Initial Crystallographic Orientation Research Articles

Understanding dislocation plasticity of single crystalline Ta micropillars under dynamic loading

Recent experimental findings have shown that tantalum single crystals display strong anisotropy during Taylor impact testing in stark contrast to isotropic deformation in polycrystalline counterparts. In this study, a coupled dislocation dynamics and finite element model was developed to simulate the complex stress field under dynamic loading of a Taylor impact test and track the intricate evolution of the dislocation microstructure. Our model allowed us to investigate detailed motion of dislocations and their mutual interactions and the effect of varying simulation parameters, such as sample size, initial dislocation density, crystallographic orientation, and temperature. Simulation results show good agreement with experimental observations and shed light on the mechanical response at small-scale under extreme loading conditions. In addition, resolved shear stress analysis incorporating the effect of shear stress from impact was performed to quantitatively support and provide a means to understand the model predictions of the impact foot shape.

Effect of Initial Position and Crystallographic Orientation on Grain Selection Procedure in Z-Form Selector for Ni-Based Single-Crystal Superalloy.

The grain selection process in a Z-form selector for Ni-based single-crystal superalloy was simulated using a macro-scale ProCAST software (2013 version) coupled CAFE module combined with an experiment to investigate the grain selection procedure and mechanism with different grain positions and crystal orientation relationships. A non-stationary solidification process was found in the Z-form selector, and the liquid-solid (L-S) interface was tilted in the same direction as the selector channel during directional solidification. Given that the grain boundary was parallel to the Z-form selector, the overgrowth rate of the bi-crystal in the selector channel was very low. The initial position of the bi-crystal in the selector channel has a greater effect on the overgrowth rate than the effects of primary and secondary orientations. The grain selection was a result of the coupling of the competitive grain growth effect and geometrical restriction effect. Finally, the selection grain mechanism within the Z-form selector was discussed, coalescing the temperature field and the grain competition growth mechanism.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Coupled effects of crystallographic orientation and void shape on ductile failure initiation using a CPFE framework

Ductile failure reveals to be an anisotropic phenomenon, for which the proper mechanism has not been clearly addressed yet in the literature. In this paper, the effects of some key anisotropy factors on ductile failure initiation, detected by void coalescence and plastic strain localization, are investigated using unit-cell computations based on crystal plasticity finite element method. The studied anisotropic effects are induced by the combination of initial crystallographic orientations and void shapes. Therefore, single crystals with three different initial orientations and polycrystalline aggregates with three different initial crystallographic textures are respectively considered. A single void with either spherical, prolate or oblate shape is assumed to be preexisting at the center of each unit cell. By contrast to previous analyses in the literature, plastic strain localization is predicted in the present study on the basis of bifurcation theory. To cover a wide range of stress states, the simulations are performed under two macroscopic loading configurations: proportional triaxial stressing, characterized by constant stress triaxiality and Lode parameter, and proportional in-plane straining, specified by constant strain-path ratio. The obtained results show that the combined anisotropic effects play an important role in the occurrence of void coalescence and plastic strain localization, as well as in the competition between them.

