Oxidation Behavior of K4750 Alloy at Temperatures Between 750 °C and 1000 °C

doi:10.1007/s40195-021-01235-z

Abstract

Abstract:

This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750 °C-1000 °C in air for 100 h-1000 h by isothermal oxidation tests. The oxidation-kinetic curves were plotted by the static discontinuous weight gain method. Observation and identification of oxidation products were conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD). X-ray photoelectron spectrometer (XPS) was also used to analyze the chemical state of elements and the distribution in depth. The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law. The average oxidation rate below 900 °C was less than 0.1 g/m ²·h which meant the alloy was at a complete anti-oxidation grade, and the alloy was at an anti-oxidation grade at 1000 °C. The predominant surface oxide was Cr₂O₃, and a double layer structure of the oxide scale was observed at all tested temperatures as time increased. The outer oxide layer contained continuous Cr₂O₃ and a small amount of oxides mixed TiO₂ and NiCr₂O₄, while the inner oxide layer was composed with Al₂O₃. The oxidation process could be interpreted by the competitive oxidation of different elements. The diffusion rate of Ti through Cr₂O₃ layer increased with increasing temperature, and thus the generation of TiO₂ was advantageous. The dispersed TiO₂ reaching a certain amount destroyed the continuity of the surface oxide layer, which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.

Key words: Ni-based superalloy, Oxidation, Titanium, Internal oxides

Yan-Li Liu, Kun-Lei Hou, Mei-Qiong Ou, Ying-Che Ma, Kui Liu. Oxidation Behavior of K4750 Alloy at Temperatures Between 750 °C and 1000 °C[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(12): 1657-1668.

Figures/Tables 16

Table 1 Chemical composition of K4750 alloy (wt%)

C	Cr	Fe	Al + Ti + Nb	Ti/Al	W	Mo	Ni
0.10	19.90	4.30	5.77	2.5	3.14	1.43	Bal.

Fig. 1 Variation of specific mass gain with oxidation time of K4750 alloy at a 750 °C-800 °C, b 900 °C-1000 °C

Fig. 2 Fitting results of lg ΔM-lg t of K4750 alloy

Table 2 Oxidation kinetic equation parameters of K4750 alloy after oxidation at 750 °C-1000 °C

Temperature (°C)	n	K_p (mgⁿ /(cm²ⁿ h))
750 °C	2.19	1.92 × 10^-5
800 °C	3.21	8.92 × 10^-5
900 °C	2.24	3.21 × 10^-3
1000 °C	2.30	0.0763

Table 3 Oxidation rate K′ of K4750 alloy oxidized at 750-1000 °C for 100-1000 h

Oxidation rate, K′(g/m²·h)	100 h	200 h	300 h	400 h	500 h	1000 h
750 °C	0.007	0.004	0.003	0.002	0.002	0.001
800 °C	0.024	0.015	0.009	0.009	0.007	0.005
900 °C	0.068	0.045	0.035	0.031	0.027	0.020
1000 °C	0.248	0.160	0.125	0.112	0.111	0.072

Fig. 3 XRD patterns of K4750 alloy oxidized at 750 °C-1000°C for a 300 h, b 1000 h

Fig. 4 SEM images of oxides on the sample surface after oxidation at a 750 °C, b 800 °C, c 900 °C, d 1000 °C for 100 h-1000 h

Fig. 5 EDS results of the oxidation products at 1000 °C for 500 h: a attachments on the crucible walland b spallation in the crucible

Fig. 6 Line scanning results of K4750 alloy oxidized at a 750 °C, b 800 °C, c 900 °C, d 1000 °C for 300 h

Fig. 7 XPS results of the oxidescale after oxidation at 1000 °C for 1000 h

Fig. 8 Results of XPS sputtering of K4750 alloy after oxidizing at a 750 °C, b 800 °C, c 900 °C, d 1000 °C for 5 h

Fig. 9 Cross section SEM images of K4750 alloy after oxidizing at a 750 °C, b 800 °C, c 900 °C, d 1000 °C for 100 h-1000 h

Fig. 10 Cross section micrograph and EDS mapping results of K4750 alloy after oxidizing at a 800 °C/500 h, b 1000 °C/500 h

Table 4 EDS analysis results of different points in Fig. 10b (wt%)

Point	Ni	Cr	Fe	Ti	Al	Nb	W	O
1	1.38	6.01	0.33	44.32	0	4.21	0.12	36.91
2	1.56	61.55	0.56	3.88	0.48	2.76	1.06	28.16
3	4.01	1.28	0.06	57.69	3.49	0	0.66	32.81
4	6.45	1.50	0.79	0.42	37.04	2.16	0.59	37.91

Fig. 11 a EPMA image, b-g corresponding elemental mappings of the sample after oxidation at 900 °C for 500 h

Fig. 12 Schematic diagram of oxide scale formation in K4750 alloy: a initial stage, b steady stage below 800 °C, c steady stage above 900 °C

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