MICROSTRUCTURAL CHARACTERISTICS ASSOCIATED WITH HIGH- STRAIN-RATE PLASTIC DEFORMATION IN THE ELECTROFORMED COPPER LINER OF SHAPED CHARGES

The microstructures of electroformed copper liners of shaped charges that had undergone high-strain-rate deformation were observed by optical microscopy （OM） and scanning electron microscopy （SEM）. Meanwhile, the orientation distribution of the grains in the recovered jet was examined by electron backscattering Kikuchi pattern （EBSP） technique. EBSP analysis reveals that the fibrous texture observed in the as-electroformed copper liners disappeared after explosive detonation deformation. OM observation shows that the microstructure evolves system- atically from the jet center to its perimeter during cooling from high temperatures after explosive detonation deformation. This microstructural characteristic is similar to that of solidification, i.e. there exist equiaxed grains in the center of the jet and significant columnar grains around the equiaxed grains. The result reveals that there is melting-related phenomenon in the jet center. Corresponding microhardness variations from the jet center to its perimeter is also determined. All the phenomena can be explained by a strong gradient of temperature across the section of the jet during plastic deformation at high-strain-rate.

A Model of Dynamic Recrystallization in Alloys during High Strain Plastic Deformation

Recrystallized grains, less than 200 nm in diameter were observed in heavily shear zones of a high strength low alloy steel and a Ni-based alloy, and Also grain refinement, less than 3 μm in diameter was made in high purity aluminum by ECAE at ambient temperature. The experimental results showed that high strain rate and large deformation could induce dynamic recrystallization.Based on dislocation dynamics and grain orientation change enhanced by plastic deformation,a model for the recrystallization process is developed. The model is used to explain the ultra fine grains which are formed at a temperature still much lower than that for the conventional recrystallization

Deformation mechanism of fine grained Mg-7Gd-5Y-l.2Nd-0.5Zr alloy under high temperature and high strain rates

Fine grained Mg-7Gd-5Y-1.2Nd-0.5Zr alloy was investigated by dynamic compression tests using a Split Hopkinson Pressure Bar under the strain rates in the range 1000-2000 s^(-1) and the temperature range 293-573 K along the normal direction.The microstructure was measured by optical microscopy,electron back-scattering diffraction,transmission electron microscopy and X-ray diffractometry.The results showed that Mg-7Gd-5Y-1.2Nd-0.5Zr alloy had the positive strain rate strengthening effect and thermal softening effect at high temperature.The solid solution of Gd and Y atoms in Mg-7Gd-5Y-1.2Nd-0.5Zr alloy reduced the asymmetry of α-Mg crystals and changed the critical shear stress of various deformation mechanisms.The main deformation mechanisms were prismatic slip and pyramidal(a)slip,{102}tension twinning,and dynamic recrystallization caused by local deformation such as particle-stimulated nucleation.c 2020 Published by Elsevier B.V.on behalf of Chongqing University.

On Numerical Modelling of Industrial Powder Compaction Processes for Large Deformation of Endochronic Plasticity at Finite Strains

