Peak Flow Stress Research Articles

Research on the High-Temperature Hot Compressive Deformation Behavior of Ni-Based Superalloy GH3128

Intermediate heat exchanger (IHX), which transfers the heat generated in the reactor core to the secondary loop, is one of the key structural components of the very high-temperature gas-cooled reactor (VHTR). The Ni-based superalloy GH3128 has good high-temperature strength and so is a promising main structural material for the IHX. In this paper, the flow stress behaviors and the deformation microstructure of superalloy GH3128 were investigated by high-temperature compression tests conducted at various temperatures (950–1150 °C) and strain rates (0.001–10 s−1), and the processing maps were analyzed in order to establish the hot deformation constitutive model and obtain the optimum hot forming condition. The results show that (1) both flow stresses and peak flow stresses increase along with the increase of strain rate or decrease of temperature, (2) GH3128 has excellent hot workability, (3) the dynamic recovery (DRV) plays the dominant role during the dynamic softening process due to the high stack fault energy, and (4) the optimum hot forming condition of GH3128 should be defined in the temperature of 1150 °C and strain rate range of 0.01–0.056 s−1. This work contributes to the application of GH3128 alloy on IHX structure.

Impact of γ′(Ni3(Al,Ti)) phase on dynamic recrystallization of a Ni-based disk superalloy during isothermal compression

The present work aims at investigating the impact of γ′(Ni3(Al,Ti)) phase on the dynamic recrystallization (DRX) behavior of a Ni-based turbine disk superalloy FGH4096 during hot compression. Two different treatments were selected to obtain γ′ phase with different size and morphology. For the first treatment by slow cooling rate after solution treatment the coarsening of γ′ phases occurred with morphologies of dendrites, fireworks and long strips, and simultaneously the grain boundaries of microstructure were serrated. The second heat treatment yielded alloy with fine and round shape γ′ phase. During sub-solvus compression, both the peak flow stress and the critical strain for the initiation of DRX for the alloy with coarse γ′ phase were lower than that of the alloy with fine γ′ phase. The volume fraction of DRX grains for the alloy samples with coarse γ′ phase was higher than that of samples with fine γ′ phase particles. Secondary electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to find the reason for this phenomenon, and finally dislocation induced coarse γ′ phase DRX nucleation mechanism was proposed.

Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures

A systematic study of the effect of δ phase precipitate morphology on the hot deformation behavior and microstructural evolution in nickel superalloy Inconel 718 is presented. Isothermal compression tests at fixed nominal strain rates and temperatures relevant to industrial forging (0.001–0.3 s−1 and 990–1040 °C) were used. Three distinct initial microstructures have been examined: (I) solution treated, (II) a microstructure with finely dispersed particulate δ precipitates, and (III) a microstructure containing dense network of intragranular and grain boundary δ platelets. The peak flow stress associated with these various microstructures has been rationalized using a single, temperature-compensated power law. This clearly demonstrates opposition of the external applied stress by an internal back stress related to the initial δ phase morphology and apparent delta solvus temperature. Post-peak flow softening is attributed to dynamic recrystallization, aided by the dissolution of finer precipitates in material containing particulate δ phase, and to a certain degree of mechanical grain refinement caused by distortion and offsetting of grain segments where a dense δ-platelet structure exists.

Dynamic tensile properties of a single crystal Nickel-base superalloy at high temperatures measured with an improved SHTB technique

In this study, to characterize the high-rate tensile properties of a Nickel-base single crystal superalloy at high temperatures, an improved technique for conducting split Hopkinson tensile bar (SHTB) tests at high temperatures (up to 1200°C) is developed. A Quick Hook Joint method to rapidly mount the specimen to the SHTB system is adopted in the technique. Firstly, the specimen is heated to target temperature while keeping the bars at the room temperature. Then the Quick Hook Joint method is used to quickly assemble the specimen to the SHTB by a synchronically dual-actuator assembled system, immediately before loading is imposed to the specimen. The cold contact time (CCT) experienced by the specimen is measured by laser sensors and the cold contact process is numerically simulated. The results show that no evident temperature decrease in the specimen gauge section and no significant temperature increase in the loading bars. Finally, tensile experiments of a single-crystal Nickel-base superalloy over a temperature range of 298–1473K and over a strain-rate range of 0.001–3000s−1 are conducted. Anomalous high temperature peak of flow stress is noticed in the experimental results, and the peak of the flow stress shifts to higher temperature as the strain rate increases. Anomalous temperature valley of the fracture strain is formed over the selected temperature range, and the valley of the fracture strain also shifts to higher temperature as the strain rate increases. A model is applied to quantitatively describe the strain-rate effect on the anomalous peak of the flow stress over a wide range of strain rates.

