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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 41907 (07)
- University: Sharif University of Technology
- Department: Materials Science and Engineering
- Advisor(s): Asgari, Sirous
- Abstract:
- Previous observation and models on the origin of linear hardening behavior in FCC polycrystals are critically reviewed. To reveal the draw backs of the previous models, selected results of an investigation on the evolution of microstructure during simple compression testing of two FCC polycrystals, Inconel 625 superalloy and AISI 316L stainless steel are reported. It is found that while a number of FCC polycrystals show linear hardening behavior, the evolution of the underlying microstructure may be quite different. It is argued that, in contrast to the current belief, deformation twinning may not be the sole cause of linear hardening in low SFE FCC polycrystals. It is suggested that only FCC crystals with sufficiently low twinning stress are expected to form mechanical twins after certain amount of initial plastic strain. In other FCC polycrystals that show linear hardening behavior, competitive mechanisms such as formation of Lomer-Cottrell locks and other non-coplanar dislocations (i.e. forest dislocations) may be dominant hardening mechanism. The effectiveness of these slip barriers in FCC polycrystals is expected to increase with the tendency of the material to slip planarity. Therefore, parameters such as SFE, temperature, Peierls stress and short range ordering are proposed to have a contribution to the linear hardening response of FCC polycrystals. For elucidating the real role of shear transformations in total hardening rate of FCC polycrystals, the plastic behavior and underlying microstructural evolution of two austenitic stainless steels, AISI 304L and AISI 316L, are investigated and reported. A rather similar four-stage hardening behavior was observed in both alloys except for the magnitude of hardening rate which was higher for AISI 304L during the linear hardening regime. Transmission electron microscopy and X-ray diffraction studies showed that deformation twinning was the dominant microstructural feature in the deformed AISI 316L samples. In AISI 304L alloy, however, martensitic transformation is the additional hardening micromechanism. This may justify the higher strain hardening rates observed in this alloy compared to that of AISI 316L. Based on the data and the model presented in this work, it is proposed that these shear deformation modes may both contribute to the linear hardening response of low stacking fault energy face centered cubic alloys. For isolation the effect of different parameters that has been suggested in the first examinations, two model alloys, Cu-6wt%Al and Cu-12wt%Mn, is designed and the similar mechanical and microstructural examinations are conducted on the prepared alloys. True stress vs. true strain responses of Cu-6wt%Al and Cu-12wt%Mn alloys shows that while Cu-6wt%Al alloy shows typical mechanical response of low stacking fault energy alloys, Cu-12wt%Mn alloy behaves similar to medium to high stacking fault energy alloys. These findings clearly show that while short range ordering triggers slip planarity, it has a minor effect on total dynamic recovery of these copper alloys. These findings suggest that the main parameter to be concerned in determining linear hardening behavior should be stacking fault energy and homologous temperature
- Keywords:
- Plastic Deformation ; Transmission Electron Microscope ; Mechanical Twinning ; Face Centered Cubic (FCC)Alloy ; Short-Range Order
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