Slip Activity of Persistent Slip Bands in early Stages of Fatigue Life of Austenitic 316L Steel (original) (raw)
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AFM and SEM-FEG study on fundamental mechanisms leading to fatigue crack initiation
International Journal of Fatigue, 2015
Early stages of surface relief evolution of persistent slip markings (PSMs) in polycrystalline 316L austenitic stainless steel cycled with constant plastic strain amplitude at 93, 173 and 573 K were studied using atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM-FEG). Qualitative and quantitative data on the morphology of PSMs, occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported for all temperatures. PSMs start in all cases as surface extrusions which are later accompanied by formation of intrusions. This finding is discussed with respect to the point defect formation within areas of localized cyclic slip and primarily to their mobility at different temperatures. Consequences of migration of respective point defects for surface relief formation and the conditions for creation of fatigue crack embryos, i.e. sharp intrusions are highlighted.
AFM study of surface relief evolution in 316L steel fatigued at low and high temperatures
Procedia Engineering, 2010
Surface relief of austenitic stainless 316L steel cycled with constant plastic strain amplitude at 93, 173 and 573 K to different early stages of fatigue life was studied using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM-FEG). Characteristic features of surface relief topography and the evolution of persistent slip markings (PSMs) were documented for all temperatures. At 173 and 573 K PSMs consist of growing extrusions accompanied later by parallel intrusions while at the lowest temperature only static extrusions were found. In addition to distinct true PSMs fine slip markings with peakto-valley topography covering homogeneously different parts of grains and deformation induced martensite were detected in 316L steel fatigued at 93 K. Experimental results on the PSM topography and the kinetics of extrusion growth are discussed in relation to point defect models of surface relief formation leading to fatigue crack initiation.
AFM and TEM study of cyclic slip localization in fatigued ferritic X10CrAl24 stainless steel
Acta Materialia, 2004
Atomic force microscopy and high resolution scanning electron microscopy were applied to the study of surface relief evolution at emerging persistent slip bands (PSBs) in individual grains of ferritic X10CrAl24 stainless steel cycled with constant plastic strain amplitude. Only the combination of both methods can reveal the true shape and fine details of extrusions and intrusions. Quantitative data on the changes of the surface topography of persistent slip markings and on the kinetics of extrusion growth during the fatigue life were obtained. Transmission electron microscopy of surface foils revealed PSBs with the typical, well-known ladder structure. Experimental data on cyclic slip localization in PSBs are compared with those in fcc metals and discussed in terms of vacancy models of surface relief evolution and fatigue crack initiation.
Materials Science and Engineering: A, 2008
The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.
The role of cyclic slip localization in fatigue damage of materials
Le Journal de Physique IV, 1993
Cyclic slip localization is demonstrated in several classes of materials as copper single crystals, polycrystals and in a duplex austenitic-ferritic stainless steel, in measuring the cyclic stress-strain response, studying the surface relief evolution and the internal dislocation structures. The characteristic structure of the persistent slip bands accommodating high local plastic strain amplitude is correlated with the extrusion/intrusion formation on the surface, which results in fatigue crack nucleation, short crack growth, dominant crack formation, its growth and fracture. These stages represent the physical manifestation of the fatigue damage.
Philosophical Magazine, 2005
The early stages of formation of persistent slip markings in fatigue are analysed, using threedimensional discrete dislocation dynamics modelling. The surface displacements due to slip are computed using a specific post-processing method. Fatigue simulations under various strain ranges and grain sizes have been performed. The resulting surface slip markings and their evolutions are analyzed quantitatively, in terms of the marking height and thickness. A detailed scheme for persistent slip marking formation and morphology is proposed, in relation to the persistent slip-band dislocation arrangements present within the grain. The simulations show the crucial role of these arrangements for the extrusion-intrusion growth and localisation of slip at the band edges. Local stress concentrations inside the crystal and their relations to damage initiation are also analysed. The results provide insights for an original micro-crack initiation scheme, combining different initiation mechanisms as described in the literature.
Nucleation of Slip Bands near Twin Boundary in High-Cycle Fatigue
JSME International Journal Series A, 2004
In order to investigate the criterion for the persistent slip band (PSB) formation near a Σ3(111) coherent twin boundary, where is a preferential site for formation of PSB, a highcycle fatigue test is carried out at room temperature using a copper bicrystal specimen with the twin boundary. PSBs are observed near the boundary in the connection with the evolution of extrusion/intrusion on the specimen surface. The slip behavior is different from that predicted by the Schmid factor. A finite element method (FEM) analysis is conducted for the bicrystal, and it reveals that the increase of resolved shear stress on the specific slip system due to the constraint of deformation between the crystals is the main cause for promoting the nucleation of PSB near the twin boundary. By taking into account the interaction between the primary and the secondary slip systems, it is possible to specify the formation behavior of PSB near the twin boundary.
2016
Strain controlled fatigue tests were conducted on polycrystalline copper at ∆ε/2=0.161 % and 0.255% to various fractions of fatigue life. Scanning electronic microscope (SEM) and atomic force microscopy (AFM), a relatively new tool that readily provides high resolution digitized images of surface features, were used to describe and quantitatively characterize the evolution of surface deformation during fatigue. To quantify surface deformation, a parameter γirrev is defined that is a measure of the local slip irreversibility at the surface. This parameter applies to any type of surface deformation feature, is independent of the size of the fields of view and yields information on the distribution of surface strains over the specimen gage length. From this formalism, a criterion for crack nucleation is proposed using actual AFM surface strain measurements.
Philosophical Magazine, 2017
A 3D dislocation dynamics study to ascertain the probable path of stage-I fatigue crack propagation across the persistent slip band (PSB) in austenitic stainless steel is presented. Cyclic plasticity and the resulting crack tip slip displacement (CTSD) are evaluated for cracks of varying length introduced at PSB-center and at two PSB-matrix interfaces. CTSD attains high value at either of the two interfaces irrespective of the proximity of crack front to the grain boundary. Further, a difference in microcrack propagation rate is also observed among the two interfaces. The present results assert microcrack propagation preferrentially along one of the two PSB-matrix interfaces rather than at the PSB-center. A preexisting PSB dislocation structure localises the cyclic slip for crack lengths up to approximately half of the grain depth for an applied strain range of 2 × 10 −4 .