A

cs(fc©

K

1

nn(spiaatotb

sitowtc

(

0d

Journal of Materials Processing Technology 191 (2007) 202–205

Cavitation phenomenon of commercially pure titanium

M.J. Tan ∗, X.J. Zhu, S. ThiruvarudchelvanSchool of Mechanical and Aerospace Engineering, Nanyang Technological University,

50 Nanyang Avenue, Singapore, Singapore

bstract

Cavitation caused during superplastic straining of a commercially pure titanium alloy under uniaxial test was studied. Tensile tests wereonducted with a constant strain rate of 0.01 s−1 at different temperatures. Studies show that commercially pure titanium alloy does not show gooduperplasticity due to cavitation. Many cavitations are formed at the grain boundary of triple points during tensile test at relatively low temperatures

below 600 ◦C). As strain increase, both the number of cavities and the average cavity diameter increase. However, no such phenomena can beound when the alloy deformed at higher temperatures (750 and 800 ◦C). Studies also show that the pre-existing defects are the main cause foravitation formation. These defects can be sintered when deformed at temperature above 750 ◦C. 2007 Elsevier B.V. All rights reserved.

2

(Ttd7stgHwuc0

3

utu

eywords: Titanium; Cavitation; Superplastic forming

. Introduction

Commercially pure (CP) titanium alloy is an emerging tita-ium alloy and widely used in many industries such as chemical,uclear and especially biomedical [1,2]. Superplastic formingSPF) is a cost-effective process for manufacturing complexhaped structural components [3–5] and SPF of commerciallyure titanium alloy is very attractive in many areas due to thenert nature of this alloy. To date, the number of research worknd publications concerning the superplasticity of CP Ti alloysre very limited [6]. Consequently, only little basic know-how onhe superplasticity of CP Ti alloy is available. The investigationsf its superplastic potential, related phenomena and microstruc-ure evolution are important for achieving desired mechanicalehaviors of the material.

It is well established that during superplastic deformation,ome materials develop cavities which grow and coalesce, lead-ng to cavitation damage [7–10]. Such damage in turn gives riseo premature failure of the material, thereby, limiting the usef superplastically formed components. In the present paper,e report and analyze experimental data obtained in an inves-

igation that focuses on the detailed quantitative evaluation ofavitation of the CP alloy.

∗ Corresponding author. Tel.: +65 6790 5582.E-mail addresses: mmjtan@ntu.edu.sg (M.J. Tan), pg04039432@ntu.edu.sg

X.J. Zhu), mthiru@ntu.edu.sg (S. Thiruvarudchelvan).

rd

i7ca

924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.03.078

. Experimental details

Commercially pure titanium sheets were used in this study. The compositionwt.%) of the alloy is 0.01 C, 0.16 Fe, 0.008 N, 0.108 O, 0.0015 H with the balancei. Tensile specimens with a gauge of 11 mm length, 4 mm width and 1.5 mm

hickness were machined with the tensile axis oriented parallel to the final rollingirection. Uniaxial tensile tests were performed at room temperature, 400, 600,50 and 800 ◦C with an initial strain rate of 0.01 s−1. After testing, the deformedpecimens were cooled rapidly to room temperature by forced cooling in ordero preserve the microstructure. Deformed specimens were sectioned along theauge part. The samples were then polished by silica paste and etched using 10%F + 5% HNO3 + 85% H2O for 5 s. Images for cavitation ratio measurementere first captured by optical microscope. Images were then sent for analysissing Image Pro+ software. The cavitation ratio was calculated by dividing theavity area by the total image area. In the present study, only cavities larger than.5 �m were counted, to reduce measurement errors.

. Results and discussion

The optical micrographs of the specimens deformed to fail-re are shown in Fig. 1 and the variation of cavitation ratio withemperatures is shown in Fig. 2. It can be seen from these fig-res that many cavities can be found on the sample deformed atoom temperature, 400 and 600 ◦C. These cavities are randomlyistributed and the cavity size varies over a wide range.

