Tensile Flow Stress Research Articles

Grain size gradient effect on grain boundary mediated deformation of nano-grained metals

The extraordinary mechanical performance of gradient nano-grained (GNG) structures is characterized by the heterogeneity of dislocation-based and grain boundary (GB)-based plasticity, where the grain size gradient effect on these deformation mechanisms even fracture process still remains unclear. Herein, molecular dynamics simulation is conducted on the GNG α-Fe structures to investigate the grain size gradient effect on the GB-mediated hetero-deformation mechanisms. The plastic deformation mechanism changes from GB motion (grain coarsening and merging) to GB-dislocation interaction when the grain size distribution extends from the Hall-Petch softening region to the strengthening region. With increasing grain size gradient, the ultimate tensile stress and flow stress increase in the GB motion-controlled samples, and strain partitioning between hard and soft zones is found to increase in the dislocation-GB interaction- controlled samples from an atomistic perspective. The increased strain partitioning and its accommodation process further contribute to the fracture resistance enhancement, which is demonstrated by the uniform hetero-deformation between coarse and fine grains of the GNG structure, and further avoids localized plastic deformation.

Dislocation evolution of a novel Co-Al-W based single crystal alloy with a γ-CoSS/γ’-Co3(Al, W) coherent microstructure around flow stress anomaly temperature from yielding to fracture

In this study, a novel single-crystal (SC) Co-9Al-9W-2Ta-0.02B-0.02Ce alloy with a growth direction of [001] was prepared by selecting crystal method (SCM). The tensile strength, ductility and dislocation evolution around temperatures of the flow stress anomaly from deformation to fracture were investigated. Aged at 900 °C for 100 h, the microstructure of the aged SC alloy is composed of Co solid solution γ-CoSS matrix and nano-scale cuboidal γ’-Co3(Al, W) intermetallic precipitates, while the two phases are coherent. The aged SC alloy exhibits tensile flow stress anomaly at 800 °C, with a peck 0.2% yield strength of 685 MPa. The SC alloy always shows strain-hardening state after yielding and excellent elongation larger than 18.9% from RT to 900 °C. At the stress anomaly temperature of 800 °C, the dislocation evolution is dominated by the 〈001〉 dislocation in the {001} plane of the γ’ phase pining back the dislocation in the {111} plane of the γ' phase in a strain range of ∼5%. With the further increase of the strain until fracture, the dislocation configuration is controlled by formation of the superlattice intrinsic stacking faults (SISF) through the frontal a/3 〈121〉 dislocations extending in the γ' phase. At 900 °C, the mobility of dislocation increases by the strong thermal activation and the Orowan mechanism appears due to the dislocations bowling the γ' phase. Both subsequently, reduce the strength and the strain-hardening effect of the SC alloy.

Influence of Solid Solutions on the Al2024 High-Temperature Deformation Behavior.

The mechanical properties of 2024 aluminum alloy were studied after two different tempers. The T351 temper (solution heat treatment, stress relief, and natural aging) leads to high hardness and toughness. A thermal treatment consisting of heat-treating at 280 °C for 48 h and slow cooling in a furnace, named TT temper, was performed to increase the precipitate size and their separation while minimizing the amount of solutes in solid solution, which produced the minimum hardness for an overaged Al2024 alloy and a lower tensile flow stress than for the T351 temper. The flow stress strongly decreases and the elongation to failure strongly increases for both materials above 300 °C. Differences in strain rate at a given stress in the power law regime at all temperatures for both tempers and compared with pure aluminum are attributed to the influence of solutes in solid solutions, affecting both the glide and climb of dislocations. However, the stacking fault energy, SFE, alone does not account for the hot deformation behavior. Thus, it is the synergistic effect of various solutes that affects the entire deformation process, causing a decrease of three or four orders of magnitude in strain rate for a given stress with respect to the pure aluminum matrix values.

Improvement of plastic property of Ti/Al nanowires by designing the core–shell structures

