Microscopic Deformation Mechanisms Research Articles

Anisotropy and symmetry in the elastoplastic deformation of single crystals under scratching: Unravelling the microscopic deformation mechanisms

The nanoscratch test, as an established technique for assessing material tribological properties has received significant attention. However, the symmetry and anisotropy in scratching performances as well as the quantitative correlation between the orientation-dependent deformation and inherent microscopic deformation mechanism remain unexplored. Herein, crystal plasticity simulations can quantitatively capture scratching forces, elastic recovery, and surface pile-ups, as well as accurately describe inner deformation fields and lattice rotation patterns, as confirmed by experimental results. The simulation results reveal that surface pile-up and elastic recovery mappings on (001)-, (011)-, and (111)-oriented samples exhibit eight-fold, four-fold, and six-fold symmetries, respectively. The orientation-dependent location and intension of both slip activities and lattice rotation, determine the features of macroscopic elastoplastic deformation under scratching.

Mechanisms of deformation, damage and life behavior of inconel 617 alloy during creep-fatigue interaction at 700 °C

The continuous fatigue tests and creep-fatigue tests with the imposition of strain hold at the peak strain were conducted at 700 °C on Inconel 617 alloy. The strain amplitude of 0.25 %, 0.30 %, 0.35 %, 0.40 %, and hold time of 60 s, 600 s and 1800 s were used. The cyclic deformation behavior and dynamic strain aging (DSA) were discussed. The strain localization, dislocation substructure and precipitation behavior were carefully characterized, which provided physical information to understand the cyclic deformation behavior. The dominant damage mechanism and damage interaction, responsible for the cracking behavior were identified based on the fracture surface observation and secondary cracks morphology. The cyclic life saturation effect was comprehensively elucidated from the perspective of macroscopic mechanical response and microscopic deformation mechanism.

Experimental and analytical study on the compressive behavior of PMI foam with different densities and cell sizes under intermediate strain rates

Polymethacrylimide (PMI) foam materials have great potential for applications in the field of protective energy absorption owing to their superior compressive properties. Existing studies on the mechanical behavior of PMI foams are limited, particularly for loading at intermediate strain rates. This paper presents a comprehensive experimental study of PMI foam with four different densities under quasi-static and intermediate strain rate compression (0.001 s−1 to 100 s−1). Each density included three different cell sizes to determine their effects on the compressive performance. Loads parallel and perpendicular to the foam rise direction were considered to investigate the anisotropic behavior. Intermediate strain rate tests were conducted using a high-speed hydraulic servo testing machine, which achieved a stable strain rate while ensuring consistent specimen sizes in both quasi-static and dynamic tests. In all the experiments, the compression process was captured using a high-speed camera and the macroscopic deformation mode and microscopic deformation mechanism were analyzed by combining Digital Image Correlation (DIC) and Scanning Electron Microscopy (SEM). The PMI foam with finer cell sizes exhibited an enhanced compression performance. As the strain rate increased, the strain rate effect became more evident in the high-density specimens and an increase in cell size diminished this effect. A modified analytical model incorporating the cell-size correction term was developed based on Gibson and Ashby's model. The modified model exhibited an average error of 4.9 % in the predicted plateau stress of PMI foam with different cell sizes at the same density.

Research on the shear stress and microscopic deformation mechanism of deep sea sediments under the action of tracked miner

The traction force of the tracked miner is primarily determined by the shear characteristics of deep-sea sediments. The influence of different parameters on the shear characteristics of deep-sea sediments and the particles change law of the soil–track interface are discussed. By constructing the relationship between the particles horizontal displacement and residual shear strength, a novel shear rheological damage model of deep-sea sediments considering the influence of grounding pressure is proposed, and the microscopic mechanism of shear displacement of deep-sea sediments is explained. The results show that the peak shear strength and residual shear strength increase significantly with the increase of grounding pressure, track length, grouser height and shear speed. Moreover, the particles within the soil–track interface move along the lower right of the vehicle operating direction during the shearing process under different working conditions, the change of particle spacing shows a non-linear trend, and a quantitative equation for the horizontal deformation of soil–track interface under the experimental conditions is constructed. Additionally, the new shear model has a high level of accuracy in fitting with the experimental data, the internal particle migration changes on the soil–track interface can be divided into four characteristics: compaction, failure, deformation and translation.