Fig. 2 also indicates that the cavitation ratio decreases with

ncreasing test temperature and when then temperature above50 ◦C, the cavitation ratio decreases to zero. The maximumavitation ratios are 5.2%, 4.3%, 0.55% for samples deformedt room temperature, 400 and 600 ◦C, respectively. The variation

M.J. Tan et al. / Journal of Materials Processing Technology 191 (2007) 202–205 203

Fig. 1. Microstructure of samples deformed at (a) room temper

oFd

tc

dcsiMmtalit can be seen that critical strain level for cavity initiation is10% and 15% for samples deformed at room temperature and400 ◦C, respectively. However, for samples deformed at 600 ◦C,

Fig. 2. Variation of cavitation ratio against temperatures.

f elongation-to-failure values against temperature is shown inig. 3. It can be seen from this figure that the CP titanium alloyo not show good superplasticity at low temperatures.

In order to identify the cavitation ratio and cavity initia-ion sites, detailed quantitative evaluation of cavitation wasonducted. Samples were strained to different strain levels at

Fig. 3. Variation of % elongation-to-failure against temperatures.

ature, (b) 400 ◦C, (c) 600 ◦C, (d) 750 ◦C, and (e) 800 ◦C.

ifferent temperatures and the cavities were studied from theenter of the gauge. Fig. 4 shows the SEM micrograph of theamples strain to 15% at 400 ◦C. The picture shows that cav-ties are formed along the grain boundary or the triple points.

any similar micrographs were examined from different defor-ation conditions, and in the overwhelming number of cases at

he lower strain levels, cavities were found to at grain bound-ries. The variation of cavitation ratio as a function of strainevel at different temperatures is shown in Fig. 5. From Fig. 5

Fig. 4. SEM picture of the deformed sample strained to 15% at 400 ◦C.

204 M.J. Tan et al. / Journal of Materials Processing Technology 191 (2007) 202–205

unction strain level at different temperatures.

ttassasiFcea

toipapir

amst[

Fz

epoaFf

Fig. 5. Variation of cavitation ratio as a f

he value increases dramatically to 80%. Further studies showhat above the critical strain, not only the size of cavities, butlso the population density of cavities increase with increasingtrain. The population increase is due to the emergence of newmall size cavities. This gives an indication that new cavitiesre continuously formed and are also enlarged with increasingtrain. As a result, the cavitation ratio increases with increas-ng strain. This leads to some cavities coalescence as shown inig. 6. In the present study, there was no significant evidence foravity interlinkage along to the stress axis, but there are somevidence for cavity interlinkage perpendicular to the stress axiss shown in Fig. 6.

A major characteristic of cavitation in superplastic alloys ishat cavities usually display an aligned configuration; a groupf cavities aligned in a specific direction is referred to as a cav-ty stringer. In most cases, cavities stringer align themselvesarallel to the tensile axis and in a few occasions they showlignment perpendicular to the tensile axis. However, in theresent studies, no such phenomenon can be found. These cav-ties are randomly distributed and cavity size varies over a wideange.

There are two opposing viewpoints concerning cavity nucle-tion. Some researchers [11] suggested cavities may pre-exist in

any superplastic alloys. On the one hand, several recent analy-

es have demonstrated that, at least in some superplastic alloys,here is no evidence to support the concept of pre-existing defect12].

ig. 6. Optical microstructure of the deformed sample (tensile axes was hori-ontal).

aptbtsfrhdwdwsdb

4

ao

Fig. 7. SEM picture of the as-received CP titanium alloy.

For CP titanium alloys, do the cavities nucleate on pre-xisting defects, or do they truly nucleate independently? For theresent study, the preferred viewpoint is that pre-existing defectsr weakly bonded interfaces exist in conventionally processedlloys and this is verified by the SEM micrograph as shown inig. 7, which clearly indicates that pre-existing defects can beound at the triple point of the grain boundary. These defectsre generated during hot and severe cold rolling. While somere-existing cavities are present, newly emerging cavities begino appear after a critical strain level. Above this strain, the num-er of cavities increases steadily with strain. On the other hand,he number of emerging cavities depends on the size of the con-trained zone of plastic deformation near the defect and is aunction of defect size. Generally larger defects induce moreapid decohesion with increasing strain, while smaller defectsave a smaller constrained zone and require more strain to pro-uce complete debonding [13]. So the cavity number increasesith increasing strain. The growth and coalescence of theseefects will lead to the fracture of the alloy. On the other hand,hen the CP titanium alloy is exposed to higher temperatures,

uch as 750 and 800 ◦C, the pre-existing defects can be sintereduring the heating up process. This explains why no cavities cane found for sample deformed at these temperatures.

