Applied Plastic Strain Research Articles

Nano hydride precipitation-induced disappearance of yield drop in zirconium alloy at elevated temperature

Hydrogen-induced variations in mechanical behavior of zirconium alloys impose detrimental influence on nuclear fuel cladding integrity. This work reports a disappearance of intrinsic yield drop in a recrystallized zirconium alloy following hydrogen-charging treatment. Microstructure characterizations reveal that the nano-hydrides precipitation, mediated by second phase particles Zr(Fe,Cr)2 acting as hydrogen trapping sites, leads to emission of substantial dislocations in α-matrix grains due to strong strain concentrations, as identified by high-angular resolution EBSD. These mobile dislocations preserved at elevated temperatures can maintain the applied plastic strain and impede rapid dislocation multiplication as well, a conclusion validated by comparative analysis of dislocation densities prior to and near yielding stage. These findings are expected to shed light on the underlying mechanisms governing the interaction between hydrogen and microstructural defects in Zr-based nuclear fuel cladding materials.

Microstructural characterization and corrosion-resistance behavior of friction stir-welded A390/10 wt% SiC composites-AA2024 Al alloy joints

This study examined the effect of traverse speed on the mechanical properties, corrosion-resistance behavior, and microstructure of friction stir-welded A390/10 wt% SiC composites-AA2024 Al alloy joints. The laminar flow of both materials was found to diminish in the stir zone (SZ) when the traverse speed of the tool increased from 40 to 80 mm/min, lowering their mixing rate. Large aspect ratio Si particles are broken by the tool pin-induced applied plastic strain, which turns them into refined equiaxed particles. Their aspect ratio remains unchanged in the SZ, despite their decreasing size. SiC and Si particles progressively come into view when moving from the AA2024 alloy's SZ to the composite workpieces. These changes happen abruptly as traverse speed increases due to the lack of an interfacial layer structure. The advancing side (AS)'s SZ grain size drops from 4.2 ± 0.3 μm to 1.2 ± 0.2 μm as the traverse speed drops from 80 to 40 mm/min. Increased traverse speed from 40 to 80 mm/min will result in a 5.8% decrease in elongation percentage (EP) and 8.4%, 36%, and 10.3% increases in the ultimate tensile strength (UTS), corrosion resistance, and yield strength, respectively.

Plastic-strain-induced magnetocaloric effect of Pt3Fe ordered alloy

We report a magnetocaloric effect of a plastically strained Pt3Fe antiferromagnet, in which ferromagnetism is induced due to the changes in the atomic arrangement around the {110} glide plane. The magnetic entropy change after the application of magnetic field increases with increasing applied plastic strain and shows a peak value of ∼0.1 J/K kg for an applied field of 50 kOe around the Néel temperature of 170 K. The magnetic entropy change can be due to the magnetization reversal of Fe magnetic moments in ferromagnetic domains formed around the {110} glide planes, and the peak temperature is influenced by the magnetic interaction between ferromagnetic domains and antiferromagnetic matrix. These observations suggest that a Pt3Fe chemically ordered alloy is a unique type of antiferromagnets in which the magnetocaloric effect can be induced and controlled by applied plastic strain.

Finite element analysis of the Non-Equal Channel Angular Pressing (NECAP) with different die geometries

The Non Equal Channel Angular Pressing is one of the promising severe plastic deformation techniques which is used to produce ultra-fine grained and nanostructured materials. In the present investigation, the deformation behavior of Al1100 alloy during non-equal channel angular pressing was studied using two dimensional finite element simulation. Results showed when the ratio of the width of output to input channel is higher than 0.7, the corner gap is formed in the outer side of the intersecting area of two channels and the amount of plastic strain decreases in the lower part of the sample. In contrast, when the amount of this ratio is lower than 0.7, the dead metal zone is formed in the outer region of the intersecting area and the amount of strain is increased in the lower side of sample. It is also observed that the uniformity of the applied plastic strain is decreased with the formation of dead metal zone. In addition, the amount of damage factor is increased with increasing the ratio of the width of output to input channel and the region with higher amounts of damage factor is shifted from the lower side to the middle region of deformed sample. Finally, the results of FEM simulation showed that the amount of pressing force is decreased with increasing the width of output channel.

Twinning characteristics in rolled AZ31B magnesium alloy under three stress states

Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR.

