Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys

Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys

Materials Science and Engineering A329– 331 (2002) 713– 717 www.elsevier.com/locate/msea Microstructure, wear and high temperature oxidation resistan...

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Materials Science and Engineering A329– 331 (2002) 713– 717 www.elsevier.com/locate/msea

Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys J. Sun *, J.S. Wu, B. Zhao, F. Wang Key Laboratory for High Temperature Materials and Tests of Ministry of Education, School of Materials Science and Engineering, Shanghai Jiao-Tong Uni6ersity, Shanghai 200030, People’s Republic of China

Abstract Gas nitridation of the TiAl based alloys in ammonia atmosphere was investigated in the present work. The scales of the nitrided alloys were characterized by X-ray diffraction (XRD) and electron probe microanalysis (EPMA). The evaluation of the surface hardness, wear and high-temperature oxidation resistance of the nitrided TiAl alloy was performed to compare with those of non-nitrided alloys. The results showed that the nitrided films composed of Ti2AlN as the inner layer and TiN as the outer layer on the surface of the alloys. Mechanical tests showed that the nitridation could obviously increase the surface hardness and the sliding wear resistance of the TiAl alloys. However, owing to the limited thermodynamical stability of the nitrides at high temperature in air, nitridation prior to the oxidation increased the high temperature oxidation rates of the TiAl alloys. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium aluminides; Tribological properties; Oxidation; Coating

1. Introduction Titanium aluminide based alloys have been developed for use in structural and aerospace applications due to their attractive properties, such as low density, high modulus and strength at elevated temperatures [1,2]. Besides the improvement of the ductility and strength by control of microstructure and alloying, much attention has also been paid to enhancement of the wear resistance of the TiAl based alloys by surface coating [3–6]. Among various coating methods, nitriding technique is commonly used to improve the wear resistance of metals and alloys. Meanwhile, owing to the thermodynamical stability of TiN, TiN films can be formed on the surface of the TiAl based alloys by direct nitridation. Recently, ion nitriding and gas nitriding of the TiAl alloys have been reported, respectively [7–9]. However, the wear properties and high temperature oxidation resistance of the TiAl alloys nitrided by these common techniques were not well investigated. Because atomic nitrogen decomposed from ammonia (NH3) is a more effective agent than molecular nitrogen (N2), gas * Corresponding author. E-mail address: [email protected] (J. Sun).

nitridation of the TiAl based alloys in ammonia atmosphere is investigated in the present work. Emphasis is placed on the evaluation of the wear properties and high-temperature oxidation resistance of the nitrided TiAl alloys. The relationships between the structure of the nitridation film and the wear properties and the high temperature oxidation resistance of the TiAl alloys are also discussed.

2. Experimental A conventional tungsten arc-melting technique was employed to prepare the alloy with a composition of Ti–47Al –2Nb –2Cr –0.2Si (at.%) in argon atmosphere. The alloys were melted and re-melted at least four times. Homogenizing annealing of the cast ingots was performed at 1000 °C for 100 h. The alloys were cut into samples and polished to a 0.3 mm surface finish for the nitriding tests. The samples were cleaned with acetone prior to introduction into the nitriding furnace. The nitriding of the TiAl based alloys was performed in a reaction chamber consisted of a quartz tube set inside a tube furnace held at 840 and 960 °C for different time (10 –50 h). The tube was filled with ammonia

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flowing at a rate of 2× 10 − 2 mm3 h − 1 for an hour, then heated to the desired temperatures. After nitriding, X-ray diffraction (XRD) and electron probe microanalysis (EPMA) were used to identify the phases present in the nitrided films. Also, the Knoop hardness of the alloys after nitridation was determined at 25 g load.

The wear properties of the nitrided films were evaluated on an Amsler test machine using a block-on-ring setup. The counterface ring was made of steel coated by a hard layer with a hardness of HRC 65. The load for wear test was 1.3 kg and the sliding speed was 523 mm s − 1 under unlubricated condition. In order to examine the high temperature oxidation resistance of the alloys after nitridation, isothermal oxidation was performed in air for 10–100 h at 900 and 1000 °C. The weight gains during the high-temperature oxidation was plotted vs. oxidation time. XRD and EPMA were also used to characterize the oxides formed on the surface of the TiAl alloys.

