Dynamic Plastic Deformation Research Articles

Nucleation and thickening of shear bands in nano-scale twin/matrix lamellae of a Cu–Al alloy processed by dynamic plastic deformation

Microstructural evolution associated with the shear banding in nano-scale twin/matrix (T/M) lamellae of a Cu–Al alloy processed by means of dynamic plastic deformation was investigated using transmission electron microscopy (TEM) and high-resolution TEM. The development of a shear band was found to be a two-stage process, namely a nucleation stage resulting in a narrow band composed of nano-sized (sub)grains intersecting the T/M lamellae, followed by a thickening stage of the narrow band into adjacent T/M lamellae regions. The nucleation stage occurred within a narrow region of an almost constant thickness (100–200 nm thick, referred to as “core” region) and consisted of three steps: (1) initiation of localized deformation (bending, necking, and detwinning) against the T/M lamellae, (2) evolution of a dislocation structure within the detwinned band, and (3) transformation of the detwinned dislocation structure (DDS) into a nano-sized (sub)grain structure (NGS). On the two sides of a core region, two transition layers (TRLs) exist where the T/M lamellae experienced much less shear strain. The interface boundaries separating the core region and the TRLs are characterized by very large shear strain gradients accommodated by high density of dislocations. Increasing shear strains leads to thickening of shear bands at the expense of the adjoining T/M lamellae, which is composed of thickening of the core region by transforming the TRLs into the core region with DDS and NGS, analogous to steps (2) and (3) of the nucleation process, and outward movement of the TRLs by deforming the adjoining T/M lamellae. Grain sizes in the well-developed shear bands are obviously larger than the lamellar thickness of original T/M lamellae.

Enhanced fracture toughness and strength in bulk nanocrystalline Cu with nanoscale twin bundles

Bulk nanocrystalline pure Cu samples with embedded nanoscale twin bundles were prepared by means of dynamic plastic deformation (DPD) at cryogenic temperature. By adjusting the DPD parameters, we increased the volume fraction of nanotwin bundles, resulting in an increase in both tensile strength and fracture toughness. The enhanced strength is attributed to the strengthening effect of the nanotwin bundles. The highly anisotropic nanotwin lamellar structures are found to be effective in energy absorption and arresting crack propagation during fracture. Coarse and deep dimples associated with obvious recrystallization underneath were detected in the fracture surfaces, owing to the presence of the nanotwin bundles. The enhancement of fracture toughness is closely correlated with the formation of these deep dimples.

A new concept for producing ultrafine-grained metallic structures via an intermediate strain rate: Experiments and modeling

A novel experimental methodology to produce ultrafine-grained metallic microstructures, which is applied on aluminum is proposed in this work. In fact, the ultrafine-grained aluminum polycrystal is made from commercial purity powder by a combination of hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 μm. The material is then subsequently impacted, using a falling weight at a maximum impact velocity of 9.2 m/sec. The resulting material shows a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture perpendicular to the impact direction. The mechanical properties of the impacted material are then characterized under compression tests at room temperature under a strain rate of 10 −4 s −1. The effect of the change of the deformation path on the mechanical response parallel and perpendicular to the impact direction is also investigated. These results are discussed in relation with microstructure. Further, a new extension of a micromechanical approach developed by Abdul-Latif et al., [2] is proposed to predict the grain size effect on the enhancement of the mechanical strength of polycrystals. Within the framework of small strain hypothesis, the elastic anisotropy of the grain and grain rotation are neglected for the sake of simplicity. The local inelastic deformation heterogeneity is determined through the slip theory. It is assumed that the yield strength increases linearly with decreasing grain size as in Hall–Petch relationship. It is obviously recognized that the model with its new extension describes fairly well the effect of the grain size on the strain–stress behavior of the sub-micrometer aluminum.

Grain orientation dependence of deformation twinning in pure Cu subjected to dynamic plastic deformation

A clear grain orientation dependence of deformation twinning has been identified in coarse-grained copper subjected to dynamic plastic deformation. Deformation twins tend to occur in grains with orientations near the [0 0 1] corner but not in grains near the [1 0 1] corner, which can be explained in terms of a Schmid factor analysis. This finding provides a guideline for the design of initial texture in exploring the structural refinement via deformation twins. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Enhanced impact toughness of magnesium alloy by grain refinement

The impact toughness of Mg–3Al–Zn alloy was systematically investigated with the focus on the effect of grain size. The impact toughness is greatly improved as the mean grain size is reduced to less than 3 μm, because the dynamic strength and dynamic plastic deformation ability are greatly enhanced. The superior dynamic strength of fine-grained samples may be explained in terms of grain size effect, and the high dynamic plastic deformation ability probably results from the suppression of twinning.

