Inhomogeneous Plastic Strain Research Articles

Effects of T1/T2 precipitates on deformation behavior and microstructure evolution of AA2099 Al–Li alloy: experimental investigation and crystal plasticity finite element modeling

In the present study, effects of T1/T2 precipitates on microstructure characteristics and texture evolution of various AA2099 Al–Li alloy (AA2099-A and AA2099-B) samples are systematically investigated by conducting uniaxial compression experiments at room temperature, followed by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) measurements as well as crystal plasticity finite element (CPFE) simulation. The heat treatment for AA2099-A sample consists of solution treatment at 540 °C/1 h and artificial aging at 250 °C/1 h followed by water quenching, while the heat treatment for AA2099-B sample includes solution treatment at 540 °C/1 h and artificial aging at 250 °C/24 h followed by water quenching. Experimental results demonstrate that although dynamic recovery (DRV) serves as the major restoration mechanism for both samples, AA2099-A sample with T1 phase possesses obviously higher mechanical response by comparison with AA2099-B sample with T1/T2 phase. This observation shows a strong precipitate hardening effect derived from these dense and large-sized T1 precipitates in AA2099-A sample. Numerical results based on CPFE simulation demonstrate the occurrence of inhomogeneous plastic strain within the constructed grain-based representative volume element (RVE) model during uniaxial compression. Further statistical analysis on the coefficient variant about equivalent plastic strain verifies this issue and demonstrates that plastic strain distributes more homogeneously in AA2099-A sample than in AA2099-B sample. These dense and large-sized T1 precipitates are responsible for this phenomenon and they result in the fast rotation of grain orientation from <111>//ED to <110>//ED during uniaxial compression.

A microstructure‐based homogenization model for predicting the low‐cycle fatigue initiation life of GH4169 superalloy

AbstractA microstructure‐based homogenization model is proposed for simulating the cyclic plasticity and predicting the low‐cycle fatigue (LCF) crack initiation life of GH4169 superalloy. Classical crystal plastic model (CPM) with a simple softening model is used at the grain level. Then, the transition from grain level to polycrystal level is based on the conservation of virtual work between the two levels. The Eshelby's formulation is applied in the model. Especially, local influences of grain interactions are considered by introducing the external Eshelby's tensor. Relatively precise macroresults and microresults as the finite element method can be provided by the present model with less computational cost. Grain volume averaged fatigue indicator parameters (FIPs) with considering the effect of inclusions are formed to predict the LCF crack initiation life, and a fold‐line fitting model is proposed to substitute for the cycle‐by‐cycle simulation. Predicted lives fit well with the experimental data for both the strain loading and stress loading simulations. Scatter of the life can also be predicted by the model and overwhelming influences of the incubation stage on the variability of LCF initiation life can be captured. It is shown that the inclusions and the inhomogeneous plastic strain are responsible for the scatter of the incubation stage.

EBSD- and ECCI-based Assessments of Inhomogeneous Plastic Strain Evolution Coupled with Digital Image Correlation

EBSD- and ECCI-based Assessments of Inhomogeneous Plastic Strain Evolution Coupled with Digital Image Correlation

In Situ Micromechanical Characterization of Metallic Glass Microwires under Torsional Loading

Small-scale metallic glasses have many applications in microelectromechanical systems (MEMS) and sensors which require good mechanical properties. Bending, tensile and compression properties of metallic glasses at micro/nano-scale have been well investigated previously. In this work, by developing a micro robotic system, we investigated the torsional behavior of Fe-Co based metallic glass microwires inside a scanning electron microscope (SEM). Benefiting from the in situ SEM imaging capability, the fracture behavior of metallic glass microwire has been uncovered clearly. Through the postmortem fractographic analysis, it can be revealed that both spiral stripes and shear bands contributed to the fracture mechanism of the microscale metallic glass. Plastic deformation of the microwires include both homogenous and inhomogeneous plastic strain, which began with the liquid-like region, then a crack formed because of shear bands and propagated along the spiral direction. Although the metallic glass microwire broke in brittle mode, the shear strain was not lower than that of conventional metal wires. Moreover, we found an inverse relationship between the plastic strain and the loading rate.

