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ISIJ International Vol. 64 (2024), No. 4

ISIJ International
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Grid List Abstracts
オンライン版ISSN: 1347-5460
冊子版ISSN: 0915-1559
発行機関: The Iron and Steel Institute of Japan

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キーワードランキング

22 Dec. (Last 30 Days)

ISIJ International Vol. 64 (2024), No. 4

Preface to the Special Issue on “New Developments in Elucidation of Hydrogen Embrittlement Phenomena from the Incubation Stage to Fracture”

Kenichi Takai

pp. 619-619

DOI:

10.2355/isijinternational.64.619

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Effects of the Addition of Alloying Elements on Hydrogen Diffusion and Hydrogen Embrittlement in Martensitic Steel

Tomohiko Omura, Takumi Oyama

pp. 620-629

DOI:

10.2355/isijinternational.ISIJINT-2023-191

抄録

We investigated the effects of the substitutional alloying elements on the hydrogen diffusion and embrittlement properties of as-quenched and subsequently tempered martensitic steels containing Cr, Mo, Mn, and Ni. Hydrogen diffusion coefficients (Ds) of the as-quenched steels, measured via hydrogen permeation tests under cathodic hydrogen charging at 24°C, reduce as a function of the concentration of the added element. The reductions of D are higher for steels comprising Cr and Mo than those for steels containing Mn or Ni. Variation in D is explained based on the index Cr + Mo + 0.01 Mn + 0.1 Ni (at%) at the substitutional element concentration in the solid-solution state. Tempering heat treatments in the range from 200 to 600°C demonstrate small effects on D. Additionally, the formation of residual austenite reduces Ds of the steels comprising Mn or Ni. Regarding the resistance to hydrogen embrittlement, the results of the slow strain rate test under hydrogen charging imply that the addition of Cr or Mo slightly decreases the fracture stresses of the as-quenched and subsequently tempered steels at the same strength levels as that of the base steel. The detrimental effects of Cr or Mo addition are attributed to embrittlement along the grain boundaries or the increase in the concentration of absorbed hydrogen due to the decrease in D. Mn addition significantly decreases the fracture stress primarily owing to the grain boundary embrittlement. In contrast, Ni addition slightly affects the susceptibilities of the steels to hydrogen embrittlement.

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Effect of Ammonium Thiocyanate on Hydrogen Absorption by Cathodic Charging in Weak Alkaline Solution

Takuya Kamisho, Ryuta Ishii, Masayuki Tsuda

pp. 630-636

DOI:

10.2355/isijinternational.ISIJINT-2023-269

抄録

To clarify the mechanism underlying hydrogen absorption enhancement by adding ammonium thiocyanate (NH4SCN) during cathodic charging in a weak alkaline solution, we constructed a kinetic model of the hydrogen evolution reaction and the hydrogen absorption reaction and performed analysis by fitting the kinetic model to the cathodic current density and steady-state permeation current density obtained from hydrogen permeation tests. Hydrogen permeation tests were conducted on pure iron by constant potential polarization at the hydrogen entry side in a weak alkaline solution with or without NH4SCN. The results indicated that NH4SCN enhances hydrogen absorption into pure iron as a result of the increasing surface hydrogen coverage on iron due to the suppression of the chemical hydrogen recombination reaction (Tafel reaction) and the electrochemical hydrogen desorption reaction (Heyrovsky reaction) in this condition.

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Effect of Stretch-forming on Hydrogen Diffusion Behavior in High-strength Steel Sheet

Hayato Nishimura, Saya Ajito, Tomohiko Hojo, Motomichi Koyama, Ken-ichi Fujita, Yuki Shibayama, Hiroshi Kakinuma, Eiji Akiyama

pp. 637-644

DOI:

10.2355/isijinternational.ISIJINT-2023-149

抄録

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated using a newly developed hydrogen visualization technique with an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. Hydrogen permeation through the steel sheet could be monitored via the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated based on changes in the brightness of the Ir complex. Additionally, this hydrogen visualization technique was applied to a stretch-formed steel sheet using a hemispherical punch to simulate the press-forming of automotive structural components. The hydrogen breakthrough time around the top of the specimen increased and then decreased as the distance from the top increased. Based on the plastic strain distribution of the specimen calculated using the finite element method, the hydrogen breakthrough time increased with the plastic strain. The introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, thus increasing the hydrogen breakthrough time.