Compaction processes are one the most important par ts of powder forming technology. The main applications are focused on pieces for a utomotive, aeronautic, electric and electronic industries. The main goals of the compaction processes are to obtain a compact with the geometrical requirements, without cracks, and with a uniform distribution of density. Design of such proc esses consist, essentially, in determine the sequence and relative displacements of die and punches in order to achieve such goals. A.B. Khoei presented a gener al framework for the finite element simulation of powder forming processes based on the following aspects; a large displacement formulation, centred on a total and updated Lagrangian formulation; an adaptive finite element strategy based on error estimates and automatic remeshing techniques; a cap model based on a hard ening rule in modelling of the highly non-linear behaviour of material; and the use of an efficient contact algorithm in the context of an interface element fo rmulation. In these references, the non-linear behaviour of powder was adequately desc ribed by the cap plasticity model. However, it suffers from a serious deficiency when the stress-point reaches a yield surface. In the flow theory of plasticit y, the transition from an elastic state to an elasto-plastic state appears more or less abruptly. For powder material it is very difficult to define the locati on of yield surface, because there is no distinct transition from elastic to ela stic-plastic behaviour. Results of experimental test on some hard met al powder show that the plastic effects were begun immediately upon loading. In such mater ials the domain of the yield surface would collapse to a point, so making the di rection of plastic increment indeterminate, because all directions are normal to a point. Thus, the classical plasticity theory cannot deal with such materials and an advanced constitutive theory is necessary. In the present paper, the constitutive equations of powder materials will be discussed via an endochronic theory of plasticity. This theory provides a unifi ed point of view to describe the elastic-plastic behaviour of material since it places no requirement for a yield surface and a ’loading function’ to disting uish between loading an unloading. Endochronic theory of plasticity has been app lied to a number of metallic materials, concrete and sand, but to the knowledge of authors, no numerical scheme of the model has been applied to powder material . In the present paper, a new approach is developed based on an endochronic rate independent, density-dependent plasticity model for describing the isothermal deformation behavior of metal powder at low homologous temperature. Although the concept of yield surface has not been explicitly assumed in endochronic theory, it is shown that the cone-cap plasticity yield surface (Fig.1), which is the m ost commonly used plasticity models for describing the behavior of powder materi al can be easily derived as a special case of the proposed endochronic theory. Fig.1 Trace of cone-cap yield function on the meridian pl ane for different relative density As large deformation is observed in powder compaction process, a hypoelastic-pl astic formulation is developed in the context of finite deformation plasticity. Constitutive equations are stated in unrotated frame of reference that greatly s implifies endochronic constitutive relation in finite plasticity. Constitutive e quations of the endochronic theory and their numerical integration are establish ed and procedures for determining material parameters of the model are demonstra ted. Finally, the numerical schemes are examined for efficiency in the model ling of a tip shaped component, as shown in Fig.2. Fig.2 A shaped tip component. a) Geometry, boundary conditio n and finite element mesh; b) density distribution at final stage of

Nucleation mechanisms of dynamic recrystallization in Inconel 625 superalloy deformed with different strain rates

The effects of strain rates on the hot working characteristics and nucleation mechanisms of dynamic recrystallization (DRX) were studied by optical microscopy and electron backscatter diffraction (EBSD) technique. Hot compression tests were conducted using a Gleeble-1500 simulator at a true strain of 0.7 in the temperature range of 1000 to 1150 °C and strain rate range of 0.01 to 10.00 s?1. It is found that the size and volume fraction of the DRX grains in hot-deformed Inconel 625 superalloy firstly decrease and then increase with increasing strain rate. Meanwhile, the nucleation mechanism of DRX is closely related to the deformation strain rate due to the deformation thermal effect. The discontinuous DRX (DDRX) with bulging of original grain boundaries is the primary nucleation mechanism of DRX, while the continuous DRX (CDRX) with progressive subgrain rotation acts as a secondary nucleation mechanism. The twinning formation can activate the nucleation of DRX. The effects of bulging of original grain boundaries and twinning formation are firstly gradually weakened and then strengthened with the increasing strain rate due to the deformation thermal effect. On the contrary, the effect of subgrain rotation is firstly gradually strengthened and then weakened with the increasing strain rate.

Twin recrystallization mechanisms in a high strain rate compressed Mg-Zn alloy

Static recrystallization of a high strain rate compressed Mg-1 Zn(wt.%)alloy was investigated using electron backscattered diffraction(EBSD).A novel 53°1010 structure was observed in the as-deformed alloy,which showed a{1012}-{1012}double twin relationship with the matrix.When the as-deformed alloy was annealed at 200°C,the{1011}compression twins and{1011}-{1012}double twins showed a higher priority to recrystallize.In addition,the coarse{1012}tension twins and their inner double twins were preferentially to recrystallize,while the lenticular tension twins had little impact on the recrystallization.Therefore,obtaining more compression twins or coarse twins instead of lenticular tension twins can be an effective approach to manipulate recrystallization process in deformed Mg alloys.

Analytical large deformation shear strength for bolted rough discontinuous rock

Presented a new analytical model for studying the shear-tensile large deforma-tion behavior near the vicinity of joint interface for bolted rough discontinuous rock, and presented the formulation estimating global shear strength for bolted joints under shear-ing-tensile loads. The analytical strength curves of bolts contribution on reinforced discon-tinuous rocks as the function of joint displacements or deformation angle of a bolt at rock joints was obtained. Based on Barton’s equation on JRC roughness profiles, the theoreti-cal shearing strength of bolted rough joints was also established. Test results on bolted granite and marble specimen confirm the validity of the analytical approach.

