Slip Transmission Research Articles

Understanding slip activity and void initiation in metals using machine learning-based microscopy analysis

The use of machine learning techniques to supplement traditional data analysis in mechanics and materials research can improve the understanding of microstructure-property relationships. Identification of key microstructural features or correlation between deformation mechanisms and material response can be discerned that might otherwise have been overlooked. Motivated by the possibilities of gaining additional insight into the process of void nucleation in polycrystalline metals, several machine learning techniques are applied to the analysis of mesoscopic deformation mechanisms as determined by experimental characterization and modeling. Results from crystal plasticity modeling, experimental microstructural analysis, and theoretical models of slip transmission are combined to test a hypothesis regarding fatigue-induced void nucleation. Unsupervised spectral clustering was used with results from crystal plasticity simulations to characterize slip system activity for different crystallographic orientations. The slip system activity as determined by the clustering analysis was then fed into a K-nearest neighbor classifier to quantify the probability of slip transmission across different grain boundaries of interest and analyze grains containing fatigue-induced voids. An unique and unanticipated result from the unsupervised clustering analysis shows that including a group of partially-active slip systems was more appropriate than using the binary classification of active/non-active. Predicted slip activity behavior in a face-centered cubic material was shown to differ significantly from that of a body-centered cubic material due to non-Schmid effects. The outcome of the overall analysis was that grains containing fatigue-induced voids were more likely to be surrounded by grains with orientations that inhibited slip transmission according the Lee-Robertson-Birnbaum (LRB) criteria. Finally, it is demonstrated that smaller datasets using limited simulation results were equally effective at predicting a similar outcome when additional physical descriptors for the slip system activity are used.

Deformation and phase transformation in polycrystalline cementite (Fe3C) during single- and multi-pass sliding wear

Cementite (Fe3C) plays a major role in the tribological performance of rail and bearing steels. Nonetheless, the current understanding of its deformation behavior during wear is limited because it is conventionally embedded in a matrix. Here, we investigate the deformation and chemical evolution of bulk polycrystalline cementite during single-pass sliding at a contact pressure of 31 GPa and reciprocating multi-pass sliding at 3.3 GPa. The deformation behavior of cementite was studied by electron backscatter diffraction for slip trace analysis and transmission electron microscopy. Our results demonstrate activation of several deformation mechanisms below the contact surface: dislocation slip, shear band formation, fragmentation, grain boundary sliding, and grain rotation. During sliding wear, cementite ductility is enhanced due to the confined volume, shear/compression domination, and potentially frictional heating. The microstructural alterations during multi-pass wear increase the subsurface nanoindentation hardness by up to 2.7 GPa. In addition, we report Hägg carbide (Fe5C2) formation in the uppermost deformed regions after both sliding experiments. Based on the results of electron and X-ray diffraction, as well as atom probe tomography, we propose potential sources of excess carbon and mechanisms that promote the phase transformation.

Dislocation interactions at the grain boundary in FCC bicrystals: An atomistically-informed dislocation dynamics study

To understand the underlying mechanisms that control the mechanical properties of nanostructured metals, an insight into the role of the grain boundary in dislocation-driven plastic deformation is vital. The grain boundary has been observed as a dislocation source, sink, or having no effect, which in turn, gives rise to different macroscopic mechanical responses. With this motivation, atomistic simulations and three-dimensional dislocation dynamics simulations were performed to investigate dislocation interactions at various grain boundaries and their role in the plastic deformation of face-centered cubic (FCC) bicrystalline micropillars. The atomistically-informed dislocation dynamics simulations show that bicrystalline samples containing a high angle grain boundary (HAGB) display hardening and higher flow stresses compared to single crystals, while micropillars with a coherent twin boundary (CTB) show similar flow stresses to the reference single crystalline samples. This is due to the transparency of the grain boundary to slip transmission, which is observed in the atomistic simulations. Interestingly, allowing dislocation glide on the grain boundary exhibits a decrease in flow stress as slip transmission becomes easier.

Dislocation transmission across Σ3{112} incoherent twin boundary: a combined atomistic and phase-field study

Grain boundaries (GBs) in polycrystalline materials act as impediments to dislocation motion and result in strengthening. Understanding slip transmission through GBs, specifically twin boundaries, is essential to understand the plastic deformation behavior of polycrystalline fcc materials. In this study the interaction between a glide dislocation and Σ3{112} incoherent twin boundary (ITB) in copper is investigated using a combined atomistic and mesoscale approach. The material parameters and structure of the GB in the mesoscale phase field dislocation dynamics (PFDD) model are informed from Molecular Statics (MS) simulations. The structural unit of the ITB consists of an array of three partial dislocations. The interaction between a glide dislocation impinging on each of the GB partial dislocations is investigated using both PFDD and Molecular Dynamics (MD) with two boundary conditions. Transmission planes predicted by both PFDD and MD (NVT) are in agreement, and show that not all transmission events are direct. Critical transmission stresses predicted by PFDD are in the range of 276 MPa to 1380 MPa, while MD predictions are in the range from 100 MPa to 700 MPa. The PFDD and MD predictions of slip transmission are explained using dislocation theory based on isotropic linear elasticity.

