Constant Plastic Strain Amplitude Research Articles

Characterization of dislocation microstructures in near-[formula omitted] single crystal copper using high-voltage scanning transmission electron microscopy

Metallic structures are prone to failure caused primarily by fatigue-related damage to the structural material. Fatigue behavior is strongly correlated with dislocation structures in metals. Therefore, investigating the formation of dislocation structures is essential for understanding the fatigue process. The dislocation structure in face-centered cubic crystals has a strong orientation dependence. However, the detailed characteristics and evolution process of dislocation structures of fatigued [1‾11] single crystal copper are still insufficient. This study investigated dislocation structures in near-[1‾11] single crystal copper cyclically deformed at room temperature under different constant plastic shear strain amplitude (γpl) to analyze the formation mechanism of dislocation structures and their relationship with fatigue behavior. Characteristic dislocation structures were characterized by adopting multiple planes using a high-voltage scanning transmission electron microscope. With increasing shear strain amplitude, a vein-like structure along the (1‾11) plane gradually developed into a wall structure. When γpl ≥ 1 × 10−3, deformation bands and cell bands were formed to accommodate the higher plastic strain. The state of cell-band development significantly affected the cyclic softening of the single crystal copper. A positive linear relationship existed between the shear stress amplitude and the reciprocal of the cell width.

Influence of Plastic Strain Control on Martensite Evolution and Fatigue Life of Metastable Austenitic Stainless Steel

Metastable austenitic stainless steel was investigated during fatigue tests under strain control with either constant total or constant plastic strain amplitude. Two different material conditions with coarse-grained and ultrafine-grained microstructure were in focus. The influence of plastic strain control of the fatigue test on both the martensitic phase transformation as well as on the fatigue lives is discussed. In addition, an approach for calculating the Coffin–Manson–Basquin parameters to estimate fatigue lives based on strain-controlled tests at constant total strain amplitudes is proposed for materials undergoing a strong secondary hardening due to martensitic phase transformation.

Cyclic deformation and dislocation structure evolution of a copper bicrystal with components rotating gradually along the grain boundary

Abstract The push –pull cyclic deformation of a copper bicrystal with components rotating gradually along the grain boundary (GB) was carried out under constant plastic strain amplitude at room temperature in air. The evolution of slip morphology and dislocation structure was observed by scanning electron microscopy electron channelling contrast technique. It was shown that slip morphology and dislocation structure in one of the grains changed gradually along the GB. This phenomenon can be understood based on the local irreversible rotation of crystals, which exists during the symmetrical push– pull loading. When the secondary slip was localized to form the secondary persistent slip bands, which interacted with the regular walls in the second type of deformation band, then the cell structure was formed. The second type of deformation band is one kind of sites for cell formation.

Influence of ECAP Processing on Fatigue Crack Nucleation on ZK60A Magnesium Alloy

Low-cycle fatigue tests were conducted on ZK60A magnesium alloy that were processed by equal-channel angular pressing (ECAP) method. The ECAP processing was carried out on a hot-extruded ZK60A alloy at a die temperature of 473K. Total pass number was four and the processing route was Route Bc. One of the ECAPed sample was annealed at 573K to induce grain coarsening. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of εpl=2×10-3, in air at room temperature. A hysteresis loop shape during the low-cycle fatigue tests depended on the sample: a tension-compression asymmetric shape was recorded in the as-drawn specimen having the tensile axis parallel to the extruction direction, while a symetric shape was seen in case of the tensile axis perpendicular to the extrusion direction. Deformation twinning during fatigue was most probably attribute to the asymmetric hysteresis loop shape. Because the asymmetric hysteresis loop was absent in the ECAPed specimen, it is possible that grain refinement suppressed the deformation twinning in the fatigue test. In the as-drawin specimen having the tensile axis parallel to the extrusion direction, a small crack was observed near a deformation twin. By contrast, a fatigue crack in the as-ECAPed specimen was generated along a shear band that was inclined to the tensile axis at 45°. After the ECAPed specimen was annealed at 573K, an intense shear banding and a subsequent crack nucleation were moderated. Accordingly, it can be said that the low-temperature annealing on the ECAPed ZK60A magnesium alloy reduced the fatigue crack nucleation sites.

