Three-dimensional Crystal Plasticity Research Articles

Anisotropic formability and deformation mechanism of near-α titanium alloy sheet under continuous nonlinear strain paths at high temperature

In the present study, the anisotropic formability and underlying deformation mechanism of near-α TA32 titanium alloy sheet under continuous nonlinear strain paths (CNSPs) at high temperature were investigated in depth. To achieve this goal, a new experimental method combining hot gas bulging with step-combined dies was proposed, and the hot CNSPs of metal sheets can be flexibly and conveniently realized by changing the number and shape of step-combined dies. Based on this method, the anisotropic deformation behavior and forming limits of TA32 sheet under fifteen CNSPs were tested at 800 ℃ with a strain rate of 0.001 s−1. Then, two advanced constitutive models with different scales were embedded into the classical Marciniak-Kuczyński (M-K) theory to predict the forming limits of TA32 sheet under different strain paths: the macro-scale viscoplastic model coupled with Hill48 yield criterion and non-associated flow rule (NAFR) as well as the meso-scale three-dimensional crystal plasticity finite element (CPFE) model coupled with cellular automata (CA). The results demonstrate that the CPFE-CA-MK coupled model exhibits higher accuracy in predicting the forming limits of TA32 sheet under linear and continuous nonlinear strain paths. Especially for the tension-tension strain paths, the CPFE-CA-MK coupled model improves accuracy by at least 3.1 % compared to macro-scale models. Due to the material anisotropy, the initial inclination angle of the groove in the CPFE-CA-MK model is closely related to the strain path and significantly affects the prediction accuracy. Based on the CPFE simulation, the effects of anisotropy and strain path change on the dislocation slip mode of different texture components was analyzed in depth, which provides a theoretical guidance for the optimization of hot forming process of TA32 titanium alloy complex components.

Formation of plate-like high-strain region in polycrystalline α-titanium and its relation to high-strain bands on surface

The investigation of heterogeneous deformation mechanisms in materials is important for understanding fracture mechanisms. In this study, uniaxial tensile deformation of polycrystalline α-titanium was numerically reproduced by three-dimensional crystal plasticity finite element analysis, and the following were revealed for the first time: (i) plate-like high-strain regions form inside the specimen, which are typically observed as band-like high-strain regions (high-strain bands: HSBs) on the surface of and cross-sections in α-titanium specimen, and (ii) the formation of HSBs also affects the work-hardening rate near the yielding point. The reasons for the difference in the formation mechanism of HSBs between the surface and inside were also discussed. The results obtained showed that active slip systems that contribute to the formation of HSBs may differ between the surface and inside of specimen, and strain in HSBs on the specimen surface is higher than that inside. The distributions of HSBs at the surface of and inside the specimen are determined by distributions of resolved shear stress. These results are important for understanding the differences in the fracture mechanisms between the surface and interior of materials.

Spatially-resolved cluster dynamics modeling of irradiation growth

We develop here a spatially resolved, three-dimensional continuum model coupling cluster dynamics (SR-CD) and crystal plasticity to investigate irradiation growth in zirconium. The model uses scale separation to divide the population of the irradiation cluster into mobile and immobile families. Small interstitial and vacancy clusters are modeled using anisotropic reaction–diffusion equations. Among the immobile clusters, an atomistically-informed vacancy cluster to vacancy loop transition is taken into account. The coupling between the evolution equation of CD and the plastic deformation of the material is two-fold, with stress-informed bias factors and local inelastic strains computed from the evolution of the evolving cluster population. The numerical implementation of the model utilizes the finite element method to analyze both single-crystal and polycrystalline samples. The growth strains that are computed align well with the experimental data provided by Carpenter for single-crystal Zr. Furthermore, the transformation of a vacancy cluster into a complete vacancy loop, occurring at a size of 14 nm, is in agreement with experimental observations and atomistic simulations. The density, size, and growth rate of the dislocation loops, denoted as 〈c〉 and 〈a〉, also exhibit good agreement with transmission electron microscopy (TEM) analysis of irradiated Zr and its alloys. Our findings demonstrate that there is a spatial correlation between the growth of these dislocation loops and growth strains, significantly influenced by the crystal size. To explain the expansion of the 〈a〉 axis and the contraction of the 〈c〉 axis in irradiated Zr, it is necessary to consider the diffusion anisotropy difference (DAD) of mobile interstitial species. We show that the PWR Kearns parameters, specifically fr = 0.63, ft = 0.32, fa = 0.05, confer enhanced irradiation resistance to Zr along the principal directions when compared to single crystals. Additionally, reducing the grain size to nanograins further enhances the resistance to irradiation-induced growth, particularly along the direction with the highest volume fraction of basal poles [0001].

