Grain growth simulation with the second phase particle pinning for heat-affected zone of microalloyed steel welds

The second phase particle dispersed in microalloyed steel has different effects on grain growth depending on their size and volume fiaction of the second phase particles which will change during welding thermal cycles. The particle coarsening and dissolution kinetics model was analyzed for continuous heating and cooling. In addition, based on experimental data, the coupled equation of grain growth was established by introducing limited size of grain growth with the consideration of the second phase particles pinning effects. Using Monte Carlo method based on experimental data model, the grain growth simulation for heat-affected zone of microalloyed steel welds was achieved. The calculating results were well in agreement with that of experiments.

Investigation of magnetization reversal process in pinned CoFeB thin film by in-situ Lorentz TEM

Exchange bias effect has been widely employed for various magnetic devices.The experimentally reported magnitude of exchange bias field is often smaller than that predicted theoretically,which is considered to be due to the partly pinned spins of ferromagnetic layer by antiferromagnetic layer.However,mapping the distribution of pinned spins is challenging.In this work,we directly image the reverse domain nucleation and domain wall movement process in the exchange biased Co Fe B/Ir Mn bilayers by Lorentz transmission electron microscopy.From the in-situ experiments,we obtain the distribution mapping of the pinning strength,showing that only 1/6 of the ferromagnetic layer at the interface is strongly pinned by the antiferromagnetic layer.Our results prove the existence of an inhomogeneous pinning effect in exchange bias systems.

A novelty strategy induced pinning effect and defect structure in Ni-rich layered cathodes towards boosting its electrochemical performance

Layered Ni-rich transition metal oxide is treated as the most promising alternative cathode due to their high-capacity and flexible composition.However,the severe lattice strain and slow Li-ion migration kinetics severely restrict their practical application.Herein,a novelty strategy induced pinning effect and defect structure in layered Ni-rich transition metal oxide cathodes is proposed via a facile cation(iron ion)/anion(polyanion)co-doping method.Subsequently,the effects of pinning effect and defect structure on element valence state,crystal structure,morphology,lattice strain,and electrochemical performance during lithiation/delithiation are systematically explored.The detailed characterizations(soft X-ray absorption spectroscopy(sXAS),in-situ X-ray diffraction(XRD),etc.)and density functional theory(DFT)calculation demonstrate that the pinning effects built-in LiNi_(0.9)Co_(0.05)Mn_(0.05)O_(2)materials by the dual-site occupation of iron ions on lithium and transition metal sites effectively alleviate the abrupt lattice strain caused by an unfavorable phase transition and the subsequent induction of defect structures in the Li layer can greatly reduce the lithium-ion diffusion barrier.Therefore,the modified LiNi_(0.9)Co_(0.05)Mn_(0.05)O_(2)exhibits a high-capacity of 206.5 mAh g^(-1)and remarkably enhanced capacity retention of 93.9%after 100 cycles,far superior to~14.1%of the pristine cathodes.Besides,an excellent discharge capacity of 180.1 mAh g^(-1)at 10 C rate is maintained,illustrating its remarkable rate capability.This work reports a pinning effect and defect engineering method to suppress the lattice strain and alleviate lithium-ion kinetic barriers in the Ni-rich layered cathodes,providing a roadmap for understanding the fundamental mechanism of an intrinsic activity modulation and structural design of layered cathode materials.

Influence of Ti(C,N)precipitates on austenite growth of micro-alloyed steel during continuous casting

Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.

The effect of V-Nb on the growth of austenite grains in 17CrNiMo6 gear steel

This study researches the effect of V-Nb on the growth of austenite grains in 17CrNiMo6 carburized gear steel. Results show that the carbonitride in V and Nb acts as second-phase particles in the steel, which can be used to block the migration of grain boundaries and the thinning of the austenite grains. This causes the crystals in the V-Nb microalloy 17CrNiMo6 steel to coarsen and the temperature to rise, thus reducing the cost of the carbonization that follows processing on the gears.

Enhancing corrosion resistance of ZK60 magnesium alloys via Ca microalloying: The impact of nanoscale precipitates

Enhancing corrosion resistance of Mg-Zn alloys with high strength and low cost was critical for broadening their large-scale practical applications. Here we prepared solutionized, peak-and over-aged ZK60 alloys with and without microalloying Ca(0.26 wt.%) to explore the effects of nanoscale precipitates on their corrosion behavior in detail via experimental analyses and theoretical calculations. The results suggested the peak-aged ZK60 alloy with Ca addition showed improved corrosion resistance in comparison with the alloys without Ca,owing to the contribution of Ca on the refinement of precipitates and increase in their number density. Although the precipitates and Mg matrix formed micro-galvanic couples leading to dissolution, the fine and dense precipitates could generate “in-situ pinning” effect on the corrosion products, forming a spider-web-like structure and improving the corrosion inhibition ability accordingly. The pinning effect was closely related to the size and number density of precipitates. This study provided important insight into the design and development of advanced corrosion resistant Mg alloys.

