Microstructural and Textural Evolution by Continuous Cyclic Bending and Annealing in a High Purity Titanium

Microstructural and Textural Evolution by Continuous Cyclic Bending

and Annealing in a High Purity Titanium

Yoshimasa Takayama, Taku Miura

_{, Hajime Kato and Hideo Watanabe}

Department of Mechanical Systems Engineering, Utsunomiya University, Utsunomiya 321-8585, Japan

Microstructural and textural evolution by continuous cyclic bending (CCB) and annealing in a high purity titanium sheet has been investigated. The hardness distribution through thickness in the CCBent sheet exhibits V-shape with marked difference between the surface and the inside. More deformation twins are observed in the surface of the sheet subjected by more CCB passes. After annealing, the texture in the sheet which was CCBent in rolling direction is randomized, while the as-received sheet has typical textural components in rolling and recrystallization of the rolled titanium sheet. On the other hand, in the CCBent sheet in transverse direction, development of the rolling texture component is found after annealing. Further, the textural evolution is compared with that in a commercial purity titanium.

(Received April 28, 2004; Accepted July 26, 2004)

Keywords: continuous cyclic bending, crystallographic orientation, titanium, scanning electron microscopy/electron back scatter diffraction pattern (SEM/EBSP) analysis, deformation twin

1. Introduction

Continuous cyclic bending (CCB) has been proposed as a straining technique that can produce the high strain on the surface and the low strain on the inside for sheet materials.1)

It was reported that the CCB process and the subsequent annealing make it possible to produce the gradient micro-structure with the coarse-grained surface layer and the fine-grained central layer in the sheet of an Al-4.7 mass%Mg-0.7 mass%Mn alloy (A5083). For the aluminum alloy sheet consisting of such layers improvement of fatigue properties has been found quite recently.2)_{Moreover, it was also found}

that preferred orientation was formed by the CCB and subsequent annealing in surface layers of the sheets of an Al-Mg-Mn alloy,3)_{a pure aluminum}4)_{and a commercial purity}

(CP) titanium.4,5) _{In the CP-titanium the transverse texture}

(TD//[0001]), which was observed in a cold-rolled titanium with a comparatively low reduction of 30%,6)_{appeared after}

the CCB and annealing in the surface layers.

The aim of the present paper is to investigate micro-structural and textural evolution after the CCB process and the subsequent annealing in a high purity (HP) titanium sheet, compared with that in the commercial purity titanium. Further, mechanism of the texture evolution is discussed briefly.

2. Experimental Procedure

The rolled sheet of high purity titanium used was produced by the Sumitomo Titanium Corporation, Japan. Chemical composition of the sheet is listed in Table 1. The as-received

sheet of 1.5 mm thick had been manufactured by the process of arc melting, casting, hot rolling, cold rolling and annealing. Workpiece was machined parallel to rolling (RD) or transverse direction (TD) with a 500 mm or 350 mm length, a 20 mm width and a 1.5 mm thickness, and then was cyclic-bent continuously up to the maximum number of passes,NCCB¼100. The CCB was mainly carried out using a roll driven machine as illustrated in Fig. 1, where true strain on the surface given by a single pass is calculated as 0.05463. The previous type or the roll traveling CCB device1,3) _{was also used for comparison. From the CCBent}

workpiece specimens with the size of about 10mm

10mm1:5mm were prepared for microstructure and texture analyses.

Microstructure, texture and microtexture were examined on the surface using the optical microscopy and the scanning electron microscope/electron backscatter diffraction pattern (SEM/EBSP) technique7) _{by TSL Orientation Image}

Mi-croscopy (OIM) system.

3. Results and Discussion

3.1 Continuous cyclic bending properties

Figure 2 shows change in dimensions after continuous cyclic bending (CCB) using both types of the CCB in the

Table 1 Chemical composition of a high purity titanium used (mass ppm).

Fe Ni Cr Al Mn O N C H Ti

16.7 1.7 0.3 1.5 0.2 243 20 15 14 Bal.

Roll 5 Roll 2

Roll 1

Roll 4

Roll 3

50 18 18 18 18 50

φ36.36 φ32

R1=80.50

R2=40.25

1

1

1.5

t=1.5

Advancing direction

Fig. 1 Roll driven CCB machine.

*_{Graduate Student, Utsunomiya University}

titanium workpiece. Cumulative true strain is taken as abscissa in the figure since true strains per a single pass are not the same for both the CCBs. In the roll traveling CCB, the dimensional change of the workpiece is considerably large, in particular, increase in length reaches 6.9% at a cumulative strain of 5.281. Because the increase in length for a commercial purity (CP) titanium was only 1.5% at the same strain,5) _{such dimensional change probably depends on the}

tensile strength. On the other hand, change in length due to the roll driven CCB reaches 2.0% at a strain of 5.463 even for the HP-titanium. Decreases in width and thickness are less than 0.1% and 0.4%, respectively. Therefore, the use of the roll driven machine results in the CCB closer to an ideal one accompanied with no dimensional change.

