Influence of grain size on the small fatigue crack initiation and propagation behaviors of a nickel-based superalloy at 650 °C

doi:10.1016/j.jmst.2019.03.023

Abstract

Abstract:

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached $\widetilde{1}$0 times the grain size.

Key words: Fatigue crack initiation, Small crack, Crack growth rate, Grain size effect, High temperature

Xumin Zhu, Congyang Gong, Yun-Fei Jia, Runzi Wang, Chengcheng Zhang, Yao Fu, Shan-Tung Tu, Xian-Cheng Zhang. Influence of grain size on the small fatigue crack initiation and propagation behaviors of a nickel-based superalloy at 650 °C[J]. J. Mater. Sci. Technol., 2019, 35(8): 1607-1617.

Figures/Tables 21

Table 1 Chemical compositions of GH4169 alloy (wt%).

Ni	Cr	Mo	Al	Ti	Nb	Mn	Si	C	Fe
53.02	19.05	3.00	0.50	0.93	5.11	0.03	0.10	0.033	Bal.

Table 2 Processing parameters used in HTs (AC denotes air cooling).

HT1	960 °C for 1 h, AC to 720 °C
HT2	1050 °C for 1 h, AC to 720 °C
HT3	1090 °C for 1 h, AC to 960 °C, 960 °C for 1 h, AC to 720 °C

Fig. 1. Shape and dimensions of SENT specimens used in fatigue testing.

Fig. 2. Microstructures of GH4169 alloys after different HTs: (a) HT1; (b) HT2; (c) HT3.

Fig. 3. Probability density functions of grain size of (a) HT1 and (b) HT2 alloys.

Table 3 Grain size and tensile properties of GH4169 alloys after different HTs.

HT	d_r (μm)	d_a (μm)	σ_uts (MPa)	σ_y (MPa)	e (%)
HT1	5-21	12	1189.1	1024.4	10.56
HT2	29-82	48	989.5	833.8	26.88
HT3	31-208	83	974.2	810.6	32.40

Table 4 Fatigue lives and crack initiation lives of SENT specimens.

Specimen	HT condition	n	N_f (cycle)	N₀ (cycles)	N₀ -N_f (cycle)	Test condition
1	HT1	1	19973	8000	11973	Replica
2	HT2	6	5979	800	5179	Replica
3	HT3	4	5705	400	5305	Replica
4	HT1		21195			No-replica
5	HT2		5838			No-replica
6	HT3		5531			No-replica

Fig. 4. Images from replicas after (a) 13900, (b) 14500, (c) 16000, (d) 18,400 and (e) 19,100 cycles showing initiation and growth process of main crack at surface of SENT specimen HT1 with average grain size of 12 μm.

Fig. 5. Images from replicas after (a) 2200, (b) 3100, (c) 3600 and (d) 4600 cycles showing initiation and growth process of main cracks at surface of SENT specimen HT2 with average grain size of 48 μm.

Fig. 6. Initiation sites of three main cracks at surface of SENT specimen HT3 with average grain size of 83 μm after 1600 cycles: (a) crack 1; (b) cracks 2 and 3.

Fig. 7. Variations of lengths of main cracks along with number of fatigue cycles for specimens (a) HT1, (b) HT2 and (c) HT3.

Fig. 8. Variations of (a) crack length and (b) normalized crack length of different SENT specimens along with cycle ratio.

Fig. 9. Variations of crack growth rates along with crack length of specimens (a) HT1, (b) HT2 and (c) HT3.

Fig. 10. Variations of crack growth rates against normalized crack length, a/da of three different SENT specimens.

Fig. 11. (a) SEM image of entire fractured surface of HT1 specimen, high magnification SEM images of (b) region A and (c) region B.

Fig. 12. (a) SEM image of entire fractured surface of HT2 specimen, high magnification SEM images of (b) region A and (c) region B.

Fig. 13. Fracture surface morphologies of HT3 specimen in (a) small crack and (b) long crack stages.

Fig. 14. Crack 6 at surface of HT2 specimen: (a) intergranular cracking caused by impingement of slip bands upon grain boundary; (b) fracture morphology near region A; (c) fracture morphologies near region B.

Fig. 15. Variation of crack initiation life N0 along with fatigue life Nf of SENT specimens tested at room temperature and 650 °C.

Fig. 16. Variation of crack growth rates along with length of (a) crack 2 and (b) crack 4 at surface of specimen HT2, where the blocking effects of grain boundaries and twins on the deceleration of small crack growth can be found.

Fig. 17. Crack growth rate as a function of stress intensity factor range ΔK for specimens HT1, HT2 and HT3, where the data of long crack [32] is included for comparison.

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