TiAl intermetallic alloys based on the γ-phase are being investigated as promising aerospace and automotive materials, which have comparative mechanical properties with nickel-based superalloy, while its density is only half of the latter, and thus becomes the attractive structural material for high-temperature application [1, 2, 3]. The application of TiAl alloys in aero-engine would be beneficial to weight reducing of machine parts and the enhancing of thrust load. However, the poor oxidation resistances of TiAl alloy are the major stumbling blocks for the application at high temperature (>700 °C), and still need to be improved to meet the performance of components such as turbine plates and compressor parts of advanced gas turbine engines [4, 5]. It is known that classical bulk alloying methods are deleterious to substrate properties, and therefore, various coatings have been applied on TiAl alloys to extend the oxidation lifetime, because the surface modification brings little effect on the mechanical performance of the alloy substrates [6, 7].
The double-glow plasma surface alloying technology, also known as Xu-Tec process, is an innovative method developed on the basis of the plasma nitriding and sputtering . It can effectively improve the surface performance, such as microhardness, wear resistance, corrosion resistance and oxidation resistance of metals or alloys. Our previous research has revealed that double-glow plasma surface chromizing and molybdenumizing was an effective way to improve the high-temperature oxidation resistance of titanium alloy. The effect of Cr on high-temperature oxidation in titanium alloy depends on the content. When the content of Cr is low, Cr will dissolve in TiO2 lattice to improve the diffusion speed of O, Ti and Cr ions in TiO2 lattice. When the content is more than the maximum solubility of Cr in TiO2, Cr will form an oxidation film on the surface of TiO2 to hinder the O ions transport . However, Mo is the common alloying element to make stable the β phase of titanium alloy. And because of the same lattice structural with β-Ti and unlimited solid solution in titanium alloy, Mo has a good solution strengthening effect on titanium alloy [10, 11].
Based on the researches mentioned above, our work presents a study of the oxidation resistance of Cr–Mo alloyed layer using double-glow plasma surface alloying technology on γ-TiAl alloy. The purposes were observing the evolution of morphology and constituent of the oxide scale during the isothermal tests, tracking the difference brought by the addition of Cr and Mo, and the mechanism responsible for the improvement in oxidation resistance after double-glow plasma surface alloying was also discussed for industrial applications.
Materials and methods
The casting TiAl alloy for this study was produced by induction shell melting and the chemical composition of this alloy used in this experiment was given in Table 1. The ingot was cut into 14 mm × 14 mm × 3 mm. The γ-TiAl alloy sample was used as the substrate material (the cathode). The Cr–Mo (50Cr–50Mo, wt.%) plate (Φ100 mm × 5 mm) was used as the target for supplying the alloying element (the source).
The principle of double-glow plasma surface alloying was as follows : a target electrode (TE) and a substrate electrode (SE) were mounted in a vacuum chamber (anode). The anode and the target (TE) were connected to pulse power supplies, and the anode and the substrate (SE) were connected to DC power supplies. The potential difference between the substrate and the target resulted in an unequal electronic potential hollow cathode effect. Once heated to a given temperature, the ions or atoms sputtered from the target were deposited on the substrate due to the negative bias which was lower than that of the target. The ions and atoms were diffused into the matrix at the elevated temperature; thus, an alloyed layer was developed on the substrate.
The parameters of double-glow plasma surface alloying treatment were shown in Table 2. Cr and Mo atoms or ions from the target due to Ar+ bombardment travel to the TiAl surface and diffuse into it to form a surface-alloyed layer. Prior to the alloying treatment, the TiAl samples were bombarded at the gas pressure of 20 Pa for 15 min for surface cleaning or activation, and then heated for 3 h at 48 Pa for the alloying treatment when the temperature was kept in the range of 900–1,000 °C stably.
Microstructural analysis and tests
After double-glow plasma surface alloying treatments, the samples were sectioned, mounted, polished and etched in a solution of 7 vol.% HNO3, 0.6 vol.% HF for metallographic examination. The surface morphology was observed by the MM-6 horizontal metallurgical microscope with the image analysis system. The surface and cross-sectional microstructures were investigated by Germany LEO-type field emission scanning electron microscopy, and the alloying elements distribution were measured by means of energy spectrum analysis attached. The phase identification was determined by Bruker D8-ADVANCE X-ray diffraction with Cu Kα radiation over a range of 2Ɵ from 10° to 90°. The test was not done by glancing angle.
