J. Cent. South Univ. (2020) 27: 2530-2537

DOI: https://doi.org/10.1007/s11771-020-4478-8

Microstructure and hot corrosion resistance of Si-Al-Y coated TiAl alloy

LI Yong-quan(李涌泉)^{1, 2}, XIE Fa-qin(谢发勤)², YANG Shao-lin(杨少林)¹

1. School of Materials Science & Engineering, North Minzu University, Yinchuan 750021, China;

2. School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: In order to obtain a high-performance surface on TiAl alloy that can meet the requirements in hot corrosion environment, Si-Al-Y coatings were fabricated by pack cementation process at 1050℃ for 4 h. Corrosion behaviors of the TiAl alloy with and without Si-Al-Y coatings are compared to illustrate the factors and corresponding mechanism in molten salt environment of 25wt% K₂SO₄ and 75wt% Na₂SO₄ at 900 °C. The obtained Si-Al-Y coating was mainly composed of a TiSi₂ outer layer, a (Ti, X )₅Si₄ and (Ti, X )₅Si₃ ( X represents Nb or Cr element) middle layer, a TiAl₂ inner layer and a Al-rich inter-diffusion zone. The inter-phase selective corrosion containing corrosion pits extending along α₂ phase from lamellar interfaces in hot corrosion tested TiAl alloy was observed. However, by being coated with Si-Al-Y coating, the hot corrosion performance of TiAl alloy was improved remarkably.

Key words: TiAl alloy; Si-Al-Y coating; structure; hot corrosion

Cite this article as: LI Yong-quan, XIE Fa-qin, YANG Shao-lin. Microstructure and hot corrosion resistance of Si-Al-Y coated TiAl alloy [J]. Journal of Central South University, 2020, 27(9): 2530-2537. DOI: https://doi.org/10.1007/ s11771-020-4478-8.

1 Introduction

Possessing low density, high specific strength, excellent high temperature performance and other superior properties, TiAl alloys have been considered important high-temperature structural materials to be potentially used in industrial and aeroengine turbomachineries [1, 2]. For instance, TiAl alloys are being considered to replace Ni-based superalloy in the temperature range of 1073-1173 K in aero-engine, to achieve a component weight reduction over 50% [3, 4]. However, the low chemical activity of Al hinders the selective oxidation to form protective Al₂O₃ scale, and the resulted poor oxidation and corrosion resistance of TiAl has been a major barrier to its practical applications and commercialization [5, 6].

In recent years, remarkable advancements have been made in solving the aforementioned problem through adding alloying elements such as B, Nb, Cr, Al and W [7-10]. Unfortunately, further alloying usually leads to mechanical properties deterioration of TiAl alloys [11]. Therefore, a protective coating is necessary to ensure the long-term mechanical and structure integrity of TiAl alloys in oxidation and corrosion environments. For instance, in ocean environment, the deposited sodium sulfate and sylvite can cause serious corrosion and accelerate the degradation of TiAl alloys [12].

Among various coating systems, Si-Al composite coatings are considered to be suitable for high temperature protection of TiAl alloys by forming a dense, compact and stable SiO₂+Al₂O₃ scale [12]. Thus, intensive studies involved in preparation technologies, microstructure control, high temperature oxidation and hot corrosion behavior, as well as the related mechanisms, etc., have been carried out in recent years [13-18]. WANG et al [13] studied the high temperature oxidation behavior of a Ti(Al,Si)₃ composite coating prepared on TiAl alloy by cold spray method, and found that the weight gain of the obtained coating after oxidation at 900 °C for 1000 h is lower than 2.2 mg/cm². SWADZBA et al [14] have reported the excellent cycle oxidation resistance of Si-Al composite coating prepared on TiAl alloy by slurry method. Many other studies have also confirmed the promising high temperature oxidation resistance of Si-Al composites [15-18]. However, the Si-Al composite coatings are considered to be poor in hot corrosion resistance. It is demonstrated that although aluminum helps to improve the high-temperature oxidation resistance of the Si-Al coatings by forming protective Al₂O₃ in the scale, it is not conducive to the hot corrosion resistance of the coatings [19, 20]. WANG et al [20] have reported that dense and protective scale can hardly form on the Si-Al coating in chlorides and sulphates containing environment in temperature range of 1050-1150 °C, mainly attributed to the rapid dissolution of the aluminum oxides by reacting with chlorine, and the formed volatile AlCl_x would cause secondary damages to the coating. To overcome the obstacle, adding rare earth element Y in the coating should be an effective way, as its rare earth effects (REEs) were reported to be beneficial for reducing the grain size, promoting the formation of protective and adhesive scales at high temperatures [21, 22].

