The improvement of hydrogen embrittlement (HE) is a key problem for transition-metal silicides. Although C40 TMSi2 disilicides are attracted candidates for ultrahigh-temperature applications, the HE mechanism of TMSi2 is unclear. Importantly, the role of hydrogen on the structural configuration, elastic modulus, and hardness of TMSi2 is entirely unknown. To reveal the HE, we study the role of hydrogen in TMSi2 (TM = Nb, Mo, and W) based on the first-principles calculations. Four H-doped sites are considered in detail. The calculated results show that hydrogen is favorable to occupy the octahedral interstitial site because the C40 TMSi2 layered structure is favorable to absorb hydrogen. H-doping results in lattice expansion of c-axis compared with the a-axis and b-axis. H-doping obviously reduces the elastic modulus and hardness of TMSi2 due to the interaction between hydrogen and TMSi2. In addition, H-doping changes the electronic properties of MoSi2 and WSi2.

The high precision offered by small-scale mechanical testing has allowed the relationships between mechanical behavior and specific microstructural features to be determined to an unprecedented degree. However, of most interest to scientists and engineers is often the behavior of materials under service conditions in an extreme environment, such as high/low temperatures, high strain rates, hydrogen atmosphere, or radiation. In this article, we detail progress made to adapt nanomechanical testing systems and techniques to observe materials behavior in situ in extreme environments.

T91 ferritic/martensitic (F/M) steel is an expected structural material candidate for Gen IV liquid lead-bismuth cooled nuclear reactors. However, molten lead-bismuth eutectic alloy (mLBE) often causes liquid-metal embrittlement (LME) of F/M steels. Although prior austenite grain boundaries and martensite block boundaries were reported to be preferential sites for LME, the mechanism of LME in a T91/LBE couple is yet to be comprehensively understood. In this paper, the effect of mLBE on T91 steel was investigated using micropillar compression tests. mLBE corrosion was found to cause a significant decrease in yield stress. The micropillar made from mLBE-corroded specimen was significantly sheared along high angle boundaries. In addition, EDS analysis demonstrated the presence of Pb/Bi at these high angle boundaries.

The present investigation focused on evaluating the effects of different amounts of precipitates on tension and hardness properties of a Ti–6Al–4V alloy, for this, two sets of samples with bimodal and equiaxed microstructures were obtained by heat treatment process. After this, the samples were aged at 545 °C for holding times of 100 and 200 h. Observation of the aged samples in the scanning electron microscope revealed fine α2 (Ti3Al) particles precipitating in the α matrix of the alloy. Uni-axial tension tests and Vickers microhardness measurements were conducted on the unaged and aged samples. Experimental results show that ultimate strength and hardness of bimodal and equiaxed microstructures are significantly affected by the heat treatment applied to produce such microstructures. Also, it was found that the precipitation of α2 particles drastically reduced the plastic response of the Ti–6Al–4V alloy.

The wettability of the 304L steel is an important parameter in Liquid Metal Embrittlement studies. Empirically, it is found to be greatly enhanced by pre-exposure to oxygenated liquid sodium. The corrosion interface formed during exposure to sodium has been analyzed at the nanoscale by transmission electron microscopy using the focused ion beam sampling. A thin layer of sodium chromite (NaxCrO2 with x ≤ 1) is detected at the interface validating wetting on an oxide mechanism for the enhanced wetting after pre-exposure. Fracture micromechanisms and the crack path of liquid sodium-embrittled austenitic steel 304L at 573 K have been investigated down to the nanoscale. High-resolution orientation mapping analyses immediately below the fracture surface show that abundant martensitic transformations (γ → α) and twinning occur during deformation of austenite. The preferential crack path is intergranular along the newly formed γ/γ interfaces. It is concluded that these transformations play a major role in the fracture process.

Advanced reduced activation ferritic/martensitic steels and oxide dispersion-strengthened steels exhibit significant radiation embrittlement under low temperature neutron irradiation. In this study we focused on atom probe tomography (APT) of Eurofer97 and ODS Eurofer steels irradiated with neutrons and heavy ions at low temperatures. Previous TEM studies revealed dislocation loops in the neutron-irradiated f\m steels. At the same time, our APT showed early stages of solid solution decomposition. High density (1024 m–3) of ∼3–5 nm clusters enriched in chromium, manganese, and silicon atoms were found in Eurofer 97 irradiated in BOR-60 reactor to 32 dpa at 332°C. In this steel irradiated with Fe ions up to the dose of 24 dpa, pair correlation functions calculated using APT data showed the presence of Cr-enriched pre-phases.