Nanocrystallization behaviour of a ternary amorphous alloy during isothermal annealing： a Monte Carlo simulation

The nanocrystallization behaviour of Zr70Cu20Ni10 metallic glass during isothermal annealing is studied by employing a Monte Carlo simulation incorporating with a modified Ising model and a Q-state Potts model. Based on the simulated microstructure and differential scanning calorimetry curves, we find that the low crystal-amorphous interface energy of Ni plays an important role in the nanocrystallization of primary Zr2Ni. It is found that when T〈T1max （where T1max is the temperature with maximum nucleation rate）, the increase of temperature results in a larger growth rate and a much finer mierostrueture for the primary Zr2Ni, which accords with the microstructure evolution in ＂flash annealing＂. Finally, the Zr2Ni/Zr2Cu interface energy σG contributes to the pinning effect of the primary nano-sized Zr2Ni grains in the later formed normal Zr2Cu grains.

Role of Cain hot compression behavior and microstructural stability of AlMg5 alloy during homogenization

Effects of a minor Ca addition on microstructural stability and dynamic restoration behavior of AlMg5 during hot deformation were investigated.They were studied using scanning electron microscopy(SEM),differential scanning calorimetry(DSC),electron backscatter diffraction(EBSD) analyses and transmission electron microscopy(TEM).JMatPro package was used for simulation of the solidification path of the alloys.The results show that the addition of Ca does not affect the microstructure and hot compression behavior of the as-cast samples.However,it prevents the drastic grain growth during homogenization.It is found that coarse grains of Ca-free alloy promote the dynamic recovery and slow down the dynamic recrystallization during hot compression.Also,the particle stimulated nucleation is suggested as the main nucleation mechanism of new recrystallized grains for hot compressed Ca-free alloy On the other hand,the microstructure of the hot compressed Ca-added alloy is greatly affected by the presence of Al4Ca intermetallics.The formation of Al4Ca phase is predicted by JMatPro and revealed by DSC,SEM and TEM studies.Finally,it is found that the presence of Al4Ca precipitates on the grain boundaries of Ca-added alloy prevents the growth of a(Al) by Zener pinning effect and results in the stability of microstructure during homogenization.

Macro-superlubricity in sputtered MoS_(2)-based films by decreasing edge pinning effect

To date,MoS_(2) can only be achieved at microscale.Edge pinning effect caused by structure defects is the most obvious barrier to expand the size of structural superlubricity to macroscale.Herein,we plan to pin edge planes of MoS_(2) with nanospheres,and then the incommensurate structure can be formed between adjacent rolling nanoparticles to reduce friction.The sputtered MoS_(2) film was prepared by the physical vapor deposition(PVD)in advance.Then enough Cu_(2)O nanospheres(~40 nm)were generated in situ at the edge plane of MoS_(2) layers by liquid phase synthesis.An incommensurate structure(mismatch angle(θ)=8°)caused by MoS_(2) layers was formed before friction.The friction coefficient of the films(5 N,1,000 r/min)was~6.0×10^(−3) at the most.During friction,MoS_(2) layers pinned on numerous of Cu_(2)O nanoparticles reduced its edge pinning effect and decreased friction.Moreover,much more incommensurate was formed,developing macro-superlubricity.

Improving cycling stability of Bi-encapsulated carbon fibers for lithium/sodium-ion batteries by Fe_(2)O_(3) pinning

Bi draws increasing attention as anode materials for lithium-ion batteries and sodium-ion batteries due to its unique layered crystal structure,which is in favor of achieving fast ionic diffusion kinetics during cycling.However,the dramatic volume expansion upon lithiation/sodiation and an insufficient theoretical capacity of Bi greatly hinder its practical application.Herein,we report the Fe_(2 )O_(3) nanoparticle-pinning Bi-encapsulated carbon fiber composites through the electrospinning technique.The introduction of Fe_(2 )O_(3) nanoparticles can prevent the growth and aggregation of Bi nanoparticles during synthetic and cycling processes,re s pectively.Fe_(2)O_(3) with high specific capacity also contributes to the specific capacity of the composites.Consequently,the as-prepared Bi-Fe_(2)O_(3)/carbon fiber composite exhibits outstanding long-term stability,which delivers reversible capacities 504 and 175 mAh/g after1000 cycles at 1 A/g for lithium-ion and sodium-ion batteries,respectively.