Microstructures and mechanical properties of a Cu–Zn alloy subjected to cryogenic dynamic plastic deformation

The microstructures and mechanical properties of a Cu–Zn alloy subjected to dynamic plastic deformation (DPD) at liquid nitrogen temperature (77 K) were investigated. The planar dislocation activities, deformation twinning and shear banding dominate the plastic deformation of samples with different strains, respectively. Twin boundary spacing decreases with strain and nearly saturates when the strain exceeds 1.2. Shear bands occur within high-density twin region at higher strains, inside which nano-sized grains are formed. Tensile tests show that the DPD brass exhibits a high strength and a limited tensile ductility. Deformation twins play an important role in the strengthening while the appearance of shear bands obviously reduces the ductility of materials.

Investigation of the properties of asphalt and its mixtures containing flame retardant modifier

The flame retardancy of asphalt binder was improved by adding flame retardants in previous research. The research was conducted to assess the performance of two kinds of flame resistant asphalt mixture compared with the control mixture in laboratory. Differential scanning calorimetry (DSC) and thermogravimetry (TG) were employed to investigate the effect of aluminium hydroxide (ATH) on the thermal properties of asphalt binder. The mixture was designed using Superpave (Superior Performing Pavement) methods and related pavement performance was evaluated. The wheel-tracking was employed to research the dynamic plastic deformation performance. Experimental results illustrated that two kinds of flame resistant asphalt binders have better flame retardancy and all the pavement performances of two kinds of modified asphalt mixture can meet the Superpave design requirements. Therefore, it was feasible to prepare asphalt mixture with the excellent flame retardancy and pavement performance using ATH modified asphalt binders.

Enhanced fracture toughness of bulk nanocrystalline Cu with embedded nanoscale twins

We report an enhanced fracture toughness of bulk Cu sample processed by dynamic plastic deformation with a composite microstructure of nanoscale grains and embedded nanoscale twin bundles. The improvement in fracture toughness is mainly due to the embedded twin bundles which generate elongated deep fracture dimples that consume much higher fracture energy.

Microstructure and Mechanical Properties of Commercial Purity HIPed and Crushed Aluminum

Ultrafine-grained aluminum microstructures were processed from commercial purity powder by combining hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2µm. The material was then subsequently impacted, using a falling weight at a strain rate of 300s-1. The resulting material showed a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture parallel to the impact direction. The mechanical properties of the impacted material were subsequently characterized under compressive tests at room temperature at a strain rate of 10-4s-1. The effect of the change of the deformation path on the mechanical response parallel (DN) and perpendicular (DT) to the impact direction was also investigated. These results are here discussed in relation with microstructure and texture evolution.

Effect of thermal annealing on mechanical properties of a nanostructured copper prepared by means of dynamic plastic deformation

Thermal annealing of a nanostructured Cu sample processed by means of dynamic plastic deformation (DPD) induces static recrystallization (SRX) accompanied by a drop in strength and enhanced ductility. An obvious increment in tensile uniform elongation appears when the volume fraction of SRX grains exceeds 80%. The strength–ductility combination of the annealed nanostructured samples is superior to that of strain-induced ultrafine-grained Cu, owing to the finer grains and the presence of nanoscale twin bundles generated from the DPD.

STRAIN RATE DEPENDENCE OF DYNAMIC FLOW STRESS OF 2017 ALUMINUM ALLOY AT VERY HIGH STRAIN RATES

To estimate the strain rate dependence of the dynamic flow stress of aluminum alloys, 2017-O and - T4, high strain rate and rate reduction tests are performed at strain rates ranging from about 1000/sec to 30000/sec. A steep increase in the flow stress is observed for 2017-O at the strain rate of about 3000/sec. For 2017-T4, the above phenomenon is observed at the strain rate above about 10000/sec. A simplified model for dislocation kinetics under dynamic plastic deformation is employed which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag. It is expected that the steep increase in the flow stress observed at high strain rates is attributed to the rate dependence of the viscous drag on the dislocation motion.

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m −2. An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.

