Different Stages Of Strain Hardening Research Articles

Investigating build orientation-induced mechanical anisotropy in additive manufacturing 316L stainless steel

Build orientation-induced mechanical anisotropy is critical for additive manufacturing metals. To leverage the existing research on anisotropy between Z and X/Y build orientations, this work focuses on the anisotropic behavior within 2D planes. Therefore, tensile coupons of 316L stainless steel (316L-SS) were fabricated in five different build orientations (X, Y, XY45°, Z, and ZX45°) using the laser powder bed fusion process with a pulsed laser. Different characterization techniques at various length scales were employed to investigate the mechanisms of mechanical anisotropy. According to the electron back-scattered diffraction (EBSD) study, X and Y samples had a larger fraction of <001> and <110> grains, whereas XY45° coupons had a larger fraction of <111> grains along the tensile axis. In contrast, Z samples exhibited a dominant <110> texture parallel to the tensile axis. X-ray diffraction analysis revealed that all the samples had a single austenitic phase but different dislocation densities. The tensile results of all samples showed higher yield strength and comparable ductility with those in the literature. Among all the build orientations, XY45° coupons had the highest yield strength due to the large fraction of <111> grains oriented along the tensile axis and the highest dislocation density of 2.2 × 1015 m−2. In contrast, the highest ductility of Z samples was caused due to twinning-favored <110> crystallographic texture along the tensile axis. The onset and termination of different stages of strain hardening were mainly affected by the cellular sub-grain structure and crystallographic texture, as well as their interactions with dislocation generation and evolution. These findings indicated that crystallographic texture and dislocation density were the dominant contributions to the mechanical anisotropy followed by grain morphology.

DISLOCATION STRUCTURE IN A LOCALIZED DEFORMATION BAND FORMED DURING TENSION OF A NORMALIZED 09G2S STEEL SPECIMEN

A transmission electron microscopy study was carried out to examine the dislocation structure of normalized steel 09G2S that exhibits a strain aging effect. In a specimen stretched to the yield point, the maximum dislocation density is observed in the center (active zone) of the localized deformation band and decreases by about an order of magnitude at the band edges. The structure of these band regions, which are at different stages of strain hardening, is analyzed. The Burgers vector of dislocations of the primary slip system is determined; the dislocation ends form ordered flat dipole walls, leading to slip polygonization. The relationship between the specimen structure at the yield point and at the ultimate load is considered.

Surface modification of sub-eutectic silumin by a pulsed electron beam

The research has pointed out that the electron-beam irradiation induces the formation of an approximately 90 μm thick surface layer, where primary inclusions dissolve. There are two defect sub-structure types in the irradiated samples: cells and columns. An important outcome to emerge from the study is that the fracture of an irradiated sample occurs at greater (by ≈7 %) applied stress and higher (by ≈13 %) plastic strain than in the as cast silumin. Stress-strain curves of samples under study were presented in the coordinates s-e0.33. As a result, straight-line segments were detected, which characterize different stages of strain hardening in the material. Strain-stress curves of untreated samples in the coordinates s(e0.33) show three straight-line segments referring to three stages of hardening. Stress-strain curves of irradiated samples also demonstrate three stages of hardening; however, two additional strain hardening stages of silumin (intermediate and before-fracture) are found out. The study suggests disappearing of local compression zones in both samples but at various values of strain, moreover, local zones in irradiated samples are not registered at greater strain. In this context it is concluded that the irradiation of АК5М2 alloy samples contributes to the stabilization of local micro-zones with compressive strain.

Stages and Irregularities of Plastic Strain in Austenitic Stainless Steel within a Temperature Range of 143 &amp;lt; t &amp;lt; 420 k

This work aims at investigating the stages and irregularity of plastic strain in austenitic stainless steel 12Kh18N9T within a temperature range of 143 &lt; T &lt; 420 K. The localized plastic strain fronts at different stages of strain hardening have been visualized and monitored via digital speckle photography (DSP). This method consists in the high-accuracy recording of the displacement field via tracking any material surface changes and the subsequent comparison of speckle images acquired under uniaxial tension. The stress-strain diagrams were found to strongly vary with a decrease in temperature due to strain-induced-phase transition. The analysis of localized strain distributions revealed the emergence of a mobile system of equidistant strain localization foci at the strain hardening stage. The temperature dependence of plastic strain localization parameters at the linear strain hardening stage was established as well.