Deformation Behaviour of Cold-Rolled Ni/CNT Nanocomposites

Metal matrix nanocomposites (MMNCs) reinforced by carbon nanotubes (CNTs) are good candidates to produce structural components in the mobility industry, given their unique properties. The manufacture of these components can involve plastic deformation. Therefore, it is crucial to understand whether reinforcement can influence the deformation behaviour of these nanocomposites. Thus, this work aims to study the deformation behaviour of MMNCs, given their importance and the lack of studies on this topic. Although nickel is not the most widely used metal as a matrix of nanocomposites, it presents mechanical properties superior to other matrices, such as aluminium. In addition, this metal has proven to establish a strong interface and integration of carbon nanotubes, making it an exciting material for the production and study of these nanocomposites. In that sense, nickel matrix nanocomposites are reinforced by 1.00 %vol. CNTs were produced by powder metallurgy using ultrasonication as a dispersion/mixture method. For comparison purposes, a nickel matrix was produced under the same conditions. Samples with and without CNTs were cold-rolled with thickness reductions between 10 and 60% (logarithmic strains between 0.11 and 0.92) to investigate the deformation behaviour. Microstructural characterization was performed using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Microhardness tests were applied to evaluate their mechanical properties. The results revealed that the nanocomposites exhibited a softening for small strains (0.11 and 0.22). This decrease in hardness was attributed to the decline in dislocation density observed by EBSD, due to the rearrangement and annihilation of pre-existing dislocations that originated during production. A possible inversion can explain the decrease in dislocation density when minor strains are applied in the dislocation or deformation trajectory, known as the Bauschinger effect. The difference in the texture evolution of the nanocomposites can be explained by the initial crystallographic orientations, which are influenced by the presence of CNTs.

Relationships between 3D grain structure and local inhomogeneous deformation: A laboratory-based multimodal X-ray tomography investigation

Relationships between the three-dimensional (3D) grain structure and the local strain distribution within individual grains are investigated. Multimodal X-ray tomography, i.e. attenuation tomography (ACT) and diffraction contrast tomography (LabDCT), was applied, for the first time using a laboratory instrument, to non-destructively characterize the grain structure and the local plastic deformation behavior of a fully recrystallized Al-4mass%Cu alloy. The evolution of the internal strain distribution during tensile deformation was measured in-situ by means of a microstructural feature tracking (MFT) method based on ACT. By a correlation analysis of the microstructural parameters of 855 grains, it was investigated if and how the initial crystallographic orientations and sizes of the grains affect the local deformations. In addition, effects of grain boundaries were analyzed. Only weak correlations are found. It is suggested that specific interactions between neighboring grains, which depend on parameters such as grain shapes and orientation differences, are of critical importance for the development of the local strains.

Prediction and Validation of Stress Triaxiality Assisted by Elasto-Visco-Plastic Polycrystal Model

The ΔEVPSC numerical code based on the elasto-visco-plastic HEM (Homogeneous Effective Medium) provides a multiscale constitutive modeling framework that is suitable for describing a wide range of mechanical behaviors of polycrystalline metals. In this study, an AA6061-T6 aluminum sample was chosen to validate the predictive capability of the ΔEVPSC stand-alone (ΔEVPSC-SA) code on stress triaxiality and its evolution until fracture. The model parameters were calibrated by fitting the uniaxial flow stress-strain curve, and the initial crystallographic orientation distribution (COD) was obtained using X-ray diffraction and electron backscattered diffraction (EBSD) methods. The statistical representativeness of the COD was further examined by comparing the experimental R-values with model predictions based on a set of CODs obtained via the two mentioned diffraction methods. The results suggest that the X-ray scan does not represent the texture very well, and instead, an entire cross-sectional EBSD scan is required, even though the texture gradient along the through-thickness direction is not very significant. The model-calculated triaxiality based on the ΔEVPSC-SA code was verified by comparison with the experimental results from the uniaxial tension, the notched tension, and the plane strain tests. The results were in good agreement with the ΔEVPSC finite element (FE) simulation results and other similar experimental results reported in the literature.

Effect of Texture and Temperature on Strain‐Induced Martensitic Transformation in 304 Austenitic Stainless Steel

The austenitic stainless steel's remarkable mechanical properties are caused by twinning‐induced plasticity and transformation‐induced plasticity mechanisms. Numerous studies focus on stacking fault energy's effect, which is affected by various factors, to interpret and control these mechanisms. However, crystallographic orientation is also an important parameter for mechanical properties in metals. This study compares the mechanical properties and microstructural features of 304 austenitic stainless steel, focusing on the effect of initial texture and deformation temperature. Microstructural characterization is identified by an interrupted tensile test based on strain, tensile direction, and temperature conditions, and X‐ray diffraction and electron back‐scattered diffraction analysis are performed. The results show that the mechanical features and strain‐induced martensitic transformation rate depend on the tensile directions. In addition, this trend is maintained irrespective of the temperature conditions. The attribute reason is that the difference in the Taylor factor and the formation rate of the deformed band structure is induced by the initial crystallographic orientations. Moreover, a decrease in temperature significantly increases the dislocation densities and abundant twins and transformed martensites formation. Furthermore, the yield and tensile strengths are enhanced while the elongation decreased with the tensile strains.

