects the resultant texture.
Recent advances in electron-back-scattered-diffraction
(EBSD) techniques provide a powerful method for charac-
terizing local texture, twin relationships, etc. and thus offer
significant promise to provide answers to suchquestions [16].
The objective of the present study, therefore, was to utilize
such techniques in order to obtain a firm understanding of
the details of deformation twinning systems in commercial-
0921-5093/$ see front matter 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2005.03.019
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210 Y.B. Chun et al. / Materials Science and Engineering A 398 (2005) 209219
purity titanium (CP-Ti) under cold rolling conditions and to
establish how twinning affects the formation of basal and
other types of textures.
2. Experimental procedures
Thematerial used in this work was commercial-purity tita-
nium received as 12-mm-thick hot-rolled and annealed plate
whose measured composition is given in Table 1. Samples
measuring 150 mm 200 mm were cold rolled by reversing
the rolling direction between each pass at room temperature
to a total thickness reduction of 90% in a two-high mill with
220 mm diameter rolls using a rolling speed of 13.8 m/min.
During each pass, the thickness was reduced by 0.2 mm with
the aid of oil lubrication.
Following cold working, optical microscopy, EBSD anal-
ysis and TEM were conducted on transverse cross-sections
cut from the rolled samples. For optical microscopy and
EBSD analysis, specimens were mechanically polished andthen electro-polished in a solution consisting of 5 ml per-
chloric acid and 95ml methanol at 30 V and 40 C. Subse-
quently, the samples were etched with a solution consisting
of 1 ml HNO3, 2 ml HF and 40 ml H2O.
Grain-boundary character distributions (GBCD) in the
rolled specimens were established via EBSD using a Hi-
tachi 3400S field emission gun scanning electron micro-
scope (FEG-SEM) and TSL-OIMTM software. The statisti-
cal certainty of the EBSD analysis, especially for the highly
strained materials, is significantly affected by the level of
confidence index (CI) for which the software allowed during
post-processing of measured EBSD data. Preliminary EBSDexperiment for cold rolled -Ti revealed that the fractions
of random high angle boundaries decreased with increasing
CImin (the minimum CI allowed in EBSD post-processing)
in the range of CImin from 0 to 0.1. This is mainly due to
random orientation relationship between incorrectly indexed
points (generallyhaving lowCI) and theirneighboring points.
In the range of CImin higher than 0.1; however, the overall
aspect of misorientation angle distribution was unaffected by
CImin. Based on these, any measured points whose CI is less
than 0.1 were excluded from the analysis of the EBSD data
in the present study.
To determine the substructure developed during rolling,
TEM analysis was performed using a Philips CM200 trans-
mission electron microscope. Specimens for TEM were
Table 1
Chemical composition of commercial-purity titanium program material
Element Composition (wt.%)
H 0.0015
C 0.005
N 0.01
O 0.06
Fe 0.02
Ti Balance
thinned to 60m and then twin-jet electro-polished at 30 V
and 40 C using the solution previously described.
The textures developed during rolling were quantified us-
ing a Rigaku RINT2500 X-ray diffractometer. For this pur-
pose, five pole figures ((1 0 1 0), (00 0 2), (10 1 1), (11 2 0)
and (10 1 2)) were obtained from the plate/sheet surface
using the Schulz reflection method. Using the five incom-plete pole figures so obtained, the orientation distribution
function (ODF) was calculated with the commercial pro-
gram LaboTexTM based on the arbitrarily defined cell (ADC)
method [17]. From the ODFs, complete pole figures were
reconstructed. Euler angles were represented with reference
to a crystal coordinate system consisting of X = [2 1 10],
Y= [0 1 1 0] and Z = [0002].
3. Results
3.1. Starting material
Optical microscopy showed that the starting material com-
prised single-phase, equiaxed-Ti with an average grain size
of 30m (Fig. 1(a)). In addition, XRD analysis revealed
peaksonlyforthe-phase, and back-scatter-electron imaging
in the SEM confirmed thatthere was nosecondphase (such as
-phase). These analytical results indicated that the program
material (as-received CP-Ti) was indeed composed solely of
-phase despite being commercial grade, most likely due to
the low levels of impurities (Table 1). In particular, the level
of iron, a potent -stabilizer in titanium alloys, was approx-
imately 200 wppm, or only half the maximum solubility of
Fe in the -phase (400 wppm), thus resulting in a very lowprobability for the retention of-phase at room temperature.
