International Journal of Plasticity最新文献

Although blades with a deviation angle of less than 15° between the blade stacking axis and the [001] orientation are qualified in the industry, the creep life of samples near [001] orientation exhibits significant anisotropy at intermediate temperatures. Those crystals having orientations within 15° from precise [001] exhibited significantly longer lives when their orientations were closer to the [001]-[101] boundary of the stereographic triangle than to the [001]-[111] boundary. Here, we first investigated the orientation rotation path of specimens near [001] orientation during creep at 750 °C/750 MPa, then revealed the dominant slip systems at different creep stages. Subsequently, we evaluated the effect of orientation deviation from precise [001] on creep properties. Finally, our research revealed the orientation sensitive mechanism of creep life in near [001] oriented Ni-based single crystal superalloys at intermediates.

Wire and arc additive manufacturing (WAAM) enables rapid near-net-shape fabrications of large-size parts and in-situ remanufacturing in many industry sectors. A comprehensive understanding of the fatigue failure mechanism of WAAM titanium alloys is a prerequisite for their widespread use in critical structural components subject to fatigue load. Here, the fatigue crack growth behavior of WAAM TA15 material is investigated. Fatigue crack growth tests are performed using compact tension specimens sampled from different locations and with different crack orientations of the WAAM TA15 block. The fatigue crack growth rate (FCGR) data exhibit two governing rates separated by a transition stress intensity factor value, $\Delta K_n$, and the degrees of fluctuation of the FCGR data in the two regimes are notably different. A piecewise log-linear model is first proposed by incorporating the Heaviside step function and $\Delta K_n$ into the classical Paris’ model, allowing for the transition $\Delta K_n$ to be determined by the data. The potential causes of the transition $\Delta K_n$ are phenomenologically inferred via fractography and surface roughness profiling results. The critical microstructure affecting the value of $\Delta K_n$ is identified by relating the crack tip cyclic plastic zone size at $\Delta K_n$ to the sizes of main microstructures. The cause of different degrees of fluctuations in the two regimes separated by $\Delta K_n$ is inferred by examining the microstructures within the plastic zone. The microstructural mechanisms of the local FCGR reduction and fluctuation are further identified and explained.

Ceramic-metal composites, or cermets, exhibit beneficial properties resulting in their use in many industrial applications. One challenge with cermets is mismatches in the coefficient of thermal expansion (CTE) values between the ceramic and metal phases that lead to residual stresses after processing, plasticity in the metal phase, internal stresses, and instability after thermal cycling. In order to make predictions of these properties to inform the design of cermets, we employ an incremental elasto-viscoplastic, self-consistent formulation to calculate the thermal, elastic, and plastic strains in two-phase polycrystalline cermet materials. This framework is extended to include temperature dependent properties, which are called implicitly within the temperature-dependent, incremental elasto-viscoplastic, self-consistent (TE-VPSC) model. Temperature-induced cooling and thermal cycling simulations are conducted using the TE-VPSC framework to study the residual stresses and plastic strains in the metal phases. Two materials are discussed in detail exhibiting stark differences based on the CTE between their ceramic and metal phases, WC/57-vol% Cu (exhibiting a pronounced CTE mismatch) and Y₂O₃/27-vol% Nb (exhibiting a negligible CTE mismatch). The model demonstrates high residual stresses in the Cu phase during processing and reverse plasticity leading to recovery of plastic strain during thermal cycling of the WC/Cu cermet. Moreover, the model demonstrates relatively low residual stresses and plasticity in Y₂O₃/Nb and a thermal stability point of 1251 °C, below which no plasticity develops in the cermet. We employ the TE-VPSC model as a design tool for cermets to systematically investigate the effects of process-induced microstructure variations (volume fraction, grain aspect ratio, and crystallographic texture are investigated) and compositional differences (19 compositions are explored) on the residual stress, degree of plasticity in the metal phase, and thermal stability point. The computational efficiency of the TE-VPSC framework makes it a desktop design tool that can be used to quantify the impact of changing composition, processing, and thermo-mechanical loading on the performance of the cermet, which can help reduce the number of time intensive and costly high temperature experiments.

