A new method has been developed for testing high cycle and very high cycle fatigue properties of materials at the micro-scale based on micro-cantilevers. Focused ion beam was employed to cut micro-cantilevers with triangular cross-section into the surface of a selected grain in a bulk polycrystalline commercial pure Titanium. The bulk specimen was pre-examined by EBSD, so the crystal orientation of all micro-cantilevers were known. The bulk sample containing the micro-cantilevers was then attached to a high power ultrasonic generator, which can generate mechanical vibration at the frequency 20KHz. The bulk of the specimen moves with the ultrasonic generator, but the micro-cantilever lags somewhat behind. The resulting deflections generate cyclic stress in the micro-cantilevers. The high vibration frequency means it can easily test into the high cycle and very high cycle regime. The design challenge is to generate enough stress to cause fatigue in these ultra-small specimens because the stress amplitude achieved in vibration is inverse to the cantilever size. Previous finite element modelling and experiments had shown that the classic micro-cantilever with uniform cross-section can only generate stress of a few MPa, even with the acceleration as high as 10⁷m/s². Instead, we designed a new ‘hammer-shaped' micro-cantilever with a widened region at the free end to significantly increase the inertia. This design now generates sufficient stress and enables fatigue testing even in Titanium, which is a challenging material due to the high strength to weight ratio (both high stress and low density require higher acceleration).
SN curves in the testing range from 10⁵ to 10⁸ cycles have been obtained for 2 mu;m wide micro-cantilever single crystal Ti samples using a step test protocol. The stress to failure decreases systematically as the number of cycles to failure increases. However, there is strong dependence on the crystal orientation with the fatigue strength at 10⁷ cycles for test pieces cut along the direction being approximately twice that of those cut in the direction. The fatigue strength of micro-fatigue test is significantly lower than the static strength measured on micro-cantilevers of identical size using a nanoindenter. Due to the small specimen size, it is suggested that the results reflect the behavior of fatigue nucleation rather than propagation.

This talk presents an experimental investigation of the stress relaxation and cracking mechanisms in ultrathin nanocrystalline metals (Au and Al). Tensile tests that include stress relaxation segments were performed on ultrathin (<100 nm) specimens with micron-sized dogbone shapes, using a MEMS-based nanomechanical testing setup. The setup allows quantification of stress and strain (and their evolution with time) as well as in situ TEM observations of the governing mechanisms. The results (based on in situ TEM observations and activation volume calculation as well as evolution of strain rate with applied stress) show that the transient relaxation deformation in 100-nm-thick Au (average grain size: 75 nm) is highly localized and dominated by intergranular and intragranular dislocation activities. These dislocation activities slow down within a timescale of 1 hour, leading gradually to a diffusion-dominated deformation (steady-state creep). In contrast, the 30-nm-thick Au films (with a much smaller average grain size) do not show these sustained dislocation activities; instead their deformation behavior appears to be dominated by grain boundary activities. In addition to the stress relaxation properties, this talk will discuss the cracking behavior in ultrathin nanocrystalline gold and aluminum films.

