Cracks In Nickel Research Articles

Improvement of Probabilistic Fracture Mechanics Analysis Code PASCAL-SP Regarding Stress Corrosion Cracking in Nickel Based Alloy Weld Joint of Piping System in Boiling Water Reactor

Abstract In the past few decades, cracks because of stress corrosion cracking (SCC) have been detected in the dissimilar weld joints using nickel-based alloy in piping system of boiling water reactors (BWRs). Thus, the structural integrity assessment for such weld joints has become important. Nowadays, probabilistic fracture mechanics (PFM) analysis is recognized as a rational method for structural integrity assessment because it can consider inherent uncertainties of various influencing factors as probability distributions and quantitatively evaluate the failure probability of a cracked component. The Japan Atomic Energy Agency has developed a PFM analysis code PASCAL-SP for a weld joint of piping system in nuclear power plant. This study improves the analysis functions of PASCAL-SP for weld joint using nickel-based alloy in BWR susceptible to SCC. As an analysis example of the improved version of PASCAL-SP, the failure probability of a weld joint is quantitatively evaluated. Furthermore, sensitivity analyses are conducted concerning the effect of leak detection and in-service inspection. From the analysis results, it is concluded that the improved version of PASCAL-SP is useful for structural integrity assessment.

Microcrack initiation mechanism in hydrogen induced cracking in nickel

ABSTRACT It is well known that cracks are observed on the surface without load when pure nickel is left in ambient air after hydrogen charge. It has been due to the decomposition of nickel hydride generated during hydrogen charging. In the present work, microcracks were investigated at the surface and inside the specimen with suppressing the decomposition of nickel hydride in polycrystalline pure nickel. As a result, microcracks initiate in the nickel matrix near the hydride inside the specimen before hydride decomposition. The microcracks grow toward the specimen’s surface as hydride decomposition.

Crack Initiation Mechanism of Hydrogen Induced Cracking in Nickel

Cracks are induced during hydrogen charging in Nickel. We have reported that the cracks initiate near the boundary between nickel matrix and nickel hydride formed near the surface by hydrogen charging. The driving force is considered to be residual stress caused by hydride formation. In the present paper, the stress distribution has been calculated near the boundary using Finite element method to clarify the crack initiation mechanism. As a result, the high-stress area corresponded to the area where the cracks were observed in the experiment.

Susceptibility of different crystal orientations and grain boundaries of polycrystalline Ni to hydrogen blister formation

The effects of crystallographic orientations, grain boundaries (GBs) and the possible contribution of dislocations to the diffusion of hydrogen were studied. Our experimental results show that the (001) crystal orientation has the minimum number of hydrogen induced blisters compared to other crystal orientations. We observed formation of blisters along the slip traces after plastic deformation and along special GBs. It shows that the interaction between dissolved hydrogen and lattice defects (e.g. dislocations and GBs) could cause void formation and ultimately induce intergranular and/or transgranular cracks in nickel. In this work we analyzed hundreds of GBs mostly with a misorientation of less than 60°. It was observed that the random GBs with a misorientation between 30 and 40° have the fastest hydrogen diffusion rate and are very sensitive to hydrogen segregation. In contrast, random GBs with a misorientation below 25°, low angle GBs and the coincidence site lattice (CSL) GBs are much less prone to blister formation.

Elucidating the contribution of mobile hydrogen-deformation interactions to hydrogen-induced intergranular cracking in polycrystalline nickel

Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen (H)-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile H-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for H-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.

Effects of phosphorus content on generation and growth of cracks in nickel–phosphorus platings owing to thermal cycling

We observed crack generation and structural changes in electroless nickel–phosphorus (Ni–P) plating layers formed on copper-metalized silicon nitride substrates both during thermal cycling from − 40 to 250 °C and during storage (not cycling) at 250 °C in order to investigate the effect of the phosphorus contents on crack generation and growth in the Ni–P platings. The used platings contained phosphorus at three different contents: 2.1 wt% [Ni–P(low)], 6.5 wt% [Ni–P(med)], and 10.9 wt% [Ni–P(high)]. The generation time and the amount of cracks were strongly dependent on their phosphorus contents. More cracks appeared after thermal cycling than after storage at 250 °C. In Ni–P(low), cracks were generated after 200 thermal cycles, whereas no cracks were observed even after 250 h of storage at 250 °C. In Ni–P(med) and Ni–P(high), both during thermal cycling and storage at 250 °C, cracks formed during or after crystallization of the amorphous layers. These results suggest that the primary factors affecting the generation of cracks in electroless Ni–P platings are crystallization of the Ni–P platings and repeated changes in thermal stress.

