The Effects Of Grain Boundary Character Distribution On The Tensile Mechanical Properties Of A 316L Stainless Steel

316L stainless steel has been widely used in petrochemical,chemistry,architecture and nuclear industry due to a combination of good corrision resistance and mechanical properties.However,with the stainless steel service conditions becoming increasingly complex,the demands for its performance are increasing in many industrial fields.Especially for stainless steel as a structural material is concerned,the need to further improve the mechanical properties should be considered.As one of the important structural features of polycrystalline materials,the grain boundary?GB?has an important influence on the performance of stainless steels.The grain boundary engineering?GBE?can enhance the grain-boundary-related properties of metal materials which have face centered cubic?FCC?structure with low stacking fault energy?SFE?by changing the grain boundary character distribution?GBCD?.In this dissertation,the effects of thermo-mechanical processing?TMP?on the GBCD of 316L stainless steels were studied.Different TMPs were applied to316L stainless steels to obtain GBE specimens and Non-GBE specimens with similar average grain sizes.Subsequently,the effects of the GBCD after GBE processing on the tensile mechanical properties at room and elevated temperatures at different strain rates were investigated.The results are as follows:?1?The proportion of low-?CSL???29,coincidence site lattice?GBs can be increased to more than 70%in the GBE specimen.Meanwhile,the large-size highly-twinned grain-cluster microstructure was formed through the treatment of GBE.All of the grains inside of this kind of cluster have?3ⁿ?n=1,2,3…?mutual misorientations.During the recrystallization process,the?3 and low-?CSL GBs are developed associating with the recrystallization nuclei growth.After full recrystallization,the specimens were in the grain growth stage.The proportion of low-?CSL GBs decreased with grain growth due to being swept away of the existing low-?CSL GBs.?2?The GBE processing leading to the enchanced proportion of low-?CSL GBs displayed increased uniform elongation in tensile test at room temperature,and the differences in uniform elongations between the GBE and non-GBE specimens(UE_GBE–UE_Non-GBE)increase with the decreasing of strain rates.The result shows that low-?CSL GBs are much more effective for improving uniform plastic deformation capability of the GBE specimens under the condition of low strain rates.?3?During the process of deformation at room temperature,the GBE specimen showed lower hardness and more homogeneous strain distribution.This indicates that high proportion of low-?CSL GBs in the interior of the grain-clusters has a weak interaction with dislocation.Therefore,in the GBE specimen,dislocations can easily directly transmit?3 twin boundaries,preventing the piling up of dislocations.The distribution of Schmid factor and Taylor factor demonstrate a dominating slip activity in the GBE specimen.The large and deep dimple structure also indicates the improvement of plasticity by GBE processing.?4?The high temperature tensile test shows that the GBCD after GBE process affects dynamic strain aging?DSA?behavior of 316L stainless steel.In the temperature range of DSA?300-500°C?,the occurrence of serrated flow in the GBE specimen is occurs later and these serrations are smaller,especially at low strain rate.Under the condition of higher strain rate,the severe serrated flow behavior makes the GBE and Non-GBE specimens have similar plasticity,while the GBE specimen has relatively high proportion of low-?CSL GBs resulting in a more homogeneous strain distribution which leads to better plasticity at lower strain rate.?5?The GBE specimen with larger deformation amount of grains leads to the higher deformation stored energy which accordingly increased the driving force of dynamic recrystallization?DRX?at 900°C.DRX can reduce the formation of dislocation accumulation and stress gradient during thermal deformation and effectively weaken the stress concentration.In addition,the DRX grains show obvious Goss texture orientation which can promote the further development of DRX in the process of high temperature deformation.The dynamically recrystallized grains have a higher value of Schmid factor,indicating that the GBE specimen is initially softer.The fracture surface also suggested improved plasticity of GBE specimens.