Intense Plastic Strain Research Articles

Development of microstructure of asymmetrically rolled AA3003 aluminium sheets with Zr addition

Asymmetric rolling is a method of severe plastic deformation, which can produce large volumes of deformed material and is easy to implement for industrial use. Asymmetric rolling with different circumferential velocities of rolls gives rise to intense plastic shear strains, resulting in grain refinement and improving mechanical properties. An aluminium alloy from AA3003 series enhanced by 0.16 wt% Zr was produced by twin-roll casting in industrial conditions. A part of specimens was annealed at 450∘C in order to precipitate fine dispersion of Al3Zr particles, which may enhance recrystallization resistance at elevated temperatures. Materials were subjected to cold rolling and to asymmetric rolling in several steps. Both methods result in a substantial elongation and fragmentation of grains. The grain refinement is more distinctive after asymmetric rolling. During isochronal annealing Vickers microhardness of materials decreases. The main drop occurs in the asymmetrically rolled material between 400 and 450∘C. In the annealed material the main drop connected with recrystallization is shifted by 50∘C to higher temperatures due to the presence of Al3Zr particles. Recrystallization occurs in the cold-rolled material between 450 and 500∘C. The imposed deformation by rolling is lower in this specimen and hence the driving force for recrystallization is also lower than in the asymmetrically rolled material. After recrystallization, the grain size of both cold-rolled and asymmetrically rolled materials is comparable. However, grains are still elongated in the rolling direction.

Effect of processing route on grain refinement and mechanical properties in equal channel angular pressing of aluminium alloy AA7068

Influence of different processing routes on microstructure of aluminum alloy AA7068 during equal channel angular pressing (ECAP) up to four passes is investigated. Equalchannel angular pressing is one of the best severe plastic deformation methods to achieve ultra-fine grain structure by subjecting metal to an intense plastic strain through a simple shear. A significant amount of grain refinement as occurred by passing aluminium alloy samples through ECAP split die at 200°C temperature from first pass up to four passes with two different processing routes such as route A and route Bc. The experimental results revealed that out of two different processing routes, route Bc where the samples are rotated by an angle of 90° in the same direction after each pass, is able to achieve rapid microstructure evolution with a better grain refinement and an equiaxed microstructure as compared to that of route A. Mechanical properties such as hardness and tensile strengths of the ECAP processed samples via route A and route Bc are also revealed that route Bc is the optimum process parameter which induces better mechanical properties into the metal.

Characteristic investigation of trilayered Cu/Al8011/Al1060 composite: Interface morphology, microstructure, and in-situ tensile deformation

In this work, Cu/Al8011/Al1060 trilayered composite with an ultrathin copper layer was fabricated successfully by the hybrid process of cast-rolling and hot-roll-bonding. The effects of heat treatment were investigated on the microstructure and mechanical behavior of the composite. The results showed that annealing temperatures above 300 ​°C led to the formation of new intermetallic compounds that joined their initial counterparts. Also, the interface growth was fast above 340 ​°C. It was found that the annealing temperature of 320 ​°C led to the optimal tensile properties of the composite. Through rolling, hard-brittle IMCs imposed intense plastic strains on the surrounding regions, resulting in further grain refinement of both metals in the vicinity of the interface. Although the kernel average misorientation of the trilayered composite was almost identical with the Cu/Al8011 bimetal, its amount was considerably decreased after heat treatment, as the distorted structure changed into a nearly strain-free one using the restoration phenomenon. The SEM-based in-situ tensile test showed that crack initiation occurred within the IMCs and propagated into the Cu strip, leading to its early fracture. However, the strength-displacement curve during in-situ tensile deformation did not decrease significantly due to the ultrathin copper layer.

