Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering

Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering

Journal Pre-proof Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering Dongjun Wan...

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Journal Pre-proof Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering Dongjun Wang, Hao Li, Wei Zheng

PII:

S1005-0302(19)30316-0

DOI:

https://doi.org/10.1016/j.jmst.2019.07.037

Reference:

JMST 1721

To appear in: Received Date:

26 May 2019

Revised Date:

26 June 2019

Accepted Date:

14 July 2019

Please cite this article as: Wang D, Li H, Zheng W, Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering, Journal of Materials Science and amp; Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.07.037

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Research Article

Oxidation behaviors of TA15 titanium alloy and TiBw reinforced TA15 matrix composites prepared by spark plasma sintering

Dongjun Wang a, b, *, Hao Li b, Wei Zheng c

a National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

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c Xi’an Space Engine Company Limited, Xi’an 710100, China

* Corresponding author. Tel.: +86 451 86413917; Fax: +86-451-86415716. E-mail address: [email protected] (Dongjun Wang).

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[Received 26 May 2019; Received in revised form 26 June 2019; Accepted 14 July 2019]

The purpose of this study is to investigate the oxidation behaviors of the TA15 titanium alloy and

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TiBw/TA15 composite with network microstructure in the temperature range of 873-1073 K. The

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results show that the oxidation kinetics of the TA15 titanium alloy and TiBw/TA15 composite follows different laws at various oxidation temperatures. Moreover, the effective activation energy Q

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for oxidation of the TA15 titanium alloy and TiBw/TA15 composite is determined to be 299±19.9 kJ/mol and 339±8.31 kJ/mol at the temperature of 973-1073 K, respectively. The experimental

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achievements of oxidation kinetics and oxide scales formed in the test temperatures indicate that the

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TiBw/TA15 composite exhibits a higher oxidation resistance than TA15 titanium alloy. A schematic diagram of oxidation mechanism is established to further reveal the oxidation process for TiBw/TA15 composite at elevated temperatures.

Keywords: Composite materials; Powder metallurgy; Sintering; Kinetics; Oxidation

1. Introduction Titanium and titanium alloys are widely used in aerospace industry [1-3] and bio-medicine [4] because of their excellent characteristics, such as high specific strength, good high temperature performance, excellent corrosion resistance and good biocompatibility. Moreover, aerospace industry is still the prime consumer of titanium and its alloys up to now due to the combination of good mechanical properties and low density. As a kind of near-α type titanium alloy with high aluminum

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equivalent, TA15 titanium alloy consisting of the hexagonal close packed (hcp) α phase and the body centered cubic (bcc) β phase possesses the considerable thermal stability, moderate room and high temperature strength and good welding performance, which is widely used to manufacture the key

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load-bearing structural components in aircrafts and engines [5]. However, in recent years, along with

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the increasing of thrust-weight ratio for modern aero-engine, the service temperature of engine material is further promoted. The application of conventional titanium alloys (e.g. TA15) at higher

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temperature is limited owing to the weak oxidation resistance and strength loss.

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Owing to their considerable properties, isotropy and low cost [6-9], the discontinuous reinforced titanium matrix composites (DRTMCs) have attracted extensive attention, especially produced by

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in-situ method. Traditionally, researchers have always obtained homogeneous distribution of reinforcement within titanium matrix for further improving the performance of DRTMCs in the past

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few years [8, 10]. Nevertheless, numerous DRTMCs with a homogeneous microstructure exhibit a limited increasing in strength with a remarkable drop in ductility. Recently, Geng et al. [11] have successfully fabricated a new type of TiBw/TC4 composite with novel network structure by means of low-energy powder mixing and reactive hot-pressing (RHP). The research results indicate that the DRTMCs with network structure not only possess the advantages of high specific strength, specific

stiffness, and good high temperature stability of traditional DRTMCs, but also have better comprehensive mechanical properties, especially the ductility [11-13]. Compared with the TC4 titanium alloy, the maximum service temperature of TiBw/TC4 composite with network structure can be increased by 100-200 K [11-14]. In addition, according to the research results [15,16], the TiBw/TA15 composite with network-like reinforcements also exhibit excellent high-temperature mechanical properties. It is also of importance to note that the fabrication of TiBw/TA15 composite

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with network structure using spark plasma sintering (SPS) process requires relatively lower temperature and shorter time in comparison with RHP method, i.e. 1100 °C and 10 min for the SPS [16] versus 1300 °C and at least 1 h for RHP [17].