Characterization of dislocation microstructures in cyclically deformed [001] copper single crystals using high voltage scanning transmission electron microscopy

Low-cycle fatigue tests of copper single crystals with the [001] stress axis were performed at room temperature under constant plastic shear strain amplitudes between γpl=3.5×10−4 and 1.0×10−2. Characteristic dislocation structures, which are highly dependent on a given plastic strain amplitude, were observed by high voltage scanning transmission electron microscopy. A vein-like dislocation structure, having a parallelepiped shape with two longitudinal (100) and (001) sides, was periodically formed along the [010] direction at around γpl=1.0×10−3. Labyrinth structure with the (100) and (001) dislocation walls was then gradually developed with increasing plastic strain amplitude.

Influence of local crystallographic configuration on microcrack initiation in fatigued 316LN stainless steel: Experiments and crystal plasticity finite elements simulations

Local crystallographic configurations (also referred to as local micro-texture) which promote transgranular micro-crack initiation in 316LN stainless steel in low cycle fatigue are studied. Specimens were subjected to tension-compression with constant plastic strain amplitude, in air, at room temperature, during 5000 cycles (i.e. about 20% of the fatigue life). The first part of this work is devoted to a statistical analysis of slip marks and cracks observed at surface of one fatigued specimen using scanning electron microscope (SEM), in a region composed of about 1000 grains. 95 micro-cracks initiated along persistent slip markings detected in this region are analyzed with respect to different characteristics of grains, especially crystallographic orientation, measured using electron backscatter diffraction (EBSD). From the detailed analysis of the numerous data derived from these observations and measurements performed only at surface, the two main significant factors which are found to favour crack formation are the grain size and the orientation of the activated slip system with respect to the surface. Indeed, the mean size of grains which contain cracks is almost twice the one of the remaining grains. Moreover, for most grains in which cracks are observed, the angle between the normal to the surface and the activated Burgers vector (resp. the normal to the activated slip plane) lies in the range [30°, 50°] (resp. [55°, 70°]). No other characteristic was found to provide significant and direct information in order to identify initiation sites. Thus, in the second part of this work, the analysis of initiation sites is performed using additional information relative to three-dimensional (3D) aspects of the microstructure. 3D characterisation of the polycrystalline microstructure and some cracks in one fatigued specimen was achieved using serial-sectioning technique combined with SEM and EBSD. As an example, the study of one specific crack and its surrounding microstructure is presented, including crystal plasticity finite element (CPFE) simulation based on 3D mesh of the polycrystal in this studied region (composed of 386 grains). It is found that the predicted plastic slip activity is more intense within the grain where cracks have been actually observed. This study illustrates that CPFE simulations can provide consistent prediction of slip activity at surface of polycrystals, at least qualitatively, if the actual 3D microstructure is taken into account.

AFM and SEM-FEG study on fundamental mechanisms leading to fatigue crack initiation

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 573K 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.

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 of Fe-20%Cr alloy processed by equal- channel angular pressing

Low-cycle fatigue properties were investigated on Fe-20%Cr ferritic stainless steel processed by equal channel angular pressing (ECAP). The Fe-20%Cr alloy bullets were processed for one to four passes via Route-Bc. The ECAPed samples were cyclically deformed at the constant plastic strain amplitude εpl of 5x10−4 at room temperature in air. After the 1-pass ECAP, low-angle grain boundaries were dominantly formed. During the low-cycle fatigue test, the 1-pass sample revealed the rapid softening which continued until fatigue fracture. Fatigue life of the 1-pass sample was shorter than that of a coarse-grained sample. After the 4-pass ECAP, the average grain size reduced down to about 1.5 μm. At initial stage of the low-cycle fatigue tests, the stress amplitude increased with increasing ECAP passes. At the samples processed for more than 2 passes, the cyclic softening was relatively moderate. It was found that fatigue life of the ECAPed Fe-20%Cr alloy excepting the 1-pass sample was improved as compared to the coarse-grained sample, even under the strain controlled fatigue condition.

Quantitative measurements of dynamic precipitation during fatigue of an Al–Zn–Mg–(Cu) alloy using small-angle X-ray scattering

The room temperature cyclic deformation behaviour of an Al–Zn–Mg–(Cu) alloy was examined in the under-aged and peak-aged states. The tests were performed under constant plastic strain amplitude loading in tension–compression mode, and the effect of cumulative strain (number of cycles), strain rate and plastic strain amplitude was examined. Pronounced cyclic hardening was observed in the under-aged states, and characterization of the microstructure using small-angle X-ray scattering illustrated that this was due to profuse dynamic precipitation at room temperature during cycling. Regardless of deformation conditions, the radius of the dynamically formed GP zones remained constant at 4–5Å, and only the volume fraction of the zones increased during cycling. The strain, strain rate and plastic strain amplitude dependence of the evolution of the volume fraction of zones forming dynamically was interpreted in terms of a nucleation-dominated reaction, and it is shown that the behaviour can be satisfactorily rationalized when the role of strain-induced vacancies is considered.

