Microscopic Deformation Mechanisms Research Articles

Trans-scale rough surface contact model based on molecular dynamics method: Simulation, modeling and experimental verification

Contact interfaces widely exist in mechanical structures from macroscopic to microscopic scales and play an important role in understanding the properties and dynamics of the structures. To obtain the contact behavior, it is essential to include the plastic deformation characteristics on rough surfaces (caused by dislocation in asperity), while they are not properly considered in the traditional macroscopic models. This work presents a trans-scale rough surface contact model with the combination of the macroscopic model and the microscopic deformation mechanism by MD simulation. (I) Based on the molecular dynamics (MD) simulation results of a single-asperity normal contact, the relationship between the critical interference and the asperity size is fitted. The results show that the critical interference of the asperity is affected by its size, especially at the microscopic scale. Large asperity produces small critical interference. (II) A trans-scale rough surface contact model was established based on the fitted curves, which considered the joint distribution of asperity radius and height and the asperity size effect. (III) The influence of parameters was analyzed, and the predicted results of the proposed model and the traditional models were compared with the experimental results and MD simulation results. The results show that compared with other traditional models, the proposed trans-scale model has better performance in describing the normal contact characteristics of rough surfaces at both macroscopic and microscopic scales, showing the ability of full-scale description.

Atomic-scale analysis of deformation behavior of face-centered cubic nanocrystalline high-entropy alloys with different grain sizes at high strain rates

In the present study, the microscopic deformation mechanism and tensile properties of Al0.1CoCrFeNi nanocrystalline high-entropy alloys (HEAs) during uniaxial tension were studied through molecular dynamics simulation, and the dependence of mechanical properties on grain size and strain rate has been explored. As the tensile strain increases from 0 to 0.1, the atoms undergo a phase transformation from the face-centered cubic (FCC) structure into the hexagonal close-packed (HCP), body-centered cubic (BCC) and amorphous structure, and the dislocation density increases gradually due to dislocation annihilation and interaction. HEA grains undergo the transformation of the inverse Hall-Petch (H–P) relation into the H–P relation after exceeding one critical size of 14.77 nm at a strain rate of 5 × 107/s, and this critical size increases with increasing strain rate. The microscopic deformation mechanism of the H–P relation is that the accumulation of dislocations at the grain boundaries leads to an increase in the strength of HEAs. In contrast, the inverse H–P relation is caused by the migration of grain boundaries, and the rotation and merging of HEA grains. The increase of strain rate positively affects the tensile properties of nanocrystalline HEAs, and the flow stress and yield strength exhibit sensitive dependence on the extremely high strain rates. In addition, the amorphization of atoms has become an important way for HEAs to deform plastically at high strain rates.

Effects of twin orientation and twin boundary spacing on the plastic deformation behaviors in Ni nanowires

• Effects of twin orientation (θ) on the Ni nanowire (NW) ductility are investigated • NW ductility with θ ∼55° increases with decreasing twin boundary (TB) spacing • In-situ TEM reveals distinct deformation behaviors in NWs with different θ • Atomistic observation reveals different roles of TB can play during deformation • The different ductility can be related with microscopic deformation mechanisms Spreading twins throughout nano metals has been proved to effectively mediate the mechanical behaviors in face-centered-cubic (fcc) metals. However, the experimental investigation concerning the roles of twin boundary (TB) during deformation is rarely reported. Here, with the joint efforts of in-situ nanomechanical testing and theoretical studies, we provide a systematic investigation regarding the effects of TB orientation ( θ , the angle between tensile loading direction and the normal of TB) and spacing on deformation mechanisms in Ni nanowires (NWs). As compared with single-crystalline counterparts, it is found that nano-twinned (nt) NWs with θ ∼0° exhibit limited ductility, whereas TB can serve as an effective blockage to the dislocation propagation. In contrast, in nt NWs with θ ∼20° and 55°, TB migration/detwinning induced by TB-dislocation reaction or partial dislocation movement dominates the plasticity, which contributes to enhanced NW ductility. Regarding nt NWs with θ ∼90°, dislocations are found to be able to transmit through the TBs, suggesting the limited effect of TB on the NW stretchability. Furthermore, decreasing TB spacing ( λ ) can facilitate the detwinning process and thus greatly enhance the ductility of NW with θ ∼55°. This study uncovers the distinct roles that TB can play during mechanical deformations in fcc NWs and provides an atomistic view into the direct linkage between macroscopic mechanical properties and microscopic deformation modes.

