Deformation inhomogeneity due to sample-anvil friction in cylindrical compression test

Finite element(FEM)analysis was used to systematically evaluate the inhomogeneity of deformation in cylindrical samples with various sample-anvil friction coefficients,m.It was found that the level of friction strongly influences the deformation homogeneity,which increases significantly with the friction coefficient although the overall geometry of the samples almost remains the same when m >0.4.The position,at which the effective strain along the maximum radial direction in a compressed sample is equal to the equivalent strain of the sample,does not vary greatly with respect to both equivalent strain of the sample and m.Hardness measurements of compressed cylindrical 5056B Al alloy samples revealed a change of effective strain distribution similar to that revealed by FEM analysis.There exists a quantitative relationship between the hardness and the effective strain if no recrystallization or recovery occurs during the compression process.

Tensile deformation behaviors of pure nickel fine wire with a few grains across diameter

Size effects on plastic deformation behaviors in uniaxial micro tension of pure nickel fine wires were investigated experimentally, including flow stress and inhomogeneous deformation behaviors. It is found that with the increase of grain size or the decrease of number of grains across the diameter, the flow stress decreases and inhomogeneous deformation degree increases. When there are less than 9.3 grains across the diameter, the flow stress decreases quickly with the increase of grain size. Then, the flow stress size effect in micro tension of fine wires is revealed by a proposed model by introducing the grain boundary size factor. These results also indicate that both the fracture strain and stress decrease with the increase of grain size. When there are less than 14.7 grains across the diameter, both the fracture strain and stress decrease quickly. This indicates that the inhomogeneous deformation degree in micro tension increases with the decrease of the number of grains across the diameter. The fracture topography tends to be more and more irregular with the decrease of the number of grains across the diameter. Then, the formation mechanism of irregular fracture topography was analyzed considering the inhomogeneous distribution of microstructure when there are a few grains across the diameter.

Size effects on plastic deformation behavior in micro radial compression of pure copper

Micro radial compression tests were carried out on cylindrical specimens of pure copper polycrystals with different grain sizes. Experimental results indicated that phenomena of decreasing forming force, increasing scatter of forming force and more irregular surface topography occurred with the increase of grain size. A modified surface model based on dislocations pile-up in surface layer grains, and a flow stress scattering formulation based on standard deviation and grain size distribution were proposed to analyze size effects on forming force in micro compression. The inhomogeneous deformation of surface layer grains was discussed by the main deformation manner of rotation. A good agreement with the experimental results was achieved.

Effect of Revolution on Inhomogeneous Deformation of IF Steel in High Pressure Torsion

The effect of revolution on inhomogeneous plastic deformation of HPT processed IF steel was investigated using experimental and simulation approaches. The results indicate that the degree of inhomogeneous plastic deformation increases as the revolutions increase along the radial direction on the transversal plane of disks. In addition, the hardness and the microstructure distributions verify the trend that the effective strain of the HPT processed disks at the early torsion stage is gradually deformed from the edge to the center with the revolutions increases.

Polycrystalline model for FE-simulation of micro forming processes

A new polycrystal model was presented from the viewpoint of polycrystal structure of the billets considering free surface effects.In the model,the billet was divided into three portions,such as free surface portion,transition portion and internal portion.The grains in free surface portion were considered the single grains,and the anisotropy of the grains was taken into account by introducing grain orientation to explain the inhomogeneous deformation.In the transition portion,the effects of the neighbouring grains were adopted in the model.The grains in the internal portion were considered the polycrystalline material.With the developed model,the upsetting deformation process was simulated by the MSC Superform software.The scatter of the flow stress and inhomogeneous deformation was observed by analysis of the model.The comparisons show that the computational results are good agreed with the experimental results.This means that the presented model is effective.

Effect of flow stress—strain relation on forming limit of 5754O aluminum alloy

A modified Swift type flow stress—strain relation was presented in order to describe the uniaxial tension test curve reasonably. The FLD-strain （forming limit diagram made up of limit strain） of 5754O aluminum alloy sheet was calculated based on the two flow stress—strain relations using Yld2000-2d yield function. By comparing the theoretical and experimental results, it is found that the calculated FLD-strain based on the modified Swift flow stress—strain relation can reasonably describe the experimental results. However, though the common Voce flow stress—strain relation can describe the deformation behavior during homogenous deformation phase accurately, the FLD-strain calculated based on it is obviously lower than the experimental result. It is concluded that the higher the hardening rate of sheet metal is, the higher the forming limit is. A method for determining the reasonable flow stress—strain relation is recommended for describing the material behavior during inhomogenous phase and the forming limit of sheet metal.

