Deformation Mechanisms Of Materials Research Articles

Effect of helium bubbles on the mobility of edge dislocations in copper

Abstract Helium bubbles can form in materials upon exposure to irradiation. It is well known that the presence of helium bubbles can cause changes in the mechanical behavior of materials. To improve the lifetime of nuclear components, it is important to understand deformation mechanisms in helium-containing materials. In this work, we investigate the interactions between edge dislocations and helium bubbles in copper using molecular dynamics (MD) simulations. We focus on the effect of helium bubble pressure (equivalently, the helium-to-vacancy ratio) on the obstacle strength of helium bubbles and their interaction with dislocations. Our simulations predict significant differences in the interaction mechanisms as a function of helium bubble pressure. Specifically, bubbles with high internal pressure are found to exhibit weaker obstacle strength as compared to low-pressure bubbles of the same size due to the formation of super-jogs in the dislocation. Activation energies and rate constants extracted from the MD data confirm this transition in mechanism and enable upscaling of these phenomena to higher length-scale models.

Revealing the Effect of Dynamic Structural Evolution on Triple Junction Deformation Mechanism: Molecular Dynamics Simulations and an Energetic Model

Abstract Triple junction (TJ) migration is a plastic deformation mechanism in polycrystalline metals mediated by the interplay between neighboring grain boundaries (GBs). Compared with GB migration, the dynamic change of TJ structure during migration and its impact on subsequent TJ behavior is often overlooked. In this study, we explore the effect of dynamic TJ structure evolution on TJ deformation mechanism in a face-centered cubic metal Au model using molecular dynamics simulations. Our analysis reveals that the dislocation accumulation at TJ due to inconsistent disconnection nucleation from neighboring GBs can influence the stick-slip behavior of TJ migration. With the dynamic change of TJ structure, the velocity of TJ migration is gradually reduced, leading to a transition from TJ migration to dislocation emission from TJ. We have developed an energetic model to evaluate the critical stresses required for TJ migration and dislocation emission, providing an explanation of the competition between TJ migration and dislocation emission. Our findings deepen the understanding of TJ-governed plastic deformation mechanisms in polycrystalline materials.

Three-dimensional phase field modeling, orientation prediction and stress field analyses of twin-twin, twin-grain boundary reactions mediated by disclinations in hexagonal close-packed metals

Crystalline defects, such as dislocations, disclinations, twins and grain boundaries, play critical roles in determining the mechanical properties of metals and alloys. In particular, with multiple competitive deformation modes activated, the mechanical behaviors of hexagonal close-packed metals are strongly influenced by the interactions and reactions of various types of defects. Despite extensive studies on the elastic interactions of defects, a theoretical framework capturing crystallographic reactions, especially reaction products and associated local stress concentration, is still unavailable. Here we suggest a disclination-based method to quantify defect reactions. By using a combination of crystallographic calculations and phase field modeling/simulations, twin-twin and twin-grain boundary reactions in hexagonal close-packed metals have been quantitatively analyzed. It has been found that partial disclinations, accompanied with other defects (e.g., {112¯6} and {112¯2} high-index twins), can be generated by defect reactions as typical byproducts. The orientation change and stress fields caused by disclination formation have been systematically calculated, which offers a rigorous mathematical foundation to explore twin-twin, twin-grain boundary reactions. By quantitatively determining defect reactions and local stress fields, our work provides new insights into the deformation mechanism and microstructure-property relationship in metallic materials.

