Afrooz Barnoush

In situ electrochemical nanoindentation of a nickel (111) single crystal : hydrogen effect on pop-in behaviour

Abstract The hydrogen effect on dislocation nucleation in Ni single crystals with (111) surface orientation has been examined with the aid of a specifically designed nanoindentation set-up for in situ electrochemical experiments. The effect of the electrochemical potential on the indent load–displacement curve, especially the unstable elastic-plastic transition (pop-in), was studied in detail. The experiments allowed the exclusion of the surface from hydrogen effects. The observations showed a pop-in load drop from an average value of 250 to 100N due to in situ hydrogen charging, which is reproducibly observed within sequential hydrogen charging and discharging. Clear evidence is provided that hydrogen atoms facilitate homogeneous dislocation nucleation.

Fracture assessment of polymethyl methacrylate using sharp notched disc bend specimens under mixed mode I + III loading

Mixed mode I/III behavior of Perspex (polymethyl methacrylate (PMMA)) is studied experimentally and theoretically in this research using a new and simple laboratory test configuration. The specimen is a circular disc containing a sharp V-notch along the diameter that is loaded by the conventional three-point bend fixture. The critical values of notch stress intensity factors (KIV and KIIIV) were obtained for the whole combinations of modes I and III simply by changing the notch inclination angle relative to the loading rollers. The value of notch fracture toughness under pure or dominantly tension loads was greater than its corresponding value under mode III or dominantly torsion loads. The experimental results were also predicted very well by employing the local strain energy density (SED) criterion.

Novel methods for micromechanical examination of hydrogen and grain boundary effects on dislocations

Most of what is known about the local interaction of dislocations with grain boundaries and hydrogen is based on transmission electron microscopy studies, which suffer from the distinct disadvantage that only extremely thin samples can be used. Recently, micropillar compression testing has become a popular means by which investigation of the size effect is conducted. This method, in combination with orientation imaging techniques, is used here to study the interaction of dislocations with a pre-selected grain boundary during the deformation of a bicrystalline specimen. Furthermore, by utilizing a custom built electrochemical cell, the micropillar compression testing can be extended to study in situ examination of micropillars charged with hydrogen. The effects of hydrogen and grain boundary on the deformation process in this small, but still bulk-like volume are presented, and our initial results reveal the value of this new technique for investigations of hydrogen embrittlement and grain boundary strengthening.

Effect of Hydrogen and Grain Boundaries on Dislocation Nucleation and Multiplication Examined with a NI-AFM

A nanoindenting AFM(NI-AFM) with an environment chamber was constructed to study the effect of hydrogen on decohesion and dislocation nucleation and the effect of grain boundaries on dislocation nucleation and multiplication. Ultra fine grained Ni single crystals were examined. It could be clearly shown that hydrogen influences the pop in width and length. Testing single grains with grain sizes below one micron at different rates inside a NI-AFM showed that the rate dependence of ultra fine grained Ni is a result of the interaction of the growing dislocation loops with the boundary.

Effect of substitutional solid solution on dislocation nucleation in Fe3Al intermetallic alloys

Dislocation nucleation in solid solutions of body-centered-cubic intermetallic Fe3Al alloys was investigated by means of nanoindentation and measurement of the pop-in load of samples with different Cr content. It is clearly shown that the Cr solute element in the Fe3Al intermetallic alloy increases the pop-in load, i.e. shear stress, required for dislocation nucleation in a previously dislocation-free region. Changes in pop-in load track closely with changes in elastic properties, which can be interpreted within the framework of the universal features of metallic bonding as a change in the interatomic potential, as proposed by Rose et al.

