oxidation effect on the mechanical behavior of thin

21ème

Congrès Français de Mécanique Bordeaux, 26 au 30 août 2013

1

Oxidation effect on the mechanical behavior of thin samples of

titanium alloy at 600°C

J. BAILLIEUXa, D. POQUILLON

a, J. HUEZ

a

a. CIRIMAT – CNRS/UPS/INPT, 4 allée Emile Monso - BP 44362 - 31030 TOULOUSE cedex 04, France

Résumé :

Les alliages de titane offrent une réelle opportunité d’évolution technologique pour l’industrie aéronautique

et spatiale. Leurs propriétés mécaniques élevées, d’un niveau comparable aux aciers, couplées à une plus

faible densité conditionnent le choix du titane pour alléger les structures. Son excellente tenue à la corrosion

permet aussi une large utilisation dans l’industrie chimique notamment. La littérature montre différentes

études sur le comportement mécanique des alliages de titane en fonction de leurs états de surface, leurs

traitements thermomécaniques et l’environnement d’usage. La forte réactivité du titane avec l’oxygène lors

de sollicitation à haute température conduit non seulement à la formation d’une couche d’oxyde mais aussi à

un enrichissement en oxygène du métal sous-jacent. Les effets de cette modification de composition chimique

sont importants sur la microstructure (-case). Dans cette étude, en partant d’éprouvettes d’épaisseur

différente en titane commercialement pur, nous nous focaliserons sur le comportement mécanique en

traction et en fluage entre 450 à 600°C. L’originalité de l’étude est de réaliser des essais mécaniques à

haute température sur des éprouvettes suffisamment minces pour ne plus être considérées comme homogènes

(substrat + -case + oxyde de titane). Ces essais sont corrélés aux analyses microstructurales dans le but

d’interpréter l’influence de l’enrichissement en oxygène sur le comportement mécanique de l’alliage.

Abstract:

Titanium alloys offer a real opportunity for technological improvement for the aerospace industry. Their

high mechanical properties, a level comparable to steel, coupled with a lower density condition the choice of

titanium to lighten structures. Excellent corrosion resistance also allows a wide use in chemical plants. The

literature shows several studies about the mechanical behavior of titanium alloys according to their surface,

their thermo-mechanical treatments and their environment of use. The high reactivity of titanium with

oxygen at high temperature not only leads to the formation of an oxide layer but also oxygen enrichment of

the underlying metal. The effects of this change in chemical composition are important for the

microstructure (-case). In this study, starting from samples of different thickness of commercially pure

titanium, we focus on the mechanical behavior in tension and creep between 450 to 600 ° C. The originality

of this study is to perform mechanical tests at high temperature on specimens thin enough to not be

considered as homogeneous (substrate + -case + titanium oxide). These tests are correlated with

microstructural analyzes carried out in order to interpret the influence of oxygen enrichment on the

mechanical behavior of the alloy

.Keywords: CP titanium, Oxidation, Mechanical behavior

1 Introduction

With the new generations of airliners Boeing 787 and Airbus A350 XWB, it is expected that aeronautical

applications will soon represent half of the annual consumption of titanium [1]. Increasing the engine

efficiency implies to expose the various components made in Ti-base alloys at higher temperatures. In this

case and in oxidizing atmosphere, Ti-base alloys form an oxide scale and dissolve a substantial amount of

oxygen behind the metal. The oxygen-rich solid solution formed is called “-case”. This solid solution is

harder and more brittle than the initial alloy. It can be observed by optical microscopy [2]. The depth of this

affected zone increases with increasing temperature and exposure duration. When this hardened layer is

formed during thermal treatments, it is often removed mechanically or chemically. Some investigations have

shown that the presence of -case can modify the mechanical properties of titanium alloys [3, 4]. For

21ème


2

example, tensile tests on the IMI834 have shown that compared with unexposed material, lower elongations

to failure were recorded for specimens exposed for 500 hours at 873K. Furthermore, low ductility of the

exposed material was associated with propagation of cracks initiated at the surface (in the α-case). Tensile

tests on Ti60 have also pointed out that micro-cracks in the α-case are crucial locations of failure initiation

[5]. The weak behavior of α-case reduces the load-bearing cross section of the specimens because of the

stress concentration in the crack tip. This effect leads to the early fracture. Therefore, the presence of oxygen

enriched layer induces a decrease in strength and ductility.

