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Journal of Materials Processing Technology 192–193 (2007) 434–438

High temperature deformation in Ti–5Al–2.5Sn alloy

M.J. Tan ∗, G.W. Chen, S. ThiruvarudchelvanSchool of Mechanical and Aerospace Engineering, Nanyang Technological University,

Singapore 639798, Singapore

bstract

Ti–5Al–2.5Sn is one of the oldest titanium alloys and has been used mainly in low-temperature applications such as hydrogen tanks and pressureessels. The study here was conducted to investigate the superplasticity, dynamic recrystallization and cavitation of this alpha-titanium alloy duringigh temperature deformation; uniaxial tensile tests were carried out at various temperatures between 600 and 900 ◦C with different initial strain

ates. Microstructure evolution during high temperature tensile testing was studied by using electron back-scattered diffraction. Recrystallizationan be observed during high temperature tensile test and this process not only decreases the average grain size but also increases the averageisorientation angle, and this aids the superplastic process. 2007 Elsevier B.V. All rights reserved.

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eywords: Alpha titanium alloy; Superplasticity; Dynamic recrystallization; E

. Introduction

For operations that require high strength levels,i–5Al–2.5Sn alloy is a good candidate, and being onef the oldest titanium alloys [1], it has gained acceptanceecades ago as a material for low-temperature applications,uch as hydrogen tanks and pressure vessels. Like other alphaitanium alloys, Ti–5Al–2.5Sn alloy cannot be age hardened,nd hence it is highly weldable. Superplastic forming (SPF) haseen developed to simplify complicated assembly structures,llowing the manufacturing of complex shaped structural com-onent [2–4]. It also offers dramatic cost and weight savingsver other conventional manufacturing processes [5]. Althoughany studies had been carried out on various titanium alloys,

here are limited studies done on the superplastic forming ofi–5Al–2.5Sn alloy. Therefore, a better understanding of theuperplasticity of this alloy is important for the successfulntroduction of this material for wider industrial applications.n the present study, the SPF and deformation behavior as wells related mechanisms of this titanium alloy are investigated.

. Experimental details

Ti–5Al–2.5Sn titanium sheets of 1.5 mm thickness were used in this study.he chemical composition (wt.%) of the alloy was 5.2Al, 2.4Sn, 0.38Fe, 0.01N,

∗ Corresponding author. Tel.: +65 67905582; fax: +65 67911859.E-mail address: mmjtan@ntu.edu.sg (M.J. Tan).

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924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.04.027

n backscattering diffraction (EBSD)

.16O2, 0.002H2, 0.01C, <0.005 and the balance was Ti. The as-received alloyas an average grain size of 7.2 �m determined by linear intercept method usingmage Pro+ software. Tensile specimens (dogbone type) with a gauge of 11 mmength, 4 mm width and 1.5 mm thickness were electro-discharged machinedith the tensile axis oriented parallel to the final rolling direction. The surfacesf the tensile sample gauge part were polished with silica paste for opticalicroscopy. Uniaxial high temperature tensile tests were performed at 600,

00, 800 and 900 ◦C, respectively, with different initial strain rates. All tests wereonducted by first heating up each sample to the desired temperature, and this wasollowed by a 3 min holding time to ensure thermal equilibrium. After testing, theeformed specimens were cooled rapidly to room temperature by forced coolingn order to preserve the microstructure. Specimens were sectioned from the gauges well as grip regions. The samples were polished by silica paste and etchedsing 10% HF + 5% HNO3 + 85% H2O (vol.%) for 5 s. For optical microscopy,he linear intercept procedure was employed for measuring the grain size. In theresent work, fine grains are defined as grains having a diameter of 10–15 �m.mages for measurement were first captured by optical microscope. Images werehen analyzed using Image Pro+ software. The electron back scattered diffractionEBSD) measurements were carried out using a scanning electron microscopeSEM: JEOL 360) equipped with a TSL EBSD system. The SEM was operatedt an accelerating voltage of 20 kV and EBSD measurements were performedith a step size of 1.0 �m.

