Dislocation Nucleation Mechanism Research Articles

The effect of chemical short-range order on incipient plastic behavior in FCC structured high entropy alloys

Chemical short-range order (CSRO) was assumed as one of the most important structure feature of high entropy alloys and the influence of CSRO on mechanical properties is a fundamental issue yet to be fully understood. In this work, we performed extensively nanoindentation experiments on CoCrFeNiAlx alloys to study the effect of CSRO on the incipient nanomechanical properties. The statistical nature of strengths at the first pop-in event was analyzed to gain insight of deformation mechanisms. All samples examined here exhibit bimodal distribution which indicate non-unique dislocation nucleation mechanisms. The bimodal distributions can be decomposed into two Gaussian distributions and the activation volumes can be obtained in the range of 0.73–1.38b3. The peaks shift to higher stress level after the development of CSRO. The heterogeneous dislocation nucleation plays a dominant role at low indentation stress with the aid of pre-existing crystalline defects. The homogeneous dislocation nucleation mechanism prevails when indentation stress close to theoretical values. The transmission electron microscopy characterization indicates the presence of chemical ordering in the aged samples. Both the degree of chemical ordering and lattice distortion are much higher in the Al containing HEAs due to the distinctive difference of properties in Al and other transition element atoms.

Dynamic mechanical behavior of ultra-high specific strength lightweight Ti61Al16Cr10Nb8V5 multi-principal element alloy

This study investigated the dynamic mechanical property and microstructure evolution of the Ti61Al16Cr10Nb8V5 (Ti61) lightweight multi-principal element alloy(MPEA) with a density of 4.83 g/cm3. After melting and rolling, the Ti61 alloy was composed of a BCC matrix phase and dispersed B2 phase, and also contained a small amount of fine needle-like FCC phase and spherical HCP phase. Mechanical testing of Ti61 alloy revealed that the ultimate tensile strength is about 1223 MPa, and the tensile elongation is about 13.2 %, with a specific strength of around 253 MPa*cm3/g. Dynamic compression testing of Ti61 alloy showed a significant strain rate strengthening effect and excellent fracture strain greater than 50 %. Calculation and microstructural observations indicated that at low strain rates ranging from 10−3/s to 10/s, the strain rate sensitivity of Ti61 alloy was 0.0087, with the main plastic deformation mechanism of dislocation nucleation, dislocation slip, and stacking faults. At high strain rates ranging from 500/s to 5000/s, the strain rate sensitivity raises to 0.0963 and the main deformation mechanism of the Ti61 alloy involved higher-density dislocation nucleation, dislocation slip, stacking faults and twinning. At strain rates of 5000/s, HCP-FCC phase transition accompanied by FCC microtwins and BCC-α2HCP phase transition occurred. Furthermore, dynamic recrystallization also occurred in Ti61 alloy. The HCP-FCC has the orientation of (0001)HCP//(11−1)FCC, [1−210]HCP//[011]FCC. The BCC-α2HCP has the orientation of {101}BCC//{0001}HCP and [111]BCC//[2−1−10]HCP. Twining and phase transition strengthen the yield stress of Ti61 alloy, while dynamic recrystallization reduced the flow stress of Ti61 alloy. Ti61 alloy has high strength, high compression fracture strain, and ultra-high specific strength, which is a new LWMPEA with great application potential in military armor.

Enhancing mechanical performance of Al0.3CoCrFeNi HEA films through graphene coating: insights from nanoindentation and dislocation mechanism analysis

The present study comprehensively elucidates the nanoindentation response of graphene-coated Al0.3CoCrFeNi high-entropy alloy (HEA), by investigating the underlying mechanism of dislocation nucleation and propagation on the atomic level. In this regard, a series of molecular dynamics (MD) simulation of nano-indentation is performed over various configurations of pristine and graphene coated Al0.3CoCrFeNi HEA substrates. To begin with, the MD simulation-derived Young’s modulus (158.74 GPa) and hardness (13.75 GPa) of the Al0.3CoCrFeNi HEA is validated against the existing literature to establish the credibility of the utilized simulation method. The post-indentation deformation mechanism of pristine Al0.3CoCrFeNi HEA is further investigated by varying substrate size, indenter size, and indentation rate, and the materials behaviour is evaluated based on functional responses such as Young’s modulus, hardness, and dislocation density, etc. In the following stage, graphene coated Al0.3CoCrFeNi HEA is nano-indented, resulting in much greater indentation forces compared to pure HEA substrates, indicating higher surface hardness (two-fold increase when compared to pristine HEA). The underlying deformation mechanism demonstrated that inducing graphene coating results in increased dislocation density and a more extensive, entangled dislocation network within the HEA substrate, which leads to strain-hardening. The combination of increased hardness, enhanced strain hardening, and prevention of pile-up effects suggests that Gr-coated HEA substrates have the potential to serve as surface-strengthening materials. The scientific contribution of this study involves extensively unveiling the deformation mechanism of graphene coated Al0.3CoCrFeNi HEA substrate on the atomic scale, which will pave the way for a bottom-up approach to developing graphene coated engineered surfaces.