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Besides the high temperature mechanical properties, oxidation is also one of the main failure

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factors of metals and alloys, which has drawn much attention in the field of high temperature structural materials. Up to date, few works have been devoted to the oxidation behavior of in situ

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DRTMCs. Furthermore, different oxidation features have been reported for various composites. For

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example, Zhang et al. [18] produced the (TiB/TiC)/Ti matrix composite with homogeneous distribution of reinforcements using ingot metallurgy and SHS techniques. According to the

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experimental results of Zhang et al. [19], in situ synthesized TiBw uniformly distributed in the matrix could improve the high temperature oxidation resistance of Ti6242 titanium alloys by promoting

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heterogeneous nucleation of oxides. And thus, the formation process of continuous oxide scale covering the oxidation samples was accelerated. Meanwhile, Brice et al. [20] found that in situ synthesized TiBw could improve the high temperature oxidation resistance of Ti-6Al-4V alloy. These results indicate that TiBw uniformly distributed in the matrix can improve the high temperature oxidation resistance of the matrix alloy. On the other hand, Huang et al. [21] found that the oxidation

resistance of TiBw/Ti60 composite with network structure was reduced when compared with the matrix alloy. Conversely, the oxidation resistance of Ti-6Al-4V/(TiC/TiB) composite with network structure was enhanced by releasing the stress existing in oxide scale [22]. Additionally, Zhang et al. [23] also studied the oxidation behavior of Ti-6Al-1.2B alloy and found that in situ TiBw deteriorated the oxidation resistance by forming evaporable B2O3 oxide. Based on above

composite with network-like reinforcement distribution in detail.

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demonstrations, it is essential to investigate the oxidation behavior and mechanism of TiBw/TA15

In this paper, the high temperature oxidation tests of TA15 titanium alloy and TiBw/TA15 composite with network structure prepared by SPS process were carried out in the range of 873-1073

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K, for understanding the effect of TiBw on the oxidation behavior of TA15 titanium alloy.

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Meanwhile, the kinetics of high temperature oxidation was also studied. The oxidation products and morphologies of the oxide scale were further characterized and observed for revealing the oxidation

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mechanisms of TiBw/TA15 composite.

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2. Experimental procedures

The TA15 titanium alloy and a 2.1 wt% TiBw/TA15 composite were fabricated by SPS process in

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this study. The chemical composition (wt%) of the TA15 alloy was 6.62 Al, 2.25 V, 1.9 Zr, 1.7 Mo, 0.04 Fe, 0.02 Si and balance Ti. The TA15 alloy samples were sintered at 1100 °C for 7 min with a

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pressure of 50 MPa in vacuum [16], while the TiBw/TA15 composite samples were sintered at 1100 °C for 10 min with a pressure of 30 MPa in vacuum [16]. Furthermore, the heating rate of 30 °C/min was used. After heating and temperature preservation, the samples were cooled in furnace. The microstructures of raw materials are shown in Fig. 1. As can be seen, the microstructure of TA15 titanium alloy selected for high temperature oxidation tests consists of equiaxed α phase, coarse α

laths and a small amount of intergranular β phase. The TiBw were in-situ synthesized at the local areas around the boundaries of TA15 powders, forming a network structure that can be divided into TiBw-rich boundary region and TiBw-lean matrix region, as shown in Fig. 1(b). The matrix of the composites is composed of α phase and intergranular β phase. Moreover, the coarse α laths intersected with each other form typical basket-weave morphology in the TiBw-lean matrix region. The oxidation samples with dimensions of 5mm×5mm×10mm were cut from the as-sintered

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TA15 titanium alloy and TiBw/TA15 composite, respectively. Then, they were conventionally ground using SiC abrasive papers and were mechanically polished using diamond polishing paste (average particle size: 2.5 μm). Subsequently, the samples were cleaned by ultrasonic in ethanol.

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After cleaning, these samples were placed in a vacuum furnace (at 473 K) to dry for 2 h and weighed

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using an OHAUS microbalance with 0.1 mg sensitivity before oxidation. The oxidation experiment was carried out in a laboratory air furnace within a temperature range of 873-1073 K up to 120 h.