The aim of this study is to quantify the effect of the -case on the mechanical properties of Ti50A CP alloy

at high temperature. Thus, we performed oxidation tests at 600°C and 700°C in order to study the oxidation

kinetics and to observe the evolution of the affected zone formed during the heat treatment. Then, knowing

the results, tensile and creep tests have been carried out on unexposed and exposed specimen at 600°C in

order to understand how the microstructure evolution impacts the mechanical behavior. By changing the

specimen thicknesses, we emphasize the role of the affected zone. The next step will be to describe the

mechanical elasto-plastic and creep laws as a function of the oxygen content. The purpose is then to describe

Young’s modulus, yield stress, elongation at break and Norton’s law coefficients as its function.

2 Material and methods

For our investigation, we chose the Ti50A which is a commercially pure titanium alloy (ASTM grade 2). Its

chemical composition is presented in Table1. Ti50A provided by TIMET was received in a sheet form (2

mm×500 mm×1000 mm) after being mill annealed and cold rolled.

Table 1: Chemical composition of Ti50A alloy, commercially pure titanium - ASTM grade 2 - (wt%).

C H Fe N O Ti

<= 0,080 % <= 0,015 % <= 0,30 % <= 0,030 % <= 0,025 % >= 98,9 %

2.1 Oxidation experimental procedure

For the oxidation tests, we have cut 10 samples in the titanium sheet with the abrasive cutter Buehler®.

These specimens (27 mm×14 mm×2 mm) have been mechanically polished using a 600 grit SiC paper, then

cleaned in ethanol and acetone, dried and weighed three times with the ultra-microbalance Sartorius®. The

oxidations have been performed at 600°C and 700°C in a Carbolite® furnace. Five samples have been

oxidized for each temperature. The oxidation conditions are described in Table 2:

Table 2 : Oxidation conditions (temperature and oxidation duration).

Ti50A oxidation / 600°C / Laboratory Air

#REF TiOx#01 TiOx#02 TiOx#03 TiOx#04 TiOx#05

Oxidation duration 5 h 27 h 100 h 190 h 355 h

Ti50A oxidation / 700°C / Laboratory Air

#REF TiOx#06 TiOx#07 TiOx#08 TiOx#09 TiOx#10

Oxidation duration 1 h 3 h 21 h 45 h 103 h

To determine the oxidation kinetics at 600°C and 700°C we have used the parabolic law commonly used for

metals and alloys since the classic works of Tammann [6] and Pilling and Bedworth [7] :

Where Δm (mg.cm-2

) is the mass gain per unit area, t (s) the oxidation duration and kp (mg2.cm

-4.s

-1) is the

parabolic rate constant. After the oxidation, each sample has been carefully weighted and then embedded in

an epoxy resin. For cross section observations, samples have been polished with the automatic polisher

Struers® using 600 down to1200 grit SiC finishing with 3µm down to 1µm diamond paste and a mixture of

colloidal silica (OP-S) with hydrogen peroxide (30%). The observations and measurements have been

performed with the optical microscope Nikon®. The hardness profile used to identify the hardened layer has

been carried out with a micro hardness Buelher® using a 10 g mass.

(1)

21ème


3

2.2 Mechanical experimental procedure

Eight tests have been carried out at 600°C (Table 3). Two specimen geometries have been used for the

mechanical test. The first one is the standard flat geometry machined by electrical discharge along the rolling

direction in the foil. Initially the specimen’s thicknesses were 2 mm but we have thinned the gauge length of

three specimens (Ti#cree04, 05 and 06) to 500 µm by machining (Dremel®) then polishing with 600 grit SiC

paper. The thermal treatments of the samples have been chosen using the results of the oxidation tests

detailed in 2.1. The oxidation conditions are reported in Table 3. Reference sample (Samples without α-case

have been polished using a 600 grit SiC paper, then cleaned in ethanol and acetone and dried before testing.