Vickers hardness on the surface of the workpiece as a function of the cumulative true strain is displayed in Fig. 3. The Vickers hardness increases drastically until a strain of

1.1, and then gradually to about 5 for both the CCBs. The latter increase in the roll driven CCB is considerably larger than that in the roll traveling CCB. This is probably attributed to following two facts. One is that practical plastic strain is small owing to small strain per a single CCB pass being accompanied with relatively large elastic strain in the roll driven CCB. Another is that the decrease of sheet thickness with the increment in the number of passes in the roll traveling CCB results in the reduction in bending curvature, and it lowers given strain or strain hardening in the late stage. It is confirmed that the surface layers are strain-hardened by about 40% using the roll driven CCB with less dimensional change. Figure 4 shows the Vickers hardness distribution along the short transverse direction of the CCBent sheet using the roll driven machine. The hardness distribution exhibits V-shape with marked difference between the surface and the inside, as has been observed in the Al-Mg-Mn alloy sheet.1) The V-shaped distribution has a better symmetry compared with that by the roll traveling CCB. Thus, the results obtained by the roll driven CCB will be mentioned below.

3.2 Microstructural evolution with the CCB and the annealing

Optical micrographs representing microstructural evolu-tion in the 100 pass CCB and the subsequent annealing are exhibited in Fig. 5. The micrographs were taken on the surface of the sheet. The microstructure in the CCBent sample includes a lot of deformation twins covering about 70% of the area (Fig. 5(a)). After annealing at 773 K for 3.6 ks, the twin density becomes smaller with more distinct boundaries (Fig. 5(b)). A small amount of the twins is observed in the microstructure of the sample annealed at 823 K (Fig. 5(c)). Grain growth seems to be retarded by the existence of the deformation twins. Higher temperature annealing accelerates grain growth in the range above 873 K. Large grains with a size over 100mm are observed after

0 1 2 3 4 5 6

–4 –3 –2 –1 0 0 2 4 6 8 Length 4N Ti –4 –3 –2 –1 0 Width Thickness

Change in dimensions (%)

Cumulative true strain,

ε

Roll traveling Roll driven

Fig. 2 Change in dimensions after continuous cyclic bending by the roll driven machine and the roll traveling device.

0 2 4

110 120 130 90 100 140 150 160 6

Cumulative true strain

Vic k ers hardness , HV / HV 4N Ti

Roll driven CCB machine

Roll traveling CCB device

Fig. 3 Relation between Vickers hardness and cumulative true strain given by CCB using the roll driven machine and the roll traveling device.

80 100 120 140 160

Position No. in ST direction

5 0 5

(Surface) (Center) (Surface)

V

ickers hardness (Load: 0.49N) / HV

HCCB=20

H_CCB=50

H_CCB=100 As–received

4N Ti

Roll driven CCB machine

annealing at 973 K (Fig. 5(f)). Figure 6 displays micro-structures showing the development of deformation twins on the transverse cross-section. More deformation twins are formed and advanced to the center as the number of the CCB passes increases. These micrographs reflect the strain

gradient corresponding to the hardness distribution in Fig. 4. Further, the gradient microstructure consisting of coarse-grained surface layers and fine-coarse-grained inside in the cross-section was found after annealing in analogy with the Al-Mg-Mn alloy.1)

3.3 EBSP analysis of deformation twins

In order to investigate formation of the deformation twins in more detail, the CCBent samples were analyzed by the SEM/EBSP technique. Figure 7 shows an EBSP map in the surface layer of the 20-pass CCBent sheet. Black area represents non-analyzable points with confidence indices not more than 0.1, which corresponds to 95% confidence of analyzed data in fcc metals.8)_{The fraction of the black area is}

measured to be about 47%. As fraction of the black area or low confidence points raised with the increase in number of the CCB passes, only the analyzed data in the 20-pass CCB will be used below. The EBSP data in Fig. 7 was treated by means of the standard cleanup procedure8)for convenience of analysis on the deformation twins. Figure 8 illustrates the modified EBSP map and analytical results in the 20-pass CCB. Figure 8(b) is a misorientation profile on the 100mm

long arrow shown in Fig. 8(a). Misorientations at the boundaries are measured to be 85 _{or 64. These}

misor-ientations refer to f11012g twin for tension in h0001i and

f11222gtwin for compression inh0001i, respectively.6,9)_Twin

boundary map reveals that onlyf11012gandf11222gtwins are observed. These twins tend to be formed in grains which have a larger Schmid factor of the prismatic slip f10110gh12210i

(Fig. 7(d)).