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Carbide Precipitation and Hydrogen Trapping Behavior in Mo and V Added Tempered Martensitic Steel

Miyuri Kameya, Shunsuke Taniguchi, Yukiko Kobayashi, Naoki Matsui, Shingo Yamasaki

pp. 645-654

DOI:

10.2355/isijinternational.ISIJINT-2023-304

抄録

To investigate the hydrogen trapping effect of the combined addition of V to Mo-added steel, 0.1C-2Mn-1.6Mo mass% steel (Steel A) and 0.1C-2Mn-1.6Mo-0.2V mass% steel (Steel B) were prepared, quenched, and tempered at 873 K. The hydrogen trapping effect was investigated by thermal desorption hydrogen analysis of hydrogen-charged specimens, and Steel B showed a higher hydrogen trapping capacity than Steel A. According to thermodynamic equilibrium calculations, hydrogen trapping site of Steel A and B after tempering were predicted as M2C carbides. However, according to TEM observation of these specimens, not only coarse M2C but fine MC carbides precipitated in Steel A, and only fine MC precipitated in Steel B. Chemical composition of these precipitates were investigated by the three-dimensional atom probe analysis. MC of both Steel A and B show a composition close to MC0.5, in which Mo is the primary element in metal sites. It was found that the carbon-site vacancy (C vacancy) ratio of MoC0.5 in the present work is higher than that of V4C3 (VC0.75). The hydrogen trapping capacity showed a good correlation with the product of the area of Fe–MC interface and the C vacancy ratio in MC. The reason of the higher hydrogen trapping capacity of Steel B than that of Steel A is considered as below. 1) The combined addition of V to Mo assisted the precipitation of MC instead of coarse M2C. 2) C vacancies in MC were increased by the partitioning of Mo into MC, and the vacancies acted as hydrogen trapping sites.

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Visualization of Hydrogen and Hydrogen-induced Defects in Tensile-deformed Pure Iron Using Hydrogen Microprint and Tritium Autoradiography

Toshiaki Manaka, Goroh Itoh, Junya Kobayashi, Shigeru Kuramoto, Yuji Hatano

pp. 655-659

DOI:

10.2355/isijinternational.ISIJINT-2023-279

抄録

To understand the process and mechanism for hydrogen embrittlement in steels, visualization of the location of hydrogen is essential. In the present study, two visualization techniques, hydrogen microprint technique (HMT) and tritium autoradiography (TAR), were applied to a pure iron sheet 20% tensile-deformed with cathodic hydrogen charging. When the specimen was covered with photographic emulsion shortly (40 min) after the deformation, HMT showed that the charged hydrogen atoms diffused out at majorly grain boundaries and minorly in the grain interiors. The TAR, conducted on the same sample but completely de-hydrogenated and then charged with tritium, revealed that hydrogen enhances the formation of vacancies or vacancy clusters with plastic deformation, which are located along grain boundaries and deformation bands and act as relatively stable trapping sites for tritium.

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Relationship between Three-dimensional Crack Morphology and Macroscopic Mechanical Properties of Hydrogen-related Fracture in Martensitic Steel

Akinobu Shibata, Yazid Madi, Jacques Besson, Akiko Nakamura, Taku Moronaga, Kazuho Okada, Ivan Gutierrez-urrutia, Toru Hara

pp. 660-667

DOI:

10.2355/isijinternational.ISIJINT-2023-316

抄録

In the present study, several parameters related to crack morphology in the case of hydrogen embrittlement were estimated by X-ray computed tomography and correlated with the macroscopic mechanical responses (J-integral and tearing modulus) obtained from the fracture mechanics tests. Even when the hydrogen content was high up to 4.00 wt ppm, unstable premature fracture did not immediately occur, and a certain crack-growth resistance could be confirmed. The three-dimensional crack morphology was not continuous with the formation of un-cracked ligaments in the uncharged specimen. In contrast, the hydrogen-related intergranular crack propagated more continuously with a smaller crack opening-displacement. The J-integral value monotonically increased with increasing estimated values of the surface area divided by the projected surface area on the macroscopic crack plane, indicating that crack meandering and branching increased the fracture energy. We defined crack-propagated thickness (standard deviation of the crack surface area at each section (parallel to the macroscopic crack plane) divided by the crack surface area) as a parameter representing crack meandering. The tearing modulus increased as the crack-propagated thickness increased, suggesting that crack meandering also increased the crack-growth resistance.

