The interaction between dislocations and the spinodal nanostructure in a 2205 duplex stainless steel
Date Issued
2015
Date
2015
Author(s)
Hsieh, Yi-Chieh
Abstract
A 2205 duplex stainless steel is susceptible to severe embrittlement after aging at 475ºC. Microstructures of the unaged and aged specimens (heat-treated at 475ºC for 64h) were investigated by different deformation tests, in order to elucidate the cause of this embrittlement and interaction between dislocations and the spinodal nanostructure. Before deformation, the aging treatment was found to bring about a spinodal nanostructure with 5-10nm interlocking domains in the δ-ferrite phase and the related dislocation characteristics were not changed by aging. After high strain and high strain rate deformation in Charpy impact tests, the dislocations in the aged δ-ferrite grain were curved individually or composed of small waves, caused by the pinning effect of the spinodal nanostructure. In-situ compression tests in a transmission electron microscope were carried out to investigate the deformation behaviors of the unaged and aged δ-ferrite nanopillars. The results indicated that the spinodal nanostructure confined the moving velocity and cross-slip of dislocations during deformation. These behaviors resulted in a high dislocation density and smooth slip-bands, leading to significant strengthening of the aged δ-ferrite nanopillars after compression. The smooth slip-plane was the (0‾1 1) with the lowest activation energy. After compression, the “cross-stitch” pattern with straight and perpendicular dislocations formed in the aged pillars and was regarded as a characteristic of the immobilization of dislocations. Through the nanoindentation test with a cross-sectional TEM analysis technique, the interaction between dislocations and the spinodal structure was found to be strain-related. On the edge of the semicircular plastic zone with a moderate strain, the straight dislocations with a cross-stitch pattern and smooth slip bands were found in the aged film, similar to the result of the in-situ nanoindentation test. At the center of the semicircular plastic zone with a high strain, the dislocations were curved and highly compacted in the aged film because of the drag effect of the spinodal nanostructure and early formed dislocations, similar to the dislocation characteristics observed in the Charpy impact test. From the investigation above, it can be concluded that the spinodal nanostructure seriously affected the dislocation behaviors, resulting in the poor deformation ability of the aged δ-ferrite phase. Therefore, the aged 2205 duplex stainless steel lost the ability to accommodate high impact energy and led to the cleavage fracture.
Subjects
Duplex stainless steel
Spinodal structure
Transmission Electron Microscopy (TEM)
In-situ nanoindentation
Focused Ion Beam microscopy (FIB)
Type
thesis
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