Nanoindentation behaviors of the AlCoCrFeNi2.1 eutectic high-entropy alloy: The effects of crystal structures

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA), as a kind of in-situ composite materials, have achieved good match of strength and plasticity due to its typical eutectic lamella structures. Although many previous studies have investigated the indentation behaviors of the AlCoCrFeNi2.1 EHEA, there are still some controversies about the hardness characteristic of different crystal structures, requiring further clarification. Accordingly, in this study, nanoindentation tests were conducted to investigate the indentation behaviors of different crystal structures (primary L12 phase, primary B2 phase, and the eutectic phase region). The residual indentation and the crystallographic orientations were characterized by the laser scanning confocal microscope, scanning electron microscope, and electron backscatter diffraction. The hardness in different crystal structures were comparatively analyzed and the material deformation mechanisms were discussed correspondingly. The statistic results showed that the eutectic phase region had the lowest hardness, and primary B2 phase had the highest hardness. Furthermore, the slip lines in the primary L12 phase were evenly distributed near the residual indentation, but the slip lines in the eutectic phase region were concentrated near the phase boundaries. These findings are expected to deepen the understanding of the microscale deformation behaviors of EHEAs.

Study on the atomic scale deformation mechanism of the h-BN coating nickel matrix composites

In this paper, molecular dynamics (MD) simulations are applied to study the contact mechanical behavior of hexagonal boron nitride (h-BN) coating/Ni matrix composites during nanoindentation, revealing the material deformation mechanism. A systematic study is carried out on the load-depth curve, temperature response curve, defect morphology, and dislocation density in indentation. The results show that the presence of h-BN coating increases the actual loading area of the matrix, effectively reduces the contact stress between the tip of the indenter and the specimen, and also induces the generation of high-density Shockley dislocations, which increases the dislocation entanglement and leads to the localized hardening of the composites, thus reducing the matrix damage. During the loading process, the temperature of the specimen before the destruction of the h-BN coating is significantly lower than that after the destruction, indicating that the coating tends to block the transfer of temperature to the interior of the specimen, preventing the occurrence of temperature-induced tissue phase changes inside the specimen, and providing good protection for the interior of the specimen.

Dynamic impact mechanical properties of red sandstone based on digital image correlation method

ABSTRACT In the process of underground blasting excavation, the safety and stability of rock mass are significantly influenced by dynamic load. To study the full-field strain and displacement behaviour of rock materials subjected to dynamic compressive force, the impact tests were done on red sandstone specimens using an electromagnetically driven Hopkinson pressure bar. At the same time, the data were collected by a high-speed camera, and non-contact measurements of the red sandstone were carried out with the digital image correlation method. The resulting images were screened and analysed, based on which the stress-strain curve, full-field strain cloud diagram and displacement cloud diagram were obtained. Results show that the dynamic peak stress increases with the increase of impact velocity. During the impact process, substantial internal uneven deformation occurs in the red sandstone close to the transmission bar’s end face. Under the impact load, the cracks initiate from the upper and lower sides near the transmission bar’s end face, followed by formation of cracks in the middle part. These cracks expand throughout the rock to form through-cracks, eventually resulting in complete failure. For rock materials, the evolution of their strain field during the failure process can reflect the initiation and propagation of their internal cracks. Therefore, the use of digital image correlation method proves effective for studying the deformation and failure mechanisms of rock. The conclusions obtained in this study provide a valuable reference for the macro-meso deformation and failure mechanisms of geotechnical materials.

Emergence of chaos in an electroactive artificial muscle PVC gel under state-varying electromechanical parameters