The mechanical behavior of nanocrystalline metals has been the subject of extensive research, especially in terms of their high strengths and wear-resistance. However, the fatigue performance of these metals has received much less attention, despite initial reports of exceptional fatigue strengths. Particularly lacking is a comprehensive understanding of the cyclically-induced deformation mechanisms, structural evolution, and fracture behaviors in nanocrystalline metals. Recent studies using various post-fracture microscopy techniques have identified extensive regions of abnormally large grains near and along macro-scale cracks - but, the exact origins of these features and their overall effects on the fatigue performance and fracture are unclear. This is due, in part, to the general difficulties in performing non-destructive in situ characterization of through-thickness microstructures during fatigue testing. In the current study, a newly developed method of detecting abnormal grain growth (AGG) using synchrotron x-ray diffraction was employed to “watch” the microstructural evolution of various nanocrystalline Ni alloys during high-cycle fatigue. This in situ fatigue approach allowed us to verify with statistical confidence that the abnormally large grains do, in fact, grow during the fatigue process (i.e. they are not pre-existing) and that they develop well before final fracture of the material. The effects of the fatigue stress conditions on the microstructural evolution and AGG were also evaluated using a range of fatigue testing conditions. Additionally, traditional cross-sectional microscopy was performed after the onset of AGG, but before final fracture, to further explore the deformation and micro-scale crack mechanisms that lead to the macro-scale fracture during fatigue.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Understanding the effects of a material&’s morphology upon the overall material performance requires a detailed understanding of its initial morphology and how it changes under external stimuli. Laboratory based X-ray computed tomography (CT) systems can image polymer foams as they are compressed, in an interrupted in situ modality. Due to the lower flux of a laboratory X-ray source, stress relaxation must be allowed to occur in the material before a tomogram can be collected. Otherwise, the residual sample motion leads to significant image blurring. This relaxation process requires ~20 minutes before the tomogram can be collected over 1⁺ hours. Important information regarding compressive performance and changes in morphology is lost during this stress relaxation period.
Synchrotron light sources, such as the Advanced Photon Source, provide a significant increase in photon flux, compared to lab-based X-ray tubes. Coupling this high flux with a high speed camera allows for X-ray radiographs to be acquired every millisecond. This generates tomographic data in ~1 second. By coupling a custom loading stage to the X-ray microscope, it is possible to study the dynamic in situ deformation of polymeric foams at a 10^-1 s^-1 strain rate.
In this study, synchrotron-based X-ray CT, at beam line 2-BM, was used to capture the morphology changes in polymeric foam materials during dynamic uniaxial compression. Twenty tomograms of each sample were acquired while simultaneously compressing the samples up to 60%, within a timeframe of 100 seconds. The materials studied included hydrogen blown silicone foam (LK3626), prilled urea silicone foam, syntactic Sylgard-184, and additively manufactured foams.
With this technique, we can measure and correlate the differences in mechanical performance of polymeric foams to their physical/morphological changes. For example, LK3626 is a soft-structured hydrogen blown silicone foam. It has a weak ligament structure which quickly buckles with almost no bending resistive force, even at low compressive strains. Also, it compresses with no Poisson effect. Syntactic Sylgard 184 is a much stiffer material, due to the reinforcement from the 50 mu;m glass microballoon pore former; therefore, it exhibits a stress/strain curve with definitive bending/buckling/compressing regions. Importantly, the true stress can be calculated from CT data by measuring the full 3D change in area for each tomographic step. Additionally, the tomographic data can be converted into a hypermesh for importing into Abaqus for explicit analysis. Direct comparison of the reconstructed slices to the modeled result adds a high level validation to the modeled uniaxial compressive performance of the material that cannot be acquired with other techniques.

Impact loading of spherical particulates occurs during materials handling, processing, and service conditions. This can lead to fracture of the particles and commutation into smaller particles. The dynamic fracture behavior of micro spherical particles of 5 brittle spherical solids, ranging from soda lime glass, to polycrystalline silicon and yttrium-stabilized zirconia (YSZ) was characterized with high speed, in situ X-ray phase-contrast imaging to examine the failure mechanisms in situ for spheroidal particles with diameters (d) from 0.600 to 2 mm. A new pulverization parameter was developed and is shown to predict the failure mechanism of the materials under fixed grips loading (i.e. overdriven in load) relating the hardness, toughness, modulus, and the size of the given particulate. The high speed behavior will be compared to quasi-static loading of particulates to demonstrate the effects of strain rate on particle fracture.

Zirconium is a material of great importance in nuclear applications, as it is commonly used for fuel cladding. One of the known factors potentially degrading the mechanical properties is hydrogen precipitation, leading to the formation of brittle zirconium hydride needles. Different hydride phases form, depending on the amount of hydrogen in these needles. The associated volume expansion of hydrides leads to an increasing applied strain on the zirconium matrix. Available observations suggest that such strains may induce the nucleation of dislocations in zirconium from the zirconium/hydrides interface, thus further altering the mechanical properties of the material. But these observations are rather limited, and little is known about the elastic limits, and the nucleation mechanisms.
To address this issue, we have performed molecular dynamics simulations of a strained Zr surface, using two different interatomic potentials (EAM and COMB). Different loading modes were tested, in order to mimic the influence of the hydride precipitate. We found that the onset of plasticity is controlled by heterogeneous nucleation of partial dislocations in the first-order pyramidal planes, immediately followed by partial dislocations in the basal plane. This surprising result can be understood from the analysis of generalized stacking fault energy surfaces [1]. The apparent blocking of the prismatic-plane slip, which unquestionably dominates in bulk zirconium, follows from the loading conditions imposed by the precipitate. The elastic limit depends on the geometric features of the surface making contact to the precipitate. In particular, it is lowered by the presence of surface steps. In specific circumstances, it is possible to favor the basal slip over the pyramidal one.
[1] C. Varvenne et al., Acta Mater. 78 (2014) 65 - 77