Hence, the possible effect of second phases on the deforma-
tion behavior of CP-Ti can be excluded from consideration.
The as-received CP-Ti plate, which had been hot rolled
and then annealed in the -phase region, had a moderate tex-
ture (Fig. 1(b)). The (0 0 0 2) pole figure revealed a bimodal
distribution of basal poles, a texture commonly found in cold
rolled pure Ti; the maximum intensity (4.4 random) was
found at locations tilted 35 from the ND toward the TD.
A second, weaker component comprising (1 1 2 0) poles at
locations tilted 15 from the RD toward the ND suggested
the development of a recrystallization texture also. In the
(1 0 1 0) pole figure, the maximum intensity was found at the
RD, indicating that a considerable amount of rolling texture,
which had developed during hot rolling, remained. From the
pole figure analysis, therefore, it was confirmed that the as-
received texture comprised both rolling and recrystallization
components.
3.2. Microstructure evolution during low-to-medium
levels of deformation
Low-to-medium levels of deformation resulted in the de-
velopment of heterogeneous microstructures due to the frag-
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Fig. 2. EBSD (inverse-pole-figure) maps for the RD direction of CP-Ti cold rolled to thickness reductions of (a) 10%, (b) 20%, (c) 30% and (d) 40%, showing
activation of deformation twins in some but not all grains. In (c) and (d), NT indicates grains without twins.
servations thus indicate that the activation of{1 1 2 2}1 1 2 3
compressive twins was most likely dependent on the ori-
entation of the matrix and the difficulty of accommodating
compression near the c-axis via slip processes. In addition,
{1 0 1 2}1 0 1 1 tensile twinning appeared to have been acti-
vated without a noticeable dependence on matrix orientation
(Fig. 4(c)). The formation tendency of particular twins in a
grain was also affected by the orientations of the surround-
ing grains in addition to that of the matrix grain because, as
shown in Fig. 4(b and c), either compressive twins or tensile
twins were generated in similarly oriented grains. For thick-
ness reductions higher than 20%; however, the dependence
of the activation of{1 1 2 2}1 1 2 3 twins on the matrix ori-
entation decreased. At the same time, other types of twins,
such as {1 1 2 1}1 1 2 6 and {1 0 1 1}1 0 1 2, were observed
occasionally.
Secondary twins were observed for thickness reduc-
tions above 20%. When the primary twins were of the
{1 1 2 2}1 1 2 3 compressive type, the secondary twins
within the primary twins were of the {1 0 1 2}1 0 1 1 ten-
sile type (Fig. 5), thus also indicating a dependence of twin
activity on parent orientation.
Pole figures determined from X-ray diffraction (XRD)
measurements revealed that the initial split-basal texture was
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Y.B. Chun et al. / Materials Science and Engineering A 398 (2005) 209219 213
Fig. 3. Grain-boundary misorientation distribution for CP-Ti cold rolled to a reduction of (a) 10%, (b) 20%, (c) 30% or (d) 40%. The peaks at 65 and 85
correspond to {1 1 2 2}1 1 2 3 compressive twins and {1 0 1 2}1 0 1 1 tensile twins, respectively. LAB: low angle boundaries of less than 15 misorientation.
transformed to a basal texture as the reduction was increased
to 40%. After 20% reduction, the original basal poles of the
bimodal distribution along the NDTD began to be dispersed
toward the ND (Fig. 6(b)). As a result, the maximum basal-
pole intensity after 3040% reduction was observed parallel
to the ND (Fig. 6(c and d)). Unlike the distribution of the
basal poles, the maximum intensities for the prism poles, al-
though not very strong, were found along the RD and were
not affected noticeably by the level of cold reduction.
