International Journal of Plasticity最新文献

This study investigates the improvement of plasticity in body-centered cubic magnesium (Mg)-lithium (Li)-aluminum (Al) alloys, crucial for lightweight structural applications. The ternary Mg-Li-Al alloys exhibits high strength but low ductility. Precipitates at grain boundaries in these alloys, linked to reduced plasticity, are examined for their crystal structure and composition. Advanced microscopic techniques reveal the transformation of precipitates and the development of specific structures at grain boundaries. Thermodynamics of element diffusion at grain boundaries are explored through first-principles calculations, and a phase-field simulation models precipitate evolution. Molecular dynamics simulations elucidate nanoscale mechanisms governing the transition from brittle to ductile fracture modes during artificial aging. The D0₃₋Mg₃Al at grain boundaries is a brittle phase, and through a 170 °C aging treatment, it induces the precipitation of lamellar α-Mg phase with D0₃₋Mg₃Al as nucleation sites. The occupancy energy of Al atoms at Li sites in α-Mg is found to be lower than that in D0₃₋Mg₃Al, leading to the dissolution of D0₃₋Mg₃Al. The α-Mg, characterized by a stronger metallic nature, exhibits a better-matched modulus with the matrix and enhanced dislocation mobility. The precipitation of α-Mg plays a pivotal role in significantly improving the ductility of the alloy. This work contributes to the understanding of the complex interplay between alloy composition, grain boundary precipitates, and plasticity, as well as brings insights to guide interfacial control in the development of advanced Mg-Li-Al alloys for structural applications.

Fatigue short crack growth governed by the crack-tip plasticity dominates the fatigue life and strength of metallic materials or structural components. Here, for the first time, we discover a new mechanism of resisting fatigue short crack growth by grain refinement near the crack-tip driven by dynamic recrystallization in a Ni-based superalloy during high-cycle fatigue. The local cumulative plastic strain plays a determining role in the crack-tip grain refinement and concurrent dissolution of nanoprecipitation. Comprehensive microstructural analysis provides the evidence that the refined grains reduce the plastic micro-strain gradient in the vicinity of the crack-tip, which causes the crack blunting and deflection towards the interface of coarse-fine grains, hence decelerating the short crack growth. Although the grain refinement reduces the local stress threshold in the fine-grained areas (FGA), the dominant effects of FGA are identified to provide additional microstructural resistance to the propagation of short cracks.

Creep resistance, which is one of the most important deformation modes, is rarely reported for refractory high entropy alloys (RHEAs). The experiment investigated the high-temperature creep mechanism of Ti-Ta-Nb-Mo-Zr RHEA prepared by laser powder bed fusion (LPBF) technology. The high cooling rate of LPBF suppresses most of the elemental segregation, but there are still over-solidified precipitates and a few continuous precipitates (CP). In the range of 923–1023 K, the stress exponent and activation energy were determined to be 3.2–3.4 and 261.5 ± 19.5 kJ/mol, respectively. Compared with other conventional alloys and HEAs, a large reduction of the minimum creep rate is found in the LPBF-built Ti_1.5Ta_0.5NbZrMo_0.5 RHEA, indicating a significant improvement in high-temperature properties. The dislocation tangles at the interface is formed during the creep process and new Zr-rich CP phases are generated in the dislocation tangles region. The interfacial dislocation tangles is the result of the interaction between dislocations and two-phase mismatch stresses. The dislocation tangles prevents dislocations from further cutting the matrix phase, which is very favorable to the high-temperature creep performance. At the same time, the formation of this dislocation tangles greatly accelerates the nucleation process and growth rate of the new CP phase. The present work provides a pathway to design novel HEAs with improved high-temperature creep resistance.

In this study, an integrated crystal plasticity finite element–phase–field (CPFE–PF) model is developed to examine dynamic recrystallization (DRX) in a dual phase Ti alloy. The CP framework is coupled with PF by updating the free energy density with energy contributions due to plasticity. The evolution of grain boundaries through evolving non-conserved order parameters in the PF model is tracked using the Allen–Cahn equation. Nucleation is allowed to occur if the dislocation density exceeds a critical value. DRX is studied in various Ti morphologies such as an $\alpha -$Ti single crystal containing a stiff elastoplastic particle, $\alpha$-Ti bicrystals with low and high misorientation between grains, an $\alpha -\beta$ bicrystal and a globular $\alpha -\beta$ Ti structure with $\beta$ phase at $\alpha -\alpha$ interfaces. For an $\alpha -$Ti bicrystal, a high misorientation facilitates the onset of DRX at the $\alpha -\alpha$ interface at a significantly lower strain than the bicrystal with low misorientation. In an $\alpha -\beta$ bicrystal, DRX is only observed at the $\alpha -\beta$ interface. For the globular morphology, nucleation is observed at both $\alpha -\beta$ interfaces and inside $\alpha$ grains, which is consistent with previous experimental observations for a similar morphology. Nucleation inside $\alpha$ grains is explained by the correlation between SSD density and misorientation indicators such as KAM and GROD at the nucleus site. To correlate slip activity with nucleation propensity immediately prior to different nucleation events, the dislocation density, shear rate and Schmid factors on different slip systems are evaluated at nucleation sites.