Die structure optimization of equal channel angular extrusion for AZ31 magnesium alloy based on finite element method

Three-dimensional（3D） geometric models with different comer angles （90° and 120°） and with or without inner round fillets in the bottom die were designed. Some important process parameters were regarded as the calculation conditions used in DEFORMTM-3D software, such as stress--strain data of compression test for AZ31 magnesium, temperatures of die and billet, and friction coefficient. Influence of friction coefficient on deformation process was discussed. The results show that reasonable lubrication condition is important to plastic deformation. The change characteristics for distributions of effective stress and strain during an equal channel angular extrusion （ECAE） process with inner angle of 90° and without fillets at outer comer were described. Inhomogeneity index （C） was defined and deformation heterogeneity of ECAE was analyzed from the simulation and experiment results. The deformation homogeneity caused by fillets at outer comer increased compared with the die without fillets. The cumulated maximum strains decrease with increasing the fillets of outer comer in ECAE die and the inner comer angle. The analysis results show that better structures of ECAE die including appropriate outer comer fillet and the inner comer angle of 90° for the die can improve the strain and ensure plastic deformation homogenization to a certain extent. The required extrusion force drops with increasing the fillet made at outer comer in ECAE die. It is demonstrated that the prediction results are in good agreement with experiments and the theoretical calculation and the research conclusions in literatures.

TEXTURE IN INHOMOGENEOUSLY ROLLED ALUMINIUM SHEET

The changs of stress state in cold rolled aluminium sheet with large pass reduction,the combining activation process of slip systems as well as the formation mechanism of corresponding shear textures were investigated.It is shown that the rolling deformation with large pass reduction produces not only the general rolling stresses but also a strong shear stress in aluminium sheet.With increasing additional shear stress the general rolling textures,i.e.{112}<111>,{123}<634>,{110}<1I2>and{110}<001>decrease,and the shear texture,i.e.{001}<110>as well as{111}fibre texture become stronger.The internal relation of these two kinds of textures was also discussed.

Mechanism of size effects in microcylindrical compression of pure copper considering grain orientation distribution

In microscale deformation, the magnitudes of specimen and grain sizes are usually identical, and size- dependent phenomena of deformation behavior occur, namely, size effects. In this study, size effects in micro- cylindrical compression were investigated experimentally. It was found that, with the increase of grain size and decrease of specimen size, flow stress decreases and inhomogeneous material flow increases. These size effects tend to be more distinct with miniaturization. Thereafter, a modified model considering orientation distribution of surface grains and continuity between surface grains and inner grains is developed to model size effects in micro- forming. Through finite element simulation, the effects of specimen size, grain size, and orientation of surface grains on the flow stress and inhomogeneous deformation were analyzed. There is a good agreement between experimental and simulation results.

Polyoxometalates-derived nanostructures for electrocatalysis application

The conversion of intermittent renewable electrical energy to chemical energy is of great importance which can not only mitigate current energy and environmental crisis but also contribute to the ongoing carbon neutrality national strategy.Electrocatalysis is serving as a low-carbon conversion technology that enables green and efficient energy conversion mainly through hydrogen evolution reaction(HER),carbon dioxide reduction reaction(CO_(2)RR),and nitrogen reduction reaction(NRR).The core of electrocatalysis is the design and construction of low-cost high-activity and high-stability electrocatalyst to drive reaction thermodynamics and kinetics.The employment of polyoxometalates(POMs)as platforms or precursors to construct different types of electrocatalysts has been widely reported.Herein,we systematically summarized the recent advances in POM-derived nanostructures for electrocatalysis application.The strategies for precursor design and electrocatalyst synthesis were briefly introduced.The morphology control,phase control,composite modulation,and heterostructure engineering in POM-derived nanostructures were presented in detail.The structure–activity relationship of POM-derived nanostructures is fully discussed for HER CO_(2)RR,and NRR applications.Finally,the current challenges and future outlooks of POM-derived nanostructures are summarized to provide insights toward high-efficiency electrocatalysts for energy conversion technologies.

Tensile Properties and Deformation Behavior of Several Cast Ni-Based Superalloys Fabricated by Different Solidification Ways

The tensile properties and deformation behavior of several cast Ni-based superalloys, respectively, in the equiaxed, columnar-crystal and single-crystal styles are comparatively studied. The effects of solidification way, heat treatment and strain rate on the tensile properties are discussed in detail. It is found that the reduction of grain boundaries by the feasible solidification ways offers cast Ni-based superalloys the potential capability of improving the mechanical properties, the ultimate achievement of which is also confirmed to lie on the appropriate modifications of chemical composition and heat treatment. The prolongation of solid solution facilitates the precipitation of fine secondary γ＇ phase, whereas the extension of high-temperature aging leads to the coarsening of secondary γ＇ phase. The combination of these two aspects has a crucial influence on the tensile properties. Under tensile applied stress, the surface grains of DZ-A alloy deform slightly, while the inner grains deform heavily. This deformation inhomogeneity is ascribed to the occurrence of cracks or oblique grains near the surface of specimens and the sliding or decohesion of grain boundaries between the surface and inner grains. Regardless of strain rate, the ILTDM （intermediate-low-temperature ductility minimum） phe- nomenon always happens in the temperature range from 400 to 600 ℃ in all the investigated alloys, the occurrence of which is closely related to the strong strain-hardening behavior in the deformation process. Finally, the interaction of slip bands which are the main deformation mode below 600 ℃ is established to be the essential reason for the strain hardening.