Texture evolution and stress formation behaviour during tensile deformation using crystal plasticity finite element method

In this work, a three-dimensional crystal plasticity (CP) model was constructed to study the texture evolution behaviour and stress distribution of AA2024 alloy under uniaxial tensile conditions. After parameter calibration, the accuracy of simulated stress–strain curve reached 98.82%, and with good convergence. It was found that the {111} plane dots of Brass in (111) pole figure predominantly prolong at an angle of approximately 35.8° with respect to the RD, while those of Copper, S, Goss and Cube scatter towards almost all directions by the way in a circular augmentation. Moreover, it was revealed that Brass orientation is more conducive to alleviating stress during deformation process, while Copper is the least effective. The situations for Cube and Goss orientations are similar.

Investigation of neighboring grain effects on load shedding in titanium alloys under cold dwell fatigue

Load shedding under cold dwell fatigue in titanium alloys has been a threat to the safety of the aero-engine since it is responsible for crack formation. The strain incompatibility between the soft-hard grain pair arises from the significant creep in the soft grain during the dwell period, which is fundamentally related to the load shedding. Such deformational heterogeneity is not only affected by the soft-hard grain itself but also by its neighboring grains, but the effect of the latter is rarely reported. In this paper, the effect of neighboring grains, including location, crystal orientation, texture, etc., on load shedding is quantitatively investigated using a three-dimensional rate-dependent crystal plasticity model by controlling the microstructural morphology of the grains. In particular, the mechanism that the dwell fatigue cracks tend to nucleate from the sample's interior rather than the free surface is revealed. Restriction of rigid body rotation from neighboring grains can further enhance load shedding. The crystal orientation of individual and group neighboring grains sitting at different locations with respect to the soft-hard grain pair is evaluated. The grain directly attached to the hard grain has the strongest influence. The statistics of load shedding are analyzed by conducting hundreds of simulations with different textures. Stronger textures are found to be able to enhance load shedding and thus deteriorate the service lifetime. The change of locations showing the highest stress during the dwell period arising from plastic deformation in neighboring grain may lead to the phenomenon of reverse load shedding. Understanding the neighboring grain effects from both the mechanistic basis and the statistical point of view is important in integrity evaluations and service life assessments of titanium components in aero-engines.

Slip system level strengthening in nano-indentation of single crystalline copper: Numerical study via a non-local CPFEM model

A three-dimensional non-local crystal plasticity finite element model (CPFEM) integrating the evolution of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) was developed to simulate the nano-indentation of single crystal copper. The simulation accurately reproduced published experimental results when considering force-depth curves, indentation size effect (ISE), anisotropic plastic deformation and sink-in phenomenon. The accumulated slip, i.e., shear strain in individual slip systems, was quantitively investigated. It is anticipated that the developed non-local CPFEM model can support future sub-micro and nanoscale manufacturing research as it enables the rapid exploration of slip system level mechanical response of a diverse range of crystalline alloys.

Microstructural effects in rate-dependent responses of smooth and notched magnesium bars

We perform three-dimensional crystal plasticity simulations of smooth and notched bar geometries made of polycrystalline hexagonal close-packed material representing a magnesium alloy. The polycrystalline microstructure is explicitly resolved to investigate the combined effect of initial texture and grain size on the rate-dependent macroscopic responses and their micromechanical underpinnings under uniaxial and multiaxial stress states. The simulations reveal that in addition to the textural effect recently investigated by Ravaji et al. (2021), grain size plays an important role in the anisotropy of macroscopic responses. For a given texture, the lateral deformation anisotropy increases with grain size refinement for all strain rates considered here. The load-deformation responses exhibit a synergistic strengthening effect in microstructures with stronger initial textures and finer grain sizes, which is enhanced with increasing notch acuity. A transition from a conventional power-law type load-deformation response to a sigmoidal load-deformation response may occur, which depends on the imposed strain rate. It is a result of the interaction between textural weakening and grain size refinement that influence extension twinning together with an equitable landscape of the different slip mechanisms. We discuss possible implications of the net material plastic anisotropy due to texture and grain size on macroscopic failure using a micromechanical basis.

