Fig. 14. TEM micrographs of Ti-IF3(1 % Si) tested under a high stress amplitude showing development of the periodically arranged dislocation wall structure near the top surface (sϭ380.9 MPa, Nϭ8 500).proximately 0.5 mm in the Si-bearing steels. Concerning the interaction between dislocations and grain boundaries during the cyclic test, detailed studies on Cu polycrystal, 17,23) Cu bicrystal 24) and Al polycrystal 25) were carried out taking into account the grain boundary characteristics. Winter et al. 17) and Luoh et al. 23) reported that dislocation free zones (DFZ) along grain boundaries were formed when the vein structures developed within grains, whereas DFZ were not formed in the case of cell structures, which is very similar to the results of the present study using Fe-Si alloy. Although further study is required, Luoh et al. 23) concluded that the multipolar dislocation loop patches (MLPs) and walls (MLWs) are built up due to grain boundary incompatible stresses providing secondary glides, which are thought to cause annihilation of dipole loops by intersecting MLPs or MLWs. Referring to this model, it is inferred that in the Si-bearing steels the difficulty of cross slip leads to the annihilation of dislocation dipole loops in the vicinity of grain boundaries by intersecting MLPs and MLWs, or dislocations simply sinks into grain boundaries. In the case of pure iron after the deformation of cold rolling, localized softening near the initial grain boundaries has been reported.
26)Further systematic quantitative study in bcc metals is necessary to clearly understand the mechanism.The present study shows that the saturated dislocation structure varies with the cyclic stress amplitude. As shown in Fig. 17, the cell width and vein width varied with the ISIJ International, Vol. 49 (2009) stress amplitude, respectively. More specifically, the cell width remained comparatively large, although varying from approximately 5 to 8 mm under low to middle-stress amplitude, and clear cell structures developed under high-stress amplitude with comparatively homogenous and fine cell sizes of 2 to 3 mm. On the other hand, the vein width varied largely under low-stress amplitude, bundled dislocations started to appear under middle-stress amplitude, and the vein width converged to 0.5 to 1 mm dimension homogeneously with further increasing stress amplitude with a channel width of about 0.5 mm. The relationship between the development of dislocation structures and stress amplitude during cyclic deformation has already been investigated from both experimental and theoretical points of view mainly on Cu. 27,28) The cyclic stress (t) of materials with periodically arranged dislocation walls is expressed by the sum of the stress necessary for dislocation to bow out into the channel (t bowing ) and interaction stress (t dip ) of screw dislocation dipoles within the channel. t bowing is expressed by Gb/d c andt dip is by Gb/4ph, where G, b, d c and h are shear modulus, Burgers vector, channel width and interval of slip plane...