Microscopic deformation mechanism and characteristics of carbonaceous slate during the creep process

Carbonaceous slate exhibits a significant creep deformation that seriously affects the construction and operation of underground projects. To investigate the microstructural changes characteristics and reveal the microscopic deformation mechanism of the carbonaceous slate during the creep process, multiple methods were performed, including the triaxial creep test, SEM and MIP. The following conclusions were drawn: The rock samples underwent three stages during the creep test: microporosity closure at a low-stress level, material densification at an intermediate stress level, and microcracks emerging and expanding to failure at the high stress. The creep deformation was particularly significant in the first and third processes. The lamellar particles are compressed or bent under stress in parallel and vertical directions, showing the anisotropic properties of deformation. The deformation of the rock sample is related to the angle between the bedding and the orientation of major principal stress, and the effect of the anisotropy decreases with the increased stress level. The sprouting and expansion of microfractures occur at high-stress levels, showing pressure dissolution of mineral particles, migration of very fine particles, and cement damage between lamellar particles. Finally, the horizontal samples formed a combined rupture surface composed of the laminar surface and the fracture surface intersecting it, showing brittle damage, while the vertical samples formed a fracture surface parallel to the laminar surface, showing a ductile damage pattern. Those results could provide the basis for a further understanding of the mechanical properties of carbonaceous slate and the improvement of its creep model and parameters. It was significant for the stability analysis and deformation prediction of engineering structures using numerical simulation.

Experimental and mechanistic investigation on the plastic anisotropic deformation behavior of α-phase titanium alloy Ti-2Al-2.5Zr

Ti-2Al-2.5Zr is widely used in piping and structural support applications, however, the rolling forming process results in anisotropic deformation during service. This behavior has implications for the manufacturing processes and structural safety assessments in engineering applications. In this study, the plastic anisotropic deformation behavior of a rolled Ti-2Al-2.5Zr plate was investigated using uniaxial tensile tests along the transverse, normal, and 45° directions. Acoustic emission, electron backscatter diffraction, and scanning electron microscopy methods were used to investigate dislocation slip and twinning mechanisms. The results indicated that different microscopic deformation mechanisms caused the significant macroscopic anisotropy of Ti-2Al-2.5Zr. The primary mechanisms involved were prismatic <a> slip, pyramidal <c+a> slip, and {10-12} extension twinning. The stress direction determined the influence of each of these mechanisms during the yielding and plastic deformation phases. Application of the visco-plastic self-consistent model established the relationship between the macroscopic mechanical responses and microscopic deformation mechanisms. It was revealed that Ti-2Al-2.5Zr achieved its optimum strength when the initial texture aligned most of the grain c-axis at angles ranging from 30° to 50° relative to the deformation direction. This finding provides a direction for the texture design of Ti-2Al-2.5Zr in engineering materials.

Microscopic deformation and fragmentation energy consumption characteristics of soils with various moisture contents using discrete element method

Increased soil moisture content commonly leads to higher resistance and energy consumption during agricultural operations. However, the impact of moisture content on soil microscopic deformation and fragmentation energy consumption characteristics remains unclear. In this study, discrete element models of sandy clay loam with different gravimetric water contents (15.0%, 18.8%, 21.6%, 24.1%, and 27.3%) were constructed for simulation experiments using coupled contact method (Hertz-Mindlin with bonding and Hertz-Mindlin with JKR). The degree of soil fragmentation and the rate of crack propagation were investigated. The results indicated that at fixed gravimetric water content, fissure rate exhibited a pattern of slow increase, rapid increase, and eventual stabilization with increasing compression displacement, while the fissure transfer coefficient showed a pattern of first increase and then decrease. At a fixed compression displacement, the increase in moisture content led to a decrease in both fissure rate and fissure transfer coefficient. The analysis of soil stress-strain characteristics and energy dissipation patterns found that soil with a gravimetric water content of 27.3% (close to soil liquid limit) exhibited no compression yield point; at a soil gravimetric water content of 15.0%, the minimum crushing energy (Udmin) was achieved (1.32 J), with a maximum elastic strain energy (Uemax) of 1.83 J. Similarly, at a soil gravimetric water content of 24.1%, the minimum crushing energy (Udmin) was reached (2.17 J), with a maximum elastic strain energy (Uemax) of 2.25 J. The comparison between actual and simulated results revealed low relative errors (<6%) in Udmin for all the above soil gravimetric water contents, suggesting the feasibility of our coupled discrete element model for assessing soil crushing energy consumption. This study revealed microscopic deformation and instability fragmentation mechanisms of soils with various moisture contents under vertical axial loads. Our findings provide novel method and strategy for developing the high-efficiency low-consumption soil fragmentation model.

