Formation Of Persistent Slip Bands Research Articles

The effect of κ-carbides on high cycle fatigue behavior of a Fe-Mn-Al-C lightweight steel

Numerous studies have demonstrated the viability of lightweight Fe-Mn-Al-C steels for exhibiting an improved balance of high strength and high ductility in automotive applications. However, their high-cycle fatigue behaviour has been scarcely studied. This work examines the effect of κ-carbides formed during the aging treatment on the high-cycle fatigue performance of an austenitic Fe-29Mn-8.7Al-1C (wt. %) steel. The material is studied in solution-treated, under-aged, and peak-aged conditions. High-cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high-cycle fatigue performance in the aged material, somewhat better than for high Mn steels. Fatigue crack formation and growth occur predominantly via a quasi-cleavage mechanism along the [1 1 1] crystallographic planes, which is also a plane for planar glide and the formation of persistent slip bands during plastic deformation. The nanoscale intragranular κ-carbides in the aged samples interact with the gliding dislocations, resulting in the shearing of nanoscale κ-carbides in a weakly coupled regime. The resistance of particles to shearing is determined by their size, volume fraction, and antiphase boundary energy (γAPB), which vary during the aging process. The aged Fe-29Mn-8.7Al-1C steel significantly improves the fatigue strength as the formation of persistent slip bands is delayed due to an additional energy barrier related to the shearing of the κ-carbides. This improvement peaks in the under-aged condition and decreases with further aging time.

How Hydrogen Affects the Formation and Evolution of Persistent Slip Bands in High‐Purity α‐Iron

The effect of hydrogen on the fatigue behavior of materials has been studied extensively during the past 100 years, but is just poorly understood due to the complex interplay between hydrogen and deformation processes. In this context, hydrogen damage of metals is becoming one of the major challenges of decarbonization. While most work focuses on f.c.c. materials, the availability of relevant results becomes sparse when considering technologically highly relevant b.c.c. metals such as structural steels. This work uses in situ electrochemical hydrogen charging of α‐iron steels to investigate the formation and evolution of intrusions and extrusions prior to fatigue crack initiation using a new charging setup by which the specimens are charged from the interior. The advantage of this innovative technique is that the surface of the specimens can subsequently be characterized using atomic force microscopy without artefacts from electrochemical charging or corrosion. Hydrogen is shown to enhance slip localization at the early stages of damage. The developed persistent slip lines are less pronounced. By means of transmission Kikuchi diffraction, it is shown that orientation gradients between cells in the dislocation structure are much weaker in the presence of hydrogen. Hence, hydrogen appears to promote slip reversibility in b.c.c. materials.

Stress-controlled fatigue of HfNbTaTiZr high-entropy alloy and associated deformation and fracture mechanisms

The stress-controlled fatigue tests are carried out at a stress ratio of 0.1 and a frequency of 10 Hz, and span both low-cycle and high-cycle regimes by varying the applied stress amplitudes. The high-cycle fatigue regime gives a fatigue strength of 497 MPa and a fatigue ratio of 0.44. At equivalent conditions, the alloy's fatigue strength is greater than all other high-entropy alloys (HEAs) with reported high-cycle fatigue data, dilute body-centered cubic alloys, and many structural alloys such as steels, titanium alloys, and aluminum alloys. Through in-depth analyses of crack-propagation trajectories, fracture-surface morphologies and deformation plasticity by means of various microstructural analysis techniques and theoretical frameworks, the alloy's remarkable fatigue resistance is attributed to delayed crack initiation in the high-cycle regime, which is achieved by retarding the formation of localized persistent slip bands, and its good resistance to crack propagation in the low-cycle regime, which is accomplished by intrinsic toughening backed up by extrinsic toughening. Moreover, the stochastic nature of the fatigue data is neatly captured with a 2-parameter Weibull model.

