Hydrogen embrittlement, revisited by in situ electrochemical nanoindentation

The fine scale mechanical probing capability of NI-AFM was used to examine hydrogen interaction with plasticity. To realize this, an electrochemical three electrode setup was incorporated into the NI-AFM. The developed ECNI-AFM is capable of performing nanoindentation as well as imaging surfaces inside electrolytes. The developed ECNI-AFM setup was used to examine the effect of cathodically charged hydrogen on dislocation nucleation in pure metals and alloys. It was shown that hydrogen reduces the pop-in load in all of the tested materials except Cu. The reduced pop-in load can be interpreted as the HELP mechanism. Classical dislocation theory was used to model the homogeneous dislocation nucleation and it was shown that H reduces the activation energy for dislocation nucleation in H sensitive metals which are not undergoing a phase transformation. The activation energy for dislocation nucleation is related to the material specific parameters; shear modulus, dislocation core radius and in the case of partial dislocation nucleation, stacking fault energy (SFE). These material properties can be influenced by H resulting in a reduced activation energy for dislocation nucleation. The universality of cohesion in bulk metals relates the reduction of the shear modulus to the reduction of the cohesion, meaning HEDE mechanism. The increase in the core radius of a dislocation due to H is a direct evidence of decrease in dislocation line energy and H segregation on the dislocation line. In the case of partial dislocations, the H can segregate on to the stacking fault ribbon and decrease SFE. This inhibits the cross slip process and enhances the slip planarity. Thus, HELP and HEDE are the two sides of a coin resulting in H embrittlement. However depending on the experimental approach utilized to probe the H effect, either HELP or HEDE can be observed. In this study, however, by utilizing a proper experimental approach, it was possible to resolve the interconnected nature of the HE.Die Motivation dieser Arbeit war es, die feinscaligen Möglichkeiten des NI-AFM zur Untersuchung des Einflusses von H auf die Plastizität einzusetzen. Es wurde mit der Integration einer elektrochemischen Dreielektrodenzelle in die Anlage die Möglichkeit der Ni-AFM Technik genutzt, in einer Flüssigkeit einsetzbar zu sein. Der hier entwickelte ECNI-AFM Versuchsaufbau wurde eingesetzt um den H-Effekt unter kathodischer Beladung auf die Versetzungsnukleation in reinen Metallen und Legierungen zu untersuchen. Es konnte für alle untersuchten Werkstoffe außer für Cu gezeigt werden, dass H die pop-in-Load reduziert. Die reduzierte pop-in-Load kann als HELP interpretiert werden. Die klassische Versetzungstheorie wurde angewendet um die homogene Versetzungsnukleation im Metall zu modellieren. Es konnte gezeigt werden, dass H in "H-sensiblen\u27; Metallen, die keine Phasentransformation durchlaufen, die Aktivierungsenergie für die Nukleation von Versetzungen senkt. Die klassische Versetzungstheorie verbindet die Aktivierungsenergie für Versetzungsnukleation mit werkstoffspezifischen Parametern, die möglicherweise durch H beeinflusst werden können. Diese Parameter sind Schubmodul, Radius des Versetzungskerns und im Falle der Nukleation von Partialversetzungen die Stapelfehlerenergie. Als Konsequenz aus Universalität der Kohaesion in metallischen Festkoerpern ist es möglich die Reduzierung des Schubmoduls mit der Reduktion der Kohäsion (HEDE) in Beziehung zu setzen. Die durch H hervorgerufene Vergrößerung des Kernradius ist ein direkter Beweis für die Verringerung der Versetzungslinienenergie und die H-Segregation entlang der Versetzungslinie. H kann im Falle von Partialversetzungen innerhalb des Stapelfehlers segregieren und dessen Oberflächenenergie herabsetzen. Dadurch wird der Quergleitprozess unterdrückt (slip planarity). Zusammengefasst kann geschlossen werden, dass HELP und HEDE die beiden Seiten einer Medaille sind, die zu H-Versprödung führen. Je nachdem, welcher Ansatz gewählt wird, um den Einfluss von H zu untersuchen, kann entweder HELP oder HEDE beobachtet werden. Innerhalb der vorliegenden Arbeit war es durch die Verwendung eines geeigneten experimentellen Ansatzes möglich, die komplizierte Natur der H-Versprödung ein Stück weiter zu entschlüsseln