The deformation mechanism of artificial muscle materials based on the natural method has always been one of the frontier topics in the field of biophysics. Electroactive PVC gel is a composite material that mimics the response of natural muscles, making it highly advantageous for various bionic mechanical applications. The existing literature primarily focuses on enhancing the performance and exploring applications of PVC gel, with particular emphasis on optimizing its ability to simulate muscle deformation. However, the existing literature lacks research on the theoretical mechanism and modeling of large deformation, particularly regarding the investigation of nonlinear dynamic behavior under state-varying electromechanical parameters. The present research establishes a novel theoretical model for PVC gel and investigates its dynamic response to voltage, mechanical force, and inertia force. Firstly, the electromechanical constitutive model of PVC gel actuator was elucidated, and the nonlinear dynamics control equation of PVC gel actuator was established based on the Gent model. Secondly, we investigate the vibration characteristics of PVC gel material by analyzing four key parameters: size, tensile strength, applied voltage amplitude, and excitation frequency. Through this analysis, we determine the relevant parameters and operational range of the PVC gel actuator when subjected to voltage excitation. Meanwhile, the phase trajectories and Poincaré maps were generated based on the control equation calculation results to investigate the vibration stability and chaotic characteristics of the PVC gel actuator under electromechanical coupling conditions. The deformation of PVC gel is influenced by various nonlinear factors, wherein the alteration in frequency induces harmonic, subharmonic, and super-harmonic resonance phenomena within the frequency response. By revising the size, voltage, and frequency of these three factors, with particular emphasis on regulating the frequency, chaotic behavior in the PVC gel will be facilitated. Finally, the bifurcation analysis of the electromechanical vibration behavior of the PVC gel actuator was carried out, and the stability of the PVC gel vibration was further quantitatively determined by the Lyapunov exponent.

A strain components-based Mohr–Coulomb fracture criterion for proportional loading

Ductile fracture criteria are crucial in the application of elastoplastic materials like advanced high-strength steels, particularly due to their tendency to fail without significant localized necking. It is aimed to figure out the unified mechanism for ductile fracture and accurately describe it with phenomenological criteria. In this study, the effect of the stress triaxiality and the Lode parameter is analyzed mathematically. The maximum shear strain and the normal strain to fracture are emphasized in the proposed strain components-based Mohr–Coulomb fracture criterion particularly designed for plane stress states, and it is assumed that fracture occurs when the combination of the maximum shear strain and the normal strain reaches the critical value. It is then extended to describe fracture with the equivalent plastic strain and the normal strain to fracture. The experimental fracture data of AA 2024-T351 and the additively manufactured Ti-6Al-4V titanium alloy are used to verify the proposed models. A fracture forming limit diagram comprising forming limit curves calibrated from stress states under different stress triaxialities is proposed based on the proposed fracture criteria for plane stress states. This study shows that a strong correlation between the maximum shear strain and the normal strain to fracture has been observed, and it can be described piecewise-linearly. Besides, the orientation of the fracture surface under both tension (including plane strain tension) and compression (including plane strain compression) can be qualitatively explained by the Mohr’s strain circles. The results provide a new insight into the intrinsic deformation and fracture mechanism of materials that fail without severe localized necking.

Effects of highly-textured and untextured polycrystalline layers on deformation mechanisms in amorphous submicron-layered materials

Amorphous alloys have high strength, but softening can lead to room temperature brittleness, limiting their applications. Typically, soft means low hardness and yield strength, and the lower the yield stress, the higher the fracture toughness. Two amorphous/crystalline multilayer samples with submicron single layer thickness were prepared. The crystalline layer of one sample is composed of highly-textured (111) nanocrystalline grains, and the other sample is untextured. Indentation experiments proved that the samples were plastically deformed, while the amorphous layer did not form shear bands with decreased layer thickness. The thickness reduction of the amorphous layer in the sample with highly-textured crystalline layers is greater than in the sample with untextured crystalline layers. The strain-hardening ability of the textured crystalline layer is higher, while the corresponding amorphous layer is deformed more severely with free volume annihilation and greater flow with more compact atoms. The amorphous layer corresponding to the untextured crystalline layer absorbs dislocations, and activates shear transformation zones (STZs), increasing the free volume near the interface. The annihilation of the free volume is less than the sample with highly-textured crystalline layers.