Large strain micromechanics of thermoplastic elastomers with random microstructures

Thermoplastic polyurethanes (TPU) are block copolymeric materials composed of plastomeric “hard” and elastomeric “soft” domains, by which they exhibit highly resilient yet dissipative large deformation features depending on volume fractions and microstructures of the two distinct domains. Here, we develop a new methodology to explore the microscopic deformation mechanisms in TPU materials with highly disordered microstructures. We propose new micromechanical models for randomly dispersed (or occluded) as well as randomly continuous hard domains (and mixtures of both dispersed and continuous hard domains), each within a continuous soft structure as widely found in representative TPU materials over a wide range of volume fractions, vhard = 26.9% to 52.2%. The micromechanical modeling results are compared to experimental data on the macroscopic large strain behaviors reported previously (Cho et al., 2017). We explore the role of the dispersed vs. continuous nature of the geometric features of the random microstructures on shape recovery and energy dissipation at the microstructural level in these phase-separated copolymeric materials.

Revealing the mechanism of ductility discrepancy between two 2205 duplex stainless steels containing layered and island austenite through in-situ experiment

A notable discrepancy in tensile ductility was observed between two 2205 duplex stainless steels (DSS) containing layered and island-like austenite microstructures. The deformation behavior of both DSS 2205 was compared through in-situ Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) techniques. The results indicated that the microscopic deformation mechanism for both DSS 2205 was governed by dislocation slip. The observed tensile ductility disparity was attributed to differences in dislocation slip behavior. In DSS 2205 with island austenite, slip transfer and blocking across grain boundaries occurred simultaneously, which was determined by the geometrical compatibility factor (m') calculations of adjacent ferrite and austenite grains. This intensified plastic inhomogeneities and promoted non-Schmid dislocation slip behavior. While in layered austenite of DSS 2205, the activation of slip system strictly adhered to the principle of maximizing the Schmid factor. Multiple slip systems in the ferrite on {110} and {112} planes were activated, inducing cross-slip, which enhanced ductility. The better ductility of DSS 2205 featuring layered austenite compared to the counterpart was further explained by quantitative analysis of geometrically necessary dislocations (GND) distribution in both phases and kernel average misorientation (KAM) values during deformation.

Study on Hot Deformation Behavior and Processing Maps of Cobalt‐Based Superalloy GH5188

True stress–strain data are obtained through the thermal compression test of the cobalt‐based superalloy GH5188 at the temperature range of 1080–1180 °C and the strain rate of 0.1–10 s−1, and the Arrhenius constitutive equation considering the effects of temperature, strain rate, and strain is established. The microscopic deformation mechanism and the appropriate process range are proposed based on the analysis of the dynamic material model and electron backscattered diffraction (EBSD). The results show that the established constitutive equation exhibits predictive ability, and the calculated thermal activation energy Q value is 446.1054 KJ mol−1; when the strain is 0.7, the GH5188 superalloy undergoes dynamic recrystallization (DRX) at 1130 °C in the form of discontinuous DRX; when the temperature is 1180 °C, the degree of DRX is significantly improved, and evenly distributed equiaxed grains are obtained; when the strain is 0.2, 0.4, and 0.6, DRX is mainly affected by the degree of deformation, and its occurrence is accompanied by the generation of Σ3. Combined with the analysis of the processing maps and EBSD diagrams, the optimal thermal deformation parameters of GH5188 alloy are determined as follows: the strain is 0.7, the deformation temperature is about 1180 °C, and the strain rate is about 1 s−1.

