Flow Stress Values Research Articles

Thermally activated plastic deformation behavior of nano oxide dispersion strengthened Fe-18Cr steel: Experiments and analysis

The objective of the present work is to investigate the ultra-high temperature (1273–1473 K) plastic deformation behavior of a nano oxide dispersion strengthened Fe-18Cr ferritic steel (n-ODS-18Cr steel) over a range of strain rates (10−2 to 10 s−1). The flow (true) stress-true strain behavior of this steel, reported for the first time, has been utilized to obtain the flow stress as a function of temperature and strain rate. From the above data, the strain rate sensitivity parameter (m) and also the activation volume have been obtained as a function of temperature (1273–1473 K). It is then demonstrated that the flow stress values obtained in the present steel over the temperature range 1273–1473 K is due to a single dominant strengthening mechanism, i.e. Orowan dispersion strengthening. Hence, the predictions of the theoretical models which assume Orowan strengthening as the dominant mechanism can be compared with the experimental flow stress data. Such a comparison indicates that the model due to Rosler and Arzt predicts the experimental data very well while the prediction of the model due to Brandes, Kovarik, Miller, Daehn and Mills is satisfactory. It is also shown that the experimentally obtained activation volume is consistent with the dislocation detachment model.

Hot Deformation Behavior and Dynamic Recrystallization Characteristics of 12Cr Ultra-Super-Critical Rotor Steel

In this paper, the constitutive model and dynamic recrystallization characteristics of 12Cr ultra-super-critical rotor steel were investigated quantitatively during hot deformation. A series of axisymmetric hot compression tests at temperatures from 900 to 1200 °C under strain rates of 0.001–1 s−1 were conducted on a Gleeble−1500D thermal simulator. Based on the experimental true stress–strain curves varying with temperature and strain rates, a complete constitutive model was established and all material parameters in the model could be expressed as a function of strain using a fifth order polynomial fit. The proposed model was verified so as to have the capability of accurately predicting the flow behaviour with an average absolute relative error of < 2.82%. Meanwhile, after hot deformation the microstructure was observed via electron backscatter diffraction technology. Then, the dependence of the characteristic parameters on the Zener–Hollomon parameter were confirmed. Furthermore, the kinetics equation of dynamic recrystallization was obtained, which included the flow stress calculated based on the evolution equation of the dislocation density during the work hardening-dynamic recovery stage. The result indicated that the predicted values for dynamic recrystallization volume fraction and flow stress were in line with the experimental values.

Characterization of Hot Deformation Behavior and Processing Maps of Ti–19Al–22Mo Alloy

The isothermal compression tests were carried out to study the hot deformation behavior and microstructure evolution of Ti–19Al–22Mo alloy. The samples were deformed in the temperature range from 1100 to 1250 °C with an interval of 50 °C, strain rate ranging from 0.01 to 1 s−1 and the height reduction of 50% using Gleeble-3800 thermal–mechanical simulator. By using this experimental data an artificial neural network (ANN) model was developed and evaluated with unseen data. Further, the developed ANN model was used to predict flow stress correction from adiabatic heating at finer intervals of strain rates and temperatures. The predicted isothermal flow stress values were utilized to construct processing maps for Ti–19Al–22Mo alloy at true strain of 0.4 and 0.6. The maximum efficiency was noticed at 1100 °C with the strain rate of 0.01 s−1 associated with dynamic recrystallization and dynamic recovery. The deformation conditions of the instability domains in processing map showed wedge cracking and flow localization. Using the processing maps safe working parameters for hot deformation of Ti–19Al–22Mo alloy was identified.

Thermally Activated Plastic Deformation Behavior of Nano Oxide Dispersion Strengthened Fe-18Cr Steel: Experiments and Analysis

The objective of the present work is to investigate the ultra-high temperature (1273 to 1473 K) plastic deformation behavior of a nano oxide dispersion strengthened Fe-18Cr ferritic steel (n-ODS-18Cr steel) over a range of strain rates (10-2 to 10 s-1). The flow (true) stress-true strain behavior of this steel, reported for the first time, has been utilized to obtain the flow stress as a function of temperature and strain rate. From the above data, the strain rate sensitivity parameter (m) and also the activation volume have been obtained as a function of temperature (1273 to 1473K). It is then demonstrated that the flow stress values of the present steel in the temperature 1273 to 1473 K is due to a single dominant strengthening mechanism, i.e. Orowan dispersion strengthening. Hence, the predictions of the theoretical models which assume Orowan strengthening as the dominant mechanism can be compared with the experimental flow stress data. Such a comparison indicates that the model due to Rosler and Arzt predicts the experimental data very well while the prediction of the model due to Brandes, Kovarik, Miller, Daehn and Mills is satisfactory. It is also shown that the experimentally obtained activation volume is consistent with the dislocation detachment model.