Physically-based constitutive modeling of NiTi intermetallic compound during hot deformation

The hot compression behaviour of a binary nitinol alloy was studied at the temperature range of 700–1000°C under strain rate of 0.1 s− 1. First, the constitutive descriptions of flow stress versus the Zener-Hollomon parameter were used to determine the hot working constants of the investigated material. Subsequently, it was shown that the theoretical exponent of 5 and the self-diffusion activation energy as the hot deformation activation energy can be used in the classical hyperbolic sine equation to describe the peak flow stress, which is in accordance to the deformation mechanism controlled by glide and climb of dislocations. This resulted in a simple and more reliable constitutive equation from the physical and metallurgical standpoints.

Characterization of hot deformation behavior of alloy 617 through kinetic analysis, dynamic material modeling and microstructural studies

The hot deformation behavior of alloy 617 has been studied by performing hot compression in a range of temperatures (1173–1473K) and strain rates (0.001–10s−1). The peak flow stress found to increase with Zener–Hollomon parameter (Z) following a hyperbolic-sine function relationship whereas peak strain followed power-law type relationship with Z. The average activation energy for the entire hot deformation domain was estimated to be 481kJmol−1. The experimental stress-strain data has been used to develop processing map employing dynamic material model. The material exhibits a wide stable domain below 0.1s−1 spanning over 1250–1473K, with a peak efficiency of ~45%. Based on the processing map and subsequent microstructural observation, the optimum hot deformation domain of alloy 617 is identified as 1323–1423K and 0.001–0.1s−1. Furthermore, microstructural observation alone has revealed that a significant DRX with grain refinement could also be obtained at high temperature (>1373K) and high strain rate (>1s−1) domain although processing map has marked this region as unstable domain.

Influence of Mo Addition on High Temperature Deformation Behavior of L12 Type Ni3Al Intermetallics

The high temperature deformation behavior of Ni3Al and Ni3(Al,Mo) single crystals that were oriented near was investigated at low strain rates in the temperature range above the flow stress peak temperature. Three types of behavior were found under the present experimental conditions. In the relatively high strain rate region, the strain rate dependence of the flow stress is small, and the deformation may be controlled by the dislocation glide mainly on the {001} slip plane in both crystals. At low strain rates, the octahedral glide is still active in Ni3Al above the peak temperature, but the active slip system in Ni3(Al,Mo) changes from octahedral glide to cube glide at the peak temperature. These results suggest that the deformation rate controlling mechanism of Ni3Al is viscous glide of dislocations by the {111} slip, whereas that of Ni3(Al,Mo) is a recovery process of dislocation climb in the substructures formed by the {001} slip. The results of TEM observation show that the characteristics of dislocation structures are uniform distribution in Ni3Al and subboundary formation in Ni3(Al,Mo). Activation energies for deformation in Ni3Al and Ni3(Al,Mo) were obtained in the low strain rate region. The values of the activation energy are 360 kJ/mol for Ni3Al and 300 kJ/mol for Ni3(Al,Mo).

Plastic deformation and dynamic recrystallization of a powder metallurgical nickel-based superalloy

The mechanisms of hot plastic deformation and dynamic recrystallization (DRX) in a powder metallurgical superalloy were investigated by compression tests in the deformation temperature range of 1020–1140 °C, and the constant strain rate range of 0.001–1.0s−1. The results showed that the stress–strain curves can indicate the intrinsic relationship between the flow stress and thermo-mechanical behavior, which can be used to reflect the internal micro-structural evolution law. All of curves can be divided into three types of DRX flow curves: single peak, cyclic behavior and steady-state flow stress curve. Based on researching of basic constitutive relation, the peak stress can be expressed as follows:σp=0.00013Z0.177. The mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature and strain rate. The inflection point of work-hardening rate curve indicates the onset of DRX. The inflection points are obvious at a low strain rate and a high temperature, while they are not existence at a high strain rate and a low temperature. The strain-rate sensitivity, and thus the deformation mechanisms, are different in the respective temperature regimes as found for the steady-state flow stress and the peak stress, and reflected by the temperature dependence of the stress exponent. Continuous dynamic recrystallization (CDRX) takes place at low deformation temperature. CDRX can lead to a partial softening in hot deformation. A great number of the dislocations are trapped by subgrain boundaries, leading to an increase in grain boundary misorientation and gradual transformation into high-angle grain boundaries (HABs). Discontinuous dynamic recrystallization (DDRX) takes place at high deformation temperature. A great number of new grains formed on the completely dynamic recrystallization microstructure.