Analyzing the onset of plasticity in Fe‐3wt.%Si

Material microstructure plays an important role in the integrity and failure of structures. The complexity of the microstructure makes the investigating of its relationship to mechanical properties and failure difficult. Industrial products have complex structural and material designs, which make such investigation challenging. To overcome this, one may study at simpler systems and focus on e.g. single crystals made of simple model materials within a limited volume. The current study aims at analyzing the onset of plasticity in single crystalline pillars of the model material Fe-3wt.%Si under compression. Modeling of these microscale testing was performed using the finite element combined with crystal plasticity (CPFEM). Excellent agreement was shown between numerical and experimental results on the global response, i.e. load/stress versus displacement/strain curves. In addition, the local mechanical behavior was investigated in more details, i.e. whether the correct slip systems are active. Please click Additional Files below to see the full abstract

A Review on the Properties of Iron Aluminide Intermetallics

Iron aluminides have been among the most studied intermetallics since the 1930s, when their excellent oxidation resistance was first noticed. Their low cost of production, low density, high strength-to-weight ratios, good wear resistance, ease of fabrication and resistance to high temperature oxidation and sulfurization make them very attractive as a substitute for routine stainless steel in industrial applications. Furthermore, iron aluminides allow for the conservation of less accessible and expensive elements such as nickel and molybdenum. These advantages have led to the consideration of many applications, such as brake disks for windmills and trucks, filtration systems in refineries and fossil power plants, transfer rolls for hot-rolled steel strips, and ethylene crackers and air deflectors for burning high-sulfur coal. A wide application for iron aluminides in industry strictly depends on the fundamental understanding of the influence of (i) alloy composition; (ii) microstructure; and (iii) number (type) of defects on the thermo-mechanical properties. Additionally, environmental degradation of the alloys, consisting of hydrogen embrittlement, anodic or cathodic dissolution, localized corrosion and oxidation resistance, in different environments should be well known. Recently, some progress in the development of new micro- and nano-mechanical testing methods in addition to the fabrication techniques of micro- and nano-scaled samples has enabled scientists to resolve more clearly the effects of alloying elements, environmental items and crystal structure on the deformation behavior of alloys. In this paper, we will review the extensive work which has been done during the last decades to address each of the points mentioned above

Hydrogen enhanced cracking studies by in‐situ electrochemical micro cantilever bending test

Hydrogen (H)-Induced degradation of metals has been a severe problem in different industrial fields. Since H has a strong tendency to segregate in structure defects, grain boundary (GB) importance becomes even more dominated in the H-embrittlement studies. GBs are considered as one of the potential sites for initiation of this catastrophic phenomenon in the polycrystalline materials. To investigate the mechanism causing H-embrittlement, a method is required to resolve the H interaction with the micro-structure and crystal defects such as GBs in the same length scale. In this study, we introduce an in-situ electrochemical micro-cantilever bending (ECCB) test of bi-crystal beams with a selected type of GBs. ECCB tests were performed using a nano-indenter with an integrated miniaturized electrochemical cell. Fe- 3wt%Si and Ni samples were used in this study. Charging the micro sized cantilevers under cathodic potential during the in situ ECCB testing, assured uniform concentration of hydrogen in the GB during bending tests. The results were compared with the bi-crystal cantilevers bent in the air. Secondary electron imaging and electron backscatter diffraction were used to analyze the deformation substructures after the test. The load-displacement curves reveal continuous decrease in the flow stress for the cantilevers bent in the presence of hydrogen. The flow stress was constant for the beams bent in air. The secondary electron images show a crack propagation in the presence of hydrogen. This method overcomes the problems that arise from out gassing of hydrogen during ex-situ testing. Furthermore, examination of hydrogen interaction with a specific type of GB is in the same microstructural length scale. Please click Additional Files below to see the full abstract

Fracture assessment of graphite components weakened by rounded V-notches and subjected to static multiaxial loading

Abstract While a large bulk of experimental results from cracked specimens of polycrystalline graphite under pure modes of loading, in particular under mode I loading, can be found in the literature, only a very limited number of tests have been carried out on notches. At the best of authors' knowledge dealing with the specific case of V-notches under mixed mode loading (tension + torsion) no results can be found in the literature. With the aim to fill this lack, the problem of mixed mode (I+III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio (i.e. the ratio between the nominal stress due to tension and that due to torsion loading). A criterion based on the local Strain Energy Density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination. The proposed criterion allows a sound assessment of the fracture loads

