Search

The grain scale dynamic interaction of dislocations with the deformation induced martensites and twins, improving the strength, ductility and formability, in 304 LN austenitic stainless steel was examined at 28 and 350 °C through microstructure mapping using electron back scatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) during interrupted tensile tests. Greater deformation induced martensites and twins form at 28 than 350 °C in the grains having higher Schmid factors. The larger geometrically necessary dislocation (GND) density, measured by EBSD, validates the excellent balance of strength and ductility at 28 than 350 °C. The rate of increase of GND density with strains and temperatures is not singular and can be explained by higher strain hardening, low dislocation cross-slip and dynamic recovery. The heterogeneity in GND distribution is greater at 28 °C due to presence of martensite, twins, refinement of the co-planar slip band spacings and formation of shear bands, fragmenting the grains with strains improving the work hardening. Finer austenite grains sustain larger GND density with temperatures and strains. Thus, GNDs are responsible for forest hardening (dissipative strain gradient strengthening) and generation of back stress (energetic strengthening) more at 28 than at 350 °C. [ABSTRACT FROM AUTHOR]

Surface mechanical treatments are known to introduce deformation heterogeneity, thus entailing a combination of excellent strength and ductility in metallic metals. Accumulative skin pass rolling (ASPR) was employed to introduce heterogeneous structure in pure copper, which significantly improved yield strength and maintained high ductility. The experimental observations of the ASPR processed copper showed that surface grains possessed a much larger magnitude of misorientations and geometrically necessary dislocations (GNDs) than the centre grains. Crystal plasticity finite element modelling was utilised to provide an in-depth understanding of microstructure evolution during ASPR. It was found that the surface grains accommodated higher shear strain and had higher slip resistance than the centre grains. The multiple active slip systems in the surface grains induce the lattice rotations and then the formation of intragranular misorientations accompanied by a high density of GNDs, while the single active slip system in the centre grains can accommodate the applied material rotation without the lattice rotation.

The defect of mixed-grain structure with millimeter-grade coarse grains (MCGs) is frequently found in heavy forgings, such as nuclear main pipes manufactured from 316LN steel. Renovation of such kind forgings can only be conducted through recrystallization. However, the allowed deformation is limited because the geometric size of forgings should be maintained during the renovation. For this purpose, the deformation heterogeneity of 316LN steel with mixed-grain structures were studied experimentally, which demonstrated that the MCGs were more inclined to deform than fine grains even if MCGs are with hard orientations. Therefore, a two-stage method was studied to refine MCGs, which takes the advantage of this deformation heterogeneity. The first stage was to impose a limited deformation to the steel at room temperature, suppressing the dynamic recovery and leading the preferentially deformed MCGs to accumulate enough deformation energy storage. The second stage was rapidly heating the deformed materials to realize static recrystallization (SRX). The results showed that the MCGs were completely refined by SRX even if the reduction rate is only 8%. The SRX mechanisms and refining assessments of MCGs under different deformation conditions were discussed. This work, thus, provides an effective method to salvage the heavy structures in engineering.

Dynamic (up to 5500 s−1) and quasi-static compression tests are conducted on a 15 wt% B 4 C particle-reinforced Al (B 4 C/Al) composite. In situ , high-speed synchrotron X-ray phase contrast imaging and digital image correlation are employed to map mesoscale deformation fields at μm and μs scales. The bulk stress–strain curves show significant strain and strain-rate hardening under dynamic compression. The strain-rate sensitivity exponent is an order of magnitude higher at high strain rates (> 10 3 s−1) than that at low strain rates (< 10 − 2 s−1). Strain field mapping demonstrates distinct compressive strain localizations for both quasi-static and dynamic loading. Nevertheless, compressive strain localizations appear denser in spacing under dynamic loading, owing to spontaneous dislocation nucleation in both weak and strong zones. This results in a higher density of geometrically necessary dislocations, which contributes to the higher strain and strain-rate hardening of B 4 C/Al under dynamic loading. The ratio of the maximum local strain to the bulk average is ~ 1.5, and the local strain-rate enhancement cannot explain the increased rate sensitivity of B 4 C/Al under dynamic loading. Therefore, the rate-dependent deformation heterogeneity dominates the strain-rate hardening of B 4 C/Al. Postmortem analyses help correlate deformation features to particle- and grain-scale microstructures, yielding consistent results with mesoscale strain fields. [ABSTRACT FROM AUTHOR]

