Hydrogen-Induced Dislocation Nucleation and Plastic Deformation of 〈001〉 and 〈11¯0〉 Grain Boundaries in Nickel.

The grain boundary (GB) plays a crucial role in dominating hydrogen-induced plastic deformation and intergranular failure in polycrystal metals. In the present study, molecular dynamics simulations were employed to study the effects of hydrogen segregation on dislocation plasticity of a series of symmetrical tilt grain boundaries (STGBs) with various hydrogen concentrations. Our study shows that hydrogen both enhances and reduces dislocation nucleation events from STGBs, depending on different GB structures. Specifically, for ⟨001⟩ STGBs, hydrogen does not affect the mode of heterogeneous dislocation nucleation (HDN), but facilitates nucleation events as a consequence of hydrogen disordering the GB structure. Conversely, hydrogen retards dislocation nucleation due to the fact that hydrogen segregation disrupts the transformation of boundary structure such as Σ9 (2 2 1¯) ⟨11¯0⟩ STGB. These results are helpful for deepening our understanding of GB-mediated hydrogen embrittlement (HE) mechanisms.

Investigation of a Novel Hydrogen Depressurization Structure Constituted by an Orifice Plate with Tesla-Type Channels.

A hydrogen depressurization system is required to supply the hydrogen to the fuel cell stack from the storage. In this study, a Tesla-type depressurization construction is proposed. Parallel Tesla-type channels are integrated with the traditional orifice plate structure. A computational fluid dynamics (CFD) model is applied to simulate high-pressure hydrogen flow through the proposed structure, using a commercial software package, ANSYS-Fluent (version 19.2, ANSYS, Inc. Southpointe, Canonsburg, PA, USA). The Peng-Robinson (PR) equation of state (EoS) is incorporated into the CFD model to provide an accurate thermophysical property estimation. The construction is optimized by the parametric analysis. The results show that the pressure reduction performance is improved greatly without a significant increase in size. The flow impeding effect of the Tesla-type orifice structure is primarily responsible for the pressure reduction improvement. To enhance the flow impeding effect, modifications are introduced to the Tesla-type channel and the pressure reduction performance has been further improved. Compared to a standard orifice plate, the Tesla-type orifice structure can improve the pressure reduction by 237%. Under low inlet mass flow rates, introduction of a secondary Tesla-type orifice construction can achieve better performance of pressure reduction. Additionally, increasing parallel Tesla-type channels can effectively reduce the maximum Mach number. To further improve the pressure reduction performance, a second set of Tesla-type channels can be introduced to form a two-stage Tesla-type orifice structure. The study provides a feasible structure design to achieve high-efficiency hydrogen depressurization in hydrogen fuel cell vehicles (HFCVs).

A crystal plasticity FEM study of through-thickness deformation and texture in a {112} <111> aluminium single crystal during accumulative roll-bonding.

In this study, a crystal plasticity finite element method (CPFEM) model was used to study the deformation behaviour in an aluminium single crystal (1 1 2)[1 1 -1] processed by accumulative roll-bonding (ARB) up to 9 cycles. The simulation followed the real ARB process based on the developed finite element model. The predicted through-thickness texture matches well with the experimental observations. The deformation behaviours, in terms of crystal rotation, shear strain and slip system activation, in the first and second cycles (conventional rolling) were unidirectional, but the deformation was altered after ARB was applied from the third cycle onwards. Such alteration was found to be caused by the thickness position change and deformation discontinuity at interfaces, which were investigated in detail. The role that interfaces play became dominant over thickness position change as increasing ARB cycles.

Investigation of Grain Refinement Mechanism of Nickel Single Crystal during High Pressure Torsion by Crystal Plasticity Modeling.

