Microstructure and hot corrosion property of a Si-Co-Y diffusion coating deposited on TiAl-Nb alloy

: Si-Co-Y diffusion coatings were deposited on TiAl-Nb alloy using pack cementation process. The influence of activators and deposition temperatures on the coating structures was investigated, alongside the coating formation process and hot corrosion performance of the optimized coating in molten salt of 25%NaCl+75%K 2 SO 4 at 850 ℃ . The results show that a dense and compact Si-Co-Y diffusion coating can be prepared on TiAl-Nb alloy, with a multi-layered structure including an outmost layer of (Ti, X )Si 2 (X represents Co, Al, Nb, Y), an outer layer composed of TiSi 2 +Ti 5 Si 4 +Ti 5 Si 3 mixtures, a middle layer of Ti 5 Si 3 , and an inner layer of TiAl 2 . The coating prepared with AlCl 3 ·6H 2 O and NH 4 Cl had many pores. Increase in deposition temperature led to a higher coating growth rate within the range of 1050-1100 o C, but temperature exceeding 1100 ℃ caused the formation of intensive holes in the coating. Hot corrosion tests at high temperatures proved that the Si-Co-Y diffusion coating prepared on TiAl-Nb alloy exhibited excellent hot corrosion resistance in 25%NaCl-75%K 2 SO 4 molten at 850 o C. A protective hot corrosion product scale composed of a TiO 2 +Na 2 SiO 3 +Na 2 TiO 3 outer layer and an Al 2 O 3 middle layer, formed on coating after hot corrosion for 50 h. The scale can effectively inhibit the inward diffusion of corrosion medial of O, Cl and S elements.


Introduction
TiAl alloys possess low density, high strength and specific strength, excellent mechanical properties, making them potential lightweight alternatives to nickel-based alloys in high-temperature structural applications [1][2][3].When serving as hot-end components in aero-engines and gas turbines, TiAl alloys need to withstand long-term exposure to high levels of air salinity, particularly in offshore or marine environments.Unfortunately, TiAl alloys exhibit relatively poor anti-hot corrosion resistance, as they prone to react with corrosive substances such as oxygen, Cl -and SO2 at high temperatures.The formed corrosion products are loose, porous, and easy to peel off, significantly compromising safety and service life [4,5].Therefore, enhancing the hot corrosion resistance of TiAl alloy is imperative to enable their use as hot end components.
Preparation of surface protective coating is an effective way to improve the hot corrosion resistance of TiAl alloys.In recent decades, various methods have been utilized to prepare protective coatings for TiAl alloys, such as aluminide coatings (TiAl3, TiAl2, etc.) [5], M-CrAlY (M=Ni, Co or NiCo) coatings [6,7], ceramic coatings [8,9], silicide coatings [10][11][12] and composite coatings doped with noble metals [13].Among the these coating systems, silicide coatings exhibit excellent thermal stability and can form a flowable and self-healing SiO2 protective scale at high temperatures, making them suitable for load-bearing applications in TiAl alloys [14].At present, there has been several studies on the high temperature oxidation resistance of the silicide coatings applied to TiAl alloys, while limited studies has focused on their hot corrosion behavior.Based on the available literature, it appears that the hot corrosion resistance of the silicide coatings is insufficient due to the continuous erosion of silicon dioxide formed by silicides through sulfates and chlorides.This hinders the formation of a protective product film, leading to ongoing internal sulfidation and eventual cracking failure.
Studies have indicated that the addition of appropriate 'third-elements' such as Co, Ni, Hf and B, etc., can effectively promote the formation of protective scale of Al2O3, Cr2O3 or SiO2 [15].REE elements such as Ce, La, and Y have been proven to be crucial for enhancing the high-temperature performance of alloy coatings by inhibiting high-temperature diffusion and delaying degradation.REEs also play a significant role in refining and purifying the coating/oxide film structure, promoting the formation of oxide films, and increasing their adhesion.Among the REEs, Y is widely distributed and commonly used as a modification element for high-temperature coatings.Wang et al. [16] have found that Y atoms, characterized by high chemical activity and low electronegativity, can effectively improve the activity of co-deposited element and facilitate their adsorption onto the substrate's surface.. Additionally, the large atomic radius of Y induces lattice distortion in the coating, increasing dislocation density and vacancies, consequently accelerate the diffusion rate of Si into the substrate.