Multimodal assessment of mechanically induced transformation in metastable multi‐phase steel using X‐ray nano‐tomography and pencil‐beam diffraction tomography

A combination of X-ray nano-tomography and pencil-beam diffraction tomography was utilized for multimodal assessment of the mechanically induced transformation of individual retained austenite grains during tensile deformation in a 0.1C-5Mn-1Si multi-phase steel. In the present study, a newly developed high energy (20 - 30 keV) and high resolution (spatial resolution of 0.16 µm in this study) X-ray nano-tomography technique was applied for the first time to the in-situ observation of a steel under external loading. The gradual transformation, plastic deformation, and rotation behaviour of the individual austenite grains were clearly observed in 3D. It was revealed that the early stage of the transformation was dominated by the stress-assisted transformation that can be associated with measured mechanical driving force, whilst the overall transformation was dominated by the strain-induced transformation that is interrelated with measured dislocation multiplication. The transformation behaviour of individual grains was classified according to their initial crystallographic orientation and size. Noteworthy was the high stability of coarse austenite grains (i.e., 2.5 μm or larger in diameter), contrary to past reports in the literature. Characteristic rotation behaviour and wide data dispersion were also observed in the case of the individual austenite grains. It was conclusively demonstrated that such characteristic behaviour partly originated from interactions with surrounding soft and hard phases. The origins of these characteristics are discussed by combining the image-based and diffraction-based information.

Combining crystal plasticity and electron microscopy to elucidate texture dependent micro-mechanisms of tensile deformation in lath martensitic steel

A modified 9Cr-1Mo steel having lath martensitic microstructure has been subjected to the hot-rolling at three different temperatures followed by a normalization at 1025°C to form different crystallographic textures after thermomechanical processing. The samples hot-rolled at 875°C, 1000°C and 1050°C showed major texture components as Goss (i.e. {110}<001>), Cube (i.e. {001}<01¯0>) and Gamma (i.e. {111}<1¯1¯2>), respectively. Next, these samples have been uniaxial tensile tested at quasi-static strain rate at room temperature, and tensile properties are evaluated. The results indicated almost similar strength levels for Goss and Cube oriented specimens, and significantly reduced strength for Gamma oriented samples. However, the Cube and Goss oriented samples showed different strain hardening rates owing to the occurrence of deformation induced twinning and anti-twinning phenomenon as revealed by the Visco-plastic self-consistent polycrystal plasticity simulations. Simulation results were validated with experimental observations using high-resolution transmission electron microscopy. Anisotropic parameters have also been simulated considering the difference in initial crystallographic orientations. Study of deformation micro-mechanism at different length scale of martensitic units (e.g., prior-austenite grain, martensitic packets, blocks, sub-blocks, and laths) revealed negligible rotations at the prior-austenite grain level, whilst the lattice rotations were found to be significant at martensitic sub-block (i.e. variant) length scale. The investigation indicated that some specific types of martensitic variants generally participated in large lattice rotation during deformation for differently textured samples.

Equal channel angular pressing of single crystal aluminum: a molecular dynamics simulation

The equal channel angular pressing (ECAP) of single-crystal aluminum was investigated using molecular dynamics to provide insights into the deformation behavior and polycrystal formation. Three samples with different initial crystallographic orientations were processed by an ECAP die with a curvature angle (Ψ) of 20° and a channel angle (Φ) of 90° at 300 K. The samples were oriented, such that [100], [110], and [111] directions were parallel to the extrusion direction (ED). The shear strain was the predominant mode of deformation during ECAP with the presence of normal strain before entering the deformation zone due to the high compression force. The sample with [100] direction oriented along the ED showed significant grain fragmentation, the highest lattice rotation but the lowest dislocation density and shear strain, whereas the sample with [111] orientation showed the opposite results. A correlation between the number of active slip systems, the magnitude of shear strain, and grain rotation angle was observed with most of the lattice rotation occurring around the transverse direction. Furthermore, the deformation and grain fragmentation processes occurring in the deformation zone were also investigated. Finally, the model could produce results similar to the results obtained experimentally in the literature.

