Dislocation Mobility Research Articles

Interplay of heat treatment and deformation temperature on the microstructural evolution and mechanical behavior of SLM AlSi10Mg alloy

This study conducts a comprehensive examination of the microstructural and dislocation dynamics of AlSi10Mg alloys under various thermal conditions, utilizing quasi in-situ Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) analyses. The research focuses on comparing the microstructural response of as-built and T6 heat-treated samples to mechanical stress at both ambient (298 K) and cryogenic (77 K) temperatures. EBSD analysis underscores the grain stability under applied strain, while TEM investigations expose notable variations in dislocation mobility influenced by temperature, with a particular emphasis on the interaction between dislocations and silicon particles at cryogenic temperatures. An observed increase in Geometrically Necessary Dislocation (GND) density at 77 K points to enhanced resistance to plastic deformation, suggesting mechanisms of cryogenic strengthening. The findings illuminate the impact of heat treatment in augmenting the mechanical properties of the alloy, shedding light on its potential applications in aerospace and cryogenic environments.

Thermal activation and dislocation dynamics in Ti–6Al–4V alloy

The tensile stress relaxation behaviour is experimentally investigated for Ti–6Al–4V alloy across a temperature range of −60, −30, 20, 120, and 230 °C. Interestingly, the results revealed that the alloy exhibited the lowest resistance to stress relaxation at 20 °C, i.e., room temperature. This resistance was found to increase with both increasing and decreasing test temperatures. The apparent activation volume values, which indicate the dislocation mean free path, increase with strain due to a decrease in activable dislocation segments. Furthermore, these values were found to be proportional to the test temperature, resulting from higher dislocation mobility at moderate temperatures (230 °C and 120 °C) than at lower temperatures. Our experimental and analysis results indicate that dislocation-mediated plastic deformation predominates in all tested temperature conditions. However, the transition from planar to wavy dislocation configuration, observed through Transmission Electron Microscopy (TEM) between temperatures of −30 and 20 °C, along with the change in activation volume, suggests a critical temperature at which dislocation mechanism changes in the Ti–6Al–4V alloy. A rapid exhaustion of a substantial amount of mobile dislocation density, which significantly contributes to time-dependent deformation was observed at 230 °C. These findings provide valuable insights into the mechanisms underlying the stress relaxation behaviour of Ti–6Al–4V alloy at different operational temperatures, offering practical implications for improved alloy design and manufacturing processes.

Uniting Ultrahigh Plasticity with Near‐Theoretical Strength in Submicron‐Scale Si via Surface Healing

AbstractAs a typical hard but brittle material, Si tends to fracture abruptly at a stress well below its theoretical strength, even if the tested volume goes down to submicron scale, at which materials are usually nearly free of flaws or extended defects. Here, via the thermal–oxidation–mediated healing of the surface that is the preferred site for cracks or dislocations initiation, the premature fracture can be effectively inhibited and the over 50% homogeneous plastic strain with the near‐theoretical strength (twice the value of the unhealed counterpart) are united in submicron‐sized Si particles. In situ transmission electron microscope observations and atomistic simulations elucidate the confinement effect from the passivated and smoothened thermal oxide, which retards the dislocation nucleation and transforms the dominant deformation mechanism from partial dislocation to the more mobile full dislocation. This work demonstrates an effective and feasible surface engineering pathway to optimize the mechanical properties of Si at small scales.

Tensile and impact behavior of 3D-printed (FeCoNi)86Al7Ti7 high entropy alloy at ambient and cryogenic temperatures

High-entropy alloys (HEAs) demonstrate superior mechanical properties at room temperature, highlighting potential for engineering applications. With the increasing demand for advanced materials, especially in modern applications facing rigorous conditions, it becomes imperative to gain profound insights into the dynamic behavior of HEAs. In this study, the (FeCoNi)86Al7Ti7 HEA, renowned for exceptional printability and room-temperature mechanical properties, has been fabricated by selective laser melting (SLM), and its tensile and impact behaviors at ambient and cryogenic temperatures are systematically investigated. Experimental results reveal enhanced tensile properties as temperature decreases, achieving a high ultimate strength of 1313.6 MPa and impressive ductility of 24.8 % at 77 K. This improvement is attributed to the synergistic deformation mechanisms, including dislocation slip, stacking faults, Lomer and Lomer-Cottrell locks, which maintain a high strain-hardening rate and thus suppress strain localization. In contrast, impact properties deteriorate due to constrained dislocation mobility at high strain rates and low temperatures, with Charpy impact energy decreasing from 8.1 J at 293 K to 5.1 J at 77 K. Consequently, cracks readily initiate at cellular boundaries or defects, propagating in an intergranular manner and inducing localized deformation. This study provides valuable insights for the future applications of 3D-printed HEAs in extreme operational environments.