Mechanics of plastic flow past a narrow-angle wedge indenter

ABSTRACTThe indentation of a metal specimen by a narrow-angle wedge produces extreme plastic deformation, with an effect akin to cutting into the metal. Simulation of such processes is challenging, and complicated by the need to model material separation along the indentation symmetry axis. Here we use an Arbitrary Lagrangian Eulerian (ALE) framework to enforce the symmetry boundary conditions (bcs) in their original, `strong’ form, as well as conventional Lagrangian FE to impose the bcs in a complementary, `weak’ form. Taken together these two cases, representing perfectly strong and perfectly weak interfaces, produce accurate bounds on the mechanical response for indentation by wedges with semi-apical angles as small as 15 degrees, and encompass intermediate cases that would require complicated models of ductile failure. The method accurately predicts the transition from the cutting pattern to the non-cutting (radially compressive) pattern as the apical angle is increased. In combination with Lagrangian particle tracking, the simulations reveal the deformation pattern as well as strain, strain-rate, and velocity fields in narrow angle indentation at high resolution. Interestingly, the strong form predicts a thin (tens of microns), near-wall layer of intense plastic strain, which has been observed recently in indentation experiments. With the exception of this feature, the strong and weak bc solutions are quite similar. The present approach reveals insights about plastic flow past narrow obstacles in a range of related problems including cone penetration and machining, and suggests using narrow-angle indentation as a way to probe material failure.

Numerical Analysis of a New Nonlinear Twist Extrusion Process

Severe plastic deformation (SPD) can produce ultrafine grained (UFG) and nanocrystalline (NC) materials by imposing intense plastic strain. One of the many options for inducing large plastic strains is to pass the material through a torsional/twist extrusion. The high-strength materials fabricated by SPD has no limit in dimension, and they can even be applied to load-carrying structural materials. Even though the method is quite successful, the industrial transfer has been limited so far because of low production efficiency and high cost. To remedy such difficulties, a new torsional extrusion process called nonlinear twist extrusion (NLTE) is introduced in this study, which has been designed based on two principles; (1) linear arrangement of the production line and (2) effective die geometry resulting in higher and more homogeneous plastic strain evolution which would give better grain refinement. The initial computational study of the designed geometry for the new extrusion process is addressed in the current study. The obtained results are discussed in detail with respect to conventional extrusion process, which is referred to as linear twist extrusion (LTE). The method is expected to offer a great potential for industrial use.

Numerical investigation of the evolution of microstructure of nickel-based alloys during plastic working

The paper considers the process of hot plastic working of a large metal billet, which causes its upsetting to a dimension specified by the manufacturing technique and the average axial deformation of 32.5%. The billet was made of heat-resistant nickel alloy Waspalloy widely used in the aircraft industry. The structural condition of the deformed alloy and, consequently, its mechanical properties depend on many factors: the degree of deformation, the rate of deformation, recrystallization in the process of deformation. This generates a need for a comprehensive study of the plastic flow in the examined material and the influence of the deformation parameters on its characteristics. The paper presents the results of a numerical investigation of the deformation and temperature states of the billet that allowed us to analyze changes occurring in the structure of nickel alloy during plastic deformation at different upsetting rates (50 and 100 mm/s). The shape of billet lateral surface and the forces required to complete the process of free upsetting are calculated as well. The numerical simulation was performed by the finite element method using the DEFORM-2D/3D software package. The average grain size and the fraction of recrystallized grains in the body of the billet were determined based on the Johnson-Mehl-Avrami-Kolmogorov model (JMAK). The results of numerical modeling showed that the greatest change in the microstructure of the alloy occurs in the region of intense plastic strains. The upsetting rate of 100 mm/s leads to the formation of structure with finer grains compared to the structure formed at the upsetting rate of 50 mm/s.

High strain rate compressive behaviors and adiabatic shear band localization of 3-D carbon/epoxy angle-interlock woven composites at different loading directions

When serving as a lightweight structural member in many areas, the dynamic mechanical behavior of fiber-reinforced polymeric matrix composites, especially under different strain rates, means a lot to the optimization of structure design as to high-speed impact. Under different loading rates and directions, we experimentally and numerically investigated the strain rate effect on the impact compressive behavior of 3-D angle-interlock woven composites (3-D AWCs) composed of carbon fiber and epoxy resin. Based on the factual geometrical architecture of 3-D AWC, the meso-scale model, which considers the interfacial damage between the reinforcements and matrix, was established to visually characterize the deformation history and damage morphologies of 3-D AWCs during impact compression process. Further, we performed high strain rate compressive tests on the split Hopkinson pressure bar (SHPB) apparatus integrated with a high-speed photography system for capturing images of progressive damage process to validate the proposed mesoscopic model. The stress-strain curves of various strain rates show strain rate effect varies depending on loading direction. The rate sensitivity on the compressive failure strength exists at weft direction and through-thickness direction except for warp direction. Moreover, both from images and finite element model results, the localized adiabatic shear band induced by intense plastic strain, initiates mainly from epoxy resin matrix and propagates along the interface at all three loading directions.