The way of coating preparation also plays an important role in the high temperature protection. Till now, substantial protective coatings have been explored by various surface modification strategies on TiAl, such as, plasma spray, vacuum fusing, magnetron sputtering, electron beam physical vapor deposition and pack cementation [23-25]. Among these techniques, the pack cementation with low operating cost, high efficiency, and applicable for a wide range of shapes and sizes, has been widely accepted and used in aluminizing, siliconization, carburizing, chromizing and bording [23]. Another outstanding advantage of pack cementation technique is that gradient coating with metallurgical bonding interfaces, characterized by high bond strength and not easy to crack, can be produced by this method, which is significant for TiAl alloy to bear complex load and thermal fatigue [26, 27]. In this paper, Si-Al-Y co-deposition coatings were prepared on a Ti-Al alloy by halide activated pack cementation (HAPC) technique. The corrosion behaviors of the TiAl alloy with and without Si-Al-Y coatings are compared to illustrate the factors and corresponding mechanism in molten salt environment.

2 Experimental procedures

The TiAl alloy with nominal composition Ti-48Al-2Cr-2Nb (at%) used for the experiments was manufactured by Beijing Institute of Aerial Materials, China. As the microstructure shown in Figure 1, the TiAl substrate with lamellar structure consists of γ-TiAl and α₂-Ti₃Al phases. All samples were machined into the rectangle blocks with dimensions of 15 mm×15 mm×3 mm, ground with 1500-SiC paper, then ultrasonically cleaned in ethanol and dried.

Figure 1 Microstructure of near lamellar TiAl alloy

According to our previous research results [28-30], pure Si, Al and Y₂O₃ were used as donor sources, and AlCl₃·6H₂O and Al₂O₃ powers were utilized as activator and filler of the pack mixture, respectively. The pack mixtures used for the pack processes were chosen as 10Si-10Al-1Y₂O₃- 8AlCl₃·6H₂O-71Al₂O₃ (wt%), and each component of powders was weighed according to the ratio and then mixed by tumbling in a ball mill sufficiently. After that, the TiAl samples were buried in the pack mixtures and then sealed in an alumina crucible with Al₂O₃-based silica sol binder. The sealed crucible was heated in an electric tube furnace from room temperature to co-deposition temperature (1050 °C) at a rate of 15 °C /min. After holding at 1050 °C for 4 h, the furnace was cooled to room temperature. The TiAl alloy was then removed from the pack and ultrasonically cleaned.

In this study, the hot corrosion tests of the TiAl alloy with and without Si-Al-Y coatings were carried out in molten salt environment of 25 wt% K₂SO₄ and 75 wt% Na₂SO₄ at 900 °C for different times. After being cooled to room temperature, the specimens were put into the distilled water to dissolve the salts adhered to surface of the samples, and then dried. The hot corrosion resistance of the specimens was evaluated via mass changes that were measured using an analytical balance (accuracy 0.1 mg).

X-ray diffraction analysis (XRD, Panalytical X’Pert PRO, 30 mA, 40 kV, CuK_α) was employed to identify the phases present in the as-coated and corroded specimens. The microstructures and chemical compositions of the constituent phases were characterized by a scanning electron microscope (SEM, JSM-6360LV, a regime of secondary and backscattered electrons, acceleration voltage of 20 kV) equipped with an energy dispersive spectroscope (EDS).