Designing grain refinement passes for multi-direction forging: A phase-field crystal study

Multi-directional forging is an effective plastic deformation method for polycrystalline grain refinement, and the degree of refinement is closely related to the number of forging passes. However, the relationship between the degree of refinement and the number of forging passes is almost ambiguous. Focusing on the forging passes, the grain refinement and dislocation nucleation mechanisms were revealed by combining the free energy variation and the strain field distribution using the dual-mode phase-field crystal model in our work. The results show that the nucleated dislocations within the grain can divide the grain into many finer grains under continuous stress. The nucleation mechanism of dislocation mainly includes two forms: grain boundary emission dislocation and atomic misalignment near the twin boundary. The strain field distribution reveals the mechanism of dislocation annihilation. The grain refinement effect of triple pass stress loading is the best, being 59 %, 18 %, and 43 % higher than that of the one forging pass, two forging passes, and four forging passes stress loading respectively. Moreover, the cause of the above phenomenon was further revealed by atomic analysis. This study reveals the response rule of forging passes to fine grain strengthening and gives feedback to guide the actual production experiment.

An atomistic study on the strain rate and temperature dependences of the plastic deformation Cu–Au core–shell nanowires: On the role of dislocations

Abstract Recently, Cu–Au core–shell nanowires have been extensively used as conductors, nanocatalysts, and aerospace instruments due to their excellent thermal and electrical conductivity. In experimental studies, various methods have been presented for producing, characterizing, and strengthening these structures. However, the mechanical behavior and plastic deformation mechanisms of these materials have not been investigated at the atomic scale. Consequently, in the present study, we carried out uniaxial tensile tests on Cu–Au nanowires at various tension rates and temperatures by means of the molecular dynamics approach. The Cu–Au interface was found to be the main site for nucleation of perfect dislocations, Shockley partials, and stacking faults due to the stress concentration and high potential energy arising from the atomic mismatch between shell and core layers. It was observed that an increase in the strain rate from 108 to 1,011 s−1 shortened the time required for the nucleation of dislocations, decreasing the dislocation density. This emphasizes that dislocation nucleation and slip mechanisms are time-dependent. Moreover, it was found that the interaction of Shockley partials can lead to the creation of lock dislocations, such as Hirth, Frank, and Stair-rod dislocations, imposing obstacles for the slip of other dislocations. However, as the tension temperature rose from 300 to 600 K, opposite-sign dislocations removed each other due to thermally activated mechanisms such as dislocation climb and dislocation recovery. Furthermore, the combination of Shockley partial dislocations decreased the stacking fault density, facilitating the plastic deformation of these structures. The yield strength and elastic modulus of the samples increased with the strain rate and substantially decreased as the temperature rose.

El-Numodis: a new tool to model dislocation and surface interactions

While surfaces are known to have a limited impact on the mechanical properties of crystalline materials at the macroscopic scale, they play a key role at small-scale behaving alternatively as sources or sinks of various plastic deformation processes. In this study, we present a new tool called El-Numodis that relies on the superposition method to couple the discrete dislocation dynamics code Numodis to Elmer, an open-source finite-element-modeling tool. After few years of development, El-Numodis allows now for the simulation of small-scale object deformation and mechanical properties based on a large set of surface-related processes including stress-free boundaries, mirrored dislocations and a Monte-Carlo based dislocation nucleation mechanism. Here we present the main features of the code as well as numerical test-cases and benchmarks going from classical boundary value problems to tensile tests on model thin film.