Detection of dislocation motion in atomistic simulations of nanocrystalline materials

A method for identifying dislocation motion in atomistic simulations is presented. While identifying static and moving dislocations within a single crystal or a combination of such is well established, the method described here is tailored to identify dislocation motion by correlating the displacements of individual atoms. This facilitates the identification of dislocation motion in complex structural arrangements, and allows the specific contribution to plastic deformation, due to dislocation motion, to be separated from that of other mechanisms. The method is applied to test cases in crystals and grain boundaries (GBs), in which irradiation-induced creep (IIC) was induced. It is shown that the method singles out the moving dislocations from among the dislocation forest at GBs, thus identifying the specific reactions driving the distortion at any given time. This enables the study of dislocation processes in the presence of realistic obstacles, and the study of the effects of microstructure on dislocation mobility. As an example of such a study, the method is applied to rule out intragranular slip, and to estimate the contribution of dislocation motion to strain, in a NC undergoing IIC.

Recent advancements and challenges in orbital‐free density functional theory

AbstractOrbital‐free density functional theory (OFDFT) stands out as a many‐body electronic structure approach with a low computational cost that scales linearly with system size, making it well suitable for large‐scale simulations. The past decades have witnessed impressive progress in OFDFT, which opens a new avenue to capture the complexity of realistic systems (e.g., solids, liquids, and warm dense matters) and provide a complete description of some complicated physical phenomena under realistic conditions (e.g., dislocation mobility, ductile processes, and vacancy diffusion). In this review, we first present a concise summary of the major methodological advances in OFDFT, placing particular emphasis on kinetic energy density functional and the schemes to evaluate the electron–ion interaction energy. We then give a brief overview of the current status of OFDFT developments in finite‐temperature and time‐dependent regimes, as well as our developed OFDFT‐based software package, named by ATLAS. Finally, we highlight perspectives for further development in this fascinating field, including the major outstanding issues to be solved and forthcoming opportunities to explore large‐scale materials.This article is categorized under: Electronic Structure Theory > Density Functional Theory Software > Simulation Methods Quantum Computing > Theory Development

A high-strength binary Mg-1.2Ce alloy with ultra-fine grains achieved by conventional one-step extrusion during 300–400 °C

Ultra-fine grained magnesium alloys are usually prepared by severe deformation and medium-low temperature deformation. In this work, it has been found that the binary Mg-1.2Ce alloy with the uniform distributed Mg12Ce precipitates smaller than 3 μm is a good candidate to obtain ultra-fine grains by conventional one-step extrusion at high temperatures. All the three Mg-1.2Ce samples extruded at 300 °C, 350 °C and 400 °C show a bimodal microstructure with high-density dislocations, whose average grain sizes are 1.19 ± 0.92 μm, 1.32 ± 1.30 μm, and 1.44 ± 1.33 μm, respectively. A linear relation of ln(d) = -0.05ln(Z)+1.27 was determined relating the average grain size (d) to the Zener-Hollomon parameter (Z). The alloy extruded at 300 °C exhibits an exceptionally high ultimate tensile strength (UTS) of 412.3 ± 5.3 MPa and yield strength (TYS) of 387.6 ± 3.2 MPa, but a low elongation after fracture (El) of 4.9 ± 0.8 %. With the extrusion temperature increasing, the tensile strength gradually decreases. For the 1.2E-400 sample, the TYS and the UTS drop to 347.2 ± 2.1 MPa and 349.4 ± 2.8 MPa, while the El increases to a more acceptable value of 12.6 ± 1.4 %. The microstructure analysis reveals that Ce atoms segregate along grain boundaries and dislocations in the Mg-1.2Ce alloy, which can limit the gliding of GBs or the slipping and climbing of dislocations. Additionally, dislocations can be pinned by Mg12Ce precipitates, thereby also restricting dislocation and grain boundary mobility. Consequently, the dynamic recrystallization (DRX) process is delayed, leading to the formation of a bimodal microstructure.