3 Results and discussion

3.1 Coating structure

Figure 2 shows a typical cross-sectional BSE image of the Si-Al-Y coating prepared by co-depositing Si, Al and Y at 1150 °C for 4 h. It can be seen that the coating was about 37 μm thick and had a multilayer structure: a TiSi₂ out layer, a (Ti, X)₅Si₄ and (Ti, X)₅Si₃(X represents Nb or Cr) middle layer, a TiAl₂ inner layer and an Al-rich interdiffusion zone. The detailed analyses of the formation processes of the coating structure and constituent phases, as well as their oxidation behavior have been reported in other articles by our group [28-31].

Figure 2 Cross-sectional BSE image of Si-Al-Y co-deposition coating

3.2 Hot corrosion behavior of Si-Al-Y coating

3.2.1 Corrosion kinetics

Figure 3 shows the mass change of TiAl alloy and Si-Al-Y coating in (Na, K)₂SO₄ molten salts at 900 °C. In the initial period of hot corrosion, the mass changes of TiAl alloy and Si-Al-Y coating were respectively about -0.52 and 1.31 mg/cm² after 1 h hot corrosion. With the increase of corrosion time to 20 h, the mass change of TiAl alloy decreased remarkably to -7.80 mg/cm². The mass change of Si-Al-Y coating increased to 6.01 mg/cm² after 14 h hot corrosion and then decreased to -3.83 mg/cm² after 50 h. From the above results, it can be found that the TiAl alloy suffered from a severe hot corrosion and a large mass loss, which was related to the spallation of corrosion scales and corrosion products. Therefore, the corrosion products on surfaces of TiAl alloy cannot provide complete protection for the TiAl substrate, and the hot corrosion resistance of the Si-Al-Y coating is better than that of the substrate.

Figure 3 Mass change of TiAl alloy and Si-Al-Y coating in (Na, K)₂SO₄ molten salts at 900 °C

3.2.2 Hot corrosion scale morphologies

The surfaces morphologies and XRD patterns of the TiAl alloy after hot corrosion in (Na, K)₂SO₄ melt at 900 °C for 1 h and 20 h are shown in Figure 4. It can be seen from Figure 4(a) that the hot corrosion scale formed on the surface is loose and cracked, and local spallasion can be clearly observed. The enlarged intact area at the right position of Figure 4(a), characterized by dark gray matrix and white granular tissues, is uniform and smooth. The phase constitution and XRD patterns shown in Figure 4(c) suggest that the corrosion products of the intact scale is TiO₂ and Al₂O₃, and the spalled area has a composition of 36.46Al-2.36S-57.11Ti-1.08Cr-2.99 Nb (at%). The XRD patterns (Figure 4(c)) and the binary Ti-Al phase diagram [9] indicate the composition of Ti₃Al₂ phase. Moreover, many grooves in the direction of the arrow were observed in the spalled area. These features could provide some evidences to the hot corrosion mechanism of the alloy, and it will be discussed later.

Figure 4 Surfaces morphologies (a, b) and XRD patterns (c) of TiAl alloy after hot corrosion in (Na, K)₂SO₄ melt at 900 °C for 1 h (a) and 20 h (b)

It can be found from Figure 4(b) that the surface morphology after hot corrosion for 20 h is similar to that after hot corrosion for 1 h. The EDS analyses determined that the intact area was mainly composed of TiO₂, and the significant increased intensity of TiO₂ diffraction peaks also revealed the formation of the TiO₂ scale. The spalled area had a typical composition of 20.30Al-7.2S-69.73Ti- 1.7Cr-1.07Nb (at%), detected by EDS analysis. Combined with the XRD pattern in Figure 4(c), the spalled area should mainly consist of Ti₃Al₂ and a small amount of TiS.

Figure 5 shows cross-sectional BSE images of the TiAl alloy after hot corrosion in (Na, K)₂SO₄ melt at 900 °C for 1 and 20 h, respectively. It can be seen from Figure 5(a) that the hot corrosion scale formed on the surface was about 1 μm thick, and EDS analysis suggested the main composition of Al and Ti oxides (arrow 1). Under the corrosion product film, many corrosion pits formed at the interface, and some of them had extended into the matrix alloy along α₂ phases (arrow 2). EDS analysis determined that the contents of O and S elements in these corrosion pits are 3.51 at% and 4.78 at%, respectively, indicating that the α₂ phase oxidized and vulcanized preferentially. The grooving morphology shown in Figure 4(a) should be a result of the corrosion film spallasion.