Statistical modeling of the effect of chemical inhomogeneity on incipient plasticity in complex concentrated alloys

In recent years, complex concentrated alloys (CCAs), also referred to as medium- or high-entropy alloys, have attracted substantial research interest due to their excellent mechanical properties including high strength, ductility, and toughness. It is known that the chemical inhomogeneity of CCAs gives rise to spatial variations in local properties such as the generalized stacking fault energy (GSFE) surface, which in turn affect their mechanical properties, but how such an inhomogeneity affects dislocation nucleation and incipient plasticity remains largely unknown and unexplored. Here, we develop a physics-informed statistical model for incipient plasticity in CCAs by combining elasticity theory for dislocation nucleation and statistical modeling of nanoindentation. Our model connects a material's fundamental properties to the statistics of incipient plasticity and is validated by the excellent agreement with the statistical data from molecular dynamics simulations of nanoindentation of CrCoNi CCA samples. By accounting for the spatial variation in the local generalized stacking fault energy surface in CCAs, our model captures the key difference in the nanoindentation-induced incipient plasticity response of CCAs compared with a conventional metal (fcc Cu) and also reproduces the trends across CCA samples with different degrees of short-range ordering. Our model also reveals a critical length scale for the underlying GSFE fluctuations which controls the overall statistics of incipient plasticity during nanoindentation of CCAs, which reflects the critical loop size of the underlying dislocation nucleation mechanism.

Superior strength-plasticity synergy in a heterogeneous lamellar Ti2AlC/TiAl composite with unique interfacial microstructure

Improving the plasticity of TiAl alloys at room temperature has been a longstanding challenge for the development of next-generation aerospace engines. By adopting the nacre-like architecture design strategy, we have obtained a novel heterogeneous lamellar Ti2AlC/TiAl composite with superior strength-plasticity synergy, i.e., compressive strength of ∼2065 MPa and fracture strain of ∼27%. A combination of micropillar compression and large-scale atomistic simulation has revealed that the superior strength-plasticity synergy is attributed to the collaboration of Ti2AlC reinforcement, lamellar architecture and heterogeneous interface. More specifically, multiple deformation modes in Ti2AlC, i.e., basal-plane dislocations, atomic-scale ripples and kink bands, could be activated during the compression, thus promoting the plastic deformation capability of composite. Meanwhile, the lamellar architecture could not only induce significant stress redistribution and crack deflection between Ti2AlC and TiAl, but also generate high-density SFs and DTs interactions in TiAl, leading to an improved strength and strain hardening ability. In addition, profuse unique Ti2AlC(11¯03¯)/TiAl(111) interfaces in the composite could dramatically contribute to the strength and plasticity due to the interface-mediated dislocation nucleation and obstruction mechanisms. These findings offer a promising paradigm for tailoring microstructure of TiAl matrix composites with extraordinary strength and plasticity at ambient temperature.

Effect of amorphous complexions on plastic deformation of nanolayered composites

Interface-induced dislocation nucleation and interface sliding are two decisive mechanisms to understand plastic deformation of nanolayered composites. Taking the Cu-Nb system as a prototype, with atomic simulations, we reveal the atomic mechanism of dislocation nucleation and interface sliding after introducing the CuNb amorphous complexions with different compositions. Under tension loading, the initial nucleation sites correspond to the localized shearing region, and the amorphous composition can regulate the ordering degree of crystalline/amorphous interfaces. Under shear loading, the pathway of interface sliding corresponds to the low-energy region of the energetic landscape, and the shear resistance increases with the increase of Cu content owing to the enhancement of the sliding barrier. These findings reveal the influence of the amorphous complexions on plastic deformation, and provide theoretical guidance for the design of nanolayered composites.

Theoretical model for yield strength of monocrystalline Ni3Al by simultaneously considering size and strain rate

AbstractTo comprehensively describe the size and strain rate dependent yield strength of monocrystalline ductile materials, a theoretical model was established based on the dislocation nucleation mechanism. Taking Ni3Al as an example, the model firstly fits results of molecular dynamics simulations to extract material dependent parameters. Then, a theoretical surface of yield strength is constructed, which is finally verified by available experimental data. The model is further checked by available third part molecular dynamics and experimental data of monocrystalline copper and gold. It is shown that this model can successfully leap over the huge spatial and temporal scale gaps between molecular dynamics and experimental conditions to get the reliable mechanical properties of monocrystalline Ni3Al, copper and gold.