The mechanical response and microscopic deformation mechanism of graphene foams tuned by long carbon nanotubes and short crosslinkers.

The mechanical response of graphene foams (GrFs) can be enhanced by both short crosslinkers (e.g. C-C bond) and long carbon nanotubes (CNTs) in experiments; however, the underlying mechanism is still unclear. Here, a coarse-grained molecular dynamics method is used to study the mechanical response and microscopic mechanism of GrF interconnected by both short crosslinkers and long CNTs (named CNT bonded GrF, CbGrF) under tension and compression, and the effect of the properties of graphene and CNTs on the mechanical properties of CbGrF is also investigated. Compared with short bonds, long CNTs play a reinforcing role at a larger tensile strain, leading to larger tensile strength and toughness. Under compression, the sliding and rotation of graphene sheets in CbGrF are prevented by long CNTs, resulting in higher compressive stiffness than that of pure GrFs. Furthermore, the tensile and compressive moduli increase by more than 300% with increasing thickness of graphene sheets from 1 to 9 layers; they increase by no more than 50% as the CNT bending stiffness increases and are almost independent of the stretching stiffness of CNTs. These results should be helpful for understanding the tunability of GrFs using both short and long crosslinkers and guiding the preparation of advanced GrF-based composites.

Ultrasonic-promoted defect activation and structural rejuvenation in a La-based metallic glass

Ultrasonic-assistant rejuvenation in a La-based metallic glass was demonstrated by means of nanoindentation for the first time. A more pronounced nanoindentation creep behavior along with mechanical softening was detected. It is proposed to be associated with the activation of flow defects with long characteristic relaxation time at a low loading rate and short characteristic relaxation time at a high loading rate during anelastic deformation process based on the Maxwell-Voigt model. Such rejuvenation behavior also leads to an unstable plastic flow and a relatively low energy barrier for atomic diffusion during the fast growth process of crystalline phases accompanied with the reduced glass transition and crystallization temperatures. Our studies might provide insights into the microscopic deformation mechanism of glassy materials, which could help in seeking the novel applications of amorphous alloys such as ultrasonic-assistant cold joining or molding.

Plastic Deformation Behavior of Metal Materials: A Review of Constitutive Models

The deformation behavior of metal materials in plastic forming is intimately related to deformation conditions, which are greatly affected by deformation rate, forming temperature, and plastic variables. Macroscopic mechanical properties research is an important basis and technical means to analyze the process parameters and deformation process of metal plastic forming. Therefore, to reveal the influence mechanism of macroscopic mechanical properties of metal materials, and establish material constitutive models under different deformation conditions, it is of great significance to choose reasonable forming parameters and prevent forming defects. There are substantial variances in the macroscopic mechanical characteristics of different materials in the deformation process. In order to accurately predict its deformation behavior, the phenomenological constitutive model, the microscopic constitutive model reflecting the microscopic deformation mechanism, and the artificial neural network constitutive model based on the neural network were constructed respectively on the basis of macroscopic mechanical tests and microscopic microstructure tests. On the basis of the existing research results, the advantages and disadvantages of phenomenological constitutive model, microscopic constitutive model, and neural network constitutive model are compared and analyzed, respectively. The research results of this paper will provide support for the selection of constitutive models for reasonably predicting the deformation behavior of metal materials.

Formability and micromechanisms of DP600 dual phase steels in electric-pulse triggered energetic materials forming