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Comparison of Crack Initiation Sites and Main Factors Causing Hydrogen Embrittlement of Tempered Martensitic Steels with Different Carbide Precipitation States

Naoki Uemura, Takahiro Chiba, Kenichi Takai

pp. 668-677

DOI:

10.2355/isijinternational.ISIJINT-2023-026

抄録

The dependence of crack initiation sites and main factors causing hydrogen embrittlement fracture on carbide precipitation states has been investigated for tempered martensitic steels with the same tensile strength of 1450 MPa. Notched specimens charged with hydrogen were stressed until just before fracture and subsequently unloaded. The crack initiation site exhibited intergranular (IG) fracture at 21 µm ahead of the notch tip as observed by scanning electron microscopy (SEM) for 0.28% Si specimens with plate-like carbide precipitates on prior austenite (γ) grain boundaries. This crack initiation site corresponded to the vicinity of the maximum principal stress position as analyzed by a finite element method (FEM). The initiation site corresponded to the triple junction of prior γ grain boundaries as analyzed by electron backscattered diffraction (EBSD). In contrast, the crack initiation site exhibited quasi-cleavage (QC) fracture at the notch tip for 1.88% Si specimens with fine and thin carbide particles in the grains. This crack initiation site corresponded to the maximum equivalent plastic strain site obtained by FEM. Additionally, the crack initiated on the inside of prior γ grain boundaries and propagated along the {011} slip plane with higher kernel average misorientation (KAM) values as analyzed by EBSD. These findings indicate that differences in carbide precipitation states changed the crack initiation sites and fracture morphologies involved in hydrogen embrittlement depending on mechanical factors such as stress and strain and microstructural factors.

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Hydrogen Content Dependence of Crack Initiation and Propagation Behavior of Hydrogen Embrittlement in Tempered Martensitic Steel

Naoki Uemura, Takahiro Chiba, Kei Saito, Kenichi Takai

pp. 678-687

DOI:

10.2355/isijinternational.ISIJINT-2023-351

抄録

Crack initiation and propagation behavior in hydrogen embrittlement fracture of tempered martensitic steel at a low hydrogen content was compared with the results at a high hydrogen content. Notched specimens charged with a low hydrogen content of 0.18 ppm and a high hydrogen content of 5.3 ppm were stressed and unloaded immediately upon reaching the maximum stress in tensile tests. At the low hydrogen content, quasi-cleavage (QC) fracture was dominant at the notch tip, and mixed intergranular (IG) and QC fractures were observed away from the notch tip. A crack initiated in the prior γ grains at the notch tip and propagated along the {011} plane. The crack initiation site corresponded to the site of maximum equivalent plastic strain. The other crack initiating on the prior γ grain boundaries was observed at a site away from the notch tip. Microvoids were formed discontinuously inclined at about 45° to the tensile axis direction between these two types of cracks observed at the low hydrogen content. In contrast, at the high hydrogen content, cracks initiated on the prior γ grain boundaries away from the notch tip. The crack initiation site corresponded to the vicinity of the region where both the principal stress and hydrogen concentration were high. These findings indicate that crack initiation at the low hydrogen content is not necessarily consistent with the site of the maximum principal stress and the local hydrogen concentration, unlike the case of the high hydrogen content.

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Grain Refinement Effect on Resistance to Hydrogen-assisted Crack Growth in Equiatomic CoCrFeNi High-entropy Alloy with Different H Charging Conditions

Taein Kong, Haoyu Wang, Taekyung Lee, Motomichi Koyama, Eiji Akiyama

pp. 688-695

DOI:

10.2355/isijinternational.ISIJINT-2023-218

抄録

The hydrogen embrittlement (HE) resistance of a fine-grained equiatomic CoCrFeNi high-entropy alloy (HEA) is investigated via tensile testing under electrochemical H charging. The HE behavior is compared with that of HEA specimens charged with 100 MPa of H gas. The fine-grained HEA shows > 40% elongation with a tensile strength of ~800 MPa under electrochemical H charging. Meanwhile, H gas-charged specimens with a uniform distribution of H show deformation twin-related intergranular cracks, whose initiation length decreases owing to grain refinement. Such small cracks, which feature blunted tips, do not significantly affect the fracture of the specimens. The electrochemically H-charged specimens exhibit numerous surface cracks because of their higher surface H content compared with that of the H gas-charged specimens. Nevertheless, similar to the case of the H gas-charged specimens, most of the cracks do not propagate significantly. In conclusion, fine-grained HEA exhibits remarkable resistance to H-related crack growth.

