Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 322 Tab. 2 lists the strain hardening coefficient and exponent of the five materials and Fig. 4 relates the cyclic strain hardening coefficient and exponent ratios with the stacking fault energy, where the ratios are the coefficient and exponent in nonproportional loading divided by those in proportional loading. The two ratios both decreased with the stacking fault energy. The interesting characteristic in the figures is that there exist thresholds in the ratios. The cyclic hardening coefficient and exponent ratios start to increase at the stacking fault energy of around 80 erg/cm 2 and the additional hardening does not occur in the stacking fault energy range above about 80 erg/cm 2 . Figure 4 : Relationship between strain hardening coefficient and exponent rations and stacking fault energy. The physical background of the large additional hardening in the materials with lower stacking fault energy is that a perfect dislocation in the materials is easy to split into two partial dislocations having a staking fault between the two partial dislocations. The elastic strain energy of the two partial dislocations is lower than the perfect dislocation. The width of the stacking fault is larger in lower stacking fault energy material. Dislocations with large width of stacking fault are difficult to make cross slip to bypass obstacles like Lomer-Cottrel locks made by interactions of slip systems, because the partial dislocations have to shrink back to a perfect dislocation for making a cross slip. Therefore, the slip characteristics of the material with low stacking fault energy tends to be planner while that with high stacking fault energy to be wavy. Fig. 5 shows SEM photographs observed on the replica films at the strain ranges shown in the figures. Unidirectional planner slip lines were observed on the photograph of 304SS in proportional loading, Fig. 5 (a), but slip lines in different directions in nonproportional loading were found, Fig. 5 (b). The maximum shear direction was fixed in proportional loading while the maximum shear direction changed in each cycle in nonproportional loading, which caused the different directional slips in 304SS in nonproportional loading. The similar slip characteristics, unidirectional slip lines in proportional loading and different directional slip lines in nonproportional loading, were found in Cu, Fig. 5 (c) and (d). No clear slip lines were found in Ni in proportional and nonproportional loadings and only the wavy surface appeared, Fig. 5 (e) and (d). Slightly curved slip lines were observed in Al in nonproportional loading but no clear slip lines were found in nonproportional loading, which was caused by the slips in many directions, Fig. 5 (g) and (h). No clear slip lines were found in 6061Al in proportional and nonproportional loadings, but the roughed surface along grain boundaries were found, Fig. 5 (i) and (j). These results indicate that, in proportional loading, the unidirectional slip lines formed in the low stacking fault energy materials like 304SS and Cu but such clear unidirectional slip lines did not formed in the high stacking fault energy materials like Ni and 6061 Al. In nonproportional loading, slip lines in different directions were found in the low stacking fault energy materials like 304SS and Cu but such clear slip lines were not found in the high stacking fault energy materials like Ni, Al and 6061Al. Only the exception was Al in proportional loading, where clear slip lines formed. The slip lines in Al may have the possibility of being a brittle fracture of oxidation layer formed on the specimen surface. Fig. 6 shows dislocation structures observed by TEM. Clear cells were found in all the tests. In 304SS and Cu, cell walls seem to have preferential orientations in proportional loading while they seem to be isotropic in nonproportional loading. The preferential orientation of cell walls may occur by the combination of the maximum shear stress direction and {111} crystallographic orientation. Isotropic cells were observed in Ni and Al and there was no clear difference in cell morphology between the proportional and nonproportional loadings. Especially, there were dislocations in the cells of Ni but no dislocations were found in the cells of Al.