This paper investigates dual-phase (DP) steel’s formability and microscopic deformation mechanism under electric-pulse triggered energetic materials forming (ETEF). It provides experimental evidence and a detailed explanation for understanding the dynamic fracture mechanism and microstructure of ETEF. The present study analyzes the macroscopic fracture mechanism of ETEF specimens. Compared with quasi-static forming, the forming limits of ETEF with different strain paths (uniaxial tension, plane strain and biaxial tension) were significantly improved: the limit major strains were increased by 40.8 %, 78.2 %, and 18.2 %, respectively. Furthermore, SEM/EBSD/TEM observations revealed the relationship between the microstructure, kernel average misorientation (KAM), dislocation density, slip mechanism, and deformation twinning of DP600 steel. It was concluded that ETEF promoted the plastic flow of ferrite and martensite, reducing the void size and delaying the fracture failure of the material. High KAM increased the overall intensity of local grain deformation and reduced the strain gradient from grain interiors to grain boundaries. Under the condition of ETEF, a high fraction of low-angle grain boundaries (LAGBs) and refined grain size were obtained, and a high fraction of LAGBs enhanced the formability of DP steel. Additionally, {012}, {013} and {104} preferred planes appeared under ETEF conditions, forming a strong < 011 > preferred orientation and activating the slip mechanism of the < 100 > fiber component. Finally, TEM confirmed that coordinated deformation of the matrix led to improved formability of the DP600 material under ETEF conditions, as evidenced by the activation of the multi-slip system and deformation of the martensitic twins.

Percolation-like transition from nanoscale structural heterogeneities to shear bands in metallic glass detected by static force microscopy

Structural heterogeneities of metallic glasses at the nanoscale have been confirmed by many experiments. Some simulations indicate that these structural heterogeneities show a direct correlation with the initiation of shear bands. However, due to the limited experimental conditions, the detailed evolution from nanoscale heterogeneities to shear bands during deformation is still unclear. Here, a tensile platform coupled with in situ static force microscopy is designed to detect the evolution of nanoscale heterogeneous viscoelasticity during deformation in a CuZr metallic glass. We observe that the dominant viscoelastic mode changes from solid-like to liquid-like with increasing tensile strain, which displays the percolation-like character. Meanwhile, the correlation length of structural heterogeneities increases with strain, and the stripe-like shear bands form when the correlation length reaches a critical value. The current findings not only present a clear picture of the evolution of structural heterogeneities with stress but also reveal the microscopic deformation mechanism of amorphous materials.

The mechanics and design of a local crystallization of amorphous for carbon material by molecular dynamics simulation

Achieving high strength and plasticity by means of microstructure manipulation of three-dimensional carbon material attracts researchers’ attention. This work is primarily on the influence of sp2 content at micro and molecular level on mechanical properties by molecular dynamics simulation. Additionally, the influence of interfaces has been investigated by three multi-nanocapsule pillars assembled with the equiaxed amorphous models. The model that has the best fit between strength and plasticity we obtained is the amorphous model with 100% multi-layer nanotubes fragments, and with no interfaces. We found that the rotating and folding of fragments are the main microscopic deformation mechanisms of amorphous carbon materials with sp2 fraction decreasing attributed to the breaking of covalent bonds, which has also been verified in experiment. The rotation of the nanocapsule and the migration of the interface contribute more in the condition of the existence of interfaces in the amorphous carbon material.

Temperature-dependent mechanical properties and the microscopic deformation mechanism of bilayer γ-graphdiyne under tension

γ-graphdiyne (γ-GDY) is a new two-dimensional carbon allotrope that has received increasing attention in scientific and engineering fields. The mechanical properties of γ-GDY should be thoroughly understood for realizing their practical applications. Although γ-GDY is synthesized and employed mainly in their bilayer or multilayer forms, previous theoretical studies mainly focused on the single-layer form. To evaluate the characteristics of the multilayer form, the mechanical properties of the bilayer γ-GDY (γ-BGDY) were tested under uniaxial tension using the molecular dynamics simulations. The stress–strain relation of γ-BGDY is highly temperature-dependent and exhibits a brittle-to-ductile transition with increasing temperature. When the temperature is below the critical brittle-to-ductile transition temperature, γ-BGDY cracks in a brittle manner and the fracture strain decreases with increasing temperature. Otherwise, it exhibits ductile characteristics and the fracture strain increases with temperature. Such a temperature-dependent brittle-to-ductile transition is attributed to the interlayer cooperative deformation mechanism, in which the co-rearrangement of neighboring layers is dominated by thermal vibrations of carbon atoms in diacetylenic chains. Furthermore, the brittle-to-ductile transition behavior of γ-BGDY is independent of loading direction and loading rate. The ultimate stress and Young’s modulus decrease at higher temperatures. These results are beneficial for the design of advanced γ-GDY-based devices.

