Evolution of Dislocation Structure and Fatigue Crack Behavior in Fe–Si Alloys during Cyclic Bending Test

Abstract

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...

Effects of Si on Microstructural Evolution and Mechanical Properties of Hot-rolled Ferrite and Bainite Dual-phase Steels

“…It has been reported that the addition of Si lowered stacking fault energy to make it more difficult for dislocations to cross slip in Fe-Si alloys. 17) Moreover, the increasing ds/de value by Si addition in interstitial free (IF) steels was confirmed due to the difficult formation of dislocation cell structure. 18,19) Thus, the solute Si atom in ferrite influenced its dislocation interactions during deformation so as to increase the strain-hardening rate, and therefore, total and uniform elongations at a given strength level.…”

Mechanical properties of Fe-rich Si alloy from Hamiltonian

“…A large-scale 3D dislocation dynamics simulation 57 suggests a cell structure formation due to the easy cross slip, which is also observed as a cell-like pattern in the plane perpendicular to the Burgers vector. 55 On the other hand, in the case of Fe-Si alloy, since the cross slip becomes difficult, a dislocation stuck to other immobile dislocations leads to work hardening. Eventually, dislocations may form the vein structure, which is usually observed in dilute Fe-Si alloys as mentioned above.…”

“…However, the framework does not answer the question why the most preferable slip system often changes by the addition of a solute atom. 48,55 In fact, in the Fe-Si dilute alloys, the {110}<111> slip system becomes the most preferable, 48,55 while the {110}<111> and {112}<111> slip systems are equally preferable in pure Fe. To answer this question, we constructed a spatiotemporal coarse-grained kMC 56 based on the energy barriers of kink nucleation and migration with and without solute Si.…”

“…It has been reported 55 that solute Si changes the work hardening rate and fatigue properties by changing the dislocation substructure. In fact, dislocations distribute in a more planar manner in the {110} planes, and the dislocation substructure becomes a vein structure, which is composed of alternate high and low dislocation density regions.…”

Influence of Cutting on the Properties of Clippings From Electrical Sheets

“…While the stator of driving engine consists of segments cut out from the sheets of grain nonoriented electrotechnical steel, the rotor is usually made by casting or sintering of powder materials, which considerably increases already high production costs. Also the resistance of these sintered materials toward dynamic fatigue is not sufficient [4,5]. Nowadays the attention is focused on the development of high-strength electrotechnical steels exhibiting the combination of demanding mechanical and good electro-magnetic properties.…”