APT study of ODS Eurofer found a significant change in the nanocluster composition after neutron irradiation to 32 dpa at 330 °C and an increase in cluster number density. APT of ODS steels irradiated with Fe ions at low temperatures revealed similar changes in nanoclusters.

These results suggest that irradiation-induced nucleation and evolution of very small precipitates may be the origin of low temperature radiation embrittlement of f\m steels.

Polyethylene terephthalate (PET) is an interesting flexible substrate material for many applications in electronics since it is inexpensive and has good mechanical and electrical properties. Due to these good properties there is increasing interest in using PET in applications involving harsh environments. However, PET has relatively poor thermal properties, which may cause reliability concerns in demanding conditions. Above its glass transition temperature, Tg, PET is susceptible to hydrolysis, which may considerably restrict its use. Hydrolysis breaks the molecular structure of PET and makes the material mechanically unreliable. This paper reports the reliability of PET films in an LCD display application and in RFID tags in thermal and humid conditions. Additionally, neat PET films were studied. Several thermal cycling and humidity tests were used. The results clearly showed that above the Tg of PET the combined effect of humidity and high temperature caused PET to become brittle leading to considerable reliability problems. However, in dry thermal cycling conditions and in humid conditions below the Tg of PET, the reliability of PET was found to be excellent even under prolonged exposure.

Susceptibility to external stress corrosion cracking (ESCC) of API X52 pipeline steel in calcareous soil containing different moisture content has been investigated using slow strain rate tests (SSRT). This type of soil is common of the state of Campeche Mexico and has a pH around 8. The results indicate that X52 pipeline steel was susceptible to external SCC only in the saturated calcareous soil, showing some micro cracks in the gage section of the SSR specimen. It was observed that some micro cracks were found at the bottom of micro-pits. Which indicate that first develop a pit and this evolved with time and micro-strain like a crack. Few micro cracks were observed as initiation of SCC close to surface failure. The mechanism of SCC may be influenced by formation and rupture film of carbonates.

The displacement per atom (dpa) has been a specific issue to evaluate the damage in the first wall of the Tokamak. Two different first wall materials were evaluated. In this study, MCNP5 code was used to obtain the neutron flux, the energy deposition and the main reaction rates, on the inboard and outboard first wall. The damage calculations were performed by the SPECTER code using the neutronic parameters obtained by MCNP5. The Tokamak reactor modeled has similar dimensions to the ITER. Tungsten and beryllium alloys were simulated on the outboard first wall. The results indicate which material has a higher resistance to be damage and dpa values for the analyzed material.

Integrity and lifetime of reactor pressure vessels are practically determined by their material resistance against fast/non-ductile failure and consequently by their radiation damage resulting in irradiation embrittlement and hardening. Mechanism of irradiation embrittlement of RPV materials depends on selected materials and operation conditions but their values depend in a large extent on reactor design, i.e. on neutron flux/fluence depending on RPV wall. Generally, new RPV design allows smaller neutron fluences but absolute value of irradiation embrittlement still depends on a choice of RPV material. Even though radiation damage (especially irradiation embrittlement) is important for RPV behavior, integrity and lifetime depends, in principle, on final value of applied fracture mechanics parameter –transition temperature. Thus, its initial value as well as its shift due to irradiation embrittlement is of interest but only the embrittlement can be affected during operation.

The effect of strain rate on hydrogen embrittlement of low alloy 4340 steel was studied using double-notched tensile samples electrochemically charged in-situ with hydrogen in 1N H2SO4 + 5 mg/l As2O3 solution. The mechanical response of samples with prior austenitic grain sizes of 10 and 40 μm and martensitic hardness of 43-52 HRC were examined after hydrogen charging times of 0-20 min. Increasing the strain rate for hydrogen charged samples resulted in decreased failure strains and increased evidence of brittle fracture. Brittle fracture surfaces for the harder samples showed primarily intergranular fracture while softer samples exhibited predominantly quasi-cleavage.