Evolution of the length scale in strain gradient plasticity

An equivalence is assumed between a microstructural length scale related to dislocation density and the constitutive length scale parameter in phenomenological strain gradient plasticity. An evolution law is formed on an incremental basis for the constitutive length scale parameter. Specific evolution equations are established through interpretations of the relation between changes in dislocation densities and increments in plastic strain and strain gradient. The length scale evolution has been implemented in a 2D-plane strain finite element method (FEM) code, which has been used to study a beam in pure bending. The main effect of the length scale evolution on the response of the beam is a decreased strain hardening, which in cases of small beam thicknesses even leads to a strain softening behavior. An intense plastic strain gradient may develop close to the neutral axis and can be interpreted as a pile-up of dislocations. The effects of the length scale evolution on the mechanical fields are compared with respect to the choice of length evolution equation.

Evaluating the heat energy dissipated in a small volume surrounding the tip of a fatigue crack

Fatigue crack initiation and propagation involve plastic strains that require some work to be done on the material. Most part of this irreversible energy is dissipated as heat and consequently the material temperature increases. The heat being an indicator of the intense plastic strains occurring at the tip of a propagating fatigue crack, the hypothesis is formed that it can be used to assess the fatigue damage accumulation rate of cracked components. Moreover, the heat energy at the crack tip is averaged according to Neuber’s finite particle concept. The aim of the present paper is to present the theoretical framework and the corresponding experimental technique to evaluate the heat energy dissipated in a structural volume surrounding the crack tip. The shape and size of the structural volume have been assumed according to the literature, even though the definition of the structural volume size of the analysed material in a fatigue sense is not the scope of the present paper. The proposed experimental technique to evaluate the averaged heat energy is based on the radial temperature profiles measured around the crack tip by means of an infrared camera. The temperature fields measured within few millimetres from the crack tip have been compared successfully with existing analytical solutions.

Experimental estimation of the heat energy dissipated in a volume surrounding the tip of a fatigue crack

Fatigue crack initiation and propagation involve plastic strains that require some work to be done on the material. Most of this irreversible energy is dissipated as heat and consequently the material temperature increases. The heat being an indicator of the intense plastic strains occurring at the tip of a propagating fatigue crack, when combined with the Neuber’s structural volume concept, it might be used as an experimentally measurable parameter to assess the fatigue damage accumulation rate of cracked components. On the basis of a theoretical model published previously, in this work the heat energy dissipated in a volume surrounding the crack tip is estimated experimentally on the basis of the radial temperature profiles measured by means of an infrared camera. The definition of the structural volume in a fatigue sense is beyond the scope of the present paper. The experimental crack propagation tests were carried out on hot-rolled, 6-mm-thick AISI 304L stainless steel specimens subject to completely reversed axial fatigue loading.

Simulation study of mechanical properties of bulk metallic glass systems: martensitic inclusions and twinned precipitates

Monolithic bulk metallic glasses (BMGs) exhibit a unique combination of mechanical properties, such as high strength and large elasticity limits, but the lack of ductility is considered the main Achilles' heel of BMG systems. To increase the competitiveness of BMGs vis-à-vis conventional structural materials, the problem of catastrophic failure via intense plastic strain localization (‘shear banding’) has to be addressed. Recent experimental observations suggest that the addition of structural heterogeneities, in the form of crystalline particles, to BMG systems hinders the catastrophic propagation of shear bands and leads to enhanced ductility. These structural heterogeneities can be introduced by either forming BMG composites, where second-phase crystalline particles accommodate applied loads via martensitic transformation mechanisms, or developing glassy alloys that precipitate crystalline particles under deformation, a process by which further deformation can be sustained by twinning mechanisms in the crystalline phase. In this work, we present a non-linear continuum model capable of capturing the structural heterogeneity in the glassy phase and accounting for intrinsic work hardening via martensitic transformations in second-phase reinforcements in BMG composites and deformation twinning in precipitated crystalline particles. Simulation results reveal that in addition to intrinsic work hardening in the crystalline phase, particle size greatly affects the overall mechanical behavior of these BMG systems. The precipitation of crystalline particles in monolithic BMGs yields two-phase microstructures that promote more homogeneous deformation, delay the propagation of incipient shear bands, and ultimately result in improved ductility characteristics.

