Drop In Flow Stress Research Articles

Subgrains, Texture Evolution, and Dynamic Abnormal Grain Growth in a Mo Rod Material

The roles of subgrains, texture, and surface energy during dynamic abnormal grain growth (DAGG) were examined in a commercial-purity Mo rod material. DAGG was observed in this material during tensile deformation at 2023 K (1750 °C). Cooling of specimens after tensile testing was sufficiently rapid to preserve both subgrain structures developed during deformation and several abnormal grains at early stages of growth. These and other microstructural features were characterized to evaluate how subgrains and boundary character influence the early stages of DAGG. Subgrains were observed in the deformed polycrystalline material but were generally absent in newly formed abnormal grains. This was identified as the cause of the sudden drop in flow stress observed at the initiation of DAGG. It is proposed that subgrain intersections with abnormal grain boundaries provide a driving pressure for DAGG. Subgrains within the deformed polycrystals were observed to locally change the boundary curvature at their intersections with abnormal grain boundaries, which likely encouraged growth of the abnormal grains into the deformed polycrystals. Abnormal grains produced by DAGG retained crystallographic orientations and boundary characters that closely resembled those of the polycrystalline material from which they grew. This suggests that neither differences in orientation nor boundary character were important to DAGG in this material.

Dynamic Uniaxial Compression of HSLA-65 Steel at Elevated Temperatures

In the present study, the dynamic response of a high-strength, low alloy Grade 65 (HSLA-65) steel, used by the United States Navy for ship hull construction, is investigated under dynamic uniaxial compression at temperatures ranging from room temperature to 1000 °C using a novel elevated temperature split-Hopkinson pressure bar. These experiments are designed to probe the dynamic response of HSLA-65 steel in its single α-ferrite phase, mixed α + γ-austenite phase, and the single γ-austenite phase, as a function of temperature. The investigation is conducted at two different average strain rates—1450 and 2100/s. The experimental results indicate that at test temperatures in the range from room temperature to lower than 600 °C, i.e. prior to the development of the mixed α + γ phase, a net softening in flow strength is observed at all levels of plastic strain with increase in test temperatures. As the test temperatures are increased, the rate of this strain softening with temperature is observed to decrease, and at 600 °C the trend reverses itself resulting in an increase in flow stress at all strains tested. This increase in flow stress is understood be due to dynamic strain aging, where solute atoms play a distinctive role in hindering dislocation motion. At 800 °C, a (sharp) drop in the flow stress, equivalent to one-half of its value at room temperature, is observed. As the test temperature are increased to 900 and 1000 °C, further drop in flow stress are observed at all plastic strain levels. In addition, strain hardening in flow stress is observed at all test temperatures up to 600 °C; beyond 800 °C the rate of strain hardening is observed to decrease, with strain softening becoming dominant at temperatures of 900 °C and higher. Moreover, comparing the high strain rate stress versus strain data gathered on HSLA 65 in the current investigation with those available in the literature at quasi-static strain rates, strain-rate hardening can be inferred. The flow stress increases from 700 MPa at 8 × 10−4/s to 950 MPa at 1450/s and then to 1000 MPa at 2100/s at a strain of 0.1. Optical microscopy is used to understand evolution of microstructure in the post-test samples at the various test temperatures employed in the present study.

Hot Workability and Superplasticity of Low-Al and High-Nb Containing TiAl Alloys

The superplastic deformation mechanism of low-Al and high-Nb containing TiAl alloy was investigated in compression mode. The experimental results showed that intense dynamic recrystallization (DRX) breaks the balance and leads to a significant drop in flow stress after the peak when deforming below 950°C. Arrhenius kinetic analysis revealed that the activation energy for superplastic compression first increased then decreased with temperature, suggesting a change in the deformation mechanism. Microstructure observations showed that, when deformed at 850°C, the deformation mechanism was grain-boundary sliding accommodated by γ-DRX, γ-intragranular deformation, and β/B2-phase decomposition, while the mechanism was grain-boundary sliding accommodated by γ-DRX, β/B2-DRX, and γ → β/B2 + α 2 phase transformation when deformed at 1000°C. After compression, the microstructure tended to be uniform, which may yield important information for the development of new deformation techniques for TiAl alloys.