We have studied the LME phenomenon for the Cu/Hg couple, from an experimental and a computational point of view. We compared the LME behavior of standard oxygen free high conductivity (OFHC) copper with Grain Boundary Engineered (GBE) copper (containing a high fraction of special Σ3 GBs). Experimentally, we find that special Σ3 GBs in copper are less prone than general GB to LME by liquid mercury. In parallel, we have investigated the difference in LME induced fracture between the symmetric Σ3(111)[110]70.5° tilt GB and the symmetric Σ5(210)[100]36.87° tilt GB by ab-initio calculations. The Hg segregation trend has been evaluated for these 2 GBs. Ab-initio tensile tests on the Σ3(111) GB with and without segregated Hg atoms have been performed. Finally solid/liquid interfaces have been modeled using ab-initio molecular dynamics (AIMD) in order to calculate solid-liquid surface energies (γSL). Using a Griffith approach, we have evaluated the energy difference γGB - 2 γSL. The LME mechanism in Cu/Hg is discussed.

Hydrogen can be used as an environmentally friendly fuel to power vehicles, electric devices, and spacecraft with water vapor as the only emission. One associated challenge is the development of safe hydrogen storage systems. Hydrogen tanks and other hydrogen infrastructure elements will be exposed to both high-pressure hydrogen and cyclic stresses. In our work, 304 stainless steel specimens were precharged with hydrogen and subjected to rotational bending fatigue with a maximum stress amplitude of 90 ksi. A diffusion model was solved to approximate the concentration of hydrogen in the specimen at the time of the test. Contrary to our previous work with simple bending fatigue tests, hydrogen precharging actually increased rotational bending fatigue life from 28,074 (Sx = 7,430, N = 103) cycles to 91,513 (Sx = 40,209, N=32) cycles, a factor of approximately 3.25. This result demonstrates that the effect of hydrogen on fatigue life can be highly situational, and great care should be taken when designing systems that will be exposed to high-pressure hydrogen under fatigue conditions.

In order to develop NiMnGa/polymer composite materials, a production of single-crystal-like NiMnGa particles is important and should be developed for better quality. Although mechanical pulverization is a promising method by utilizing intrinsic intergranular brittleness of NiMnGa polycrystalline ingots, the amount of lattice defects introduced during mechanical crushing needs to be minimized. This must be achieved by enhancement of intergranular brittleness of NiMnGa particles. In this study, the effect of Bi addition on the compressive fracture behavior of polycrystalline Ni50Mn28Ga22 was investigated where Bi was expected to be segregated to the grain boundaries in NiMnGa, similar to Bi segregation to the grain boundaries in Ni. It was found that only intergranular fracture was observed in Ni50Mn28Ga22 polycrystals with 0.3 at.% Bi addition, although a mixture of intergranular and transgranular fracture was observed in Bi-free Ni50Mn28Ga22 polycrystal. Microalloying of Bi into NiMnGa enhances intergranular embrittlement. A number of spherical particles of Bi were confirmed on the fractured surface of Bi-doped NiMnGa polycrystals. The formation of Bi particles is a proof of the grain boundary segregation of Bi in NiMnGa.

The use of light weight structural materials such as titanium in transport systems like aero planes leads to a significant reduction in fuel consumption. However, titanium and its alloys cannot be used at elevated temperatures above 500°C for several reasons. Today aero engine compressors are made of a mixture of light Ti- and heavy Ni-alloys. The improvement of Ti-alloys to withstand the conditions in the high pressure compressor i.e. temperatures above 500°C would enable the manufacturing of a compressor from titanium as a whole with all its associated benefits. Intermetallic TiAl-alloys are another class of light weight materials for several high temperature applications. The use of TiAl as low pressure turbine (LPT) blades in the last sections of a large jet engine could save up to 150 kg of weight. In the last sections of the LPT the temperature is quite moderate (max. 650°C). The improvement of the high temperature capability of TiAl would allow its use in hotter sections of the engine with additional weight reduction. Similarly, the response performance of TiAl-turbocharger rotors in automotive engines would be much faster compared to the heavy Ni-based alloys used today. Furthermore higher rotation speeds are possible. Due to the novel so called fluorine effect the oxidation mechanism of TiAl can be altered. Fluorine-treated TiAl-components are protected by an alumina layer formed during high temperature exposure in oxidizing environments. This effect can be transferred to Ti-base materials if they are enriched with aluminum in a thin surface zone. The concepts and the results of high temperature exposure experiments of treated Ti- and TiAl-specimens are presented in this paper. They are discussed in the view of a use for real components.