The Combination Addition of 2-Mercapto-5-benzimidazolesulfonic Acid and Thiourea to Watts Bath in Controllable Electro-Healing Cracks in Nickel

Based upon previous investigations on crack electro-healing by the addition of only inhibitor(s), the present paper concentrates on the exploration of the crack electro-healing behavior in pure Ni with the combination presence of an inhibitor 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) and an accelerator thiourea (TU) in Watts bath. Voltammetry was performed to characterize the electrochemical aspect when these two additives were added. Current transient and injection experiments were conducted to study the synergistic effect of MBIS and TU to Ni deposition. It is proposed that TU will displace some of the MBIS molecules adsorbed on the electrode surface, which will break up the blocking layer of MBIS, therefore avoid the formation of annual-ring-like defects that form when only MBIS is present.

Evaluating the Performance of 2-Mercapto-5-Benzimidazolesulfonic Acid in Controllable Electro-Healing Cracks in Nickel

Healing or repairing cracks in materials is of significant importance for sustainable development of the world. As the mobility of metal atoms is quite limited at service temperature, it remains a challenge to realize crack healing in metallic materials. Recently, an electrochemical process named electro-healing was introduced to heal cracks in nickel. Meanwhile，it is also recognized that to get a satisfactory recovery of strength, it is critical to minimize pores or voids in the healed crack by controlling the growth mode of healing crystals, which might be realized by the addition of additives to the healing solution. In our recent work, we found that controllable electro-healing cracks was achieved by the addition of polyethyleneimine (PEI) and saccharin to the Watts bath under free convection. Nevertheless, some defects were found along the interface between the substrate and the healing crystals when PEI and saccharin were added to the solution, which might be attributed to the lowered current efficiency. As with the presence of inhibitors, hydrogen evolution, the secondary reaction on the cathode, becomes less ignorable, the definite result of which is the reduction of current efficiency. Hence, additives with relatively higher cathode current efficiency has to be introduced to the controllable electro-healing of cracks. 2-mercapto-5-benzimidazolesulfonic acid (MBIS) has been found to an effective inhibitor in the reduction of nickel. In the present work, different concentration of MBIS was added to conventional Watts bath to systematically evaluate the performance of MBIS in the controllable electro-healing on thin pure Ni plate with a through thickness crack. Voltammograms under different concentration and convection condition were carried out to evaluate the inhibition ability of MBIS to the reduction of nickel. Rotating disk electrode (RDE) with rotation speeds of 50 rpm, 300 rpm and 2000 rpm was employed to assess the convection dependent performance of MBIS. The cathodic current efficiency and surface coverage were examined. These characterizations together with the SEM observation of the healed crack were analyzed to get the correlation between the addition of MBIS and the electro-healing process of crack. It is demonstrated that with the addition of MBIS, a hysteresis loop forms in the positive-scan and negative-scan voltammetry. With the increment of MBIS concentration, the loop becomes larger, the onset of the inhibition breakdown moves to a more negative value, illustrating that the inhibition of MBIS is concentration dependent. This is in accordance with the reported result. The inhibition is also found to be convection dependent, with a similar behavior to concentration when the rotation speed of RDE increases. One interesting phenomenon in the voltammetry of MBIS-added solution is the transition between the breakdown of inhibition (active) and the rebuilt of inhibition (passive). The phenomenon only occurs when the MBIS concentration or the rotation speed of RDE is higher than a certain value. The phenomenon has not been observed with the presence PEI at a given concentration. If a minimal flux of inhibitor is required to maintain the inhibition blocking layer in the passive state, this transition from passive to active may imply a fast mass transfer of MBIS at higher concentration or higher intensity of convection. Additionally, it is found that at a given concentration the current efficiency of MBIS is much higher than PEI but with a similar surface coverage. The morphology of the healed crack reveals that the healing crystals grows in a controllable manner. The reduction of Ni on the sidewalls has been consistently inhibited, while the growth of healing crystals has been much less restricted from the crack tips. No defects were found along the interface between the substrate and the healing crystals which is believed to be the result of the much higher current efficiency. Although, annul-ring like defects still exist, the average distance between the annual-ring of MBIS is much larger that that of PEI, indicating a longer active growth period of healing crystals from crack tips. Further in-depth analysis is still underway to understand the phenomenon.