Figure 5 Cross-sectional BSE images of TiAl alloy after hot corrosion test at 900 °C for 1 h (a) and 20 h (b)

With prolonging the corrosion time to 20 h, as seen in Figure 5(b), a corrosion scale with a multilayer structure and a thickness about 22 μm formed. A large crack can be clearly observed between the scale and the matrix. EDS analyses determined that the concentrations of Ti and O in the outer white tissue of the scale were about 35.42 at% and 64.58 at%, respectively, indicating the formation of TiO₂, while the Al content in the dark gray tissue of the middle layer was about 41.31 at%, suggesting the formation of Al₂O₃. The inner layer is mainly composed of mixed oxides of Ti and Al, and the content of S in this layer is as high as 9.43 at%.

Figure 6 shows the surfaces BSE images and XRD patterns of the Si-Al-Y co-deposition coating after hot corrosion test at 900 °C for 5 and 50 h, respectively. It can be seen that the corrosion scale formed on the surface at different time had a similar morphology, and is continuous without visible peeling. However, with the increasing corrosion time, corrosion pores increased slightly on the coating surface. XRD analysis (Figure 6(c)) revealed that the corrosion products formed on the coating were similar after oxidation for different time, mainly composed of TiO₂, SiO₂, Na₂SiO₃ and TiSi₂.

Figure 7 presents the cross-sectional BSE images of the Si-Al-Y coatings after hot corrosion test at 900 °C for 5 and 50 h. As shown in Figure 7(a), it can be easily found that slight corrosion occurred in the outer layer of Si-Al-Y coating, and the corrosion scale was about 2 μm thick and consisted of dark gray phase (arrow 3). XRD patterns (Figure 6(c)) and EDS analysis indicated the composition of Ti rich TiO₂ and a small amount of SiO₂ and Na₂SiO₃. Beneath the corrosion scale, many black punctate tissues (arrow 4) formed, with O and S concentrations of respectively 3.91 at% and 0.73 at%, determined by EDS analysis.

After being corroded for 50 h, as shown in Figure 7(b), all layers in the coating have been corroded, and a hot corrosion scale with a thickness of about 2 μm formed on the surface of the coating.

Figure 6 Surfaces BSE images (a, b) and XRD patterns (c) of Si-Al-Y coating after hot corrosion in (Na, K)₂SO₄ melt at 900 °C for 5 h (a) and 50 h (b)

The EDS analysis results determined that the scale had a typical composition of 54.76Ti-44.52O- 0.72Si at%, suggesting the formation of Ti-rich TiO₂. Moreover, a number of holes can be seen in the remaining TiSi₂ out layer and TiAl₂ inner layer, EDS line scanning results show that O and S increased significantly in the inner layer; especially, S element was enriched in both upper and lower part of the layer, where the cracks formed, as shown in the right position of Figure 7(b).

3.2.3 Hot corrosion mechanisms

Both oxides and sulfides, which originated from (Na, K)₂SO₄ molten salt, were detected in the corrosion products of TiAl alloy and Si-Al-Y coating. Since it was difficult for external O and S elements to enter the corrosion system, the O and S elements should mainly generate from the high temperature decomposition of Na₂SO₄ and K₂SO₄ according to the following thermodynamic equilibrium existing in the corrosion system [32]:

Na₂SO₄→Na₂O+SO₂+1/2O₂                          (1)

K₂SO₄→K₂O+SO₂+1/2O₂                             (2)

Actually, the molten salt is a high sulfur, low oxygen potential system.

Figure 7 Cross-sectional BSE images of Si-Al-Y coatings after hot corrosion test at 900 °C for 5 h (a) and 50 h (b)

For TiAl, oxidation occurred on the surface of the alloy at the initial stage of hot corrosion [33]:

Ti+O₂→TiO₂                                             (3)

2Al+3/2O₂→Al₂O₃                                     (4)

As the above process progressed, O and S elements penetrated the matrix interface through the loose oxide film, and reacted with α₂ phase preferentially due to the two-phase laths morphology and high activity of Ti in the α₂ phase, which means that inter-phase selective corrosion occurred, as demonstrated by the cross-sectional morphologies of TiAl alloy shown in Figure 5 and ZHAO et al [34]. Moreover, the corroded α₂ phase spread laterally in horizontal direction, leading to fracture and premature failure of TiAl alloy.