Ti alloy has the disadvantages of low elastic modulus, high yield ratio, and low plasticity, therefore, improving its plasticity is very important to promote their use. In this study, the tensile behavior of Ti/Al core–shell nanowires (NWs) in the z-axis direction of single-crystal Ti with [0001] grain-oriented HCP structure and single-crystal Al with [001] grain-oriented FCC structure was investigated using molecular dynamic (MD) simulations to explore the mechanism of enhanced ductility in Ti alloy. The results indicate that the shell thickness may significantly affect the mechanical behaviors of the NWs. For the mechanical properties of core–shell NWs, Young’s modulus, ultimate tensile strength (UTS), Specific modulus, Specific strength, flow stress, and fracture strain showed sensitivity to shell thickness. Compared with core–shell NWs, single crystal Ti NW has greater strength and higher Young’s modulus, Specific strength and UTS. By contrast, core–shell NWs have better Specific modulus and plastic properties, their flow stress and fracture strain are higher than those of single crystal Ti NW. For the single crystal Ti NW, the main plastic deformation mechanisms are shear band nucleation and recrystallization. For Ti/Al core–shell NWs with shell thicknesses of 1and 2 nm, the nucleation of the twin variants replaces the dominant position of the shear bands. As the twin boundaries (TBs) expand, the dislocation slip is activated, and grain reorientation occurs, inducing the superior plastic properties of NWs. As the shell thickness increases to 3–5 nm, the interaction between the twin variants and shear bands reduces the expansion rate of the TBs, resulting in increased flow stress and fracture strain of the NWs. This study can provide theoretical guidance for the experimental study and preparation of core–shell NWs.

Quantitatively evaluating respective contribution of austenite and deformation-induced martensite to flow stress, plastic strain, and strain hardening rate in tensile deformed TRIP steel

Transformation-induced plasticity (TRIP)-assisted steels exhibit an excellent combination of strength and ductility due to enhanced strain hardening rate associated with deformation-induced martensitic transformation (DIMT). Quantitative evaluation on the role of DIMT in strain hardening behavior of TRIP-assisted steels and alloys can provide guidance for designing advanced materials with strength and ductility synergy, which is, however, difficult since the phase composition keeps changing and both stress and plastic strain are dynamically partitioned among constituent phases during deformation. In the present study, tensile deformation with in situ neutron diffraction measurement was performed on an Fe-24Ni-0.3C (wt.%) TRIP-assisted austenitic steel. The analysis method based on stress partitioning and phase fractions measured by neutron diffraction was proposed, by which the tensile flow stress and the strain hardening rate of the specimen were resolved into factors associated with each phase, i.e., the austenite matrix, deformation-induced martensite, and the transformation rate of DIMT after differentiation, and then the role of each factor in the global strain hardening behavior was discussed. In addition, the plastic strain partitioning between austenite and martensite was indirectly estimated using the dislocation density measured by diffraction profile analysis, which constructed the full picture of stress and strain partitioning between austenite and martensite in the material. The results suggested that both the transformation rate and the phase stress borne by the deformation-induced martensite played important roles in the global tensile properties of the material. The proposed decomposition analysis method could be widely applied to investigating mechanical behavior of multi-phase alloys exhibiting the TRIP phenomenon.

Temperature and Die Angular Effect on Tensile Strength, Hardness, Extrusion Load and Flow Stress in Aluminum 6063 Processed by Equal Channel Angular Extrusion Method

Temperature and Die Angular Effect on Tensile Strength, Hardness, Extrusion Load and Flow Stress in Aluminum 6063 Processed by Equal Channel Angular Extrusion Method

Tensile behavior, microstructural evolution, and deformation mechanisms of a high Nb-TiAl alloy additively manufactured by electron beam melting

To evaluate the tensile behavior, microstructural evolution, and deformation mechanisms of Ti-45Al-8Nb alloy additively manufactured by electron beam melting, uniaxial tensile experiments were performed at a constant strain rate of 2.5 × 10−4 s−1 at various temperatures. The experimental results indicated that the tensile behavior and flow stress are sensitive to temperature. The brittle to ductile transition temperature of Ti-45Al-8Nb alloy additively manufactured by electron beam melting is between 700 and 750 ℃. Below this temperature, the fracture was predominantly trans-granular, resulting in brittle failure. In addition, the brittle failure behavior is related to the localization of deformation within the grains and stress concentrations owing to plastic incompatibility at the interface of the α2+γ phases. However, above this temperature, the fracture transformed from the trans-granular type to the mixed mode of trans-granular and ductile dimples. Dynamic recovery and recrystallization are the primary processes leading to softening behavior. The flow behavior at elevated temperatures results from the competition between work hardening and softening. Furthermore, the Chaboche model was used to describe the tensile inelastic behavior at different temperatures.