Thermomechanical processing of titanium alloys often requires complex routes to achieve the desired final microstructure. Recent advances in modeling and simulation tools have facilitated the optimization of these processing routes. However, existing models often fail to accurately predict microstructural changes at large deformations. In this study, we refine the physical principles of an existing mean-field model and propose a calibration method that uses experimental results under isothermal conditions, accounting for the actual local deformation within the workpiece. This new approach improves the predictability of microstructural changes due to continuous dynamic recrystallization during torsion and compression experiments. Additionally, we integrate the model into the commercial FEM-based DEFORM™ 2D software to predict the local microstructure evolution within hot torsion specimens thermomechanically treated by resistive heating. Validation using non-isothermal deformation tests demonstrates that the model provides realistic simulations at high strain rates, where adiabatic heat modifies temperature, flow stress and microstructure. This study demonstrates the intrinsic correlation between microstructure, flow behavior, and workpiece geometry, considering the impact of deformation history in thermomechanical processes.

Monotonic tensile and cyclic deformation behaviours are investigated under different microstructural rafting states of a SC Ni-based superalloy, with emphasis on the influences of the rafting extent, type and loading orientation. The deformed microstructures and the dislocation configurations are characterized to give a micro-based understanding on the varying of deformation behaviours due to rafting. It is found that the decreases in the initial yield point and cyclic stress amplitude are only related to the rafting extent. Nevertheless, the rafting type (namely, the plate-like and needle-like morphology) has an undeniable contribution to the shape of hysteresis loops, where the plate-like rafting morphology results in more significant Bauschinger effect than needle-like rafting morphology. The variation of monotonic and cyclic deformation induced by rafting shares affinity with the alteration of internal stress and the movement of dislocations. Afterwards, a microstructure-sensitive constitutive model with two-phase flow rules has been developed. The effect of rafting on the monotonic and cyclic stress-strain responses is captured by introduce a series of microscopic mechanisms and a micromechanics-based back stress model that considers the morphology and size of the γ'/γ two-phase structures. The developed model is used to simulate the macroscopic stress-strain responses of the SC Ni-based superalloy under different rafting states. Model predictions are in good agreement with tests, capturing the reduction of cyclic stress amplitudes and the change in hysteresis loops. Finally, the impacts of the two-phase flow rules and the micromechanics-based back stress on the simulation capability have been discussed.

Machine learning (ML) based methods have achieved preliminary success in the constitutive modeling for single crystals or homogenized polycrystals with remarkable computational efficiency. However, existing ML-based constitutive models neglect grain-level anisotropy, which limits the accurate analysis of local effects. In the current work, a temporal graph neural network (TGNN) model is proposed to simulate cross-scale deformation behaviors of polycrystals under complex loading conditions, with straightforward consideration of microstructure variation and local interaction. The TGNN-based model, a variant of Linearized Minimal State Cells (LMSCs), extends its scope from macroscopic stress response to the mechanical response and orientation evolution of all grains within the aggregate. Specifically, the polycrystalline microstructure is represented with a graph to incorporate essential features of grains, including the spatial connectivity, crystallographic orientation and deformation state. Graph neural network (GNN) is used to capture the spatial correlation of grains, and the features extracted by the GNN are further processed with LMSCs to account for the history-dependent deformation and microstructure evolution. Moreover, the representative volume element (RVE) simulation with crystal plasticity is performed to provide reliable datasets for model establishment. The proposed model demonstrates high efficiency, accuracy and self-consistency in predicting the strain-stress response and orientation evolution at the scale of both individual grain and the overall aggregate under complex loading cases, such as cyclic loading and arbitrary loading.

This paper investigates void growth and coalescence in porous ductile solids under dynamic loading condition. A physical definition for the onset of void coalescence in porous ductile solids under dynamic loading is proposed. The onset is deemed to occur when the third invariant of the tensorial form of the Hill–Mandel condition attains a zero value. The definition allows for systematic investigations on the effects of dimensionless stress rate $\kappa$ and stress state, defined by the stress triaxiality $T$ and Lode parameter $L$, on the onset of void coalescence via micromechanical analyses. The analyses reveal that the critical macroscopic effective strain for the onset of void coalescence displays an increasing–decreasing transition trend as the dimensionless stress rate increases, for all levels of $T$ and $L$ considered. The macroscopic effective strain at the transition is identified as the “ductile–brittle” transition strain. The dimensionless stress rate at which the transition strain occurs is found to be relatively constant. A mapping in the $\kappa -T$ space for $L=-1$, representative of a generalized uniaxial tension typical in spall fracture experiments, is established which depicts regions where coalescence and non-coalescence can take place, as well as the ductile–brittle regions demarcated by a ductile–brittle transition curve. The results also show that the critical void volume fraction and macroscopic effective strain at the onset of void coalescence are insensitive to inertia at high stress triaxialities at $L=-1$.