Effect of Deformation Parameters on Microstructural Evolution of GGG 40 Spheroidal Graphite Cast Iron Alloy

Studying the thermo-mechanical behavior of ductile iron is necessary to develop the rolling process for ductile iron sheet/strip production, thus, extending its application by replacing steel in several fields such as machine casing, constructional applications, etc. In order to predict the safe rolling conditions for producing sheets and strips, the thermo-mechanical behavior of a ductile iron alloy, with CE of 4.48, is studied by physical simulation of hot rolling process using Gleeble-3500 simulator. The test was conducted on specimens at a range of deformation temperatures from 800°C to 950°C while three different strain rates; namely 0.05, 0.1 and 0.5 s-1 were used. The results obtained, show minimum values of flow stresses at 850°C. By increasing the deformation temperature up to 900°C, the flow stresses increased to reach maximum values, beyond which the flow stress decreased again. A remarkable dynamic recrystallization is observed at the deformation temperatures of 850°C and 800°C with applied strain rates of 0.05 and 0.1 s-1. Gleeble test results are correlated with microstructure observations on samples quenched at their deformation temperatures, where the changes in structure and graphite morphology are reported. The deformation process at high temperatures namely 950°C and 900°C result in changing the graphite shape from a spheroidal-like to a saucer-like shape. However, by decreasing the deformation temperature to 850°C as well as 800°C, graphite with lamellar shape is observed. As a conclusion, ductile iron could be successfully deformed without cracking at the applied conditions.

Constitutive Modeling for Hot Deformation Behavior of Al-5083 + SiC Composite

In the present research, isothermal hot deformation behavior of aluminum 5083 alloy + 15 (wt.%) SiC composite was obtained through compression test on INSTRON 8801 universal tensile testing machine (UTM) under a wide temperature range of 473-773 K and strain rate range of 0.01-10 s−1. The experimental true stress–strain data were employed to establish constitutive equations based on modified Johnson–Cook (JC) model and modified Zerilli–Armstrong (ZA) model to predict the hot flow behavior of the composite. The flow stress values obtained from these two models were plotted against the experimental flow curves to check the accuracy of these models. Suitability of the models was evaluated by comparing correlation coefficient (R), average absolute relative error and relative errors of prediction. The results show that the hot flow stresses of the present material depend on temperature and strain rate significantly. Both the models give good description of the hot deformation behavior of the composite. The prediction accuracy is found to be higher for modified ZA model compared to modified JC model, though the number of materials constants involved and time needed to evaluate them to establish the model are lower for modified JC model.

The effect of size on the plastic deformation of annealed cast aluminium microwires

Monocrystalline, cast aluminium microwires of diameter (D) below 25 μm free of micromilling artefacts are annealed under protective atmosphere. Annealing increases the flow stress of all tested microwires, by an amount that is found, in a Haasen plot of measured activation area values, to be an athermal contribution to the flow stress that is attributed to surface oxide layer thickening. Microwires oriented for single slip with D ~14 μm behave as do as-cast microwires with D ~7 μm, showing a transition to higher and more stochastic flow stress values; this effect is attributed to a greater scarcity of dislocation sources after annealing.

Physics-based Constitutive Model for the Hot Deformation of 2Cr11Mo1VNbN Martensitic Stainless Steel

The hot deformation behavior of 2Cr11Mo1VNbN martensitic stainless steel is investigated through isothermal compression tests between 1173 and 1423 K with strain rates of 0.005-5 s−1. Based on experimental results, the work hardening–dynamic recovery stage and dynamic softening stage are evaluated, and a couple-stage constitutive model is developed to describe the work hardening–dynamic recovery and dynamic softening behaviors. Phenomenological constitutive models, which consider temperature, strain and strain rate, are established. A comparison between the predicted and experimental flow stress values indicates that the established models accurately describe the hot deformation behaviors for the studied supercritical steel.