X-ray tomographic observation of environmental assisted cracking in heat-treated lean duplex stainless steel

The occurrence of environmental assisted cracking (EAC) has been observed in heat-treated lean duplex stainless steel via X-ray computed tomography (X-ray CT) and cross-sectional microstructure analysis. Selective corrosion of the ferrite phase occurred at the sample surface, with internal corrosion triggered by applied plastic strain. The observed cracks mainly propagated along ferrite-ferrite boundaries, with crack arrest associated with ferrite-austenite grain boundaries. Crack propagation during EAC fracture was observed, in-operando. The sudden appearance of EAC occurred due to the suppression of crack opening by ductile austenite phase. The reduced EAC resistance was attributed to hydrogen embrittlement effects.

On the effect of strain and triaxiality on void evolution in a heterogeneous microstructure – A statistical and single void study of damage in DP800 steel

In order to improve the understanding of damage evolution in mechanically heterogeneous microstructures, like the ones of dual-phase steels, the influence of the applied stress state is a key element. In this work, we studied the influence of the globally applied stress state on the evolution of damage in such a microstructure. Classical damage models allow predictions of damage during deformation based on considerations of the material as an isotropic continuum. Here, we investigate their validity in a dual phase microstructure that is locally dominated by its microstructural morphological complexity based on a statistical ensemble of thousands of individual voids formed under different stress states. For this purpose, we combined a calibrated material model incorporating damage formation to assess the local stress state in samples with different notch geometries and high-resolution electron microscopy of large areas using a deep learning-based automated micrograph analysis to detect and classify microstructural voids according to their source of origin. This allowed us to obtain both the continuum stress state during deformation and statistically relevant data of individual void formation. We found that the applied plastic strain is the major influence on the overall number, and therefore the nucleation of new voids, while triaxiality correlates with the median void size, supporting its proposed influence on void growth. In contrast, coalescence of voids leading to failure appears related to local instabilities in the form of shear band formation and is therefore only indirectly determined by the global stress state in that it determines the global distribution, density and size of voids.

Microstructure, strength and fracture toughness of CuNb nanocomposites processed with high pressure torsion using multi-sector disks

CuNb composites were processed by high pressure torsion using specially designed disks consisting of a pre-defined number of single sector elements called multi-sector disks. By variation of the applied plastic strain the composite structure could be tuned ranging from relatively coarse multilayer structures to nanostructured laminates. In this first study the technique is introduced, the microstructural and hardness evolution of CuNb with varying strain is presented and a fracture toughness study of the finest composite structure showing good damage tolerance has been performed.

Flow stress characteristics and microstructural evolution of cast superalloy 625 during hot deformation

The study is aimed to analyze the macro- and microstructural changes of homogenized cast alloy 625 during uniaxial hot compression testing at various strain rates (ε˙= 0.01–10s−1) in a temperature range of 1050–1200 °C. A Gleeble-3800 simulator was used for conducting the tests. The peak stress analysis shows that the average activation energy of 520 kJ is required to accelerate dynamic softening over hardening in the studied alloy, where recrystallization is the rate defining process. The flow stress behavior of the alloy is studied using a physical model, and further finite element method is used as a tool to map the strain distribution. At intermediate ε˙, especially at 1s−1, an unusual flow saturation is observed without revealing any peak stress. The macrographs of these samples indicate that the applied plastic strain is localized in small regions that are occupied with fine recrystallized grains, thereby leaving large undeformed zones. Detailed characterization of the simulated samples by SEM, EBSD and TEM indicate that at intermediate strain rates of 0.1 and 1s−1, the recrystallization rate is controlled by the formation of ∑3 twins at the interface of migrating high angle boundaries. The generation of recrystallization twins continuously changes the direction and velocity of a moving boundary by forming a new dynamically recrystallized grain. It is also observed that several recrystallization fronts are created due to the presence of carbide particles in the starting material through particle stimulated nucleation. The formation of the δ-phase by incipient melting/dissolution of carbide particles is observed for the sample deformed at 1200 °C with 10s−1. The results were discussed in terms of different softening mechanisms, such as dynamic recovery and recrystallization during the hot deformation of the studied alloy 625.