3.3. Microstructure evolution during higher levels of
deformation
At yet higher levels of thickness reduction (90%), the
microstructure became more refined, but more heterogeneous
as well. After 60% reduction, elongated, coarse grains (with
a thickness of 10m) were interspersed with fine grains (de-
veloped at lower reductions due to twinning), as shown in
Fig. 7. Because the as-received CP-Ti was equiaxed with an
average grain size of 30m, the aspect ratio of the coarse
grains reflected the amount of deformation imposed dur-
ing cold rolling. EBSD analysis showed that the elongated
coarsegrains(Fig.7(c)), hadorientations in therange1 = 0,
= 3090 and 2 = 30. Also, a small amount of macro-
scopic shear banding which had not been found during re-
ductions equal to or below 40% was noted at high reductions.
After 90% cold reduction, the microstructure became
much more refined and the macroscopic shear banding was
more evident (Fig. 7(b)). The thickness of the elongated
coarse grains had been reduced to 3m. The orientation im-
age for the sample rolled to 90% reduction (Fig. 7(d)) also
showed that the lattice was so severely deformed that it was
impossible to analyze approximately 70% of the data points
via EBSD. The orientation of the elongated coarse grains
in the sample rolled to 90% reduction was in the range of
1 = 0, = 3050 and 2 = 30
, thus indicating that the
basal poles near the TD moved toward the ND as the amount
of deformation increased.
TEM analysis of CP-Ti samples rolled to 60% reduction
revealed a fine lamellar structure with high dislocation den-
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Fig. 4. EBSD pole-figure data indicating the propensity of twinning as
a function of crystal orientation in CP-Ti cold rolled 20%: (a) (00 0 2)
and (10 1 0) pole figures of untwinned grains, (b) (00 0 2) pole figure of
parent/matrix grains (dark circles) and compressive {1 1 2 2}1 1 2 3 twins
within the corresponding matrix grains (open circles) and (c) (0 0 0 2) pole
figure of parent/matrix grains (dark circles) and tensile {1 0 1 2}1 0 1 1
twins within the corresponding matrix grains (open circles).
sity in regions which had been difficult to analyze with opti-
cal microscopy or EBSD. Deformation bands composed of a
lamellar-type microstructure with a thickness of 100150 nm
were observed (Fig. 8(a)); a generally high dislocation den-sity was found within the deformation bands [18,19]. In an-
other region of the same sample, grains elongated parallel
to the RD with a thickness of 100500 nm were observed; a
high dislocation density was also found inside these grains
(Fig. 8(b)). The ring-like selected area diffraction patterns
(SADP) (upper right-hand corner ofFig. 8(b)) indicated that
the grain-boundary character in this region was high an-
gle. Elongated coarse grains with homogeneously distributed
dislocations were also observed (upper left-hand corner of
Fig. 8(a)), and similar observation was made earlier by Wag-
ner et al. [15].
The split-basal texture reappearedat higher levels of defor-
mation. The basal poles had an intensity of 3.7 (random) af-
ter60% reduction(Fig.9(a)). The split-basal texture strength-
ened with yet further cold reduction, reaching an intensity of
5.9 (random) at locations tilted 35 from the ND toward
the TD after 90% reduction (Fig. 9(b)). While the maximum
intensity in CP-Ti rolled to a 40% reduction was observed
in the (0 0 0 2) pole figure, the maximum intensity of 4.6
(random) was observed in the RDof the (1 0 1 0) pole figure
for material rolled to 60% reduction (Fig. 9(a)). The inten-
sity of prism poles was also strengthened by increasing the
amount of reduction, reaching 7.7 (random) after 90%
reduction.
Fig. 5. EBSD pole-figure data indicating the rotation of crystals due to de-
formation twinning in CP-Ti cold rolled to 30% reduction: (a) (0 0 0 2),
(b) (1 0 1 0) and (c) (1 1 2 0) pole figures showing the orientations of a par-
ent/matrix grain (dark circles), primary twin (open squares) and secondary
twin (open triangles).
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Fig. 6. XRD pole-figures measurements for CP-Ti cold rolled to reductions
of (a) 10%, (b) 20%, (c) 30% and (d) 40%. Contour levels (random): 1.5,
2.0, 2.5, . . ., 4.5.