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During the oxidation experiment, the oxidized samples were put into the alumina crucible (purity

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over 99%) with good thermal stability below 1873 K. After oxidation for the given times, the samples were removed from the furnace together with the crucibles and air-cooled to room

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temperature. Then, the samples and the crucibles were reweighed together to avoid damage of the oxide scale caused by sampling. To ensure the accuracy, two oxidation samples were prepared at

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each oxidation temperature for both materials. The phase identification of the oxide scale for TA15 titanium alloy and TiBw/TA15 composite

was carried out by Empyrean intelligent X ray diffraction using CuKα radiation. A scanning electron microscope (SEM, Quanta 200FEG) equipped with energy dispersive spectrometer (EDS) was used for the examination of microstructural characteristics and oxide scale of the samples. In order to

measure the thickness of the oxide scale, the oxidized samples were coated with a layer of Aurum, and then the oxidized samples were embedded in resin before grinding and polishing to prevent the spallation of the oxide scales.

3. Results 3.1. Oxidation behavior of TA15 titanium alloy and TiBw/TA15 composite For comparing the difference of oxidation behavior between the TA15 titanium alloy and

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TiBw/TA15 composite, the relationship between the mass gain per unit initial surface area and oxidation time was recorded at all oxidation temperatures. Fig. 2 shows a plot of the mass gain per unit initial surface area versus the oxidation time for the TA15 titanium alloy and TiBw/TA15

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composite in the temperature range of 873-1073 K. According to previous literatures [23,24], B2O3

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will evaporate slightly as the oxidation temperature reaches 1073 K. Meanwhile, the TiBw content in TiBw/TA15 composite is low in this work (2.1% mass fraction). Thus, the mass loss caused by

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evaporation of B2O3 can be neglected for the TiBw/TA15 composite at/under 1073 K. Below 973 K,

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the values of the mass gain are very small. When the temperatures are higher than 973 K, the weight gain increases rapidly with oxidation time. One can see that the oxidation rate of the TiBw/TA15

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composite with network structure is lower than that of the TA15 titanium alloy at identical temperatures, especially in the temperature range of 973-1073 K. This result suggests that the

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TiBw/TA15 composite with network structure exhibit a higher oxidation resistance compared with the TA15 titanium alloy. In addition, more mass gain is observed for the samples with increasing the oxidation temperature, as shown in Fig. 2. Compared with TC4 titanium alloy prepared by ingot metallurgy [25], the TA15 titanium alloy and TiBw/TA15 composite fabricated by SPS process exhibits a smaller weight gain when the samples were oxidized at the same parameters.

3.2. Phase identification of oxide scale Fig. 3 shows the XRD patterns of the TA15 titanium alloy and TiBw/TA15 composite after oxidation at different temperatures for 120 h. As can been seen from Fig. 3(a), except for a small amount of diffraction peaks of TiO2, no diffraction peaks of other oxidation products are found in the two materials oxidized at 873 K, revealing that weak oxidation takes place. This is consistent with the results reflected by the plot of mass gain per unit initial surface area versus the oxidation time for

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TA15 titanium alloy and TiBw/TA15 composite at the temperature of 873 K. Furthermore, the diffraction intensity of oxide increases with the increasing of oxidation temperature for the two materials. As the oxidation temperature rises to 973 K and above, the diffraction peaks of TiO2 and

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Al2O3 appear in the XRD patterns (Fig. 3(a) and (b)) and it is noted that the oxidation products are

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mainly TiO2 and Al2O3. Due to the relatively lower content of alloy elements (such as Zr, Mo, V) within the titanium alloy, no oxides of these metals are detected. Since the B2O3 is usually

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amorphous [24], the diffraction peaks of B2O3 also do not exist in the XRD patterns for the

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TiBw/TA15 composite. Similar oxidation results were also observed in other TiBw-reinforced TMCs [21,22]. It is also of interest to note that the diffraction peaks of Ti appear even at 1073 K. Although

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dense oxide scale forms at this temperature (see below), the spallation of oxide scale may make local substrate of TA15 titanium alloy exposing, leading to the existence of diffraction peaks of Ti in XRD

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patterns for the samples oxidized at 1073 K.

3.3. Morphology and composition of oxide scale Fig. 4 shows the typical surface morphologies of the oxide scales formed on the TA15 titanium alloy and TiBw/TA15 composite after oxidation at different temperatures for 120 h. It is evident that fine oxide granules are formed on the surface of the TA15 titanium alloy and TiBw/TA15 composite

at 873 K (Fig. 4(a1) and (a2)), while the surface of the TiBw within the composites becomes rough as marked by the red arrows in the Fig. 4(a2). Especially, the α phase and β phase constituting the TA15 titanium alloy and TiBw/TA15 composite matrix can be clearly distinguished. This phenomenon indicates that a very thin oxide scale is formed on the surface of the oxidation samples. The above results also suggest that the oxidation of TA15 titanium alloy and TiBw/TA15 composite is not obvious at 873 K, which agrees with the results of Figs. 2 and 3(a). When the

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oxidation temperature rises to 973 K and above, the oxide granules on the surface grow up obviously and the size of oxidation products increases with increasing oxidation temperature, as shown in Fig. 4(b1)-(d2). The observations show that the oxidation for these two materials becomes more serious

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with the increasing of temperature and the oxide scale forms gradually during oxidation. In addition,

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the oxides possess two typical morphologies: massive particles (marked by the red arrows in Fig. 4(b1)-(d2)) and short needle-like/columnar particles (marked by the yellow arrows in Fig.