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Hydrogen Effects on Fracture Resistances of Bulk Cementite Evaluated by in-situ Microbending Test during Cathodic Hydrogen Charging

Kota Tomatsu, Masahiro Sasaki, Takahiro Aoki, Tomohiko Omura

pp. 696-705

DOI:

10.2355/isijinternational.ISIJINT-2023-311

抄録

Hydrogen absorption characteristics and mechanical properties in hydrogen environment of cementite were evaluated by low-temperature thermal desorption analysis and in-situ microbending tests during cathodic hydrogen charging using bulk cementite plates obtained through a vacuum carburizing process. In the low-temperature thermal desorption analysis, no hydrogen desorption was identified up to 1073 K. In the microbending test, notched microcantilevers experienced cleavage fracture in an elastic deformation range in air. The cathodic hydrogen charging increased fracture load (i.e., fracture toughness) with appearance of plasticity while it did not change the fracture surface morphology and Young’s modulus. In the present hydrogen charging conditions, the hydrogen atoms are present only near the specimen surface because of high hydrogen migration energy in the cementite. It seems that no hydrogen desorption is detected because the hydrogen atoms are absent in most regions of the specimens. The invariance of the Young’s modulus and the fracture surface morphology can be explained by the same reason. On the other hand, it is considered that the fracture toughness is improved because the hydrogen atoms charged near notch bottom of the microcantilever enhance dislocation nucleation and glide, and cause blunting of the notch during the bending.

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Hydrogen Embrittlement Property of a 1.5 GPa Dual-Phase Steel Evaluated Using U-bending Test: A Comparison with Tempered Martensitic Steel with Identical Tensile Strength

Shuya Chiba, Motomichi Koyama, Tomohiko Hojo, Saya Ajito, Yuki Shibayama, Rama Srinivas Varanasi, Eiji Akiyama

pp. 706-713

DOI:

10.2355/isijinternational.ISIJINT-2023-287

抄録

The hydrogen embrittlement susceptibilities of 1.5 GPa-class dual-phase (DP) and tempered martensitic steels were comparatively evaluated using U-bending test. The U-bending tests showed no differences in hydrogen embrittlement susceptibility between the two steels. The martensitic and DP steels exhibited intergranular and quasi-cleavage fractures, respectively. The crack initiation site reached the outer surface of the specimen with increasing bolt-tightening displacement. Although qualitative trends were observed between the positions of the maximum principal stress peak and the crack initiation site, the two positions significantly differed, particularly in the DP steel. This difference could arise from premature hydrogen diffusion and the plastic strain effect.

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Effect of Hydrogen Concentration on Nanoindentation Softening and Hardening in Iron: Ferrite Phase of S25C and Single-crystal Fe-3wt.%Si

Shinya Taketomi, Toshiki Taniguchi, Hiroki Yamamoto, Seiya Hagihara, Sadahiro Tsurekawa, Ryosuke Matsumoto

pp. 714-721

DOI:

10.2355/isijinternational.ISIJINT-2023-250

抄録

Plastic deformation is key to understanding hydrogen embrittlement in steels. Although macroscopic to microscopic observations and analyses have reported softening and hardening behavior in the presence of hydrogen, the mechanisms causing these effects and the overall mechanisms remain unclear. Therefore, this study applies nanoindentation tests to the ferrite phase of bcc-structured polycrystalline carbon steel S25C and single-crystalline iron with 3 wt.% silicon (Fe-3wt.%Si). The change in indentation work is evaluated by varying the exposure time in air between the uncharged and hydrogen-charged materials, focusing on the hydrogen concentration. The change in work per unit indentation volume caused by hydrogen (hydrogen-induced work) is investigated, revealing that the specimens harden immediately upon hydrogen charging, and then gradually soften with increasing exposure time before returning to their original state. This variation is attributed to the change in hydrogen concentration. The softening and hardening behavior with and without hydrogen, as confirmed by nanoindentation tests, is suggested to quantitatively affect the macroscopic mechanical response, which is determined by the mobility of screw dislocations in these materials.