EBSD and ECCI Based Assessments of Inhomogeneous Plastic Strain Evolution Coupled with Digital Image Correlation

EBSD and ECCI Based Assessments of Inhomogeneous Plastic Strain Evolution Coupled with Digital Image Correlation

Development of internal inhomogeneous plastic strain during cold rolling of Al-Mg alloys

Development of inhomogeneous plastic strain in three-dimension had been measured in cold rolled aluminium-magnesium alloys that contain small lead particles, which are marker of local strain measurement, by using synchrotron radiation microtomography. Local inhomogeneous deformation during thermo-mechanical processing is very important for controlling mechanical properties of metallic materials, for example strength and formability, because localized deformation would affect not only formation of deformation microstructure but also development of recrystallized microstructure. The strain distributions in different magnesium content were compared with texture development investigated by SEM/EBSD. The mechanism of texture development affected by local deformation is reported.

Comparative Study on Damage Evolution during Sheet Metal Forming of Steels DP600 and DP1000

Two different dualphase steels of significantly different strength properties are compared with respect to microstructural configuration, chemical composition, mechanical properties, and ductile damage mechanisms. The investigated steel grades DP600 and DP1000 show remarkable differences in terms of martensite phase fractions, yield and ultimate tensile strength, and ductile damage evolution behaviour. In particular it turns out that steel DP600 experiences a relatively early ductile damage initiation, but only a moderate rate of damage evolution, whereas for steel DP1000, contrary behaviour is observed. This steel shows a comparably late ductile damage initiation with rapid subsequent damage evolution. This behaviour is expressed by the ratio of fracture strain over strain at damage initiation. Its origin lies in both the martensite phase fraction and the individual strength properties of the ferritic and the martensitic phases, which will provoke inhomogeneous plastic strain distributions in the materials’ microstructures. The more pronounced these differences are, the higher the local plastic strain peaks become, and the earlier ductile damage initiation happens. Nevertheless, with decreasing strength of the ferritic phase, its ability to withstand even large plastic strains without fracturing is strongly promoted, so that the fracture strain in steel DP600 can be shifted to higher values even though the material has undergone early damage initiation.

Stress states in individual Si particles of a cast Al–Si alloy: Micro-Raman analysis and microstructure based modeling

The stress states in Si particles of cast Al–Si based alloys depend on its morphology and the heat treatment given to the alloy. The Si particles fracture less on modification and fracture more in the heat treated condition. An attempt has been made in this work to study the effect of heat treatment and Si modification on the stress states of the particles. Such understanding will be valuable for predicting the ductility of the alloy. The stress states of Si particles are estimated by Raman technique and compared with the microstructure-based FEM simulations. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to determine the stress states in Si particles with (111) orientations. Stress states are measured in the as-received state and under uniaxial compression. The residual stress, the stress in the elastic–plastic regime and the stress which causes fracture of the particles is estimated by Raman technique. FEM study demonstrates that the stress distribution is uniform in modified Si, whereas the unmodified Si shows higher and more complex stress states. The onset of plastic flow is observed at sharp corners of the particles and is followed by localization of strain between particles. Clustering of particles generates more inhomogeneous plastic strain in the matrix. Particle stress estimated by Raman technique is in agreement with FEM calculations.

Experimental and finite element simulation study of the adiabatic shear band phenomenon in cold heading process