. Conclusions

Cavitation caused by superplastic straining of CP titaniumlloy under uniaxial tension was systematically studied. Thebservations made in this work are listed as follows:

roces

(

(

(

(

R [

[

[

M.J. Tan et al. / Journal of Materials P

1) Due to cavitation, CP titanium alloy does not show goodsuperplasticity under at low temperatures.

2) Cavitations can be found during high temperature defor-mation of CP titanium alloy and this phenomenon istemperature dependent. When the test temperature is above750 ◦C, no cavities can be found in the deformed samples.

3) Pre-existing defects are the main cause for cavitation for-mation. These defects can be sintered when deformed attemperature above 750 ◦C.

4) Cavities are formed at the grain boundary and triple points.During deformation, both the size and population density ofcavities increase with increasing strain. The maximum cav-itation ratios are 5.2%, 4.3%, 0.55% for samples deformedat room temperature, 400 and 600 ◦C, respectively.

eferences

[1] F.J. Campideli, H.E.P. Sobrinho, L. Correr, D. Goes, M. Fernando,Stress-relieving and porcelain firing cycle influence on marginal fit of com-mercially pure titanium and titanium–aluminium–vanadium copings, Dent.Mater. 19 (2003) 686–691.

[2] C.R.F. Azevedo, A.P.D. Santos, Environmental effects during fatigue test-ing: fractographic observation of commercially pure titanium plate forcranio-facial fixation, Eng. Fail. Anal. 10 (2003) 431–442.

[3] C.Y. Gao, P. Lours, G. Bernhart, Thermomechanical stress analysis ofsuperplastic formingtools, J. Mater. Prod. Technol. 169 (2005) 281–291.

[

sing Technology 191 (2007) 202–205 205

[4] H.L. Xing, K.F. Zhang, Z.R. Wang, A preform design method for sheetsuperplastic bulging with finite element modeling, J. Mater. Prod. Technol.151 (2004) 284–288.

[5] H.L. Xing, C.W. Wang, K.F. Zhang, Z.R. Wang, Recent development in themechanics of superplasticity and its applications, J. Mater. Prod. Technol.151 (2004) 196–202.

[6] X.J. Zhu, M.J. Tan, W. Zhou, Enhanced superplasticity in commerciallypure titanium alloy, Scripta Mater. (2005) 651–655.

[7] K. Kalaichelvan, R. Sivaramakrishnan, D. Dinakaran, A. Joseph Stanley,Cavity minimization and uniformity studies on superplastic forming of thineutectic Pb–Sn sheet by optimum loading and performing, J. Mater. Prod.Technol. 162–163 (2005) 519–523.

[8] Y. Huang, T.G. Langdon, Cavitation and failure in a fine-grained Inconel718 alloy having potential superplastic properties, Mater. Sci. Eng. A410–411 (2005) 130–133.

[9] D.L. Yin, K.F. Zhang, G.F. Wang, W.B. Han, Superplasticity and cavita-tion in AZ31 Mg alloy at elevated temperatures, Mater. Lett. 59 (2005)1714–1718.

10] Z.Y. Ma, R.S. Mishra, Cavitation in superplastic 7075Al alloysprepared via friction stir processing, Acta Mater. 51 (2003)3551.

11] M.J. Stowell, in: N.E. Paton, C.H. Hanilton (Eds.), Superplastic Formingof Structural Alloys, Metal Soc., Warrendale, PA, 1982.

12] A.H. Chokshi, T.G. Langdon, The influence of rolling direction on the

mechanical behavior and formation of cavity stringers in the superplasticZn–22% Al alloy, Acta Metal. 37 (1989) 715–723.

13] D.H. Bae, A.K. Ghosh, Cavity growth during superplastic flow inan Al–Mg alloy. I. Experimental study, Acta Mater. 50 (2002) 993–1009.