Research on microhardness prediction of 304 stainless steel turning based on dislocation density

The microscopic dislocation slip and deformation of the machined surface layer of 304 stainless steel during turning leads to severe work hardening, and the study of the microstructural evolution of the surface layer of turned 304 stainless steel is of high importance to investigate microscopic plastic deformation and reduce work hardening. Most existing studies relating to the microstructural evolution of machined surfaces have used two-dimensional (2D) orthogonal cutting models. In this paper, we proposed a method to quickly build a three-dimensional (3D) bevel cutting finite element model and ABAQUS 3D cutting simulation models using temperature-displacement coupling algorithm, which can solve the problems of complex geometric modeling and low computational efficiency of ABAQUS 3D. In addition, a dislocation evolution modeling method was proposed through direct iterative optimization of intrinsic parameters, a subroutine of dislocation density evolution prediction model was written in accordance with finite element secondary development, and a joint simulation-based inverse identification method of dislocation evolution rate parameters was developed to minimize the error between simulated and experimental values of cutting forces. We compared the simulation results and the results of microstructure morphology analysis, verified the effectiveness of the inverse identification method and the dislocation density evolution prediction model, and then investigated the effect of dislocation evolution rate parameters α∗, β∗, k0 on the main cutting force, the microhardness, the dislocation density and the grain size. The microhardness changes of the cut surface were predicted using the microscopic dislocation deformation mechanism. By comparing the experimental data, we found that the predicted microhardness values of the machined surface at different cutting amounts were well consistent with the experimental data, thus verifying the credibility of the microhardness prediction model. Subsequently, the effect of cutting parameters on microhardness values, the average depth of slip zone, the equivalent plastic strain and the strain layer depth was analyzed.

Hardening and toughening effects of intermediate nanosized structures in a confined amorphous alloy film

• Intermediate nanosized structure (termed as “nano shear bands” (NSBs)) between microscopic shear transformation zones (STZs) and macroscopic shear bands (SBs) was observed in the crystalline-layer confined amorphous (CLCA) NiW alloy films. • NSBs promoted multiple and controlled shear banding deformation, hence leading to the enhanced crystallization. • NSBs possessed both hardening and toughening effects for the CLCA films. The plastic deformation of amorphous alloys is well known to be localized into shear bands (SBs), which are believed to stem from the atomic-scale flow defects, i.e., shear transformation zones (STZs). Yet, the bridge between the mesoscopic SBs and the atomic-scale STZs remains poorly understood. In this work, through thermally activating pronounced β relaxations in the well-designed crystalline-layer confined amorphous (CLCA) NiW alloy films, we experimentally captured and observed an intermediate nanosized structure termed as “nano shear bands” (NSBs) with a typical size of 1–2 nm in thickness and 5–10 nm in length. The influences of such NSB structures on the macroscale deformation behavior were systematically investigated. It was found that NSBs lead to both hardening and toughening effects for the CLCA films, as they promote multiple and controlled shear banding deformation, which results in enhanced crystallization. The intermediate NSB structure could connect the microstructural characteristics and macroscopic plasticity in amorphous alloys and may provide new insights for understanding the microscopic deformation mechanism of amorphous alloys as well as tuning/designing their properties.

Micro-Mechanical Investigation of Interfacial Debonding in Carbon Fiber-Reinforced Composites Using Extended Finite Element Method (XFEM) Approach.

The interface debonding in carbon fiber-reinforced polymers is analyzed and evaluated using the extended finite element method (XFEM). In order to accurately evaluate the bonding properties between fibers and matrix, different tests were carried out, including the multiple tests for different orientations to study longitudinal, transversal, and shear properties of unidirectional carbon fiber-reinforced composites. Extensive experimentation has been performed in all the different groups and categories with different dimensions and parameters in order to ascertain the values of strength and the prediction of the damage to the structure. The experimental and numerical comparison provided significant trends and data to evaluate the mechanical properties of the interface. The values of stiffness and strength are compared and validated. Development of Representative Volume Element (RVE) for progressive damage model to these damage phenomena has already been performed as a feasibility study for the model, though it is not included in this particular paper. The results of this research for all the experimental and numerical sets can serve as reliable data in the microsimulation of devices and sensitive parameters that include carbon fiber-reinforced light metal matrix composites and makes a better investigative model that contributes to various conditions. It further offers an investigation of the microscopic deformation mechanisms in the composites.