Mechanical Characteristics of a Roll-Bonded Cu-Clad Steel Sheet Processed Through Incremental Forming

Incremental sheet forming (ISF) is an emerging cold forming process with a high economic payoff. In this study, the influence of the ISF process on the tensile properties of Cu-clad Steel sheet is analyzed experimentally. Depending on the forming conditions, the post-ISF values of yield strength, tensile strength, and ductility are found to range from 161 to 430, 288 to 449 MPa, and 4.93 to 16.62 pct, respectively. These properties show a strong dependence on several correlated quantities such as the applied plastic strain, grain size, and mean residual stress. As the plastic strain increases from 0.08 to 0.8, the width of the grain decreases from 8.11 to 5.78 µm and the residual stress increases from − 15 to − 131 MPa. In comparison to the unformed sheet, ISF enhances the yield strength (4.6 to 117 pct) and tensile strength (0.6 to 73 pct). The nature of change in ductility (− 67 to 117 pct), however, depends on the process conditions, especially the state of the unformed sheet (i.e., rolled/annealed) and the value of applied strain. In general, the rolled (or prestrained) sheet upon ISF experiences an increase in ductility. The annealed sheet, however, sees a drop in ductility when the forming strains are low (e.g., 0.4). X-ray diffraction (XRD) and electron dispersive spectroscopy (EDS) analyses confirm that no new phase formed during ISF.

Dislocation-type evolution in quasi-statically compressed polycrystalline nickel

The nature of dislocation generation as a function of applied plastic strain in quasi-statically compressed polycrystalline pure nickel has been studied experimentally at ambient temperature. First, to ensure representative datasets of the geometrically-necessary dislocation densities (ρGND) associated with non-uniform plastic deformation, measurements over large (several millimeter square) areas were made using Hough-based EBSD methods. In addition, the total dislocation density (ρT) responsible for the overall work hardening is estimated from the measured flow stress based on Taylor's hardening model. Next, the statistically stored dislocation (SSD) density (ρSSD) is calculated by subtracting the GND density from the total dislocation density. The results demonstrate that in quasi-statically deformed nickel: i) the measured GND density varies linearly as a function of plastic strain in the range between 0.05 and 0.46; although Ashby's model predicts linearity for GND density evolution over entire range of strains, this study does not cover strains below 0.05; ii) the SSD density increases at a rate much faster than GND density; and iii) the SSD density exceeds the GND density at above 0.09 plastic strain. Both i) and ii) are in agreement with Ashby's prediction, while the magnitudes of GND density (iii) differ from Ashby's model prediction, particularly at large applied strains. Overall, this study enables the interplay of GNDs and SSDs in the hardening of nickel to be gleaned in a quantitative sense. This study illustrates that GNDs are the more important for the strength of polycrystalline metals in the early stages of work hardening, whereas SSDs contribute more to the strength at larger strains. Over the range of strain in this study, work hardening is predominantly through rapid multiplication of SSDs; whereas the GND density is initially higher than SSD density at 0.05 probably due to non-linear evolution of GNDs at low strains (<0.05), which will be the subject of future investigation.

Anelastic behavior of Mg-Al and Mg-Zn solid solutions

The anelastic strain measured using tensile loading-unloading loops in pure Mg and in binary solid solutions of Mg-Al and Mg-Zn, with contents of 0.5, 2 and 9at% Al and 0.8 and 2.3at% Zn, was correlated with available data for the area fractions and number densities of twins, for applied strains of up to 3%. For pure Mg, no anelastic strain was observed up to about 4% area fraction of twins and at an applied plastic strain of ~1.7×10−4, after which it increased rapidly, levelling off at a plastic strain of ~1%. The alloys followed the same pattern, but from much smaller minimum area fractions of twinning, <1% for Mg-Zn, and ~2% for Mg-Al, at applied plastic strains of ~2.5×10−4 and ~3.1×10−4, respectively. The anelastic strain saturated at a maximum value of ~0.002 for all alloys, save for the 9%Al for which it reached a much larger level (~0.004). The correlation with the number density of twins followed similar patterns. For a given alloy, the magnitude of the anelastic effect can be related to the applied stress in excess of that required to initiate microplasticity. The results are discussed in terms of the solid solution hardening and softening effects upon basal and prism slip in the dilute alloys, and of short range order upon slip and twinning in concentrated Mg-Zn.