Synergistic effect of microstructure and defects on the initiation of fatigue cracks in additively manufactured Inconel 718

Fatigue cracks in additively manufactured (AMed) Inconel 718 (IN-718) in machined surface condition often initiate from persistent slip bands (PSBs) unlike other popular AM alloys such as 17-4 PH stainless steel or Ti-6Al-4V, where fatigue crack initiation is exclusively from volumetric defects; therefore, a competition between PSB- vs. defect- mediated crack initiation clearly exists. To shed light on the factors governing the competition, this study investigates the characteristics of cyclic strain localization, PSB formation, and crack initiation via crystal plasticity (CP) modeling of cyclic loading on polycrystalline aggregates which are then validated by experiments. A physics-based, free slip distance (FSD) dependent slip strength evolution law is proposed, which is shown to enable the CP model to simulate the heterogeneous strain distribution in IN-718. Implementing a crack initiation criterion based on strain contrast, the locations and lives for crack initiation can also be calculated. It is shown that both FSD and resolve shear stress influence the strain localization and crack initiation behaviors. The distribution of a localization parameter calculated based on the multiplication of FSD and Schmid factor within a grain is found to correlate well with the locations of PSBs. The maximum values of the localization parameters within a microstructure are shown to correlate well with the experimentally obtained crack initiation lives. The presence of volumetric defects in IN-718 generally do not impact strain localization behavior unless their size is large compared to the grain size.

Amorphous and Anomalous Persistent Slip Bands Formation in Tungsten Under Repeated High-Flux Hydrogen Plasma Loads

Amorphous and Anomalous Persistent Slip Bands Formation in Tungsten Under Repeated High-Flux Hydrogen Plasma Loads

Surface relief evolution and fatigue crack initiation in René 41 superalloy cycled at room temperature

Surface relief evolution and fatigue crack initiation of René 41 superalloy was studied. The material was tested in two thermodynamical states: solutionised and aged. Cylindrical specimens were cycled at total strain amplitude 0.8% at room temperature. In order to study relevant mechanisms resulting in surface relief formation and eventually fatigue crack initiation, cycling was interrupted at definite number of elapsed cycles. After each interruption, the specimen surface was observed by means of scanning electron microscopy and thin lamellae were cut out of the surface and observed by means of transmission electron microscopy. Cyclic deformation was found to localise since the first cycle, causing the formation of persistent slip bands and persistent slip markings in the form of slip steps, extrusions and later intrusions as well. Continuous cycling led to the formation of new persistent slip markings as well as widening of the already existing ones. The role of persistent slip markings in fatigue crack initiation at room temperature was found to be crucial in this superalloy. The results were thoroughly discussed in relation to theoretical models of surface relief formation.

Experimental and analytical investigation on service life of film cooling structure for single crystal turbine blade

In this study, regarding the film cooling structure of single crystal (SC) turbine blade, the life tests simulating the service condition are conducted, and the life prediction modeling as well as damage mechanism analysis are carried out. In life test, the SC specimen with multiple film cooling holes is designed to simulate the geometrical characteristic of film cooling structure. The creep-fatigue loading spectrum characterizing a typical flight process is employed. The loading system is developed to simulate the cyclic service load in laboratory environment. Meanwhile, the strain distribution of film cooling structure is measured using digital image correlation (DIC) method. After that, the life tests are conducted, and eight groups of test lives are obtained. Taking the strain distribution measured by DIC as input, the model based on theory of critical distance (TCD) is improved to predict the service lives of film cooling structure. Furthermore, the crack nucleation mechanism of film cooling structure is discussed. Based on the microscopic observation, it can be found that the microcrack nucleation around hole edge is induced by persistent slip bands (PSBs). The matrix channel that significantly influences the dislocation accumulation rate and PSBs formation is chosen as the concerned microstructure. Under the effect of stress distribution of film cooling structure, the uneven spatial distributions of mean values and standard derivations for matrix channel width are quantificationally revealed. And the distribution of dislocation density on film cooling structure is measured using electron back-scatter diffraction (EBSD). It can be found that the concentration of dislocation density exists around the hole edge, and the distribution of dislocation density is quite unsmooth due to the scatter of matrix channel width. These findings can be employed in the life prediction and fault diagnosis for film cooling structure under service condition.