Transient Slow Slip Characteristics of Frictional‐Viscous Subduction Megathrust Shear Zones

AbstractThe deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tectonic tremor. Observations from exhumed rocks suggest this region of the subduction interface is a shear zone with frictional lenses embedded in a viscous matrix. Here, we use numerical models to explore the transient slip characteristics of finite‐width frictional‐viscous megathrust shear zones. Our model utilizes an invariant, continuum‐based, regularized form of rate‐ and state‐dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D heterogeneous continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface melange) with inclusions (clasts) of varying distributions and viscosity contrasts with respect to the surrounding weaker matrix. The entire shear zone exhibits the same velocity‐weakening RSF parameters, but the lower viscosity matrix has the capacity to switch between RSF and viscous creep as a function of local stress state. Results demonstrate a mechanism in which stress heterogeneity in these shear zones both (a) sets the “speed limit” for earthquake ruptures that nucleate in clasts such that they propagate at slow velocities; and (b) permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. For reasonable input parameters, modeled events have moment‐duration statistics, stress drops, and rupture propagation rates that overlap with some natural slow slip events. These results provide new insights into how geologic observations from ancient analogs of the slow slip source may scale up to match geophysical constraints on modern slow slip phenomena.

Correlation between Taylor Model Prediction and Transmission Electron Microscopy-Based Microstructural Investigations of Quasi-In Situ Tensile Deformation of Additively Manufactured FeCo Alloy

Within this research, the multiscale microstructural evolution before and after the tensile test of a FeCo alloy is addressed. X-ray µ-computer tomography (CT), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM) are employed to determine the microstructure on different length scales. Microstructural evolution is studied by performing EBSD of the same area before and after the tensile test. As a result, langle001rangle||TD, langle011rangle||TD are hard orientations and langle111rangle||TD is soft orientations for deformation accommodation. It is not possible to predict the deformation of a single grain with the Taylor model. However, the Taylor model accurately predicts the orientation of all grains after deformation. {123}langle111rangle is the most active slip system, and {112}langle111rangle is the least active slip system. Both EBSD micrographs show grain subdivision after tensile testing. TEM images show the formation of dislocation cells. Correlative HRTEM images show unresolved lattice fringes at dislocation cell boundaries, whereas resolved lattice fringes are observed at dislocation cell interior. Since Schmid’s law is unable to predict the deformation behavior of grains, the boundary slip transmission accurately predicts the grain deformation behavior.

A convergence relationship between slip transmission and grain boundary migration in nanocrystalline nickel

In the present study, a geometric factor M proposed to predict slip transmission across grain boundaries (GBs) was lent to explore the factors influencing the stress-driven GB migration behaviors in nanocrystalline nickel under high strain rate loading. By confronting the relationship between slip systems in neighboring grain of the samples before and after high-rate loading, it is found that the geometric factor M of the remaining nano-grains in the impacted samples shifts to smaller values. This apparent correlation between GB migration and the geometric factor M can be understood in terms of two factors: the angle α between slip plane traces on the GB and the residual dislocation deposited at the GB plane to ensure the Burgers vector unity. Both contribute to slip transmission across GBs.

Under-stoichiometric cementite in decomposing binary Fe-C pearlite exposed to rolling contact fatigue

In this study, we investigate the fundamentals of deformation-driven cementite decomposition during rolling contact fatigue in a binary Fe-0.74C (wt.%) steel with pearlitic microstructure. To reveal the involved nano-scale mechanisms, we apply transmission electron microscopy and atom probe tomography, as well as the combination of both techniques on the same probe volume for correlative structural and chemical analysis. After ~32,500 individual ball contacts under a nominal Hertzian contact stress of ~1,250 MPa, cementite lamellae are consistently depleted in carbon (C) down to ~20 at.%, which is a reduction of 20% in comparison to the original stoichiometric composition of 25 at.% before deformation. By electron diffraction on atom probe tip volumes, we show that this under-stoichiometric cementite still maintains its crystal structure. The potential effect of temperature increase during rolling contact fatigue on cementite decomposition is addressed by carrying out annealing experiments at 150°C and 250°C for 30 minutes after rolling contact fatigue. Our results show that slip transmission across cementite lamellae is primarily responsible for the observed C-depletion in cementite. We quantitatively discuss slip transmission from ferrite to cementite using slip trace analysis and correlate the amount of slip activity with the observed C-depletion. In this way, we explain the formation of under-stoichiometric cementite by combined slip transmission and solute drag by dislocations, where C is transported out of cementite by dislocation gliding through cementite lamellae. Only afterward, cementite decomposition in terms of phase dissolution is suggested to occur by the dislocation-shuffle mechanism.