Sequential coupling simulation for electromagnetic–mechanical tube compression by finite element analysis

A sequential coupled field analysis of electromagnetic tube compression process has been performed by using the finite element method (FEM). A 2D axisymmetric electromagnetic model based on the magnetic vector potential is proposed for the calculation of magnetic field and magnetic forces; and Newmark integration method is used to calculate the dynamic plastic deformation of tube in the mechanical model. In each time step during the simulation, the transient magnetic forces acting on the tube are obtained from the electromagnetic model, which are then used as input load to the mechanical model. According to the tube deformation calculated, the geometry of the electromagnetic model is updated. Therefore, the sequential coupling simulation is realized between the electromagnetic model and the mechanical model, whose results are more accurate than the loosing coupling simulation method. The radial deformation along the outer generatrix of deformed tube wall is presented as a function of time and the plastic strain energy of electromagnetic tube compression is analyzed.

Numerical Simulation of Dynamic Plastic Response of Cylindrical Steel Lining‐concrete Structure Under Internal Intense Blast Loading

This paper presents numerical simulation results of cylindrical steel lining‐concrete structure’s dynamic plastic response under internal intense blast loading by ANSYS/lS‐DYNA finite element software. The emphasis is put on the dynamic plastic deformation and three‐dimensional stress conditions of steel lining‐concrete structure. It is shown that maximal plastic deformation of steel lining‐concrete structure results from the first action of shock wave on internal surface of the structure. For the steel lining of the structure, radial stress is compressive stress, while hoop and axial stress is tension stress. For concrete of the structure, three‐dimension stress is compressive stress near the out surface of steel lining. Radial stress of concrete at intense shock wave seems to decay as an exponential function with increasing of scaled‐distance.

High strength and high electrical conductivity in bulk nanograined Cu embedded with nanoscale twins

A bulk nanograined Cu sample embedded with nanoscale twins is produced by means of dynamic plastic deformation at cryogenic temperatures. It exhibits a tensile yield strength of 610MPa and an electrical conductivity of 95% IACS at room temperature. The unique combination of a high strength and a high conductivity is primarily attributed to the presence of a considerable amount of nanoscale twins which strengthen the material significantly while having a negligible influence on electrical conductivity.

Microstructural evolution and nanostructure formation in copper during dynamic plastic deformation at cryogenic temperatures

The microstructural evolution and formation mechanism of nanostructures in bulk pure Cu samples induced by dynamic plastic deformation (DPD) at high strain rates and cryogenic temperatures were investigated using transmission electron microscopic characterization. Three different mechanisms were identified for the plastic deformation and microstructural refinement, including dislocation manipulation and rearrangement, deformation twinning forming nanoscale twin/matrix (T/M) lamellae in bundles, and shear banding in the T/M lamellae. An increasing tendency of deformation twinning and shear banding was observed at higher strains. For strain ε = 2.1, a mixed nanostructure is formed in the DPD Cu bulk sample with nanoscale T/M lamellae making up about 33% of the volume and nano-sized grains making up about 67%. The nanograins can be classified into three types in terms of their origin: (i) nanograins derived from fragmentation of nanoscale T/M lamellae with an average transverse size of about 47 nm; (ii) nanograins in shear bands with an average transverse size of about 75 nm; and (iii) nanograins derived from dislocation cells with an average transverse size of about 121 nm. The high density of deformation twins induced by high strain rates and cryogenic temperatures in DPD, distinct from that in conventional severe plastic deformation, plays a crucial role in formation of the nano-sized grains.

Effect of Strain Rates on the Transformation Behavior of Ni-Ti Alloy

In order to clarify the effect of strain rates on phase transformation behaviors of Ni-Ti alloy, a compressive test using a cylindrical specimen of polycrystalline Ni-Ti alloy of Ti-50.69 at% Ni was carried out at a high strain rate and a low strain rate. The transformation temperatures were determined by a differential scanning calorimeter (DSC) using a sample cut from a compressed specimen. The transformation temperatures of the specimens before deformation were Ms= 303 K, Mf = 287 K, As = 297 K and Af = 319 K, respectively. The compressive test was carried out using specimen heated from liquid nitrogen temperature to room temperature. A universal testing machine as a static test apparatus and a Split Hopkinson Bar apparatus for a dynamic test were used. The specimen had a reoriented martensite phase after deformation because the superelastic effect was not observed upon unloading. Two reverse transformations during heating and a forward transformation during cooling were observed by DSC measurement. The first reverse transformation corresponds to that of thermal-induced martensite by immersion in liquid nitrogen and the second reverse transformation corresponds to that of reoriented martensite with slips in a polycrystalline matrix introduced by plastic deformation. The reverse transformation of the martensite phase with a slip exhibited strong strain rate dependency. Plastic strains and strain rate had strong influence on the shape recovery. The interaction between the temperature elevation by a conversion of plastic work and slip generated by dynamic plastic deformation is a complicated problem.