3D discrete dislocation dynamic investigations of persistent slip band formation in FCC metals under cyclical deformation

The formation of persistent slip bands (PSBs) at early stages of fatigue plays an important role in fatigue damage. In order to get a better understanding of the physics involved at the dislocation scale, 3D discrete dislocation dynamic simulations are set up in the case of an isolated surface grain of Cu cyclically loaded in single slip. Simulations reveal the complex transformation process of the dislocation microstructure from a homogeneous distribution into an organized microstructure made of slip bands mainly composed of dislocation dipoles and prismatic loops. These slip bands are the embryo of the PSBs observed for large number of cycles. The localization mechanism evidenced by the simulations is well explained from a stress field analysis which highlights the role of cross-slip. A saturation regime is reached after a few cycles during which dislocation microstructure, dislocation densities and height of extrusions/intrusions remain unchanged. Furthermore, the linear relationship between the volume fraction of slip bands and the imposed plastic strain amplitude is retrieved by the simulations and the stability of the slip bands is verified by decreasing the plastic strain amplitude after the saturation regime is reached. Finally, when compared to AISI 316L stainless steel, Cu shows a tendency to build more bands with relatively larger thicknesses which is attributed to the differences in the cross-slip probability.

Correlation of the low and high frequency fatigue responses of pure polycrystalline copper with mechanisms of slip band formation

Ultrasonic fatigue testing at 20 kHz was developed to accelerate fatigue tests and explore the very high cycle fatigue range. However, the use of ultrasonic fatigue systems raises the open question of the impact of frequency on the material fatigue response. In this paper, the impact of the loading frequency on the fatigue response of face-centered cubic polycrystalline copper is investigated. Firstly, experiments with conventional fatigue devices inducing loading frequencies lower than 100 Hz were carried out and compared with previously published results obtained with an ultrasonic fatigue machine at 20 kHz. At a stress amplitude of 100 MPa, the number of cycles required for failure was found to be more than 200 times greater for tests at 20 kHz than for tests at frequencies below 100 Hz. Secondly, for both types of fatigue tests, the conditions in stress amplitude and number of cycles needed for the emergence of the early slip markings were investigated. The early slip marking S–N curve was found to have the same sensitivity as the failure S–N curve to the frequency effect. Thirdly, the different possible reasons responsible for this frequency effect are discussed. It is concluded that the physical mechanisms of slip marking formation is preserved while their kinetics is sensitive to the loading frequency. Finally, two simplified models were considered and for the first time, the frequency effect on the fatigue response of copper is quantitatively correlated with the occurrence of time-dependent dislocations cross slip and vacancies production/diffusion involved in the persistent slip band formation for face-centered cubic structure. The obtained results could be extended to a full class of materials where slip band formation plays a key role in the crack initiation process while suggesting no frequency effect for materials with a lower degree of plasticity development at crack initiation (High Strength Steels for example).

The Effects of Grain Size and Strain Amplitude on Persistent Slip Band Formation and Fatigue Crack Initiation

The maximum persistent slip band (PSB) width in cyclic loading is experimentally proven to be a material property in this paper. The dependence of the maximum PSB width on grain size, selected as a microstructural property of the material, is tested and reported. The effect of strain conditions, which conversely is not a material property of copper, is also included in the study. The results demonstrate that the maximum PSB width of annealed copper (grain size = 104.2 µm) is statistically the same within this class of grains, and is statistically larger than that of the stress-relieved copper (grain size = 14.2 µm). In contrast, the PSB width evolution is not sensitive to different strain conditions. These findings advance the fundamental understanding of how persistent slip band formation is influenced by variations in fatigue mechanical stimulus and metallurgical characteristics.