By impeding dislocation motion, the irradiation-induced dislocation loops cause irradiation hardening and further embrittlement of plasma-facing tungsten in fusion reactors, leading to its performance degradation. But fundamental questions regarding the mechanisms remain to be clarified and predictive model for loop hardening remains to be built. In this paper, interactions between gliding edge dislocations and interstitial dislocation loops (with Burger vector b_L = ½<111>) are studied using atomistic simulations. The influences of b_L orientations, dislocation-loop intersection positions, loop sizes, and loading conditions (temperature and strain rate) on the interactions are systematically calculated and analyzed. Results show a large variety of interaction mechanisms, depending mainly on the relative orientations of b_L to dislocation slip plane, while slightly affected by loading conditions. Although loops with b_L parallel to the plane can be easily swept away by gliding dislocations, loops with b_L inclined to dislocation slip plane can strongly pin the gliding dislocation by forming a sessile 〈100〉 segment, which would bend the dislocation line into a screw dipole. Thus, high stress is required for the dislocation line to break away from the inclined loops by cross-slip of each individual arm of the screw dipole coupled with glide of the 〈100〉 segment. On the other hand, increasing temperature and/or decreasing strain rate hardly change the above mechanisms, but monotonically reduce the obstruction by these loops. Simplifying the complex motion of the edge dislocation pinned by the inclined loops as a thermally-activated process of a 1/2[111] edge dislocation overcoming barriers, a hardening model for the inclined loops is proposed. This model well describes the dependence of loop strength on loop sizes, temperatures and strain rates. The model is then applied to predict irradiation hardening at experimental strain rates, and it shows reasonable agreement with experimental results.

Polysynthetically twinned (PST) TiAl single crystal with lamellar structures exhibits great mechanical properties at room temperature. Therein twin boundaries (TBs) are important for achieving optimized ductile and fatigue performance of PST TiAl, but their role and the associated mechanism are elusive. Herein, we decipher the role of true TB (TTB) and pseudo TB (PTB) by a combined atomistic simulation and mesoscopic modeling, and find that PTB could remarkably improve room-temperature flow stress and cyclic stability of TiAl single crystal. It is revealed that dislocations pile up at PTB while unobstructedly traverse TTB. The emergency of back stress and the movement of dislocations along PTB contribute to the strengthening mechanism. The flow stress of TiAl single crystal with PTB is 34% higher than that with TTB. It is further found that as the twin thickness decreases, the flow stress of TiAl single crystal with TTB initially increases and then decreases (i.e., inverse Hall–Petch like behavior), whereas that with PTB always increases owing to the extra back stress and interfacial stress (i.e., Hall–Petch like behavior). Atomistic-informed mesoscopic theoretical models are then proposed to describe the flow stress as a function of twin thickness. Under cyclic loading, PTB is found to facilitate strain delocalization of TiAl single crystal during plastic deformation and thus noticeably improve the cyclic stability. These findings should shed light on achieving strong TiAl alloys with enhanced fatigue performance by the introduction and design of PTB.

Multi-principal-element alloys (MPEAs) based on 3d-transition metals show remarkable mechanical properties. The stacking fault energy (SFE) in face-centered cubic (fcc) alloys is a critical property that controls underlying deformation mechanisms and mechanical response. Here, we present an exhaustive density-functional theory study on refractory- and copper-reinforced Cantor-based systems to ascertain the effects of refractory metal chemistry on SFE. We find that even a small percent change in refractory metal composition significantly changes SFEs, which correlates favorably with features like electronegativity variance, size effect, and heat of fusion. For fcc MPEAs, we also detail the changes in mechanical properties, such as bulk, Young's, and shear moduli, as well as yield strength. A Labusch-type solute-solution-strengthening model was used to evaluate the temperature-dependent yield strength, which, combined with SFE, provides a design guide for high-performance alloys. We also analyzed the electronic structures of two down-selected alloys to reveal the underlying origin of optimal SFE and strength range in refractory-reinforced fcc MPEAs. These new insights on tuning SFEs and modifying composition-structure-property correlation in refractory- and copper-reinforced MPEAs by chemical disorder, provide a chemical route to tune twinning- and transformation-induced plasticity behavior.