Influence of the distance of a collisional cascade to an edge dipole in [formula omitted]-Fe on dislocation mobility and defect production

In this work we studied the effects of 20 keV collision cascades in alpha iron with a 1/2〈111〉{110} edge dipole using molecular dynamics. We analysed three different cases: a) the primary knock-on atom (PKA) is centred between both dislocations, b) the PKA is closer to one of the dislocations, and c) the PKA is on top of one of the dislocations and directed towards it. Our calculations show enhanced formation of vacancy clusters in the bulk for intermediate distances due to the absorption of self-interstitials by the dislocation during the collision cascade reducing the recombination between vacancies and self-interstitials. This effect results in the formation of jogs at the dislocation and the consequent climb due to the absorption of the self-interstitials. As a result, there is an unbalance between vacancies and self-interstitials produced by the collision cascade, with a significantly larger number of vacancies than self-interstitials in the bulk and the formation of large vacancy clusters. When the collision cascade develops directly on top of the dislocation, jogs are formed due to both the absorption of vacancies and self-interstitials, inducing climb and with the total dislocation length increasing up to several nanometers.

Mechanism of solute hardening and dislocation debris-mediated ductilization in Nb-Si alloy

Niobium (Nb) is sensitive to even minute quantities of silicon (Si) solutes, which are known to induce pronounced hardening. However, the underlying mechanism for hardening remains elusive since the effect of Si solutes on dislocation behavior is unclear. Here, using tensile testing, in-situ microscopy and nanomechanical testing, the behavior of dislocations in dilute Nb-Si alloys, containing from 0 at.% to 0.8 at.% Si, is investigated. We show that the hardness, strength and strain hardening rate increase from two to four times, while the uniform elongation in tension only reduces 50 % as the Si content increases. Dislocations evolve from complex entangled patterns in Nb to parallel long-straight screw dislocation-dominated structures in Nb-Si alloys. In-situ indentation reveals that the origins of the marked hardening in Nb-Si alloy are the reduction of dislocation mobility and cross-slip propensity. Large densities of dislocation debris-superjogs and loops introduced throughout the sample during warm rolling and annealing are found to provide active internal dislocation sources, which explain the minimal ductility loss seen in these Nb-Si alloys. These findings can help guide the alloy design of high-performance refractory materials for extreme temperature applications.

Excellent strength-ductility synergy of Cu-Al alloy with a gradient nanograined-nanotwinned surface layer

Gradient nanostructured (GNS) metallic materials have shown significant potential in achieving excellent mechanical properties. However, the underlying strengthening mechanisms resulting from plastically inhomogeneous deformation in GNS materials, particularly those involving nanotwinned structures, remain inadequately understood. In this study, a gradient nanograined-nanotwinned (GNNT) surface layer was generated on a Cu-Al alloy using the severe plastic deformation technique known as single-point diamond turning. Uniaxial tensile and localized micropillar compression tests were conducted to compare the mechanical properties of GNNT samples with conventional gradient nanograined (GNG) and uniform coarse-grained (CG) Cu/Cu-Al counterparts. The results revealed that the GNNT samples exhibit excellent strength-ductility synergy, with a yield strength of approximately 329 MPa, tensile strength of around 477 MPa, and uniform ductility of about 48 %, thereby demonstrating remarkable strain hardening capacity and mechanical stability. These characteristics are further supported by the observed strong back stress effect, well-preserved mobile dislocation density, and effective suppression of grain coarsening in the GNNT surface layer. In contrast to the evident mechanically induced grain coarsening through grain boundary (GB) migration in the GNG surface layer, grain coarsening through twin boundary (TB) migration in GNNT surface layers is effectively inhibited by Lomer-Cottrell (L-C) locks formed by TB-stacking fault and TB-TB intersections, resulting in significantly enhanced structural stability. Furthermore, the L-C locks also extensively contribute to the high density of mobile dislocations observed in the GNNT samples. These findings provide valuable insights into the optimization of GNS structures for achieving excellent strength-ductility synergy in metallic materials.

