Hardening effects of sheared precipitates on {1121} twinning in magnesium alloys

The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the(110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length(in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning(non-sheared precipitate), {1121} twinning(sheared precipitate) and basal dislocations(non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.

Electrostatic removal of ultrafine carbon black with square-grooved collecting plates:Insights for capturing black carbon emitted from ships

Ships and other mobile pollution sources emitted massive ultrafine and low-resistivity particles containing black carbon(BC),which were harmful to human health and were difficult to capture by conventional electrostatic precipitators(ESPs).In this study,nanoscale carbon black was adopted as simulated particles(SP)with similar physicochemical properties for black carbon emitted from ships(SP-BC)to investigate the feasibility of using an ESP with square-grooved collecting plates for the removal of SP-BC at low backpressures.The increased applied voltage significantly improved the total collection of SP-BC whereas may also promote the conversion of relatively larger particle size SP-BC into nano-size below 20nm.The outlet number concentration of SP-BC under 27 kV at 130℃was three times that of the inlet.While the reduction of the flow rate could strengthen the capture of SP-BC below20 nm,and under the combined action of low flow rate and maximum applied voltage,the collection efficiency of 20-100 nm SP-BC could exceed 90%.In addition,the escape and capture characteristics of SP-BC under long-term rapping were revealed.The square-grooved collecting plate could effectively restrain the re-entrainment of collected SP-BC generated by rapping,and the nanoscale SP-BC was trapped in the grooves after rapping.The results could provide insights into the profound removal of massive nanoscale black carbon emissions from mobile sources.

Bioinspired anti-freezing 3D-printable conductive hydrogel microfibers for highly-sensitive and wide-range detection of ultralow and high strains

Soft strain sensors that can transduce stretch stimuli into electrical readouts are promising as sustainable wearable electronics.However,most strain sensors cannot achieve highly-sensitive and wide-range detection of ultralow and high strains.Inspired by bamboo structures,anti-freezing microfibers made of conductive poly(vinyl alcohol)hydrogel with poly(3,4-ethylenedioxythiphene)-poly(styrenesulfonate)are developed via continuous microfluidic spinning.The microfibers provide unique bamboo-like structures with enhanced local stress to improve both their length change and resistance change upon stretching for efficient signal conversion.The microfibers allow highlysensitive(detection limit:0.05%strain)and wide-range(0%-400%strain)detection of ultralow and high strains,as well as features of good stretchability(485%strain)and anti-freezing property(freezing temperature:-41.1°C),fast response(200 ms),and good repeatability.The experimental results,together with theoretical foundation analysis and finite element analysis,prove their enhanced length and resistance changes upon stretching for efficient signal conversion.By integrating microfluidic spinning with 3D-printing technique,the textiles of the microfibers can be flexibly constructed.The microfibers and their 3D-printed textiles enable highperformance monitoring of human motions including finger bending and throat vibrating during phonation.This work provides an efficient and general strategy for developing advanced conductive hydrogel microfibers as highperformance wearable strain sensors.

Molecular Dynamics Simulations of Displacement Cascade in Ni-Based Concentrated Solid Solution Alloys

Single-phase concentrated solid solution alloys(SP-CSAs),including high-entropy alloys,have received extensive attention due to their excellent irradiation resistance.In this work,displacement cascade simulations are conducted using the molecular dynamics method to study the evolution of defects in Ni-based SP-CSAs.Compared with pure Ni,the NiCr,NiCo,and NiCu alloys exhibit a larger number of Frankel pairs(FPs)in the thermal peak stage,but a smaller number of surviving FPs.However,the NiFe alloy displays the opposite phenomenon.To explain these different observations for NiFe and other alloys,the formation energy and migration energy of interstitials/vacancies are calculated.In the NiFe alloy,both the formation energy and migration energy barrier are higher.On the other hand,in NiCr and other alloys,the formation energy of interstitials/vacancies is lower,as is the migration energy barrier of interstitials.The energy analysis agrees well with previous observations.The present work provides new insights into the mechanism behind the irradiation resistance of binary Ni-based SP-CSAs.