The pack cementation technique, characterized by simple process ability, versatility, and high repeatability, is essentially a gas-phase deposition technique.With using this method, compact silicon-based coatings or modified silicon-based coatings with strong bonding to the substrate can be conveniently prepared on TiAl-Nb alloys.In this study, a Si-Co-Y coating was prepared on a TiAl-Nb alloy by pack cementation method.The structural characteristics and hot corrosion resistance of the coatings prepared with different activators and temperatures were investigated.The primary goal was to develop a protective coating with good anti-hot corrosion performance for TiAl-Nb alloys, and provide support for promoting the application of TiAl-Nb alloys in high-temperature structural components.

Experimental Materials and Methods
The TiAl-Nb substrate with a composition of Ti45Al-8Nb-0.3Ywas prepared by vacuum consumable arc-melting method.Before smelting, the raw materials are sequentially subjected to degreasing→ acid cleaning→ alkali cleaning→ alcohol cleaning to remove surface oxides and contaminants.The smelting equipment used is a self-made high-temperature, high-vacuum water-cooled crucible arc melting furnace, with a tungsten rod (arc gun) as the cathode and the copper crucible as the anode.During smelting, high-purity argon gas is filled into the furnace to prevent the oxidation of highly active elements such as Ti, Al, and Y, and suppress the volatilization of low-melting-point elements of Al.To minimize the component segregation, each alloy component was re-melted for 3 times.
The specimens to be coated were cut from the ingot into cuboid measuring 4 mm×4 mm×3 mm.Before packing, each surface of the specimens was polished using 1000 # SiC paper, and then ultrasonically cleaned in an acetone bath.According to the previous research of our team, the pack powders of 15Si-10Co-3Y-4M-68Al2O3 (wt.%) were employed, in which, Si, Co and Y powders were used as the donor sources, M represents the activators of NaF, NaCl, NH4Cl and AlCl3•6H2O, and Al2O3 were used as the filler.All the powders were produced by Sinopharm Chemical Reagent Co., Ltd in Shanghai, China.The total weight of the pack powders utilized for each sample was 50 g.Before packing, the powders were ball milled in a planetary ball mill to be fully mixed and refined.Each sample to be coated was buried nearly in the center of the pack powders contained in an alumina crucible with a capacity of 50 ml, which was then sealed with Na2SiO3.The sealed crucible was then placed in an atmosphere furnace.After vacuuming, furnace was heated to 1080 ℃ at a rate of 10℃/min for 0~4 h.
After coating, each face of the samples was slightly brushed, and then ultrasonically cleaned in an acetone bath to remove the residual pack powders.
Hot corrosion testing was conducted in an SX2-2.5-12Atype muffle furnace at a temperature of 850 ℃ for 1 h, 10 h, 25 h and 50 h, using 25% NaCl+75% K2SO4 (wt.%) as the corrosion medium.During hot corrosion, the specimens were placed in a corundum crucible after heating without changing mass to obtain the spalling corrosion products.The corrosion medium was promptly replenished by adding drops of the corrosion solution every 12 h to the specimen surface.The specimens after hot corrosion was placed in distilled water for 10 min before weighing, during which it was slightly stirred to dissolve the surface salt, and then dried before weighing.
An electronic balance with an accuracy of 0.1 mg was used to measure the mass change of the specimens before and after oxidation, and the average value was determined by weighing the sample five times.X-ray diffraction (XRD, Panalytical X'Pert PRO, Cu Kα) was employed to identify the constituent phases of the coatings and their hot corrosion products.A scanning electron microscopy (SEM, JSM-6360LV) equipped with an energy dispersive spectroscopy (EDS) were employed to identify the microstructure and compositions of the coatings and their hot corrosion products.

Effects of the activators
Fig. 1 presents the cross-sectional BSE images and element distribution curves the Si-Co-Y diffusion coatings prepared with NaF, NH4Cl, AlCl3•6H2O and NaCl activators at 1080 ℃ for 4 h.Fig. 2 shows the surface XRD patterns of the coatings prepared with different activators, as well as the outer layer of the coating prepared using NaF activator.Upon observing the microstructure of the coating as a whole, it is noted that the coatings prepared with NaF, AlCl3•6H2O and NH4Cl exhibit a greater coating thickness prepared with NaCl.Furthermore, the coatings prepared with NaF and NaCl display a denser microstructure when compared to those prepared with AlCl3•6H2O and NH4Cl.2θ/(º) structure of the coating.Fig. 3 presents the cross-sectional BSE images and elemental concentration profiles of the coatings prepare at 1050 ℃ and 1100 ℃ using NaF as the activator Fig. 4 shows the surface XRD of the corresponding coatings.