Predicting porosity distribution effects on the orientation induced plastic anisotropy of ductile solids: A crystal plasticity investigation

This paper investigates the role of crystallographic orientation and heterogeneous void distribution on void growth using three-dimensional crystal plasticity simulations. Face-centered cubic crystal structure is chosen as the model system. The role of crystallographic orientation on void growth is mapped by modeling single crystals as cubic unit cells with an embedded spherical void. Reminiscent of real aggregates, porosity is then re-distributed in single crystals to assess the role played by heterogeneous distribution of voids. Simulations are coupled with statistical analysis tools to decode the complex interplay involved in orientation dependent anisotropic plasticity and stochastic parameters governing heterogeneous void distribution like void size and placement. The applicability of the obtained results for single crystals is appraised for polycrystals as well by embedding the porous crystal in a randomly textured polycrystalline aggregate. Statistical analysis is repeated for all the considered orientations of the central grain to highlight the intricacies associated with plastic anisotropy-randomness in the heterogeneous microstructure. The results reveal interesting variations in mechanical strength, ductility and void growth rate in the three-dimensional orientation space. Analysis of configuration-property linkage reveals critical variables regulating the material strength and ductility for single crystals as well as polycrystals. Ramifications of the obtained results in designing strong and tough ductile components for practical applications are discussed.

Slip localization behavior at triple junctions in nickel-base superalloys

Incipient slip localization in the vicinity of hundreds of grain boundary triple junctions (TJs) in a lightly deformed nickel-base superalloy IN718 is studied using a combination of three-dimensional (3D) crystal plasticity finite element (CPFE) modeling, high resolution digital image correlation (HR-DIC) and 3D electron back-scatter diffraction tomography (3D EBSD). A 3D reconstruction method enables identification of thousands of TJs and correspondence of any observed slip bands with their originating TJ lines below the specimen surface. We present a large-scale CPFE model of the experimental 3D microstructure composed of high-fidelity representation of the TJ lines and the boundaries and interiors of the parent grains and use it to calculate the local micromechanical response and slip activity of all TJs at the onset of macroscopic yielding. Statistical analysis of the calculated quantities reveal TJs develop larger stress concentration and grain-average re-orientation than grain interiors and grain boundaries, however no substantial differences in cumulative slip were found among these microstructural regions. We find that TJs with observed slip bands generate lower grain-average re-orientation, fewer active slip systems, and more localized slip on a single system than those without. The distinctions in the reorientation and slip activity are stronger in TJs that experience more intense slip.

Three-dimensional crystal plasticity and HR-EBSD analysis of the local stress-strain fields induced during twin propagation and thickening in magnesium alloys

Present work focuses on analysis of the stress and strain fields inside and around the individual {10–12} twin in magnesium alloy. The 3D crystal plasticity model represents twin as an ellipsoidal inclusion surrounded by the matrix. Five different twin thicknesses and three different lateral twin lengths are used for stress/strain analysis. The simulations are complemented with experimental observations using high-resolution electron backscattered diffraction. The simulations and experiments show a similar distribution of the shear stress and the spatial activity of individual slip systems (basal, prismatic, pyramidal). Plasticity induced inside the twin is dominantly caused by the prismatic dislocations slip and does not influence twin back stress which is identical to pure elastic twin. The twin with larger lateral dimension requires lower equilibrium stress which suggests anisotropic twin propagation and increased thickness of such twins. The lateral twin propagation is mostly influenced by prismatic and pyramidal slip in the twin vicinity. The twin thickness can reach a maximal level that is driven by the critical resolved shear stress values for dislocation slip with the significant influence of basal slip.

