Dislocation Structure and Strain Localisation in Nickel Single Crystals Cyclically Deformed at 77 K

Abstract

An experimentally based investigation is presented of the dislocation structure and of glide effects occurring in single slip oriented nickel crystals cyclically deformed at 77 K until saturation of the stress amplitude. Special attention is paid to a comparison of slip and structure phenomena observed in fatigue tests at 77 K and those found after cycling at room temperature (RT) and elevated temperatures. At strain amplitudes within the plateau region of the cyclic stress‐strain curve, where at higher temperatures in the crystal two structure types co‐exist, at 77 K nearly the entire specimen volume is occupied by one structure “phase”, a dislocation‐“condensed” wall configuration. On different scale levels the main characteristics of this extended wall structure were found to be independent of the imposed amplitude, and they turned out to fit in the temperature dependence of the structure features of the ladder‐like wall phase characterising the zones of intense slip (persistent slip bands) at RT and elevated temperatures. At 77 K the strain is localised in narrow slip bands (SBs) in the same way as at higher temperatures, although there is no indication of a “two‐phase” structure. From the experimental findings it is concluded that WINTERs “two‐phase” model remains valid, when averaging the plastic strain values over all SBs and over a sufficient number of cycles.

Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth

“…Typically, the local plastic shear strain amplitude in the PSBs is about 1%, compared with the much lower value of about 10 −4 in the matrix [40,41]. This behaviour is representative of FCC materials like copper or nickel and is studied best on single crystals [8][9][10][11][12][13][14][15][42][43][44]. PSBs develop only if well-defined thresholds of the plastic strain and the stress amplitude are exceeded.…”

Cyclic Slip Irreversibilities and the Evolution of Fatigue Damage

“…Again, these studies were mainly performed on copper crystals [52,[65][66][67][68] but more recently also on nickel single crystals [69,70] and polycrystals. [71,72] Thus, Watt et al [65] and Basinski et al, [66] using optical microscopy, concluded that slip steps generated in one half-cycle almost disappeared in the subsequent reverse half-cycle.…”

“…These results were substantiated by the Basinskis [22,67] with respect to the nonuniformity of cyclic strains within PSBs and the much larger local strains in so-called persistent slip ''lines.'' Even more detailed information was obtained by the Dresden group [69,70] by atomic force microscopy on fatigued nickel single crystals, who quantified an entire spectrum of local strains within PSBs and emphasized that considerable variations in local strain occurred within one and the same PSB as cycling was continued. Subsequently, Weidner et al combined atomic force microscopy (AFM) and SEM and characterized the half-cycle slip activity in PSBs in nickel polycrystals and the variations in the course of fatigue in considerable detail.…”

“…In the last 25 years, several groups have studied the surface reliefs of fatigued metals in more detail, making use of novel techniques that had not been available in most of the very early investigations, such as SEM, contamination line profile analysis by SEM, [26,58,98] AFM, [40,41,69,70,[99][100][101][102] and SEM applied to specimens sectioned with the so-called sharp corner technique developed by the Basinskis [22,60,67,103] and subsequently applied also by Laird and co-workers [59,104] or by the similar micromilling technique of Neumann's group. [18,105] In general, the surface profiles of PSBs observed by different authors have been found to be quite complex and to differ in details that depend on the type of fatigue testing and on the observational techniques employed.…”

Atomic force microscopy of the intense slip localization causing fatigue crack initiation in polycrystalline brass

“…Characterization of nanodamaged surfaces of fatigued metals has been revolutionized by SEM applied in the electron channelling contrast (ECC) mode [9], which can be complemented with nanoscale characterization methods like AFM to investigate slip line damage caused by heterogeneous PSB plasticity, as illustrated by studies of fatigue crack initiation in polycrystalline copper [10], nickel single crystals fatigued at 77 K [11] (where PSBs are not observed) and industrial alloys [12,13]. Here we present the first results of an experimental study using grain orientation determination by electron back-scattering diffraction (EBSD), ECC imaging of fatigue-induced microstructure and characterization of surface topography by AFM.…”