Together with the coating prepared at 1080 ℃, as shown in Figs.1(a) and (a1), it is seen that the coatings prepared at different deposition temperatures possess similar structure.As the deposition temperature increased, there was a noticeable increase in coating thickness.Moreover, intensive holes formed in the middle and inner layers of the coating prepared at 1100 ℃.According to the EDS analysis results of the coating cross-sections at different temperatures and the surface XRD patterns (Figs. 2 and 4), it can be concluded that there were no significant differences in the composition of the various coatings.They all consisted of an outmost layer of TiSi2, an outer layer of TiSi2+Ti5Si3+Ti5Si4, a middle layer of Ti5Si3, and an inner layer of TiAl2.The formation of diffusion coatings is generally considered to be mainly controlled by two processes.The first step mainly involves the formation, transmission, and adsorption of the active atoms from the deposited elements, which is mainly influenced by the vapor partial pressure of the halides of the deposited elements procduced in the pack.During the packing process, the activator reacts with the deposited elements at high temperatures, leading to the formation of the respective vapor halides.Deposition of a single element is relatively straightforward when the partial pressures of the vapor halides of the deposited element are high.However, simultaneous depositing multiple elements becomes more challenging since it requires maintaining a reasonable range of partial pressures for each deposited element [15].Fig. 5 presents the calculated equilibrium partial pressures of the main vapor halides of the deposited elements using different activators.These calculations are based on the Gibbs free energy of each independent reaction and state equation.It is observed that the equilibrium partial pressures of the vapor halides of Si and Co in the pack using NaF, NH4Cl and AlCl3•6H2O as the activators are generally higher than that those using NaCl as the activator.In other words, the equilibrium partial pressures of the vapor halides produced in the NaCl containing pack are generally low, resulting in thinner coating thickness as shown in Fig. 1(d).However, numerals void formed in coatings prepared by NH4Cl and AlCl3•6H2O.This should be mainly attributed to the formation of Cl2, which was released by decomposition of NH4Cl and AlCl3•6H2O, and can penetrate into the substrate at high temperatures and react to form volatile halides, as shown by Eqs of ( 1) and (2) [19].In NaF containing pack, the partial pressures of Si containing species are generally higher compared to those of Co and Y containing species.Among these species, the partial pressures of Y containing species are the lowest.These observations suggest that the transport and adsorption of Si atoms on the surface of the substrate should be easier compared to Co and Y atoms.Consequently, deposition of Si atoms is considered to be more favorable compared to Co and Y. Fortunately, the differences in partial pressures between Si, Co, and Y-containing halides fall within a reasonable range, providing the necessary thermodynamic conditions for co-deposition.
The second step of the coating formation process mainly involves the inward diffusion or reaction diffusion of the active atoms within the substrate.Once the active atoms were adsorbed on the surface of the substrate, the diffusion temperature, the reactivity between the deposited atoms and the substrate, and the diffusion activation energy of the deposited elements become the main factors influencing the growth of the coating.According to the empirical formula Q=32Tm (Tm is the melting point of the material), the growth rate of the diffusion coating is lower at the lower temperature of 1050 ℃, resulting in a thinner coating thickness.Increasing the deposition temperature to a higher temperature of 1100 ℃ resulted in a larger coating thickness.However, excessively high diffusion rate can result in an insufficient of the deposition atoms in the pack, leading to the formation of intensive pores in the coating.It is also important to note that the atomic radius and melting points of Co and Si are similar and much lower than that of Y.As a result, the diffusion rates of Si and Co within the TiAl-Nb alloy will be higher compared to that of the Y atoms.
From the perspective of the reactivity of deposited elements with the TiAl-Nb substrate at a fixed temperature, it is observed that the standard formation Gibbs free energy of Ti5Si3 is the lowest, followed by Ti5Si4 and TiSi2.On the other hand, the formation of Co-Si and Co-Al compounds exhibits relatively higher energies.Considering that the partial pressures of Si containing halide species are much higher compared to Co and Y containing halide species (Fig. 5), it is reasonable to deduce that Ti5Si3 will initially form in the early stage of co-deposition, followed by Ti5Si4 and TiSi2.Thus, Ti5Si3 acts as the growth frontier of the diffusion coating, leading to a multi-layer structure as presented in

Morphology of hot corrosion products
Fig. 7 shows the surface and cross-section BSE images, along with the EDS analysis maps, of the hot-corrosion products scale formed on TiAl substrate and Si-Co-Y diffusion coating after hot corrosion for 50 h.Fig. 8 presents the surface XRD patterns of the hot corrosion product scales.It can be seen from Fig. 7 (a) that the hot corrosion products formed on TiAl-Nb substrate are mainly composed of strip-like dark gray phase with small amount of bright white phase embedded within it.
