Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 333 slip systems under nonproportional loading reduces the cell size and results in the additional hardening. The results also imply that microstructures other than cell structure have almost no influence on the additional hardening. M ICROSTRUCTURE IN VARIOUS STRAIN WAVEFORMS AT HIGH TEMPERATURE icrostructures of 304 SS were also studies at 923K on the specimen fatigued using 14 strain paths shown in Fig. 15. The TEM photographs are not presented here but the same microstructure map as Fig. 19 is shown in Fig. 21, where the results at room temperature are also shown by the dashed line. The figure clearly shows that the cell boundary at 923K shifted downwards from that at room temperature. The low positioned boundary indicates that cells were easily formed by lower principal stress and lower nonproportional factor at high temperature than at room temperature, which results from the activated mobility of dislocations at high temperature. The relationship between the principal stress range and the cell size at 923 K is shown in Fig. 22, where the results at room temperature are superimposed in the figure. The same principal stress range at 923K formed smaller cell size by about 4 times compared with room temperature. The smaller cell size results from the activated mobility of dislocations at high temperature. The cell structure is a structure with lower elastic energy than the dislocation bundle and dispersed dislocations. Figure 21 : Microstructure map represented with principal strain range and nonproportional factor. Figure 22 : Relationship between principal strain range and cell size. C ONCLUSIONS 1. FCC materials with low stacking fault energy showed large additional hardening but those with high stacking fault energy little additional hardening. The amount of the additional hardening results from the slip system of material. Materials with low stacking fault energy yielded planner slips and severe interactions of the slip systems caused large additional hardening. Those with high stacking fault energy gave wavy slips and a little interactions of the slip systems caused little additional hardening. The planner slip was caused by a perfect dislocation splitting into two partial dislocations and cross slip of the dislocation was difficult because two partial dislocations should shrink back to a perfect dislocation to make cross slip. 2. Ladder and maze structures were developed in 304SS cyclically loaded in tension and torsion at 923K. The microstructures were an anisotropic microstructure and the structures showed cross hardening. These microstructures were easily rearranged to another microstructure by switching the loading mode. Only a cell structure was formed in cruciform loading path and the cell structure was not completely rearranged by changing the loading mode. The small cell structure showed isotropic stress response. The amount of the additional hardening was caused by the reduction in cell size. 3. Stacking fault, cell, twin and dislocation bundles were observed in 304SS cyclically loaded under 14 proportional and nonproportional strain paths at room temperature. There was a critical boundary of the cell formation in a diagram presented with principal strain range and nonproportional factor. Hall-Petch relationship held between the principal stress range and mean cell size, indicating the additional hardening mainly resulted from the reduction in cell size. M