Defect evolution induced by low-dose neutron irradiation and elastoplastic deformation mechanism of indium-doped GaN materials

The low-dose neutron irradiation defect evolution and nanoindentation response of indium (In)-doped gallium nitride (GaN) materials are investigated in this work, which is of great significance for improving the reliability and safety of nitride semiconductors in neutron irradiation environments, as well as for clarifying their mechanical properties. A numerical simulation approach based on molecular dynamics (MD) is utilized to construct wurtzite GaN models comprising Indium doping concentrations up to 2 %. Nanoindentation simulations are performed to analyze the elastoplastic deformation mechanisms of the c-plane and m-plane In-doped GaN materials. The low-dose neutron irradiation process of In-doped GaN is further simulated, and its structural changes and defects evolution are analyzed. The indentation hardness of the c-plane GaN decreases and its Young's modulus increases as the doping concentration increases. Increases in both the indentation hardness and Young's modulus are seen for the m-plane GaN with the increasing doping concentration. Moreover, the doping of indium suppresses the plastic deformation both in the c-plane and m-plane GaNs. Two types of prismatic loops formation mechanisms are unraveled, i.e. the “nested shear loops pinch-off” mechanism in the m-plane perfect GaN, and the “lasso-shape shear loop pinch-off” mechanism in the m-plane In-doped GaN. When subject to low-dose neutron irradiation, In-doped GaN materials exhibit higher resistance to the generation of irradiation defects. As the doping concentration increases, the amounts of phase transformation, Frenkel pairs, and amorphization decrease.

Review of indentation size effect in crystalline materials: Progress, challenges and opportunities

Nanoindentation has been widely utilized for measuring mechanical properties at small scales over the past three decades. Indentation size effect (ISE) is a crucial phenomenon in nanoindentation testing of crystalline materials, which is usually manifested as an increase of hardness with a decrease of indent size. Tremendous efforts have been made to understand ISE with the aim of eliminating its influence on determining the intrinsic mechanical properties. More importantly, ISE is a key phenomenon for exploring the unique deformation mechanisms of crystalline materials at the nanoscale. We critically review the experimental observations of ISE in crystalline materials, focusing on the influences of the indenter geometry and crystal structure. In addition, the mechanistic models proposed for ISE as well as recent findings by numerical simulations are also examined in detail to explore the origins of ISE and identify the unsolved challenges.

Effect of Grain Size on Nanometric Cutting of Polycrystalline Silicon via Molecular Dynamics Simulation.

The grain size effect is an important factor in determining the material removal behavior of polycrystalline silicon (p-Si). In the present study, to improve the understanding of nanoscale machining of p-Si, we performed molecular dynamics simulation of nanometric cutting on a p-Si workpiece and discussed the grain size effect on material removal behavior and subsurface damage formation. The simulation results indicate that when cutting on the polycrystal workpiece, the material removal process becomes unstable compared with single crystals. Higher removal efficiency, less elastic recovery and higher frictional coefficient are observed as the average grain size decreases. In the subsurface workpiece, when the grain size decreases, slip along grain boundaries merges as a nonnegligible process of the plastic deformation and suppresses the elastic deformation ahead of the cutting tool. It is also revealed that when cutting on a polycrystal workpiece with smaller grains, the average stress decreases while the workpiece temperature increases due to the impediment of heat transfer by grain boundaries. These results could provide a fundamental understanding in the material deformation mechanism of p-Si during nanoscale machining.

Study on hot deformation behavior and recrystallization mechanism of an Al-6.3Zn-2.5Mg-2.6Cu-0.11Zr alloy based on machine learning