Understanding ferrite deformation caused by austenite to martensite transformation in dual phase steels

During the production of DP steels, the volume expansion and shape change accompanying the austenite to martensite transformation is accommodated by the deformation of surrounding ferrite grains. The extent of the deformation in ferrite grains ultimately affects the mechanical properties of DP steels. Using electron backscatter diffraction measurements, this study identifies the characteristics of martensite which govern the extent of transformation induced deformation of ferrite grains. It was found that small austenite grains tend to transform into martensitic variants having a close-packed plane parallel relationship with adjacent ferrite grains, thus achieving relatively easy slip transmission and resulting in a long-range deformation of ferrite grains. Ferrite grains can also exhibit a short-range deformation limited to the vicinity of the ferrite/martensite interface, which is primarily governed by martensite carbon content.

Intergranular micro-deformation behavior of a medium strength 7xxx aluminum alloy

Compression experiments were designed to examine the intergranular deformation behaviors of 7xxx aluminum alloy at a micro-scale. Band structures formed in grains upon deformation were characterized by electron backscatter diffraction technique, and the local stored energy was analyzed in a quantitative way. In addition, the deformation behaviors of the six pairs of adjacent deformed grains show that, the dislocation slips are transmissible between some pairs of grains but terminated in front of other grain boundaries. Such slip transmission is influenced by the Schmid factor, the strain coordination factor mʹ, and the degree of strain concentration between grains, according to the analysis of kernel average misorientation.

In-situ investigation of tensile behaviors of Ti–6Al alloy with extra low interstitial

Abstract Tensile behaviors of Ti–6Al with extra low interstitial (ELI) have been systematically investigated using the in-situ tensile testing monitored by the scanning electron microscopy (SEM). The electron backscatter diffraction (EBSD) technique was employed to reveal the detailed deformation mechanisms during the in-situ tensile process. The results showed that multiple slip activities occurred during the in-situ tensile process. Moreover, prismatic and basal slips dominated the slip activities. To accommodate the increased macrostrain under the tensile loading, coordinated deformation mechanisms were involved in the tensile process, including crystalline orientation rotation, slip transmission and deformation twinning. α grains were rotated about 9.8 (±4)° for convenience of slip activation to accommodate the macrostrain when the tensile displacement reached to 0.4 mm. Slip transmission supplied the intergranular coordinated deformation which avoided the localized stress concentration at grain boundaries. Deformation twinning accommodated the intragranular strain in α grains with hard orientations. Furthermore, the mechanisms for microcrack formation was thoroughly investigated by combining the in-situ SEM observations with the EBSD technique. Localized stress concentration at the grain boundaries derived from the geometric incompatibility between neighboring α grains were mainly responsible for the microcrack formation in Ti–6Al ELI alloy.

Role of equilibrium and non-equilibrium grain boundary stress fields on dislocation transmission

This study presents an approach to investigate the influence of intergranular stresses induced by equilibrium and non-equilibrium grain boundaries (GBs) on dislocation transmission via the discrete dislocation dynamics simulation method. First, a disclination-based construct is used to describe the equilibrium intergranular stress fields of two symmetric tilt GBs. Then, slip transfer through these GBs is simulated via an algorithm that predicts dislocation transmission considering geometric and plastic dissipation criteria. Results show that the locations for transmission and the glide of residual dislocations are strongly correlated with GB stresses. Next, ordered structural defects in the equilibrium GBs are modified using a new approach to simulate disruptions in GB structure caused by dislocation transmission, creating non-equilibrium GBs. Slip transmission preferentially occurs near the non-equilibrium defects, illustrating the effect of modified GB screening characteristics due to prior dislocation transmission. The present work provides a basis for physically representative modeling of GBs and dislocation-GB phenomena in mesoscale simulations of polycrystalline plasticity.

