Investigation of the high-temperature fatigue mechanism related to altered local short-range order in AL6XN austenitic stainless steel

Abstract

An increase in crystal local short-range order (SRO), characterized by obvious diffuse intensity maxima at 1/3{422} in the [111]-axis selected area diffraction pattern, is observed in AL6XN austenitic stainless steel subjected to high-temperature low-cycle fatigue deformation. SRO is an ordered fcc structure with nanometer size, which can be well explained by the ordered arrangement of Cr/Mo atomic layers segregated on the {422} crystal plane in austenite. Many planar slip bands are observed in the fatigue sample, the distribution of SROs is highly related with the distribution of dislocations. In situ high-energy X-ray diffraction (HE-XRD) experiments are performed to investigate the deformation behavior of both as-received and fatigued samples under uniaxial loading. The lattice strain evolution reveals an interesting lattice expansion in the initial elastic stage, which provides direct evidence of a decrease in SRO during further uniaxial tensile deformation, indicating that the deformation modes greatly affect the SRO transition. We believe that the high-temperature fatigue damage originates mainly from the accumulation of planar slip bands, accompanied by a local increase in SRO, which promotes work hardening near the final fatigue fracture.

Keywords

• High-temperature fatigue
• Short-range order
• Planar slip band
• In situ high-energy X-ray diffraction
• Austenitic stainless steel