Stacking fault induced hardening and grain size effect in nanocrystalline CoNiCrFeMn high-entropy alloy

A broad variation of stacking fault energy (SFE) in high entropy alloys (HEAs) gives rise to rich deformation mechanisms and unique mechanical properties of HEAs. In this paper, we aim to reveal the interplay between grain boundaries and stacking faults in governing the strength and flow stress of CoNiCrFeMn HEA. We carried out atomistic simulations for the tension of polycrystalline CoNiCrFeMn HEA of different grain sizes from 3.0 to 48.6 nm at different temperatures. The tensile flow stress of the HEA follows the traditional Hall–Petch (HP) relation till grain size is down to 15.0 nm. Massive stacking faults contribute to the extra hardening and render a rather gradual drop of flow stress with increasing grain size: In the regime governed by HP relation, partial dislocations emitting from grain boundaries are primarily leading partials; and resulted stacking fault (SF) can serve as barriers to dislocation gliding on intersecting planes. This mild hardening mechanism complements strong hardening from grain boundaries. We expect SF induced hardening could be general in polycrystalline HEAs. The findings reported here may provide a basis and engineering guidance for strengthening and toughening the design of a broad range of HEAs characterized by a wide spectrum of SFEs.

Flow stress and work hardening behaviour of Mg-3Al-1Zn alloy

In the present study, the uniaxial tensile flow stress behaviour and work hardening behaviour of AZ31B alloy has been investigated across a wide range of temperature (25 °C–250 °C) at quasi-static strain rates (10−1·s−1–10−3·s−1). The flow stress behaviour of the AZ31B sheet has been analysed at different temperatures, quasi-static strain rates and sheet orientations. It has been found that flow stress behaviour is highly influenced by the variation of temperatures, strain rates, and sheet orientations. The various important mechanical properties such as yield strength, ultimate tensile strength, percentage elongation, strain hardening exponent, and strain rate sensitivity have been determined. There has been a significant improvement in ductility (201.71%) of AZ31B alloy at 250 °C compared to 25 °C. The decrement in yield strength and ultimate strength has been found to be 47.52% and 53.66%, respectively. It has been observed that the AZ31B alloy is found to be more strain rate sensitive at elevated temperature (m = 0.11, at 250 °C) as compare to 25 °C. The various empirical hardening equations such as Hollomon, Ludwik, Ludwigson, Swift, and modified Voce have been formulated for a wide range of temperatures, strain rates, and sheet orientations. The K-M plots show three-stage of work-hardening behaviour for AZ31B alloy. Based on the performance comparison of different models, the Swift and Modified Voce efficiently predict the flow curve with correlation (more than 99%) at different temperature ranges. The fracture surface is analysed by FESEM. The quasi cleavage and river-like pattern are observed for the room temperature fractured specimen. The increase in temperature converts the mode of fracture from cleavage to ductile fracture.

Hot Deformation Behavior and Strain Rate Sensitivity of α+β Brass Sheet by Uniaxial Material Constitutive Equations

The present work proposes a systematic procedure for evaluation of high temperatures deformation and formability of α+β Brass undergoing the uniaxial tensile test conditions. Firstly, uniaxial tensile tests were conducted on Universal Testing Machine (UTM) with loading capacity of 100 KN at temperature of 773K, 873K and 973K with a quasi-static strain rates of 0.001s-1, 0.01 s-1 and 0.1s-1. Hot tensile flow stress behaviors have been affected significantly by test temperatures and strain rates for Brass. Drop-in yield and ultimate tensile strength have been observed at approximately 58 % and 68 % with a rise in test temperature from 773 K to 973 K. Around 30% improvement has been observed in % elongation with rise in test temperature. Further, flow stress has been predicted by most popular Johnson Cook (JC) uniaxial constitutive model at wide range of temperatures (773K, 873K and 973K) and strain rates (0.001s-1,0.01 s-1 and 0.1s-1). Further, yield loci have been plotted at various temperatures using Hill 1948 and Barlat 1989 yield function. Barlat 1989 has followed experimental results correctly in all test temperatures.

Strengthening contributions of dislocations and twins in warm-rolled TWIP steels

A twinning-induced plasticity (TWIP) steel was processed by warm rolling with different reduction ratios to elevate yield strength while maintaining considerable ductility. To investigate the effect of prior warm rolling on the deformation mechanisms during subsequent tensile straining, the contributions of individual strengthening mechanisms to the tensile flow stress of the as-received (AR) and warm-rolled (WR) steels were decoupled, based on the measured dislocation density evolutions determined by synchrotron X-ray diffraction (XRD) experiments. Whereas dislocation multiplication invariably governs the maximum flow stress of the AR and WR steels, deformation twinning becomes increasingly important for the strain hardening of steels with larger warm rolling reductions. In addition, twinning kinetics is enhanced by the pre-existing high dislocation densities caused by warm rolling, leading to an enhancement of twinning-induced hardening. The present work provides further insight into the influence of warm rolling on the deformation mechanisms of TWIP steels.