Effects of Al and V on deformation behavior and microscopic mechanism of titanium alloy at ultra-low temperature using molecular dynamics

Abstract A uniaxial tensile was executed with the molecular dynamics method, while the interaction between Ti, Al and V was described by hybrid potential. The influences of Al and V elements on the plastic deformation behavior and microscopic deformation mechanism of Ti-Al-V alloy were studied under ultra-low temperatures. The results show that the high content of Al and V elements leads to a sudden decrease of plasticity at 300 K. The plasticity is slightly reduced with increasing Al content when the content of V is 3.3~6.2 wt.% at 77 K. However, with the temperature reduced to 50 K, Al promotes the activation of dislocation when the content of V is high (6.2 wt.%) and hinders the activation of dislocation when V content is low (0.4 wt.%). At high V content (6.2 wt.%), the increase in plasticity with increasing Al content is more pronounced. Therefore, the strength and plasticity of Ti6.5Al6.2V increase with decreasing temperature.

The Horizontal Bearing Characteristics and Microscopic Soil Deformation Mechanism of Pile-Bucket Composite Foundation in Sand

The pile-bucket composite foundation represents an innovative foundation form that surpasses the horizontal bearing performance of both single bucket-shaped foundations and pile foundations. The intricate interplay between piles and buckets introduces the complexity of the factors influencing the bearing performance of composite foundations under horizontal loads. In this paper, the indoor model tests were conducted to investigate the effects of relative density and pile-to-barrel diameter ratio on the horizontal bearing capacity and surrounding soil pressure of the pile-bucket composite foundation. A sensitivity analysis on the bearing characteristics of the pile-bucket foundation was performed using ABAQUS/CAE 2020 software. The results reveal a consistent variation in load–displacement curves across diverse diameter ratios of piles to buckets. The pile-bucket diameter ratio significantly impacts the horizontal bearing characteristics of the composite foundation. Reducing the pile-bucket diameter ratio improves the horizontal bearing capacity of the composite foundation. When the diameter ratio of piles to buckets diminishes to ≤0.317, the influence of this ratio on bearing performance becomes markedly pronounced. The displacement range of the surface soil decreases with an increase in relative density, while the influence depth of the surrounding soil of the composite foundation significantly decreases as the pile-to-barrel diameter ratio decreases.

Evolution of Microstructure and Fe/Fe3C Interface Structure in Cold‐Drawn Pearlitic Steel Wires

The microstructure, texture, and dislocation configuration of cold‐drawn pearlitic steel wires were investigated by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). In addition, Fe/Fe3C interface structure at the atomic level was clearly observed and analyzed by high‐resolution TEM (HRTEM), and the microscopic deformation mechanism of cementite lamellae at different drawing strains is revealed. The results show that pearlite lamellae turn to the drawing direction to form ferrite &lt;110&gt; fiber texture, and the cementite and ferrite lamellae are simultaneously thinned with the increase of drawing strain. During the cold drawing process, shear‐bands(S‐bands) are formed in the cementite lamellae; the dislocation of ferrite transits from single slip to multi‐slip stage, and the dislocation density increases. After drawing, the lattice structures of cementite and ferrite phases show the evolution from single‐crystal to polycrystal, and the cementite lamellae are divided into a multilayer structure: central relative large‐size cementite crystal and outside relative small‐size cementite crystal layer; the dissolution of cementite lamellae in some regions results in the lattice expansion of ferrite phase and the occurrence of neck‐down regions in the cementite lamellae.

