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Investigations on Cyclic Deformation and Damage Behavior of AL6XN Super-Austenitic Stainless Steels

Author: MaChaoQun
Tutor: LiXiaoWu；ZhangZheFeng
School: Northeastern University
Course: Materials Physics and Chemistry
Keywords: AL6XN austenitic stainless steel Cyclic deformation Uniaxial deformation Stress amplitude Plastic strain amplitude Grain boundary Twin boundary Fracture Dislocation structure
CLC: TG142.71
Type: Master's thesis
Year: 2010
Downloads: 61
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Abstract

As a kind of newly-developed stainless steel materials with a broad application background and excellent performance, the A16XN super-austenitic stainless steel would be inevitably subjected to uniaxial or cyclic loads during the actual service. Therefore, studies on its mechanical and fatigue behavior are of particular importance for its further development and practical applications. But until now, no systematical investigation in this area has yet been performed. In light of this, the A16XN super-austenitic stainless steel was selected in the present work as the target material to examine its uniaxial deformation and damage behavior at different strain rates, stressing upon the cyclic deformation and damage behavior under constant stress amplitude control and constant plastic strain amplitude control.A16XN stainless steel shows a low strain rate sensitivity of its uniaxial deformation behavior, and dislocation slip is the main uniaxial deformation feature. All fracture surfaces induced by tensile deformation at different strain rates can be divided into two parts, i.e., fibrous zone and shear lip zone. The fibrous zone consists of dimples with a bimodal size. It is noted that the compressive yield strength and tensile yield strength are basically comparable.The curve of fatigue life versus stress amplitude of the AL6XN stainless steel meets the Basquin relationship, while the curve of fatigue life versus plastic strain amplitude follows the Coffin-Manson relationship. This material exhibits monotonically sustained cyclic softening behavior at low plastic strain amplitudes. However, when the plastic strain amplitude is as high as 5×10-3s-1, the stress response curve of this material enters into an striking softening stage just after an initial hardening stage. No obvious stress saturation phenomenon appears.For the fatigue deformation under constant stress amplitude control, as the applied stress amplitude is below the tensile yield strength, the surface deformation feature is primarily manifested by slips in gains and twins, and zigzag slips crossing some grains and twins, and etc. However, when the applied stress amplitude is slightly higher than its tensile yield strength, the secondary cracks with different patterns are also found, such as transgranular cracks, intergranular cracks, transgranular/intergranular mixed-model cracks and so on. For the low-cycle fatigue deformation under constant plastic strain amplitude control, the secondary crack features exhibit a certain varying rule with increasing strain amplitude. At low strain amplitudes, the majority of cracks are transgranular ones, strictly propagating along the slip bands. At moderate strain amplitudes, the crack growth path is more complicated, and cracks are mainly intergranular/transgranular mixed-model ones. As the plastic strain amplitude is raised up to high level, secondary cracks extend mainly along the grain boundaries. In addition, when the plastic strain amplitude is higher than 3×10-3, the cracks initiating along twin boundaries also become apparent.The fatigue fracture features of the AL6XN steel under constant stress amplitude control are more or less related with the applied stress amplitude. As the stress amplitude changes from lower values than the yield strength to ones higher than the yield strength, the micro-features with brittle cleavage steps and brittle fatigue striations in the fatigue crack source zone changes gradually into features with regular fatigue striations and the formation of secondary cracks. The micro-characteristics in the fatigue crack growth zone are basically featured by the formation of ductile fatigue striations, and the micro-features in the final rapid fracture area for all samples correspond to the formation of dimples. The fatigue fracture features of the AL6XN steel under constant plastic strain amplitude control present quite similar situation.The low-cycle fatigue dislocation arrangements have also been observed using the electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and a transmission electron microscopy (TEM). It is found that with increasing strain amplitude, the dislocation structures with planar slip type (e.g. planar slip bands) are evolved gradually into those with wavy slip type (e.g. veins, dislocation cells and persistent slip band ladders and etc.). Secondary twins form in some deformation concentration areas. Furthermore, the dislocation at the crack tip has been detected by SEM-ECC. It is found that the major microstructural features around the crack (tip) are dominated by the elongated dislocation cells, which form in great number in the grains near the crack (tip). These elongated cells seemingly tend to develop away from the crack but towards the crack growth direction.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > Special performance of steels > Stainless steel, acid-resistant steel
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