Hot deformation behavior of Zr-1%Nb alloy: Flow curve analysis and microstructure observations

The hot deformation behavior of Zr-1%Nb alloy has been studied at temperatures of 770–850°C and strain rates of 10−2 to 1s−1. To minimize friction during the tests at high temperature, the ends of the specimens were coated with graphite powder and a thin layer of mica was placed between the face of the specimen and the anvils. In order to investigate the warm working behavior of Zr-1%Nb alloy, all data were analysed stepwise and the step-by-step procedure was provided. The occurrence of the dynamic recrystallization (DRX) phenomena at different working conditions was analysed by the work hardening rate analysis. The stress-strain curve of many samples was identical to the typical DRX stress-strain curve which consists of a single peak stress followed by a gradual decrement toward the steady-state stress. Microstructural observations showed that at low temperatures and high strain rates the flow curves can be explained by localized deformation as a consequence of shear band formation. The transmission electron microscopy (TEM) analysis showed that the DRX has taken places through a continuous mechanism. At high Zener-Hollomon (Z) parameter a drop in flow stress was detected as a consequence of adiabatic deformation heating. The warm working constants of this alloy were determined through the general constitutive equations. Furthermore, after analyzing the work hardening behavior of this alloy, the activation energy for deformation of Zr-1%Nb alloy was calculated as 514kJ/mol.

Flow behaviour and constitutive modelling of a ferritic stainless steel at elevated temperatures

The flow behaviour of a ferritic stainless steel (FSS) was investigated by a Gleeble 3500 thermal-mechanical test simulator over the temperature range of 900–1100 °C and strain rate range of 1–50 s−1. Empirical and phenomenological constitutive models were established, and a comparative study was made on the predictability of them. The results indicate that the flow stress decreases with increasing the temperature and decreasing the strain rate. High strain rate may cause a drop in flow stress after a peak value due to the adiabatic heating. The Zener-Hollomon parameter depends linearly on the flow stress, and decreases with raising the temperature and reducing the strain rate. Significant deviations occur in the prediction of flow stress by the Johnson-Cook (JC) model, indicating that the JC model cannot accurately track the flow behaviour of the FSS during hot deformation. Both the multiple-linear and the Arrhenius-type models can track the flow behaviour very well under the whole hot working conditions, and have much higher accuracy in predicting the flow behaviour than that of the JC model. The multiple-linear model is recommended in the current work due to its simpler structure and less time needed for solving the equations relative to the Arrhenius-type model.

Effect of Ultrasonic Vibration on Compression Behavior and Microstructural Characteristics of Commercially Pure Aluminum

The principle objective of this research is to investigate the modeling of compression behavior and microstructural evolution of pure aluminum in the ultrasonic-assisted compression test. A dislocation density-based constitutive model was developed based on the existing frameworks and calibrated using experimental data to predict the stress-strain response of pure aluminum during UAC tests. An experimental set-up was designed to work at resonance condition with frequency of around 20 kHz and variant longitudinal vibration amplitudes at the range of 0~20 μm. The verified model and experimental samples were used for parameter studies and the study of grain formation of aluminum after conventional and ultrasonic upsetting. Results showed that the developed constitutive model was able to predict compression behavior of aluminum suitably. An increase in the flow stress drop, residual flow stress, and dislocation density occurred when the applied vibration intensity was raised. In addition, it was observed that the more homogenous microstructure with nearly equiaxed grains and also the higher microhardness values can be achieved when ultrasonic vibration is imposed on samples during compression test.

Characterization of hot compression behavior of a new HIPed nickel-based P/M superalloy using processing maps

Abstract Isothermal forging was a critical step process to fabricate the high-performance nickel-based superalloy. The temperature and strain rate served the most critical role in determining its microstructure and mechanical properties. In this article, we employed the hot compression to simulate the isothermal forging process upon the temperature ranging from 1000 °C to 1100 °C in combination with a strain rate of 0.001–1.0 s− 1 for a new P/M nickel-based alloy. The activation energy was determined as 903.58 kJ/mol and the processing maps at a strain range of 0.4–0.7 were developed. The instability domains were more inclined to occur at strain rates higher than 0.1 s− 1 and manifested in the form of adiabatic shear bands. The map further demonstrated that the regions with peak efficiency of 55% were located at 1080 °C/0.0015 s− 1 and 1095 °C/0.014 s− 1, respectively. Obvious dynamic recrystallization could be detected at the strain rate 0.01 s− 1 leading to a significant flow stress drop and the grain growth was remarkably triggered under 1100 °C. The findings can shed light on the forging processing optimization of the new nickel-based superalloy.

An elasto-plastic self-consistent model with hardening based on dislocation density, twinning and de-twinning: Application to strain path changes in HCP metals

In this work, we develop a polycrystal mean-field constitutive model based on an elastic–plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flow-stress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on (1) the flow stress drop when reloading from one path to another, (2) deformation twin activation, (3) de-twinning during a reversal strain path change, and (4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

A metallurgical approach to individually assess the rheology of alpha and beta phases of Ti–6Al–4V in the two-phase domain

An original hot torsion procedure is presented using Ti–6Al–4V in the two-phase domain. Kinetics of phase transformation are used to obtain stress–strain curves in off-equilibrium states. The individual influences of phase fraction and of temperature are quantitatively assessed in the two-phase domain. Between the two forging temperatures 1143K (870°C) and 1228K (955°C), a 95MPa stress difference is observed. The paper shows that this flow stress drop is mainly due to thermal activation, by 80MPa, while 15MPa is due to the change of phase fraction. The stresses in each phase are individually assessed and stress–strain curves for each phase are obtained within the two-phase domain.