4. Discussion
4.1. Deformation twinning at low-to-medium levels of
deformation
The activation of twinning in the present material exhib-
ited a strong dependence on the level of deformation. For
thickness reductions less than or equal to 40%, twinning was
active, whereas for higher deformations dislocation slip was
the sole mechanism of deformation. The occurrence of twin-
ning was confirmed by the peaks in the misorientation distri-
bution at 65 and 85, which correspond to {1 1 2 2}1 1 2 3
compressive twinning and {1 0 1 2}1 0 1 1 tensile twinning,
respectively (Fig. 3). Fig. 3(a and b) also reveal that com-
pressive twinning was more prevalent than tensile twinning
at reductions less than 20%. This result does not necessar-
ily mean that the critical shear stresses for the two twin-
ning systems were different, for the imposed deformation
and the crystallographic orientation of the grains also play
a key role in the activation of a particular twinning system.
For the undeformed CP-Ti program material, the basal poles
were preferentially distributed along the ND, which is sub-jected to a compressive strain during rolling (Fig. 1(b)); very
few grains had basal poles parallel to the RD along which a
tensile strain is imposed. Therefore, the combination of the
initial texture andthe state of deformation imposed duringflat
rolling resulted in the preferential activation of compressive
twins.
An explanation for the activation of {1 0 1 2}1 0 1 1 ten-
sile twins, despite the unfavorable texture, focuses on the
value of the critical shear stress for such a deformation mode.
In related work, for example, Tenckhoff[20] established the
twinning activity in pure zirconium by determining the ini-
tial orientation and lattice rotations of 19 grains during cold
rolling. In this earlier work, {1 1 2 2}1 1 2 3 compressivetwins and {1 0 1 2}1 0 1 1 tensile twins were found to be
activated in grains with their basal poles inclined by 050
and 5090, respectively, to the ND. A similar analysis in
the present work, using EBSD and focusing on a much larger
number of grains (62 grains) (Fig. 4(b)), confirmed Tenck-
hoffs observation for the case of {1 1 2 2}1 1 2 3 compres-
sive twins. The {1 0 1 2}1 0 1 1 tensile twins, however, were
activated in grains which did not have an obvious orien-
tation relationship to the imposed plane-strain deformation
(Fig. 4(c)). This result may be interpreted to be a result of
a comparatively low critical shear stress for tensile twinning
compared to that required for compressive twinning and per-haps a slip. This hypothesis was confirmed by the nature of
the secondary twins. As shown by EBSD analysis (Fig. 5),
secondary twins of the{1 0 1 2}1 0 1 1 type nucleated within
the primary compressive twins of the {1 1 2 2}1 1 2 3 type
whose thickness ranged from 1 to 5m. It is well known that
the propensity for twin formation is significantly reduced as
grainsize decreases[8,21,22]. Therefore, theformation of the
secondary tensile twins within the fine primary twins would
be feasible only if the critical shear stress for the tensile twins
were very small.
According to Paton and Backofen [23], the formation ten-
dency of particular twins in -Ti is affected by temperature:
{1 1 2 2}1 1 2 3 type compressive twins at room tempera-
ture whereas {1 0 1 1}1 0 1 2 type compressive twins above
400 C.Thepresentresultisinsupportofthisearlierreport.In
case of the tensile twins, the critical shear stress is reported
to be low for the {1 1 2 1}1 1 2 6 type twins compared to
the {1 0 1 2}1 0 1 1 type [24]. In the contrary, however, the
latter type tensile twins were dominant in the present result.
Christian and Mahajan [22] suggested that the favorable con-
ditions for twin formation are a low twin shear and a small
extent of atomic shuffling. Yoo [25] calculated the two pa-
rameters for the hcp crystals, the result of which is shown in
Table 2. The type of twins found in the present result can be
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Fig. 7. Microstructures of CP-Ti cold rolled to reductions of (a) 60% (optical micrograph), (b) 90% (optical micrograph), (c) 60% (EBSD-orientation image)
and (d) 90% (EBSD-orientation image). Insert: stereographic color key for the rolling direction inverse-pole-figure maps shown in (c) and (d).