应变梯度引起的极化畴结构减少铁电材料脆性

铁电材料的力电耦合特性以及在纳米尺度的快速失效导致其剪切过程很难研究,而材料的脆性也限制了这些材料在铁电器件中的潜在应用.在这项工作中,我们使用分子动力学方法模拟了剪切和畴演变的动态过程,揭示了这些畴结构如何影响材料的力电性能.在原子尺度上,一个明显的极化旋涡状的畴结构出现在剪切带的尖端,两侧分别有180°和90°的畴壁.由应变梯度引起的挠曲电效应导致了这些复杂的畴结构.施加外部电场阻碍畴的翻转,并通过减少积累的能量减少了失配应变区域,加速了剪切过程.结果表明极化旋涡畴阻碍了剪切破坏过程,这启示我们可以通过设计畴结构来增韧铁电材料.

金属材料辐照缺陷演化的分子动力学研究进展

辐照条件下,高能粒子在金属材料内部引入稠密的辐照缺陷,导致材料力学性能严重退化,缩短材料服役寿命,是辐照材料研究的关键问题.辐照缺陷多处在纳米尺度,故分子动力学方法是模拟辐照缺陷的有力工具,近年来被广泛用于研究辐照缺陷演化.论文总结了金属材料中辐照缺陷演化的分子动力学研究进展,介绍了级联碰撞、点缺陷、空洞、氦泡、Frank位错环、层错四面体等辐照缺陷,及其与位错、晶界等微结构的相互作用.分子动力学方法揭示的机制与模型,深化了学界对辐照效应的认识,有助于提高辐照材料力学性能和设计耐辐照材料.

Hierarchical-vortex polarization domain pattern in nano polycrystalline ferroelectric

Domain pattern is the carrier of electromechanical property.A novel domain pattern will open a gate for ferroelectric nano device.A distinctive topological domain pattern termed as hierarchical vortex(Hvo)has been found in polycrystalline ferroelectric based on the first-principles-based atomistic method.The Hvo pattern displays a unique structure,which is a flux-closing vortex encircle an anti-vortex or a vortex and anti-vortex pair(VA).Each Hvo structure could be regarded as a single vortex to forming a vortex-anti-vortex pair with anti-vortex or forming a vortex-vortex array with the vortex.The mechanism of HVo obtained in polycrystalline ferroelectric has been found that the grain boundary(GB)equals the domain wall when the first-order vortex is in the vortex.The HVo will open a new view of the domain topology pattern and its evolution.

Adhesive Contact of a One-Dimensional Hexagonal Quasicrystal Half-Space Punched by a Spherical Indenter

This paper develops the adhesive contact theory for a one-dimensional hexagonal quasicrystal half-space punched by a spherical indenter on the basis of the classical adhesive contact models involving the Johnson–Kendall–Roberts(JKR)model,the Maugis–Dugdale(MD)model and the Derjaguin–Muller–Toporov(DMT)model.By using the superposition principle combined with the Griffith energy balance,all the significant physical quantities for adhesive contact,such as the energy release rate,indentation force,penetration depth,contact radius and pull-out force,are obtained for different models.The result for the DMT model is derived from the MD solution through a limiting procedure.A numerical calculation is carried out to verify the present analytical solutions,to compare different contact models,and to analyze the influence of the phason field on the results.It is indicated that the effect of the phason field on the result for the MD model is pronounced,especially for a small contact radius.However,the phason effect on the JKR and DMT results is not significant.The present solution can serve as a theoretical basis for nano-indentation and atomic force microscopy to measure the material properties of quasicrystals.

Decomposition of(c+a) Dislocations in Magnesium Alloys

In this paper,molecular dynamics simulations are performed to investigate the decomposition of(c+a)dislocations on both pyramidal-Ⅰ and pyramidal-Ⅱ planes.The pyramidal-Ⅰ dislocations are decomposed into(c)and(a)dislocations under shear stress at 0-400 K,which all reside on the basal plane.At 500-700 K,the dislocations are transited onto the basal plane at zero stress,then decomposed into(c)and(a)dislocations under shear loading.In particular,at 700 K,the dislocation is possibly decomposed spontaneously at zero stress.For the pyramidal-Ⅱ dislocations,the core is glissile below 400 K.At 500 K,the dislocation is transited onto the basal plane under shear loading.At 600-700 K,basal(c+a)dislocation is formed at zero stress,but then decomposed under shear loading.The dislocation core energy is calculated to explain the observations.It is found that the energy of decomposed(c+a)dislocation is high,the energy of pyramidal(c+a)dislocation is intermediate,and the energy of basal(c+a)dislocation is low.Our results provide new insights into the behaviors of pyramidal dislocations and temperature effects.