Modeling of hot deformation behavior of high phosphorus steel using Johnson-Cook model

To describe the hot deformation behavior of high phosphorus steel, isothermal hot compression test was performed using Gleeble 3800 thermo-mechanical simulator in the temperatures ranging from 750-1050°C at constant strain rates of 0.001-10 s−1. Based on the experimental results, Johnson-Cook model was developed to predict the values of flow stress. Johnson-Cook model shows a fairly good agreement with experimental and predicted values. Average absolute relative error (AARE), and relative error (RE) of JC model are calculated as 24.356%, and - 19.97% to 33.179%, respectively.

Characterization of the Hot Deformation Behavior and Microstructure Evolution of a New γ-γ′ Strengthened Cobalt-Based Superalloy

Recently, cobalt-based γ-γ′ microstructured superalloys have attracted attention. However, studies on their processing behavior [i.e., processing maps (the variation of strain rate sensitivity (m) with temperature)] are limited. Thus, the high-temperature flow behavior of a γ-γ′ Co-30Ni-10Al-5Mo-2Ta-2Ti-5Cr (at. pct) superalloy was investigated using isothermal compression tests between 1348 and 1498 K at strain rates from 0.001 to 10 s−1. The m contour map was generated using the experimental flow stress values, which were used to locate the optimum hot workability and desired microstructural processing range. A strong dependence of m on the deformation parameters (temperature, strain rate, and strain) was observed. A maximum m value of around 0.3 at 1460 K to 1498 K and strain rates of 0.01 to 0.5 s−1 was found. The deformed samples show a fully recrystallized microstructure at high m. Unstable domains showed the formation of cavities at the grain boundary triple points and cracks along the grain boundaries at high strain rates (1 to 10 s−1), corresponding to m < 0.10. A constitutive model was developed using an Arrhenius hyperbolic sine function, yielding an apparent activation energy of 540 ± 30 kJ mol−1 for hot deformation. This study indicates reasonable formability under certain conditions below the solvus, thus opening possibilities for further thermomechanical treatment.

Hot Deformation Behaviors of Cu/Al Laminated Composites

In order to research the hot deformation behaviors of Cu/Al laminated composites, isothermal hot compression tests were conducted on a Gleeble-1500D thermo-mechanical simulator in the temperature range of 300 ∼ 450°C and strain rate range of 0.01 ∼ 1 s-1. After hot compression, the cooperative deformation behaviors between Cu and Al were discussed. The effects of deformation temperature and strain rate on the deformation behaviors were described using an Arrhenius-type constitutive equation. Furthermore, considering the effect of strain, the constitutive model was modified. The predicated flow stress values obtained from the modified constitutive equation could be well in accord with the experimental values. High correlation coefficient and low average absolute relative error demonstrate the high accuracy of the constitutive model developed in this work.

Constitutive modelling and processing map analysis of tungsten heavy alloy (92.5 W-5.25Ni-2.25Fe) at elevated temperatures

Tungsten Heavy Alloys (WHA) are finding increasing applications in the kinetic energy penetrators. For developing the accurate simulation models of deformation processes on WHA, it is essential to properly understand the flow stress behavior of these alloys considering the combined effects of strain, strain rate and temperature. This paper focuses on developing various constitutive models for 92.5 W-5.25Ni-2.25Fe alloy by using the flow stress data at six different temperatures (298 K, 573 K, 773 K, 973 K, 1173 K, 1373 K) and three different strain rates (1600, 2500, 4000 s−1). Four constitutive models, namely, modified Johnson-Cook (m-JC), modified Zerilli-Armstrong (m-ZA), modified Arrhenius (m-Arr) and modified Khan-Huan-Liang (m-KHL) models have been developed to predict the flow behavior. The predictions of these constitutive models have been compared with the experimental values using statistical measures like correlation coefficient, average absolute error and its standard deviation. Based on these statistical measures, m-Arr and m-ZA models have been found to be better models for predicting the flow stress values. In addition, using the flow stress curves, the strain rate sensitivity values have been computed for determining the efficiency of power dissipation and instability parameter for the deformation process. Superimposing the efficiency map over the instability map, the processing maps have been developed for better understanding of response of a material to the imposed experimental parameters.