Elasto-thermoviscoplastic finite element analyses of cold upsetting and forging processes of S25C steel with dynamic strain aging considered

A practical methodology is presented to characterize the thermoviscoplastic flow stress at larger strain over the temperature range of cold metal forming using tensile and compression tests. Its importance is emphasized for non-isothermal finite element (FE) analysis of automatic multi-stage cold forging (AMSCF) process where maximum strain and strain rate exceed around 3.0 and 200/s, respectively. The experimental compressive flow stress is first characterized using traditional bilinear C-m model with high accuracy. It is employed for describing the closed-form function model to extrapolate the experimental flow stress over the experimentally uncovered ranges of state variables. The strain effect on the flow stress is then improved using the experimental tensile flow stress accurately calculated at large strain and room temperature. A complicated flow behavior of S25C characterized by its dynamic strain aging features is expressed by the presented methodology, which is utilized to analyze the test upsetting and AMSCF processes by the elasto-thermoviscoplastic finite element method for revealing the effects of flow stresses on the process.

Damage constitutive model of pure copper at different annealing temperatures

Aiming at the problem of damage evolution of pure copper during the plastic deformation, the normalized shape factor is introduced based on the RO model (Ramberg-Osgood model). The mesoscopic damage constitutive model of pure copper at different annealing temperatures is established and the tensile deformation of industrial pure copper at different annealing temperatures is analyzed. The results show that the error between the calculated value and the experimental value of the damage constitutive model, based on normalized shape factor, at different annealing temperatures, is less than 10%. The model can effectively reveal the tensile damage evolution behavior of industrial pure copper and accurately predict the plastic tensile flow stress of industrial pure copper at different annealing temperatures. The hardening coefficient and hardening exponent in the model are closely related to the annealing temperature of the material. The annealing temperature has little effect on the hardening exponent and has a significant effect on the hardening coefficient and the hardening coefficient decreases with the increase in annealing temperature.

Analysis of Tensile Deformation Behaviour of Dual Phase -590 Steel at Different Temperatures and Strain Rates

ABSTRACT An accurate uniaxial constitutive model is necessary to analyse the plastic deformation behaviour of metal and for reliable finite element simulations in sheet metal forming operations. In the present study, the hot uniaxial tensile flow stress analysis of dual phase (DP590) steel thin sheet has been carried out under a quasi-static strain rates of 0.1–0.001/s at the temperature range of 723–1073 K. The flow stress behaviour is significantly affected by test temperatures, strain rates and sheet orientations. Further, the Fields–Backofen constitutive flow relationship has been used to investigate the flow stress behaviour. This predicted flow stress curves shows a good fit to the experimentally measured stress at the work-hardening stage. Furthermore, a modified Fields–Backofen constitutive equation with a softening term has been considered to predict the flow behaviour. The capability of models has been evaluated based on the correlation coefficient (R), average absolute error AAE (Δavg) and root mean square error (RMSE). A better prediction of flow stress behaviour is observed by modified modified Fields–Backofen model with R = 0.9552, Δ = 0.034 and s = 0.0564.

Analysis of material properties and strain hardening behaviour of brass at different temperature and quasi-static strain rate conditions

ABSTRACT The α-Brass alloy is gaining special attention for electrical and thermal conductivity applications because of being easily formed/workability and retaining high strength after forming. This makes crucial to understand the plastic deformation behaviour and work hardening behaviour of a material for reliable finite element simulations. In present study, the hot uniaxial tensile flow stress analysis of α-Brass alloy thin sheet has been carried out under a quasi-static strain rates of 0.1–0.001/s at the temperature range of 300 K–773 K. Flow stress behaviour has significantly affected with test temperatures, strain rates and sheet orientations. Anisotropy in terms of yield stress variation and % elongation by in-plane anisotropy and Anisotropy index have been evaluated. Further, stress–strain relation has been analysed by popular empirical strain hardening equation namely, Hollomon and Swift. The work hardening behaviour of alloy has been well-defined by Swift relationship compare to Hollomon empirical relation.

A physically-based structure-property model for additively manufactured Ti-6Al-4V

A physically-based, mixed-phase structure-property model is presented for microstructure-sensitivity of tensile stress-strain response, including yield stress, ultimate tensile strength, uniform elongation and flow stress (strain hardening), for additively manufactured Ti-6Al-4V. The interdependent effects of solutes, grain size, phase volume fraction and dislocation density are explicitly included. Solid-state phase transformation and dislocation density evolution are incorporated to simulate the effects of martensite dissolution and α-β transformation at high temperature. Predictions are validated by comparison with measured tensile test data for (i) effects of additive manufacturing process conditions (such as build orientation and sample size) on tensile properties, based on the microstructure attributes inherited from the process, and (ii) the effect of temperature on tensile stress-strain response across a broad range of temperatures. The model is thus applicable for rapid process-structure-property prediction, in conjunction with AM process modelling, to capture the effects of key manufacturing variables and for process optimization.