Texture Effect on Fatigue Crack Propagation Behavior in Annealed Sheets of an Al-Cu-Mg Alloy

The effects of texture, including Brass, Goss + Cube and Cube, on the fatigue crack growth behavior in annealed Al-Cu–Mg alloy sheets were characterized in this investigation. Results showed that the Goss + Cube sheet possessed the greatest fatigue threshold value and the lowest fatigue crack propagation (FCP) rate, and the Brass sheet presented the smallest threshold value and the greatest FCP rate among the three textured sheets. SEM results showed more slip bands and more tortuous crack path formed in Goss + Cube sheet during cyclic loading, and EBSD results showed that numerous dramatic crack deflections occurred when fatigue crack propagated across Goss-grain boundary in Goss + Cube sheet, indicating that the grain orientations including Goss and Cube had positive effect on enhancing the FCP resistance. On the one hand, Goss and Cube grains have more high Schmid factor slip systems than Brass grain, promoting persistent slip bands formation and externally applied energy consumption, consequently giving rise to a high resistance to FCP. On the other hand, Goss grains could induce crack deflection with large twist angles, resulting in more energy consumption during the cracking process and thereby enhance the FCP resistance.

Hierarchical microstructure for improved fatigue properties in a eutectic high entropy alloy

Microstructural hierarchy can enable enhanced properties in high entropy alloys. A unique microstructure was observed in 50% cold-rolled and heat-treated eutectic high-entropy alloy (EHEA). Formation of recrystallized FCC grains and B2 precipitates in FCC lamellae with different microstructural length scale constitute the hierarchy in cold-rolled and heat-treated EHEA. Monotonic loading and high cycle fatigue properties of both microstructures were compared. In as-cast microstructure, the fatigue crack originated in FCC lamellae with the formation of persistent slip bands. The B2 precipitates in cold-rolled and heat-treated alloy delayed the crack initiation and improved fatigue properties.

Cyclic Deformation Microstructure in Heavily Cold-Drawn Austenitic Stainless Steel

Cyclic deformation microstructure of the heavily cold-drawn austenitic stainless steel is significantly influenced by the spacing between mechanical twins introduced by prior cold drawing. Well-developed dislocation cells form between mechanical twins with the spacing larger than about 800 nm. Persistent slip band (PSB)-like structure with ladders takes place between mechanical twins spacing from 300 to 800 nm. Few dislocations occur between neighboring mechanical twins with spacing less than about 100 nm. Pre-existing mechanical twins and deformation bands segment austenitic grains, facilitating multi-slip and consequently suppressing PSB formation.

Dislocation Structure in a Single Crystal Nickel Base Superalloy During High Cycle Fatigue at 870 °C

Abstract Dislocation structure and its impact on the mechanical behavior of a γ′ strengthened nickel base single crystal alloy under high cycle fatigue (HCF) at 870 °C was studied. The results indicate that HCF lifetime declines with increase of applied stress amplitude. In the early stages, the fatigue deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. During mid-term fatigue stages, most of the dislocations formed in the matrix are located in the γ/γ′ interfaces, whose intersection and reaction produce new dislocation segments and three-dimensional dislocation networks. The Burgers vectors of the dislocations in the network are 1/2 . The interaction of cyclic stress with high temperature induces the precipitation of homogeneous globular γ′ particles, which is beneficial to fatigue strength. At the end of fatigue test, the cyclic stress leads to the formation of persistent slip bands moving through the γ matrix channels and γ′ particles. Dislocation shearing through the γ′ phase is found occasionally. The extrinsic stacking faults is also observed.

