Dislocation mechanisms of fatigue crack initiation in high-cycle fatigue are formulated, with special consideration given to those material properties which determine the cyclic slip mode. The models developed are related to pertinent experimental observations, referring mainly to copper mono- and polycrystals fatigued at room temperature. In particular, the following topics are considered: (1) the origin of cyclic slip irreversibilities in the bulk and near the surface, (2) computer simulations of surface roughening by random irreversible slip processes in planar- and wavy-slip materials, and (3) fatigue crack initiation by cyclic strain localization in persistent slip bands (PSBs) in mono- and polycrystalline wavy-slip materials. The evolution of the surface profile at emerging PSBs is described by a new semiquantitative model which is compared with other models. The model distinguishes between the rather rapid formation of extrusions and the more gradual development of surface roughness. It is shown that specific predictions regarding differences between PSB surface-profiles in mono- and polycrystals are borne out fully by the observations. In the case of copper polycrystals, the experiments show that PSBs are not only responsible for slip-band cracking in PSBs but also induce intergranular cracks. A model is proposed according to which cracks initiate as a result of the piling-up of PSB-matrix interface dislocations against the grain boundaries. Furthermore, the supporting role of environmental interaction and/or deformation-induced diffusion and grain boundary sliding is discussed.
I<. DIFFERT e t al. : Models of Particle Destruction in Fatigued Alloys 95 phys. stat. sol. (a) 104, 95 (1987) Dedicated to Prof. Dr. A. SEEGER on the occasion of his 60th birthday I n cyclic deformation experiments prcaipitation-hardened alloy3 in many cases show a softening effect after a hardening period. The softening occurs because tho precipitates are destructed by gliding dislocations. For the destruction mainly two mechanisms are suggested. i) Disordering of ordered particles or ii) dissolution a,fter repearted cutting by gliding dislocations. In this paper models for mechanism ii) are suggested which ascribe the destruction of particles in "wavy" slip materials t o irreversible glide of dislocations and in "planar" slip materials to the cutting of t h e precipitates by series of dislocations.Bei Wechselverformungsexperimenten an ausscheidungsgeharteten Legierungen folgt in vielen Fallen auf eine anfiingliche Verfestigung eine Entfestigung. Letztere ist darauf zuruckzufiihren, daS gleitende Versetzungen die Teilchen irreversibel durehschneiden. Dabei konnen 1. geordnete Ausscheidungen entordnet werden oder 2. Ausscheidungen in Scheiben zerlegt werden. I n beiden Fallen nimmt die verfestigende Wirkung der Teilchen ab. In dieser Arbeit wird der 2. Fall betrachtet. E s werden Modelle untersucht, die die Zerstorung der Teilchen im Falle ,,welliger" Gleitung irreversiblen Gleitprozessen einzelner V~rsetzungen und im Falle ,,planarer" irreversibler (ilcitung von Versetzungsgruppen zuschreiben.
I . IntrodixctionCyclically strained two-phase alloys may or niay not show a softening effect after a hardening period. The softening effect occurs when the precipitates of the second phase are penetrated by dislocations. As Calabrese and Laird [l, 21 showed, the possibility of penetration depends on nature, size, and distances of the particles. In the case of softening, which will be considered in this paper, during the initial cycles the particles hinder the dislocation movement and lead to an additional hardening effect as compared to pure materials. Subsequently the dislocations penetrate and destroy the particles, which leads to a softening effect. Calabrese and Laird [l] suggested as the major mechanism for softening the disordering of particles of ordered structure. More recently Steiner et a1.[3] showed convincingly that the softening effect can also be caused by the dissolution of the particles. They studied Cu-2 atyo Co single crystals where the coherent precipitates consist of nearly pure cobalt.Many pure metals and alloys show during fatigue the evolution of persistent slip bands (PSBs) in which the pIastic deformation is localized. The softening effect arises from disordering or dissolution of particles which lie within active PSBs..~ l) Heisenbergstr. 1, D-7000 Stuttgart 80, FRG. 2, Martensstr. 5 , D-8520 Erlangen, FRG.
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