3. Results and discussion

3.1. Nitridation

Fig. 1. The XRD pattern of the TiAl-based alloys nitrided in ammonia atmosphere at 940 °C for 50 h.

Nitrided films were detected on the surface of the TiAl based alloys at all temperatures for different periods. The typical XRD pattern of the film on the alloys is shown in Fig. 1. It is seen that besides the diffraction peaks of the constituents (TiAl and Ti3Al) of the alloys, diffraction peaks of the TiN and Ti2AlN occurred in the XRD pattern. Fig. 2 shows the back scattered electron images and the compositional line scan of the nitriding film cross-section of the alloys treated at 940 °C for 50 h, where Ti2AlN could be distinguished as the inner layer formed under the TiN outer layer. Chu et al. had investigated the ion nitriding of the TiAl alloys and they also found that Ti2AlN layer formed under the outer layer of TiN [7]. A similar phenomenon was confirmed in the nitriding of TiAl alloys in nitrogen gas and solubility of Al was detected in the TiN outer layer [9]. In addition, AlN and other new Al-rich intermetallic compounds, such as Al2Ti and Al3Ti in the sub-scale were not found at the nitrides/matrix interface in the present work. Generally, the thickness of the films increased mainly with increase of treating temperatures and also with increase of time. The nitriding behaviors of TiAl alloys in ammonia atmosphere could be explained by the mechanism proposed by these authors, in the case of the ion-nitriding and N2-gas nitriding [7,9].

3.2. Hardness and wear properties

Fig. 2. SEM cross-sectional micrograph (a) and EPMA compositional analyses for Ti, Al and N (b) of the nitrided films on the TiAl-based alloys.

The variations of the Knoop hardness of the nitrided TiAl based alloys vs. nitriding temperatures and nitriding time were plotted in Fig. 3. Compared with the non-nitrided alloy, the Knoop hardness of the nitrided alloys was markedly increased with the nitriding temperature and time. Increase in hardness resulted from the hard TiN and Ti2AlN layers on the surface of the alloys. As shown in Fig. 2, the maximum thickness of

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Fig. 3. Knoop hardness of the TiAl alloys versus nitridation time at 860 and 940 °C.

Fig. 4. Comparison of the weight loss of the nitrided TiAl alloys after gas nitridation at 940 (a) and 860 °C (b), and of the non-nitrided TiAl alloy (c).

Fig. 5. Weight gains versus oxidation time for the nitrided TiAl and non-nitrided alloys exposure at 900 and 1000 °C.

the nitrided films in the present work was about 4 mm. On the other hand, the depth of the indenter trace was estimated to be  0.9 mm at 25 g load. The ratio of the film thickness to the trace depth was less than 5. Thus, the apparent hardness of the nitrided films was influenced by the soft substrate of TiAl based alloys, and was considered to be proportional to the thickness of the nitided films, which was related to the temperatures and time of nitridation. When the nitridation time prolonged to 50 h at temperature of 940 °C, the Knoop hardness value of 1286 kg mm − 2 was obtained in the present work.

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Wear properties are generally related to the surface hardness of the alloys. Formation of nitrided films on the surface of the alloys increased the Knoop hardness and was also thought to be able to improve the wear resistance of the alloys. Fig. 4 shows the wear properties of the TiAl alloy nitrided at 860 and 940 °C for 50 h, respectively, and of the non-nitrided TiAl alloys. It can be seen that the weight loss of the nitrided alloy was much lower than that of the non-nitrided alloy at sliding speed of 523 mm s − 1 for 10 min, which means gas nitridation increased the wear resistance of the TiAl based alloys. The wear resistance of the nitrided alloys also increased with an increase of the nitridation temperature. After the wear tests, observations of the morphology of the wear tracks showed a large number of parallel traces with different depth on the non-nitrided alloys and the worn protrusions along the tracks on the nitrided TiAl alloys, respectively. The results in this work are consistent with those in the literature [3,8]. It is noteworthy that the hydrogen decomposed from ammonia might diffuse into the TiAl alloys, which would have an influence on the mechanical properties of the alloys. This is required to be further investigated.