A hybrid dislocation

ABSTRACT The shear of a crystalline interface by a matrix dislocation is an elementary phenomenon of plasticity that generates a three-dimensional elastic field difficult to express. However, in the case of a flat interface and two elastic isotropic media, the present work shows that this field can be described starting from an explicit form of the Green tensor associated with a point force exerted in one of the crystals. Indeed, the knowledge of this tensor makes it possible to express the displacement field of a dislocation in the final form of a surface integral. Because the shear is limited by a matrix branch and an interfacial branch, the dislocation can be called ‘hybrid dislocation’ or HD. If the two branches are straight, the field is that of an angular HD. Moreover, if the crystals are identical, the field becomes that of an angular matrix dislocation whose field is well known since Yoffe (1960). By applying the superimposition theorem, it becomes possible to determine the field of more complex configurations, such as those sometimes observed during the transmission of the plastic slip through the interfaces.

Effect of Strain Rate on the Microstructure and β-Texture Evolutions in β-Processed Forging of a Near-β Titanium Alloy

The effect of strain rate on the β texture evolution during two-step hot forging of Ti-6246 alloy was investigated. The two-step forging consisted of 15% or 50% prior-β forging at 980°C and subsequent 60% or 25% forging at 870°C in the (α + β) dual-phase region. The total compression ratio was 75%, and the investigated strain rates were 0.01 and 1.0 s−1. The β forging texture showed typical {001} and {111} body-centered cubic textures. With increasing compression ratio in the (α + β) region and at a strain rate of 0.01 s−1, the amount of precipitated α phase increased. Dynamic recrystallization was rarely observed after forging in the (α + β) region at a strain rate of 0.01 s−1. Large amounts of α precipitates lowered the {001} β texture intensity through slip transmission between the α and β phases under the Burgers orientation relationship. However, in specimens forged at a strain rate of 1.0 s−1, as the compression ratio in the β single-phase region increased, the growth of dynamic-recrystallized β grains was promoted at the prior-β grain boundaries, where α-phase precipitation was not substantial. These effects resulted in a higher {001} texture intensity of the β phase in specimens forged at 1.0 s−1 compared with that of the β phase in specimens forged at 0.01 s−1.

Influence of reversible and non-reversible fatigue on the microstructure and mechanical property evolution of 7075-T6 aluminum alloy

Evolution of microstructural features and defects caused by high cycle fatigue loading have the potential to affect post-fatigue global behavior in different ways depending on the material, fatigue loading parameters, and subsequent loading strain rate. This work focuses on the influence of reversible and non-reversible, macroscopic elastic fatigue on the microstructure and subsequent mechanical properties of 7075-T6 aluminum alloy. Subsurface material obtained from interrupted fatigue experiments is used for both microstructural characterization and post-fatigue mechanical property evolution. Crystal plasticity modeling, in conjunction with machine learning, is used to help determine mechanisms contributing to the mechanical property evolution.It was found that the subsequent mechanical behavior was dependent on both the fatigue stress ratio and subsequent loading strain rate. There was a 7% decrease in quasi-static strength after being fatigued with a stress ratio of zero. However, quasi-static strength remained unchanged after being fatigued at other stress ratios. In addition, dynamic tension experiments did not reveal any decreases in strength due to prior fatigue loading. Fatigue-induced voids potentially developed in grains where slip transmission across the grain boundary was less likely to occur. Iron-rich inclusion cracking took place throughout the bulk fatigued material and was mainly limited to larger sized inclusion particles, but there was no evidence it contributed to significant mechanical property changes. Results from this work show how different stress ratios and degrees of dislocation reversibility during high cycle fatigue loading influence the microstructure and post-fatigue global behavior of 7075-T6 aluminum alloy.

A molecular dynamics study of dislocation-interphase boundary interactions in FCC/BCC phase transformation system

Industrial alloys are often strengthened via the formation of second phase during phase transformation due to the strong barrier of the interfaces between the second phase and the matrix, or interphase boundaries (IPBs), for dislocation propagation. In the present work, molecular dynamics simulation was employed to reveal the atomistic processes of the interactions between the lattice dislocations and face-center-cubic (FCC)/body-center-cubic (BCC) IPBs, including the image force on the lattice dislocation, slip transmission and other local reactions between the lattice dislocation and interfacial dislocations. It is found that the image force always attracts BCC lattice dislocation towards the IPB due to the difference in elastic properties of the two phases. With the presence of external force, four dislocation/IPB interaction results were observed among various dislocation-IPB interactions. Detailed analysis were made regarding how influencing factors such as resolved shear stress, continuity of slip systems, local dislocation reaction and dislocation core spread, affect dislocation-IPB interaction results. The present work provides some new insight into an in-depth understanding of how and in what ways IPBs can affect the plastic deformation in alloy systems.