Tensile properties and strain hardening mechanism of Cr-Mn-Si-Ni alloyed ultra-strength steel at different temperatures and strain rates

To reveal the relationship between the microstructural morphologies, tensile behaviors, and strain hardening mechanism of Cr-Mn-Si-Ni alloyed ultra-strength steel, uniaxial tensile tests of a Cr-Mn-Si-Ni alloyed steel were conducted at room temperature (RT) and elevated temperatures with different strain rates in this study. When the tensile test was carried out at RT, increased strain rate led to increased dislocation density of the tensed Cr-Mn-Si-Ni alloyed steel specimen and resulted in increased value of yield strength. Because the dislocation entanglement and strain hardening of tensed Cr-Mn-Si-Ni alloyed steel were inhibited by increased strain rate, decreased value of ultimate strength was measured. During tensile test at RT, the dislocations rearranged and transformed to low-angle grain boundaries and high-angle grain boundaries sequentially. Above microstructural evolution resulted in microstructural refinement of Cr-Mn-Si-Ni alloyed steel. The strain hardening of Cr-Mn-Si-Ni alloyed steel during tensile test at RT was divided into four stages according to the calculated values of strain hardening rate. The microstructural evolution behaviors including accumulation of dislocations, generation and transformation of slips took place in different stages of strain hardening were affected significantly by the strain rate. Based on the true stress-true strain curves, the effects of the tensile temperature and strain rate on the values of the yield strain, fracture strain, yield strength, and ultimate strength of Cr-Mn-Si-Ni alloyed steel tensed under different experimental conditions were studied. A two-stage constitutive model of Cr-Mn-Si-Ni alloyed steel associated with dislocation density was established based on the experimental results.

A phenomenological model based on nanostructured dislocation cluster interactions to predict the work hardening behavior of cryodeformed materials

In the present work, a phenomenological model was developed to understand the different stages of strain hardening in cryodeformed FCC materials with different stacking fault energy (SFE) level, i.e., high, medium, and low. The strain hardening in the materials consists of three stages, i.e. (i) easy-glide (stage-I) (ii) hardening (stage-II) and (iii) dynamic recovery (DRV) (stage-III). The proposed model considers the microstructure of cryodeformed materials which contains a mixture of discrete dislocation clusters including twin boundaries, and randomly arranged dislocations (dislocation debris). The model output predicts the work hardening behavior by using three deciding parameters: (i) normalized nanostructured dislocation cluster velocity (vDC)nor, (ii) normalized nanostructured dislocation cluster acceleration (aDC)nor and (iii) normalized dislocation debris velocity (vDD)nor. The deformation mechanisms during stage-II of strain hardening can be predicted by the nature of (aDC)nor curve. The cyclic trend in the (aDC)nor curve indicates the simultaneous occurrence of dislocation-based strengthening and DRV. The mono-cyclic (aDC)nor curve indicates hardening behavior due to combined effect of twin and dislocations-based deformation. The (aDC)nor curve fails to segregate stage II and stage III of deformation. In order to identify the transition from stage II and stage III mode of deformation, the relationship between (vDC)nor and (vDD)nor was formulated. A new parameter: DRV factor (Sx), was used to estimate the fraction of DRV in the materials. The uniaxial stress vs strain curves at varying strain rates and defect densities calculated from XRD analysis were used as input parameter for the model. A transmission electron microscopy (TEM) was performed to validate the dislocation interactions concluded from the model.