This paper presents the outcomes of a comprehensive experimental, metallurgical and finite element (FE) simulation study to characterize the development of adiabatic shear band (ASB) phenomenon in steel cold heading (CH) process. The main objective of this work is to investigate the complex interplay of different process and material parameters on the ASB development stages inside the cold headed parts.In this work, the drop weight compression test (DWCT) was selected to simulate the CH process impact loads on specimens machined from 1038 steel and 1018 steel. Series of DWCTs were performed under different impact loading conditions. The goal of these tests is to achieve different deformation levels and introduce ASBs at different stages.To reach a full understanding of this complex phenomenon, the FE simulation analysis was used to support the metallurgical examination of the DWCT specimens. The FE analysis provided important details about the changes of different material and process parameters at the critical zones inside the ASBs.This study confirmed that the ASB is mainly a thermo-mechanically controlled phenomenon. The ASBs develop in three stages: homogeneous plastic strain, inhomogeneous plastic strain, and strain localization. The ASB development stage depends mainly on the status of the competition between the work hardening and the thermal and geometrical softening mechanisms inside the bands. The domination of the softening mechanisms at advanced levels of deformation triggers a self-catalytic strain localization and material strength degradation process that leads to failure inside the band.In general, the metallurgical and finite element analysis investigation revealed that under impact loads, three ASBs can develop simultaneously inside the cold headed parts; lower, upper and central ASBs. As the deformation continues; the development of the lower and upper bands slows down and contributes in the rapid development of the adjacent central ASB. This study confirmed that the ASB has a canonical structure which leads to an ASB that can experience different development stages along the same band simultaneously.This study proved that the shape and the type of ASBs in cold headed parts depend highly on material's properties. The metallurgical and finite element analysis revealed that the higher the strength of the tested steel, the easier to form a narrow ASB that reaches the localization stage at low deformation levels. In contrast, ductile steels experience wider ASBs when subjected to the same deformation levels. These bands require higher levels of deformation to reach the localization stage in comparison to higher strength steels.

Textured optical synthetic sapphire

This paper discusses features of the inhomogeneous plastic strain of synthetic sapphire. A mechanism is proposed for forming its texture and cone of optic axes. The comparative characteristics of textured synthetic sapphire and its single-crystal analog are presented.

Moving inelastic discontinuities and applications to martensitic phase transition

In this work, equations of the kinetics and kinematics are developed for heterogeneous materials containing inelastic discontinuities with moving boundaries. From the derived free energy and the power of external forces one obtains the driving force acting on the moving boundary. Introducing the interface operators and some hypothesis on inelastic fields, one gets the driving force for the formation of an ellipsoidal domain. The theoretical model is illustrated by the derivation of nucleation and growth conditions of a martensitic plate inside an inhomogeneous plastic strain field. The obtained results are combined with a study of the kinetics and kinematics to derive the constitutive equation of an austenitic single crystal, from which the overall behavior of polycrystalline TRIP steels is deduced using the self-consistent scale-transition method. Comparison with experimental data shows a good agreement.

Couplings between plasticity and martensitic phase transformation: overall behavior of polycrystalline TRIP steels

Abstract Several couplings between plasticity and martensitic phase transformation are at the origin of remarkable properties of ductility and toughness in the case of TRIP steels. A micromechanical model is developed to predict the conditions of nucleation and growth of a martensitic microdomain inside an inhomogeneous plastic strain field. More explicit relations are developed in the case of a simple shear test where a heterogeneous plastic strain field leads to a significant decrease of the critical stress for martensitic transformation. The obtained results are combined with a kinetics and kinematics studies to derive the constitutive equation of an austenitic single crystal from which the overall behavior of a polycrystalline steel is deduced using the self-consistent scale transition method. Comparison with experimental data shows a good agreement.

Producing bulk residual stresses in gas turbine blades

Inhomogeneous plastic strain has been used to produce a pattern of bulk compressive stresses that counteract macrodefect formation and growth in machine components, which increases the working life. Studies have been made on the residual stresses in a blade caused by uneven heating or compression. The stresses have been calculated by the finite-element method. The residual-stress pattern parameters have been related to the geometrical dimensions of the heat-or force-treated zone and the extent of the treatment.

The Formation of Similar Folds by Inhomogeneous Plastic Strain, with Reference to the Fourth Phase of Deformation Affecting the Dalradian Rocks in the Southwest Highlands of Scotland

Recent work on the formation of folds is reviewed critically and found inadequate to account for the similar folding of inhomogeneous sequences. The geometry of the fourth-phase structures, typical of the later secondary phases of folding, is described from an area where the prior attitudes of the earlier primary structures can be inferred. Four stages in progressive secondary deformation can be recognized. The structures formed are, ideally, (1) conjugate sets of phyllitic (strainslip) cleavages, (2) conjugate sets of similar folds with axial planes parallel to one or other of the cleavage sets, (3) single sets of similar folds formed by the suppression of one or other of the conjugate sets of folds, and (4) single sets of composite folds. Flinn's (1962) methods indicate that the deformation was biaxial, at least in the less intense stages. A Compatibility Principle is outlined whereby the strains undergone by adjacent limbs of a similar fold can be related. It is applied to a simplified model of the c...