On the nature of acoustic emission and internal friction during cyclic deformation of metals

A strong correlation between acoustic emission (AE) and internal friction (IF) has been established experimentally during rapid hardening of model copper polycrystals and single crystals deformed cyclically at constant plastic strain amplitudes. The difference between these two interrelated phenomena becomes pronounced when strain localization occurs in persistent slip bands or deformation bands. We demonstrate that the extension of the Kocks–Mecking–Estrin constitutive modelling approach to the cyclic deformation mode allows calculation of the major characteristics of both IF and AE in the same universal dislocation-based phenomenological framework, highlighting the fundamentally similar nature of both phenomena originating from elastic energy dissipation during plastic deformation.

AFM and FIB Study of Cyclic Strain Localization and Surface Relief Evolution in Fatigued f.c.c. Polycrystals

Atomic force microscopy (AFM) and focused ion beam technique (FIB) were adopted to study the early stages of surface relief evolution in 316L steel and polycrystalline copper fatigued with constant plastic strain amplitudes at different temperatures (316L steel at 93, 173 and 573 K; copper at 83, 173 and 295 K). Qualitative and quantitative data on the morphology and shape of persistent slip markings (PSMs), occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported. They are discussed in relation with recent physically based theories of surface relief formation leading to fatigue crack initiation.

Slip Activity of Persistent Slip Bands in early Stages of Fatigue Life of Austenitic 316L Steel

Flat specimen of 316L steel was cyclically pre-deformed with constant plastic strain amplitude to early stage of fatigue life relevant to the period of cyclic strain localization and fatigue crack initiation. To document slip activity and reversibility/irreversibility of persistent slip bands (PSBs) in situ experiments in the high-resolution SEMFEG under special imaging conditions were performed. The half-and full-cycle slip activity and distribution of plastic strain within PSBs in individual grains were investigated via slip steps generated in half-and full-cycle deformation after intermediate vibration polishing. After completion of in situ tests the surface topography in identical locations was quantitatively documented using atomic force microscopy (AFM).

Low-temperature fatigue behaviour and development of dislocation structure in aluminium single crystals with single-slip orientation

Al single crystals oriented for single slip were cyclically deformed under constant plastic strain amplitudes between 1 × 10−3 and 5 × 10−2 at 77 K. Al single crystals showed hardening to saturation at all applied shear stress amplitudes. The resultant cyclic stress–strain curve (CSSC) showed a stress plateau in a range of plastic strain amplitude from 2 × 10−3 to 2 × 10−2. Surface observation revealed that multiple slip systems were active even at the strain amplitude in the plateau region. At plastic strain amplitudes corresponding to the plateau of the CSSC, persistent slip bands (PSBs) were formed parallel to the primary slip plane. In the PSBs, well-developed dislocation walls parallel to the {100} planes were observed. The microstructure in the PSBs was explained by the fact of multiple activation of the primary and critical slip systems. The above results indicate that the high stacking fault energy of Al is an important factor affecting the fatigue behaviour even at 77 K.

The effect of cyclic prestraining on the fatigue life and the microstructural evolution of fatigued copper polycrystals

Almost all engineering materials undergo a mechanical pre-treatment during forming process. For this reason, the effect of prestraining on fatigue life and microstructure evolution was investigated at different stages of cycling up to failure. In this context, several specimens were tested under various plastic strain amplitudes in air and vacuum environments. The obtained results showed that the fatigue life is not affected by prestraining. In addition, microscopic analysis revealed that the characteristic dislocation microstructure of a virgin specimen fatigued at constant plastic strain amplitude is never observed after prestraining. Furthermore, the equiaxed dislocation cell structure tended to form very quickly with cumulative plastic deformation.