Thermal conductivity reduction of crystalline silicon by high-pressure torsion.

We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm−1 K−1 to approximately 7.6 Wm−1 K−1). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.

Effect of roll speed ratio on deformation characteristics of IF steel subjected to differential speed rolling

The paper investigates the effect of the roll speed ratio on the deformation characteristics of interstitial free (IF) steel processed by the differential speed rolling (DSR) technique. An intense plastic strain induced by a single DSR with various roll speed ratios ranging from 1:1 to 1:4 for the lower and upper rolls was successfully imparted to the samples, resulting in the formation of fine elongated ferrite grains as the roll speed ratio increased. Observations of the preferred orientation using electron back-scattered diffraction and transmission electron microscopy revealed that the {110} slip was readily activated as the dominant deformation mode in order to accommodate the intense plastic strain imposed by the DSR. In addition, the microhardness values of the IF steel sample deformed via DSR under a roll speed ratio of 1:4 increased, and this is discussed on basis of the frictional force between the roll and the sample during DSR deformation.

On constructing the theory of ductile fracture near friction surfaces

A criterion of ductile fracture is proposed, which takes into account the singular character of theoretical solutions near the maximum friction surfaces and the emergence of a thin layer with intense plastic strains near surfaces with high friction stresses in real processes of metal forming. The equation for the thickness of the layer with intense plastic strains and the fracture criterion include the strain rate intensity factor, apparently, characterizing the intensity of physical processes that occur in a thin material layer near the friction surfaces. Some experimental data are used to determine the thickness of this layer. The ductile fracture criterion is analyzed by solving the problem of strip extrusion under conditions of plane strain deformation.

厚い試料を用いた高圧ねじり(HPT)加工および高圧すべり(HPS)加工におけるひずみ導入分布と結晶粒超微細化

Intense plastic strain is imparted into disk- and ring-shape samples using high-pressure torsion (HPT) which is known as a typical process of severe plastic deformation. The grain size is refined to the submicrometer and/or nanometer range. The grain refinement is also achieved using high-pressure sliding (HPS) which was developed recently for rectangular sheet samples under high pressure as in HPT. However, the samples with the thickness thinner than 1 mm are usually used for both HPT and HPS processes but no attempt has been made for samples with the thickness thicker than 1 mm. This study is then initiated to investigate how homogeneously the strain is developed and the grain refinement is achieved in the thicker samples. High purity Al (99.99%) is used with ring-shape and rectangular sheet samples having thicknesses of 2 and 4 mm. It is shown that a region of intense plastic strain develops at the center height and expands to both upper and lower edges with straining. The grain refinement is achieved at such an intense plastic region with dislocation-free grains with the sizes of a few micrometers.

Integrated Experimental, Atomistic, and Microstructurally Based Finite Element Investigation of the Dynamic Compressive Behavior of 2139 Aluminum

The objective of this study was to identify the microstructural mechanisms related to the high strength and ductile behavior of 2139-Al, and how dynamic conditions would affect the overall behavior of this alloy. Three interrelated approaches, which span a spectrum of spatial and temporal scales, were used: (i) The mechanical response was obtained using the split Hopkinson pressure bar, for strain-rates ranging from 1.0×10−3 s to 1.0×104 s−1. (ii) First principles density functional theory calculations were undertaken to characterize the structure of the interface and to better understand the role played by Ag in promoting the formation of the Ω phase for several Ω-Al interface structures. (iii) A specialized microstructurally based finite element analysis and a dislocation-density based multiple-slip formulation that accounts for an explicit crystallographic and morphological representation of Ω and θ′ precipitates and their rational orientation relations were conducted. The predictions from the microstructural finite element model indicated that the precipitates continue to harden and also act as physical barriers that impede the matrix from forming large connected zones of intense plastic strain. As the microstructural FE predictions indicated, and consistent with the experimental observations, the combined effects of θ′ and Ω, acting on different crystallographic orientations, enhance the strength and ductility, and reduce the susceptibility of 2139-Al to shear strain localization due to dynamic compressive loads.

