Neutron Diffraction Study of Low-Cycle Fatigue Behavior in an Austenitic-Ferritic Stainless Steel

doi:10.1007/s40195-015-0319-4

Abstract

Abstract:

By performing in situ neutron diffraction experiments on an austenitic-ferritic stainless steel subjected to low-cycle fatigue loading, the deformation heterogeneity of the material at microscopic level has been revealed. Based on the in situ neutron diffraction data collected from a single specimen together with the mechanical properties learned from the ex situ micro-hardness, a correlation has been found. The performance versus diffraction-profile correlation agrees with the cyclic-deformation-induced dislocation evolution characterized by ex situ TEM observation. Moreover, based on the refined neutron diffraction-profile data, evident strain anisotropy is found in the austenite. The high anisotropy in this phase is induced by the increase in dislocation density and hence contributes to the hardening of the steel at the first 10 cycles. Beyond 10 fatigue cycles, the annihilation and the rearrangement of the dislocations in both austenitic and ferritic phases softens the plastically deformed specimen. The study suggests that the evolution of strain anisotropy among the differently oriented grains and micro-strain induced by lattice distortion in the respective phases mostly affect the cyclic-deformation-induced mechanical behavior of the steel at different stages of fatigue cycles. The stress discrepancy between phases is not the dominant mechanism for the deformation of the steel.

Key words: Duplex steel, Neutron diffraction, Fatigue, Plastic deformation, Microstructure

Ming-Wei Zhu, Nan Jia, Feng Shi, Bjørn Clausen. Neutron Diffraction Study of Low-Cycle Fatigue Behavior in an Austenitic-Ferritic Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(10): 1247-1256.

Figures/Tables 12

Table 1 Chemical composition of the duplex stainless steel (in wt%)

C	Si	Mn	P	S	Cr	Ni	Mo	N	Fe
0.03	0.8	1.2	0.035	0.02	25.0	7.0	4.0	0.3	Bal.

Fig. 1 Schematic presentation to show the arrangement of the in situ neutron diffraction measurements

Fig. 2 Stress amplitude as a function of number of loading cycles for the austenitic-ferritic steel

Fig. 3 Responses of lattice strains to the fatigue cycles along the loading direction and the transverse direction for different reflections in austenite (γ) and ferrite (α) (the duplex steel is at the macro-strain of 1%; |ɛ max-ɛ min| denotes the intergranular strain among the differently orientated grains in each phase): a γ phase, loading direction; b γ phase, loading direction; c α phase, loading direction; d α phase, loading direction

Fig. 4 Responses of diffraction peak widths to the fatigue cycles for different reflections in austenite (γ) and ferrite (α): a γ phase, loading direction; b γ phase, loading direction; c α phase, loading direction; d α phase, loading direction

Table 2 Diffraction elastic constants (E) and Poisson’s ratio (ν) in the subset of grains with their reflection plane {hkl} normal to the scattering vector, determined by the Kröner model for austenite (γ) and ferrite (α)

	{200}γ	{200}α	{220}γ	{211}α	{311}γ
E (GPa)	152	181	211	228	184
ν	0.33	0.321	0.264	0.275	0.294

Fig. 5 Isotropic model calculated stress of the constituent phases and stress discrepancy between phases in the duplex steel as a function of number of fatigue cycles

Fig. 6 Responses of micro-hardness to the fatigue cycles for the constituent phases and the duplex steel

Fig. 7 TEM image of the as-received duplex steel showing low dislocation density in both austenitic and ferritic phases

Fig. 8 Microstructures of the duplex steel after the first cycle of fatigue tests: a pileups of planar dislocations and accumulation of dislocation arrays in austenitic grains; b, c substructure containing stacking faults that appear as parallel fringe patterns between two partial dislocations in austenite; d only a slight increase in dislocation density is identified in the interior of ferritic grains

Fig. 9 Microstructures of the duplex steel after the tenth cycle of fatigue tests: a, b a denser distribution of planar arrangement of dislocations and some loop-like dislocations in the austenite; c ferrite is dominated by screw dislocations; d dense dislocations are identified at some phase boundaries

Fig. 10 Microstructures of the duplex steel after the 225th cycle of fatigue tests: a, b the hierarchical dislocation structure is formed in austenite; c the dislocation density in ferrite is lowered; d in some ferritic grains, a cellular dislocation structure is formed and a much lower dislocation density is observed at the cell interiors than the cell boundaries

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