These specimens with and without α-case have been used for tensile and creep tests on the MTS

electromechanical test machine with a 5KN load cell. These tests have been performed at 600°C in a

radiation furnace equipped with 6 halogen lamps. The mechanical conditions are reported in Table 3:

Table 3 : Tensile and creep tests performed. The thickness of the gauge length is indicated, as well as strain rate for tensile

tests and stress level for creep tests. The affected zone proportion (AZP) is estimated using results of 2.1.

Tensile Tests

(600°C)

#REF Heat

Treatment

Thickness Strain rate Affected zone

proportion (AZP)

Ti#tens01 none 2 mm 10-2

s-1

0 %

Ti#tens02 52 h/700 °C 200 µm 10-2

s-1

55 %

Creep Tests

(600°C)

Stress

Ti#cree01 none 2 mm 40 MPa 0 %



Ti#cree04 52 h/700 °C 500 µm 40 MPa 25 %

Ti#cree05 52 h/700 °C 500 µm 30 MPa 25 %

Ti#cree06 52 h/700 °C 500 µm 25 MPa 25 %

The second geometry used for the others specimen is a beam (2 mm×25 mm×0.2 mm). Samples have been

cut in the foil along the rolling direction with the cut-off machine Struets®. Then they have been thinned

down to 200µm with the mechanical polishing system Logitech® [8]. The α-case growth has been performed

with the same furnace described in the oxidation experimental procedure. This geometry has been used

especially for the micro tensile test machine developed by D. Texier [8]. In this device, the specimens are

maintained by self-gripping jaws. After performing the polishing described for the optical microscope

observations, microstructures have been analyzed with a LEO scanning electron microscope. For the creep

tests analysis, the coefficient of sensitivity to stress n has been determined with the Norton law used in creep

test by plotting the logarithm of the strain rate as a function of the logarithm of the stress [9].

3 Results

3.1 Results of oxidation

After the oxidation tests at 600°C and 700°C, the mass gain has been measured for each sample. These

results are given in Table 4. For both temperatures, the curves Mass gain = f (time) appears as a parabolic

curve. As a consequence, it has been possible to determine a parabolic constant kp for both temperature

thanks to equation (1). For the Ti50A, we have calculated: kp = 6.4×10-7

mg2.cm

-4.s

-1 at 600°C and kp =

1.5×10-5

mg2.cm

-4.s

-1 at 700°C.

The oxide layer has been analyzed with the X-Ray diffractometer Seifert 3000. Rutile seemed to be the only

titanium dioxide formed during the oxidation. The figure 1 shows a cross section of the sample TiOx#03.

The micrograph on the right, which comes from the TiOx #03 section, presents three different areas. The

first one is the Ti50A substrate (a). Then, we can observe an area with an optical contrast when compared to

the substrate (b). This area, according to literature is the zone with the highest concentration of oxygen and

corresponds to the α-case area. The last area observed is the oxide scale (c) which growths on the sample

surface. However, the microhardness profiles performed along the cross section showed an interesting result:

For all the tested samples, the hardened layer is larger than the area (b) corresponding to the α-case observed.

Thus, we can highlight a fourth zone deeper than the area presenting a different optical contrast. So the zone

21ème


4

affected by oxygen inward diffusion - the affected zone - is larger than the zone labeled (b) and revealed by

optical micrograph.

Ti50A / Oxidation in air / 600°C

#REF Time (h) Oxide (µm)

thickness

α-case observed (µm)

thickness

Mass gain Affected zone (µm)

thickness

TiOx#03 100 1±0.5 9±2 0.44 mg.cm-2

22±2

TiOx#04 190 2±0.5 14±2 0.62 mg.cm-2

26±2

TiOx#05 365 4±0.5 19±2 0.92 mg.cm-2

36±4

Ti50A / Oxidation in air / 700°C

#REF Time (h) Oxide (µm)

thickness

α-case observed (µm)

thickness

Mass gain Affected zone (µm)

thickness

TiOx#07 3 1±0.5 8±2 0.37 mg.cm-2

18±2

TiOx#08 21 4±0.5 15±2 0.97 mg.cm-2

35±2

TiOx#09 45 6±0.5 38±3 1.48 mg.cm-2

50±4

TiOx#10 103 10±1 55±3 2.41 mg.cm-2

80±5

We have measured the thicknesses of the oxide scale, of the α-case and of the affected zone for each sample

except for TiOx#01, 02 and 06 (not observable). Values are reported in Table 4. For all our tests and

interpretations, we have considered this affected zone like the layer to be taken into account. These results

allowed us to choose the preoxidizing treatment conditions for mechanical tests in order to have a significant

AZP between substrate and affected zone to observe the resulting mechanical effects.