The incremental elasto-visco-plastic self-consistent polycrystal model ($\Delta$EVPSC) was utilized to describe the constitutive behavior of dual-phase 980 (DP980) steel. A simple baseline modeling approach was chosen: the hardening behavior of each constituent phase in the DP980 steel was described by a simple Voce hardening law without explicitly considering the back stress; and it was assumed that using the same single crystal elastic modulus for ferrite and martensite is sufficiently representative. The adequacy of this baseline modeling approach was evaluated by comparing various mechanical experimental data with model predictions in terms of the stress vs. strain curves obtained from uniaxial tension, tension-compression, and loading-unloading-loading (LUL) tests. Additionally, the evolution of experimental lattice strain data reported in literature was used to validate the phase-specific Voce hardening parameters. Despite its minimalistic modeling description, the baseline $\Delta$EVPSC model successfully captured key features: 1) the Bauschinger effect, 2) the decrease in chord modulus, and 3) the non-linearity in the stress vs. strain curves resulting from the LUL test. All three mentioned characteristics are crucial for accurate prediction of springback in sheet metals. The $\Delta$EVPSC model, interfaced with a finite element solver (Abaqus/standard) as the user material subroutine, was employed to simulate the Numisheet93 benchmark problem. The strip of DP980 was first U-drawn followed by springback. The model-predicted springback profile aligned well with the experimental results only when stress relaxation was properly considered, resulting in improved predictive accuracy compared to predictions based on a distortional plasticity model.

Restrained by the strength-ductility tradeoff, it is still challenging to develop advanced high-strength low carbon low alloy (LCLA) steels with superior strength-ductility combinations and cost-effectiveness to satisfy industry demands. In this study, an innovative 2-cyclic quenching and partitioning (Q&P) heat treatment was developed to produce a novel LCLA steel with the optimized microstructure, in which a bimodal grain size distribution across various constituent phases was achieved. Tensile test results show that the 2-cyclic Q&P LCLA steel exhibits excellent mechanical properties with a uniform elongation, close to 18 %, nearly triple that of conventional Q&P LCLA steel while maintaining a tensile strength above 1 GPa. To reveal the underlying mechanisms of such exceptional strength-elongation synergy, the detailed deformation behaviors of the developed LCLA steel were characterized while the evolution of hetero-deformation-induced (HDI) stress and effective stress was investigated from the perspective of the dislocation model. It is indicated that, with increasing strain, the heterogeneous structures promote strong strain partitioning which leads to extensive geometrically necessary dislocations (GNDs) pile-ups at hetero-interface and persistently strong HDI strengthening effect, and produce the coordinated deformation among constituent phases to realize dislocation forest strengthening, collectively contributing to the enhanced work hardening capacity and hence overcoming the strength-ductility tradeoff. This study provides a new processing strategy for developing strong and ductile LCLA steels.

Prone H reduction is considered an important factor in the poor corrosion resistance of Mg and its alloys, while the reduced H simultaneously impacts their mechanical properties whose mechanism is still unclear. It can be experimentally found that the elongation of Mg charged with atomic H is 2.76 % greater than that in air. To reveal the underlying physics, multi-scale modeling combining first-principle calculation, molecular dynamic/static (MD/MS) simulation, and crystal plasticity finite element method (CPFEM) is first employed to elaborate the influence of H on Mg at different length scales. The first-principle results show that the Prism-I {10$\overline{1}$0} exhibits the most corrosive nature with an effective H adsorption density that reaches 18 nm⁻² and its diffusion barrier is only 0.156 eV H⁻¹. Conversely, the Basal {0001} has the best surficial H resistance. After H infiltration into the Mg matrix, the generalized stacking fault energies of most twining planes decrease by 2.26 % ∼18.49 %. Especially for the Basal {0001}, the H not only lowers its stacking fault energy to -7.13 J m⁻², but also impedes its cleavage cracking along [10$\overline{1}$0] according to the MD/MS simulation. The presence of H within the grains induces early initiation of stacking fault and elevates the critical stress at the crack tips. The CPFEM modeling reveals that the difference in twining growth is concentrated within 4 % strain. The H addition promotes the twining of Mg, however, following 4 % strain, the relative activity of planes in the Mg/Mg-H models is consistent.