High Temperature Deformation Behavior and Constitutive Modelling for 05Cr17Ni4Cu4Nb Stainless Steel

The high temperature deformation behavior of 05Cr17Ni4Cu4Nb stainless steel was investigated at the temperatures from 1000 to 1200°C and strain rates from 0.01 to 10s-1 on Gleeble-3500 thermo-simulation machine. The stress-strain curves at lower strain rate (0.01-0.5 s-1) exhibit a single peak stress, indicating a typical dynamic recrystallization (DRX) behavior of the steel, but at higher strain rates (10 s-1), the temperature rise due to the heat liberated from severe plastic deformation leads to the final drop of flow stress. Further microstructural observation confirmed the occurrence of DRX behavior, the average grain sizes decreased with the increasing strain rate and the decreasing deformation temperature, and the higher the deformation temperature, the larger the DRX degree. A new constitutive equation incorporating the effect of the strain on the deformation behavior was proposed based on the Arrhenius-type equation, in which the material constants α, n, Q and A were found to be polynomial functions of strain. The stress-strain relations of 05Cr17Ni4Cu4Nb steel predicted by the proposed constitutive equation agreed well with experimental results.

OS1712 高ひずみ速度域における鉄の活性化体積のひずみ速度依存性

Strain rate reduction tests are made on high purity iron containing of 14ppm C, S20C and S45C at very high strain rates. In order to evaluate quantitatively the strain rate dependence of the dynamic flow stress at each strain rate, decrement strain rate tests are performed at strain of about 0.20 at the strain rates above about 1 × 10^4/sec. The reduction ratios caused by the collision between an impact bar and decelerators are 0.44 and 0.586 of the strain rate before reduction. Activation volumes are determined from the amounts of the flow stress drop obtained from the reduction tests and the relation of a thermal activation process. The magnitude of activation volumes determined for the given materials depends not only on the strain rate but also on the difference in the carbon contents.

Thermomechanical process optimization of U-10 wt% Mo – Part 1: high-temperature compressive properties and microstructure

Nuclear power research facilities require alternatives to existing highly enriched uranium alloy fuel. One option for a high density metal fuel is uranium alloyed with 10wt% molybdenum (U–10Mo). Fuel fabrication process development requires specific mechanical property data that, to date has been unavailable. In this work, as-cast samples were compression tested at three strain rates over a temperature range of 400–800°C to provide data for hot rolling and extrusion modeling. The results indicate that with increasing test temperature the U–10Mo flow stress decreases and becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature, the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. Room temperature X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy analysis of the as-cast and compression tested samples were conducted. The analysis revealed that the as-cast samples and the samples tested below the eutectoid transformation temperature were predominantly γ phase with varying concentration of molybdenum, whereas the ones tested above the eutectoid transformation temperature underwent significant homogenization.

Modeling and simulation of polymer melts flow in the extrusion process of plastic profile with metal insert

The extrusion technology of plastic profile with metal insert is recently an advanced plastic processing method whose products keeps rising today for their excellent performance. However, the related fundamental research on polymer forming mechanism in the extrusion process of plastic profile with metal insert is lagging behind. With the development of computational fluid dynamics (CFD) theory, numerical method becomes an effective way to investigate such complex material forming problems as in the polymer extrusion process. In the present study, the mathematical model for three-dimensional non-isothermal viscous flow of the polymer melts obeying a Carreau model is developed based on the CFD theory. The Williams–Landel–Ferry equation is employed to involve the temperature dependence of material parameters. A decoupled numerical algorithm based on the penalty finite element method is conducted to predict the rheological behaviors of polymer melts within the complex flow channel. The streamline upwind/Petrov–Galerkin scheme is employed to improve the computational stability for the calculation of temperature field. Based on the theoretical model, the essential flow characteristics of polymer melts in the extrusion process of plastic profile with metal insert is investigated. The distributions of principal field variables like flow velocity, melt temperature, flow stress and pressure drop are predicted. The effects of die structure parameters including the intake angle and the distribution section length upon the melts flow patterns are further discussed. The variations of melt rheological properties versus different processing conditions like the volume flow rate and the metal insert moving velocity are also investigated. Some advice on practical processing operations of the extrusion process of plastic profile with metal insert is accordingly put forward based on the numerical results.

