Cyclic Slip Localization Research Articles

Importance of γ′ shearing in low‐cycle fatigue of a wrought superalloy

AbstractThe present study deals with the fatigue behavior of a wrought nickel‐based superalloy René 41. Uniaxial, fully reversed low‐cycle fatigue tests were conducted at room and elevated (800°C) temperatures under total strain control. Cyclic response was analyzed using the statistical theory of the hysteresis loop, and scanning and transmission electron microscopies were employed to study relevant damage mechanisms. At room temperature, cyclic deformation was found to localize to slip bands since the first cycle, and most slip bands were formed very early in the fatigue life. Hence, shearing of the strengthening phase, coherent γ′ precipitates, was found to be significant only in the first few cycles. At 800°C, fatigue life was affected by an environmental attack resulting in a surface oxide layer. Cyclic deformation was less localized; nevertheless, cyclic slip localization in persistent slip bands and resulting γ′ shearing contributed to cyclic softening throughout the fatigue life.

Cyclic deformation and cracking behavior of 316LN stainless steel under thermomechanical and isothermal fatigue loadings

Isothermal fatigue (IF) tests at 250 °C, 350 °C and 450 °C, and thermomechanical fatigue (TMF) tests of 316LN stainless steel within the temperature range of 250–450 °C under in-phase (IP) and out-of-phase (OP) mechanical strain-temperature phase angles were carried out. Comparison of the cyclic deformation behavior, microcrack nucleation mechanisms, crack growth and failure behavior were performed between the IF test at 450 °C (IF-450 °C) and TMF tests. Results showed that the cyclic deformation behavior was governed by the combination of thermally activated recovery process, dynamic strain aging (DSA) and nitride precipitations for IF-450 °C and TMF tests. The occurrence of DSA and nitride precipitations including Z-phase and MX phase were found to promote the planar slip of dislocations and enhance the cyclic stress response. The occurrence of recovery process, however, offset the strengthening effects caused by DSA and precipitations, accelerating the softening stage. The intergranular cracking was attributed to the strong impingement of slip bands on severely oxidized grain boundaries, which can be found only on the specimen surface or near-surface regions. The transgranular cracking, resulting from the cyclic slip localization which was characterized by the formation of persistent slip markings (PSMs) in the form of surface intrusions and extrusions was enhanced by the presence of brittle oxide. The ultimate failure was still dominated by the transgranular propagation.

Microcrack nucleation and early crack growth of a nuclear grade nitrogen alloyed austenitic stainless steel X2CrNiMo18.12 under thermomechanical fatigue loading

The continuous and interrupted in-phase thermomechanical fatigue (IP-TMF) tests under mechanical strain-controlled mode were conducted to investigate the microcrack nucleation mechanisms and early growth behavior of a nuclear grade nitrogen alloyed austenitic stainless steel (ASS) X2CrNiMo18.12. The temperature interval of 250–450 °C, cycling period of 120s and strain amplitude of 0.8% were used. The principal deformation and damage mechanisms activated in the present work are attributed to dynamic strain aging (DSA), thermal recovery and oxidation. The oxidation-assisted intergranular cracking was attributed primarily to the slip-bands impingement and the thermal mismatch between oxide scales and substrate. The formation of persistent slip markings (PSMs) in the form of extrusions and intrusions due to occurrence of cyclic slip localization also provided a favored site for microcrack nucleation. Additionally, both the oxidized grain boundaries and PSMs acted as the preferential path for the early crack growth leading to the final failure.

Basic Mechanisms Leading to Fatigue Failure of Structural Materials

General features of the damage evolution in cyclic loading of structural materials are summarized. The attention is paid to the comparison of the damage mechanisms in materials that are used for service both at room and at elevated temperatures, namely austenitic stainless steel Sanicro 25. Principal mechanisms leading to fatigue fracture at room and at elevated temperature are documented. While cyclic slip localization is a decisive process in the initiation of fatigue cracks at room temperature, the localized oxidation plays an important role in isothermal high temperature cyclic loading. Specific mechanisms of the early fatigue damage in thermomechanical cyclic loading are studied. The in-phase thermomechanical loading leads to the intergranular crack initiation due to preferred grain boundary oxidation and intergranular crack growth. The out-of-phase thermomechanical loading results in oxide cracking and localized oxidation of the metal in the area of cracked oxides and to transgranular crack growth. The damage mechanisms can explain differences in fatigue life under various loading and temperature conditions.