The EDS analysis maps reveal that dark gray phase has a typical composition of 22.51Ti-0.17Al-0.56Nb-76.70O-0.05Na-0.01K(at.%), and the bright white phase has a typical composition of 15.02Ti-3.08Al-5.26Nb-72.88O-3.69Na-0.06K-0.01Cl(at.%).Combined with the XRD pattern in Fig. 8, the dark gray phase should be TiO2 and the bright white phase is rich in NaNbO3.The cross-section BSE images of the corrosion products formed on the TiAl-Nb substrate, as shown in Fig. 7 (a1), reveal that the hot-corrosion product scale with a thickness exceeding 200 μm, appears rather loose.
Additionally, noticeable cracking also formed between the products scale and the substrate.The EDS analysis maps confirm that the hot-corrosion product scale of the TiAl-Nb substrate is mainly consisted of TiO2 and Al2O3 mixtures, without the formation of a dense and continuous Al2O3 layer.Research by Guan et al. also reported that a single dense protective Al2O3 layer can hardly form on TiAl-Nb alloys when hot corrosion in a mixed molten salt of K2SO4 + NaCl.
From Fig. 7(b), it is seen that the hot-corrosion product scale formed on the Si-Co-Y diffusion coating is mainly composed of small blocky gray phases.EDS analysis determined that these phases possess a typical composition of 26.40Ti -65.56O-0.61Nb-2.35Al-0.11Si-0.16Co-4.56Na-0.23Cl-0.02K(at.%).Combined with XRD pattern in Fig. 8, The superficial zone of the hot-corrosion product scale formed on Si-Co-Y diffusion coating is mainly composed of TiO2, Na2SiO3 and Na2TiO3.The cross-sectional image in Fig. 7(b1) and its EDS analysis maps show that the hot-corrosion product scale exhibit double layered structure that is rather dense and tightly adherent to the residual coating.The thin outer layer has a typical composition of 25.14Ti-66.01O-4.71Al-3.37Na-0.21Nb-0.17Si-0.08Y-0.13Co-0.03Cl-0.15S(at.%), confirming the formation of TiO2, Na2SiO3 and Na2TiO3 once again.Beneath the outer layer, an inner layer with a typical composition of 33.89Al-60.81O-The hot corrosion behavior of TiAl-Nb alloys in the NaCl + K2SO4 molten salt environment has been reported in details in References [20]- [25].Briefly, in the early stage of hot corrosion, preferential corrosion of the α2 phase with O2 would occur due to its high activity [19].The oxides produced during this process then reacted with NaCl to form Cl2 [23].The as-generated Cl2 was considered to be a highly corrosive agent that reacted with the metal elements, leading to the formation of volatile species such as TiCl4, AlCl3.These volatile chlorides with low melting points and high vapor pressures (the boiling point of TiCl4 is 135.9 ℃ and the boiling point of AlCl3 is 178 ℃) would volatilize outward to a region with an appropriate oxygen partial pressure, initiating the oxidation to form oxides and meantime releasing Cl2, S2, SO3 or SO2.Consequently, a large number of voids, holes and even cracks, could be easily generated, creating more path ways for transport of corrosive agents and oxygen.
Therefore, throughout the entire reaction process, chlorine acted as a catalyst, accelerating the failure of the corrosion product scale [24].
In the case of the Si-Co-Y diffusion coating under hot corrosion, the TiSi2 outer layer came into direct contact with corrosive medium of O2, Cl2 and S at the initial stage of hot corrosion, and underwent reactions at high temperatures [25,26]: S2 and Cl2 generated by reactions (19) to (22) can infiltrate into the coating and react with Ti, Si again, resulting in a self-sustaining sulfurization/oxidation process.Thus, S2 and Cl2 can act as catalysts, accelerating the oxidation process of the coating.Fortunately, SiO2 possesses low solubility and good stability in the salts, and can hardly dissolve in the melt as either basic or acidic solutes [27,28].
Additionally, the amorphous silicate can also play a beneficial role to seal the loose TiO2, and enhanced the protective nature of the corrosion product scale.The formed oxides of TiO2, SiO2 (or silicate) etc., can also react with Na2O through the following reactions [29] Therefore, the hot corrosion product scales shown in Figs.7(b1) and Fig. 8 revealed the formation of Na2SiO3 and Na2TiO3.The amorphous silicate generated from SiO2 possesses a certain degree of fluidity at high temperatures and can adhere to the surface, forming a continuous scale that act as the first barrier to enhance the protective performance of the corrosion product scale.
