Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 332 In Cases 10 and 11, Fig. 18 (h) and (i), many stacking faults or martensite were observed. There is little difference between a stacking fault and martensite formation. A stacking fault forms when one atomic plane is dropped off and when more than two atomic planes have a disordering it is a different structure which may be martensite. Since 304SS is a material of low stacking fault energy, slip is planner as shown in Fig. 5 and there are many partial dislocations which make a stacking fault between them. Long stacking faults exceeding several subgrains in length were formed in Case 10 with short stacking faults formed within cells in Case 11. The long stacking faults were formed by the several box nonproportional straining and which hindered the cell formation, while, in Case 11, the cells were formed earlier than the stacking faults and stacking faults were stopped by the cell boundaries. For Case 13, fine cells are found and they are rather close to subgrain since the cell boundaries are rigid and misorientation angle between cells is rather large. This strain path made resulted in clear cells and rigid cell boundaries. Fig. 19 is a microstructure map showing the cell, dislocation bundle and stacking fault boundaries as functions of maximum principal strain range and nonproportional factor for all the strain paths. In the figure, solid symbols indicate tests in which only cells were observed, while open symbols represent tests in which cells and other dislocation structures were found. Asterisks indicate tests where stacking faults were observed and the number at the data indicate the strain path number shown in Fig. 15. This figure shows that stacking faults were observed in almost all the tests and did not depend on the principal strain range and nonproportional factor. 304 SS is a low stacking fault material and a dislocation easily split into partial dislocations, making a stacking fault between them as discussed earlier. A partial dislocation glides on the slip plane, and a stacking fault arises between the partial dislocations. Many stacking faults seem to be generated by this mechanism. There is a critical combination of strain range and nonproportional factor for forming cells indicated by the solid line. In the region above the line, the microstructure is dominated by cells with other microstructure in addition to cells observed for the test conditions below the solid line. Figure 19 : Microstructure map represented with principal strain and nonproportional factor. Figure 20 : Relationship between principal stress range and cell size. Fig. 20 shows the relationship between cell size and the maximum principal stress range for all the tests where the cell structure was observed. The mean cell size was determined by the Heyn method (JIS G005), observing 3 or 4 locations of each specimen. Maximum principal stress range (   ) and mean cell size ( d ) can be approximated by a straight line for all of the strain histories. The relationship is, σ    n m d (4) The values of m and n are 975MPa and  0.57, respectively when d is measured in μ m. The value of exponent is close to  0.5, so that the Hall-Petch relationship holds in proportional and nonproportional loadings. As shown in Fig. 18, various microstructures were formed under nonproportional loading. However, the results in Fig. 20 indicate that the additional hardening in nonproportional loading is mainly caused by the reduction in cell size. The severe interaction of