Nanostructure, Plastic Deformation, and Influence of Strain Rate Concerning Ni/Al2O3 Interface System Using a Molecular Dynamic Study (LAMMPS)

The plastic deformation mechanisms of Ni/Al2O3 interface systems under tensile loading at high strain rates were investigated by the classical molecular dynamics (MD) method. A Rahman-Stillinger-Lemberg potential was used for modeling the interaction between Ni and Al atoms and between Ni and O atoms at the interface. To explore the dislocation nucleation and propagation mechanisms during interface tensile failure, two kinds of interface structures corresponding to the terminating Ni layer as buckling layer (Type I) and transition layer (Type II) were established. The fracture behaviors show a strong dependence on interface structure. For Type I interface samples, the formation of Lomer-Cottrell locks in metal causes strain hardening; for Type II interface samples, the yield strength is 40% higher than that of Type I due to more stable Ni-O bonds at the interface. At strain rates higher than 1×109 s-1, the formation of L-C locks in metal is suppressed (Type I), and the formation of Shockley dislocations at the interface is delayed (Type II). The present work provides the direct observation of nucleation, motion, and reaction of dislocations associated with the complex interface dislocation structures of Ni/Al2O3 interfaces and can help researchers better understand the deformation mechanisms of this interface at extreme conditions.

Methods of dislocation structure characterization in AIIIBV semiconductor single crystals

The development pace of advanced electronics raises the demand for semiconductor single crystals and strengthens the requirements to their structural perfection. Dislocation density and distribution pattern are most important parameters of semiconductor single crystals which determine their performance as integrated circuit components. Therefore studies of the mechanisms of dislocation nucleation, slip and distribution are among the most important tasks which make researchers face the choice of suitable analytical methods. This work is an overview of advanced methods of studying and evaluating dislocation density in single crystals. Brief insight has been given on the main advantages and drawbacks of the methods overviewed and experimental data have been presented. The selective etching method (optical light microscopy) has become the most widely used one and in its conventional setup is quite efficient in the identification of scrap defects and in dislocation density evaluation by number of etch pits per vision area. Since the introduction of digital light microscopy and the related transfer from image analysis to pixel intensity matrices and measurement automation, it has become possible to implement quantitative characterization for the entire cross-section of single crystal wafers and analyze structural imperfection distribution pattern. X-ray diffraction is conventionally used for determination of crystallographic orientation but it also allows evaluating dislocation density by rocking curve broadening in double-crystal setup. Secondary electron scanning electron microscopy and atomic force microscopy allow differentiating etch patterns by origin and studying their geometry in detail. Transmission electron microscopy and induced current method allow obtaining micrographs of discrete dislocations but require labor-consuming preparation of experimental specimens. X-ray topography allows measuring bulky samples and also has high resolution but is hardly suitable for industry-wide application due to the high power consumption of measurements.Digital image processing broadens the applicability range of basic dislocation structure analytical methods in materials science and increases the authenticity of experimental results.

Methods of dislocation structure characterization in A IIIB V semiconductor single crystals

The development pace of advanced electronics raises the demand for semiconductor single crystals and strengthens the requirements to their structural perfection. Dislocation density and distribution pattern are most important parameters of semiconductor single crystals which determine their performance as integrated circuit components. Therefore studies of the mechanisms of dislocation nucleation, slip and distribution are among the most important tasks which make researchers face the choice of suitable analytical methods. This work is an overview of advanced methods of studying and evaluating dislocation density in single crystals. Brief insight has been given on the main advantages and drawbacks of the methods overviewed and experimental data have been presented. The selective etching method (optical light microscopy) has become the most widely used one and in its conventional setup is quite efficient in the identification of scrap defects and in dislocation density evaluation by number of etch pits per vision area. Since the introduction of digital light microscopy and the related transfer from image analysis to pixel intensity matrices and measurement automation, it has become possible to implement quantitative characterization for the entire cross-section of single crystal wafers and analyze structural imperfection distribution pattern. X-ray diffraction is conventionally used for determination of crystallographic orientation but it also allows evaluating dislocation density by rocking curve broadening in double-crystal setup. Secondary electron scanning electron microscopy and atomic force microscopy allow differentiating etch patterns by origin and studying their geometry in detail. Transmission electron microscopy and induced current method allow obtaining micrographs of discrete dislocations but require labor-consuming preparation of experimental specimens. X-ray topography allows measuring bulky samples and also has high resolution but is hardly suitable for industry-wide application due to the high power consumption of measurements. Digital image processing broadens the applicability range of basic dislocation structure analytical methods in materials science and increases the authenticity of experimental results.