Damage accumulation and crack initiation detection based on the evolution of surface roughness parameters

Cyclic strain localization generates sharp surface slip markings in the form of depressions and elevations on the surface of materials. The process gives rise to the formation of persistent slip bands and results in immanent changes that manifest themselves in the form of surface roughening. The heights and depths of these extrusions and intrusions grow during cyclic loading up to a critical value leading to crack initiation. In this study, we investigate the evolution of the surface roughness parameters starting from pristine specimens and ending in final fracture in fully-reversed cyclic bending tests. Results are presented for both low-and high-cycle fatigue that covers a wide range of surface finish. Two types of contacting (via a stylus) and no-contacting (optical) profilometers were used in this investigation. The most sensitive and useful surface roughness parameter for the assessment of fatigue growth in low-and high-cycle fatigue is identified, and it is shown that results can be utilized to detect the onset of fatigue crack nucleation. For this purpose, a surface roughness criterion for detecting crack initiation at different applied loads is introduced.

Analysis of crack tip transformation zone in austenitic stainless steel laser-MIG hybrid welded joint

In this study, fatigue crack growth rates in weld center and Heat Affected Zone (HAZ) of laser-MIG hybrid welded joint are compared using standard compact tension (CT) specimens. Electron backscatter diffraction (EBSD) is applied to analyze crack tip transformation zone by characterizing phases, plastic strains and crystallographic orientations. This study provides interesting insights into the growth of fatigue cracks in austenite steel welded joint at micro-scale. Analysis of the plastic strain and Schmid factor indicates that mechanical energy provided by external load transforms into internal crystal energy firstly, which is stored within grains in the form of strain energy, then partly relieves to provide mechanical driving force for martensite transformation. With plastic strain accumulating within individual grain, dislocations begin to glide on slip system resulting in formation of persistent slip bands (PSB), then metastable austenite grains transform into martensite. Thus, strain-induced martensite transformation increases resistance to crack growth by dissipating energy and crack closure effect. Different crack growth rate in weld center and HAZ is attributed to variant volume of strain-induced martensite. Shielding effect produced by neighboring harder ferrite decreases martensite in weld center, as confirmed by nanoidentation and Atomic Force Microscope (AFM). Additionally, martensite phase is found to have a Kurdjumov-Sachs (K-S) orientation relationship with austenite phase according to Inverse Pole Figure (IPF) and Pole Figures (PF) derived by EBSD.

Dependence of hardening and saturation stress in persistent slip bands on strain amplitude during cyclic fatigue loading

Cyclic loading of FCC single crystals leads to the formation of persistent slip bands (PSBs) over a wide range of applied strain amplitudes. The hardening and saturation stress associated with cyclic loading of Cu single crystals are reproduced with 3-D discrete dislocation dynamics simulations. Evolution of the dislocation microstructure in PSB channels and walls during cyclic loading is shown to explain key features of plastic deformation. Screw dislocation segments are found to deposit edge components near PSB channel walls resulting in the nucleation of half-loops that expand into neighbouring channels. The saturation stress is found to be 34 MPa at an applied strain amplitude, , within the range of experimental observations. It is shown that once PSBs are formed, subsequent cycling at lower strain amplitudes does not lead to their elimination, but the cyclic stress–strain behaviour is modified. At lower values of of previously formed PSBs, the saturation stress is found to decrease to 24 MPa at . Plastic energy dissipation in hysteresis loops is also significantly reduced but not removed. However, some reverse plasticity is shown to take place upon unloading at low values of , and is a direct result of strain recovery of loops that expand into neighbouring channels during the loading phase.