The objective of this work is to show some new and original methods to determine the fracture toughness in brittle metals. In this case the analysis was performed for tungsten due to its thermo-physical properties, tungsten is a suitable material for plasma facing applications in the future fusion reactors. However, its structural use is compromised due to its inherent brittleness. Therefore, to improve this critical feature is relevant to accurately measure its fracture toughness as a real property independent of geometrical parameters.
The most common methods to produce cracks such as fatigue, indentation microfracture or electro-discharge machining cannot be used here because they are difficult to apply, do not produce reproducible cracks or induce extended thermal damage in the material. For these reasons, in a first part of this work four experimental methods, that gradually approach a crack-like notch, were compared to determine the effect of the notch root radius on the measured fracture toughness of a nanostructured bulk tungsten material. Three-point bending tests were performed on these four types of specimens with notches introduced with: a classical diamond disc, a diamond wire, a razor blade and an ultra-short pulsed laser. The results showed that the best alternative is to introduce a notch with a femtosecond pulsed laser. This method, which produces crack-like notches without thermal damage, possesses good reproducibility, high accuracy and reliable fracture toughness. It was previously used on ceramics but there is no evidence of its use on metals.
Additionally, the ultra-short pulsed laser was used to introduce notches in tungsten foils of 0.1 mm thickness to perform in situ tensile tests inside a scanning electron microscope and determine the fracture toughness. Two types of geometries were also produced in the foils: single-edge and double-edge notched specimens. The introduction of the notches is a challenging task because of the reduced size of the samples and their slenderness. Single-edge notched specimens, easier to produce, were not suitable to provide accurate fracture toughness since the slenderness and the non-symmetric geometry resulted in complex stresses and momentum apart from the uniaxial tensile being applied to the sample. Double-edge notched specimens, however, produced accurate fracture toughness results, although to introduce both notches aligned at both sides of the specimens involves higher complications during the notching process.

Irradiation in a fast neutron environment is known to degrade the mechanical properties of materials, therefore, to ensure safe operation it is necessary to undertake measurements of these properties. To minimise doses to personnel during post irradiation testing it is appropriate to minimise the sample mass and select small test specimens. In addition, in materials with a complex microstructure, it is necessary to undertake these measurements at the required length-scale to provide input parameters for computer modelling. In this paper, we undertake measurements using micro-metre length scale cantilever beam specimens prepared by ion beam milling in a Helios Dualbeam workstation. These cantilever beams were loaded using a customised system developed at the University of Bristol. This allows load-displacement to be measured and the associated mechanical properties, such as the elastic modulus and flexural strength to be determined. With these techniques, it is possible to observe specimens throughout the duration of a test.
Pile Grade A (PGA) graphite was used as moderator and structural material within the core of UK gas-cooled Magnox reactors. It is an anisotropic, polygranular graphite comprising aligned filler particles in a matrix of graphitised binder with around 20 vol.% porosity caused by the manufacturing process. Irradiated and radiolytically oxidised PGA graphite samples were extracted from the fuel channels as part of an overall monitoring programme to support safe operation. Three cylindrical samples (7 mm height by 12 mm dia.) trepanned from bricks with increasing distance from the centre of the reactor were selected to provide a decreasing neutron dose. The microstructure, in particular the fine-scale porosity has been characterised using serial sectioning, high spatial resolution tomography. To extrapolate small-scale testing data to size of reactor core bricks, a microstructure-based multi-scale modelling approach has to be adopted. This requires the input parameters at a suitable length-scale to eliminate size effects introduced by the heterogeneity of the multi-scale microstructure. Measurement of load-displacement, elastic modulus and fracture strength have been made using 2 µm x 2 µm x 10 µm micro-cantilever test specimens in the dualbeam workstation to evaluate the changes in properties with neutron dose. This has revealed the effect of irradiation damage, independent of the macro-sized pores present in the PGA graphite. The data are discussed with respect to the possible mechanisms leading to changes in properties and their benefit as input parameters to multi-scale computer models.