An atomistic study of deformation mechanisms in metal matrix nanocomposite materials

With nanoscale reinforcements resulting in a high density of matrix/reinforcer interfaces, metal matrix nanocomposite (MMNC) materials behave differently from their micro-composite counterparts. An atomistic level understanding of the fundamental deformation mechanisms is necessary to link the nanoscale reinforcement attributes with the strengthening and fracture properties of the nanocomposite. Using the Aluminum-(Silicon Carbide), Al-SiC, nanocomposite as an example, we conducted classical Molecular Dynamics simulations of uniaxial tensile deformation for the composite. By varying nanocomposite design parameters, including the SiC particle size, and the particle volume fraction, we revealed the deformation mechanisms and defects evolution in nanocomposite materials. The deformation mechanisms are characterized into three subsequent mechanisms, which are (I) defect-free deformation driven by lattice distortion of the matrix, (II) dislocation-based deformation driven by dislocation nucleation and growth, and (III) failure-based deformation driven by interface separation and void growth. The nanoparticle volume fraction was found to have a major effect on dislocation-based deformation, whereas the particle size had a greater impact on the failure-based deformation. This work sheds light on the fundamental deformation mechanisms of MMNCs which may facilitate the future design of advanced nanocomposites with broader applications. • Atomistic deformation mechanisms of metal matrix nanocomposite (MMNC) materials • Molecular dynamics simulations of tensile deformation of the Al-SiC nanocomposites with varying size and volume fraction of the SiC nanoparticles • Three subsequent deformation mechanisms of MMNCs are identified as (I) defect-free mechanism, (II) dislocation nucleation & growth mechanism, and (III) interface separation mechanism

Dislocation nucleation mechanisms during nanoindentation of concentrated FeNiCr alloys: unveiling the effects of Cr through molecular simulations

Fe-based alloys with high chromium and nickel concentrations are very attractive for efficient energy production in extreme operating conditions. We perform molecular dynamics (MD) simulations of nanoindentation on fcc FeNiCr multicomponent materials. Equiatomic FeNi, Fe55Ni19Cr26, and Fe74Ni8Cr18 are tested by using established interatomic potentials and similar conditions, for the elucidation of key dislocation nucleation mechanisms and interactions. Generally, we find that the presence of Cr in these alloys reduces the mobility of prismatic dislocation loops, and increases their area, regardless of crystallographic orientation. Dislocation nucleation and evolution is tracked during mechanical testing as a function of nanoindentation strain and Kocks–Mecking continuum modeling displays good agreement with MD findings. Furthermore, the analysis of geometrically necessary dislocations (GNDs) is consistent with the Ma–Clarke’s model at depths lower than 1.5 nm. The presence of Cr leads to a decrease of the GND density with respect to Cr-less FeNi samples, thus we find that Cr is critically responsible of increasing these alloys’ hardness. Post-indentation impression maps indicate that Ni–Fe–Cr compositions display strain localization and hardening due to high Cr concentration.

Atomistic understanding of incipient plasticity in BCC refractory high entropy alloys

Understanding the incipient plastic behavior of refractory high-entropy alloys (RHEAs) is crucial for their high-temperature applications. In this study, the initial dislocation nucleation and motion mechanisms in the TaTiZrV RHEA and their dependence on temperature were investigated upon nanoindentation by molecular dynamics (MD) simulations. Compared with the high stress-driven homogeneous nucleation criterion in pure BCC metals, the Zr-V short-range orders in the RHEA facilitate preferential inhomogeneous nucleation at low stress. The local compositional fluctuation not only causes intermittent slipping of screw dislocations by the trapping-detrapping mechanism but also severely reduces the moving rate of edge dislocations. In particular, the initial dislocation nucleation in the RHEA becomes more difficult with the increasing temperature. Based on the competitive mechanism between multiple-element-induced lattice distortion and point-defect-induced lattice distortion, the reason for their excellent retained mechanical properties at high temperatures was revealed. This study would provide theoretical support for the development of RHEAs in high-temperature technological applications.