Isothermal oxidation tests
The oxidation tests were performed in a high-temperature box resistance furnace in a static air atmosphere at constant 850 °C. The method was widely used for inspecting the oxidation resistance performance and the coating adhesion at the elevated temperature under air atmosphere. However, the sets of period length and oxidation temperature had no uniform standard according to different typed specimen .
Prior to oxidation test, porcelain crucibles used to hold specimens were heated for 2 h at 900 °C to remove the moisture and impurities. The total oxidation time was 100 h and the oxidized specimens were periodically weighted for ten times at a 10 h interval using a Precision Electronic Balance with an accuracy of 10−4 g. Three specimens with their own holding crucibles for one group were weighed, recorded and calculated to obtain the average mass gain values per every group. It should be pointed out that the possible stripping fractions of the surface oxides during the oxidation tests were held in the crucibles and included in the mass gain values. A period was chosen to visually inspect whether the treated surface spalling occurred, to generally evaluate the adhesion performance under the high-temperature oxidation condition. The average mass gain values obtained after 100 h were used to characterize the oxidation kinetics. The surface morphologies and cross-sectional microstructures of corroded specimens were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray powder diffraction (XRD) to identify phases of oxide scales.
Results and discussion
Surface microstructure and organization analysis
SEM micrograph and chemical composition of the treated surface was shown in Figure 1. It can be found that the surface demonstrated a dense and fine cellular structure. However, the particles were not very uniform, maybe it was due to their nucleating and growing up procedures . From the result by EDS, the surface was rich in chromium and molybdenum elements, meanwhile there were a small amount of titanium and aluminum. According to the XRD analysis shown in Figure 2(b), the alloyed surface consisted of Al8Cr5, Al5Mo, TiCr as the major phases and TiAl as the minor phase. In the meantime, the bare TiAl substrate consisted of γ-TiAl and α-Ti3Al as the major phases (Figure 2(a)). The initial bombardment of the Ar ions leaded to a decrease of substrate elements in the outmost surface and an amount of cavities were generated as well. These defeats in the top surface caused Cr and Mo diffusing easily into the substrate and the elements of Al and Ti diffusing off the substrate at high temperature simultaneously. The formation of the alloyed layer was mainly attributed to the diffusion of Cr and Mo by an inward growth mechanism possibly [15, 16].
Cross-sectional morphology and organization analysis
Figure 3 shows the results of cross-sectional scanning and composition distribution of the treated sample. The cross-sectional microstructure of the treated sample was shown in Figure 3(a). It can be found that the alloyed layer was continuous and compact. The thickness of the coating was approximated to 6 µm. The interface was characterized by the metallurgical to adhere with the substrate well and no porosity was found. Figure 3(b) shows the composition distribution along the depth of the cross section of the treated sample. It can be found that the alloying elements Cr and Mo were successfully formed on the surface. The diagram shows the Cr and Mo contents increasing gradually from the substrate to the surface, and there was gradient distribution throughout the alloyed layer.
Isothermal oxidation analysis
The isothermal oxidation kinetic curve for untreated and treated specimens was shown in Figure 4. It is clear that after exposure at 850 °C for 100 h, the mass gain for Cr–Mo alloyed samples was 3.2 mg/cm2, which was much lower than that of bare TiAl (9.1 mg/cm2), and the oxidation kinetics of the coated specimens approximately followed the parabolic rate law at all temperatures, indicating a better oxidation resistance.
The surface morphology of bare TiAl substrate after oxidation for 100 h was shown in Figure 5(a) and it appears as a loose-scale structure. The oxide scale formed on the bare TiAl was peeling off severely. The magnified view area, as shown in Figure 5(b), the outermost scale for bare TiAl specimen, was a Ti-rich scale by the EDS analysis. The total weight percent of Ti and O was higher than 82 wt.% in the composition. The titanium oxide formed on bare TiAl grew quickly to a big size at 850 °C, which exhibited a coarse and porous columnar structure. Meanwhile, XRD analysis in Figure 7(a) indicated that the scale was mainly TiO2 (rutile)-dominated mixture, with a small amount of Al2O3. As the oxidation went on, the composition of the scale maintained as the mixture of TiO2 and Al2O3, while the delamination became aggravated, and cracking and peeling were repeatedly observed.