The data from thermal simulation plays a significant role in understanding the deformation behavior and deformation mechanisms of materials, and provides guidance for the design of hot deformation processes. In this paper, thermal simulation experiments were performed on an Al-6.3Zn-2.5Mg-2.6Cu-0.11Zr alloy, and the microstructure after hot deformation were characterized by Electron Backscatter Diffraction (EBSD) technique. The thermal simulation data and microstructural characteristics were analyzed and studied using machine learning technique: the Backpropagation Artificial Neural Network (BP-ANN) method, correlation analysis and K-means clustering analysis. A model for predicting not only flow stress but also dislocation density was established using the BP-ANN method, which exhibits good prediction accuracy. The R² and AARE values are 90.19 % and 6.87 % for the prediction of dislocation density, and are 98.67 % and 6.96 % for the prediction of flow stress. Subsequently, correlation analysis was performed to investigate the relationship between energy dissipation efficiency η and deformation parameters as well as microstructural characteristics, revealing that energy dissipation rate is correlated positively with the content of dynamical recrystallization (DRX) and high angle grain boundary (coefficients of 0.90 and 0.82) and negatively with the dislocation density and the Zener-Hollomon parameter (coefficients of −0.98 and −0.97). Additionally, K-means clustering analysis was applied to study the relationship between the energy dissipation efficiency and softening mechanisms, it was found that the softening mechanisms changed with the variation of η value, dynamic recovery (DV), geometric DRX (GDRX) and continuous DRX (CDRX) dominated when η>0.38; DV, CDRX and discontinuous DRX(DDRX) dominated when 0.28<η<0.38; DV dominated when η<0.28. The results show that machine learning techniques are useful tools in mining thermal simulation data, and more information can be obtained than traditional data mining methods.

Prediction and Rational Design of Stacking Fault Energy of Austenitic Alloys Based on Interpretable Machine Learning and Chemical Composition

Accurately predicting the stacking fault energy (SFE), as one of the crucial factors influencing the material deformation mechanism, is a focal point in research. This study utilizes measured SFE values from the literature on austenitic alloys to establish a predictive model for the relationship between chemical composition and SFE using machine learning techniques. Among five compared machine learning algorithms, the extremely randomized trees algorithm demonstrates the highest prediction accuracy. Incorporating atomic features further enhances the model's performance. Subsequently, feature selection is conducted using correlation analysis, recursive elimination, and exhaustive search to obtain the optimal subset of features. The interpretability of the model is analyzed using Shapley additive explanations values, providing rankings of feature importance and critical thresholds for feature influence. Finally, rational design of SFE is demonstrated by adjusting alloy composition, exemplified by Fe–17.5Cr–15.6Ni–2.5Mo–0.02Si–0Co steel.

Tuning generalized planar fault energies to enable deformation twinning in nanocrystalline aluminum alloys

As deformation twins have a profound impact on the plastic flow and mechanical properties of metallic materials, enhancing deformation twinning in face-centered cubic (FCC) metallic materials has long served as a unique microstructure design strategy to attain an extraordinary strength-ductility synergy. Deformation twinning, however, rarely occurs in pure FCC Al and its alloys since its generalized planar fault energies (GPFEs) are almost unaffected by most soluble alloying elements such as Mg, Zn and Cu. Here we successfully tune the GPFEs of a nanocrystalline Al-Mg alloy by alloying with Zr, Fe or Y element, and enable deformation twinning in the Zr-, Fe- and Y-containing alloys. Based on a combined analysis of microscopic observations, modeling and ab initio calculations, we find a strong grain-size-dependent twinning (i.e., twinning occurs in preferable grains having sizes in the range ∼20–40 nm), as well as only one single twinning plane (i.e., twinning occurs in single, parallel atomic planes) for twin formation rather than intersecting twinning planes (i.e., twinning occurs in multiple, unparallel atomic planes) usually observed in coarse-grained FCC materials. This interesting twinning behavior is further observed to be accompanied by grain rotations, producing defective twin boundaries. Our experimental results extend the current understanding of the plastic deformation mechanisms in nanograined metallic materials, and will guide microstructure design of twinnable nanograined Al alloys with an improved strength-ductility synergy.

Study on the tensile properties and deformation mechanism of high-temperature resistant nitrogen-containing nickel-based welding material deposited metal

This article uses a new type of nitrogen-containing nickel-based flux cored welding wire as the experimental material to study the tensile properties and deformation mechanism of the deposited metal from room temperature to 900 °C. The results show that the tensile strength gradually decreases with the temperature. The plasticity gradually increases, but the lowest value appears at 700 °C, which is the phenomenon of intermediate temperature brittleness. M23C6 carbides begin to decompose at 700 °C. More carbonitrides are precipitated within the crystal with temperature. In the low-temperature zone (T ≤ 600 °C), the main deformation mechanism is the unit dislocation a/2<110> cutting precipitation phase. In the medium-temperature zone (600 °C < T ≤ 750 °C), initiation of Orowan bypass mechanism for dislocations. In the high-temperature zone (800 °C ≤ T ≤ 900 °C), the slip and climbing mechanism of dislocation is the main deformation mechanism.