Phase field crystal simulation of gap healing at nanoscale

The phase field crystal method is used to simulate the healing process of the central gap of three-dimensional bcc crystal material under compressive strain at the atomic level. It is found that during the healing process of the central gap, the gap protrudes at both ends of it, leading to dislocation nucleation and vacancy formation. Through the mechanism of dislocation nucleation and dislocation emission, the thickness of gap is reduced layer by layer, and finally the connection and closure of the lattice atoms on up and down surface of the gap are achieved, and the surface healing of the central gap is realized. According to the sharpening and passivation mechanism of the lattice atomic planes at both ends of the gap, the elliptic shape gap is approximated to calculate and analyze the influence of the change of stress intensity factor during the gap healing, and the critical condition of the gap dislocation emission is determined.

Chemical short-range ordering regulated dislocation cross slip in high-entropy alloys

High-entropy alloys (HEAs) have received extensive interest owing to their unusual mechanical properties as a result of the extreme compositional complexity. Understanding the dislocation behavior under the influence of chemical disorder, especially chemical short-range ordering (SRO), which is commonly found in HEAs, is fundamental to the rational design of high-performance alloy systems. Here, we systematically investigate the effect of disorder and SRO on the regulation mechanism of dislocation nucleation and cross-slip in HEAs through a combination of Monte Carlo methods, molecular dynamic (MD) simulations, and theoretical analysis. Our results demonstrate that the incipient plasticity of HEAs under nanoindentation is mainly controlled by the nucleation and cross-slip of Shockley partial dislocation loops, which are found to be highly dependent on SRO. In contrast to traditional strengthening, SRO leads to a significant increase in the stable nucleation radius of the dislocation loop and cross-slip resistance. Strengthening is then achieved due to the extension of the dislocation nucleation period and the high resistance for dislocation cross-slip. Aided by the transition path analysis and theoretical model, we show that the local compositional changes originating from SRO, specifically for strong Co-Cr pairs, are the dominant factors governing the dislocation properties and accordingly, the strengthening effects. Finally, the SRO strengthen effect is further validated against different alloy systems with different SRO types. These findings suggest that the elasticity and strength of HEAs can be improved through the prolonging of the nucleation period of dislocations resulting from SRO, dictating an avenue for enhancing the mechanical properties of HEAs. • The incipient plasticity of HEAs under nanoindentation is controlled by the nucleation and cross-slip of partial dislocation loops. • Strong SRO increases the nucleation energy barriers and the stable nucleation length is longer simultaneously. • Dislocation cross-slip is inhibited by SRO. • SRO increase the critical transition radius from dislocation slip to cross slip. • Strength originating from the interaction between solute and dislocation is enhanced by strong SRO.

Nano-mechanical properties and onset of plasticity in Fe–Mn–Al–Ni–C duplex lightweight steel processed by high temperature compression

In the present study, nano-indentation tests have been performed on the Fe–Mn–Al–Ni–C duplex lightweight steel following hot compression at 1223 K and 1423 K at a strain rate of 0.1 s−1. The nano-hardness values of various composite regions are determined and correlated with the complex deformed microstructural features, strain gradient, elemental partitioning, intergranular/intragranular precipitates and lattice distortion effect. The estimated elastic modulus value of FCC and BCC matrix regions in the studied steel is relatively higher than those reported for other conventional duplex steels, highlighting the contribution of intragranular B2 and к-carbide precipitates on the overall elastic modulus. Additionally, the deformation behavior of the matrix regions reveals that the onset of plasticity is governed by both homogeneous dislocation nucleation mechanism and multiplication of pre-existing dislocations under the indenter tip. The BCC + B2 region exhibits higher maximum shear stress (τmax) value as compared to the FCC + к-carbide region, indicating that the plastic deformation would first initiate in the latter region. This is related to the pre-existing high strain gradient in the soft FCC + к-carbide region that would facilitate the onset of plasticity more easily as compared to the BCC + B2 region during indentation. Moreover, it is observed that the elasto-plastic regime of the indentation loading curve exhibits random pop-in events associated with the resistance generated by the various intragranular precipitates in the FCC and BCC matrix regions, against the dislocation motion during nano-indentation test.

Study on the effects of H on the plastic deformation behavior of grain boundaries in nickel by MD simulation

It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress.