A comparative study on fatigue indicator parameters for near‐<i>α</i> titanium alloys

Nucleation of in‐service cracks leads to detrimental consequences for structural components of near‐α titanium alloys subjected to fatigue loads. Experimental observations show that the fatigue initiation facets usually form in certain crystallographic orientation ranges of “hard” primary α grains which differ between pure and dwell fatigue loads. In this manuscript, a comparative study has been performed using several fatigue indicator parameters (FIPs) to assess their ability to predict the location of fatigue crack nucleation in near‐α titanium alloy microstructures. All selected FIPs are implemented within the same polycrystalline plasticity finite element modeling framework to facilitate one‐to‐one comparisons. Comparison on predictability of critical initiation locations and their crystallographic orientations is studied for incorporated FIPs under pure and dwell fatigue. The critical locations predicted by some FIPs were found to be close to each other, and consistent with the crystallographic orientation ranges from fractography measurements, in addition to the range transition from pure to dwell fatigue loads. Critical locations from slip driven FIPs are obtained to be several grains away from that of the former ones and are inclined to capture orientations of slip traces from experiments.</p

Crystal Plasticity Finite Element Modeling of the Influences of Ultrafine-Grained Austenite on the Mechanical Response of a Medium-Mn Steel

Medium manganese (medium-Mn) steel, one of the third-generation advanced high-strength steels (AHSS), delivers impressive mechanical properties such as high yield strength, ultimate tensile strength, and uniform elongation. One notable feature of medium-Mn steels is the presence of ultrafine-grained (UFG) austenite, achieved through phase transformation from the parent martensite phase during intercritical annealing. While, in general, UFG is considered a strengthening mechanism, the impact of UFG austenites in medium-Mn steel has not been fully studied. In this manuscript, we advance our previous work on crystal plasticity simulation based on the Taylor model to consider fully resolved high-fidelity microstructures and systematically study the influence of the UFG austenites. The original microstructure with UFG is reconstructed from a set of serial electron backscatter diffraction (EBSD) scans, where the exact grain morphology, orientation, and phase composition are preserved. This microstructure was further analyzed to identify the UFG austenites and recover them to their parent martensite before the intercritical annealing. These two high-fidelity microstructures are used for a comparative study using dislocation density-based crystal plasticity finite modeling to understand the impact of UFG austenites on both the local and overall mechanical responses.</p

Uncertainty Quantification for Microstructure-Sensitive Fatigue Nucleation and Application to Titanium Alloy, Ti6242

Microstructure of polycrystalline materials has profound effects on fatigue crack initiation, and the inherent randomness in the material microstructure results in significant variability in fatigue life. This study investigates the effect of microstructural features on fatigue nucleation life of a polycrystalline material using an uncertainty quantification framework. Statistical volume elements (SVE) are constructed, where features are described as probability distributions and sampled using the Monte Carlo method. The concept of SVE serves as the tool for capturing the variability of microstructural features and consequent uncertainty in fatigue behavior. The response of each SVE under fatigue loading is predicted by the sparse dislocation density informed eigenstrain based reduced order homogenization model with high computational efficiency, and is further linked to the fatigue nucleation life through a fatigue indicator parameter (FIP). The aggregated FIP and its evolution are captured using a probabilistic description, and evolve as a function of time. The probability of fatigue nucleation is measured as the probability that the predicted FIP exceeds the local critical value which represents the ability of material to resist the fatigue load. The proposed framework is implemented and validated using the fatigue response of titanium alloy, Ti-6Al-2Sn-4Zr-2Mo (Ti-6242).</p

A crystal plasticity approach to understand fatigue response with respect to pores in additive manufactured aluminium alloys

A crystal plasticity finite element modelling method integrated with a stored energy density criterion is utilized to comparatively investigate fatigue crack nucleation behaviour and quantify fatigue life with respect to different pore types in AlSi10Mg fabricated by selective laser melting. Representative microstructural models show that fatigue crack nucleation exhibits high sensitivity to both gas/keyhole and lack of fusion pores, but particularly the latter, which leads to much lower fatigue life at high stress levels. Multi-intragranular slip system activations occurring at the sharp corners of lack of fusion pores contribute to substantial increase in local geometrically necessary dislocation density. Together with the rapid accumulation of slip, these drive high local stored energy density at the tips of lack of fusion pores. For gas/keyhole pores, high stresses lead to pore-induced shear band formation which shifts the origin of crack nucleation away from the pore to other microstructural features. At low stresses, fatigue life for lack of fusion and gas/keyhole pores tend to converge but remain shorter than for pore-free microstructures.</p

Characterization of the strain rate sensitivity of basal, prismatic and pyramidal slip in Zircaloy-4 using micropillar compression

The slip strength of individual slip systems at different strain rates will control the mechanical response and strongly influence the anisotropy of plastic deformation. In this work, the slip activity and strain rate sensitivity of the basal, prismatic, and pyramidal slip systems are explored by testing at variable strain rates (from 10−4 s−1 to 125 s−1) using single crystal micropillar compression tests. These systematic experiments enable the direct fitting of the strain rate sensitivities of the different slips using a simple analytical model and this model reveals that deformation in polycrystals will be accommodated using different slip systems depending on the strain rate of deformation in addition to the stress state (i.e. Schmid's law). It was found that the engineering yield stress increases with strain rate, and this varied by slip systems. Activation of the prismatic slip system results in a high density of parallel, clearly discrete slip planes, while the activation of the pyramidal slip leads to the plastic collapse of the pillar, leading .to a ‘mushroom’ morphology of the deformed pillar. This characterization and model provide insight that helps inform metal forming and understanding of the mechanical performance of these engineering alloys in the extremes of service conditions.</p

