Influence of Thermal Exposure at 700oC on the Microstructure and Morphology in the Surface of β-Solidifying γ-TiAl Alloys
LIU Renci1(), WANG Peng1,2, CAO Ruxin1,2, NI Mingjie1,2, LIU Dong1, CUI Yuyou1, YANG Rui1
1.Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
LIU Renci, WANG Peng, CAO Ruxin, NI Mingjie, LIU Dong, CUI Yuyou, YANG Rui. Influence of Thermal Exposure at 700oC on the Microstructure and Morphology in the Surface of β-Solidifying γ-TiAl Alloys. Acta Metall Sin, 2022, 58(8): 1003-1012.
β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B has attracted considerable attention owing to its higher strength and excellent creep resistance at elevated temperature. Indeed, its application temperature is much higher than that of Ti-48Al-2Cr-2Nb. Because γ-TiAl alloys are exposed to air at elevated temperatures for a long time during application, an oxidation layer is formed in the surface. The oxidation layer, which is potentially harmful to the mechanical properties of the crack nucleation sites, was observed near the surface. Concerning the β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B, it has median Nb content and low Al content. Additionally, a considerable β0-phase with lower Al content is retained. To better understand the influence of the composition and microstructure on the oxidation behavior of γ-TiAl alloys, it is necessary to investigate the oxidation behavior and microstructure evolution in the surface of β-solidifying γ-TiAl alloys during thermal exposure. In this study, samples of β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B were obtained by investment casting and thermal exposure at 700°C for different times, and the oxidation behavior and microstructure of different phases in the surface were compared. The results showed that the constituents of the oxidation layer on the surface varied with the exposure time. The volume fractions of TiO2-R, α-Al2O3, Ti2AlN, and Nb2Al increased by increasing the exposure time. Metastable κ-Al2O3 was detected in the sample exposed for a short time, but it was transformed into α-Al2O3 after exposure for 200 h. Moreover, metastable Ti4O7 and TiAl2O5 were detected in samples exposed for 200 and 500 h. The microstructures, morphologies, and heights of oxidations in the surface of a specific phase are different, varying by increasing the exposure time. These variations are related to the different oxidation behaviors during thermal exposure, i.e., the γ-phase experienced selective oxidation after a short time exposure, α2-phase changed from internal oxidation to selective oxidation when the exposure time reached 200 h, while the β0-phase suffered internal oxidation during the entire exposure. The different oxidation behaviors of each specific phase contributed to the different Al contents. Dispersed TiO2 was formed during internal oxidation, and it kept growing during thermal exposure, forming a continual layer at the end. The continual Al2O3 layer was formed during selective oxidation, in which the Ti element was rejected in the reaction interface. When the content produced during the internal oxidation of the Ti element reached a critical value, dispersed TiO2 was formed and kept growing to form the continual layer. The alternating formation of continual Al2O3 and TiO2 layers resulted in the layer structure observed in the surface.
Table 1 Chemical composition analysis results of Ti-43.5Al-4Nb-1Mo-0.5B (TNM-0.5B) alloy casting
Fig.1 OM (a) and backscattered electron (BSE) (b) images of initial microstructures of TNM-0.5B alloy
Position
Specific constitute
Al
Ti
Nb
Mo
B
1
Equiaxed β0
20.64
60.96
11.86
6.54
0
2
Equiaxed γ
31.20
56.00
11.10
1.70
0
3
α2 + γ lamellar
27.55
59.54
9.96
2.95
0
4
Boride
8.06
43.91
13.98
3.17
30.88
Table 2 Chemical composition of different phases in the initial microstructure of TNM-0.5B alloy in Fig.1b
Fig.2 XRD spectra in the surface of samples thermal exposed at 700oC in air for different time
Fig.3 Secondary electron images of sample surfaces after thermal exposed at 700oC for 0.5 h (a), 10 h (b), 200 h (c), and 500 h (d)
Position
Phase
Constitute
O
Al
Ti
Nb
Mo
1
Equiaxed β0
30.96
16.93
44.11
6.33
1.67
2
Equiaxed γ
21.99
22.99
47.11
7.05
0.85
3
γ lamellar
24.47
20.75
46.51
6.95
1.33
4
α2 lamellar
28.67
18.28
45.35
6.27
1.43
5
γ lamellar
42.31
18.37
34.29
4.50
0.53
6
α2 lamellar
46.86
19.76
28.79
3.82
0.77
7
Equiaxed β0
51.00
4.38
43.96
0.66
0.00
8
Equiaxed γ
44.42
14.32
37.00
3.74
0.52
Table 3 Chemical composition of oxide in the surface of exposed samples in Fig.3
Fig.4 Surface profiles of samples thermal exposed at 700oC in air for 0 h (a), 100 h (b), 200 h (c), and 500 h (d)
Fig.5 Linear profiles (a) and heights of specific phase (b) in the surface of samples thermal exposed at 700oC in air for different time
Fig.6 OM images of microstructures in the surface of samples thermal exposed at 700oC in air for 10 h (a), 100 h (b), 200 h (c), and 500 h (d) (Insets in Fig.6b show the γ and β0 phase layer characteristics of oxidation front, respectively)
Fig.7 BSE images of microstructures near the reaction interface of samples thermal exposed for 10 h (a), 100 h (b), and 500 h (c), and element distributions of Al (d), Ti (e), O (f), N (g), Mo (h), and Nb (i) near the interface of sample thermal exposed for 500 h (Main elements in specific layer of oxidation were summarized in the right of Fig.7f; rectangulars in Fig.7d, e, h, and i show the main elements distributions near the reaction interface of β0 phase)
Fig.8 Evolution mechanism of microstructure, composition distribution, and morphology near the reaction interface of TNM-0.5B alloy during thermal exposure for 0 h (a), 10 h (b), 100 h (c), 200 h (d), and 500 h (e)
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