A crystal plasticity model for porous HCP crystals in titanium alloys under multiaxial loading conditions

In this paper, an efficient and effective crystal plasticity model is proposed for porous HCP crystals subject to a variety of multiaxial loading conditions. These conditions include (i) uniaxial, biaxial, and triaxial, (ii) tension and compression, (iii) low and high triaxiality, and (iv) axisymmetric and non-axisymmetric loadings. The framework is based on a combination of variational homogenization, phenomenological extensions, and assumptions motivated by observations from the high-fidelity micromechanical analysis. A novel penalty-free algorithm is employed to reach and maintain a specified stress state while performing representative volume element (RVE)-based crystal plasticity finite element (CPFE) analysis with porosity. The RVE studies demonstrate that the initial porosity, crystallographic orientation, and stress states have a significant effect on the homogenized mechanical responses of microscopically porous RVEs. The proposed porous crystal plasticity model is developed and calibrated using a database generated from the results of micromechanical RVE analysis. The calibrated porous model is reasonably effective in predicting the response of porous crystalline RVEs.

Combining Crystal Plasticity and Electron Microscopy to Elucidate Texture Dependent Micro-Mechanisms of Tensile Deformation in Lath Martensitic Steel

A modified 9Cr-1Mo steel having lath martensitic microstructure has been subjected to the hot-rolling at three different temperatures followed by a normalization at 1025 °C to form different crystallographic textures after thermomechanical processing. The samples hot-rolled at 875 °C, 1000 °C and 1050 °C showed major texture components as Goss (i.e. {110} ), Cube (i.e. {001} ) and Gamma (i.e. {111} ), respectively. Next, these samples have been uniaxial tensile tested at quasi-static strain rate at room temperature, and tensile properties are evaluated. The results indicated almost similar strength levels for Goss and Cube oriented specimens, and significantly reduced strength for Gamma oriented samples. However, the Cube and Goss oriented samples showed different strain hardening rates owing to the occurrence of deformation induced twinning and anti-twinning phenomenon as revealed by the Visco-plastic self-consistent polycrystal plasticity simulations. Simulation results were validated with experimental observations using high-resolution transmission electron microscopy. Anisotropic parameters have also been simulated considering the difference in initial crystallographic orientations. Study of deformation micro-mechanism at different length scale of martensitic units (e.g., prior-austenite grain, martensitic packets, block, sub-block, and laths) revealed negligible rotations at the prior-austenite grain level, whilst the lattice rotations were found to be significant at martensitic sub-block length scale. The investigation indicated that some specific types of martensitic variants generally participated in large lattice rotation during deformation for differently textured samples.

Initial texture dependence of nanocrystalline omega phase formation during high pressure torsion of commercially pure titanium

The present investigation studied the influence of initial texture on large strain deformation behavior of hexagonal close packed (HCP) titanium in high pressure torsion (HPT). Discs of two distinct initial textures comprising of near basal and near prismatic-pyramidal orientations were subjected to HPT at room temperature. X-ray diffraction (XRD) revealed the formation of omega phase in HPT processed samples with the fraction of omega phase increasing with increase in strain. Transmission electron microscopy and precession electron diffraction showed the formation of nanocrystalline grains of alpha and omega phases in 5 turns samples for both the orientations. Although omega transformation initiated first in the prismatic-pyramidal textured sample, the basal textured sample showed higher omega volume fraction at higher strain. The mechanical response based on micro-hardness measurements is different for the aforementioned two different initial textures. This is because hardness depends not only on the omega phase fraction, but also on the grain size and distribution of the nanocrystalline omega phase. Further, the relative contributions of strengthening imparted by the omega phase and hardening caused by the accumulated strain component determines the overall hardness profile. Also, synchrotron XRD measurements were performed to estimate crystallite size, microstrain and dislocation densities. The latter analysis coupled with the microstructural analysis indicates that initial texture and crystallographic orientation relationships of omega transformation influence the distribution of omega phase and strain partitioning between the alpha and omega phases, resulting in a strong texture dependence of hardness response.