Table 2
Twinning shear and shuffling parameters of the various twinning systems in
titanium [25]
Twinning systems Shear, s qa Remarks
{1 0 1 2}1 0 1 1 0.174 4 Tensile twin
{1 0 1 1}1 0 1 2 0.099 8 Compressive twin
{1 1 2 2}1 1 2 3 0.219 6 Compressive twin
{1 1 2 1}1 1 2 6 0.630 2 Tensile twin
a Shuffling parameter.
explained in terms of thetwo parameters: the{1 0 1 1}1 0 1 2
type compressive twins and the {1 1 2 1}1 1 2 6 type tensile
twins are difficult to activate in Ti due to the large shuffling
parameter and the high twinning shear, respectively. In con-
trast, the {1 1 2 2}1 1 2 3 type compressive twins and the
{1 0 1 2}1 0 1 1 type tensile twins are easily activated be-
cause of their small shuffling parameter and low twin shear,
respectively.
The formation of deformation twins during cold rolling
contributed to the significant refinement in microstructure
and hence reduced the effective slip length. The initial CP-Ti
material used in this work presented a favorable condition for
deformation twinning because the grain size was relatively
large. Consequently, the number density of twins increased
with the imposed deformation. Formation of numerous me-
chanical twins and intersection among these twins divide the
grain interior, resulting in microstructural refinement. Addi-
tional grain refinement occurred by the almost simultaneous
formation of secondary and the tertiary twins in addition to
subdivision of twins due to crossing twins. Twinning became
saturated at 40% thickness reduction (Fig. 2(d)) at which the
effective grain size had been reduced to such a large extent
that twinning was impossible. In contrast, grains whose basal
poles were inclined from the ND toward the TD by 4090
were not susceptible to twinning (Fig. 4(a)). These grains
deformed mainly by dislocation slip and, as a result, became
elongated grains, which were comparatively larger than thosethat underwent twinning. The coexistence of fine twinned
grains and large grains that had undergone slip alone, there-
fore, resulted in an inhomogeneous microstructure in CP-Ti
cold rolled to low-to-medium levels of reduction. This inho-
mogeneity in microstructure persisted to high deformation
(Fig. 7).
4.2. Texture evolution
Slip in titanium occurs most readily along the a direc-
tion on prism and basal planes. However, a slip alone can-
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Fig. 8. Transmission-electron micrographs of 60% cold rolled CP-Ti show-
ing (a) shear bands formed in a fine, elongated grain and (b) fine grain
structure with highly dislocated boundaries. TEM foil was normal to the
TD. Note the ring pattern of the SAD in (b) indicating that boundaries were
mostly of high angle.
Fig. 9. XRD pole-figure measurements for CP-Ti cold rolled to reductions
of (a) 60% and (b) 90%. Contour levels (random): 1.5, 2.0, 2.5, . . ., 7.5.
not satisfy the von Mises requirement of five independent
deformation modes to accommodate an externally imposed
strain [26,27]. Although the activation of twinning accom-
modates plastic deformation along the c direction, heavy
deformation above 40% suppresses further twin formation
due to the reduced grain size introduced by prior twinning.
The absence of additional twinning during the large defor-mation of titanium has also been reported by Philippe et al.
[28] and Mullins and Patchett [29]. Therefore, another defor-
mation mechanism is required to accommodate strain above
40% thickness reduction. Otherwise, it would be impossi-
ble to accommodate uniform plane-strain deformation in all
crystallites during rolling. In the present work, it appears that
the latter case pertained in that non-uniform, macroscopic
shear banding was activated as shown in Fig. 7. In speci-
mens reduced by 60% or more, numerous shear bands were
present. The particular shear deformation is known to occur
when the grain orientation is unfavorable for slip or where a
fine lamellar structure is predominant [3032]. Considering
the fine deformed microstructure and the lack of sufficientslip systems in CP-Ti, the observed deformation via shear
banding during heavy deformation is as expected.
In the present work, two principal types of textures were
found: a basal texture (Fig. 6(d)), developed during low-to-
intermediate rolling reductions (40%), and a split-basal
texture (Fig. 9), found at high reductions (to 90%). Using
a Taylor-type (isostrain) crystal-plasticity model, Thornburg
and Piehler [7] suggested that the basal texture originated
from a combination of prism a and pyramidal c + a slip.