Dislocation Density-Based Multiscale Modeling of Deformation and Subgrain Texture in Polycrystals

In this work, a viscoplastic fast Fourier transform (FFT)-based code is combined with a continuum dislocation dynamics (CDD) framework to analyze the mechanical behavior of polycrystalline MgAZ31 material under unidirectional tensile test. A crystal plasticity formulation including the size effects through a stress/strain gradient theory, dislocation density flux among neighboring grains and grain boundary back stress field is implemented into the CDD and coupled with VPFFT for this purpose. Then, an electron backscatter diffraction-based orientation image microscopy of a sample microstructure is applied as an input to the code. The model predicts, among other things, distributions of stress, strain, mobile dislocation density, geometrically necessary dislocation and stress–strain behavior. The numerical findings are compared with experimental results, and the micromechanical behavior of the polycrystal is discussed regarding dislocation density evaluation in different stages of strain hardening.

Influence of hydrogen on the localization of plastic strain in low-carbon steel

The localization of plastic strain in the extension of 08ps low-carbon steel is compared after hot rolling and after electrolytic saturation with hydrogen by means of a three-electrode electrochemical cell. Analysis of the strain curves shows that they include a plasticity region, a stage of linear strain hardening, and a stage of parabolic strain hardening (Taylor hardening). Double-exposure speckle photography is used to visualize the localized strain zones. On that basis, the quantitative deformation characteristics may be obtained. In other words, the displacement-vector field in a plane sample under extension may be determined, and then the components of the plastic distortion tensor (the local elongation exx, shear exy, and rotation ωz) may be calculated. The basic types of plastic flow localization are identified at different stages of strain hardening, and their parameters are determined.

Influence of hydrogen on the localization of plastic strain in low-carbon steel during electrolytic saturation

Attention focuses on the macroscopic localization of the plastic deformation and structure in polycrystals of low-carbon 08ps steel sheet after hot rolling as a result of electrolytic saturation by hydrogen in a thermostatic three-electrode cell at constant potential. The basic types of macroscopic localization of the plastic deformation and its parameters (the velocity and wavelength) at different stages of strain hardening are determined by two-position speckle photography. By optical and electronic microscopy, the influence of interstitial hydrogen atoms on the defect substructure and morphology of cementite is investigated. The formation of dislocational substructure is considered. Torsional flexure of the α-phase lattice is discovered, corresponding to flexural extinction contours. The basic sources of the stress fields are the boundaries of the grains and fragments.

Regularities of Macroscopic Localization of Plastic Deformation in the Stretching of a Low-Carbon Steel

The special features of plastic deformation localization in the stretching of polycrystals of low-carbon steel 08 ss after hot rolling and electrolytic saturation with hydrogen are investigated. The main types and parameters of plastic flow localization in different stages of strain hardening are determined by the method of double-exposure speckle photography.

Strain-hardening and warm deformation behaviors of extruded Mg–Sn–Yb alloy sheet

Abstract Strain-hardening and warm deformation behaviors of extruded Mg–2Sn–0.5Yb alloy (at.%) sheet were investigated in uniaxial tensile test at temperatures of 25–250 °C and strain rates of 1 × 10−3 s−1–0.1 s−1. The data fit with the Kocks–Mecking type plots were used to show different stages of strain hardening. Besides III-stage and IV-stage, the absence of the II-stage strain hardening at room temperature should be related to the sufficient dynamic recrystallization during extrusion. The decrease of strain hardening ability of the alloy after yielding was attributed to the reduction of dislocation density with increasing testing temperature. Strain rate sensitivity (SRS) was significantly enhanced with increasing temperature, and the corresponding m-value was calculated as 0.07–0.12, which indicated that the deformation mechanism was dominated by the climb-controlled dislocation creep at 200 °C. Furthermore, the grain boundary sliding (GBS) was activated at 250 °C, which contributed to the higher SRS. The activation energy was calculated as 213.67 kJ mol−1, which was higher than that of lattice diffusion or grain boundary self-diffusion. In addition, the alloy exhibited a quasi superplasticity at 250 °C with a strain rate of 1 × 10−3 s−1, which was mainly related to the fine microstructure and the presence of the Mg2Sn and Mg2(Sn,Yb) particles.