Stability of Fatigued Dislocation Wall Structure in Coarse-Grained and Ultrafine-Grained Aluminum against Monotonic Tensile Deformation

Coarse-grained (CG) and ultrafine-grained (UFG) pure aluminum samples fabricated by equal channel angular pressing (ECAP) were cyclically deformed at 77K under constant plastic strain amplitude. Monotonic tensile tests were performed at 300 and 77K soon after the fatigue tests. In spite of the increase in the tensile strength of fatigued CG Al, tensile ductility decreased remarkably in comparison to that of as-annealed CG Al. On the other hand, the ultimate tensile strength (UTS) and tensile ductility of UFG Al were much higher at 77K than those at 300K. Furthermore, UFG Al at 77K maintained high UTS and high tensile ductility even after fatigue tests. Microstructural observation has revealed that dislocation wall structure formed in fatigued CG Al persists after the monotonic tensile tests. However, dislocation wall structure formed in fatigued UFG Al disappeared during early stages of monotonic tensile tests at both 300 and 77K. These results indicate that the dislocation wall structure in UFG Al is unstable against succeeding monotonic tensile deformation. [doi:10.2320/matertrans.M2012280]

Stability of austenitic 316L steel against martensite formation during cyclic straining

Solution-annealed AISI 316L steel was fatigued with constant plastic strain amplitudes at room temperature and under various conditions at depressed temperatures down to 113K to reveal its stability against deformation-induced martensite formation. Microstructural changes induced by fatigue were characterized by transmission electron microscopy (TEM), electron channeling contrast imaging (ECCI) and electron backscattering diffraction (EBSD) techniques. Neutron diffraction and magnetic induction method were adopted for quantification of martensite content. Deformation-induced martensite formation in the bulk of material was evidenced for low temperature cyclic straining under various conditions. Room temperature cycling, even with high plastic strain amplitudes, results in a local very limited martensite formation in areas closely linked with the long fatigue crack growth.

Fatigue behavior and microstructure of an Al-Mg-Sc alloy at an elevated temperature

Al-Mg-Sc alloy polycrystals bearing Al3Sc particles with different sizes, i.e. 4, 6 and 11 nm in diameter, have been cyclically deformed at 423 K under constant plastic-strain amplitudes, and the microstructural evolution has been investigated in relation to the stress-strain response. Cyclic softening after initial hardening is found in specimens with small particles of 4 and 6 nm, but no cyclic softening takes place in specimens with larger particles of 11 nm. These features of cyclic deformation behavior are similar to the results previously obtained at room temperature. Transmission electron microscopy observations reveal that dislocations are uniformly distributed under all applied strain amplitudes in the specimens containing large particles of 11 nm, whereas slip bands are formed in the cyclically softened specimens bearing smaller particles. The cyclic softening is explained by a loss of particle strength through particle shearing within strongly strained slip bands. The 6 and 11 nm Al3Sc particles have a stronger retardation effect on the formation of fatigue-induced stable dislocation structure than 4 nm particles at 423 K.

Low-Cycle Fatigue Behavior of an Al-Mg-Si Alloy with and without a Small Addition of Sc

Low-cycle fatigue behavior of a wrought Al-0.8wt%Mg-0.7wt%Si alloy with and without 0.27wt%Sc has been investigated at room temperature under constant plastic-strain amplitudes. After peak-aging treatments, both the alloys had fine lath-shaped β' precipitates. In the Sc-containing alloy, spherical Al3Sc precipitates of about 11 nm in diameter were co-existed. The alloy with Sc exhibited cyclic hardening to saturation, while the alloy without Sc showed clear cyclic softening after initial hardening. Transmission electron microscopy observation revealed that slip band structures were developed in the Sc-free alloy. Within the slip bands, shearing of the β' precipitates by moving dislocations was often observed. The cyclic softening in the alloy without Sc can then be explained by a loss of precipitation strengthening effect through the precipitation destruction within strongly-strained slip bands. In the Sc-bearing alloy, owing to the existence of non-shearable Al3Sc precipitates, dislocations were uniformly distributed, resulting in the absence of the cyclic softening.

Cyclic Softening of Cu-Ni-Si Alloy Single Crystals under Low-Cycle Fatigue

Cu-2.2wt%Ni-0.5wt%Si alloy single crystals were grown by the Bridgman method and aged at 723 K for 10 h to form Ni2Si precipitates. Fully reversed tension-compression fatigue tests were conducted on the aged single crystals with a single slip orientation under constant plastic-strain amplitudes at room temperature. Cyclic softening occurred at plastic-strain amplitudes between 2.5x10-4 and 2.5x10-2. Using the maximum stress amplitude in each cyclic hardening/softening curve, a pseudo cyclic stress-strain curve (CSSC) was obtained. The CSSC was found to exhibit a plateau region with a stress level of about 167 MPa. Transmission electron microscopic observation revealed the formation of persistent slip bands (PSBs) in the plateau regime. It was found that the Ni2Si precipitate particles were intensively sheared by glide dislocations within the PSBs and were eventually re-dissolved into the Cu matrix. The macroscopic cyclic softening can be attributed to the local softening induced by the re-dissolution of the Ni2Si particles in the PSBs.