Novel in situ nanomechanical tests: a new insight into the hydrogen embrittlement

Hydrogen embrittlement is a complicated process hard to investigate due to the volatile nature of the hydrogen atom and different states it can exist in the metals. Therefore, to reveal the underlying mechanism of the hydrogen embrittlement, we designed and performed “critical experiments.” In this paper, we will present some of our novel approaches used to study the hydrogen embrittlement in different alloy systems. We specifically used in situ hydrogen charging combined with nanoindentation, microcantilever bending, and tensile testing to observe the hydrogen embrittlement at various microstructural length scales. Additionally, we used high-resolution microstructural characterization techniques including, High-resolution Electron Backscattered Diffraction (EBSD), Electron Channeling Contrast Imaging (ECCI), transmission EBSD (t-EBSD) and Transmission Electron Microscopy. Please click Additional Files below to see the full abstract

Temperature-dependent mechanical properties of Tin+1CnO2 (n = 1, 2) MXene monolayers: a first-principles study

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas. One of the least explored properties of MXenes is their mechanical properties. While the basic elastic properties of MXenes have been studied by first-principles, the effects of temperature on the elastic properties have never been explored. In this study, we investigate temperature-dependent structural and mechanical properties of the titanium-containing MXenes (Tin+1CnO2 (n = 1, 2)) based on the first-principles calculations combined with quasi-harmonic approximation. The effective Young's modulus of a single layer of Ti2CO2 and Ti3C2O2 is calculated to be 565 and 482 GPa, respectively, at 0 K. By increasing temperature to 1000 K, Young's moduli of Ti2CO2 and Ti3C2O2 decrease to 469 GPa and 442 GPa, respectively, which indicates a larger reduction in stiffness in thinner MXenes at higher temperatures. Our calculations of the temperature-dependent bond strengths within MXenes showed that titanium and carbon atoms in Ti3C2O2 form stronger bonds than Ti2CO2 and atomic bonds in Ti2CO2 lose their stiffness more than Ti3C2O2 with increasing temperatures. The Debye temperature of these monolayers is also calculated to provide a comparison of the thermal conductivity between these monolayers, in which the results show that the Ti3C2O2 has a higher thermal conductivity than Ti2CO2. Our calculated electronic properties results of the monolayers are also shown that the electrical conductivity of the monolayers would not change with temperature. Our study extends MXenes applications to high-temperature applications, such as structural composite components and aerospace coatings

Effect of hydrogen on the nucleation and motion of dislocations

Conventional mechanical tests are costly, time consuming, and due to their large scale, not very successful in obtaining mechanistic information. In contrast, the local method like nanoindentation, compression or bending test of micro pillars have comprehensive ranges of possibilities to achieve an essential understanding about the influence of substitutional atoms and/or interstitial atoms (e.g., hydrogen and nitrogen) on the mechanical properties like Young´s modulus, Gibbs free energy for homogeneous dislocation nucleation, dislocation line energy and also friction stress. These methods allow us to measure the mechanical behavior in simulated environments and atmospheres close to the routine industrial applications. Nanoindentation was applied for studying the sensitivity of various materials like nickel, Cu, steels and iron aluminides to hydrogen embrittlement, as it offers sufficiently high resolution in determining load and displacement and works effectively non-destructive. Nevertheless, the method of in-situ nanoindentation suffers from the complexity of the stress field below the nanoindenter. Furthermore, a novel method was developed where miniaturized compression samples are machined using focused ion beam (FIB) milling and loaded in a nanoindenter system equipped with a flat diamond punch. This method is able to probe mechanical properties on the micrometer and sub-micrometer scale under nominally uniaxial loading. Additionally, very small volume of pillar guarantees a fast and homogeneous distribution of hydrogen. More recently the influence of hydrogen on the elastic properties and interaction of dislocations was studied using the in-situ bending test of micro pillars (see Figure 1). The advantage of the bending test is the presence of high tensile stress in the pillar during the test. It is in contrast to other techniques like in-situ nanoindentation or micropillar compression tests with the compressive stress field which works as driving force for the hydrogen diffusion out of the highly stressed region

Evaluation of microstructural and environmental effects on the hydrogen uptake and desorption in high-strength carbon steels: A thermal desorption spectroscopy study

publishedVersio

Role of cementite morphology on corrosion layer formation of high-strength carbon steels in sweet and sour environments

publishedVersio