Atomistic simulation of the influence of semi-coherent interfaces in the V/Fe bilayer system on plastic deformation during nanoindentation

Semi-coherent interfaces can have a strong influence on the mechanical behavior of bilayer systems, which is seen very clearly under indentation conditions where a well-defined plastic zone interacts directly with the interface. The main aim of this work is to study the influence of a semi-coherent bcc/bcc interface in the V/Fe bilayer system with molecular dynamics (MD) simulations. In particular, the influence of the V layer thicknesses on the apparent hardness of bilayer system is investigated. Our results show that the deformation behavior of pure V and pure Fe resulting from the MD simulations is in good agreement with the literature. Moreover, the MD simulations reveal a significant enhancement of the hardness of V/Fe bilayer system for thinner vanadium layers, resulting from the crucial role of the semi-coherent interface as a barrier to dislocation propagation. This is seen from a detailed analysis of the interaction of mobile dislocations in the plastic zone with misfit dislocations in the interface. Our work shows that dislocation pile-ups at the interface and formation of horizontal shear loops are two key mechanisms dominating the rate and magnitude of plastic deformation and thus contributes to our understanding of mechanical behavior of bilayer systems with semi-coherent interfaces.

The strength and ductility mechanism of gradient fine grains prepared by surface severe plastic deformation

The gradient fine grains on low-carbon steel surface were produced via ultrasonic impact treatment, which led to excellent combination of high strength and ductility. Results showed that the grain size had a gradient increasing from 120 nm at top surface to 5–8μm at the matrix. In the gradient fine grain region, the grain boundary misorientation is in the range of 2–15°. Compared with original microstructure, the surface gradient fine grains can achieve a higher work hardening rate. A digital image correlation(DIC) technology was employed to analyze variations in strain during the tensile deformation. It is demonstrated that the surface gradient distribution of grain size can reduce the stress concentration, resulting in higher ability for uniform deformation. Such a capability is attributed to the decreased dislocation density and Schimid factor due to gradient fine grains distribution. Compared with the original microstructure, a number of dislocation walls (or cells, or tangles) can hinder the movement of dislocations. In addition, the surface gradient fine grains have faster recovery and proliferation of mobile dislocation density, which can lead to higher work-hardening ability. As a result, the surface gradient fine grains can attain a combination of excellent strength and ductility.

Trade-off between local chemical order and lattice distortion in affecting dislocation motion in NbTiZr multi-principal element alloys

Local chemical order (LCO) is energetically favorable in multi-principal element alloys (MPEAs) due to the chemical affinity difference among constituent species. Here we show that the increase of LCO is often accompanied by a decrease of lattice distortion in body-centered-cubic (BCC) MPEAs. This leads to a trade-off between LCO and lattice distortion in their effects on retarding the glide of edge and screw dislocations. With a random solid solution without LCO as a reference, the emergence of LCO at the expense of lattice distortion could weaken the local trapping force for dislocation glide and promote dislocation mobility. However, when the LCO becomes sufficiently large and robust, it acts to hinder dislocation glide. We found that the mobility of edge dislocation is more affected by local lattice distortion, while the kink-pair based screw dislocation motion is more governed by LCO. We further quantitatively evaluate the contribution of LCO and lattice distortion to the overall critical shear stress for dislocation glide using analytical models. For the present equiatomic NbTiZr MPEAs, the critical shear stress is dominated by lattice distortion when the LCO is still limited to chemical short-range order (CSRO). However, LCO could even surpass lattice distortion to become a major contributor to the critical shear stress once it grows to a sufficiently large size. Our work indicates that increasing the degree and extent of LCO may provide an effective means to regulate dislocation motion, shedding light on how to use LCOs to improve the strength of BCC MPEAs.

Multiscale modeling of dislocation-mediated plasticity of refractory high entropy alloys

Refractory high entropy alloys (RHEAs) have drawn growing attention due to their remarkable strength retention at high temperatures. Understanding dislocation mobility is vital for optimizing high-temperature properties and ambient temperature ductility of RHEAs. Nevertheless, fundamental questions persist regarding the variability of dislocation motion in the rugged energy landscape and the effective activation barrier for specific mechanisms, such as kink-pair nucleation and kink migration. Here we perform systematic atomistic simulations and conduct statistical analysis to obtain the effective activation barriers for the mechanisms underlying various types of dislocation motion in a typical RHEA, NbMoTaW. Moreover, a stochastic line tension model is developed to calculate the activation barrier with substantially reduced computational costs. By incorporating the effective activation barriers into the crystal plasticity model, a multiscale simulation framework for predicting the mechanical properties of RHEAs is established. The ambient temperature yield strength of NbMoTaW is well-predicted by the kink-pair nucleation mechanism of screw dislocations, while the strengthening originating from screw dislocations does not predominate at high temperatures. Our work provides a robust foundation for atomistic studies of effective dislocation behaviors in random solution solids, elucidating the intricate relationship between microscopic mechanisms and macroscopic properties.