The Meso-inhomogeneous Deformation of Pure Copper under Tension-Compression Cyclic Strain Loading

The investigation based on experiments and crystal plasticity simulation is carried out to undertake research on mesodeformation inhomogeneity of metals under cyclic loading at grain level.Symmetrical tension-compression cycle tests are performed on pure copper specimens to observe the inhomogeneous distribution of slip deformation and its evolution with cycle number.Cyclic hardening process and stable hysteretic behavior of pure copper under cyclic loading are simulated by applying a crystal plasticity constitutive model including nonlinear kinematic hardening associated with the polycrystalline representative volume element(RVE)constructed by Voronoi tessellation.Inhomogeneous deformation processes of materials under six different strain amplitudes are simulated by 1600 cycles,respectively.We discuss the variation law of the inhomogeneous meso-deformation distribution of material with the increase in cycle number,and research the rationality of characterizing the inhomogeneous deformation distribution and variation with the statistical standard deviation of the micro-longitudinal strain or the statistical average of the first principal strain based on the statistical analysis of the inhomogeneous deformation of the polycrystalline RVE model during the cycling process.It is found that these two parameters are related to and approximately inversely proportional to the length of measuring gauge.

Numerical Simulation of Microstructure Evolution for SA508-3 Steel during Inhomogeneous Hot Deformation Process

Based on hot compression tests by a Gleeble-1500D thermo-mechanical simulator, the flow stress model and microstructure evolution model for SA508-3 steel were established through the classical theories on work hardening and softening. The developed models were integrated into 3D thermal-mechanical coupled rigid plastic finite element software DEFORM3D. The inhomogeneous hot deformation （IHD） experiments of SA508 3 steel were designed and carried out. Meanwhile, numerical simulation was implemented to investigate the effect of temperature, strain and strain rate on microstructure during IHD process through measuring grain sizes at given positions. The simulated grain sizes were basically in agreement with the experimental ones. The results of experiment and simulation demonstrated that temperature is the main factor for the initiation of dynamic recrystallization （DRX）, and higher temperature means lower critical strain so that DRX can be facilitated to obtain uniform fine microstructure.

Analysis of meso-inhomogeneous deformation on a metal material surface under low-cycle fatigue

A polycrystalline Voronoi aggregation with a free surface is applied as the representative volume element（RVE）of the nickel-based GH4169 superalloy.Considering the plastic deformation mechanism at the grain level and the Bauschinger effect,a crystal plasticity model reflecting the nonlinear kinematic hardening of crystal slipping system is applied.The microscopic inhomogeneous deformation during cyclic loading is calculated through numerical simulation of crystal plasticity.The deformation inhomogeneity on the free surface of the RVE under cyclic loading is described respectively by using the following parameters：standard deviation of the longitudinal strain in macro tensile direction,statistical average of first principal strains,and standard deviation of longitudinal displacement.The relationship between the fatigue cycle number and the evolution of inhomogeneous deformation of the material’s free surface is investigated.This research finds that：（1）The inhomogeneous deformation of the material free surface is significantly higher than that of the RVE inside;（2）the increases of the characterization parameters of inhomogeneous deformation on the free surface with cycles reflect the local maximum deformation of the RVE growing during cyclic loading;（3）these parameters can be used as criteria to assess and predict the low-cycle fatigue life rationally.

Deformation behavior of Al-rich metallic glasses under nanoindentation

To clarify the deformation behavior of Al-rich metallic glasses（MGs）, two kinds of Al-rich MGs（i.e. bulk and ribbon samples） with different frozen-in excess volume have been analyzed under nanoindentation.It was found that, with the decrease of frozen-in excess volume, the serration behavior becomes inconspicuous together with the increase of hardness. Further, shear transformation zones（STZs）, related to the occurrence of shear banding, have been evaluated by different methods： the cooperative shearing model（CSM）, the rate-jump method（RJM） and the dynamic-mechanical response（DMR）. In contrast,the STZ volumes, calculated by the RJM, increase from 2.77 nm^3 in the bulk to 3.59 nm^3 in the ribbon,which are in good agreement with 2.60 nm^3 obtained from the icosahedral supercluster medium-range order structure model in Al-rich MGs. This result reflects that an intrinsic correlation exists between the formation of STZs and the medium-range orders（MROs）. Moreover, the variation trend of the STZ volume was analyzed in terms of the frozen-in excess volume content.