Effect of Deformation Twinning on Micro Structure and Texture Evolution During Cold Rolling of CP...

8/7/2019 Effect of Deformation Twinning on Micro Structure and Texture Evolution During Cold Rolling of CP Titanium

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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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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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Effect of Deformation Twinning on Micro Structure and Texture Evolution During Cold Rolling of CP...

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