As hot corrosion progresses, both Ti and Al elements underwent outward diffusion.Dudziak et al. [30] have found that diffusion of Ti is slightly faster compared to Al within the temperature range of 750-950 ℃.As a result, Ti reacted with the inward diffusing oxygen first, followed by the reaction of Al with oxygen.This diffusion mechanism effectively explains the dual-layer structure of the corrosion product scale, which consists of an outer layer of TiO2+Na2SiO3+Na2TiO3 and an inner layer of Al2O3.
Once a dense and continuous Al2O3 layer formed in the corrosion product scale, the inward diffusion of corrosive media such as O, S, Cl, etc., will be effectively inhibited.Studies have shown that both Co and Y can promote the selective oxidation of Al to form a continuous Al2O3 protective layer [15,31,32].
During hot corrosion process, there was an external diffusion of Al element from TiAl2 inner layer of the Si-Co-Y coating.Under the synergistic effect of Co and Y, a continuous and dense corrosion product scale mainly composed of Al2O3 layer was formed, Zhou et al. [33] found that the addition of Y can promote the diffusion of Co in during coating formation process, resulting in an even distribution of Co. Wei et al. [5] also have found that Co has the ability to delay the diffusion of S, which slows down the sulfidation-oxidation rate, thereby improving the hot corrosion resistance of the coating.

Conclusion
(1) Si-Co-Y diffusion coating with a dense structure and close bonding with the substrate can be prepared on TiAl-Nb alloy by pack cementation process at 1080 ℃, using NaF as the activator.The activator and deposition temperature imposed obvious influences on the microstructure of the coating: the coating prepared with NaF, AlCl3•6H2O and NH4Cl exhibited higher growth rate compared to the one prepared with NaCl.However, there were many pores formed in the coating prepared with AlCl3•6H2O and NH4Cl.Increase in the deposition temperature within the range of 1050-1100 ℃ led to a higher coating growth rate; but a higher temperature of 1100 ℃ results in the formation of intensive holes in the coating.
(2) The coating prepared with NaF at 1080 ℃ for 4 h is dense and compact, which has a multi-layer structure of (Ti,X)Si2 (X represents Co, Al, Nb, Y) outmost layer, an outer layer composed of TiSi2+Ti5Si4+Ti5Si3 mixtures, a Ti5Si3 middle layer and a TiAl2 inner layer. (

Fig. 2 XRD
Fig. 2 XRD patterns conducted on (a) the surfaces of the Si-Co-Y coatings prepared with different activators and (b) the outer layer of the coating prepared with NaF as the activator.The XRD pattern in Fig. 2(b) was obtained by stripping the coating from its original surface by about 10 μm using 1500 #SiC paper, and then conducted XRD analysis

Figs. 1 and 3 .Fig. 5 Fig. 6 Fig. 6
Figs. 1 and 3.It is worth noting that the cobalt-silicide compounds such as Co2Si, CoSi and CoSi2 can hardly form during this period because the Gibbs formation energies of Co reacting with Ti5Si3 or Ti5Si4 to form these compounds are positive values universally higher than 200 kJ/mol at the employed deposition temperatures.However, once the TiSi2 outmost layer is formed, the formation of cobalt-silicide compounds through Eqs.(6) to (8) became possible.The above analysis revealed that the formation process the Si-Co co-deposited coatings on TiAl-Nb alloy follows an orderly progression of depositing Si first, followed by Co and Y.In this process, the inward react-diffusion of Si into the substrate is the dominated factor for the growth of the co-deposition coating.kJ/mol 588.6 -≈ C) ° (1080 G Δ ; Si Ti = 3Si + 5Ti 3 5Si Ti 3 5 -0.93Ti-0.62Nb-0.11Y-0.28Cl-0.09S(at.%) is observed, indicating the formation of a dense Al2O3 layer.Such an Al2O3 inner layer may be a resulted of the outward diffusion of Al from the TiAl2 layer in the coating, as the TiAl2 layer has undergone a certain degree of degradation compared to the original coating.From the EDS analysis maps, it can be observed that the hot-corrosion product scale primarily contains Ti, Al, O, Nb and Si elements.The presence of S element, which is considered a main invader, is slightly enriched at the interface of the scale and the remained coating.This indicates that only a few S ions have successfully permeated into the scale.Fortunately, there is very limited Cl element, which is considered the main factor causing cracking, can be observed in the scale and the remained coating.This suggests that the inward diffusion of Cl ions has been effectively suppressed.