Research progress and prospects of plastic thermoelectric materials

In recent years, significant progress has been made in the research of plastic thermoelectric materials, for example, Ag&lt;sub&gt;2&lt;/sub&gt;S-based alloys. These materials exhibit excellent room-temperature plasticity due to their low slipping barrier energy and high cleavage energy, with synergistic enhancements in plasticity and thermoelectric properties achievable through alloying and doping strategies. The latest study on Mg&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;-based single crystals demonstrated superior performance in terms of plastic deformation capability and room-temperature thermoelectric properties. Microstructural characterization and theoretical calculation have revealed the crucial role of dislocation glide in the plastic deformation process of Mg&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt; single crystals, especially, the low slipping barrier energy observed in multiple slip systems. Importantly, the Te-doped single-crystalline Mg&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt; shows a power factor of ~55 μW cm&lt;sup&gt;–1&lt;/sup&gt; K&lt;sup&gt;–2&lt;/sup&gt; and &lt;i&gt;ZT&lt;/i&gt; of ~0.65 at room temperature along the &lt;i&gt;ab&lt;/i&gt; plane, which exceed those of the existing ductile thermoelectric materials. These findings not only deepen the understanding of microscopic deformation mechanisms in plastic thermoelectric materials but also establish an important foundation for optimizing material properties and developing novel flexible thermoelectric devices. Future applications of these materials in practical devices still face challenges in thermal stability, chemical stability, and interfacial contact. Addressing these issues will promote the application of plastic thermoelectric materials in the field of flexible electronics.

Microscopic deformation mechanism and residual plastic accumulation in cross-connection area in nano-grooving process of Zr-based metallic glass

Metallic glass plays a significant role in precision micro-molding and has increasingly gained attention in surface micro-nano structure processing technology. However, the current cutting processes of metallic glass surface micro/nanostructures are still facing several issues, such as burrs and defects. The research on surface structure defects at the nanoscale, specifically residual plastic accumulation in the cross-connection area (PACA), has not been explored. This paper analyzes the material removal process of a nano-groove from the Cu-Zr metallic glass surface, along with the deformation and chip separation mechanism under nanoscale conditions. The results indicate that there are no burrs or defects in cross-areas of surface micro/nanostructures for nano-scale cutting depths. PACA is identified as the primary factor that affects shape accuracy of nanometer microstructures. The study also clarifies the influences of machining parameters (such as tool geometry, workpiece temperature, and cutting process repeats) on material removal process and provides an optimization scheme for solving PACA in cross-areas. These findings offer direct theoretical guidance in understanding the material shear deformation mechanism and chip separation process of Cu-Zr metallic glass during the nano-grooving process, which will aid in solving PACA in cross-areas.

Analysis of microscopic deformation mechanism of SiCp/Al composites induced by ultrasonic vibration nanoindentation

Ultrasonic vibration machining technology affords environmentally friendly dry cutting without employing a cutting fluid and has been applied to the macro-scale dry cutting precision machining of SiCp/Al composites. However, the high-frequency vibration on the atomic-scale deformation mechanism of such materials remains unclear. Hence, this paper combines the molecular dynamics simulations (MD) with ultrasonic vibration indentation tests to investigate the effect of ultrasonic vibration on the multiscale deformation of SiCp/Al composites. The results demonstrate that the vibration amplitude exceeding the lattice constant (4.05 Å) of Al induces the plastic flow of Al atoms after breaking through the interatomic force. On the one hand, the ultrasonic high-frequency vibration energy accelerates the interfacial failure and the SiC particle fragmentation and promotes the dislocation movement to form the dislocation loop. On the other hand, compared with conventional indentation, ultrasonic vibration energy reduces the FCC phase transition rate by up to 40.8% and improves the toughness of the composites. Meanwhile, the high-frequency impact energy promotes the material to produce lattice distortion and subgranular grains, where grain slippage and lamination faults occur at the grain boundaries. Besides, the maximum depth of the material impact layer is about 1.45 times that of a conventional indentation, which contributes to the material being removed efficiently. The results of this research provide potential insights into ultrasonic vibration-assisted micro and nano removal processing of SiCp/Al composites, which could help to expand the efficient and precise clean processing of this type of material.

Strength and Deformation Behavior of Graphene Aerogel of Different Morphologies

Graphene aerogels are of high interest nowadays since they have ultralow density, rich porosity, high deformability, and good adsorption. In the present work, three different morphologies of graphene aerogels with a honeycomb-like structure are considered. The strength and deformation behavior of these graphene honeycomb structures are studied by molecular dynamics simulation. The effect of structural morphology on the stability of graphene aerogel is discussed. It is shown that structural changes significantly depend on the structural morphology and the loading direction. The deformation of the re-entrant honeycomb is similar to the deformation of a conventional honeycomb due to the opening of the honeycomb cells. At the first deformation stage, no stress increase is observed due to the structural transformation. Further, stress concentration on the junctions of the honeycomb structure and over the walls occurs. The addition of carbon nanotubes and graphene flakes into the cells of graphene aerogel does not result in a strength increase. The mechanisms of weakening are analyzed in detail. The obtained results further contribute to the understanding of the microscopic deformation mechanisms of graphene aerogels and their design for various applications.