Evaluating the Performance of 2-Mercapto-5-Benzimidazolesulfonic Acid in Controllable Electro-Healing Cracks in Nickel

The performance of 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) in a Watts bath in crack electro-healing was investigated. Potentiostatic and galvanostatic voltammetry, current efficiency measurements were performed for electrochemical investigation. The morphology of the healed crack indicates that the healing crystals grow in a controllable manner with the addition of MBIS under forced convection with the healing crystals fill preferentially in the crack tip to the crack sidewalls. No obvious defects were observed along the interface between the substrate and the healing crystals, which is attributed the much higher current efficiency of MBIS. Annual-ring like defects are observed but with a much longer width compared with PEI. The formation mechanism of annual ring is discussed. Possible approach to get defect-free crack healing is also prospected.

An Electrochemical Investigation on Additives to the Controllable Electro-Healing Cracks in Nickel

In the present work, polyethyleneimine (PEI), 2-mercapto-5-benzimidazolesulfonic acid (MBIS), MBIS and thiourea (TU) were added respectively to the conventional Watts bath to investigate the controllable electro-healing on pure Ni plate with a through thickness crack. It is demonstrated that PEI has a strong inhibition effect on the Ni reduction, which resulted in the formation of large visible defects along the interface between the substrate and the healing crystals. MBIS has a similar inhibition effect on Ni reduction, but with a promoted current efficiency. The addition of TU to the MBIS added solution depolarized the Ni reduction, resulting in an enhanced growth of healing crystals from sidewalls of the crack.

Investigations of twin boundary fatigue cracking in nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Implant retrieval studies have indicated that the primary cause of failure in stainless steel devices is fatigue, and time or cycles required for fatigue crack initiation often consumes the majority of implant lifetime. Stainless steels with significant nitrogen additions have shown an improved fatigue response, but have also shown a peculiar preference for fatigue crack initiations at or along annealing twin boundaries in the face-centered cubic (FCC) materials. In a recent comparison study on cold-worked implant grade stainless steels, a number of fatigue crack initiations were found along former annealing twin boundaries on both nitrogen-stabilized austentitic (HNASS) and nickel-stabilized austenitic steels. Further investigations were warranted to determine the crystallographic conditions present around these annealing twin boundary cracks, since not every twin boundary showed crack initiation. The present study examined the crystallographic conditions present around each of the former annealing twin boundary cracks relative to the applied loading direction. It was determined that the former annealing twin boundary cracks showed the complete range of misorientation deviations allowed by the Brandon criterion. The textures of the cracked twin boundaries were found to be random relative to the overall global textures of the materials. Most of the cracked twin planes in the HNASS steel were shown to be high angles, and in many cases were nearly perpendicular to the material surface. The nickel-stabilized steel showed a preference for lower twin plane inclination angles relative to the material surface. High Schmid factors were shown for all grains surrounding the cracked twin boundaries indicating each grain was oriented favorably for slip relative to the applied loading direction. A high Taylor factor mismatch was also shown across most of the cracked twin boundaries in both steels indicating strong difference in expected yield response for each of the grains which suggest localized strain incompatibility was another important factor in twin boundary cracking.

An Eectrochemical Investigation on Additives to the Controllable Electro-Healing Cracks in Nickel