Chemically Induced Phase Transformation in Austenite by Focused Ion Beam

A highly stable austenite phase in a super duplex stainless steel was subjected to a combination of different gallium ion doses at different acceleration voltages. It was shown that contrary to what is expected, an austenite to ferrite phase transformation occurred within the focused ion beam (FIB) milled regions. Chemical analysis of the FIB milled region proved that the gallium implantation preceded the FIB milling. High resolution electron backscatter diffraction analysis also showed that the phase transformation was not followed by the typical shear and plastic deformation expected from the martensitic transformation. On the basis of these observations, it was concluded that the change in the chemical composition of the austenite and the local increase in gallium, which is a ferrite stabilizer, results in the local selective transformation of austenite to ferrite.

Investigation of the role of grain boundary on the mechanical properties of metals

Compression testing of micropillars was used to investigate the gain boundary effect on the strength of metals which is especially interesting in ultra fine grained and nanocrystalline metals. Single and bicrystal micropillars of different sizes and crystallographic orientations were fabricated using a focused ion beam system and the compression test was performed with a nanoindenter. A reduction of the pillar size as well as the introduction of a grain boundary results in an increase in the yield strength. The results show that the size and the orientation of different adjoining crystals in bicrystalline pillars have an obvious effect on dislocation nucleation and multiplication.

Materials and corrosion trends in offshore and subsea oil and gas production

The ever-growing energy demand requires the exploration and the safe, profitable exploitation of unconventional reserves. The extreme environments of some of these unique prospects challenge the boundaries of traditional engineering alloys, as well as our understanding of the underlying degradation mechanisms that could lead to a failure. Despite their complexity, high-pressure and high-temperature, deep and ultra-deep, pre-salt, and Arctic reservoirs represent the most important source of innovation regarding materials technology, design methodologies, and corrosion control strategies. This paper provides an overview of trends in materials and corrosion research and development, with focus on subsea production but applicable to the entire industry. Emphasis is given to environmentally assisted cracking of high strength alloys and advanced characterization techniques based on in situ electrochemical nanoindentation and cantilever bending testing for the study of microstructure-environment interactions.

Hydrogen Effect on Nanomechanical Properties of the Nitrided Steel

In situ electrochemical nanoindentation is used to examine the effect of electrochemically charged hydrogen on mechanical properties of the nitride layer on low-alloy 2.25Cr-1Mo martensitic structural steel. By application of this method, we were able to trace the changes in the mechanical properties due to the absorption of atomic hydrogen to different depths within the compound and diffusion layers. The results clearly show that the hydrogen charging of the nitriding layer can soften the layer and reduce the hardness within both the compound and the diffusion layers. The effect is completely reversible and by removal of the hydrogen, the hardness recovers to its original value. The reduction in hardness of the nitride layer does not correlate to the nitrogen concentration, but it seems to be influenced by the microstructure and residual stress within the compound and diffusion layers. Findings show that nitriding can be a promising way to control the hydrogen embrittlement of the tempered martensitic steels.

Hydrogen-enhanced cracking revealed by in situ micro-cantilever bending test inside environmental scanning electron microscope

To evaluate the hydrogen (H)-induced embrittlement in iron aluminium intermetallics, especially the one with stoichiometric composition of 50 at.% Al, a novel in situ micro-cantilever bending test was applied within an environmental scanning electron microscope (ESEM), which provides both a full process monitoring and a clean, in situ H-charging condition. Two sets of cantilevers were analysed in this work: one set of un-notched cantilevers, and the other set with focused ion beam-milled notch laying on two crystallographic planes: (010) and (110). The cantilevers were tested under two environmental conditions: vacuum (approximately 5 × 10−4 Pa) and ESEM (450 Pa water vapour). Crack initiation at stress-concentrated locations and propagation to cause catastrophic failure were observed when cantilevers were tested in the presence of H; while no cracking occurred when tested in vacuum. Both the bending strength for un-notched beams and the fracture toughness for notched beams were reduced under H exposure. The hydrogen embrittlement (HE) susceptibility was found to be orientation dependent: the (010) crystallographic plane was more fragile to HE than the (110) plane. This article is part of the themed issue ‘The challenges of hydrogen and metals’.