The microcracking phenomenon in ceramics initiates above a critical grain size due to anisotropy in the coefficient of lattice thermal expansion. This microcrack network can result in an increase in light scattering, electrical and thermal resistance, and a decrease in the bulk coefficient of thermal expansion-even to the point of negative thermal expansion. The tunable material properties of microcracked ceramics provide a unique design opportunity but in order to utilize these materials, there is a need to understand the effect of mechanical loading on microcrack populations. In the current study, we probe the effects of microcracks in ceramics by varying the microstructure in beta-eucryptite samples. We mounted specimens with varying grain sizes into a microtensile apparatus and loaded until failure. The resulting stress-strain curves exhibit nonlinear behavior and irrecoverable strain accumulation. Digital image correlation (DIC) was used to measure the two-dimensional strain fields under applied stress for noncontact microstrain measurements. We develop a constitutive model that predicts the evolution of microcrack density within a linear elastic continuum and then applied the model to fit the experimental tensile curves. The constitutive model accounts for the accumulation of irrecoverable strain due to the increasing crack density and the evolving effect of linear-elastic modulus due to changes in crack density.

Stiction failures in radio frequency (RF) micro-electro-mechanical systems (MEMS) based devices can sometimes be attributed to stress relaxation in the metallic thin films that make up the architecture. Efforts to mitigate the extent of the relaxation through novel manufacturing and engineering techniques are critical to production of highly reliable MEMS devices. We have shown previously via thin film bulge mechanical testing that both viscoplastic and viscoelastic deformation behavior are present during the stress relaxation of a typical FCC metal film. Our prior results indicate that the mechanism responsible for the viscoelastic component is dislocation-based, but literature reports based on substrate curvature measurements imply that surface diffusion could be responsible for the viscoplastic component. In the present work, the mechanism responsible for relaxation in both regimes was explored using thin film bulge testing of metal films with and without surface layers. Surface modifications to gold films such as TiO₂ passivation layers, Al₂O₃ atomic layer deposition (ALD) coatings, and SiN_x support layers were each evaluated in an iso-strain relaxation test on the bulge tester. It was found that surface diffusion was not the dominant mechanism for relaxation under the small-strain isothermal conditions typically experienced by a MEMS device in service, but rather it is hypothesized that a dislocation double-kink nucleation mechanism controls both components of deformation

Adhesion and bonding of multilayer systems continue to be the topic of interest in the coatings and membrane industry. Generally, simple techniques such as peel testing have been used to evaluate film adhesion properties. The ability to perform controlled, repeatable, and rapid assessment of the adhesion properties in film systems, especially systems with more than two layers, is of utmost importance for formulation and application development.
In this work we present a new analytical approach to evaluate layer to layer adhesion strength for multilayered systems. A high-throughput mechanical testing (HTMECH) technique is used to measure the impact performance of polymeric films under a wide range of strain conditions. Delamination in multilayered systems is induced by puncturing the film at increasing impact speeds. Ongoing study on the mechanism of failure in the systems with varying levels of adhesion will be discussed.

Nanomechanical measurements, particularly nanoindentation, have transitioned from a purely academic research instrument into a tool for examination of complex industrial processes. In an academic setting proper design of experiments allows for the simple elimination of variables. This is not possible in an industrial setting where a complex engineering material such as the multilayers in a hard disc film stack are the accumulation of 1000&’s of engineering hours. For these complex cases both statistical information and data on the failure mechanisms are required. Here, both ex and in situ measurements are performed and then correlated, using a co-deposited hard drive film stack. Indentation depths range from 0.5 to 10 nm, with the analysis utilizing both the approach and retract curves to describe the crack growth/separation between the diamond indenter and the carbon film. Additionally, the elastic-plastic transition is thoroughly explored in an effort to understand the effects of a protective coating on a substrate and the loss of magnetic information during plastic deformation.

