1 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi′an 710072 2 Sinosteel Xi′an Heavy Machinery Co. Ltd., Xi′an 710077
Cite this article:
Yuxiang ZENG,Xiping GUO,Yanqiang QIAO,Zhongyi NIE. EFFECT OF Zr ADDITION ON MICROSTRUCTURE AND OXIDATION RESISTANCE OF Nb-Ti-Si BASE ULTRAHIGH-TEMPERATURE ALLOYS. Acta Metall Sin, 2015, 51(9): 1049-1058.
Nb-Ti-Si base in situ composites which consist of Nb solid solution (Nbss) and silicides (a-Nb5Si3, b-Nb5Si3, g-Nb5Si3 and/or Nb3Si) phases, have shown great potential as alternative materials to Ni-based superalloys due to their high melting points (beyond 1700 ℃), good formability, low density (6.6~7.2 g/cm3) and high strength. However, a major hindrance to the applications of these alloys at elevated temperatures is their poor oxidation resistance. Alloying is an effective method to improve the integrated properties of the alloys, especially for the oxidation resistance. Up to now, many beneficial elements such as Ti, Al, Cr and Sn have been employed to ameliorate their oxidation resistance. Nevertheless, there is no systematic and comprehensive investigation on the effect of Zr contents on the microstructure and oxidation behavior of the alloys based on Nb-Ti-Si system. The aim of this work is to clarify the effects of Zr contents on phase selection, microstructure and high temperature oxidation resistance of Nb-Ti-Si based alloys in detail. The constituent phases, microstructure and composition of the alloys under as-cast state and after oxidation were investigated by OM, XRD, SEM and EDS. Thus, six Nb-Ti-Si base ultrahigh-temperature alloys with compositions of Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr (x=0, 0.5, 1, 2, 4, 8, atomic fraction, %) were prepared by vacuum non-consumable arc-melting. The results show that the alloys with different Zr contents are mainly composed of Nbss and g-(Nb, X)5Si3 (X represents Ti, Hf, Cr and Zr). However, the addition of Zr has an obvious affect on the microstructure of Nb-Ti-Si base alloys. Both the sizes and amounts of primary g-(Nb, X)5Si3 increase with increase in Zr contents. Alloys with different Zr contents were oxidized at 1250 ℃ for 1~50 h, respectively. It is found that both adhesion and compactness of the scales are improved effectively by increase in Zr contents. The scales of alloys with higher Zr contents (x=4 and 8) after oxidation for 50 h show an obvious layered structure: the outmost layer is only composed of TiO2, the middle layer mainly consists of ZrO2, TiNb2O7 and TiO2, and the inner layer is mainly comprised of Si-rich oxides. The mass gain per unit area and the thickness of the scale after oxidation decrease with increase in Zr contents in the alloys, indicating that the addition of Zr can improve the oxidation resistance of the alloys significantly.
Fig.1 XRD spectra of the as-cast Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr alloys (0Zr, 0.5Zr, 1Zr, 2Zr, 4Zr and 8Zr represent x=0, x=0.5, x=1, x=2, x=4 and x=8 alloys, respectively. Nbss is Nb solid solution)
Alloy
Phase
Nb
Ti
Si
Cr
Al
Hf
Zr
0Zr
Nbss
69.4
19.8
1.7
5.1
3.0
1.0
-
g-(Nb, X)5Si3
41.5
17.3
33.9
1.2
2.2
3.9
-
2Zr
Nbss
60.7
24.1
1.6
8.3
3.1
1.8
0.4
g-(Nb, X)5Si3
40.1
15.0
35.1
0.4
2.5
3.9
3.0
8Zr
Nbss
66.8
20.4
1.5
4.5
3.1
1.5
2.2
g-(Nb, X)5Si3
25.9
16.0
34.9
0.7
3.0
4.0
15.5
Table 1 EDS analysis of chemical compositions of Nbss and g-(Nb, X)5Si3 in 0Zr, 2Zr and 8Zr alloys
Fig.2 BSE images of the as-cast 0Zr (a), 0.5Zr (b), 1Zr (c), 2Zr (d), 4Zr (e) and 8Zr (f) alloys (The plus symbol in Fig.2e indicates a three-phase eutectic zone with low melting point)
Fig.3 Isothermal oxidation kinetics curves of alloys with different Zr contents at 1250 ℃ (t—time)
Fig.4 Macro-morphologies of 0Zr (a), 0.5Zr (b), 1Zr (c), 2Zr (d), 4Zr (e) and 8Zr (f) alloys oxidized at 1250 ℃ for 50 h
Alloy
Phase
O
Nb
Ti
Si
Cr
Al
Hf
Zr
0Zr
Nbss
56.6
31.3
8.6
0.1
1.9
1.2
0.3
-
g-(Nb, X)5Si3
20.9
36.1
11.4
26.2
0.8
1.9
2.7
-
2Zr
Nbss
53.0
32.5
10.2
0.1
2.1
1.3
0.3
0.5
g-(Nb, X)5Si3
19.3
31.2
14.5
26.9
1.2
2.0
3.4
1.5
8Zr
Nbss
52.5
31.5
2.4
0.4
4.0
6.8
0.5
1.9
g-(Nb, X)5Si3
14.6
29.1
10.8
29.0
0.3
2.3
2.7
11.2
Table 2 EDS analysis of chemical composition of Nbss and g-(Nb, X)5Si3 in internal oxidation zones of 0Zr, 2Zr and 8Zr alloys oxidized at 1250 ℃ for 5 h in Fig.6
Fig.5 XRD spectra of scales of 0Zr, 2Zr and 8Zr alloys oxidized at 1250 ℃ for 1 h (a), 5 h (b) and 20 h (c)
Fig.6 BSE images of cross-sections of scales (a, c, e) and internal oxidation zones (b, d, f) of 0Zr (a, b), 2Zr (c, d) and 8Zr (e, f) alloys oxidized at 1250 ℃ for 5 h
Fig.7 XRD spectra of scales of 0Zr, 2Zr and 8Zr alloys oxidized at 1250 ℃ for 50 h
Alloy
Point
Phase
O
Nb
Ti
Si
Al
Cr
Hf
Zr
0Zr
1
Ti-rich oxide
73.7
9.0
9.4
1.5
1.4
3.9
1.1
-
2
Ti2Nb10O29
75.4
16.5
4.4
2.2
0.5
0.3
0.7
-
2Zr
3
Ti2Nb10O29
69.6
21.2
6.3
0.1
0.5
0.6
0.7
1.0
4
Si-rich oxide
74.3
9.8
2.8
11.6
0.3
0.2
0.5
0.5
5
Ti-rich oxide
67.4
9.8
12.6
0.5
2.1
5.4
1.3
0.9
8Zr
6
TiO2
71.7
8.1
11.6
0.1
1.0
5.0
0.6
1.9
7
ZrO2
72.6
6.2
1.0
5.7
0.9
0.2
2.8
10.6
8
TiNb2O7
72.4
15.4
9.5
0.1
0.4
0.1
0.5
1.6
9
Si-rich oxide
73.4
4.5
1.4
17.9
2.0
0.2
0.1
0.5
Table 3 EDS analysis of chemical compositions of phases in scales and internal oxidation zones of 0Zr, 2Zr and 8Zr alloys oxidized at 1250 ℃ for 50 h corresponding to points 1~9 in Fig.8
Fig.8 BSE images of cross-sections of scales of 0Zr (a, b), 2Zr (c, d) and 8Zr (e, f) alloys after oxidation at 1250 ℃ for 50 h at low (a, c, e) and high (b, d , f) magnification
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