A theoretical and experimental study of deformation mechanism dictated by disclination-dislocation coupling in Mg alloys at different temperatures

Dislocations and disclinations are fundamental topological defects within crystals, which determine the mechanical properties of metals and alloys. Despite their important roles in multiple physical mechanisms, e.g., dynamic recovery and grain boundary mediated plasticity, the intrinsic coupling and correlation between disclinations and dislocations, and their impacts on the deformation behavior of metallic materials still remain obscure, partially due to the lack of a theoretical tool to capture the rotational nature of disclinations. By using a Lie-algebra-based theoretical framework, we obtain a general equation to quantify the intrinsic coupling of disclinations and dislocations. Through quasi in-situ electron backscatter diffraction characterizations and disclination/dislocation density analyses in Mg alloys, the generation, coevolution and reactions of disclinations and dislocations during dynamic recovery and superplastic deformation have been quantitatively analyzed. It has been demonstrated that the obtained governing equation can capture multiple physical processes associated with mechanical deformation of metals, e.g., grain rotation and grain boundary migration, at both room temperature and high temperature. By establishing the disclination-dislocation coupling equation within a Lie algebra description, our work provides new insights for exploring the coevolution and reaction of disclinations/dislocations, with profound implications for elucidating the microstructure-property relationship and underlying deformation mechanisms in metallic materials.

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.

Mechanical Behaviour of Photopolymer Cell-Size Graded Triply Periodic Minimal Surface Structures at Different Deformation Rates.

This study investigates how varying cell size affects the mechanical behaviour of photopolymer Triply Periodic Minimal Surfaces (TPMS) under different deformation rates. Diamond, Gyroid, and Primitive TPMS structures with spatially graded cell sizes were tested. Quasi-static experiments measured boundary forces, representing material behaviour, inertia, and deformation mechanisms. Separate studies explored the base material's behaviour and its response to strain rate, revealing a strength increase with rising strain rate. Ten compression tests identified a critical strain rate of 0.7 s-1 for "Grey Pro" material, indicating a shift in failure susceptibility. X-ray tomography, camera recording, and image correlation techniques observed cell connectivity and non-uniform deformation in TPMS structures. Regions exceeding the critical rate fractured earlier. In Primitive structures, stiffness differences caused collapse after densification of smaller cells at lower rates. The study found increasing collapse initiation stress, plateau stress, densification strain, and specific energy absorption with higher deformation rates below the critical rate for all TPMS structures. However, cell-size graded Primitive structures showed a significant reduction in plateau and specific energy absorption at a 500 mm/min rate.

Uniaxial deformation behaviors and mechanism of Ti–6Al–4V alloy with bi-oriented grains

It is of great importance to investigate the deformation and damage mechanisms of polycrystalline materials with soft crystal orientations, considering the soft grains are usually the fatigue crack initiation sites. In this study, a Ti–6Al–4V alloy fully consisted of <01 1‾ 0> and <1‾ 21‾ 0> α (soft) grains was prepared and its uniaxial deformation behavior was investigated. Interestingly, no significant asymmetry on the tension-compression yield strength was found in this alloy with bi-grain orientations, though mechanical asymmetry was generally found in the traditional Ti alloys with strong texture. After high temperature solution treatment, the preferred bi-orientations were retained and the asymmetry degree of yield strength did not change, showing an excellent stability in texture and mechanical performance. Deformation mechanisms during tension and compression were studied and discussed. This work provides a new understanding on the deformation and damage mechanisms in the grains with soft crystal orientations, and may guide the microstructural design of Ti alloy for pursuing high mechanical and fatigue performances in the future.