Experimental Investigation and Constitutive Modeling for Hot Deformation Behavior of 2219 Al-Alloy Microalloyed With Silver

2219Al and 2219Al + 0.1 wt % Ag alloys were processed by casting route. The hot compression tests were carried out at constant true strain rates and temperatures in the range of 10−3 to 101 s−1 and 300–500 °C, respectively. Flow stress of the alloy decreases with the addition of silver. The flow stress of both alloys increases with the decrease in deformation temperature and the increase in strain rates. Constitutive models correlating the peak flow stress with deformation temperature and strain rates for the two alloys were developed using hyperbolic–sine relationship. The activation energy for hot deformation of 2219 Al alloy decreases with the addition of silver. Comparison of the predicted and experimental values of peak flow stress reveals that 92% of the data could be predicted within a deviation error of ±10% indicating good predictive capability for the developed constitutive relationships.

Determination of critical condition for initiation of dynamic recrystallisation in Zr-1%Nb alloy

The critical stress and strain for initiation of dynamic recrystallisation (DRX) in Zr-1%Nb alloy have been determined by employing two different approaches. The work hardening rate versus stress curve and the natural logarithm of work hardening rate versus strain were used for this analysis. To evaluate the onset of DRX in Zr-1%Nb alloy, the hot deformation behaviour of this alloy has been studied in the temperature range of 770–850 °C and strain rates range 10−2 to 1 s−1. To reduce friction during the tests, the ends of samples were coated with graphite powder and a thin layer of mica placed between the surface of samples and the anvils. Nevertheless, the flow stress values were corrected to eliminate the effects of friction and adiabatic heating during the tests. The outcomes of work hardening rate analysis showed that the first method (work hardening rate versus stress) is the best method to determine the critical conditions of initiation of DRX. All in all, for Zr-1%Nb alloy, the dynamic recrystallisation was found to start when the normalised stress and strain reach the values of 0.91 and 0.87, respectively. Furthermore, the transmission electron microscopy (TEM) analysis showed that the DRX took place through a continuous mechanism, and optical microstructures confirmed that at low temperature and high strain rate the flow curves could be explained by localised deformation and shear band formation.

Using spherical indentation to measure the strength of copper-chromium-zirconium

Precipitation hardened CuCrZr will be used in heat-sink components in the ITER tokomak and is a primary candidate for EU DEMO. The measurement of mechanical properties of irradiated CuCrZr using conventional, standardised techniques is difficult due to the challenges involved in working with radioactive material and the relatively large specimen size required. Spherical nano-indentation offers a technique to measure stress-strain properties from far smaller volumes than conventional tests. In this work, CuCrZr has been heat-treated at different temperatures to vary the Cr precipitate size and spacing. Spherical nano-indentation using multiple tip radii was then used to produce stress-strain curves for all samples, from which values of initial flow stress were calculated. It was found that there was a strong indentation size effect (ISE) in the stress required to initiate plasticity, however at higher indentation strains the flow stress became constant for tip radii, R, ≥8 μm. This suggests that at the initiation of plastic deformation the ISE is dominated by dislocation source activation but in later stages the interaction with microstructural material length-scales dominate the measured mechanical strength. The mechanical response of these small-scale tests is governed by multiple mechanisms, which convolute interpretation of data and must be considered when measuring the effects of irradiation on the mechanical properties.

Effects of Strain Rate and Texture on the Tensile Behavior of Pre-strained NiCr Microwires

The stress–strain behavior and strain rate sensitivity of pre-strained Ni80Cr20 (Ni20Cr) were studied at strain rates from 4.8×10–4 s–1 to 1.1×10–1 s–1. Specimens were prepared through cold drawing with abnormal plastic deformation. The texture of the specimen was characterized using electron backscatter diffraction. Results revealed that the ultimate tensile strength and ductility of the pre-strained Ni20Cr microwires simultaneously increased with increasing strain rate. Twinning-induced negative strain rate sensitivity was discovered. Positive strain rate sensitivity was present in fracture flow stress, whereas negative strain rate sensitivity was detected in flow stress values of σ0.5% and σ1%. Tensile test of the pre-strained Ni20Cr showed that twinning deformation predominated, whereas dislocation slip deformation dominated when twinning deformation reached saturation. The trends observed in the fractions of 2°-5°, 5°-15°, and 15°-180° grain boundaries confirmed that twinning deformation dominated the first stage.