Effect of distribution characters of ferrite/bainite on deformation compatibility in dual-phase pipeline steel: Experimental and numerical study

The morphology distribution of microstructure has an important influence on the match of the strength and plasticity of dual-phase pipeline steel. Bainite and PF dual-phase steels with layered and network morphology were obtained by combining the Thermomechanical Control Process (TMCP) and inter-critical isothermal heat treatment. A novel experimental-numerical methodology studied the effect of phase distribution characters on the deformability of PF and bainite dual-phase pipeline steel. The plastic damage features and microscopic strain and stress distribution in both network and layered microstructure were clarified. The obvious differences in the microstructure evolution for both samples were captured by a quasi-situ tensile test. The PF phase in the network morphology sample was believed the main deformation phase, but the deformation of the PF phase in the layered morphology sample was restrained. Two three-dimensional (3D) crystal plasticity finite element models (CPFEMs) were established based on the electron backscatter diffraction (EBSD) data. The simulation results indicated that the strain field in layered microstructure was more uniform than that in network microstructure, and the continuous internal stress relaxation (ISR) in the bainite phase led to plastic damage at low tensile strain level; The strain field in network microstructure mainly existed in the PF phase, the deformation of the PF phase can be fully utilized. When changing the tensile x-direction of the layered microstructure to the z-direction, the strain field showed an obvious anisotropy, and loading in the z-direction increased the risk of interlaminar crack formation. • Quasi-situ tensile test was used to study the deformation behavior • Strain/stress distribution characteristics were clarified using CPFEM method • Uniform strain distribution and ISR in layered microstructure decreased its plasticity • z-axis loading increased the risk of interlaminar crack for layered microstructure

Crystal plasticity simulation of in-grain microstructural evolution during large deformation of IF-steel

High-resolution three-dimensional crystal plasticity simulations are used to investigate deformation heterogeneity and microstructure evolution during cold rolling of interstitial free (IF-) steel. A Fast Fourier Transform (FFT)-based spectral solver is used to conduct crystal plasticity simulations using a dislocation-density-based crystal plasticity model. The in-grain texture evolution and misorientation spread are consistent with experimental results obtained using electron backscatter diffraction (EBSD) experiments. The crystal plasticity simulations show that two types of strain localization features develop during the large strain deformation of IF-steel. The first type forms band-like areas with large strain accumulation that appear as river patterns extending across the specimen. In addition to these river-like patterns, a second type of strain localization with rather sharp and highly localized in-grain shear bands is identified. These localized features are dependent on the crystallographic orientation of the grain and extend within a single grain. In addition to the strain localization, the evolution of in-grain orientation gradients, misorientation features, dislocation density, kernel average misorientation, and stress in major texture components are discussed.

Phase field study on the microscopic mechanism of the cyclic degradation of shape memory effect in nano-polycrystalline NiTi shape memory alloys

A three-dimensional non-isothermal and crystal plasticity based phase field model was proposed to simulate the cyclic deformation of nano-polycrystalline NiTi shape memory alloys (SMAs) reflecting their one-way shape memory effect (OWSME) and stress-assisted two-way effect (SATWSME), and then the microscopic mechanism of the cyclic degradation of shape memory effect (SME) was systematically revealed. The proposed model was numerically implemented in a two-dimensional plane strain nano-polycrystalline NiTi SMA system, where each grain was divided into a grain-interior (GI) phase and grain-boundary (GB) phase, and the GI phase was set to be embedded in the GB phase. For the GI phase, a crystal plasticity based phase field model was newly established by considering four inelastic deformation mechanisms, i.e., martensitic transformation (MT), martensitic reorientation (MR), austenitic plasticity caused by dislocation slipping and martensitic plasticity caused by dislocation slipping and deformation twinning, simultaneously; for the GB phase, a visco-plastic model was used to describe its plastic deformation. The simulated results show that: during the cyclic deformation of nano-polycrystalline NiTi SMAs involving the OWSME or SATWSME, the local internal stress caused by the MR or MT can lead to the accumulation of plastic deformation, which further increases the fraction of the martensite variants with favorable orientations by inducing a specific internal stress field after cooling and then results in the accumulation of residual martensite phase after heating; the accumulation of the irrecoverable strain mainly from the martensitic twinning and the decrease of recoverable strain cause the cyclic degradation of OWSME, which is further dependent on the loading rate and loading level; the SATWSME of the SMAs can exhibit a cyclic strengthening during the cyclic loading with a relatively low constant stress, while a cyclic degradation with a relatively high stress, which results from the competition between the transformation strain and the irrecoverable strain mainly caused by the austenitic plasticity that increases gradually during the cyclic deformation involving the SATWSME. These new findings are very helpful to clarify the physical nature of the cyclic degradation of SME and then alleviate the functional fatigue of NiTi SMAs by modulating the material design.