. Results and discussion

The variation of percent elongation-to-failure values as aunction of strain rate at different temperatures is shown in Fig. 1.

t can be observed from this figure that the optimum condition foreformation of this alloy is at 800 ◦C with an initial strain rate of.0001/s. A maximum elongation of 245% can be obtained underhis condition. It was also observed from this figure that over

M.J. Tan et al. / Journal of Materials Processing Technology 192–193 (2007) 434–438 435

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ig. 1. Effects of varying temperatures against elongation-to-failure percent-ges.

he temperature range of 600–800 ◦C, the percent elongation-to-ailure generally increases with slower strain rates. Conversely,he elongation-to-failure decreases with slower strain rates for00 ◦C.

An additional test was carried out at 800 ◦C with a strainate of 0.00006/s, and the percent elongation-to-failure decreasesramatically at this point.

Fig. 2 shows the graph of log (flow stress) against log (strainate) at different temperatures. The strain rate sensitivity value,, can be determined from the slope of the curves. The m values

pproached 0.33 and 0.37, at 800 ◦C with an initial strain ratef 0.001–0.0001/s and at 900 ◦C with an initial strain rate of.01–0.001/s, respectively. The m values for the other tests wereelow 0.3. It was noted that the m values for superplastic alloysre generally above 0.33. If the m values fall below 0.5, it indi-ates that dislocation creep is dominant, whereas values below.3 signify the presence of diffusion creep. In order to ensure thatrain boundary sliding is the predominant deformation mecha-ism, it is desirable for the m values to be above 0.5. Therefore,he m values in Fig. 2 agree with the results of the tensile testsnd it can be deduced from this figure that grain boundary slid-ng is not the predominant mechanism taking place during theests.

The presence of fine grain sizes aids grain boundary slid-ng. Hence, it is crucial that the growth of the grain size isontrolled when the experiments are being carried out at high

emperatures. Both Figs. 3 and 4 show that the sample from00 to 800 ◦C (for 0.0001/s), and also from an initial strainate of 0.001–0.0001/s (for 800 ◦C) has the largest amount

ig. 2. Graph of log (flow stress) against log (strain rate) at different tempera-ures.

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ig. 3. Variation of average grain size with temperatures at 0.0001/s strain rate.

f grain growth at the gauge region. On the other hand, therain growth at the other temperatures and strain rates are moreradual.

The microstructures of the deformed samples were exam-ned by optical microscope. Typical microstructures of the gripnd the gauge parts of samples deformed with an initial strainate of 0.0001/s across the varying temperatures are shown inig. 5. It revealed that the formation of cavities declines with

ncreasing temperatures. This could be attributed to sinteringffects taking place during the process of the tensile tests. Withhese effects, existing defects in the material, such as voidsr cavities, will sinter due to the high working temperatures.dditionally, with higher temperatures, the grains become more

quiaxed, in particular the gauge region, as we go from 700, 800o 900 ◦C, via dynamic recrystallization. This indicates that theriving force for dynamic recrystallization at higher tempera-ures, but with restrained grain growth. The ability of the grainso remain equiaxed also aids the occurrence of grain boundaryliding. The microstructure of the gauge part at 700 ◦C with.0001/s strain rate also show the onset of dynamic recrys-allization, gradual increased grain size thereafter to higheremperatures.

At 900 ◦C, a new phase was generated. This was later deter-ined to be the beta phase at the grain boundaries as the heating

emperature was beyond the alpha to beta transus temperature,hich is at 883 ◦C.The microstructures of the grip and the gauge parts of samples

eformed at 800 ◦C at varying strain rates are shown in Fig. 6.hey show that the formation of cavities declines with slower

train rates. This is because at slower strain rates, the time takenor the sample to fail at the same elongation increases. Withonger heating duration, the process has a sintering effect, thusemoving any existing voids or cavities in the material, and this

Fig. 4. Variation of average grain size with strain rates at 800 ◦C.

436 M.J. Tan et al. / Journal of Materials Processing Technology 192–193 (2007) 434–438

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Fig. 5. Microstructures of samples (strain rate of 0.0001/s) failing at (a) 600

lso leads to the occurrence of grain growth. Consequently, itas noted that dynamic recrystallization increased with slower

train rates and the grains also become less elongated in theensile direction at 800 ◦C.

To further investigate the evolution of the microstructuralgrain boundary) characteristics during the tensile tests, EBSDas employed. Fig. 7 shows that there is a shift in grain bound-

ry angles toward the higher angles from the grip region tohe gauge region for the 800 ◦C, 0.0001/s strain rate samples,ndicating recrystallization. As noted by Winning [6], a largeraction of low angle grain boundaries signifies the presence

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b) 700 ◦C; (c) 800 ◦C; (d) 900 ◦C. (Left): grip region; (right) gauge region.

f deformed microstructure, while recrystallized microstruc-ure is indicated by the large fraction of high angle grainoundaries. Henceforth, the increase in grain boundary mis-rientation and the conversion of low angle boundaries intoigh angle boundaries that occurred between the grip and theauge samples, can be attributed to dynamic recrystallizationnd its consequent grain boundary sliding during the tensile tests.

hus, by thermomechanical means, the grian boundary charac-

er and misorientation can be engineered to modify improvehe properties of the materials [7–9], and in this case to obtainuperplasticity.

M.J. Tan et al. / Journal of Materials Processing Technology 192–193 (2007) 434–438 437

Fig. 6. Microstructures of the samples at 800 ◦C: (a) 0.01/s; (b) 0.001/s; (c) 0.0001/s; (d) 0.00006/s.

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Fig. 7. Histograms showing the misorientation angle distributions of the (a)

. Conclusions

For this study, the high temperature deformation of thelpha titanium Ti–5Al–2.5Sn alloy was examined. The alloypecimens were subjected to tensile tests under various work-ng temperatures and strain rates, to examine the potential foruperplasticity in the alloy. The microstructural changes of theaterial were also studied at various stages of the project and

he following conclusions can be drawn:

. The optimum deformation condition for this alloy is 800 ◦C

with an initial strain rate of 0.0001/s. the maximumelongation-to-failure at this condition is 245%.

. The strain rate sensitivity exponent is 0.33 at theoptimum condition and grain boundary sliding is

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sample at 800 ◦C with 0.0001/s; (b) gauge sample at 800 ◦C with 0.0001/s.

not the predominant deformation mechanism of thealloy.

. Cavitation decreases with higher temperatures and lowerstrain rates. It was found that these conditions can inhibitthe formation of internal cavities.

. Higher strain rates tend to generate more dynamic recrys-tallization; and temperatures plays is important at lowertemperatures up to the recrystallization temperature, afterwhich grain growth is detrimental for higher high temperatureductility.

eferences

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4 Proc

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[ical Loads, Institut fur Metallkunde und Metallphysik, RWTH Aachen,

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(1999) 272.4] H.W. Hayden, S. Floreen, P.D. Goodall, Metall. Trans. 3A (1972) 833.5] X.J. Zhu, M.J. Tan, S. Thiruvarudchelvan, Effects of oxidation on the ductil-

ity of Ti–6Al–4V alloy during superplastic forming, Mater. Forum 29 (2005)233–237.

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6] W. Myrjam, S. Carmen, Influencing Recystallization Behavior by Mechan-

Germany, 2006.7] Palumbo G. US Patent No. 5702543, 1997.8] T. Watanabe, Res. Mech. 11 (1984) 47.9] A.J. Schwartz, W.E. King, J. Met. 50 (1998) 50.