Micromachining imposed subsurface plastic deformation in single-crystal aluminum

Mechanical removal of metal induces deformation and changes to microstructural characteristics of the newly created surfaces. The mode and extent of deformation can be difficult to predict since it depends on the local crystallographic orientation, which varies significantly for polycrystalline metals. In this work, we analyzed the deformation mode and extent beneath machined surfaces of different crystallographic orientations. This was accomplished by orthogonal micromachining of single-crystal aluminum along six different crystallographic orientations orthogonal to the sample [111] zone-axis, followed by electron backscatter diffraction (EBSD) analysis to evaluate the resulting subsurface microstructure and crystal lattice rotation. The results indicate that differences in the initial material crystallographic orientation produce significant variations in the depth of deformation (compared to the uncut chip thickness), the degree of grain refinement and the extent of lattice rotations. We grouped the orientation as “hard” or “soft” based on the measured cutting force. The soft orientations exhibit deformation modes consisting of shear bands and lattice rotations; whereas hard orientations exhibit deformation modes consistent with strain hardening: localized dynamic recrystallization, highly entangled dislocations and minimal crystal lattice rotations. The depth of subsurface deformation for some orientations was extensive, reaching depths far greater than the uncut chip thicknesses. Overall, we conclude that the cutting force required to machine a given orientation does provide some insight on the local deformation mode, and orientations can be easier or harder to machine based on local susceptibility to shear and lattice rotation.

Void growth and morphology evolution during ductile failure in an FCC single crystal

Void growth and morphology evolution are studied using a 3D representative volume element with a spherical void embedded in an FCC single crystal. The plastic flow contours are studied to determine the scenarios leading to fully plastic flow and plastic flow with elastic region. Further, the effect of anisotropy on void growth is studied through three initial crystallographic orientations (ICOs) [100], [110], & [111] with respect to loading direction. Void growth and macroscopic stress variations with applied strain are obtained from our simulations. It is observed that the peak stress corresponds to rapid void growth initiation. The peak stress is found to be dependent on void volume fraction and ICO. Furthermore, an additional geometrical parameter, diagonal distortions $$(D_{d_{i}})$$ is introduced to classify the non-spheroidal void shapes observed in deformed anisotropic crystal.

Rotational self-alignment of graphene seeds for nanoribbon synthesis on Ge(001) via chemical vapor deposition

The chemical vapor deposition of CH4 on Ge(001) results in the anisotropic synthesis of graphene nanoribbons that are aligned to Ge⟨110⟩ and have faceted armchair edges, sub-10 nm widths, and lengths greater than 100 nm. The utilization of small graphene seeds to initiate nanoribbon synthesis provides control over the nanoribbon placement and orientation. However, in order to exclusively grow nanoribbons and suppress the concomitant growth of lower aspect ratio crystals, it is imperative to control the crystallographic orientation of the seeds with respect to the Ge lattice. Here, we demonstrate that when seeds are less than 18 nm in diameter, they are able to rotate upon annealing at 910 °C prior to nanoribbon synthesis. The effect of this rotation on the resulting nanoribbons’ orientation is characterized as a function of the diameter and initial crystallographic orientation of the seeds. The seeds preferentially rotate to an orientation in which an armchair direction of their lattice is parallel to Ge⟨110⟩—subsequently maximizing the anisotropy in growth kinetics. By exploiting this seed rotation phenomenon, we demonstrate the fabrication of seamless nanoribbon meshes and gain understanding that will affect future efforts to create arrays of unidirectionally aligned nanoribbons.