A general model for crack growth from initial defect in Very-High-Cycle Fatigue

Abstract: It is well-known in the literature that internal defects play a major role in the Very-High-Cycle Fatigue (VHCF) response of metallic materials. Generally, VHCF failures nucleate from internal defects characterized by a limited size. Unexpectedly, it has been found that cracks can grow from the initial defect even if the Stress Intensity Factor (SIF) is quite below the characteristic threshold for crack growth. Even though researchers unanimously accept this singular experimental evidence, they still dispute about its physical justification. Different micromechanical explanations have been proposed in the literature: local grain refinement, carbide decohesion, matrix fragmentation, hydrogen embrittlement, numerous cyclic pressure and formation of persistent slip bands are the most famous proposals. Regardless of the specific micromechanical explanation, it is generally acknowledged that a weakening mechanism occurs around the initial defect, thus permitting crack growth below the SIF threshold. The present paper proposes an innovative approach for the quantitative modeling of the weakening process around the initial defect. The proposed model considers an additional SIF that reduces the SIF threshold of the material. Starting from a very general formulation for the additional SIF, possible scenarios for crack growth from the initial defect are also identified and described. It is theoretically demonstrated that, depending on the scenario, a VHCF limit may also be present and its final formulation recalls the well-known expression previously proposed by Murakami.

Molecular dynamics simulation of persistent slip bands formation in nickel-base superalloys

Persistent slip band (PSB) is an important and typical microstructure generated during fatigue crack initiation. Intensive work has been done to investigate the mechanisms of the formation of persistent slip bands since the 1950s when Wadsworth[1] observed the fatigue fracture in copper. Simulations have indicated that PSBs formation during fatigue crack initiation is related to the dislocation driving force and interaction. In this paper, a molecular dynamics (MD) simulation associated with embedded atom model (EAM) is applied to the PSBs formation in nickel-base superalloys with different microstructure and temperature under tensiletensile loadings. Five MD models with different microstructure (pure γ phase and γ/γ′ phase), grain orientation ([1 0 0][0 1 0][0 0 1] and [1 1 1][ $$\overline{1}$$ 0 1][1 $$\overline{2}$$ 1]) and simulation temperature (300 K, 600 K, 900 K) were built up in these simulations. Our results indicated that within the γ phase by massive dislocations, pile-up and propagation which can penetrate the grain. Also, it is found that the temperature will affect the material fatigue performance and blur PSBs appearance. The simulation results are in strong agreement with published experimental test result. This simulation is based on the work[2]. The highlights of the article include: 1) investigation of the PSB formation via molecular dynamics simulation with three different parameters, 2) conduct of a new deformation and velocity combination controlled simulation for the PSB formation, 3) high-performance computing of PSB formation, and 4) systematic analysis of the PSB formation at the atomic scale in which the dislocation plays a critical role.

Simulation of the Kitagawa-Takahashi diagram using a probabilistic approach for cast Al-Si alloys under different multiaxial loads

This article describes a microstructural-based high cycle fatigue strength modelling approach applied to different cast Al-Si alloys used in an automotive context. Thank to different casting processes (gravity die casting and lost foam casting), associated with several heat treatment (T7 and Hot Isostatic Pressing-HIP), three alloys with very different microstructures have been obtained. In a vast experimental campaign undertaken to investigate the fatigue damage mechanisms governing these alloys under different multiaxial loading conditions, it was shown that the principal crack initiation mechanisms for the porosity-free alloy are either the formation of persistent slip bands (PSB) or the rupture and/or debonding of eutectic particles. For the porosity-containing alloys, the fatigue damage is always controlled by crack growth from pores. In order to take into account these fatigue damage mechanisms, a probabilistic model using a combination of the Dang Van and a modified LEFM criteria is proposed. The modified LEFM criterion is able to take into account the influence of the grain size on the threshold of the stress intensity factor.It is shown that for the porosity-free alloy, the predictions are good for combined tension-torsion loads with R=-1. However, because the crack initiation mechanisms are not the same depending on the hydrostatic stress, the predictions are non-conservative for the uniaxial and equibiaxial tension loads with R=0.1. For the porosity-containing alloys, the predictions are very good for the uniaxial, combined tension-torsion and equibiaxial tension loads with both R=-1 and R=0.1. As observed experimentally, the proposed model can also predict a more pronounced effect of casting porosity for the uniaxial and combined tension-torsion loads, when compared to pure torsion loads.