Anisotropic deformation mechanisms in textured nanotwined Cu under shock loading

In this paper, a model of nanotwinted Cu with [111] texture is constructed, and the anisotropic response mechanism and microscopic deformation mechanism are investigated under shock loading with the impact velocity of 0.5 km/s, 1 km/s, 1.5 km/s and 2.5 km/s from 0°, 45° and 90° loading directions by molecular dynamics simulations. It is found that the textured nanotwinned Cu has obvious anisotropy under 0.5 km/s, 1 km/s and 1.5 km/s, and there are secondary yield and two-stage plastic relaxation under 90° loading. However, the anisotropic response is not obvious under hypervelocity impact (2.5 km/s). Under the shock loading, a large number of intrinsic stacking fault appear, among which the content of 0° and 45° is relatively higher than 90° shock. Under different loading direction, the behavior of multiple twins and dislocation motion varies. Specifically, at 0° loading, multiple twins emit dislocation motion simultaneously, leading to the formation of a large number of faults. At 90° loading, the twin boundary effectively blocks dislocation motion, resulting in less deformation. Under 45° loading, the deformation mode differs from both 0° and 90° loading, causing the original twin boundary to be destroyed and recombined into a new twin lamellar layer with varying thickness. This process creates intrinsic layer faults as well.

Strengthening mechanism of Ni–Cu nanotwins with void under different tensile directions based on Molecular Dynamics simulation

Defects such as voids and inclusions are inevitably inserted in the process of improving the mechanical properties by adjusting the microstructure, which results in alteration of deformation mechanism and ductile failure of the material. Although this is a macroscopic phenomenon, it is related to the mechanism at the atomic scale. In this paper, the uniaxial tension processes of Ni–Cu nanotwins containing a void are simulated using the molecular dynamics method, and the effects of the loading direction of perpendicular and parallel to the twin boundary (TB) on the mechanical properties are investigated, and the emphasis is on the hardening behavior after yielding. The results show that the yielding of the material is attributed to the emission of dislocation shear loops on the void surface. The elastic modulus and yield strength are greater under the loading direction of perpendicular to TB. The peak stress is equal to the yield stress under perpendicular loading, while the peak stress is greater than the yield stress under parallel loading. In addition, the hardening behavior is found after yielding under two tensile directions, but the hardening mechanisms are different. For perpendicular loading, the hardening behavior is controlled by the TB and the Lomer-Cottrel lock. While for parallel loading, the hardening behavior is controlled by the stacking faults that distributes uniformly and symmetrically along the TB. The general conclusions obtained may provide new insights into the microscopic properties and deformation mechanisms of nanotwins in depth.

Effect of hot isostatic pressing on the microstructures and mechanical properties of TiC/GTD222 composite fabricated by selective laser melting

In this work, hot isostatic pressing (HIP) treatment was performed on the TiC/GTD222 nickel-based composite prepared by selective laser melting (SLM). The effect of HIP on the microstructures and mechanical properties of the SLMed composite was investigated using XRD, SEM, EBSD and TEM. The results show that the microstructures of both SLMed and HIP-SLMed composites are composed of γ matrix, γ′ phase and carbides. The HIP treatment can significantly promote the formation of nanoscale carbides and γ′ phase, and simultaneously decrease the dislocation density of the composite. The yield strength and tensile strength of the HIP-SLMed composite increase by approximately 75 MPa and 189 MPa, respectively, while the elongation remains similar to that of the SLMed composite. The enhanced mechanical properties of the HIP-SLMed composite can be primarily attributed to the precipitation strengthening of the γ′ phase and the Orowan strengthening effect of the carbides. The microscopic deformation and strengthening mechanisms of the composites were also studied in detail.