Tuning the Structural and Optical Properties of SILAR-Deposited Cu_2O Films Through Zn Doping

Undoped and Zn-doped Cu2O films were deposited onto glass substrates using successive ionic layer adsorption and reaction（SILAR） technique with different Zn doping levels（0, 1, 2, 3, 5 and 10 wt%）. The structural,optical, and surface morphological studies were carried out and reported. The structural study revealed that the crystalline quality is gradually enhanced up to 5 wt% of Zn doping level, and then quality begins to degrade for further increase in doping level. Moreover, the preferential orientation changes from（111） to（110） for the highest doping level were examined. Optical study shows that the transmittance（65%） and optical band gap values are maximum（2.41 e V） when the Zn doping level is at 5 wt%. The photoluminescence study confirms the presence of various defects in the Cu2O matrix and also the variation obtained in the optical band gap from the transmittance data. SEM images revealed the annealinginduced changes in the surface morphology of the films.

Ablative behavior and mechanism of SiC_(f) /SiBCZr composites prepared by PIP process

SiC_(f)/SiBCZr composites were prepared by polymer precursor impregnation and pyrolysis process with near stoichiometric ratio SiC fiber preform as reinforcement phase and SiBCZr multiphase ceramic precursor as impregnating reagent.The results highlighted that the SiC_(f)/SiBCZr composites exhibited excellent ablative properties after ablative tests at 1200℃/3600 s and 1400℃/3600 s,and the strength retention rates of the composites reached 90%and 85%,respectively.This was mainly due to the liquid sealing effect of the ablative products represented by B2O_(3) and SiO_(2)∙B_(2)O_(3),which inhibited the ablative reaction by reducing the diffusion rate of the oxidation medium,and the solid pinning effect of the substances represented by SiO_(2),ZrO_(2),and ZrSiO_(4),which could play high viscosity and high strength characteristics to improve anti-erosion ability.The above-mentioned SiC_(f)/SiBCZr composites with corrosion resistance,oxidation resistance,and ablative resistance provided a solid material foundation and technical support for the development of reusable spacecraft hot-end components.

Growth kinetics of MgH2 nanocrystallites prepared by ball milling

MgH2 is one of promising hydrogen storage materials due to its high hydrogen capacity of 7.6 wt%.However,MgH2 nanocrystallites easilygrow up during hydrogen absorption-desorption cycling,leading to deterioration of hydrogen storage properties.To clarify the growth kinetics of MgH2 crystallites,the growth characteristics of MgH2 nanocrystallites are investigated in this work.The growth exponents of MgH2 nanocrystallites in pure MgH2 and MgH2-10 wt% Pr3 Al11 samples are evaluated to be n=5 and n=6,respectively.Meanwhile,their activation energies for crystallite growth are also determined to be109.2 and 144.2 kJ/mol,respectively.The increase of growth exponent and rise of activation energy for crystallite growth in MgH2-10 wt% Pr3 Al11 composite are ascribed to the presence of nano-sized Pr3 Al11phase.

Fragility under shocking: molecular dynamics insights into defect evolutions in tungsten lattice

Tungsten has promising applications in high-radiation,high-erosion and high-impact environments.Laser peening is an effective method to enhance the surface mechanical properties of tungsten materials.However,the ultrafast dynamic mechanism of defect evolutions induced by laser shockwave in tungsten lattice is unclear.Here,we investigated the evolutions and interactions of various defects under ultrafast compressive process in tungsten lattice using molecular dynamic method.The results confirm the brittleness of tungsten and reveal that void can reduce the yield strain and strength of the tungsten lattice by accelerating defect mesh extension and promoting the dislocation nucleation around itself.Dislocation density is increased with compressive strain rate.Meanwhile,dislocation multiplication and motion reduce the elastic stage and play a dominant role during the plastic deformation of tungsten lattice.Additionally,void can disrupt the dislocation displacement and promote the pinning effect on dislocations by defect mesh extension.

Effect of Niobium on Dynamic Recrystallization Behavior of 5%Ni Steel

The single-pass hot compressions of two 5%Ni steels with and without niobium addition at different temperatures of 800-1 150℃and strain rates of 0.01-1s-1 were performed by using a Gleeble-3500 thermal simulator and the effect of niobium on the dynamic recrystallization （ DRX ） behavior was analyzed.The results showed that the niobium addition of 0.04% can retard DRX in 5%Ni steel significantly by increasing the activation energy for DRX from 394 to 462kJ / mol.The critical strain required for starting DRX in 5%Ni steel was increased by 0.04-0.10 with niobium addition when the steel was deformed at a strain rate of 0.01s-1 and temperatures varied from 950 to 1 000℃.The critical temperature required for starting DRX in 5%Ni steel was also increased from 1 000 to 1 050℃ with niobium addition when the steel was deformed at a strain rate of 0.1s-1 .Such a retarded DRX occurring in Nb-added 5%Ni steel can be attributed to the pinning effect of precipitates containing niobium.