• One-layer RVE generated directly using EBSD increases simulation efficacy. • Geometrical deployment of martensite hinders ferrite stretch and localize strain. • Mobile dislocation density evolution causes stress and strain heterogeneity in ferrite. • Misorientation of ferrite grains are highly loading dependent. • Martensite vicinity intensifies dislocation interactions and evolution in ferrite. In this study, we employ a physics-based crystal plasticity finite element method (CPFEM) approach to study the microstructural response of DP600 steel to various loading types. With the pursued approach, it is possible link microstructural changes in the dual phase (DP) steel to its macroscopic response in a convenient and efficient manner. Microstructural data were acquired first from experimental work via the electron backscattered diffraction (EBSD) method. DP crystallographic features were then fed to a numerical hybrid calculation subroutine combining a comprehensive physics-based crystal plasticity scheme for ferrite along with an isotropic plasticity scheme for martensite defined on a single-layer representative volume element (RVE). The developed single-layer 3D mode, while allowing for high computational efficiency, can capture the stress/strain distribution, revealing the microscopic features of the deformation process including the physical evolution of the dislocation density, retexturing, and morphology. The single-layer RVE physics-based method employed herein establishes the strong correlation between morphology, dislocation density and deformation response of the material under various loading types. Specifically, findings reveal that the tendency of the ferrite phase neighboring the martensite regions in the DP steel to retexture is much more profound than pure ferrite when subjected to the same loading conditions, pointing out to the steep strain gradients in the former component. Martensite phase facilitates considerable heterogeneity in the stress and strain distribution, increases the misorientation polarization as well as leading to local strain hardening in the neighboring ferrite volume that stem from profound increase in spatial dislocation density. Stress-strain curves under various loading schemes are simulated and discussed in the light of the experimental findings in literature, revealing the dependence of the microstructural evolution on the loading type. [Display omitted] [ABSTRACT FROM AUTHOR]