In the initial stage of oxidation, a thin and discontinuous Al2O3 scale with a small amount of TiO2 was formed on the bare TiAl surface due to the low free energy of Al2O3. The activity of aluminum is negatively deviated from its concentration, and TiO2 is superior to Al2O3 in growth kinetics. With the development of oxidation, TiO2 crystals grew at the interface between the scale and the substrate when the flux of oxygen diffusion through the Al2O3 scale exceeded that of Al diffusion toward the interface. Then TiO2 soon became the main constituent of scale as the time prolonged . The preferential oxidation of Ti resulted in the enrichment of Al beneath the TiO2 dominant outer layer, which was prone to cracking due to a loose and porous rutile structure. And the preliminary Al2O3 layer was broken under a compressive stress produced by TiO2 grains, which induced a multilayer structure: outer TiO2 layer/Al2O3 + TiO2 layer/Al-rich layer. When the oxide scale grew to a critical thickness, it began to crack due to a stress produced by different thermal expansion coefficients between the oxide scale and the substrate. The alternation of formation and spallation of the oxide scale led to a linear growth with the exposure time. The observed oxidation process was also consistent with other reports [6, 18, 19].
The surface morphology of Cr–Mo alloying layer after oxidation for 100 h was shown in Figure 6. However, at the starting stage for the treaded sample, Cr2O3 and MoO3 were the main body of the oxide scale due to the high Cr and Mo concentration at the top surface of the alloyed layer. TiO2 then joined before Al2O3 because of its superiority in the concentration and kinetics. As the oxidation developed, the scale turned into a mixture of Cr2O3, MoO3, Al2O3 and TiO2, and the proportions of Al2O3 and TiO2 changed alternatively just like what occurred in the oxidation process of bare TiAl alloy for the similar reason. The melting point of MoO3 is 795 °C , so when oxidized for 100 h at 850 °C, the oxide scale finally became a mixture of Cr2O3, Al2O3 and TiO2, as that was traced out by the XRD result in Figure 7.
Then Cr2O3 was also a protective oxide like Al2O3, but its stability was inferior to the latter. Cr2O3 had a considerable mutual solubility with Al2O3 due to their similar corundum structures. Therefore, the contribution of Cr2O3 to the integrity of oxide scale was also vital for the improvement of oxidation resistance. In addition, it also had almost equal PBR (Pilling–Bedworth ratio) with TiO2. These were beneficial to reduce the brittleness of oxide scale and avoid the cracking and peeling off. And some part of the alloyed layer was still solid solution of Cr in TiAl and the solute Cr functioned as doping element, which could prevent the permeation of oxygen into TiAl substrate and promote the preferential oxidation of Al [20, 21]. Then it provided a protective function. Although the affinity to oxygen for Al was higher than that for Mo, the element of Mo due to its enrichment which could substitute for Ti in the surface, together with Al, was oxidized to form MoO3 and Al2O3, respectively. Because MoO3 would evaporate during the oxidation, Al2O3 was retained in the surface simultaneously, Ti transported toward the surface and reacted with oxygen anions to form TiO2, resulting in an Al2O3-rich scale created on the alloyed surface. The growth of the oxide scale was dependent on the inward diffusion of oxygen as soon as the formation of thin and continuous Al2O3 scale. Obviously, it is more difficult for the oxygen anions to transport through Al2O3 scale than TiO2 grains because Al2O3 contains less defects and paths. Consequently, the effect of Mo addition on promoting the formation of Al2O3 scale was significant and at the same time a low oxidation rate was obtained [11, 22].
A Cr–Mo alloyed layer was successfully formed on the TiAl alloy surface by the double-glow plasma surface alloying technique. The alloyed layer mainly consisted of Al8Cr5, Al5Mo, TiCr and TiAl phases, which was continuous and compact. The alloyed TiAl showed a better isothermal oxidation resistance at 850 °C in air for 100 h. The introducing of Cr and Mo into the TiAl alloy changed the formation and composition of oxides scale, and therefore the oxidation mechanism. A protective and adherent oxide scale was an Al2O3, Cr2O3 layer mixed with TiO2 grains. The protection function against the high-temperature oxidation could be attributed to the strong adhesion of oxides mixture, as well as the reduction of the oxygen solubility in the alloyed layer, and finally as solute doping elements to promote the growth of protective Al2O3.
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About the article
Published Online: 2016-06-23
Published in Print: 2017-07-26
This project was supported by the National Natural Science Foundation of China (grant no. 51175247), the Natural Science Foundation for Young Scientists of Jiangsu Province, China (grant no. BK20140819), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.