Solution strengthening is a well-known approach to tailoring the mechanical properties of structural alloys. Ultimately, the properties of the dislocation/solute interaction are rooted in the ...electronic structure of the alloy. Accordingly, we compute the electronic structure associated with, and the energy barriers to dislocation cross-slip. The energy barriers so obtained can be used in the development of multiscale models for dislocation mediated plasticity. The computed electronic structure can be used to identify substitutional solutes likely to interact strongly with the dislocation. Using the example of a-type screw dislocations in Mg, we compute accurately the Peierls barrier to prismatic plane slip and argue that Y, Ca, Ti, and Zr should interact strongly with the studied dislocation, and thereby decrease the dislocation slip anisotropy in the alloy.
•We develop a crystal plasticity model considering the effect of dislocation source.•Dislocation behavior can be reproduced by the crystal plasticity model.•Differences in dislocation behavior cause ...discontinuous yielding and a yield point drop.•The yield stress is affected by the size of the dislocation source.
Ultrafine-grained metals (UFGMs) produced by warm- or cold-rolling under severe plastic deformation have attracted interest as high-strength structural materials. UFGMs with a grain size less than 1μm exhibits remarkable material and mechanical properties, and a computational model predicting these properties is desired in the field of materials science and engineering. In order to clarify the utility of UFGM numerically, it is important to investigate the size effects of metallic materials that depend on initial grain size. It is assumed that such unusual mechanical properties originate in grain size and the enormous volume fraction of the grain boundary. When grains are of the submicron order, dislocation loops are hardly generated from Frank–Read sources smaller than the grain size. Grain boundaries play an important role in dislocation dynamics. In this study, we develop a crystal plasticity model considering the effect of the grain boundary and dislocation source. In order to predict variation of critical resolved shear stress (CRSS) due to grain boundaries or dislocation sources, information on dislocation source and grain boundary is introduced into a hardening law of crystal plasticity. In addition, FE simulation for FCC polycrystal is used to analyze stress–strain responses such as increased yield stress and yield point drop, from the viewpoint of grain size and dislocation density. We thoroughly investigate the effect of dislocation behavior on the material properties of UFGMs.
Although body-centered-cubic (bcc) metals and alloys are ubiquitous as structural materials, they are brittle, particularly at low temperatures; however, the mechanism of their brittle fracture is ...not fully understood. In this study, we conduct a series of three-dimensional molecular dynamics simulations of the cleavage fracture of α-iron. In particular, we focus on mode-I loading starting from curved crack fronts or the so-called penny-shaped cracks. In the simulations, brittle fractures are observed at cleavages on the {100} plane, while the initial cracks become blunted on other planes as a result of dislocation emissions. Our modeling results agreed with a common experimental observation, that is, {100} is the preferential cleavage plane in bcc transition metals. In addition, dislocation emissions from the crack front were analyzed; the result supported the notion that plasticity in the vicinity of the crack front determines the preferential cleavage plane.
Body-centered-cubic (BCC) transition metals are ubiquitous structural materials, and their mechanical degradation under irradiation is significantly influenced by the stability and mobility of the ...lattice defects. In this study, we analyzed the self-interstitial atoms (SIAs) in BCC molybdenum (Mo) and tungsten (W) in comparison with other BCC transition metals utilizing the first-principles method; particularly, we focused on uncommon dumbbells whose direction are inclined from 〈111〉 toward 〈110〉 on the {110} plane. Such a direction is not stable in the group 5 BCC metals (i.e., vanadium, niobium, and tantalum) or in α-iron. Our first-principles relaxation simulations indicated that inclined dumbbells were more energetically favored than common 〈111〉 dumbbells in Mo, while this is not necessarily the case for W. However, a certain degree of lattice strain, such as shear or expansive strain, could make inclined dumbbells more favored also in W, suggesting that the lattice strain can substantially influence the migration barrier of SIAs in these metals because inclined dumbbells generally have a larger migration barrier than 〈111〉 dumbbells. We also elucidated the mechanism of the inclination using the electronic charge density; the charge density map of the perfect crystals suggested that the anti-bonding state of electrons along the 〈111〉 direction is likely to cause the instability of 〈111〉 dumbbells, and the charge density map near dumbbells suggested how 〈111〉 dumbbells are inclined toward the 〈110〉 direction.