Functional coatings protect substrates from wear, add special electronic and optical properties to the substrates and change their gas permeability. Their functionality, however, is often limited by fracture. This contribution focuses on the flavor of fracture patterns as they arise from an interplay between the cleanliness of the substrate, the adhesion between the two substrates, the amount of ductility that the coating exhibits and the coating microstructure. The examples given range from brittle materials such as diamond like carbon, silica, titania, silicon, ink-jet printable materials to ductile coatings such as Cu, Au, Al and some of their alloys. Special analysis methods are based on in-situ tensile testing and include the measurement of electrical resistivity, AFM, SEM, optical microscopy, Raman microscopy as well as reflectance difference spectroscopy.

Currently the majority of commercial MEMS devices are fabricated out of silicon, but the development of metal MEMS alloys that possess enhanced mechanical and functional properties, requisite dimensional stability, and the ability to be shaped on the microscale would greatly expand the design space for MEMS in applications ranging from microturbine engines, to power generation, high frequency switching, microheaters and high temperature sensors. Advanced metallic alloys are especially attractive in MEMS applications that require high density, electrical and thermal conductivity, strength, ductility and toughness. Here we report the mechanical behavior of sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) thin film alloys. The as-deposited films go down as single-phase nanocrystalline solid solutions and possess ultra high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and the nucleation of Mo-W precipitates. Films annealed at 600^oC or 800^oC for 1hour still showed brittle behavior similar to the as-deposited film. Interestingly, films annealed at 1,000^oC for 1hour were found to exhibit perfect elastic-plastic deformation behavior with strength greater than 1.2 GPa and approximately 10% tensile ductility. The ultra high strength observed in the as-deposited films and significant ductility measured in the annealed films suggest that sputtering and subsequent heat treatment may offer an attractive option for depositing metallic MEMS materials with tailorable mechanical properties.

Hydrogen embrittlement (HE) affects the cracking behavior of metals by influencing the crack nucleation and growth process, which leads to disastrous consequences. Considering that dislocations are the carrier of plastic deformation, which avoids catastrophic failure by brittle fracture, the crucial role of dislocations in HE is enlightened. Hence, in order to understand the fundamental process governing HE, it is essential to clarify the effect of HE on dislocation activity in the fracture process zone of the crack. In an elastic-plastic solid, the dislocation activity in the plastic zone of the crack is limited, either by the dislocation nucleation rate, or by the dislocation mobility.
Recent nanomechanical testing methods, based on nanoindentation, provide a versatile tool for studying both dislocation nucleation and motion. Especially with integration of an electrochemical setup into the nanoindenter, we investigated the hydrogen effect on the activation energy and activation volume for dislocation nucleation as well as interaction force of dislocation with each other that controls their mobility. In this paper, we will present the application of this method, the so called electrochemical nanoindentation, for studying the nanomechanical aspects of HE.

Dislocation organizations and slip band patterns developed under strengthening nickel crystals at room temperature was studied with different content of hydrogen. Complementary observations by AFM and TEM carried out at the same plastic strain yield correlations between different structural parameters of dislocation organization in strengthening nickel oriented for single slip, <135> and multiple slip, <001>. The impact of these correlations on the slip localization and the length scale (geometric necessary boundary, GNB spacing) which affects the hardening rate is demonstrated and discussed in relation with the wavelength of long range internal stresses and the mean free path of mobile dislocations. Additionally, it is shown that hydrogen content reduces the length scale because of a decrease of the stacking fault energy and cross-slip probability on the one hand and a shielding effect on internal stresses on the other. A similitude law between hardening rate and inverse of GNB spacing is evidenced and offers the opportunity to discuss the effect of solutes on hardening rate. In opposite, the impact of hydrogen on the equiaxe cell size and IDB (incidental dislocation boundary) size seems to be moderated for <001> multiple slip orientation. If our work gives a first quantitative insight on the impact of hydrogen on the characteristic dimensions of dislocation cell structures, the dynamics of these effects remain unclear and require more investigations.
[1] A. Oudriss, X. Feaugas, Length scales and scaling laws for dislocation cells developed after monotonic deformation of (001) nickel single crystal, Inter. J. of Plasticity, (2015), in revision.
[2] G. Girardin, C. Huvier, D. Delafosse, X. Feaugas, Correlation between dislocation organization and slip bands: TEM and AFM investigations in hydrogen-containing nickel and nickel-chromium, Acta Materialia, 91 (2015) 141-151.
[3] A. Oudriss, J. Creus, J. Bouhattate, F. Martin, X. Feaugas, Impact of dislocation distribution on hydrogen diffusion and trapping in tensile strengthening nickel (100) single crystal, Acta Materialia, (2015) in progress.