A micromechanical analysis of intergranular stress corrosion cracking of an irradiated austenitic stainless steel

Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micromechanical analysis of IGSCC of an irradiated austenitic stainless steel is performed in this study to assess local cracking conditions. A 304L proton irradiated sample tested in PWR environment and showing intergranular cracking is investigated. Serial sectioning, Electron BackScatter Diffraction (EBSD) and a two-step misalignment procedure are performed to reconstruct the 3D microstructure over an extended volume, to assess statistically cracking criteria. A methodology is also developed to compute Grain Boundary (GB) normal orientations based on the EBSD measurements. The statistical analysis shows that cracking occurs preferentially for GB normals aligned with the mechanical loading axis, but also for low values of the Luster-Morris slip transmission parameter. Micromechanical simulations based on the reconstructed 3D microstructure, FFT-based solver and crystal plasticity constitutive equations modified to account for slip transmission at grain boundaries are finally performed. These simulations rationalize the correlation obtained experimentally into a single stress-based criterion. The actual strengths and weaknesses of such micromechanical approach are discussed.

Tensile behavior and deformation mechanism for Ti–22Al–25Nb alloy with lamellar O microstructures

In this research, the tensile behavior and deformation mechanism of two microstructures for Ti–22Al–25Nb alloy are studied by in-situ tensile test and electron backscatter diffraction (EBSD) experiment. Due to the higher B2 phase content and the long and continuous slip line, the solution-treated microstructure has better plasticity. For the aged microstructure, the slip line is short and scattered due to the obstruction of acicular O phase, which leads to the higher strength. In addition, the slip mode of B2 phase includes single system slip, double system slip and cross slip. {110}<111> slip and {112}<111> slip for B2 phase can be activated at room temperature. Meanwhile, the potential slip systems of O phase are summarized. The slip mode of O phase includes single system slip and double system slip, and the former is the most important one. All three types of slip systems (prism ‘a’ slip, basal ‘a’ slip, and pyramidal ‘c+a’ slip) for O phase can be activated at room temperature. Moreover, there is a good slip transmission between B2 and O phases. B2 phase slip can induce single system slip and double system slip of O phase. The Burgers orientation relation (BOR) between B2 and O phases is conducive to the slip transmission.

Crystallographic Orientation Dependence of Mechanical Responses of FeCrAl Micropillars

Iron-chromium-aluminum (FeCrAl) alloys are used in automobile exhaust gas purifying systems and nuclear reactors due to its superior high-temperature oxidation and excellent corrosion resistance. Single-phase FeCrAl alloys with a body centered cubic structure plastically deform through dislocation slips at room temperature. Here, we investigated the orientation dependence of mechanical responses of FeCrAl alloy through testing single-crystal and bi-crystal micropillars in a scanning electron microscopy at room temperature. Single-crystal micropillars were fabricated with specific orientations which favor the activity of single slip system or two slip systems or multiple slip systems. The strain hardening rate and flow strength increase with increasing the number of activated slip system in micropillars. Bi-crystal micropillars with respect to the continuity of slip systems across grain boundary were fabricated to study the effect of grain boundary on slip transmission. The high geometrical compatibility factor corresponds to a high flow strength and strain hardening rate. Experimental results provide insight into understanding mechanical response of FeCrAl alloy and developing the mechanisms-based constitutive laws for FeCrAl polycrystalline aggregates.

A complete grain-level assessment of the stress-strain evolution and associated deformation response in polycrystalline alloys

Polycrystalline alloys are used pervasively across structural applications contingent upon extensive experimental testing. A statistically representative number of tests are necessary to expose the variability in the material's performance, as a result of non-uniform microstructures and associated micromechanical fields. In a more direct means of capturing this pertinent information, multi-modal experimental techniques are presented to measure and track the complete micromechanical state, evolving during loading, of each and every grain within the regions of interest. Specifically, a combination of high-energy X-ray diffraction microscopy and digital image correlation coupled with electron backscatter diffraction are conducted on a specimen for each of the alloys, Haynes 282 and Ti7Al. The results of the multi-modal analysis definitively demonstrate that the degree of heterogeneity increases with deformation level and is used to assess the number of grains necessary for a representative volume element description of the stress state for each of these materials. Moreover, higher resolution imaging is used for identification of the slip system activity and subsequently used to study slip transmission events. An accurate knowledge of the resolved shear stress in adjacent grains (grain interactions) is demonstrated to be a key descriptor of the slip transmission events.