Fatigue Damage Mechanism of AL6XN Austenitic Stainless Steel at High Temperatures

doi:10.1007/s40195-020-01020-4

Abstract

Abstract:

By the combination of transmission electron microscope, neutron diffraction and small-angle neutron scattering methods, mechanical fatigue behavior of AL6XN austenitic stainless steel was investigated in the temperature range of 400-600 °C. At 400 °C, in addition to the occurrence of dynamic strain aging, the formation of short-range order was evidenced from the forbidden electron diffraction spot of 1/3 {422} in face-centered cubic (fcc) structure viewed down [111] zone axis, which facilitate the planar slip mode of dislocation and result in the work hardening during the fatigue deformation. The fatigue damage is mainly dominated by the accumulation of planar slip band and the interaction among various slip systems. With increasing temperature, precipitates of chi phase, Laves phase and sigma phase were formed during the fatigue tests at 500 and 600 °C. An increase in precipitation content at 600 °C has also been confirmed by both scanning electron microscope and small-angle neutron scattering analysis. The dislocation pileup originating from the uncoordinated deformation between precipitate and austenitic matrix is an important fatigue damage leading to crack. The continuous cycle softening behavior was also observed on the fatigue curve at 600 °C, which is considered to be caused by dynamic recovery.

Key words: Precipitates, Low-cycle fatigue, Austenitic stainless steel, Small-angle neutron scattering, Planar slip bands

Yanyan Hong, Penglin Gao, Hongjia Li, Changsheng Zhang, Guangai Sun. Fatigue Damage Mechanism of AL6XN Austenitic Stainless Steel at High Temperatures[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(6): 799-807.

Figures/Tables 10

Table 1 Chemical composition (wt%) of AL6XN ASS

C	Cr	Ni	Si	Mn	Mo	N	Cu	Fe
0.016	21.6	24.0	0.36	0.23	6.1	0.21	0.10	Bal.

Fig.1 Initial microstructure a and inverse pole figure (IPF) b of AL6XN ASS (the white lines represent the twins), c tensile stress-strain curves of AL6XN ASS at elevated temperatures

Fig.2 Cyclic stress responses (on the logarithm scale) for AL6XN stainless steel at 400 °C a, 500 °C b, 600 °C c as a function of cycles

Fig.3 Dislocation structures and electron diffraction pattern observed in the fatigue sample at 400 °C: a dislocation tangles, b planar slip bands, c diffraction pattern in [111] zone axis

Fig.4 Microstructures observed in the fatigue samples at 500 °C a-c, 600 °C d-f: three different types of precipitates

Fig.5 a SANS results of AL6XN ASS: the scattering intensity versus the scattering vector q, b, c SEM micrographs of precipitates in fatigue samples at 500 and 600 °C, respectively

Table 2 SV obtained from Porod fitting

	Original sample	400 °C fatigue sample	500 °C fatigue sample	600 °C fatigue sample
S_V	0.0024 ± 7.7E - 5	0.0025 ± 1.3E - 4	0.0036 ± 1.3E - 4	0.0070 ± 1.7E - 4

Fig.6 TEM bright field micrographs of the original AL6XN ASS a, and the fatigue samples of AL6XN ASS at 500 °C b, 600 °C c

Fig.7 Micro-crack originated from grain boundary a, micrograph of dislocation structures b in the fatigue sample at 600 °C

Fig.8 Neutron diffraction patterns a, conventional Williamson-Hall plots b, of the fatigue samples at elevated temperatures, c micrograph of Laves phase in fatigue sample at 600 °C

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