Experimental analysis and mechanical model of interaction between warm frozen silt and pre-bored grouted planted nodular pile

To reveal interaction mechanism on the interface between the pre-bored grouted planted nodular pile (PGPN) and warm frozen ground, and to compare its bearing performance with that of cast-in-place pile, shear tests on the interface of the two types of pile were carried out at negative temperature under different influencing factors through self-designed test devices. Combined with the single factor method and response surface method, the influence laws of various factors and interaction on the adfreezing force of the interface were analyzed. The micro-deformation mechanism of the interface could be revealed by the binary medium model, and volume breakage rate and Weibull distribution function were introduced to establish the constitutive equation of the interface. The results showed that the cemented soil slurry could strengthen the frozen soil around the pile, The shear zone of the interface appeared on the frozen ground around the pile rich in ice film, and the shear stress-displacement curve showed a strain softening type and included four stages. Among all the influencing factors, temperature has the most significant effect on adfreezing force. The freezing strength has an exponential relationship with temperature, and the water content and normal stress have a linear relationship with freezing strength. Compared with the traditional cast-in-place pile, PGPN pile has higher bearing capacity and more stable deformation. The proposed interface constitutive model scientifically reveals the microscopic deformation mechanism of the shear zone. The model data fit well with the test results, which could provide valuable reference for the design and calculation of the bearing capacity of the pile foundation in warm frozen ground.

Molecular dynamics study of microscopic deformation mechanism and tensile properties in AlxCoCrFeNi amorphous high-entropy alloys

Molecular dynamic (MD) simulations have been performed to explore the microscopic deformation mechanism and tensile properties of AlxCoCrFeNi amorphous high-entropy alloys (HEAs) during uniaxial tension. The effects of Al concentration, strain rate and temperature have been taken into account. Three amorphous HEAs with different aluminum concentrations were prepared by rapidly cooling from a high temperature of 3000 K at a cooling rate of 20 K/ps. The results of radial distribution function (RDF) and common neighbor analysis (CNA) show that the prepared amorphous HEAs do not contain crystalline structure. The uniaxial tensile results show that the increase in temperature impairs the tensile properties of all amorphous HEAs over the span of 300–1200 K, including Young's modulus, yield stress and yield strain. Especially at 1200 K, the intense thermal motion of the atoms makes them easier to migrate, which severely weakens the mechanical properties of amorphous HEAs. The atomic shear strain increases with increasing temperature and is uniformly distributed. In the range of the aluminum molar ratio of 1.0–3.0, the aluminum concentration has little influence on the Young's modulus and yield stress of the amorphous HEAs. Moreover, the increase in strain rate contributes to the increase in tensile properties of all amorphous HEAs with different aluminum concentrations, but it has no significant influence on the Young's modulus in the strain rate range of 108-1010/s.

Long-term creep and microscopic deformation mechanisms of sandstone using rock rheology creep equipment and scanning through high-resolution 3D XRM

Abstract Since rock is heterogeneous, hence its behavior is always variable. The governing parameters in rock deformations are time, applied load, composition, water content, temperature and loading conditions including confinement and loading rate. Time-dependent deformation study is extremely important for the prevention of hazards such as rockburst, roof fall and collapse. In the current study, sandstone samples from Kouzidong Mine China are analyzed using long-term rheological creep tests and advanced microscopic scanning using 3D X-ray microscopy (XRM). Time is the main governing factor in rock rheological creep study. The current study focuses on the microscopic deformation of selected rock using rheological creep and microscopic analysis. Rock behaves differently under different applied loads; therefore, two different loads are applied on two sandstone samples. Two samples are scanned before and after rheological creep testing. The uniaxial constant load applied on the first sample is 40% of the uniaxial compressive strength of the rock (σ1 = 40%σc) for one month. The behavior of the first sample shows an instantaneous creep and stable creep. The maximum uniaxial strain is 0.0307. The scanning results show evidence of microfractures in sandstone sample 1 at a lower load. There is evidence of grain movement and boundary alteration. The grain movement depends on the bonding between different grains. The second sample is loaded at 50% of the uniaxial compressive strength (σ1 = 50%σc). The maximum uniaxial strain for sample 2 is 0.0408. Creep behavior is the same and the microscopic deformation is enhanced with increasing applied load.