Cyclic Plastic Response and Damage in Materials for High Temperature Applications*

The characteristic changes of the cyclic plastic stress-strain response of stainless steel and superalloys derived from the analysis of hysteresis loop at different temperatures are reported. The evolution of the surface relief in strain controlled cycling is documented using high resolution SEM and TEM. The mechanisms leading to cyclic strain localization, formation of surface relief, fatigue crack initiation and early crack growth are discussed. The study of the low cycle fatigue behavior of these materials was concentrated mostly to the analysis of the stress-strain response, internal structures, fatigue life and crack growth (1-3). In recent years, considerable attention has been attracted to the study of the early damage mechanisms in crystalline materials, starting with the cyclic slip localization and resulting finally in the initiation of fatigue cracks and their early growth (4, 5). In this contribution, the methods allowing to study the sources of cyclic stress and early fatigue damage at room and elevated temperatures are presented and used to reveal details of the mechanisms of cyclic plastic straining and fatigue damage evolution in austenitic steels and nickel superalloys. 1. Cyclic Plastic Stress-Strain Response. Cyclic plasticity of materials is usually studied in strain- controlled cycling with constant strain rate, since stress response of the material depends not only on temperature but also on the strain rate. In a standard low cycle fatigue experiment, several specimens are cycled in a symmetrical cycle with constant total or plastic strain amplitudes, and their stress response is recorded and evaluated. Hysteresis loops can be stored in computer memory, and stress, strain and plastic strain amplitudes are evaluated during the fatigue life. The plot of the stress amplitudes vs. number of loading cycles represents the fatigue hardening/softening curves. The plot of the saturated stress amplitude (or stress amplitude at half-life) vs. strain or plastic strain amplitude represents the cyclic stress-strain curve. In order to find the sources of the cyclic stress, more detailed analysis is necessary. Hysteresis loop and the analysis of its shape using the generalized statistical theory (6) can yield information on the sources of the cyclic

Low cycle fatigue behavior of Sanicro25 steel at room and at elevated temperature

Austenitic heat resistant Sanicro 25 steel developed for high temperature applications in power generation industry has been subjected to strain controlled low cycle fatigue tests at ambient and at elevated temperature in a wide interval of strain amplitudes. Fatigue hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated at room temperature and at 700°C. The internal dislocation structures of the material at room and at elevated temperature were studied using transmission electron microscopy. High resolution surface observations and FIB cuts revealed early damage at room temperature in the form of persistent slip bands and at elevated temperature as oxidized grain boundary cracks. Dislocation arrangement study and surface observations were used to identify the cyclic slip localization and to discuss the fatigue softening/hardening behavior and the temperature dependence of the fatigue life.

Cyclic Slip Localization and Crack Initiation in Crystalline Materials

Cyclic plastic straining in crystalline materials is localized to persistent slip bands (PSBs) and results in formation of persistent slip markings (PSMs) consisting of extrusions and intrusions. Intensive plastic strain in PSBs results in dislocation interactions and formation of point defects. The extended model based on point defect formation, migration and annihilation is presented describing surface relief formation in the form of extrusion-intrusion pairs. Point defect migration and resulting mass transfer is the principle source of cyclic slip irreversibility leading to crack-like defects - intrusions. Fatigue cracks start in the tip of sharp intrusions.

Strain localization and fatigue crack initiation in ultrafine-grained copper in high- and giga-cycle region

Initiation of fatigue cracks and early crack propagation in high-cycle and giga-cycle region in ultrafine-grained copper prepared by equal channel angular pressing was experimentally investigated. The cyclic slip localization takes place in cyclic slip bands whose length substantially exceeds the grain size. The mechanism of the crack initiation under load controlled cycling does not require the dynamic grain coarsening often indicated in literature. Fatigue cracks initiate in slip bands, which develop predominantly in zones of near-by oriented grains. The damage mechanism consists in localized slip resulting in development of cavities and voids arranged along the planes of highest cyclic shear stress and in formation of surface relief. A process of growth and linking of cavities and voids produced by the irreversible cyclic slip by dislocation movement generating point defects governs the early stage of the development of fatigue cracks. Sufficiently large cracks created by this mechanism and lying in suitably oriented long slip bands finally transform into fatigue cracks propagating by common opening mode with a plastic zone generated at their tip.

Microstructural extremes and the transition from fatigue crack initiation to small crack growth in a polycrystalline nickel-base superalloy

The fatigue behavior of the nickel-base superalloy René 88 DT has been investigated at room temperature with fully reversed loading in an ultrasonic fatigue apparatus operating at a frequency close to 20kHz. A characterization protocol based on the electron backscatter diffraction technique has been developed to identify the combination of microstructural features within crack initiation sites and surrounding neighborhoods that leads to the transition from initiation to early small crack growth. Surface grains that were more than three times the average grain size, that were favorably oriented for cyclic slip localization and that also contained Σ3 twin boundaries inclined to the loading axis were most favorable for fatigue crack initiation. Fatigue cracks subsequently grew in grain clusters within which grains are misoriented by less than 20° relative to the initiation grains. More highly misoriented neighboring grains resulted in crack arrest. The material characteristics that promote crack initiation and small crack growth exist only at the extreme tails of the microstructural distributions. The implications for modeling of fatigue life and fatigue life variability are discussed.