A Flow Stress Model of 300M Steel for Isothermal Tension.

To investigate the effect of hot working parameters on the flow behavior of 300M steel under tension, hot uniaxial tensile tests were implemented under different temperatures (950 °C, 1000 °C, 1050 °C, 1100 °C, 1150 °C) and strain rates (0.01 s−1, 0.1 s−1, 1 s−1, 10 s−1). Compared with uniaxial compression, the tensile flow stress was 29.1% higher because dynamic recrystallization softening was less sufficient in the tensile stress state. The ultimate elongation of 300M steel increased with the decrease of temperature and the increase of strain rate. To eliminate the influence of sample necking on stress-strain relationship, both the stress and the strain were calibrated using the cross-sectional area of the neck zone. A constitutive model for tensile deformation was established based on the modified Arrhenius model, in which the model parameters (n, α, Q, ln(A)) were described as a function of strain. The average deviation was 6.81 MPa (6.23%), showing good accuracy of the constitutive model.

Ductile fracture behavior and flow stress modeling of 17-4PH martensitic stainless steel in tensile deformation at high temperature

The ductile fracture behavior of 17-4PH martensitic stainless steel at high temperature was investigated based on the experimental data of hot uniaxial tensile test. The elongation and reduction of area indicated that the ductility was mainly enhanced during the deformation at the temperature 1150 °C–1200 °C with high strain rate more than 0.1 s−1. The hot flow stress curves were obtained from the tensile experiments, and the true strain of necking initiation was determined by analyzing the contours of fractured specimens. Then, the true stress-true strain data of the deformations before necking were adopted to establish the tensile flow stress model. In order to express the combined influences of the strain, strain rate and temperature on flow stress, a modified Johnson-Cook model was proposed to describe the hot constitutive behavior of the presented steel. It indicates that reasonable agreements between the model-predicted results and the experimental data were achieved.

Construction of tensile stress-strain curves for solid bars pre-deformed by gradient shear strain

Cai Chen, Beygelzimer Y., Kulagin R., Davydenko O. Construction of tensile stress-strain curves for solid bars pre-deformed by gradient shear strain. Material working by pressure. 2020. № 1 (50). P. 114-118.
 Strain hardening curves were derived for the first time for tension of cylindrical solid bars in the presence of a linear strain gradient produced by severely pre-straining by torsion. An analytical formula was obtained, which enables constructing the local stress-strain curve. Data obtained for pure copper samples pre-twisted to different levels of shear strain were used. A saturation stage not known hitherto was found for large strains.
 One particularity of the present results is that for low tensile strains the tensile flow stress after torsion agrees well with the flow stress in monotonic tension.
 Another particularity of the results is that the tensile flow stress is constant after torsion at large strains, starting from about 1.5 strain. This effect was observed for the first time because the present technique is the first one to provide access to tensile flow stress after large strain torsion.

Monotonic and cyclic anisotropies of an extruded Mg–Al–Ca–Mn alloy plate: Experiments and crystal plasticity studies

Mechanical anisotropies of an extruded Mg–Al–Ca–Mn alloy plate were investigated under both monotonic and cyclic conditions through a combination of experiments and crystal plasticity finite element simulations. To this end, monotonic tensile, compression as well as fully-reversed strain-controlled and load-controlled fatigue tests were conducted on specimens aligned either in the extrusion or transverse direction. Plastic activities of the deformation mechanisms for the different loading conditions were quantitatively analyzed based on the results of numerical simulations. The asymmetrical mechanical response between tension and compression along the two loading directions was found to be associated with the lack of axi-symmetry of the relatively strong basal texture. In addition, the peculiar and lower tensile flow stress in the transverse direction was explained by the activation of deformation twinning in grains with their c-axis aligned along the loading direction. Under low-cycle fatigue conditions, a lower asymmetry of stress–strain hysteresis curves was observed in the transverse direction due to the occurrence of twinning/detwinning in both the tensile and compressive portions. High-cycle fatigue experiments revealed lower fatigue lives in the transverse direction than in the extrusion direction but similar fatigue limits. Crystallographic analyses of multiple secondary cracks revealed crack initiation from basal slip, twin bands, grain boundaries and non-metallic inclusions. The evaluation of slip-induced and twin-induced fatigue criteria from the results of fatigue simulations of explicitly modeled polycrystals suggested that the most detrimental crack initiation mechanism was crack initiation from basal slip as it predicted a significantly lower initiation life in the transverse direction than in the extrusion direction, as observed experimentally.