Localization of plastic deformation in alloyed γ-iron single crystals electrolytically saturated with hydrogen

On chromonickel γ-iron single crystals with \(\left[ {\bar 111} \right]\) orientation and low packing-defect density, the plastic-flow localization during electrolytic saturation with hydrogen within a three-electrode electrochemical cell is investigated, with constant controllable cathode potential. On the plastic-flow curve for the extension of single crystals in the initial state (without hydrogen), beyond the transition from elasticity to developed plastic flow, linear strain hardening and then parabolic (Taylor) strain hardening may be observed. The plastic-flow curve for single crystals of austenitic steel saturated with hydrogen includes a small projection and a flow trough, stages of linear strain hardening and parabolic strain hardening, and a prefailure stage. Saturation of \(\left[ {\bar 111} \right]\) single crystals with hydrogen reduces the yield point, increases the plasticity to failure by a factor of 1.3, and suppresses necking in crystals oriented for multiple slip. By double-exposure speckle photography, the basic types of plastic flow location at different stages of strain hardening may be identified, in the presence and absence of hydrogen, and the corresponding parameters may be determined. Hydrogenation of chromonickel γ-iron single crystals intensifies the localization of deformation and leads to considerable changes in the characteristic distances between the plastic-shear bands and the localized-strain zones.

Mechanical properties and nanostructures in a duplex stainless steel subjected to equal channel angular pressing

The nanostructure and resultant mechanical property were studied in an UNS S32304 duplex stainless steel subjected to equal channel angular pressing. The successive refinement of original grains was observed to happen in dual phases, i.e., austenite and ferrite, during 4-pass pressing. Both phases exhibit similar lamellar features of microstructures by the formation of dislocation boundaries and their misorientation evolution with increasing strains. The deformation twinning and martensitic transformation as well were observed to happen in austenite. The transversal spacing of elongated grains is 79nm in austenite and 130nm in ferrite, respectively. After annealing at 973K for 20min, the elongated grains tend to become equiaxed, and the average size of nanograins increases to 108nm in austenite and 235nm in ferrite, respectively. The tensile tests show that the yield strength of the dual-phase nanostructure is 1460MPa after 4-pass pressing, as compared to 403MPa of its coarse-grained counterpart, but with uniform elongation of merely 2%. After annealing at 973K, the uniform elongation increases to 7% with the yield strength of 1100MPa. In addition, different stages of strain hardening were observed in various microstructures during tensile testing, and the corresponding plastic responses were discussed.

Effect of friction stir welding (FSW) parameters on strain hardening behavior of pure copper joints

Effect of tool rotation rate and traverse speed on strain hardening behavior of friction stir welded (FSWed) copper joints were investigated using hardening capacity and strain hardening exponent concepts. Kocks–Mecking type plots were used to show different stages of strain hardening. FSWed samples reveals higher hardening capacity and lower strain hardening exponent relative to base metal. With increasing rotation rate and/or decreasing traverse speed, FSWed samples show higher hardening capacity and lower strain hardening exponent. The strain hardening behavior was discussed by dislocation density and grain size variation during FSW. In addition, the microstructure and mechanical properties of the samples were investigated in detail. Four different zones were observed in microstructure of FSWed joints. The joints showed finer grain structure and weaker mechanical properties relative to base metal due to dynamic recrystallization and lower dislocation density.

Strain-Hardening Behavior of Mg-Li-Zn Alloy Thin Sheets under Tension

This work examined the effects of Li content on the strain-hardening behaviors of three varieties of Mg−Li−Zn alloys containing approximately 6 wt%, 9 wt%, and 10 wt% of Li. Tensile tests were carried out on specimens in the directions of 0, 45 and 90° to the rolling direction. Kocks–Mecking type plots were constructed to illustrate different stages of strain-hardening. The cold-rolled Mg−6Li−1Zn (designated as LZ61) alloy sheet showed stage II and stage III strain-hardening behaviors at room temperature. The specimens of Mg−9Li−1Zn (designated as LZ91) and Mg−10Li−1Zn (designated as LZ101) alloy sheets did not show stage II strain-hardening. Higher initial strain-hardening rates were observed in the 90° direction for these alloys as a result of the cold-rolled fibrous structure affording stronger barriers to dislocation movements in this direction.