The corrosion resistance of metals relies on a passivating surface layer of dense and adherent oxide. However, the integrity of such a protective oxide is compromised in the presence of excess hydrogen which can cause blistering and eventual spallation of the oxide film. So far it remains unclear as to how a nanoscale gas bubble manages to reach its critical size at the metal/oxide interface before the oxide layer can deform. Using in situ environmental transmission electron microscopy, we have discovered that once the aluminum metal/oxide interface is weakened by the segregating hydrogen, rampant surface-diffusion of Al atoms sets in to form numerous gas-accumulating cavities on the metal side driven by Wulff reconstruction. The morphology and growth of these metal-side cavities are found to be highly orientation sensitive. The surface oxide layer remains unyielding until the metal-side cavities grow to a critical size above which the accumulated gas pressure become strong enough to blister the oxide layer. Our findings have broad implications for coating performance in nuclear, petroleum, and transportation industries, and can help optimize the material design strategies to alleviate a broad range of hydrogen induced interface failures. ( Xie et al, Nature Materials, 2015)

By using multi-scale simulation techniques, we probed the role of hydrogen-vacancy complexes on nucleation and growth of proto nano-voids upon dislocation plasticity in α-Fe. Our atomistic simulations reveal that, unlike a lattice vacancy, a hydrogen-vacancy complex is not absorbed by dislocations sweeping through the lattice. Additionally, this complex has lower lattice diffusivity; therefore, it has a lower probability of encountering and being absorbed by various lattice sinks. Hence, it can exist metastably for a rather long time. Our large-scale atomistic simulations show that when metals undergo plastic deformation in the presence of hydrogen at low homologous temperatures, the mechanically driven out-of-equilibrium dislocation processes can produce extremely high concentrations of hydrogen-vacancy complex (10^-5~10^-3). Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. The current work provides the possible route for the experimentally observed nano-void formation in the context of hydrogen-induced failure and also bridge the link from the atomic-scale events to the macroscopic failure.

Metastable austenitic stainless steels such as type 304 suffer from severe hydrogen embrittlement (HE), whereas the type 310S stable austenitic steel exhibits little degradation in ductility because of deformation localization in the presence of hydrogen. This difference between these steels is related to the intrinsic tendency towards deformation-induced martensitic transformation. Moreover, fractographic observation has often shown quasi-cleavage and planar fracture features in hydrogenated austenitic stainless steels. The cleavage is presumably formed near martensite laths and/or annealing twin boundaries. However, the role of martensite and twin boundaries in the HE of metastable austenitic steels is controversial. Therefore, this study using microtension testing has been employed to analyze the HE in single crystals and twinned bicrystals, with a particular focus on the effect of the deformation-induced martensite transformation.
The material used in this study was a solution-treated 304 stainless steel with an average grain size of 60 mu;m. Microtension specimens with gauge section dimensions of 20 mu;m × 20 mu;m × 50 mu;m were fabricated using a focused ion beam. Single-crystalline specimens were prepared so that the loading direction (LD) is close to the [123] and [111] directions, denoted as specimen A and B, respectively. For the bi-crystalline specimen, the twin boundary was arranged perpendicular to the LD. A set of specimens was electrochemically charged with hydrogen prior to microtension testing. A microtension test was performed at room temperature in air and at a loading rate of 0.1 mu;m s^minus;1. After failure, the deformed microstructure was studied by electron back scatter diffraction (EBSD).
In the uncharged single-crystalline specimen A, yielding occurred at a stress of 170 MPa, and an ultimate strength of 490 MPa and 130% strain-to-failure were attained through a three-step strain hardening process. Specimen B exhibited a high yield stress but a low strain-to-failure owing to significant strain hardening. For both orientations, the yield stress increased but the ductility decreased drastically when pre-charged with hydrogen. For the hydrogen-charged single- and bi-crystalline specimens, the fracture morphology exhibited quasi-cleavage and a planar facet fracture, respectively. The EBSD observation of the deformation microstructures suggests that hydrogen-induced fractures occurred along the martensite habit plane and