Cyclic plasticity and strain localization in cast γ-TiAl based alloy

Cyclic strain localization and observation of surface relief in nearly lamellar cast γ-TiAl based alloy Ti–48Al–2Nb–2Cr–0.82B (all in at. %) cycled in strain control were experimentally studied. Hysteresis loop area, the cyclic hardening curves, the cyclic stress-strain curve and VH parameter were measured. Surface relief of cyclically strained specimens was observed using scanning electron microscopy. The variations of the VH parameter indicate the cyclic slip localization. The localization has a pronounced effect on surface relief formation. Persistent slip markings are formed on the surface along interlamellar interfaces γ/α2 and γ/γ. These locations were nuclei for fatigue cracks in the interior of grains. Experimental results concerning the loop shape parameter and surface relief are used to discuss the role of cyclic slip localization on the cyclic stress-strain response and fatigue crack initiation.

Initiation and propagation of fatigue cracks in cast IN713LC superalloy

Abstract Fatigue life, initiation and propagation of cracks at 800 °C in a cast Ni-base superalloy IN 713LC were experimentally studied in high-cycle fatigue region. Load symmetrical cycling and cycling with high tensile mean load were applied. Both crystallographic crack initiation resulting in long Stage I crack growth and non-crystallographic Stage II propagation were observed. High scatter of fatigue life data was explained by: (i) variability in microstructural conditions for crystallographic crack initiation and propagation and by (ii) influence of casting defect size distribution. The fractographic observation supports the slip band decohesion mechanism of crack initiation and an important role of cyclic slip localization in persistent slip bands.

Dislocation Structures in Nickel Based Superalloy Inconel 792-5A Fatigued at Room Temperature and 700°C

Dislocations structures were studied using transmission electron microscopy in specimens of cast polycrystalline nickel based superalloy Inconel 792-5A cyclically strained up to failure with constant total strain amplitudes at temperatures 23 and 700 °C. Spatial arrangement of dislocations in individual grains was determined using the technique of oriented foils. Planar dislocation arrangements in the form of bands parallel to the {111} planes were observed in specimens cycled at both temperatures. The bands showed up as thin slabs of high dislocation density cutting both the γ channels and γ´ precipitates. They correspond to areas of cyclic slip localization. Experimental results concerning the dislocation structure are used to discuss the effect of temperature on the cyclic stress-strain response and fatigue life.

AFM and TEM study of cyclic slip localization in fatigued ferritic X10CrAl24 stainless steel

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.

Cyclic hardening and slip localization in single slip oriented copper crystals

A static-dynamic model is applied to the interpretation of slip localization modes observed in a systematic study of the evolution of dislocation structures in single slip intermediate amplitude fatigue in copper deformed at room temperature. The model assumes that veins do not deform plastically, unless their critically soft interiors are destabilized to produce an embryonic PSB. This assumption is shown to be fully consistent with slip homogeneity. The model predicts that the vein structure remains active during cyclic saturation. This prediction explains the present novel observation that in intermediate amplitude fatigue, after the initial formation of PSBs by «collapse» of matrix walls, triggered by long-range internal stresses, there is continued formation of new PSBs by «splitting» of matrix veins. The model shows that the long-range internal stresses in the thin PSB lamellae and in the more extended wall structures produced by this process are about equal. Nevertheless, the model and microscopy suggest convincingly that minimization of dislocation line energy in the dynamic structure between glissile walls and veins controls the condensation mode and the equilibrium wall spacing of PSBs

The role of cyclic slip localization in fatigue damage of materials

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 present the physical manifestation of the fatigue damage

Cyclic stress-strain response of polycrystalline copper in a wide range of plastic strain amplitudes

Polycrystalline copper was cyclically strained with constant plastic strain amplitude from a very wide interval (3×10 −6−1×10 −2). Fatigue hardeing-softening curves and the plot of the loop shape parameter vs. the number of cycles were evaluated. Fatigue softening observed in low amplitude cycling was studied in detail. Measurements of the loop shape parameter which indicated the cyclic slip localization were accompanied by surface relief observations and electronmicroscope observations of the dislocation structure. Several mechanisms of the fatigue softening were considered. The basic cyclic stress-strain curve was evaluated and the difference between this curve and curves using apparent saturation values of the stress amplitude were discussed.