Micromechanical testing of focused ion beam (FIB) machined cantilevers was used to study oxidised grain boundaries in Ni-alloy 600. The Ni-alloy 600 samples were exposed in simulated PWR primary water at 325°C for 4500h with a hydrogen partial pressure of 30kPa. The FIB was used to machine small cantilever beams at the selected sites in the Ni alloy 600, cut so that the beam contained a selected grain boundary close to the built-in end. The FIB was also used to make a pre-crack, 700 nm deep, on the grain boundary. Cantilevers were loaded at the free end using a nanoindenter. Cantilevers milled in the un-oxidised sample yielded, and did not fracture. The specimens containing oxidised grain boundaries fractured at the boundary after small amounts of plasticity. Load vs. displacement data were used to calculate the fracture toughness of the oxidised grain boundaries. The fracture toughness associated with fracture of grain boundary oxide for these cantilevers was in the range 0.73-1.82MPa (m)1/2, with an average value of 1.3MPa (m)1/2. We believe this to be the first time the fracture toughness of an oxidised grain boundary has been determined.

We discuss the relative complexity and computational cost of several popular many-body empirical potentials, developed by the materials science community over the past 30 years. The inclusion of more detailed many-body effects has come at a computational cost, but the cost still scales linearly with the number of atoms modeled. This is enabling very large molecular dynamics simulations with unprecedented atomic-scale fidelity to physical and chemical phenomena. The cost and scalability of the potentials, run in serial and parallel, are benchmarked in the LAMMPS molecular dynamics code. Several recent large calculations performed with these potentials are highlighted to illustrate what is now possible on current supercomputers. We conclude with a brief mention of high-performance computing architecture trends and the research issues they raise for continued potential development and use.

Steel quenching from the austenite region is a widely used industrial process to increase strength and hardness through the martensitic transformation. It is well known, however, that it is very likely that macroscopic distortion occurs during the quenching process. This distortion is caused by the rapidly varying internal stress fields, which may change sign between tension and compression several times during quenching. If the maximum internal stress is greater than the yield stress at given processing temperature, plastic deformation will occur and, depending on its magnitude, macroscopic distortion may become apparent.

The complex interaction between thermal contraction and the expansion resulting from the martensitic transformation is behind the sign changes in the internal stress fields. Variations in the steel composition and cooling rate will result in a number of different paths, which the internal stresses will follow during processing. Depending on the route followed, the martensitic transformation may hinder the thermal stresses evolution to the point where the stress fields throughout the component may actually be reverted. A different path may support the thermal stresses evolution further increasing their magnitude. The cross-sectional area also affects the internal stresses magnitude, since smaller areas will have further trouble to accommodate stress, thus increasing the distortion. Additionally, the bainitic transformation occurring during relatively slow cooling rates may have an important effect in the final stress field state.

A finite-element (FE) model of steel quenching has been developed in the DEFORM 3D simulation environment. This model has taken into account the kinetics of both austenite-bainite and austenite-martensite transformations in a simplified leaf spring geometry. The results are discussed in terms of the optimal processing parameters obtained by the simulation against the limitations in current industrial practice.

In a 2.25Cr1.5W heat-resistant alloy, it is shown that the time to intergranular failure under tensile stress t can be expressed by , where t0 is the constant of proportionality, n is the stress exponent, and Q is the activation enthalpy. It is shown that the dimples observed at elevated-temperature intergranular fracture surfaces are not the micro-ductile fracture areas but the interfaces between the grain boundary carbides and the neighboring grains. It is also shown that the segregation concentration of solute atoms is much higher at the grain boundary carbide interfaces than at the carbide-free grain boundaries. Under tensile stress, the elevated-temperature intergranular cracking occurs through the decohesion of grain boundary carbide interfaces, which is followed by the eventual carbide-free grain boundary cracking.

A nonlinear field projection method has been developed to study nanometer scale mechanical properties of grain boundaries in nanocrystalline FCC metals. The nonlinear field projection is based on the principle of virtual work, for virtual variations of atomic positions in equilibrium through nonlocal interatomic interactions such as EAM potential interaction, to get field-projected subatomic-resolution traction distributions on various grain boundaries. The analyses show that the field projected traction produces periodic concentrated compression sites on the grain boundary, which act as crack trapping or dislocation nucleation sites. The field projection was also used to assess the nanometer scale failure processes of Cu Σ5 grain boundaries doped with Pb. It was revealed that the Pb dopants prevented the emission of dislocations by grain boundary slip and embrittles the grain boundary.