Impact of Si addition on high-temperature oxidation behavior of NiAlHf alloys

doi:10.1016/j.jmst.2019.04.023

Abstract

Abstract:

Silicon (Si) and reactive elements (REs) like hafnium (Hf) have favorable effects on improving the high-temperature oxidation resistance. To reveal the interaction effect between them, Si with different contents was added into β-NiAlHf alloy in this study. Cyclic oxidation behavior at 1200 °C was investigated. Results show that Si can increase the Hf-rich precipitates and suppress Hf outward diffusion, thus inhibit the RE effect leading to worse oxidation resistance. However, NiAlHf-5Si alloy exhibited a lower oxidation rate and better spallation resistance mainly because of the existence of micro-cracks and Ni₂Al₃ phases which provided more rapid diffusion paths for Al³⁺.

Key words: Intermetallic NiAl, Rare earth element, Thermal cycling, Oxidation, Segregation

Jing Jing, Jian He, Hongbo Guo. Impact of Si addition on high-temperature oxidation behavior of NiAlHf alloys[J]. J. Mater. Sci. Technol., 2019, 35(9): 2038-2047.

Figures/Tables 12

Table 1 Actual chemical compositions of as-annealed alloys identified by EPMA analysis (at.%).

Alloy	Ni	Al	Si	Hf
NiAlHf	53.36	46.64	—	—^a
NiAlHf-1Si	51.96	46.99	0.98	—^a
NiAlHf-3Si	51.58	45.93	2.49	—^a
NiAlHf-5Si	49.80	45.27	4.93	—^a

Fig. 1. XRD patterns of NiAlHf, NiAlHf-1Si, NiAlHf-3Si, NiAlHf-5Si alloys after annealing at 1300 °C for 24 h.

Fig. 2. Back-scattered electron surface images of the heat-treated alloys: (a) NiAlHf; (b) NiAlHf-1Si; (c) NiAlHf-3Si; (d) NiAlHf-5Si. The insets are the high magnification images in each alloy.

Table 2 Chemical compositions of white Hf-rich areas in Fig. 2 identified by EDS (at.%).

H	Ni	Al	Si	Hf
NiAlHf	38.54	22.32	—	39.14
NiAlHf-1Si	36.88	—	35.95	27.17
NiAlHf-3Si	36.16	—	38.86	24.98
NiAlHf-5Si	34.42	—	28.84	36.75

Fig. 3. Metallographic morphologies of heat-treated alloys: (a) NiAlHf; (b) NiAlHf-1Si; (c) NiAlHf-3Si; (d) NiAlHf-5Si. The insets are the high magnification images in each alloy.

Fig. 4. Oxidation kinetic curves of four alloys at 1200 °C during 100 h: (a) mass gain curves, in which solid/dotted lines denote weighing results with/without crucible respectively and the inset shows mass gain of the first hour; (b) fitted lines of square mass gain (with crucible) vs. time.

Fig. 5. XRD patterns of four alloys oxidized at 1200 °C for 100 h.

Fig. 6. SEM microstructure evolutions of four alloys: (a-c) NiAlHf; (d-f) NiAlHf-1Si; (g-i) NiAlHf-3Si; (j-l) NiAlHf-5Si. The insets denote higher magnification of dotted rectangular areas.

Fig. 7. Back-scattered electron patterns of alloy oxide scale surface (a, c, e, g) and cross section (b, d, f, h) after 100 h cyclic oxidation at 1200 °C for NiAlHf (a, b), NiAlHf-1Si (c, d), NiAlHf-3Si (e, f) and NiAlHf-5Si (g, h).

Table 3 Chemical compositions of bright white areas in Fig. 7 identified by EDS (at.%).

Area	O	Ni	Al	Si	Hf
1	57.82	1.52	36.11	—	4.55
2	53.14	4.05	30.82	1.21	10.78
3	57.99	2.37	15.98	2.72	20.93

Fig. 8. Luminescence spectra of alloys after short-term oxidation at 1200 °C (a, c, e, g) and their corresponding surface morphologies after 30 min oxidation (b, d, f, h) for NiAlHf (a, b), NiAlHf-1Si (c, d), NiAlHf-3Si (e, f) and NiAlHf-5Si (g, h).

Fig. 9. TEM analysis of as-annealed NiAlHf-5Si: (a) TEM bright-field image; (b) EDS results of circles in (a); (c) low-magnification TEM bright-field image; (d) β-NiAl [012] zone axis SAED pattern from (c).

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