Healing or repairing cracks in materials is highly desired not only from economic considerations but also for their sustainable and reliable applications. For metallic materials, crack healing is challenging due to the limited mobility of metallic atoms in solid state at ambient temperature. Recently, it is reported1 that an electrochemical process named electro-healing is capable of repairing through-thickness cracks in nickel where metallic ions in electrolytes were introduced as the healing agent. However, in plain healing solutions when there is no additives, the healing crystals grow following a general mode, in which the healing crystals grow oppositely from the two sidewalls of the crack towards the meeting line (plane), where randomly-located micrometer-sized voids or smaller are sometimes observed, see Fig.1 (a). As these voids are detrimental to the load-bearing ability of the healed crack, it is essential to control the growth of the healing crystals to minimize the number of voids that left after electro-healing2. It is well-established that the addition of inhibitors or accelerators to an electrolyte is capable of suppressing or accelerating the metal deposition rate. In the present work, two inhibitors: polyethyleneimine (PEI) and 2-mercapto-5-benzimidazolesulfonic acid (MBIS), and an accelerator thiourea (TU) were added respectively to the conventional Watts bath to investigate the controllable electro-healing on pure Ni plate with a through thickness crack. Rotating disk electrode (RDE) was introduced to evaluate the electrochemical behavior in crack tips and crack center. Voltammograms and galvanostatic measurements were carried out to evaluate the inhibition effect of PEI, MBIS, synergistic effect of MBIS+TU to the reduction of Ni ions. The cathodic current efficiency was also investigated. SEM observations were used to characterize the morphology of the healed cracks. It is demonstrated that the addition of PEI greatly inhibited the reduction of Ni ions. This inhibition is convection dependent. As a result, the growth of healing crystals from the sidewalls was much restricted compared to that from the crack tips((ν s <<ν t ) when strong convection was employed to the solution. No meeting line was observed in the healed crack, instead, annual-rings and some large visible defects were found in the healed crack(Fig.1(b)). The former is believed to be the results of fluctuation of PEI diffusion flux, while the latter is attributed to the hydrogen inclusion corresponding the much reduced current efficiency. MBIS exhibited a similar inhibit effect to Ni reduction to PEI when added to the Watts bath. The surface adsorption kinetics of MBIS is also controlled by transportation. The difference is that the solution with MBIS demonstrated much higher current efficiencies compared with PEI when similar amount of inhibitor were added(100µM). For example, at a current density of 1 A/dm2, the current density of solution with PEI is about 70 %, while that with MBIS is about 97 % when the rotation speed of RDE is 50 rpm. Consequently, except annual rings, no visible defects were spotted in the acquired morphology (Fig.1(c)). From the linear sweep voltammetry (LSV), it is observed that as an accelerator, the addition of TU to the MBIS containing solution demonstrated a depolarization effect to the Ni reduction,the growth of healing crystals from sidewalls was much enhanced when TU was presented. A meeting line appeared again(Fig.1(d)). But different from that of plain solution without additives, there is no voids or pores left along the meeting line after electro-healing. Instead of that perpendicular to the meeting line, the healing crystals grew at an acute angle to the meeting line, which is believed to be the consequence of simultaneous growth from crack sidewalls and the crack tip. It is expected that a satisfactory recovery of load bearing ability will be accomplished after the controllable electro-healing with the co-addition of both MBIS and TU. Fig. 1 SEM morphologies of cracks after electro-healing in different solutions: (a) Plain Watts bath without additives(general growth mode); (b) 35µM PEI, (c) 50µM MBIS and (d) combination of 50µM MIBS and 20µM TU. Black arrows indicate the growth direction of the healing crystals, white arrows in (a) indicate pores and voids. White dot-dashed line in (a) and (d) designates the meeting line. Watts bath contains: 1M NiSO4•6H2O, 0.2 M NiCl2•6H2O and 0.5 M H3BO3.Ref.X.G. Zheng, Y.N. Shi and K. Lu, Materials Science & Engineering A, 561, 52 (2013).X.G. Zheng, Y.N. Shi and K. Lu, J. Electrochem. Soc.,160, D289(2013)

An Investigation on Polyethyleneimine and Saccharin on Controllable Electro-Healing Cracks in Nickel

To minimize pores or voids that left after electro-healing by plain solution without additives, controllable electro-healing was investigated in pure Ni plates with a through-thickness crack in a Watts bath by adding polyethyleneimine (PEI) and saccharin. Controllable electro-healing means that the growth velocity of healing crystals from crack tips is much faster than that from sidewalls of the crack, which reduces the number of pores owing to depletion of healing solution. Rotating disk electrode (RDE) was introduced to evaluate the electrochemical behavior in crack tips and crack center. Voltammograms and galvanostatic measurements indicated that PEI exhibits a convection-dependent inhibition effect on Ni deposition. The addition of saccharin will much enhance the inhibition behavior of PEI, controllable electro-healing was achieved with the presence of both PEI and saccharin.