A virtual laboratory based on full-field crystal plasticity simulation to characterize the multiscale mechanical properties of AHSS

In this work, we proposed a virtual laboratory based on full-field crystal plasticity (CP) simulation to track plastic anisotropy and to calibrate yield functions for multiphase metals. The virtual laboratory, minimally, only requires easily accessible EBSD data for constructing the highly-resolved microstructural representative volume element and macroscopic flow stress data for identifying the micromechanical parameters of constituent phases. An inverse simulation method based on a global optimization scheme was developed to identify the CP parameters, and a nonlinear least-squares method was employed to calibrate yield functions. Mechanical tests of advanced high strength steel sheet under various loading conditions were conducted to validate the virtual laboratory. Three well-known yield functions, the quadratic Hill48 and non-quadratic Yld91 and Yld2004-18p yield functions, were selected as the validation benchmarks. All the studied functions, calibrated by numerous stress points of arbitrary loading conditions, successfully captured both the deformation and strength anisotropies. The full-field CP modeling correlated well the microscopic deformation mechanism and plastic heterogeneity with the macromechanical behavior of the sheet. The proposed virtual laboratory, which is readily extended with physically based CP models, could be a versatile tool to explore and predict the mechanical property and plastic anisotropy of advanced multiphase materials.

Microstructure and nanomechanical behavior of individual phase in β-solidifying Ti-43Al-5Nb-3.5Cr-1Zr alloy

A near fully lamellar Ti-43Al-5Nb-3.5Cr-1Zr alloy with different lamellar orientations was investigated by electron backscattered diffraction and nanoindentation measurements to study the influence of lamellar orientation on the nanohardness of α2/γ lamellae in a β-solidifying TiAl alloy. The results indicate that nanohardness of the α2/γ lamellae exhibits apparent plastic anisotropy, which is simultaneously determined by the oriented angle θ1 between the indentation direction and the lamellar plane and the deformation characteristics of the α2 and γ phase. Furthermore, the nanohardness of the individual lamellar colony decreases with an increase in the oriented angle from hard (0°) to soft (45°) orientations. Based on the analysis of the Schmid factor, the nanohardness value of the hard oriented α2/γ lamellae with a small Schmid factor is greater than the soft orientation with a large Schmid factor. The pile-up morphology on the γ phase is determined to be higher than that on the B2 phase at the interface through atomic force microscopy characterizations, which is attributed to the stronger plastic deformability of the γ phase. Therefore, the results can be successfully applied to understand the microscopic deformation mechanism of near fully lamellar β-solidifying TiAl alloy from a new perspective.

The Role of Graphene in Graphene-Filled Carbon Nanotube Foam and the Corresponding Microscopic Deformation Mechanism

The Role of Graphene in Graphene-Filled Carbon Nanotube Foam and the Corresponding Microscopic Deformation Mechanism

Change of Acoustic Emission Characteristics during Temperature Induced Transition from Twinning to Dislocation Slip under Compression in Polycrystalline Sn.

In this study, acoustic emission (AE) measurements on polycrystalline tin as a function of temperature at different driving rates under compression were carried out. It is shown that there is a definite difference between the acoustic emission characteristics belonging to twinning (low temperatures) as well as to dislocation slip (high temperatures). The stress averaged values of the exponents of the energy probability density functions decreased from = 1.45 ± 0.05 (−60 °C) to = 1.20 ± 0.15 (50 °C) at a driving rate of , and the total acoustic energy decreased by three orders of magnitude with increasing temperature. In addition, the exponent γ in the scaling relation SAE~DAEγ (SAE is the area and DAE is the duration) also shows similar temperature dependence (changing from γ = 1.78 ± 0.08 to γ = 1.35 ± 0.05), illustrating that the avalanche statistics belong to two different microscopic deformation mechanisms. The power law scaling relations were also analyzed, taking into account that the detected signal is always the convolution of the source signal and the transfer function of the system. It was obtained that approximate values of the power exponents can be obtained from the parts of the above functions, belonging to large values of parameters. At short duration times, the attenuation effect of the AE detection system dominates the time dependence, from which the characteristic attenuation time, τa, was determined as τa ≅ 70 μs.