Microstructure Evolution and Flow Stress Model of a 20Mn5 Hollow Steel Ingot during Hot Compression.

20Mn5 steel is widely used in the manufacture of heavy hydro-generator shaft due to its good performance of strength, toughness and wear resistance. However, the hot deformation and recrystallization behaviors of 20Mn5 steel compressed under high temperature were not studied. In this study, the hot compression experiments under temperatures of 850–1200 °C and strain rates of 0.01/s–1/s are conducted using Gleeble thermal and mechanical simulation machine. And the flow stress curves and microstructure after hot compression are obtained. Effects of temperature and strain rate on microstructure are analyzed. Based on the classical stress-dislocation relation and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of 20Mn5 steel. Comparisons between experimental flow stress and predicted flow stress show that the predicted flow stress values are in good agreement with the experimental flow stress values, which indicates that the proposed constitutive model is reliable and can be used for numerical simulation of hot forging of 20Mn5 hollow steel ingot.

Hot Deformation Behavior and a Two-Stage Constitutive Model of 20Mn5 Solid Steel Ingot during Hot Compression.

20Mn5 steel is widely used in the manufacture of heavy hydro-generator shaft forging due to its strength, toughness, and wear resistance. However, the hot deformation and recrystallization behaviors of 20Mn5 steel compressed under a high temperature were not studied. For this article, hot compression experiments under temperatures of 850–1200 °C and strain rates of 0.01 s−1–1 s−1 were conducted using a Gleeble-1500D thermo-mechanical simulator. Flow stress-strain curves and microstructure after hot compression were obtained. Effects of temperature and strain rate on microstructure are analyzed. Based on the classical stress-dislocation relationship and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of 20Mn5 steel. Comparisons between experimental flow stress and predicted flow stress show that the predicted flow stress values are in good agreement with the experimental flow stress values, which indicates that the proposed constitutive model is reliable and can be used for numerical simulation of hot forging of 20Mn5 solid steel ingot.

The microstructural origin of work hardening stages

The strain evolution of the flow stress and work hardening rate in stages III and IV is explored by utilizing a fully described deformation microstructure. Extensive measurements by transmission electron microscopy reveal a hierarchical subdivision of grains by low angle incidental dislocation boundaries (IDBs) and medium to high angle geometrically necessary boundaries (GNBs). This universal evolution is demonstrated for nickel, copper, and aluminum deformed by cold rolling from strains of 0.05–5.5. Microstructural morphology evolves with increasing strain through a transition resulting in a lamellar cell-block structure aligned with the deformation. This transition is caused by the emergence of new slip systems and a stable texture. Four parameters describe the microstructure, the misorientation angle across each boundary type and their respective spacing. Universal scaling characterizes the normalized distributions of three separate parameters. A new scaling law connects the strain evolution of two strength parameters: the dislocation density of IDBs and the spacing between GNBs. Strengthening mechanisms and strength contributions for those two parameters are expressed respectively as a linear addition of the classical Taylor and Hall-Petch formulations. Model predictions agree closely with experimental values of flow stress and work hardening rate in stages III and IV. Strong connections between the evolutionary stages of the deformation microstructure and work hardening rates create a new (modern) basis for the classic problem of work hardening in metals and alloys. These connections lead the way for the future development of ultra high strength ductile metals produced via plastic deformation.

Dynamic Recrystallization Behavior of AISI 422 Stainless Steel During Hot Deformation Processes

In this work, hot compression tests were performed to investigate the dynamic recrystallization (DRX) process of a martensitic stainless steel (AISI 422) at temperatures of 950, 1000, 1050, 1100 and 1150 °C and strain rates of 0.01, 0.1 and 1 s−1. The dependency of strain-hardening rate on flow stress was used to estimate the critical stress for the onset of DRX. Accordingly, the critical stress to peak stress ratio was calculated as 0.84. Moreover, the effect of true strain was examined by fitting stress values to an Arrhenius type constitutive equation, and then considering material constants as a function of strain by using a third-order polynomial equation. Finally, two constitutive models were used to investigate the competency of the strain-dependent constitutive equations to predict the flow stress curves of the studied steel. It was concluded that one model offers better precision on the flow stress values after the peak stress, while the other model gives more accurate results before the peak stress.