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Effect of Residual Stress on Hydrogen Embrittlement at Sheared Edge

Yuji Sakiyama, Tomohiko Omura, Takashi Yasutomi, Takayuki Harano, Kengo Noami

pp. 722-731

DOI:

10.2355/isijinternational.ISIJINT-2023-227

抄録

The residual stresses at a circular punched end face in tempered martensitic high-strength steel sheets were investigated using triaxial stress analysis via X-ray diffraction. The maximum principal stress and its direction were calculated from the measured nine stress components. The relationship between the directions of the maximum principal stress and hydrogen cracks was verified by generating hydrogen cracks on the punched end face in the same specimen using cathodic hydrogen charging. The direction of the cracks was perpendicular to that of the maximum principal stress. This result indicates that hydrogen embrittlement at the sheared end face is caused by the maximum principal stress. Moreover, the distribution of the residual stresses toward the thickness direction and the relationship between residual stresses and tensile strength of the specimens were investigated. The maximum principal stress on the punch side was lower than that on the dice side. Unlike the maximum principal stresses, the normal stresses did not increase monotonically with the tensile strength of the specimens. Therefore, it was concluded that investigating the maximum principal stress at any area between the dice side and a line located midway from the end face and dice side is crucial for considering the hydrogen embrittlement criteria.

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Overview of Hydrogen Effects on γ−ε Martensitic Transformation in Steels

Motomichi Koyama, Satoshi Iikubo, Rama Srinivas Varanasi

pp. 732-741

DOI:

10.2355/isijinternational.ISIJINT-2023-243

抄録

This paper presents an overview of our recent works on the effects of hydrogen on γ-ε martensitic transformations in steels. The study first discusses how hydrogen impacts these transformations. While hydrogen suppresses thermally-induced γ–ε martensitic transformation, it increases the fraction and number density of deformation-induced ε-martensite and decreases its thickness. Secondly, we discuss the effects of γ–ε martensitic transformations on hydrogen kinetics. The study also highlights the significance of low hydrogen diffusivity in the hexagonal-close-packed (HCP) lattice of pure iron, demonstrating the effectiveness of ε-martensite in resisting hydrogen. Moreover, the characteristic behavior of the HCP phase-related diffusionless transformation from a hydride is discussed. We believe that this overview will assist in developing hydrogen-resistant steels and in exploring new microstructural control concepts using hydrogen.

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Delayed Fracture Enhanced by Martensite Transformed from Retained Austenite in Ultra-high Strength Steel Sheet

Katsutoshi Takashima, Takamasa Nishimura, Ken’ichi Yokoyama, Yoshimasa Funakawa

pp. 742-750

DOI:

10.2355/isijinternational.ISIJINT-2023-339

抄録

Delayed fracture of ultra-high strength complex phase (CP) steel sheets containing either a small amount of retained austenite or substantially no retained austenite was investigated by a sustained tensile-loading test during hydrogen charging. The microstructures of both specimens are composed of martensite and bainitic ferrite. Most of the retained austenite exists as a secondary phase in the bainitic ferrite or near prior austenite grain boundaries. In comparison with the specimen containing substantially no retained austenite, the amount of absorbed hydrogen in the specimen containing a small amount of retained austenite increases under the same charging conditions, and the time to fracture increases under the same applied stress lower than the yield stress. Upon pre-deformation before the sustained tensile-loading test, the retained austenite transforms to martensite, and the time to fracture decreases significantly. Moreover, the morphology of the fracture initiation area also changes from a mixture of quasi-cleavage and intergranular to quasi-cleavage. In the fractured specimens without pre-deformation, many cracks initiate in martensite and at prior austenite grain boundaries, although a small number of cracks are also observed in the bainitic ferrite and at the interface between different phases. Upon pre-deformation, the number of cracks in martensite increases, particularly in the specimens containing martensite transformed from retained austenite (transformed martensite). The probable reason for the decrease in the time to fracture in the specimens subjected to pre-deformation is that cracks readily nucleate in martensite and transformed martensite. The results of the present study indicate that pre-transformed martensite substantially enhances delayed fracture.