Elevated-temperature creep properties and deformation mechanisms of a non-equiatomic FeMnCoCrAl high-entropy alloy

This study investigates the high-temperature creep behavior of a novel (Fe50Mn30Co10Cr10)91Al9 non-equiatomic high-entropy alloy (HEA) spanning the temperature range of 873–973 K and stress conditions from 50 to 90 MPa. Microstructural examinations and theoretical analyses are conducted to elucidate the performance parameter and deformation mechanisms of the HEA under sustained high-temperature conditions, providing an theoretical basis for application prospects. The results indicated that the cold-rolled and annealed alloy exhibited excellent creep resistance under conditions of 923 K/50 MPa and 873 K/50 MPa. The creep mechanism of the HEA at 923 K was identified as being primarily controlled by dislocation climb, as determined by stress exponent fitting calculations. Differing from common equiatomic HEAs, creep in these cases is controlled by solute atoms and mobile dislocations mechanisms. Microstructural characterization validated identified creep trends and deformation mechanisms, revealing diverse micro-mechanisms, such as dislocation jogs and new phase precipitation under various deformation conditions. Additionally, the 'climb-dominant mechanism model' was used to discuss the self-consistency of experimental parameters, providing in-depth insights into the contribution of elemental parameter variations to dislocation climb, as well as the reasoning behind creep performance concerning stress and temperature dependencies.

Microstructural evolution and formation of fine grains during fatigue crack initiation process of laser powder bed fusion Ni-based superalloy

This study reports the fatigue failure mechanism at very-high-cycle fatigue (VHCF) regime of laser powder bed fusion (LPBF) GH4169 superalloy through a series of detailed microstructural characterizations, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and energy dispersive spectrometer (EDS). Microstructural characterization of the solution and double-aging post-treated initial material exhibits process-induced imperfections (mainly pores), as well as γ′, γ′′ and δ precipitates, and carbide. The fatigue tests were performed using an ultrasonic fatigue tester (20 kHz). Fatigue fracture analysis suggests there exists a competitive fatigue failure mechanism with surface flaw initiation and internal pore initiation, corresponding to high-cycle fatigue (HCF) and VHCF regime, respectively. Focused ion beam (FIB) samples taken within the fatigue initiation area (FIA) revealed grain refinement and precipitate dissolution behavior. Based on the characterization results, the fatigue crack initiation mechanism was hypothesized: During numerous cyclic loading, the mobile dislocations shear γ′ and γ′′ precipitates, causing their dissolution and local chemical and mechanical alterations near internal pores. This enables twinning and promotes sub-grains formation. Sub-grains refine via localized continuous dynamic recrystallization (CDRX), forming a fine-grained layer that leads to crack initiation. This work reveals how precipitate dissolution contributes to VHCF crack initiation in LPBF GH4169 superalloy, highlighting the potential for extending alloy lifespan by adjusting precipitates and LPBF defects and incorporating these factors into fatigue life predictions for enhanced accuracy.

The Advanced Assessment of Nanoindentation-Based Mechanical Properties of a Refractory MoTaNbWV High-Entropy Alloy: Metallurgical Considerations and Extensive Variable Correlation Analysis

In the present study, a thorough examination of nanoindentation-based mechanical properties of a refractory MoTaNbVW high-entropy alloy (RHEA) was conducted. Basic mechanical properties, such as the indentation modulus of elasticity, indentation hardness, and indentation-absorbed elastic energy, were assessed by means of different input testing variables, such as the loading speed and indentation depth. The obtained results were discussed in terms of the elasto-plastic behavior of the affected material by the indentation process and material volume. Detailed analysis of the RHEA alloy’s nanoindentation creep behavior was also assessed. The effect of testing parameters such as preset indentation depth, loading speed, and holding—at the creep stage—time were selected for their impact. The results were explained in terms of the availability of mobile dislocations to accommodate creep deformation. Crucial parameters, such as maximum shear stress developed during testing (τmax), critical volume for dislocation nucleation (Vcr), and creep deformation stress exponent n, were taken into consideration to explain the observed behavior. Additionally, in all cases of mechanical property examination and in order to identify those input testing parameters—in case—that have the most severe effect, an extensive statistical analysis was conducted using four different methods, namely ANOVA, correlation matrix analysis, Random Forest analysis, and Partial Dependence Plots. It was observed that in most of the cases, the statistical treatment of the obtained testing data was in agreement with the microstructural and metallurgical observations and postulates.