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4(b1)-(d2)). According to the previous literatures [26,27] and the XRD patterns, the short

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needle-like/columnar particles are Al2O3 phase and the massive particles are TiO2 phase. One can also see that the size of oxidation products formed in the TiBw/TA15 composite is relatively smaller

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when compared with those of TA15 titanium alloy, as shown in Fig. 4(b1) and (b2), Fig. 4(c1) and (c2), Fig. 4(d1) and (d2), respectively. These results suggest that the oxidation of TA15 titanium

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alloy is more serious than the TiBw/TA15 composite at the same oxidation temperature and indicate that the oxidation resistance of the TiBw/TA15 composite with network structure is higher than that of TA15 titanium alloy. It is of importance to note that the spallation of oxide scale occurs preferentially for the TA15 titanium alloy when the samples oxidized at 1023 K, as shown in the blue- ellipse region in the inset

of Fig. 4(c1). After spallation of the surface oxide scale, repeated oxidation process of the underlying materials can be easily generated. At 1023 K, the oxide scale of TiBw/TA15 composite is complete and continuous. This phenomenon also demonstrates that the TiBw/TA15 composite possess a higher oxidation resistance compared with TA15 titanium alloy. Furthermore, it is found that the network microstructure of reinforcements can still be clearly observed (the low magnification SEM micrograph in Fig. 4(c2)), although the typical features of TiBw disappear (the high magnification

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SEM micrograph in Fig. 4(c2)). As the oxidation temperature rises to 1073 K, the spallation of oxide scale occurs in both materials (blue- ellipse regions in the insets of Fig. 4(d1) and (d2)). Meanwhile, one can see that the oxide scale of TiBw/TA15 composite is more compact (the high magnification

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SEM micrograph in Fig. 4(d2)) in comparison with that of TA15 titanium alloy (the high

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magnification SEM micrograph in Fig. 4(d1)).

In order to determine the thickness of the oxide scale, the cross-section of the samples oxidized at

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973 K and 1023 K is observed, as shown in Fig. 5. When the samples oxidized at 973 K, it can be

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observed that the oxide scales for the TA15 titanium alloy and TiBw/TA15 composite are relatively thin, as shown in Fig. 5(a1) and (b1). These thin oxide scales can effectively decrease the oxidation

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rate by protecting the titanium matrix from oxidizing. Moreover, the thickness of the oxide scales for the TA15 titanium alloy and TiBw/TA15 composite is about 6.46 ± 0.19 μm and 4.03 ± 0.75 μm at

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the same oxidation temperature of 973 K, respectively, which further confirms that the oxidation resistance of the TA15 titanium alloy is inferior to the TiBw/TA15 composite. With the oxidation temperature rising to 1023K, the thicker and layered oxide scales are noted in Fig. 5(c1) and (d1). At this temperature, the layered oxide scales with different compositions (i.e. the oxide scale and the transition layer) can be visually distinguished from each other. Similar oxide scales with layer

morphology were also reported in previous literatures [21,22]. The thickness of the oxide scale increases to 23.97 ± 2.41 μm and 11.01 ± 1.33 μm for the TA15 titanium alloy and TiBw/TA15 composite (Fig. 5(c1) and (d1)), respectively. From the above results, it can be found that the thickness of the oxide scale increases with increasing oxidation temperature, as well as the thickness of the transition layer. For better understanding the oxidation behavior of the TA15 titanium alloy and TiBw/TA15

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composite, the elemental analysis of the samples oxidized at 973 K and 1023 K for 120 h are conducted using EDS and the results are also shown in Fig. 5. According to the cross-sectional image and the EDS line scanning, it can be seen that an Al-rich region exists at the exterior of the oxide

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scales, while an relative Al-lean region is also found at the beneath of the Al-rich region, as shown in

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the Fig. 5. Moreover, in the Al-lean region, the weight percent of Ti is much higher than other metal elements. Combining with the XRD results in Fig. 3, it can be identified that the outer oxide layer is

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composed of the Al2O3 phase and TiO2 phase (the TiO2/Al2O3 layer), while the inner layer is mainly

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composed of the TiO2 phase (the TiO2 layer). Comparing the Fig. 5(a2) and (b2), it is found that the oxide scale for TA15 titanium alloy has a lager width of the Al-rich region and Al-lean region than

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that of TiBw/TA15 composite. Meanwhile, one can also see that the width of the Al-rich region and Al-lean region increases with increasing oxidation temperature, especially for TA15 titanium alloy,

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as shown in Fig. 5(c2) and (d2). Since the higher oxidation temperature will lead to faster diffusion and in turn wider oxide layer, as the oxidation temperature rises, both the aluminum content of the outer oxide scale and the width of the Al-rich region increase for TA15 titanium alloy and TiBw/TA15 composite (Fig. 5). Based on the above results, the growth of TiO2/Al2O3 layer is determined to be controlled mainly by outward diffusion of aluminum and titanium atoms, while the

growth of inner TiO2 layer is dominated by oxygen ingress diffusion for both materials during oxidation.

4. Discussion 4.1. Oxidation kinetics of TA15 titanium alloy and TiBw/TA15 composite In order to further investigate the difference of oxidation behavior of TA15 titanium alloy and TiBw/TA15 composite at high-temperature, the analysis of the oxidation kinetics for these two

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materials was conducted. According to the classical high-temperature oxidation theory, generally, the mass gain in titanium alloys can be described by a rate equation [21]: (∆𝑀)𝑛 = 𝑘p 𝑡

(1)

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where ΔM (mg/cm2) is the mass gain per unit area, kp (mg cm-2 h-1) is the parabolic rate constant, n is

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the rate exponent and t (h) is the oxidation time. Owing to the low oxidation mass gain and poor linear relationship between ln(ΔM) and ln(t) for the TA15 titanium alloy and TiBw/TA15 composite

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at 873 K, the linear relationship between ln(ΔM) and ln(t) for these two kinds of materials was used

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to calculate the rate exponents n and parabolic rate constants kp in the temperature range of 973-1073 K. The rate exponents and calculated parabolic rate constants obtained from the plots of ln(ΔM) vs.

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ln(t) (Fig. 6(a)) are listed in Table 1. Based on Eq. (1), the kinetic model is linear if n = 1 and parabolic if n = 2 under a constant temperature condition. As can been seen from Table 1, the

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oxidation kinetics of the TA15 titanium alloy obeys parabolic law and parabolic-linear laws at 973 K and 1023-1073 K, respectively. On the other hand, the oxidation kinetics of the TiBw/TA15 composite obeys parabolic law and parabolic-linear laws at 973-1023 K and 1073 K, respectively. Based on above experimental date, one can see that the morphologies of the oxide scale exhibiting obviously differences at various oxidation conditions are related to the oxidation kinetics. When the

oxidation kinetics follows the parabolic law, relatively dense oxide scales without spallation (Fig. 4(b1), (b2) and (c2)) are formed at oxidation temperatures of 973 K and 973-1023 K for TA15 titanium alloy and TiBw/TA15 composite, respectively. As the oxidation temperature rise to 1023-1073 K and 1073 K for TA15 titanium alloy and TiBw/TA15 composite, a parabolic-linear oxidation law is followed, which promotes the formation of a thicker oxide scale with spallation. In addition, it can also be found from Table 1 that the calculated oxidation rate constants of the

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TA15 titanium alloy are higher than those of the TiBw/TA15 composite at 973-1073 K. This result further indicates that the TiBw/TA15 composite possesses higher oxidation resistance than the TA15

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titanium alloy.

𝑘p = 𝑘0 exp (−

𝑄 ) 𝑅𝑇

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Moreover, the parabolic rate constant kp follows an Arrhenius relation [28]: (2)

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where Q is the effective activation energy for oxidation, k0 is the constant for a given material, T is the absolute temperature and R (8.314 J mol-1 K-1) is the gas constant. Fig. 6(b) shows the fitted plots

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of ln(kp) versus 1/T of these two materials. One can see from Fig. 6(b) that the fitting of the curves is good and the results have high reliability. The slope of the plots yields the activation energy of

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oxidation. As can be seen, the effective activation energy Q of oxidation for the TA15 titanium alloy

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and TiBw/TA15 composite is 299±19.9 kJ/mol and 339±8.31 kJ/mol, respectively. The Q value of TiBw/TA15 composite is evidently higher than that of the TA15 titanium alloy. It is well known that the activation energy of oxidation reflects the critical energy required for the oxidation process of materials. The larger activation energy of oxidation will result in a more difficult oxidation process of multi-component alloy. According to the EDS line scanning results, the growth of TiO2/Al2O3 layer is mainly controlled by outward diffusion of aluminum and titanium atoms, while the growth of

inner TiO2 layer is dominated by oxygen ingress diffusion for both materials. Therefore, the oxidation process of the two materials is affected significantly by diffusion. Since the TiBw/TA15 composite has higher oxidation resistance than the TA15 alloy, it is deduced that the atomic diffusion of TA15 alloy is easier than that of TiBw/TA15 composite when they are oxidized at the same temperature. Additionally, the TA15 titanium alloy has thicker oxide scale than TiBw/TA15 composite when oxidized at the same temperature, which could further proves this analysis. When

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the oxidation temperature increases, the diffusion of atoms and the rate of oxidation are promoted, especially for TA15 titanium alloy owing to its lower effective activation energy for oxidation. Thus, the growth of oxide scale for TA15 titanium alloy is faster, resulting in a thicker oxide scale.

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4.2. Mechanism of improved oxidation resistance for TiBw/TA15 composite

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In order to clarify the difference of initial stage for the oxidation process, Table 2 shows the EDS

the Fig. 4(a1) and (a2)).

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results of the TA15 titanium alloy and TiBw/TA15 composite oxidized at 873 K for 120 h (marked in

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One can see that the oxygen content of point D is much higher than that of point C for the TiBw/TA15 composite. While there is no significant difference of the oxygen content for point C of

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TiBw/TA15 composite, and point A and B of TA15 titanium alloy. This result indicates that the

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TiBw-rich boundary regions are more easily to be oxidized. Subsequently, the observations of surface for the TiBw/TA15 composite samples oxidized at 1023 K for 2 h and 10 h are performed, as shown in Fig. 7(a) and (b). As can be seen from Fig. 7(a), in the initial stage, the oxidation of the TiBw-rich boundary region (relatively bright contrast) is more serious in comparison with the TiBw-lean region (relatively dark contrast). When the oxidation time further extends to 10 h, the TiBw-lean region is gradually oxidized (Fig. 7(b)), forming a complete and continuous oxide layer.

According to the above results, a simplified diagram of oxidation process has been established to further reveal the oxidation resistance for TiBw/TA15 composite, as shown in Fig. 7(c)-(f). Fig. 7(c) is the three-dimensional photograph of the TiBw/TA15 composite for high temperature oxidation experiments. It is noted that the TiBw reinforcements distribute at the boundary of three-dimensional spatial network structure. Then, the oxidation of a network unit on the TiBw/TA15 composite surface is typically investigated, marked by the red dotted circle in the Fig. 7(c) and an initial stage of

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oxidation diagram for a network unit is shown in Fig. 7(d). Due to the difference of thermal expansion coefficient between TiBw and the alloy matrix, the TiBw-rich boundary regions possess more lattice defects at high temperature. It is noteworthy that there are also lattice defects in TiBw

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[29,30]. Therefore, in the initial stage of oxidation process (low temperature and long time or high

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temperature and short time), these lattice defects can provide more nucleation sites for oxides and the nucleation rate of oxide at lattice defect in the TiBw-rich boundary regions is promoted. Under this

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situation, the oxidation process of TiBw-rich boundary regions is accelerated and continuous oxide

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scale is formed in the initial stage of oxidation, as shown in Fig. 7(d). After that, the further oxidation of the matrix beneath the TiBw-rich boundary regions can be hindered by the continuous oxide scale.

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With extending the time, the oxidation in the TiBw-lean regions became more serious, as shown in Fig. 7(a) and (b). Fig. 7(e) exhibits the oxidation diagram of A-A sectional view marked by the red

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arrows in Fig. 7(c). One can see that there are numerous TiBw reinforcements beneath the TiBw-lean regions due to the three-dimensional spatial network structure, as shown in Fig. 7(e). Besides the continuous oxide scale forms in TiBw-rich boundary regions, there also exists some oxide granules in the TiBw-lean regions. As the oxidation proceeds, these oxide granules on the surface of the samples grow up gradually, and the oxygen also continues to diffuse into the matrix. Due to the

existence of the TiBw beneath the TiBw-lean regions (Fig. 7(e)), a large number of nucleation sites for oxides will be provided, and then a local continuous oxide scale is formed rapidly, which inhibits the internal diffusion of oxygen and the external diffusion of aluminum and titanium. During further oxidation, the local continuous oxide scale resulted from TiBw three-dimensional spatial network structure can gradually hinder the diffusion and protect the matrix layer by layer until a dense oxide scale forms. Thus, more critical energy for oxidation of TiBw/TA15 composite is required and the

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growth of oxide scale controlled by the diffusion of atoms for TiBw/TA15 composite is lower than that of TA15 titanium alloy, causing that TiBw/TA15 composite exhibit higher oxidation resistance. Fig. 7(f) shows the formation diagram of layered oxide scales for the TiBw/TA15 composite at

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higher temperatures or longer oxidation time. According to the XRD results, the oxidation products

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are mainly TiO2 phase due to its smaller activation energy for growth (TiO2 phase is 59.5 kJ/mol, Al2O3 phase is 502.4 kJ/mol) [31] and relatively higher atomic fraction of Ti in the alloy. Once the

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TiO2 phase forms, the concentration of aluminum will take place at the boundary between the TiO2

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granules and substrate. Moreover, the higher the oxidation temperature, the faster the oxidation reaction, which will further promotes the concentration of aluminum. On the other hand, the

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concentration gradient of oxygen leads to the oxygen diffusion from the atmosphere/(TiO2/Al2O3) interface to the TiO2/substrate interface during oxidation process. Based on the Van't Hoff equation,

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Gibbs free energy ΔG of the oxidation reaction can be written as follow: ∆G = ∆𝐺 𝜃 − 𝑅𝑇 ln 𝑃O2

(3)

where ΔGθ is the standard Gibbs free energy, R (8.314 J mol-1 K-1) is the gas constant, T is temperature and PO2 is oxygen partial pressure on the surface. According to the Van't Hoff equation, the oxidation reaction of aluminum and titanium at the atmosphere/(TiO2/Al2O3) interface can be

written as ΔG1: 1 ∆𝐺 1 = ∆𝐺 θ − 𝑅𝑇 ln 𝑃O2

(4)

1 where the 𝑃O2 is oxygen partial pressure of the atmosphere/(TiO2/Al2O3) interface. Meanwhile, the

oxidation reaction of aluminum and titanium at the TiO2/substrate interface can be written as ΔG2: 2 ∆𝐺 2 = ∆𝐺 θ − 𝑅𝑇 ln 𝑃O2

(5)

2 where the 𝑃O2 is oxygen partial pressure of the TiO2/substrate interface. Then, combining the Eqs.

∆𝐺 1 − ∆𝐺 2 = 𝑅𝑇 ln

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(4) and (5) yields the Eq. (6): 2 𝑃O2 1 𝑃O2

(6)

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As to the EDS line scanning results, the oxygen diffuse from the atmosphere/(TiO2/Al2O3) 2 1 interface to the TiO2/substrate interface. Thus, the relationship between the 𝑃O2 and 𝑃O2 can be

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described by the Eq. (7):

(7)

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2 1 𝑃O2 ≪ 𝑃O2

From Eqs. (6) and (7), it can be derived that ΔG=ΔG1-ΔG2 <0. As a result, the reduction of the Gibbs

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free energy for the system promotes the outward diffusion of aluminum and titanium. Furthermore, it will be more favorable to form Al2O3 phase at the gaps of massive TiO2 particles with the increasing

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of oxidation temperature (Fig. 4(b1)-(d2)). Meanwhile, the oxygen diffuses inward through the

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matrix and reacts with titanium to produce TiO2, forming the Al-lean region as shown in Figs. 5 and 7(f). Finally, the transition layer moves forward gradually and a dense layered oxide scales is formed.

5. Conclusions

(1) The oxidation kinetics of the TiBw/TA15 composite follows parabolic law and parabolic-linear laws at 973-1023 K and 1073 K, respectively. Moreover, the oxidation kinetics of the TA15 titanium alloy obeys parabolic law and parabolic-linear laws at 973 K and 1023-1073 K,

respectively. (2) The oxidation rate of the prepared composite is much lower than that of the TA15 titanium alloy. The effective activation energy Q of the TA15 titanium alloy and TiBw/TA15 composite is 299±19.9 kJ/mol and 339±8.31 kJ/mol at the temperature range of 973-1073K, respectively. (3) During oxidation at temperature range of 973-1073 K, the growth of TiO2/Al2O3 layer is

TiO2 layer is dominated by oxygen ingress for both materials.

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controlled mainly by outward diffusion of aluminum and titanium atoms, while the growth of inner

(4) The TiBw/TA15 composite possess a higher oxidation resistance because of the TiBw distributed at the boundary of the alloy matrix exhibiting three-dimensional spatial network structure.

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A schematic diagram of oxidation mechanism is established to reveal the oxidation process for

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Acknowledgments

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TiBw/TA15 composite at elevated temperatures, which is superior to that of TA15 titanium alloy.

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This work was financially supported by the National Natural Science Foundation of China (Nos.

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51674093 and 91860122).

References

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[1] N. Khanna, J.P. Davim, Measurement 61 (2015) 280-290. [2] J.C. Williams, E.A. Starke Jr., Acta Mater. 51 (2003) 5775-5799. [3] P. Singh, H. Pungotra, N.S. Kalsi, Mater. Today 4 (2017) 8971-8982. [4] A. Gao, R.Q. Hang, L. Bai, B. Tang, P.K. Chu, Electrochim. Acta 271 (2018) 699-718. [5] C. Leyens, M. Peters, Titanium and Titanium Alloys: Fundamentals and Applications, Wiley-VCH, Weinheim, 2003. [6] Z.Y. Ma, S.C. Tjong, L. Gen, Scr. Mater. 42 (2000) 367-373. [7] Y. Tanaka, J.M. Yang, Y.F. Liu, Y. Kagawa, Scr. Mater. 56 (2007) 209-212. [8] S. Gorsse, D.B. Miracle, Acta Mater. 51 (2003) 2427-2442. [9] I. Sen, S. Tamirisakandala, D.B. Miracle, U. Ramamurty, Acta Mater. 55 (2007) 4983-4993.

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Fig. 1. Microstructures of TA15 titanium alloy (a) and TiBw/TA15 composite (b).

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Fig. 2. Relationship between mass gain and time of the TA15 titanium alloy and TiBw/TA15 composite.

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Fig. 3. XRD patterns of the TA15 titanium alloy and TiBw/TA15 composite after oxidation: (a) 873-973 K; (b) 1023-1073 K.

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Fig. 4. Typical surface morphologies of the oxide scales after oxidation at different temperatures for 120 h: (a1) 873 K for TA15; (a2) 873 K for TiBw/TA15; (b1) 973 K for TA15; (b2) 973 K for TiBw/TA15; (c1) 1023 K for TA15;

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(c2) 1023 K for TiBw/TA15; (d1) 1073 K for TA15; (d2) 1073 K for TiBw/TA15.

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Fig. 5. Images of cross-section for the samples after oxidation and the EDS line scanning results: (a1, a2) 973 K for

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TA15; (b1, b2) 973 K for TiBw/TA15; (c1, c2) 1023 K for TA15; (d1, d2) 1023 K for TiBw/TA15.

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Fig. 6. Plot of ln(ΔM) vs. ln(t) (a) and the Arrhenius plot of parabolic rate constant (kp) for oxidation of the two materials in the temperature range of 973-1073 K (b).

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Fig. 7. Typical surface morphologies of the oxide scales for TiBw/TA15 composite after oxidation at 1023 K with 2 h (a), 10 h (b), the three-dimensional photograph of the TiBw/TA15 composite (c), the initial stage of oxidation diagram for a network unit (d), the oxidation diagram of cross-section for a network unit (e), and the formation illustration of layered oxide scale (f).

Table 1 Rate exponents n and calculated parabolic rate constants kp of the TA15 titanium alloy and TiBw/TA15 composite oxidized in the temperature range of 973-1073 K. n

TA15 TiBw/TA15

973 K 2.52 2.07

kp (10-3×mg cm-2 h-1)

1023 K 1.45 2.01

1073 K 1.75 1.89

973 K 6.51 3.63

1023 K 54.6 31.3

1073 K 220 186

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Materials

Table 2 EDS analysis results of the TA15 titanium alloy and TiBw/TA15 composite at oxidation temperature 873 K (points shown in Fig. 4(a1) and (a2)).

Ti 73.93 75.92 74.74 71.85

Al 5.59 5.93 5.85 1.87

O 17.01 15.52 16.33 24.23

Zr 2.16 1.89 1.73 0.78

Mo 1.32 0.73 1.05 0.89

V 0.00 0.00 0.00 0.11

B — — 0.30 0.27

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Points A (TA15) B (TA15) C (TiBw/TA15) D (TiBw/TA15)