The excellent properties of ultra-fine grained (UFG) materials are relevant to substantial grain refinement and the corresponding induced small grains delineated by high-angle grain boundaries. The present study aims to understand the grain refinement mechanism by examining the nickel single crystal processed by high pressure torsion (HPT), a severe plastic deformation method to produce UFG materials based upon crystal plasticity finite element (CPFEM) simulations. The predicted grain maps by the developed CPFEM model are capable of capturing the prominent characteristics associated with grain refinement in HPT. The evolution of the orientation of structural elements and the rotations of crystal lattices during the HPT process of the detected differently oriented grains are extensively examined. It has been found that there are mainly two intrinsic origins of lattice rotation which cause the initial single crystal to subdivide. The correlation between the crystallographic orientation changes and lattice rotations with the grain fragmentation are analyzed and discussed in detail based on the theory of crystal plasticity.

Enhanced surface Fenton degradation of BPA in soil with a high pH.

Although the heterogeneous Fenton process of iron-bearing minerals has been widely studied due to its potential use for the removal of organic pollutants, the transformation mediated by Fe species in soil particles remains poorly understood. Here, we compared the removal of bisphenol A (BPA) from soil using a Fenton system at low and high pH values. At low pH value, the BPA removal rate decreased with increasing pH value; this result was consistent with the amount of soluble Fe(II) and surface-bound Fe(II) observed in the soil systems. In contrast, an increased BPA degradation efficiency was observed at high pH, which is different from the traditional Fenton system. The electron spin resonance analysis verified that the high BPA degradation rate was attributed to enhanced ·OH generation. The binding environments of the Fe species in the soil for different reaction pH values were investigated by using Mössbauer spectroscopy combined with selective chemical extraction. A mixed-valence Fe(II) phase was observed at pH 12.0 and accounted for 12% of the total Fe content. The results indicate that in addition to the well-studied soluble Fe(II) and surface-bound Fe(II), structural Fe(II) located in the newly formed secondary precipitates may play a more important role in the generation of ·OH, especially at high pH values. These findings may provide insights into the utilization of Fe-bearing soil minerals as a renewable source for the degradation of organic pollutants over a wide pH range.

Tribological Behavior of Aqueous Copolymer Lubricant in Mixed Lubrication Regime.

Although a number of experiments have been attempted to investigate the lubrication of aqueous copolymer lubricant, which is applied widely in metalworking operations, a comprehensive theoretical investigation at atomistic level is still lacking. This study addresses the influence of loading pressure and copolymer concentration on the structural properties and tribological performance of aqueous copolymer solution of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) (PPO-PEO-PPO) at mixed lubrication using a molecular dynamic (MD) simulation. An effective interfacial potential, which has been derived from density functional theory (DFT) calculations, was employed for the interactions between the fluid's molecules and iron surface. The simulation results have indicated that the triblock copolymer is physisorption on iron surface. Under confinement by iron surfaces, the copolymer molecules form lamellar structure in aqueous solution and behave differently from its bulk state. The lubrication performance of aqueous copolymer lubricant increases with concentration, but the friction reduction is insignificant at high loading pressure. Additionally, the plastic deformation of asperity is dependent on both copolymer concentration and loading pressure, and the wear behavior shows a linear dependence of friction force on the number of transferred atoms between contacting asperities.

Experiment and molecular dynamics simulation of nanoindentation of body centered cubic iron.

Experiments and molecular dynamics (MD) simulations have been conducted to investigate the nanoindentation behaviours of iron with body centered cubic (BCC) structure. The experiments show that the indentation hardness decreases with the indentation depth and it changes sharply for a small depth. Two cases with different crystallographic orientations have been simulated. The indentation plane is (010) for Case I and (111) for Case II, respectively. The calculated harness (17.4 GPa for Case I and 22.6 GPa for Case II) are in reasonable agreement with the experimental value (24.2 GPa). The simulation results show that the crystallographic orientation significantly influences the indentation deformation. Case I and Case II exhibit different deformation patterns. The indentation force and the hardness in Case I are smaller than Case II. It is also found that the pileup around the indenter is mainly formed along [110] direction for both cases.