In Situ Studies of Deformation and Fracture in Sputtering Copper Film

Nanocrystalline copper films were prepared by sputtering and then in situ straining experiments were performed using a trans- mission electron microscope. Macroscopically, these copper films exhibited very low ductility (<l%). Dislocation activity was limited in regions far from propagating cracks. Near stable growing cracks, considerable local plasticity was observed. The evidence of slip ac- tivity both within grain interiors and in grain boundaries was also observed. Although some dislocation; moved very fast, others showed rates much lower than those typically measured for bulk copper. Fracture was intergranular, but not brittle. It occurred by linking of microcracks. Microcracks formed within a micrometer or so ahead of the main crack tip, usually within a grain boundary. Linking then took place by the easiest available path.

Analysis of the Motion and Deformation Characteristics along the Zhangjiakou-Bohai Fault

We have collected GPS data in the period of 1999-2007 from the Crustal Motion Observation Network of China along the Zhangjiakou-Bohai fault and its adjacent regions to study the characteristics of present-day crustal horizontal motion velocities in the research zone.Strain rate components are computed in the spheric coordinate system by the least square collocation method.According to the spatial distribution of the principal strain rate,dilation rate and maximum shear strain rate derived from GPS measurements,this paper analyses the deformation of the subordinary faults of the Zhangjiakou-Bohai fault.The principal compression strain rates are apparently greater than the principal extension strain rates.The larger shear strain rate is mainly in and around the Xianghe,Wenan and Tangshan areas in Hebei Province.According to the profiles across different segments of the Zhangjiakou-Bohai fault,the three segments glong the Zhangjiakou-Bohai fault show an obviously left-lateal strike-slip and compression characteristics.By analysis of the motion characteristics of the blocks,e.g.the Yanshan block,North China Plain block,Ordos block,and Ludong-Huanghai block in and around the North China region,this paper speculates that the dynamics of the motion styles of Zhangjiakou-Bohai fault may directly come from the relative movement between the Yanshan block and the North China plain block,and the ultimate dynamics may be the results of the collison between Indian plate and Eurasian plate,and the persistent northeastward extrusion of the Indian plate.

Regional Tectonic Deformation Background and Medium- and Short-Term Precursors to the Minle-Shandan Earthquakes1

Using GPS observations of horizontal movement from 2001 to 2003 and the cross-fault mobile short-levelling data of 1988～2003, and with the aid of the improved negative dislocation model and the time-varying curve of strain intensity ratio of fault deformation, the regional tectonic deformation background and medium- and short-term precursors related to the preparation of the Minle-Shandan earthquakes of M S6.1 and M S5.8 on October 25, 2003 are investigated. The results reveal that, under the background of the wide-range deformation adjustment, short-term relaxation and recovery caused by the Kunlun Mountains earthquake of M S8.1, the hypocenters of the earthquakes are located on the north edge of the shear stress enhancement zone between the compressional locked segments of block boundary fault, a place which may represent an accelerated strain accumulation. An obvious anomaly of strain intensity ratio appeared in short-levelling measurements crossing over the fault at the Shihuiyaokou site, the closest to the epicenters, 3 months before the occurrence of the earthquakes. In addition, the variation in number of anomalies from 10-odd days to months before the earthquakes in the entire monitoring area and the anomaly concentration and local enhancement relative to near source in the 3 months before the earthquakes are regarded to be precursors to the two events.

A Review of Residual Stress and Deformation Modeling for Metal Additive Manufacturing Processes

A metal additive manufacturing process results in a nearly net-shaped fabrication of parts directly from digital data.A local heat source melts the deposited material,and a part is built layer-by-layer.Residual stress and de-formation are critical issues experienced by additively manufactured parts.Modeling the additive manufacturing process provides important insights and can help determine an optimal build plan so as to minimize residual stress formation.Various approaches have been used for modeling of residual stresses,ranging from high-fidelity models to simplified models,for quicker results.This paper provides a state-of-the-art review of the approaches used to numerically model residual deformation and stresses in structures built using additive manufacturing.Fur-thermore,it describes the physical causes of residual-stress generation in an additively manufactured structure.

Elastic deformation behavior of CuZrAlNb metallic glass matrix composites with different crystallization degrees

The room temperature brittleness has been a long standing problem in bulk metallic glasses realm.This has seriously limited the application potential of metallic glasses and their composites.The elastic deformation behaviors of metallic glass matrix composites are closely related to their plastic deformation states.The elastic deformation behaviors of Cu48-xZr48Al4Nbx（x=0,3at.%）metallic glass matrix composites（MGMCs）with different crystallization degrees were investigated using an in-situ digital image correlation（DIC）technique during tensile process.With decreasing crystallization degree,MGMC exhibits obvious elastic deformation ability and an increased tensile fracture strength.The notable tensile elasticity is attributed to the larger shear strain heterogeneity emerging on the surface of the sample.This finding has implications for the development of MGMCs with excellent tensile properties.