Predicting the complete tensile properties of additively manufactured Ti-6Al-4V by integrating three-dimensional microstructure statistics with a crystal plasticity model

A multiscale finite element model integrating microstructure statistics with an enhanced three-dimensional (3D) crystal plasticity model including damage has been developed to predict the complete tensile stress-strain response of Ti-6Al-4V manufactured by selective laser melting. A statistically equivalent representative volume element (SERVE) was constructed from an orthogonal set of electron backscatter diffraction (EBSD) images capturing key statistical information such as the spatial distribution of the grain size and the crystallographic and morphological orientation of the grains. Working from this 3D SERVE as a statistical model of the real microstructure, the crystal plasticity finite element (CP-FE) method was then used to predict the complete tensile stress-strain response of the material, including damage post necking. For the first time, crack-band theory (CBT) was incorporated into the CP-FF model to accurately predict ductility and to minimize mesh size sensitivity, which is a common issue in existing models. The effects of the 3D SERVE size on the stress-strain response was investigated through a rigorous sensitivity analysis. The model was calibrated using one sample and validated against a second sample with a different microstructure and properties. The new multiscale model provides a basis for a comprehensive Integrated Computational Materials Engineering (ICME) tool to enable the rational design of new high-performance materials.

An additively manufactured and direct-aged AlSi3.5Mg2.5 alloy with superior strength and ductility: micromechanical mechanisms

An AlSi3.5Mg2.5 (wt%) alloy with excellent mechanical properties was produced via laser powder bed fusion in this study. The yield strength, tensile strength, and elongation of this as-built AlSi3.5Mg2.5 alloy reach about 406 MPa, 501 MPa, and 8.6%, respectively. These properties are dramatically superior to the current additively manufactured Al-Si-Mg alloys. A direct-aging treatment at 170°C for one hour increases the yield strength and ductility further to about 417 MPa and 11.0%, respectively, with the tensile strength remaining the same level. The microstructures and strengthening mechanisms of the as-built and direct-aged samples were investigated systematically. The underlying micromechanical mechanisms of the as-built and direct-aged samples were examined based on a combination of in-situ synchrotron X-ray diffraction and three-dimensional crystal plasticity modeling. The as-built AlSi3.5Mg2.5 alloy possesses a fine microstructure, including fine grains and nano-sized Mg2Si and Si precipitates. After direct-aging treatment, additional Mg2Si and Si precipitate out. Besides, element diffusion upon aging treatment causes migration of cell boundaries and relaxation of residual stress. The direct-aging treatment leads to an increased Orowan strengthening, dislocation strengthening, and load-bearing strengthening effects. Moreover, the variations of microstructure and residual stress after the aging treatment change the dislocation behavior and increase the dislocation storage capacity, causing an increased ductility. Nevertheless, the aging treatment does not alert the type of damage and fracture. This study provides valuable insights to tailor the microstructure and mechanical properties of additively manufactured Al-Si-Mg alloys.

Meso-modelling study of the mechanical response and texture evolution of magnesium alloy during hot compression

The mechanical response and texture evolution of magnesium alloy during hot deformation are closely related to the heterogeneous meso-deformation. The improved Voce hardening law was developed and introduced into the crystal plasticity (CP) theory, and a reconstruction method of textured representative polycrystal was proposed. Three-dimensional crystal plasticity finite element model (3D-CPFEM) was established to meso-simulate the hot compression deformation of ZK61 Mg alloy. The influence of the temperature and strain rate on mechanical response was investigated, and the corresponding texture evolution and the texture transformation mechanism was revealed through meso-simulation. The results show that the 3D-CPFEM with the improved Voce hardening law and the initial texture characteristic can well predict the mechanical response and texture evolution under different temperatures and strain rates during hot compression of ZK61 Mg alloy. The basal plane texture of the ZK61 tubular blank is transformed into the basal fiber texture after hot compression, where the basal plane of grains is deflected gradually from the radial-axial plane to the radial-tangential plane. The texture transformation is attributed to two reasons, i.e., the orientation deflection of the grains with larger basal slip systems Schmid factor (SF) and the orientation retention of the grains whose c-axis tends to be parallel to the axial direction (AD) of the tubular blank. The change of the grain orientation corresponding to the basal plane texture component of the tubular blank has little contribution to the formed basal fiber texture.