The effects of pre-straining conditions on fatigue behavior of a multiphase TRIP steel

To assess the fatigue properties of transformation-induced plasticity (TRIP) steels with various strain histories, a multiphase TRIP780 steel (0.2C–1.7Mn–1.7Si–0.04Al in wt pct) containing 17.3vol pct initial retained austenite was studied. Specimens were pre-strained to 0%, 5%, or 15% in uniaxial tension at room temperature, −20, or 80°C, then subjected to fully-reversed room-temperature fatigue cycling at total strain amplitudes of 0.2–0.6%. The low-cycle fatigue (LCF) life of the TRIP780 steel was relatively independent of prior strain history with respect to the applied strain amplitude. However, the large loss in ductility due to pre-straining to 15% resulted in a decrease in LCF life with respect to the applied plastic strain amplitude. Decreasing the pre-strain temperature (in the range of 80 to −20°C) increased martensite content but did not significantly reduce fatigue life. The amount of austenite formed during fatigue was comparable or slightly larger than the amount formed at equivalent strains in tension.

フェライト・パーライト鋼のへき開破壊靱性予測モデルの構築

A numerical model to quantitatively predict cleavage fracture initiation in ferrite-pearlite steel is proposed. The model is based on microscopic fracture process of three stages; Stage-I: formation of fracture origin in pearlite colony, Stage-II: propagation of the pearlite crack into ferrite matrix, and Stage-III: propagation of the crack across ferrite grain boundary. In the proposed model, fracture conditions are formulated by the probability of pearlite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage-III. Ferrite grains and cementite particles are assigned based on their distributions into each volume element. Applied plastic strain and stress of each volume element are calculated by finite element analysis. Cleavage fracture is assumed to initiate at the time when the fracture conditions of the all stages are satisfied in any one of the volume elements. Cleavage fracture toughness of three point bend test is simulated by the proposed model. The numerical predicted results of fracture toughness show good agreement with experimental ones. The bottleneck process of cleavage fracture is then evaluated by the number of arrested micro-cracks until all of the fracture process is satisfied. Influence of ferrite and pearlite size on cleavage fracture toughness is evaluated. It is shown that steel with finer pearlite colony is tougher, and then the developed model can reproduce the size effect of cleavage fracture toughness. Based on the aforementioned results, the validation and the effectiveness of the proposed model are found out.

Development of a novel method for the backward extrusion

In this study, a new method of backward extrusion using small diameter billet is proposed. In this new process, the die setup consists of three main parts of the fix-punch, the moveable punch and the matrix. The fix-punch has been used to decrease the cross section of applied billet and finally reducing the total force of the process. To investigate the capability of this process, experimental and finite element (FE) methods were used. The results showed that the first advantageous of the new process is that the load is reduced to about less than a quarter in comparison with the conventional backward extrusion process. This higher reduction in the required force is due to reduce of the cross section of the initial billet. EF results showed that while needing lower loads, the applied plastic strain through the processed sample is about two times higher than that in the sample processed via conventional backward extrusion. This is the second advantageous of the process. Eventually, the most important advantages of the novel method of backward extrusion are the lower process force, imposing higher effective strain and a better strain homogeneity through the tube length. This new process is also very promising for producing ultra fine grained (UFG) samples because the higher level of shear strains.

Stress Measurement of an Austenitic Stainless Steel Foil by cos&lt;sup&gt;2&lt;/sup&gt;χ&lt;sub&gt;&lt;/sub&gt; Method Using Polychromatic Laboratory X-Rays

Elastic-plastic deformation properties of austenitic stainless steel foils were evaluated by using polychromatic laboratory X-rays. The transmitted optical system was used for stress measurement. The diffraction elastic constants for several diffraction planes were measured under monotonic loading by the cos2χ method. The diffraction energy decreased almost linearly with increasing cos2χ, and the slope of the cos2χ diagram decreased with increasing applied stress. Measured diffraction elastic constants were compared with the theoretical values calculated by the Kröner model. The experimental value agreed well with the theoretical value. The lattice strain measured during plastic deformation depended on the diffraction plane. The full width at half maximum increased with applied plastic strain. From the the diffraction-plane dependence of the lattice strain, the full width at half maximum and the diffraction intensity can be evaluated using polychromatic laboratory X-rays.