A microstructure-sensitive analytical solution for short fatigue crack growth rate in metallic materials

Short fatigue crack growth in engineering alloys is among the most prominent challenges in mechanics of materials. Owing to its microstructural sensitivity, advanced and computationally expensive numerical methods are required to solve for crack growth rate. A novel mechanistic analytical model is presented, which adopts a stored energy density fracture criterion. Full-field implementation of the model in polycrystalline materials is achieved using a crystallographic crack-path prediction method based on a local stress intensity factor term. The model is applied to a range of Zircaloy-4 microstructures and demonstrates strong agreement with experimental rates and crack paths. Growth rate fluctuations across individual grains and substantial texture sensitivity are captured using the model. More broadly, this work demonstrates the benefits of mechanistic analytical modelling over conventional fracture mechanics and recent numerical approaches for accurate material performance predictions and design. Additionally, it offers a significant computer processing time reduction compared with state-of-the-art numerical methods.</p

Synergistic coupling of thermomechanical loading and irradiation damage in Zircaloy-4

Abstract This work addresses in-situ synergistic irradiation and thermomechanical loading of nuclear reactor components by linking new mechanistic understanding with crystal plasticity finite element modelling to describe the formation and thermal and mechanical annihilation of dislocation loops. A model of pressurised reactor cladding is constructed to extract realistic boundary conditions for crystal plasticity microstructural sub-modelling. Thermomechanical loads are applied to the sub-model to investigate (i) the unirradiated state, (ii) synergistic coupling of irradiation damage and thermal annihilation of dislocation loops, (iii) synergistic coupling of irradiation damage without thermal annihilation of dislocation loops, and (iv) a post-irradiated state. Results demonstrate that the synergistic coupling of irradiation damage and thermomechanical loads leads to the early onset of plasticity, which is exacerbated by the thermal annihilation of dislocations, while the post-irradiated case remains predominantly elastic due to substantial irradiation hardening. It is shown that full synergistic coupling leads to localisation of quantities linked with crack nucleation including geometrically necessary dislocations and stress

Understanding the hydride precipitation mechanism in HCP Zr polycrystals: a micromechanical approach

This study focuses on the hydride precipitation in zirconium polycrystals during thermo-mechanical cycles. The precipitation and dissolution of mesoscale hydrides in Zircaloy-4 is modelled using crystal plasticity finite element methods supported with DFT-informed zirconium lattice hydrogen concentration. Results for a tri-crystal case show the effects of crystallography, thermo-mechanical load and elasto-plastic anisotropy on hydride nucleation and growth. Analyses of polycrystalline models provide new insights into the complex precipitation process of hydrides in Zircaloy-4 with explicit representation of experimental observations that lay the foundation for further research in this field. Micromechanical findings demonstrate the importance of microstructure, pre-thermal condition, and hydrogen concentration limit on hydride precipitation. Overall, the study provides a deeper understanding of hydride formation during industrially relevant reactor conditions. Graphical abstract</p

Simulation of crystal plasticity in irradiated metals: A case study on Zircaloy-4

A classical crystal plasticity formulation based on dislocation slip was extended to include the mechanisms of dislocation channelling, with associated strain softening which is observed in many alloys post irradiation. The performance of the model was evaluated against experimental data on Zircaloy-4, which included engineering stress-strain response and high-resolution digital image correlation strain mapping. Variants of the model were developed to evaluate the influence on the strain hardening law used, comparing hardening based on a linear relationship for effective plastic strain with that based on the evolution of geometrically necessary dislocations. In addition, governing equations for simulating the interaction between gliding dislocations with various types of irradiation defect were investigated; this included the comparison of isotropic and anisotropic interactions based on the resultant reaction segments for each interaction. It was shown that the engineering stress-strain response measured by experiment could be captured by many of the model variants, but the simulation of characteristic strain heterogeneity was more sensitive to the model used. Direct modelling of the HRDIC experiments indicated that the model successfully predicted the activation of slip systems in many cases and exhibited localised strain distribution as observed in the experiment. In all models localised kink band formation was predicted, which is not observed experimentally, which highlights limitations in modelling of softening materials with a local crystal plasticity approach and a required area of development going forward.</p

Exploring the hydride-slip interaction in zirconium alloys

Hydrogen pick-up and hydride precipitation can lead to embrittlement and fracture strength reduction of nuclear fuel cladding tubes made of Zircaloy. Plastic deformation of hydride packets and its interaction with local plasticity in the zirconium matrix is a key linkage of microstructure feature to structural integrity of hydrided polycrystalline bulk Zircaloy. This work focuses on explicit representation of hydride packets from high spatial resolution electron backscatter diffraction onto a crystal plasticity finite element model for capturing and understanding slip localisation near hydride-matrix phase boundaries, based on the extracted material property of hydrides. The mechanisms behind slip evolution including slip nucleation, slip transfer, and slip inhibition are studied by combined high-resolution digital image correlation and crystal plasticity results. Through assessing various slip transfer parameters, new slip transfer criterion is proposed for α/δ phase boundaries. Prior to slip transfer criterion, local micromechanical quantities, specifically shear stress and stored energy density, are necessary to drive and provide pathway for subsequent slip transfer at α/δ phase boundaries.</p