Three-dimensional crystal plasticity simulations using peridynamics theory and experimental comparison

A three-dimensional (3D) peridynamics (PD) model of crystal plasticity (CP) is presented for predicting the fine-scale localization in polycrystalline microstructures undergoing elastoplastic deformation. Microscale data from electron microscopy and digital image correlation have indicated that slip localizations arise early in deformation and act as precursors to mechanical failure and fracture. However, classical numerical approaches such as crystal plasticity finite element methods (CPFEM) are generally unable to predict the emergence and distribution of such localizations. Alternatively, the PD formulation has attracted significant attention for its unique treatment of deformation in the presence of high strain gradient fields. In this paper, a mesh-free non-ordinary state-based PD technique is developed for simulating the elasto-plastic deformation of 3D polycrystalline aggregates of a magnesium alloy. This work presents the details of 3D polycrystal plasticity modeling using PD theory with experimental and CPFEM comparisons. The results from this model are validated against published experimental data for the stress-strain response and texture evolution. The crystal plasticity peridynamic (CPPD) models are successful in simulating grain averaged strains seen in the experiment and depict well-resolved regions of strain localization.

A Method to Incorporate Grain Boundary Strength and its Effects on Plastic Deformation in FCC Polycrystals

A three-dimensional (3D) crystal plasticity finite element method (CPFEM) is developed to investigate the effect of grain boundary strength on heterogeneous strain partitioning in FCC polycrystals. The proposed method incorporates electron backscatter diffraction (EBSD) maps into finite element analyses. The numerical analysis accounts for crystallographic texture, its evolution and 3D grain morphologies. Furthermore, grain boundaries are also mapped with a special finite element framework that allows material properties to be assigned to the grain boundaries. The material parameters and the grain boundary strengths are obtained by calibration to experimental uniaxial tension curves for single and polycrystals. Numerical simulations of uniaxial tension are performed and the effects of grain boundary strength on the onset of non-uniform deformation is investigated. The predicted local strain evolution is compared with corresponding experimental results from digital image correlation (DIC) measurements of an AA5754 aluminium sheet. The results showed that an excellent agreement was reached when the grain boundary properties were set so that the hardness was five times that of the average polycrystal response.

A three-dimensional microstructure-based crystal plasticity model for coarse-grained and harmonic-structured Ti–6Al–4V under monotonic and cyclic shear loading

This paper develops a microstructure-based numerical model to simulate the mechanical behavior of homogeneous coarse-grained (CG) and harmonic-structured (HS) Ti–6Al–4V under monotonic and cyclic simple shear loading conditions. This model incorporates a crystal plasticity model describing the deformation behavior of lamellar $$\alpha +\beta $$ colonies and a scale transition rule called ‘ $$\beta $$ -rule’ which enables to deal with the huge contrast between grain sizes in fine-grained and coarse-grained regions in HS Ti–6Al–4V. Besides, the numerical model can be used to consider the effects of microstructural features, such as the slip geometry for lamellar $$\alpha +\beta $$ colonies, length scale dependence of hardening, or anisotropic strength of the slip systems. The model is implemented into a finite element code (Cast3M) via a UMAT subroutine. The strengthening effect of harmonic structure design is studied based on the numerical results for HS Ti–6Al–4V and homogeneous CG Ti–6Al–4V. The simulation results are in good agreement with the experimental observations.

A Numerical Study of Strain-Rate and Triaxiality Effects in Magnesium Alloys

We report a preliminary computational study of the rate-dependent behavior of a polycrystal magnesium alloy under varying levels of stress triaxiality conditions. Smooth and notched round bar specimens with a strong initial texture are considered to achieve different levels of stress triaxiality. Full three-dimensional crystal plasticity simulations are conducted, which mimic tensile Kolsky bar experiments. The results indicate that the material rate sensitivity couples with the stress state to produce qualitatively different macroscopic responses that are governed by the interacting microscale deformation mechanisms. While the smooth specimens show macroscopic strain localization resulting in stress softening immediately following the initial yield, notched bars exhibit increasingly stable responses with increasing notch acuity. Deformation anisotropy is tempered with increasing stress triaxiality and strain rate. A micromechanical analysis of the deformation activities is presented to explain the macroscale responses. Stress triaxiality distributions in the notch regions provide insights into probable damage mechanisms as a function of the imposed strain rate.