Insights into microstructure evolution and fracture mechanisms of welded joints of high strength steel patchwork components

The hot roll bending patchwork components of B1500HS uncoated ultra-high strength steel processed at industrial scale were welded with filler wire. The microstructure, grain orientation, mechanical properties and fracture behavior of the joint were investigated. The results showed that the weld metal (WM) was mainly composed of lath martensite and lower bainite (LB). Various microstructure appeared in different positions of the HAZ, among which martensite-austenite (M-A) constituents in the incompletely quenched zone and tempered martensite (TM) in the tempered zone were the key factors determining the performance of the joint. The welded joint exhibited complex structure and anisotropic mechanical response due to the decarburization and rolling texture of the original base material (BM). In the tempered zone, lath merge and boundaries disappear lead to the annihilation of dislocations, carbide precipitation leads to reduction of mobile dislocations, and recrystallization and recovery eliminate work hardening. The reduction of the effect of dislocation strengthening results in the formation of a softening belt at 6 mm from the center of the weld, with a minimum hardness of 266 HV. The tensile strength of the joint in the rolling direction (RD) was 983 MPa, and the fracture occurred in the softening belt. Both brittle and ductile fracture features were observed. The reason why the fracture occurs in the high temperature tempered zone instead of the incompletely quenched zone was the difference in the effect of “constraint strengthening”. Fracture occurred through delamination cracking along the “weak interface” owing to precipitated high-hardness cementite particles and the unfavorably oriented ferrite cleavage planes.

Deformation mechanisms and their role in the lack of ductility in the refractory-based high entropy alloy AlMo0.5NbTa0.5TiZr

The deformation mechanisms in the refractory high entropy alloy AlMo0.5NbTa0.5TiZr have been investigated, with their potential impact on ductility. Specifically, the modes of plasticity active in the B2 matrix and in an alloyed version of the phase Al4Zr5 found at grain and subgrain boundaries were explored by use of nano-indentation. For the B2 phase, a combination of weak-beam dark-field and superlattice imaging revealed dissociation of 〈111〉 superdislocations into two superpartial dislocations, bounding an anti-phase boundary (APB), following the reaction: a〈111〉= a/2〈111〉+APB+a/2〈111〉. The dislocations in the B2 phase exhibited long, relatively straight screw segments and short edge segments, such that the mobility of the edge dislocations appears to be higher than that of the screw segments. Evidence of a B2 to bcc slip transmission process has been observed through use of stereo-pair imaging and anaglyph construction. This process involves the transmission of slip from the B2 matrix, followed by the decorrelated motion of the two a/2〈111〉 dislocations in the bcc precipitate, demonstrated by the increased separation of the dislocations within the bcc phase. Regarding the deformation of the Al4Zr5 intermetallic phase, it was found that indentation of this phase resulted in no evidence of dislocation generation and motion, and cracking at the edges of the given indent. It has been concluded that the lack of plasticity of this phase is the cause of the room temperature brittle nature of this alloy.

Effects of terraces and steps on the 4H-SiC BPD-TED conversion rate: A reaction pathway analysis

The practical use of 4H-SiC as a semiconductor material alternative to Si has been investigated by several researchers. However, a key challenge impeding its practical implementation is the elimination of killer defects in the epitaxial layer, such as basal plane dislocations (BPDs), which cause bipolar degradations. The conversion of BPDs into threading edge dislocations is crucial to reduce detrimental mobile dislocations. However, their underlying atomistic mechanisms remain unclear. In this study, the effects of the step height and distance from the step on the contraction of BPDs were determined using a reaction pathway analysis. Notably, the step height did not affect the contraction, and the activation energies for the contraction of the partial dislocation pairs with Burgers vectors closed toward the step were 0.4 (C face) and 0.3 eV (Si face) lower than those for expansion. Conversely, for the partial dislocation pairs with Burgers vectors open toward the step, the activation energies for contraction were 0.4 (C face) and 0.2 eV (Si face) higher than those for expansion. Furthermore, the effect of the step diminished when the distance from the step exceeded 3 nm. The results suggest that the steps prevented contraction, and longer terraces reduced this preventive effect. Therefore, a surface morphology with fewer steps and longer terraces would increase the conversion rate. Furthermore, a low-off-angle substrate and surface polishing would increase the conversion rate, whereas step bunching slightly would decrease it. Macrosteps would decrease the conversion rate as the average distance from the surface to BPDs increased.