Figure 1: [1] Schematic diagram of a representative cross section of heat treated samples with the affected zone revealed by

microhardness test and the -case (b). [2] Optical micrograph of TiOx#03 sample with the corresponding microhardness

profile and the case (b) observed by optical contrast.

3.2 Results of mechanical tests

Tensile tests results performed at 600°C are reported in Figure 2. For the reference specimen Ti#tens01

without affected zone, the tensile curve observed was the one expected at this temperature for Ti50. Data

obtained for all the other tests showed mechanical properties modifications due to the preoxidizing

treatments performed. The curve for sample for which the AZP represented 55% showed an increase of the

yield stress and an aspect similar to the curve obtained for alloy hardened by cold working.

Results of the creep tests performed at 600°C with exposed and unexposed specimens at high temperatures

are presented in Figure 3 and 4. In both cases, it has been possible to carry out a Norton analysis (Figure 5)

to determine the coefficient of sensitivity to stress n. For the specimens with 25 % of AZP (Ti#cree04, 05

and 06), we got the value of n = 5. For the specimens without affected zone, a value n= 4 gave a better fit of

the experimental data.

Table 4: Ti50A oxidation results with the thickness of the different layers when determined.

ND

TD

21ème


5

Creeps results showed a large decrease in creep life and ductility for the heat treated samples. At each creep

stress, the time to failure decreased systematically with the specimens preoxidized. The larger the affected

zone the more strain to failure was reduced.

4 Discussion

The parabolic rate constants kp are in good agreement with the values found in the literature [11-13]

Oxidation tests allowed us to predict the α-case formation. By plotting the evolution of the affected zone in

function of the oxidation duration, we observe a parabolic curve. The thickness of this layer is diffusion-

controlled and was roughly constant all around a given sample. The depth of the affected zone increases with

increasing the oxidation duration or the temperature. In [14] a direct correlation has been evidenced between

hardness measurement and oxygen content. Using these results, we have chosen to take into account this

affected layer, which is larger than the area revealed by optical micrograph, as affected zone for the

mechanical tests. However oxygen analyses profiles are required to determine the real amount of oxygen in

each area.

Because of the reduction of the load-bearing section with the affected zone, it was expected a yield stress

decrease for heat treated specimens. Though, we have evidenced an increase of this yield stress for the

specimen with a 200 m thickness and more than 50% of AZP. This hardening effect was also observed for

creep tests as, for a given stress, the creep strain was lower for samples with a significant affected area.

Creep tests confirmed a lower elongation to failure and a diminution of creep life for specimens with α-case

0,00E+00

2,00E+04

4,00E+04

1 2 3

Without affected zone

section with 25% ofaffected zone

Figure 2: Tensile tests performed at 600°C, Ti#tens01 (no

heat treatment, thickness 2 mm), Ti#tens02 (55% AZP,

thickness 200 µm,).

Figure 3: Creep tests results on specimens (TI#cree04, 05 and

06) preoxidised at 700°C during 52 h (25% of affected zone

proportion estimated) and specimens without α-case (Ti#cree01,

02 and 03).

0

40

80

0,00% 2,00% 4,00%

Stress (Mpa)

Strain (%)

Ti#tens01

Ti#tens02

Figure 5: Norton fit of minimum creep rate for creep

tests carried out at 600°C.

0%

30%

60%

0,E+00 2,E+04 3,E+04

Strain (%)

Duration (s)

Ti#cree01

Ti#cree02

Ti#cree03

Ti#cree04

Ti#cree05

Ti#cree06

Duration (s)

40 MPa 25 MPa

Stress

Figure 4: Failure time for creep tests carried out at 600°C.

-5,1

-4,6

-4,1

1,39 1,59

LOG(έ)

LOG(σ)

Withoutaffected zone

section with25% ofaffected zone

25 MPa 30 MPa

40 MPa

58% 35%

68%

27%

49%

27% Failure strain

30 MPa

21ème


6

up to four times faster for the 30 MPa creep stress. The modified Monkman-Grant relationship [15] (MMGR)

relates the failure time , to the minimum creep rate ̇ and the failure strain :

C’ is a temperature independent constant and m’ is close to unity. By plotting the logarithm of in

function of the logarithm of ̇ , we determined m’ and C’ for creep tests. For specimens without affected

zone, we got m’= 0.97(m’ close to 1) which reflects that the mean creep rate is linearly related to the

minimum creep rate, and C’= 0.63. For samples with 25% of AZP, we got m’= 1.13 (m>1) which mean there

is a deviation between the mean creep rate and the minimum creep rate, and C’= 0.08. These values which

need to be confirmed with other creep tests emphasize a creep mechanism modification. The Norton

coefficients need to be confirmed with other tests but were similar. The dislocation creep is predominant but

as the heat treated samples are not homogenous, a composite approach taking into account the brittle nature

of the α-case (were cracks initiated) could only explain their mechanical behavior.

5 Conclusion

This study confirms that oxygen diffusion in the Ti50 affects its mechanical behavior and has to be taken

into account for thin components. These first results show the interest of further mechanical tests on thin

specimens that allow having a significant AZP. Samples with different thicknesses and thin samples

extracted from different areas will allow us to investigate the mechanical behavior of zone with different

oxygen content. Investigations at different temperatures will also be carried out to accumulate tests results in

the aim of proposing a mechanical model with multilayer approach.

6 Acknowledgement

The authors gratefully acknowledge Damien Texier (CIRIMAT) for his technical assistance about the

thinning process and the micro tensile tests method.

References

[1] The French Titanium association, Marché et applications du titane: http://www.titane.asso.fr/ (22/03/2013)

[2] K.S. McReynolds and S. Tamirisakandala, A study on alpha-Case Depth in Ti-6Al-2Sn-4Zr-2Mo

[3] M.A. Daeubler and D. Helm, Influence of long time exposure at elevated temperatures in air on

mechanical properties of the high temperature titanium alloy, IMI834, Proc. Int. Conf. on Titanium Products

and Applications, Orlando, Florida, 1990, pp.244-255.

[4] R.W. Evans, R.J. Hull and B. Wilshire, The effects of alpha-case formation on the creep fracture

properties of the high-temperature titanium alloy IMI834, Journal of Materials Processing Technology 56,

University College, Swansea, U.K. (1996) 492-501

[5] W. Jia, W. Zeng, J. Liub, Y. Zhou, Q. Wang, Influence of thermal exposure on the tensile properties and

microstructures of Ti60 titanium alloy, Materials Science and Engineering A 530, Northwestern

Polytechnical University, China, (2011) 511–518

[6] T. G. Tammann, Z. Anorg. Chem. 111, 78 (1920)

[7] N. B. Pilling and R. E. Bedworth, J. Inst. Met. 29, 529 (1923)

[8] D. Texier, Évolution et mesure des gradients de propriétés mécaniques dans le système superalliage à

base de nickel MC2 revêtu MCrAlY, Thèse de doctorat, Institut National Polytechnique de Toulouse, 2013

[9] D. François, A. Pineau and A. Zaoui, Viscoplasticité, endommagement, mécanique de la rupture et

mécanique du contact. Lavoisier, Paris (2009)

[10] Aaron P. Broumas, Nick M. Degnan, Michael L. Meier, Oxygen Diffusion into titanium, Department of

Chemical Engineering and Materials Science University of California, Davis (2002)

[11] P. Kofstad, High Temperature Oxidation of Metals J. Wiley & Sons, New York, (1966)

[12] Y. S. Chen and C. J. Rosa, Oxidation of Metals 14, 147 (1980)

[13] A. M. Chaze and C. Coddet, Oxidation of Metals 27, 1 (1987)

[14] M. Göbel, V. A. C. Haanappel, M. F. Stroosnijder Oxidation of Metals 55, (1/2), 137 (2001)

[15] F. Povolo Comments on the Monkman-Grant and the modified Monkman-Grant relationships, Journal

of Material Science 20 (1985) 2005-2010

(2)

http://www.titane.asso.fr/

oxidation effect on the mechanical behavior of thin

Documents