3.4 Formation of preferred orientation

As mentioned in Introduction, the transverse texture was formed by the CCB and the annealing in the CP-titanium. It is

as-CCBent

823K-3.6ks

923K-3.6ks

Rolling direction

(a) (b)

(c) (d)

(e)

100 m (f)

µ

773K-3.6ks

873K-3.6ks

973K-3.6ks

Fig. 5 Microstructural evolution of the surface layer after 100 passes of the roll-driven CCB and subsequent annealing.

As-received

Rolling direction

NCCB=20

NCCB=50

NCCB=100 300µm

Fig. 6 Optical micrographs on transverse cross-section showing deforma-tion twins formed by the roll-driven CCB.

interesting to examine whether the same preferred orientation is formed in the HP-titanium. Figure 9 shows development of texture after annealing in the as-received sheet without the CCB. The as-received sheet has the two principle compo-nentsf112114gh10110iandf02225gh211110iof the rolling and its recrystallization textures. There are almost the same amounts of the two components in the as-received sample. As expected, the recrystallization texture component is devel-oped by annealing, and the similar oriented grains meet

together to make low angle boundaries.

In contrast to this, the annealing after the CCB in rolling direction (RD) affects the texture in another way. The both components decrease into one third or one fourth by the CCB and the annealing as shown in Fig. 10. The microstructure after the annealing is characterized by comparatively small grains.

On the other hand, the CCB in transverse direction (TD) and the annealing leads to evolution of the rolling texture as

(a) Modified EBSP map

(b) Misorientation profile on the arrow

(c) Twin boundary map

(d) Schmid factor map

}

012

1

{

}

2

2

11

{

0.475 0.5

0.45 0.475

0.35 0.45

0.25 0.35

0 0.25

0 100

0 50 100

Distance,

d

/

µ

m

Misor

ientation,

θ

/deg

rees

shown in Fig. 11. The rolling texture evolves remarkably though the recrystallization texture is also developed. The textural evolution is accompanied with grain growth. The development of the rolling texture after annealing was found in cross-rolled titanium sheet.10)_{The results of the textural}

analysis are summarized in Table 2. It is also confirmed that the formation of the preferred orientation is intimately related to grain growth.

The result that the different textures are developed in the CP and the HP titanium may be attributed to the difference in strength. The spring back in bending depends on purity of titanium. The difference in strength probably causes the difference in the stored strain energy. Thus, the transverse texture formation can be regarded as a result of a small amount of strain stored.

4. Summary

In the present study, the microstructural and textural

evolution by the CCB and the annealing in the high purity titanium sheet has been investigated. The results obtained are summarized as follows:

(1) The hardness distribution through thickness in the CCBent sheet exhibits V-shape with the marked differ-ence between the surface and the inside.

(2) More deformation twins are observed in the surface of the sheet subjected by more CCB passes.

(3) The texture in the annealed sheet after the CCB in RD is randomized, while the as-received sheet has principle components of the rolling and the recrystallization textures in the titanium sheet.

(4) In the CCBent sheet in TD, the rolling texture componentf112114gh10110iis developed after annealing.

Acknowledgements

The authors wish to thank the Sumitomo Titanium Corporation for providing the materials, Mr. A. Mitsutake

(a)

(b)

(c)

Rolling texture {1214}<1010>

Recrystallization texture {0225}<2110>

Fig. 9 Orientation maps showing primary components of rolling and its recrystallization textures: (a) as-received and annealed for 3.6 ks at (b) 973 K and (c) 1023 K.

(a)

(b)

(c)

Rolling texture {1214}<1010>

Recrystallization texture {0225}<2110>

and Mr. N. Saito for their experimental collaborations. This work was supported in part by the Light Metals Educational Foundation of Japan.

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5) Y. Takayama, Y. Saigo, R. Takahashi and H. Kato: J. Japan Inst. Light Metals52(2002) 566–571.

6) H. Inoue and N. Inakazu: Proc. ICOTOM8, ed. by J. S. Kallend and G. Gottstein, (The Metallurgical Society, Warrendale, 1988) 997–1004. 7) B. L. Adams, S. I. Wright and K. Kunze: Metall. Trans. A24A(1993)

819–831.

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(a)

(b)

Rolling texture {1214}<1010>

Recrystallization texture {0225}<2110>

Fig. 11 Orientation maps showing primary components of rolling and its recrystallization textures in the sheets annealed for 3.6 ks after 20 pass CCBent in transverse direction: at (a) 973 K and (b) 1023 K.

Table 2 Summary of texture analysis

Samples As-received 0P RD20P RD50P RD100P TD20P

Fraction of rolling texture 14.8% 8.6% 10.1% 3.4% 5.1% 26.2%

Fraction of recrystallization texture 13.4% 26.7% 8.0% 3.5% 4.8% 20.0%

Peak intensity 12.8 26.1 9.2 8.4 7.5 22.5