Electro-healing cracks in nickel

Healing cracks in metallic materials is challenging due to limited atomic mobility in solid state around ambient temperature. In this paper, we developed a novel crack-healing approach by means of an electrochemical process in which metallic ions in electrolyte are used as a healing agent. Pure Ni sheets with a through-thickness crack were taken as an example. Cracks with sizes in the micrometer range or larger are successfully healed by electro-healing. The electro-healing process starts with the vertical epitaxial growth of healing crystals from the original crack surfaces followed by lateral growth of healing crystals that bond with each other at atomistic level. Tensile tests exhibited that the healed samples have a comparable tensile strength as the virgin sample and some tensile ductility can be achieved for the sample of 100μm thick. Post-fracture analysis indicated that part of the crack propagated along the substrate instead of healing crystals. The healing efficiency, ranging from 96% to 33% with an increasing sample thickness, is related to the fraction of fully-healed region and the strength difference between the substrate and the healing crystals.

2Pa2-2 Evaluation of Closed Stress Corrosion Cracks in Nickel Based Alloy Weld Metal Using Subharmonic Phased Array(Poster Session)

2Pa2-2 Evaluation of Closed Stress Corrosion Cracks in Nickel Based Alloy Weld Metal Using Subharmonic Phased Array(Poster Session)

DFT studies of sulfur induced stress corrosion cracking in nickel

Abstract The sulfur induced embrittlement of nickel Σ 5 (0 1 2), a symmetrical tilt grain boundary, has been studied to understand why and how impurity atoms weaken the grain boundaries of nickel. By using first-principles calculations, the effect of concentration of sulfur has been investigated to comprehend the basic rules on the deterioration of a nickel grain boundary. We have found that the extension of the nickel grain boundary is a result of the repulsion of the segregated and neighboring sulfur atoms.

Atomistic Simulations of Dislocation - Crack Interaction

The interaction of dislocations with a static mode I crack is studied by large scale molecular dynamics simulations. The model consists of a blunted [001](110) crack in nickel, to which after relaxation at K < K Ic the displacement field of a dislocation is added. The response of the system is monitored during its evolution in the micro-canonical ensemble. The three dimensional nature of the problem requires the simulation of many millions of atoms. The great demands on the computational resources and data storage can only be met by high performance computing platforms and by the development of appropriate simulation methods. The simulations allowed to identify different characteristic processes during the interaction of the impinging dislocation with the crack. In particular, stimulated dislocation emission and cross slip processes are observed to be important for the development of a plastic zone.

Optimising visibility for the neutron radiography of titanium and nickel gas turbine components

An analytical expression for the visibility of radiographic images with phase and absorption contrast has been derived. It was used to theoretically optimise the visibility of cracks in titanium and nickel for the case of neutron radiography. The image-to-object distance, object-to-detector distance and the crack depths were varied in the optimisation. The different neutron refractive indices of the titanium and nickel produced qualitative differences in the predicted visibility as a function of the spatial frequencies of the object. Significant visibility from phase contrast was predicted for 50 μm deep cracks in nickel and 200 μm deep cracks in titanium.

In-situ scanning electron microscopy of fatigue crack behaviour in ductile materials

The activity of naturally grown short cracks in nickel was studied by in-situ measurements of the crack opening displacement in a scanning electron microscope. Plastic crack tip blunting was observed at macrocracks in single crystals as well as at naturally grown microstructurally short cracks in plastic strain control at loading amplitudes below cyclic yield. Crack tip opening hysteresis loops in nickel do not show closure due to cyclic hardening. Local yield behaviour at the crack tip was observed only after reducing the influence of the cyclic pre-history by annealing.

Hydrogen diffusion and its relevance to intergranular cracking in nickel

This investigation examines the effect of hydrogen precharging upon the fracture mode of pure nickel at 77 K. Specific attention is given to the diffusion coefficient of hydrogen along grain boundaries,D g , the critical hydrogen concentration to cause intergranular fracture, C g * , and the binding energy of grain boundaries with hydrogen,E b . Both scanning electron microscope (SEM) observations and a newly developed grain boundary diffusion model indicate that the fracture mode changes from transgranular (TG) to intergranular (IG) when the hydrogen concentration in grain boundaries reaches a critical value, in the range of 6.5 to 9.8 at. pct. The experimental results further show that the observed increment of IG cracking depth with the precharging time could be accounted for by lattice diffusion alone, thus implying that hydrogen transport in this material is not enhanced by grain boundaries. Finally, the binding energy of grain boundaries with hydrogen is found to be 11.3 to 12.3 kJ/mol.