Laser Marking System of 3D Nondestructive Anti-Counterfeiting Identifiers Based on Liquid Crystal Mask

The theoretical researches on fabrication, measurement and identification of laser marking system of 3D(three-dimension) anti-counterfeiting identifiers have been done in this paper. 3D identifiers were fabricated and 3D identifiers’ modelings were reconstructed, and binary coding of gray images were encoded by programmable control of liquid crystal mask, spatial modulating of laser beam and photolithography fabrication technics. We regarded the depth of 3D identifiers as anti-counterfeiting information, and the original marking data and anti-counterfeiting information are saved in remote database server by database technology and computer network technology，so true produces can be distinguished from fake through network. The basic theory of 3D nondestructive anti-counterfeiting identifiers based on liquid crystal mask was built, and a new technology of creating 3D anti-counterfeiting identifiers by laser shock wave was set up, which differs entirely from marking by laser ablation (or laser thermal effect). A new high-efficiency theory of detecting and identifying on 3D anti-counterfeiting identifiers by 3D identifiers’ reconstruction and binary coding was set up. The study enriches dynamic plastic deformation theory of partial high-strain-rate and anti-counterfeiting design & manufacturing theory which is also a highlight based on advanced manufacturing theory of mechanical effect on laser shock wave.

Strain rate dependence of dynamic flow stress considering viscous drag for 6061 aluminium alloy at high strain rates

In order to evaluate the strain rate dependence of the dynamic flow stress of aluminium alloys, 6061-O and -T6, high strain rate tests are performed at strain rates ranging from about 1000/s to 30000/s, and strain rate reduction tests are also conducted in the strain rate range from about 10000/s to 20000/s. A steep increase in the flow stress is observed for 6061-O at the strain rate of about 5000/s. The above phenomenon, however, is not observed for 6061-T6 in the strain rate range where the strain rate reduction tests is conducted. A simplified model for a dislocation kinetics under a dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate ranges, which reveals that the steep increase in the flow stress of 6061-O is attributed to the rate dependence of the viscous drag on the dislocation motion. It is estimated that the velocity of moving dislocation of 6061-T6 is lower than that of 6061-O at a given strain rate, and that 6061-T6 is higher in a mobile dislocation density than 6061-O. The model predicts also that the increase in the mobile dislocation density shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side.

Synergism of Electrochemical and Mechanical Factors in Erosion−Corrosion

A theoretical model was developed on basis of nonequilibrium thermodynamics, dislocation kinetics, and electrochemistry, which may lead to the quantitative assessment of material loss produced by the synergism of mechanical and electrochemical factors in an erosion-corrosion process, As predicted by this model, the synergistic effect results mainly from the interaction of two irreversible fluxes, namely, the anodic dissolution current density and the plastic flowing in the surface layer caused by dynamic plastic deformation. An enhanced anodic dissolution flux is induced by the dynamic surface plastic deformation resulting from the impingement of solid particles, which can be correlated to the wastage rate due to the mechanical erosion. Meanwhile, the anodic current present at the electrode surface, in turn, can increase the mobility of dislocation and reduce the resistance in the surface layer against plastic deformation. Such an effect is demonstrated by the hardness degradation of metals in corrosive media. Theoretical analysis indicates that the corrosion-induced hardness degradation is a linear function of the logarithm of anodic current density. The degradation of mechanical erosion resistance with decreasing hardness suggests that the corrosion-enhanced erosion may result from the degradation in hardness of target material induced by the anodic dissolution and the corresponding wastage is also a linear function of the logarithm of anodic current density. The theoretical predictions were compared with the experimental results of carbon steels obtained form the slurry-erosion tests and the micro-hardness measurements. The results indicate that the hardness degradation in corrosive media is mainly controlled by the anodic current density and is almost independent of the polarization behavior of steels.