Numerical simulations of void growth near a notch tip in ductile single crystals

The objective of this work is to study the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals under mode I, plane strain, small scale yielding (SSY) conditions. To this end, finite element simulations are performed within crystal plasticity framework neglecting elastic anisotropy. Attention is focussed on the effects of crystal hardening, ratio of void diameter to spacing from the notch and crystal orientation on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. Further, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. Also, a strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Finally, the synergistic, cooperative growth of multiple voids ahead of the notch tip is examined.

Mechanical properties of ultrafine-grain zirconium in the temperature range 4.2–300K

The mechanical properties under uniaxial compression of cylindrical samples of ultrafine-grain (UFG) zirconium (grain size ∼0.4μm) obtained by intense plastic strain (IPS)—a combination of extrusion, annealing, and drawing—have been studied in the temperature range 4.2–300K. The mechanical characteristics of UFG zirconium have been studied in two structural states and the results are compared with each other as well as with the characteristics of coarse-grain polycrystals of this material (state 1—samples after IPS, state 2—samples after IPS and annealing). It has been established that grain-size reduction under IPS substantially increases the yield stress of polycrystals (by a factor of 6 at 300K and by a factor of 4 at 77K) while maintaining a large reserve of plasticity up to 15%. Two stages of strain hardening and a decrease of the plastic-flow activation volume with increasing strain has been recorded in the strain diagrams of UFG zirconium. It is concluded that the main mechanisms determining the yield stress and strain hardening are the same for ultrafine- and coarse-grain zirconium: intragrain dislocation slip and twinning occur in parallel and interact with one another.

Locally enhanced Voronoi cell finite element model (LE‐VCFEM) for simulating evolving fracture in ductile microstructures containing inclusions

AbstractDuctile heterogeneous materials such as cast aluminum alloys undergo catastrophic failure that initiates with particle fragmentation, which evolves with void growth and coalescence in localized bands of intense plastic deformation and strain softening. The Voronoi cell finite element model (VCFEM), based on the assumed stress hybrid formulation, is unable to account for plastic strain‐induced softening. To overcome this shortcoming of material softening due to plastic strain localization, this study introduces a locally enhanced VCFEM (LE‐VCFEM) for modeling the very complex phenomenon of ductile failure in heterogeneous metals and alloys. In LE‐VCFEM, finite deformation displacement elements are adaptively added to regions of localization in the otherwise assumed stress‐based hybrid Voronoi cell finite element to locally enhance modeling capabilities for ductile fracture. Adaptive h‐refinement is used for the displacement elements to improve accuracy. Damage initiation by particle cracking is triggered by a Weibull model. The nonlocal Gurson–Tvergaard–Needleman model of porous plasticity is implemented in LE‐VCFEM to model matrix cracking. An iterative strain update algorithm is used for the displacement elements. The LE‐VCFEM code is validated by comparing with results of conventional FE codes and experiments with real materials. The effect of various microstructural morphological characteristics is also investigated. Copyright © 2008 John Wiley & Sons, Ltd.

Texture control and grain refinement of AA1050 Al alloy sheets by asymmetric rolling

Asymmetric rolling, in which the circumferential velocities of the upper and lower rolls are different, can give rise to intense plastic shear strains and in turn shear deformation textures through the sheet thickness. The ideal shear deformation texture of fcc metals can be approximated by the {0 0 1}〈1 1 0〉 plus 〈1 1 1〉∥ND orientations, among which the latter improves the Lankford values or the deep drawability. The intense shear strains can result in the grain refinement and hence improve mechanical properties. In this paper, a study has been made of effects of asymmetric rolling variables such as reductions per pass, changes in shear direction each pass, reductions in the last pass, roll rotation rate ratios, changes in shear direction in the last pass, and different frictions on the upper and lower surfaces of AA1050 Al alloy sheets on the evolution of their deformation and annealing textures and grain refinement.