フェライト鋼へき開破壊靱性予測モデルの構築

A numerical model to quantitatively predict the cleavage fracture initiation in ferrite-cementite steel is proposed. The model is based on the microscopic fracture process of the three stages; Stage-I: formation of fracture origin by cementite cracking, Stage-II: propagation of the cementite crack into ferrite matrix and formation of a cleavage crack, and Stage-III: propagation of the cleavage crack across ferrite grain boundary. Influencing factors on Stage-I is quantified by using tensile testing with circumferential notched round bar specimens. The specimens are made from steels with various ferrite and cementite sizes. Probability of cementite cracking is formulated based on the results of SEM observation and measurements. Notched three point bend interrupted tests with fractography shows the importance of Stage-III because a number of arrested micro-cracks are observed on the fracture surface. In the proposed model, fracture conditions are formulated by the ratio of cementite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage-III. Active zone is divided into finite volume elements. Ferrite grains and cementite particles are assigned based on their distributions into each volume element. Applied plastic strain and stress of each volume element are calculated by macroscopic finite element analysis. Cleavage fracture is assumed to initiate at the time when the fracture conditions of the all stages are satisfied in any one of the volume elements.

Assessment of local deformation using EBSD: Quantification of local damage at grain boundaries

Abstract Electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy was used to assess localization of the local misorientation to grain boundary. In order to quantify the degree of localization, a parameter, which was referred to as the grain boundary local misorientation, was proposed. Through crystal orientation measurements using deformed Type 316 stainless steel, it was shown that the grain boundary local misorientation increased with the applied plastic strain. Particularly, at several grain boundaries, the grain boundary local misorientation was more than 3 times the local misorientation averaged for the whole area. Surface observations revealed that the large local misorientation near the grain boundaries was attributed to the impeded slip steps rather than the number of slip steps observed on the surface. The magnitude of the grain boundary local misorientation had a week correlation with grain boundary length or grain boundary misorientation, and no correlation was found for twin boundaries. Finally, it was shown that the maximum grain boundary local misorientation could be estimated statistically, and the estimated maximum value for the specimen surface with an area of 80 mm2 was 10.6 times the averaged value.

Dynamic yielding of single crystal Ta at strain rates of ∼5 × 105/s

A magnetic loading technique was used to produce planar ramp loading of [100] and [110] orientations of single crystal tantalum to peak stresses of either ∼18 or ∼86 GPa for applied plastic strain rates of about 2 × 106/s. It was found that the dynamic elastic limit varied only slightly for factor-of-2 changes in the resulting elastic strain rates near 5 × 105/s. For wave propagation in the [100] direction, the dynamic elastic limit varied from 4.18–3.92 GPa for corresponding sample thicknesses of 0.625–1.030 mm and exhibited a slight rate dependence for the strain rate region studied. For [110] compression, the elastic limit was essentially independent of propagation distance, but exhibited a significant sample-to-sample variation; the elastic limit for this orientation varied from 2.49–3.18 GPa over sample thicknesses of 0.702–1.023 mm, with an average and standard deviation for the data of 2.93 ± 0.27 GPa. There was no apparent rate dependence in this case for the strain rates examined.

Stress Measurement of an Austenitic Stainless Steel Foil by Transmitted Polychromatic X-Rays

Deformation properties of austenitic stainless steel foils of 0.05 mm in thickness were evaluated by using transmitted polychromatic X-rays under monotonic tensile loading. A conventional laboratory X-ray equipment with a rotating Mo anode was adopted at a tube current of 40 mA and an acceleration voltage of 60kV. Soller slits with a divergence angle of 0.5 deg were attached on both divergent and receiving sides. By the preset time of 500s, enough diffraction intensity was obtained to determine the stress. The diffraction elastic constants were measured under monotonic loading by the cos2χ method. The diffraction energy decreased almost linearly with increasing cos2χ, and the slope of the cos2χ diagram decreased with increasing applied stress. Measured diffraction elastic constants were compared with the theoretical values calculated by the Kröner model. The experimental value obtained from a single peak with high intensity agreed well with theoretical one, and the standard deviation was enough small. The lattice strain measured during plastic deformation depended on the diffraction plane. For the single peak profile, the full width at half maximum increased with applied plastic strain. From the the diffraction-plane dependence of the lattice strain, the full width at half maximum and the diffraction intensity, deformation properties of the materials can be evaluated. Diffraction method of laboratory polychromatic X-rays is effective as a simple technique to measure multiple X-ray parameters.