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Examples of Fractured Steel Parts during Actual Usage due to Hydrogen Embrittlement and Consideration Regarding the Behavior of Hydrogen at the Prior Austenite Grain Boundary

Hiroshi Yaguchi

pp. 751-755

DOI:

10.2355/isijinternational.ISIJINT-2023-264

抄録

Steel parts fractured during actual use were investigated, and the mechanism of fracture for some of them was attributed to hydrogen embrittlement. This article describes a summary of the observation results fractured by hydrogen embrittlement. Fracture occurred under tensile stress within the elastic limit including residual stress. Most of the parts had a martensitic microstructure, and intergranular fracture surfaces were always observed in martensitic steels at least on part of the fracture surface. This observation indicates the importance of the role of hydrogen on the prior austenite grain boundary in hydrogen embrittlement.

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Influence of High Concentration Vacancy-Type Defects on the Mobility of Edge Dislocation in α-Iron: An Atomistic Investigation

Sunday Temitope Oyinbo, Ryosuke Matsumoto

pp. 756-764

DOI:

10.2355/isijinternational.ISIJINT-2022-356

抄録

Many vacancy-type defects (vacancy, vacancy clusters, and hydrogen-vacancy complexes) are generated in metals by plastic deformation in hydrogen environments. In this study, we use extensive molecular dynamics calculations based on a highly accurate interatomic potential to examine how vacancy-type defects affect the mobilities of edge dislocations in α-iron at a temperature range of 300–500 K and a dislocation speed Vd range of 0.1–10 m/s. Under all conditions, the edge dislocation absorbs the vacancies along the slip plane and causes them to migrate with the edge dislocation. Although the necessary shear stress to glide edge dislocation in α-iron containing vacancy increases with dislocation speed, the effect is small compared to the hydrogen effects. The dislocation absorbs the hydrogen-vacancy complex along the slip plane and causes the hydrogen and the jog to migrate with the edge dislocation at low dislocation velocity regimes (Vd ≤ 0.1 m/s). Therefore, the hydrogen-vacancy complex exerts a continuous drag effect on the dislocation. At higher dislocation speeds (Vd ≥ 1 m/s), hydrogen does not migrate with the dislocation, resulting in the formation of isolated hydrogen detached from the dislocation and diffused into the material; only vacancy is absorbed. When multiple hydrogen-vacancy complexes are arranged along the slip plane, the dislocation absorbs them if they interact with dislocation at different points rather than at a single point to avoid the formation of a large jog at the colliding segment, and the required shear stress increases as the hydrogen atoms in the dislocation core increase.

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Nucleation of Nano-sized Prismatic Dislocation Loop from Spherical Vacancy Clusters in α-iron: An Atomic-scale Study

Mugilgeethan Vijendran, Ryosuke Matsumoto

pp. 765-771

DOI:

10.2355/isijinternational.ISIJINT-2023-338

抄録

The concentration of vacancy-type defects in α-iron is increased by plastic deformation and the presence of hydrogen. This leads to the accumulation of monovacancies, either in the form of planar vacancy clusters (VCs) or small voids. The prismatic dislocation loop (PDL) can nucleate from VC as vacancies agglomerate into two-dimensional (2D) VCs and collapse due to attractive force between two interior surfaces. The transition between 2D-VC and PDL is comparatively more straightforward, requiring only a short displacement without the need for atom diffusion to reach stability. However, the most stable VC configuration is three-dimensional (3D) (spherical cluster), which have lower formation energy than 2D-VCs. Despite their stability, the transformation from 3D-VC to PDL is complex, involving the diffusion of multiple atoms. A quantitative energy barrier is established for transitioning from 3D-VC to nano-sized 1/2<111> PDLs using an approach that combines the reaction rate theory and molecular dynamics (MD) simulations. The nucleation of PDL from a spherical cluster composed by 15 vacancies is a rare event at room temperature, even under considerable compressive strain since the activation energy is 1.33 eV. In contrast, 2D-VC with 37 vacancies can be nucleated to PDL with an energy barrier of 0.61 eV.

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