As shown in the present work, however, the probability of
pyramidal c + a slip seems to be low for low-to-medium
rolling reductions because twinning can accommodate thestrain along the c axis as well as the fact that the critical
resolved shear stress for c + a slip is relatively high. There-
fore, it is expected that the main slip systems would be the
prism a and the basal a.
During large deformation (>40% thickness reduction),
a split-basal texture was formed (Fig. 9). Thornburgh and
Piehler [7] concluded that such a texture results from the
activation of both slip and twinning. The present result, how-
ever, does not support this conclusion in as much as no ad-
ditional twinning was found for reductions above 40%. This
implies that twinning did not contribute to the separation of
the basal poles from the ND toward the TD. Therefore, it may
be hypothesized that either pyramidal c + a slip (activated
to accommodate deformation along the c axis when twin-
ning is not feasible) or the shift in strain path associated with
shear banding may have contributed to the split-basal texture.
The development of texture during cold rolling was also
interpreted in terms of the 2 = 30 section of orientation dis-
tribution function maps (Fig. 10). The location of the maxi-
mumf(g) progressed from (1 = 20,= 35 and2 = 30
) in
the initial (undeformed) condition toward (1 = 0, = 35
and2 = 30) in the final 90% cold rolled condition. The ODF
results showed that the typical cold rolling texture compo-
nent (1 = 0, = 35 and 2 = 30
) started to form at low
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Fig. 10. ODF maps for the 2 = 30 section: (a) initial CP-Ti material and after cold reductions of (b) 10%, (c) 40%, (d) 60% and (e) 90%. In the deformed
specimens, the maximum intensity was observed at 1 = 0, = 35 and 2 = 30
.
Fig. 11. Variation of the maximum intensity of f(g) in CP-Ti as a function
of deformation indicating the effects of twinning and slip in weakening or
strengthening texture intensity, respectively.
reductions. With increasing cold reduction, however, the in-
tensity of this component was weakened when twinning was
activated in addition to slip, as shown in Fig. 11. By contrast,
during heavy cold rolling during which slip waspredominant,
the intensity of the cold rolling texture component increased.
Therefore, it is concluded that slip intensifies the cold rolling
texture, but twinning weakens it by randomizing the crystal
orientations.
5. Conclusions
Microstructure and texture evolution during cold rolling
of CP-Ti were studied via optical microscopy, OIM-EBSD
and TEM. The following conclusions were drawn:
1. Deformation comprising low-to-moderate thickness re-
ductions (40%) was accommodated by slip and twin-
ning, whereas slip predominated at higher reductions. The
primary twinning systems activated were {1 1 2 2}1 1 2 3
compressive twinsand {1 0 1 2}1 0 1 1 tensile twins. Sec-
ondary twins, mainly of the tensile type, were also acti-
vated, thus indicating that the critical shear stresses of
these twins is probably relatively low.
2. The activation of deformation twinning results in grain
refinement due to intersection of twins and the formation
of secondary and tertiary twins. Grain refinement due to
twinning leads to increased difficulty for twin activity,
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Y.B. Chun et al. / Materials Science and Engineering A 398 (2005) 209219 219
leading to saturation in twinning at modest reductions.
Furthermore, an inhomogeneous grain structure can be
generated because some grains may be oriented to ac-
commodate the imposed strain via slip alone.
3. During heavy deformation (thickness reductions >40%),
macroscopic shear bands develop because of the absence
of twinning and the difficulty of accommodating the im-posed plain-strain deformation via a slip alone.
4. The characteristic rolling texture of (1 = 0, = 35
and 2 = 30), a split-basal texture, is a consequence of
deformation by slip. Twinning weakens this particular
texture component by randomizing the orientations of
crystals.
Acknowledgements
The present work was performed under the auspices of the
Air Force Office of Scientific Research and its Asian Office
of Aerospace Research and Development (Dr. Kenneth C.Goretta and Dr. Craig S. Hartley, program managers) and
also of the 2004 National Research Laboratory Program of
the Korea Ministry of Science and Technology.
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