Hot Deformation Behavior and Flow Stress Prediction of Ultra Purified 17% Cr Ferritic Stainless Steel Stabilized with Nb and Ti

The hot deformation behavior of ultra purified 17% Cr ferritic stainless steel stabilized with Nb and Ti was investigated using axisymmetric hot compression tests on a thermomechanical simulator.The deformation was carried out at the temperatures ranging from 700 to 1 100℃ and strain rates from 1to 10s-1.The microstructure was investigated using electron backscattering diffraction.The effects of temperature and strain rate on deformation behavior were represented by Zener-Hollomon parameter in an exponent type equation.The effect of strain was incorporated in the constitutive equation by establishing polynomial relationship between the material constants and strain.A sixth order polynomial was suitable to represent the effect of strain.The modified constitutive equation considering the effect of strain was developed and could predict the flow stress throughout the deformation conditions except at800℃in 1s-1 and at 700℃in 5and 10s-1.Losing the reliability of the modified constitutive equation was possibly ascribed to the increase in average Taylor factor at 800℃in 1s-1 and the increase in temperature at 700℃in 5and10s-1 during hot deformation.The optimum window for improving product quality of the ferritic stainless steels was identified as hot rolling at a low finisher entry temperature of 700℃,which can be achieved in practical production.

Shear response of β-SiC bulk dependent on temperature and strain rate

The shear responses of β-SiC are investigated using molecular dynamics simulation with the Tersoff interatomic potential. Results show a clear decreasing trend in critical stress,fracture strain and shear modulus as temperature increases. Above a critical temperature, β-SiC bulk just fractures after the elastic deformation. However, below the critical temperature, an interesting pattern in β-SiC bulk emerges due to the elongation of Si-C bonds before fracture. Additionally, the shear deformation of β-SiC at room temperature is found to be dependent on the strain rate. This study may shed light on the deformation mechanism dependent on temperature and strain rate.

Bending,buckling and vibration analyses of MSGT microcomposite circular-annular sandwich plate under hydro-thermo-magneto-mechanical loadings using DQM

The purpose of this paper is to investigate the bending,buckling,vibration analyses of microcomposite circular-annular sandwich plate with CNT reinforced composite facesheets under hydro-thermo-magneto-mechanical loadings are presented using first order shear deformation theory(FSDT)and modified strain gradient theory(MSGT)that includes three material length scale parameters.Also,an isotropic homogeneous core is considered for microcomposite circular-annular sandwich plate.The generalized rule of mixture is employed to predict mechanical,moisture and thermal properties ofmicrocomposite sandwich plate.By using Hamilton’s principle,governing equations are solved by differential quadrature method(DQM)for a circular annular sandwich plate.The predicted results are validated by carrying out the comparison studies for the FGM plates by modified couple stress theory(MCST).The obtained results are given to indicate the influence of the material length scale parameter,core-to-facesheet thickness ratios,magnetic effect,thermal andmoisture effects on the dimensionless deflection,critical buckling load,and natural frequency of microcomposite circular sandwich plate.The results can be employed in solid-state physics,materials science,nano-electronics,and nano electro-mechanical devices such as microactuators,and microsensor.

The concept and realization of nanostructure fabrication using free-standing metallic wires with rapid thermal annealing

Free-standing metallic nanostructures are considered to be highly relevant to many branches of science and technology with applications of three dimensional metallic nanostructures ranging from optical reflectors,actuators,and antenna,to free-standing electrodes,mechanical,optical,and electrical resonators and sensors.Strain-induced out-of-plane fabrication has emerged as an effective method which uses relaxation of strain-mismatched materials.In this work,we report a study of the thermal annealing-induced shape modification of free-standing nanostructures,which was achieved by introducing compositional or microstructural nonuniformity to the nanowires.In particular gradient,segmented and striped hetero-nanowires were grown by focused-ion-beam-induced chemical vapor deposition,followed by rapid thermal annealing in a N2 atmosphere.Various free-standing nanostructures were produced as a result of the crystalline/grain growth and stress relief.