Modeling of finite deformation of pseudoelastic NiTi shape memory alloy considering various inelasticity mechanisms

A crystal plasticity finite element (CPFE) model is developed for pseudoelastic NiTi shape memory alloy (SMA). The model includes various inelastic mechanisms of deformation such as martensite transformation, dislocation glide in austenite phase and twinning in the martensite phase. This model is based on the finite deformation theory and predicts phase transformation, stress field and residual strain due to slip and twinning. Following the work of Anand and Gurtin (2003), the Helmholtz free energy is derived and further the thermodynamic driving forces for slip, twin and phase transformation are obtained by using Clausius-Duhem inequality. The developed constitutive model has been implemented within a user material subroutine interface VUMAT in ABAQUS™/Explicit. The activation of different inelasticity mechanisms at different temperatures and strains are first verified by performing various simulations of uniaxial tensile deformation. Then the model is calibrated and validated against the experimental stress-strain response of NiTi single crystal as reported in the literature. The single crystal constitutive model is extended for modeling the response of a polycrystal with both initial random and textured crystallographic orientations using Taylor scale transition technique. A series of simulations are performed on polycrystalline representative volume element (RVE) at various temperatures and strains. The effect of temperature and imposed strain on phase transformation and residual strain is investigated systematically. The residual strain increases due to slip in austenite phase and twinning in the martensite phases that are activated as temperature and imposed strain increase respectively. Further the uniaxial loading conditions are extended to multiaxial loading and the results are compared against the experimental data.

Modeling the evolution of microtextured regions during [formula omitted] processing using the crystal plasticity finite element method

Titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) is frequently used in the high-pressure compressor of aero engines. While exhibiting high strength at elevated temperatures, it is susceptible to dwell fatigue at temperatures below ∼473 K due in part to the presence of microtextured regions (MTRs), also known as macrozones. This work investigates the role of forging direction on the mesoscale mechanical response of MTRs. The crystal plasticity finite element (CPFE) method was used to simulate large strain compression of MTRs with different initial crystallographic and morphological orientation with respect to the axial direction of the extruded billet. These simulations included cases where the c-axis of neighboring MTRs were (i) both perpendicular, (ii) one at 45° and one perpendicular, and (iii) one parallel and one perpendicular, to the compression direction. The effectiveness of each processing direction on the breakdown of MTRs is inferred through the extent of lattice rotation and the development of internal misorientations within the MTRs. The calculations reveal that case (i) leads to the most effective MTR breakdown but the c-axis remains similarly aligned; the c-axis is more scattered in case (iii) but the extent is limited by the high critical resolved shear stress of the pyramidal slip systems. Under uniaxial compression, competitive slip system activity correlates with positive divergence of reorientation velocity field in Rodrigues’ space as well as efficient breakdown of MTR.

Coupled simulation of ferrite recrystallization in a dual-phase steel considering deformation heterogeneity at mesoscale

Microstructure-based numerical modeling of ferrite recrystallization in a cold-rolled dual-phase (DP) steel during continuous annealing has been performed by considering the deformation heterogeneity with a coupled simulation method. The plastic deformation inside the two-phase structures is firstly simulated using the crystal plasticity finite element method (CPFEM) at the grain scale with the initial grain structures and crystallographic orientations inputted from EBSD maps based on a digital material representation algorithm. The predicted local stored deformation energy is then incorporated into the cellular automaton model as the driving force for subsequent ferrite recrystallization nucleation and growth. The simulations demonstrate inhomogeneous microstructural behaviors of ferrite recrystallization owing to the microstructural deformation heterogeneity inherited from the deformed multi-phase structures. Reliable predictions regarding the recrystallization kinetics, grain size distribution and microstructure morphology can be made compared with the experimental results. The influence of annealing temperatures and heating rates is also obtained by the simulation approach.