Tensile deformation behavior of Ti–22Al–25Nb alloy at elevated temperatures

The deformation behavior of Ti–22Al–25Nb alloy at elevated temperatures and different stain rates was investigated using uniaxial tensile test. It was found that the tension was accompanied with the effect of hardening and softening, under which the stress–strain curve was characterized by a rise to a peak followed by a nearly linear drop in flow stress. The peak stress was strongly dependent on the temperature and strain rate. The underlying mechanism was clarified in terms of dislocation dynamics. Work hardening and strain rate hardening both contributed to the hardening mechanism, and the softening mode was dominated by dynamic recovery. The effect of work hardening was completely neutralized by dynamic recovery. Owing to the strain rate hardening, the alloy exhibited certain degree of superplasticity. The further drop in flow stress after the peak was due to the rise in temperature, which originated from the heat generated during deformation. The deformation mechanism was dominated by dislocation slip and climb. The misorientation distribution between β/B2 and α2 phase scarcely changed, implying a harmonious deformation of the two phases.

Crystal plasticity analysis of constitutive behavior of 5754 aluminum sheet deformed along bi-linear strain paths

Sheet specimens of aluminum alloy 5754 were deformed along a series of bi-linear, equal-biaxial and uniaxial, strain paths while simultaneously measuring stress–strain behavior. The Visco-Plastic Self-Consistent crystal plasticity model, which incorporates texture evolution, was used to simulate the flow stress and hardening behavior based in part on the measured crystallographic texture before deformation. The predicted crystallographic texture qualitatively captures the texture measured in the experiments. Including latent hardening of multiple slip planes allowed the model to explain the decrease in flow stress when changing from equal-biaxial to uniaxial deformation. However, the model captures neither the details of the drop in flow stress nor the magnitude of the plastic hardening after the change in deformation mode. This is likely due to room temperature recovery between the two segments of testing.

Modeling of Flow Stress for one New Kind of Metastable β Titanium Alloy at High Temperature

Based on the characteristics of the flow stress curves for one new kind of metastable Ti2448 titanium alloy from isothermal hot compression tests, the constitutive model was developed to describe the relation between flow stress and strain, strain rate, deformation temperature completely. During this process, the flow behavior of alloy at high temperature undergo flow softening caused by dynamic recovery (DRV) was modeled by the adopted hyperbolic sine function based on the unified viscoplasticity theory, the further drop in flow stress after the peak value in stress-strain curves was assumed to be caused by temperature rise and the constitutive model was modified accordingly. Additionally, the material constants were determined by optimization strategies, which is a new method to solve the nonlinear constitutive equation. The stress-strain curves predicted by the developed constitutive model well agree with experimental results, which confirms that the developed constitutive model gives an accurate estimate of the flow stress of Ti2448 titanium alloy and can provide an effect method to model the flow behavior of metastable titanium alloy at high temperature.

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10−3 to 100 s−1. When compressed at 10−3 s−1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10−2 s−1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10−1 s−1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s−1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.

An Approach Based on the Classical Free-Electron Theory to Study Electroplastic Effect

Based on the classical free-electron theory, the mechanism of electroplastic effect is studied at a microscopic level. A method is presented for calculating the amount of metal’s flow stress for electroplastic effect. This calculation method takes into account the free electrons exchange energy with metallic ions with a current pulse applied and considers the energy cause the change of the dislocation Gibbs free energy of activation. The results indicate that the metal’s flow stress drop associates with an enhanced plastic strain rate. At last, the effects of current parameters and microscopic parameters on the metal’s flow stress for electroplastic effect are discussed.

Effect of twinning on flow stress during initial stage of hot compression of twin roll cast Mg-5.51Zn-0.49Zr alloy

A Thermecmastor-Z hot deformation simulator, optical microscopy, XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 °C and 400 °C and a given strain rate of 10−2 s−1. The results suggest that flow stress drop during initial stage of hot compression at 300°C, generally led by dynamic recrystallization, is attributed to twinning, correspondingly to dynamic recrystallization as deformation temperature is raised to 400 °C.

Flow Curve Analysis of 17-4 PH Stainless Steel under Hot Compression Test

The hot compression behavior of a 17-4 PH stainless steel (AISI 630) has been investigated at temperatures of 950 °C to 1150 °C and strain rates of 10−3 to 10 s−1. Glass powder in the Rastegaev reservoirs of the specimen was used as a lubricant material. A step-by-step procedure for data analysis in the hot compression test was given. The work hardening rate analysis was performed to reveal if dynamic recrystallization (DRX) occurred. Many samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall toward the steady-state stress. At low Zener–Hollomon (Z) parameter, this material showed a new DRX flow behavior, which was called multiple transient steady state (MTSS). At high Z, as a result of adiabatic deformation heating, a drop in flow stress was observed. The general constitutive equations were used to determine the hot working constants of this material. Moreover, after a critical discussion, the deformation activation energy of 17-4 PH stainless steel was determined as 337 kJ/mol.