The effect of grain size on micro-void evolution and further macroscopic fracture in an austenite steel 316LN was studied through a series of experiments together with the full-field crystal plasticity finite element method (CPFEM) simulations. To probe the grain size effect on void behaviors, macroscopic tensile tests and microscopic fractography characterizations were conducted for samples with different grain sizes. A hierarchy modeling approach based on CPFEM was adopted to quantify the gran size effect accordingly. Authentic boundary conditions were enforced on the high-resolved representative volume elements (RVEs) with realistic grain structures and voids. The simulation results demonstrate that the deformation heterogeneity and the scatter of void growth increase with grain size. Via the quantitative analysis of the void dimension, an extended void growth model involving the effect of grain size was proposed on the basis of the Rice and Tracey model. The extended model adopts the Gaussian distribution to describe the non-uniform void growth induced by the grain-scale deformation heterogeneity and manifests the increase of void size deviation with grain size. The variation of void growth with grain size further leads to a transition of fracture modes. For the fine grain sample, coalescence of densely distributed voids dominates the fracture initiation, and the total void volume fraction thus plays a key role; For the coarse grain sample, however, the growth and coalescence of individual large void are critical for fracture occurrence. With the grain size affected void behavior, the ductility of the material is also shown to be grain size dependent. This study thus advances the comprehensive understanding of the micro-mechanics of ductile fracture and the relationship between microstructure and the macroscopic fracture behavior. • Identify the grain size effect on void behavior via experiments and full-field CP modeling. • Deformation and void size distribution become more uneven as grain size is increased. • An extended void growth model involving the effect of grain size was proposed. • Explain different ductile fracture mechanisms related to grain size. • Identify the relationship between ductility and grain size. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Slip transmission at grain boundaries is characterised by SEM and EBSD at early stage of deformation. • A strain gradient crystal plasticity finite element model is developed. • Criterion for slip transmission is determined from experiment and compared against CPFEM simulations. • Effect of slip transmission on flow strength and deformation localisation are evaluated using CPFEM model. This study explores the phenomenon of slip transmission across grain boundaries in coarse-grained copper. The associated effects on the deformation gradient are also investigated, using both experiment and simulation. An in-situ tensile test of a fully annealed copper specimen was conducted first using a scanning electron microscope (SEM). The deformation gradient and geometrically necessary dislocation (GND) maps at selected tensile strain values were obtained and characterized by electron backscatter diffraction (EBSD). In addition, a physically based strain gradient crystal plasticity finite element method model has been developed to simulate the early stage of tensile deformation behaviour of copper. In this model, slip transmission across the grain boundary is either allowed or blocked, depending on the alignment of adjacent grains. Slip transmission is allowed at grain boundaries between suitably aligned grains according to the Luster-Morris factor, residual Burgers vector is applied to consider the slip misalignment at GB region, and dislocation accumulation at grain boundaries is included following the Kocks-Mecking law. It was found that dislocation pile-up, lattice rotation and stress concentration were much higher near the impenetrable grain boundaries than at the penetrable ones. Due to the slip transmissions across grain boundaries, the stress and strain concentrations can be partially relaxed, and the slip transmission induced relaxation is associated with the number of active slip systems and shear strain on the best aligned slip systems. This study provides an effective strategy to investigate the early stage of plastic deformation, and a good agreement between experimental observations and simulation results has been achieved. [ABSTRACT FROM AUTHOR]

This paper focuses on the development of an appropriate Digital Image Correlation (DIC) methodology based on Image Registration and dedicated for characterizing the plastic deformation in single crystals. A pure nickel single crystal specimen is plastically deformed in tension and investigated by DIC technique. Based on the measured kinematic fields, the proposed method enables to identify the slip activity on the crystal surface and to locate precisely the slip band interfaces at microscale which behave as kinematic discontinuities. The computed displacement data are projected on a well-defined physical basis containing slip details, then the strain fields can be derived directly from a set of analytical functions. The possible errors in displacement induced by this projection approach are evaluated. Finally, some results of the evaluated strain fields are presented. It demonstrates that the developed DIC methodology allows quantitative characterization of a heterogeneous deformation process and promotes further relationships to be established between slip activity and strain field evolution in single crystals. [ABSTRACT FROM AUTHOR]

[Display omitted] • MCG grain size plays a significant role in strain partitioning. • MCGs can be refined by limited cold deformation followed by rapidly heating process. • Distinct dislocation piles up in MCG GBs under small deformation and initiates SRX. • SRX occurs at both GBs and interior of the MCG when deformation is relatively large. The defect of mixed-grain structure with millimeter-grade coarse grains (MCGs) is frequently found in heavy forgings, such as nuclear main pipes manufactured from 316LN steel. Renovation of such kind forgings can only be conducted through recrystallization. However, the allowed deformation is limited because the geometric size of forgings should be maintained during the renovation. For this purpose, the deformation heterogeneity of 316LN steel with mixed-grain structures were studied experimentally, which demonstrated that the MCGs were more inclined to deform than fine grains even if MCGs are with hard orientations. Therefore, a two-stage method was studied to refine MCGs, which takes the advantage of this deformation heterogeneity. The first stage was to impose a limited deformation to the steel at room temperature, suppressing the dynamic recovery and leading the preferentially deformed MCGs to accumulate enough deformation energy storage. The second stage was rapidly heating the deformed materials to realize static recrystallization (SRX). The results showed that the MCGs were completely refined by SRX even if the reduction rate is only 8%. The SRX mechanisms and refining assessments of MCGs under different deformation conditions were discussed. This work, thus, provides an effective method to salvage the heavy structures in engineering. [ABSTRACT FROM AUTHOR]