Tensile flow and strain-hardening behaviors of dual-phase Mg–Li–Zn alloy thin sheets

The tensile flow behavior and associated microstructure changes of the cold-rolled Mg–Li–Zn alloys with a thickness of 0.6 mm containing approximately 6 and 9 wt% of Li were examined under tension. Tensile tests were carried out on specimens in the directions of 0°, 45° and 90° to the rolling direction using an initial strain rate of 1.67 × 10 −3 s −1 at room temperature. Kocks–Mecking type plots were constructed to illustrate different stages of strain-hardening. The results showed that tensile properties could be related to the deformed microstructures. Analysis of the flow behavior and the deformed microstructures indicated that the mechanical properties were related to the deformation of BCC β-phase in the cold-rolled Mg–6Li–1Zn (designated as LZ61) alloy sheet. The β-phase did not show significant deformation in the 90° specimens, leading to a higher strength and a higher stage II strain-hardening rate in this direction in the LZ61 alloy sheet. The activity of non-basal slips resulted in the considerable deformation of the α-phase in all test directions in the cold-rolled Mg–9Li–1Zn (designated as LZ91) alloy sheet.

Cyclic behavior and microstructural stability of ultrafine-grained AA6060 under strain-controlled fatigue

The strain-controlled fatigue behavior of AA6060, a precipitation hardening aluminum alloy, was investigated in ultrafinegrained (UFG) conditions after severe plastic deformation (SPD) by equal-channel angular pressing (ECAP). Two as-processed conditions, representing different stages of strain hardening and grain refinement as well as a ductility-optimized condition, achieved by a combined ECAP and aging treatment were considered. Low-voltage scanning transmission electron microscopy on samples stopped at characteristic stages of the fatigue process was applied to investigate the microstructural development. The as-processed as well as the optimized condition showed cyclic softening, which was found to be dependent on the amount of prestrain induced by ECAP processing. This is linked to dynamic recovery processes in the microstructure, indicated by a clearer distinction of grain boundaries and a reduction of dislocations in the grain interior. For all applied plastic strain amplitudes, ranging from Δεpl/2=1×10−3 to 5×10−3, the fatigue life of the ductility-optimized condition did not reach that of the severely work-hardened counterpart. For explaining this unexpected result, the differing (size-dependent) effectiveness of precipitates for the pinning of dislocations during cyclic loading was considered.

Plastic anisotropy and strain-hardening behavior of Mg–6%Li–1%Zn alloy thin sheet at elevated temperatures

An Mg–Li–Zn (designated as LAZ61) alloy containing about 6 wt% of Li has been prepared by melting and solidification in a carbon steel crucible, and extruded at a billet preheating temperature of 200 °C. The extruded plate was then cold-rolled to a final thickness of 0.6 mm with a total reduction of approximately 82%. Tensile tests were carried out in the rolling and transverse directions and at various temperatures to explore the effects of anisotropy and temperature on mechanical properties and strain-hardening behavior. Kocks–Mecking type plots were used to illustrate different stages of strain hardening. Anisotropic behavior of LZ61 sheet were observed in the mechanical properties at all test temperatures due to the development of texture in α phase during cold-rolling and a low content of BCC β phase. The cold-rolled LZ61 alloy sheet showed stage II and stage III strain-hardening behavior at test temperatures of room temperature and 100 °C. The specimens tested at 200 °C did not show stage II strain hardening. Higher initial strain-hardening rates were observed in the transverse direction as a result of the cold-rolled fibrous structure providing more strong barriers to the dislocation movement.

Strain hardening behavior of a friction stir welded magnesium alloy

The work hardening properties of a friction stir welded (FSWed) magnesium alloy were evaluated using two modified equations of hardening capacity and strain hardening exponent where the elastic deformation stage was excluded. Kocks–Mecking type plots were used to show different stages of strain hardening. The hardening capacity of the FSWed samples was observed to be about twice that of the base alloy, while the strain hardening exponent of the FSWed samples was nearly threefold higher than that of the base alloy.

Dependence of strain hardening exponent on the volume fraction and carbon content of martensite in dual phase steels during multistage work hardening

This letter deals with the influence of volume fraction (V m ) and carbon content (C m ) of martensite in ferrite-martensite dual phase steels on the value of n for different stages of strain hardening. This particular aspect has not so far been systematically investigated and reported in the literature