There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of ...magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.
Prediction of material performance in fusion reactor environments relies on computational modelling, and will continue to do so until the first generation of fusion power plants come on line and ...allow long-term behaviour to be observed. In the meantime, the modelling is supported by experiments that attempt to replicate some aspects of the eventual operational conditions. In 2019, a group of leading experts met under the umbrella of the IEA to discuss the current position and ongoing challenges in modelling of fusion materials and how advanced experimental characterisation is aiding model improvement. This review draws from the discussions held during that workshop.
Topics covering modelling of irradiation-induced defect production and fundamental properties, gas behaviour, clustering and segregation, defect evolution and interactions are discussed, as well as new and novel multiscale simulation approaches, and the latest efforts to link modelling to experiments through advanced observation and characterisation techniques.
We report Suzaku results for soft X-ray emission to the south of the Galactic center (GC). The emission (hereafter "GC South") has an angular size of ~42' x16' centered at (l, b) ~ 0degrees.0, - ...1degrees.4) and is located in the largely extended Galactic ridge X-ray emission (GRXE). The X-ray spectrum of GC South exhibits emission lines from highly ionized atoms. Although the X-ray spectrum of the GRXE can be well fitted with a plasma in collisional ionization equilibrium (CIE), that of GC South cannot be fitted with a plasma in CIE, leaving hump-like residuals at ~2.5 and 3.5 keV, which are attributable to the radiative recombination continua of the K-shells of Si and S, respectively. In fact, GC South spectrum is well fitted with a recombination-dominant plasma model; the electron temperature is 0.46 keV while atoms are highly ionized (kT = 1.6 keV) in the initial epoch, and the plasma is now in a recombining phase at a relaxation scale (plasma density x elapsed time) of 5.3 x 10 super(11) s cm super(-3). The absorption column density of GC South is consistent with that toward the GC region. Thus, GC South is likely to be located in the GC region (~8 kpc distance). The size of the plasma, the mean density, and the thermal energy are estimated to be ~97 pc x 37 pc, 0.16 cm super(-3), and 1.6 x 10 super(51) erg, respectively. We discuss possible origins of the recombination-dominant plasma as a relic of past activity in the GC region.
Electrochemical impedance technique has been applied to study the corrosion behavior of galvanized steel under wet–dry cyclic conditions with various drying periods. The wet–dry cycles were carried ...out for the period of 336 h by exposure to alternate conditions of 1 h immersion in a 0.5 M NaCl solution and drying for various time periods (11, 7 and 3 h) at 298 K and 60% RH. During the wet–dry cycles, the polarization resistance,
R
p, and solution resistance,
R
s, were continuously monitored. The instantaneous corrosion rate of the coating was estimated from the obtained
R
p
−1 and time of wetness was determined from the
R
s values. The corrosion potential,
E
corr, was also measured only during the immersion period of each wet–dry cycle. In all cases, the corrosion was accelerated by the wet–dry cycles in the early stage, and started to decrease at a certain cycle and finally became similar to that at the initial cycle. The underlying steel corrosion commenced after the corrosion rate started to decrease. The shorter drying period in each cycle led to higher amount of corrosion of the coating because the surface was under wet conditions for longer periods. On the other hand, time to red rust appearance due to occurrence of the underlying steel corrosion became shorter as the drying period increased, although the total amount of corrosion was smaller. The corrosion mechanism of substrate steel under various drying conditions has been discussed, the galvanic coupling effect being taken into account.
PDF