Creep-fatigue interaction is identified as a primary failure mode for components operating under high temperatures. As operational durations extend, this interaction not only alters the material's microstructures but also initiates a gradual degradation in mechanical properties, significantly impacting its deformation and damage behaviors. In this work, the dynamic microstructural evolution of GH4169 superalloy during creep-fatigue was elucidated via qualitative characterization, and damage level evaluation method was subsequently developed by bridging microstructure degradation to mechanical property degradation. Creep-fatigue tests were performed at 650 °C with various tensile holding times and were interrupted at lifetime fractions of 10 %, 50 % and 80 % for further analysis and tensile evaluations. Results revealed that the prolonged exposure to holding times induced the coarsening of ${\gamma }^{″}$ precipitates alongside an increase in low-angle grain boundaries, culminating a reduction in creep-fatigue strength. The development of voids and cracks exacerbated the degradation of elongation, leading to a hybrid fracture mode encompassing both intergranular and transgranular cracking paths. Synthesizing microstructural evolutions to qualitatively categorize diverse degradation levels imparted a robust physical basis for damage evaluation. A mapping model was established to correlate the average kernel average misorientation (micro-degradation indicator) with the tensile plastic strain energy density (macro-degradation indicator). The damage level evaluation method was endowed with quantitative metrics utilizing this model, and its generality was additionally validated in P92 steel. This work offers an insight into the quantitative damage evaluations of creep-fatigue-induced degradations in materials, thereby providing a theoretical basis for the development of operation management and inspection plans of components.

Damage initiation in high-strength aluminum alloys with a precipitate-rich matrix is typically particle-driven. In AA7075-O temper, particle cracking and decohesion are the primary void nucleation mechanisms. However, the impact of particle-induced voiding on subsequent void growth and coalescence remains inadequately understood. Given that void growth and coalescence are inherently three-dimensional (3D) phenomena, conventional two-dimensional microstructure-based numerical models fail to accurately capture these damage evolution processes. The current work investigates void growth and coalescence phenomena in AA7075-O by developing 3D finite element (FE) real microstructure based models, created from plasma focused ion beam-scanning electron (PFIB-SEM) tomography and 3D electron back scattered diffraction (3D-EBSD). The models incorporate three key damage processes: particle cracking, particle decohesion, and matrix damage, to examine their effects on void growth and coalescence behavior in AA7075-O. Additionally, the influence of aluminum matrix grains on damage evolution in AA7075-O is explored. Complementary multi-scale modeling tools, along with in-situ scanning electron microscopy (SEM) and in-situ micro-X-ray computed tomography (μXCT), were employed for validation and supplementary insights. It is shown that 3D RVEs can capture the general 3D experimental trends in plastic heterogeneity and damage development at the microstructural length scale. Also, void growth and coalescence is influenced by the local stress fields, which in turn is dictated by particle morphology, particle cracking and decohesion. Particle cracking can accelerate the final specimen fracture, while particle decohesion promotes void growth but delays final coalescence. Void coalescence is shown to occur through void sheeting mechanism while the influence of grain characteristics on ductile void damage progression is found to be relatively limited.

Precipitate free zone (PFZ) consistently forms near the grain boundaries (GBs) in precipitate-strengthened alloys, significantly weakening the materials because of their intrinsic softness compared to the bulk. However, the influence of PFZ near GBs on deformation mechanism remains largely unrevealed. Here, we systematically investigate the effects of PFZ on the macroscopic mechanical behavior and the microstructure deformation mechanism of the modelled precipitate-strengthened Al-Zn-Mg-Cu alloy, using a combination approach of experiments, molecular dynamics (MD) simulations, and theoretical modeling. Four Al-Zn-Mg-Cu alloys with highly different PFZ widths are prepared by tailoring the quenching media and applying the new deformation heat- treatment process proposed by us. Experimental characterizations demonstrate that severe dislocation accumulation occurs at the interface between PFZ and bulk. Meanwhile, MD simulations further reveal that PFZ is prone to plastic deformation during tensile process, contributing to the softening of materials. The PFZ exhibits significant strain concentration, leading to the preferential formation of dislocations within PFZ rather than at GBs. It is found that the level of strain concentration and the degree of dislocation accumulation are not sensitive to the PFZ width. Based on these mechanisms, a PFZ-dependent strength model is developed to quantitatively evaluate the influence of PFZ on tensile strength by considering dynamic strengthening of PFZ. It is predicted that an increase in PFZ width greatly reduces the tensile strength, with a 21 % reduction observed when PFZ width reaches 268 nm, emphasizing the important impact of PFZ width on materials strength.