For the Si-Al-Y coating, the surface is slightly oxidized to form a thin SiO₂ and TiO₂ scale (about 2 μm) at the beginning of hot corrosion process, such a thin scale cannot effectively prevent the inward diffusion of S and O. Moreover, the Ti element has a higher diffusion rate along the grain boundaries compared with other elements, and thus, the outward diffusion of Ti caused intensive Kirkendall vacancies in the remained coating and increased TiO₂ in the scale, as demonstrated by Figure 7(b). These vacancies can act as the inward diffusion channel of S and O elements, and accelerate the corrosion process of the coating. Accompanied with the inward diffusion of S and O, Al should be the preferentially oxidized element due to its more negative oxidation Gibbs free energy, while Ti is prone to initially react with S to form TiS in the case that oxygen been consumed [35]:

S+Ti→TiS                              (5)

With the continually inward diffusion of O, TiS was then oxidized to release S, and subsequently led to internal oxidation and sulfidation.

4 Conclusions

1) A uniform and compact Si-Al-Y coating with a TiSi₂ outer layer, (Ti, X)₅Si₄ and (Ti, X)₅Si₃ ( X represents Nb or Cr element) middle layer, TiAl₂, γ-TiAl inner layer and a Al-rich inter-diffusion zone is prepared on TiAl alloy by pack cementation at 1050 °C for 4 h.

2) Inter-phase selective corrosion presents during the hot corrosion process of TiAl alloy in the molten salt corroding environment consisting of 25%K₂SO₄+75%Na₂SO₄ (mass fraction). TiO₂ and Al₂O₃ are the main products during hot corrosion of the alloy, and the oxide scale is loose and porous with poor adhesive ability to matrix alloy.

3) The Si-Al-Y co-deposition shows better hot corrosion resistance at the initial stages of hot corrosion. However, with the time prolongation, the Si-Al-Y co-deposition coating fails for the formation of internal sulfidation, leading to the occurrence of cracks.

Contributors

The overarching research goals were developed by XIE Fa-qin and LI Yong-quan. The initial draft of the manuscript was written by LI Yong-quan. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

LI Yong-quan, XIE Fa-qin and YANG Shao-lin declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

TiAl合金表面Si-Al-Y渗层的组织及热腐蚀性能

摘要：为了提高TiAl合金的抗热腐蚀性能，采用1050 °C下扩散共渗4 h的方法在TiAl合金表面制备了Si-Al-Y渗层。研究了TiAl合金基体及渗层在900 °C的25%K₂SO₄+75%Na₂SO₄熔盐(质量分数)中的热腐蚀行为，探讨了其腐蚀速率和腐蚀机理。结果表明：Si-Al-Y共渗层由外向内依次为 TiSi₂外层、(Ti,X)₅Si₄及(Ti,X)₅Si₃(X表示Nb, Cr)中间层、TiAl₂和γ-TiAl内层及富Al 的过渡层。在K₂SO₄+Na₂SO₄熔盐中，TiAl合金沿着层片状的α₂相发生了选择性的腐蚀；经过制备Si-Al-Y渗层后，TiAl合金的抗热腐蚀性能得到了显著的提高。

关键词：TiAl合金；Si-Al-Y渗层；组织；热腐蚀

Foundation item: Project(2020AAC02025) supported by the Natural Science Foundation of Ningxia Province, China; Project(51961003) supported by the National Natural Science Foundation of China; Project(TJGC2019040) supported by the Ningxia Youth Talents Supporting Program, China; Project(2020xyzc103) supported by the Foundation of North Minzu University, China

Received date: 2019-11-26; Accepted date: 2020-07-07

Corresponding author: LI Yong-quan, PhD; Tel/Fax: +86-951-2067378; E-mail: 8386595@163.com; ORCID: https://orcid.org/0000- 0001-6095-8761