3.3. High-temperature oxidation resistance As mentioned above, the gas nitridation can markedly improve the knoop hardness and the wear properties of TiAl based alloys. However, it had been reported that during the oxidation process in air, nitrides consisting of Ti2AlN and TiN formed at scale– matrix interface has a deleterious effect on the oxidation resistance of TiAl based alloys [10,11]. Therefore, the high-temperature oxidation resistance of the nitrided TiAl alloys is required to be examined. The weight gains for the oxidation of the nitrided TiAl alloys at 900 and 1000 °C vs. exposure time were plotted in Fig. 5. The weight gain data for non-nitrided alloys was also illustrated in Fig. 5 for comparison purposes. The nitrided TiAl alloys gained much more weight than the non-nitrided alloys at 900 and 1000 °C. Spallation of the scale was found on the surface of both alloys after 100 h exposure at these temperatures, especially in the nitrided alloys. The phase constitutions of the scale on the alloys were determined by XRD as shown in Fig. 6. After exposure at 1000 °C, the intensity of the diffraction peaks of TiO2 was much higher than the that of Al2O3 in the nitrided TiAl alloys, which means that the scale formed on the nitrided alloys consisted of TiO2 as the major phase and Al2O3 as the minor phase. TiN and Ti2AlN were not detected on the surface of the alloys. However, after exposure at same temperature, the height-to-height ratio between the peaks from TiO2 and Al2O3 reduced for the non-nitrided alloys, compared with the nitrided alloys. Also, no TiN and Ti2AlN were

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detected for the non-nitrided alloys. Fig. 7 shows the images and the compositional line scan of the cross sections of the oxide layer on the nitrided and untreated alloys after exposure at 1000 °C. Cross-sectional analyses showed that the oxide scale of the nitrided alloys was wide than that of the non-nitrided alloys. Although the outer layer was Al rich and Ti depleted for both alloys, more Al enrichment in outer layer was observed for the non-nitrided alloys than the nitrided alloys, which conformed to the results of XRD analyses. This also suggested that Al2O3 formed in the outer layer, whereas TiO2 formed in the inner layer for both TiAl alloys. Therefore, the high oxidation rates of the nitrided TiAl alloys could be attributed to the reduced amount of Al2O3 in the outer layer of the oxide scale. Several authors had investigated the oxidation behaviors of the TiAl based alloys exposure to air [10,11]. They found that formation of TiN and Ti2AlN at the scale– matrix interface prevents alumina from forming a continuous protective layer and increased the oxidation rate of the alloys. A thermodynamic analyses conducted by Roy et al. [12] showed that titanium oxide was more thermodynamically stable than TiN, so the nitrides could eventually be transformed into titanium oxide during the oxidation process. The layer of TiO2

generated by this process prevented the formation of the layer of Al2O3. According to these, the presence of nitrides, such as TiN and Ti2AlN, on the surface of the TiAl alloys prior to the oxidation would accelerate the transformation process described above, which resulted in the higher oxidation rates of the nitrided TiAl alloys, compared with the non-nitrided alloys.

4. Summary The gas nitriding of the TiAl based alloys in ammonia atmosphere has been investigated in the present work. The nitrided films composed of Ti2AlN as the inner layer and TiN as the outer layer on the surface of the alloys and the thickness of the nitriding films increased mainly with the increasing of nitriding temperatures and also of nitriding time. Mechanical tests showed that the nitridation could obviously increase the surface hardness and the sliding wear resistance of the TiAl alloys. However, owing to the limited thermodynamical stability of the nitrides at high temperature in air, nitridation prior to the oxidation accelerated the high temperature oxidation process of the TiAl alloys compared with the non-nitrided alloys.

Fig. 6. XRD analyses of the oxide scales of the nitrided TiAl (a) and non-nitrided alloys (b) exposure at 1000 °C for 100 h.

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Fig. 7. SEM cross-sectional micrographs and EPMA analyses of the oxide scales of the nitrided TiAl (a, b) and non-nitrided alloys (c, d) exposure at 1000 °C for 100 h.

Acknowledgements This work is sponsored by the Science and Technology Commission of the Shanghai Municipal Government.

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