Hot Compressive Deformation Behavior of TC4-DT Titanium Alloy

Hot compressive experiments of TC4-DT titanium alloy were performed on Gleeble 3500 hot simulator. The influence of hot deformation parameters on high temperature deformation behaviors were investigated, including deformation temperature (938°C~1038°C), deformation degree and strain rate (0.01s-1~10s-1). The results indicated that the peak (σp) and steady-state flow stress (σs) of TC4-DT alloy decreased with the increase of deformation temperature under the same strain rate, especially under a high strain rate. The flow stress increased sharply then decreased and kept invariant finally with the increase of deformation degree. The flow stress increased with the strain rate increasing and exhibited different characteristics in different strain rate range. The optimum conditions obtained based on this investigation of TC4-DT alloy as follows: temperature was 938°C~1008°C, stain rate was 0.01s-1~0.1s-1.

Characterization of Hot Deformation Behavior of Ti-4.5Al-3V-2Mo-2Fe Titanium Alloy

The hot deformation behavior of Ti-4.5Al-3V-2Mo-2Fe (SP-700) titanium alloy in the temperature range of 650°C~950°C and constant strain rate of 0.01, 0.1, 1 and 10s-1 has been investigated by hot compressive testing on the Gleeble-1500D thermal simulation test machine. The experimental results indicated that the hot deformation behavior of SP-700 alloy was sensitive to the deformation temperature and strain rate. The peak flow stress decreased with the increase of temperature and the decrease of strain rate. The flow curves characteristic under different deformation parameters show significant different. Analysis of the flow stress dependence on strain rate and temperature gives a stress exponent of n as 4.8235 and a deformation activation energy of Q as 410kJ/mol. Based on the dynamic materials model, the processing map is generated, which shows that the most peak efficiency domain appears at the temperature of 725°C~775°C and the strain rate of 0.001 s-1~0.003s-1 with a peak efficiency of 45% at about 750°C/0.01s-1.

Hot Deformation Processing Map and Microstructural Evaluation of the Ni-Based Superalloy IN-738LC

Hot deformation behavior of the Ni-based superalloy IN-738LC was investigated by means of hot compression tests over the temperature range of 1000-1200 °C and strain rate range of 0.01-1 s−1. The obtained peak flow stresses were related to strain rate and temperature through the hyperbolic sine equation with activation energy of 950 kJ/mol. Dynamic material model was used to obtain the processing map of IN-738LC. Analysis of the microstructure was carried out in order to study each domain’s characteristic represented by the processing map. The results showed that dynamic recrystallization occurs in the temperature range of 1150-1200 °C and strain rate of 0.1 s−1 with the maximum power dissipation efficiency of 35%. The unstable domain was exhibited in the temperature range of 1000-1200 °C and strain rate of 1 s−1 on the occurrence of severe deformation bands and grain boundary cracking.

Al-4.5Mg-0.6Mn铝合金高温变形行为研究

在Gleeble-3800热模拟试验机上对Al-4.5Mg-0.6Mn铝合金进行热压缩实验，研究该合金在变形温度为300℃~500℃、应变速率为0.0l~10 s−1条件下的热变形行为。分析了Al-4.5Mg-0.6Mn铝合金的高温流变行为。结果表明：变形温度和应变速率对Al-4.5Mg-0.6Mn铝合金流变应力影响显著，流变应力随变形温度的升高而降低，随应变速率的增加而增大。建立了Zener-Hollomon参数的本构方程，可对Al-4.5Mg- 0.6Mn铝合金在不同应变速率和变形温度下的峰值流变应力进行预测，发现本构方程预测值与流变应力实测值吻合较好。 The flow stress features of Al-4.5Mg-0.6Mn aluminum alloy were studied by the isothermal com-pression of cylindrical specimen in the temperature range of 300˚C - 500˚C and strain rate range of 0.01 - 10 s−1 with Gleeble-3800 simulated machine. The results show that flow stress is apparently influenced by both deformation temperature and strain rate, as the flow stress decreases with the increased temperature, while increases with the raise of strain rate. The constitutive equations were presented by Zener-Hollomon parameter, and using the Zener-Hollomon parameter consti-tutive equation presented, the prediction of the peak flow stress in Al-4.5Mg-0.6Mn aluminum alloy as function of strain rate, temperature and strain shows to be in good agreement with the measured flow stresses.

Hot deformation behavior and microstructure evolution of 1460 Al–Li alloy

The hot deformation behavior and microstructure evolution of 1460 Al–Li alloy were investigated by isothermal compression test conducted at various strain rates (10−3–10 s−1) and temperatures (573–773 K). The flow stress curves were corrected by considering the friction at the platen/specimen interface and the temperature change due to the deformation heating. The effects of strain, strain rate and temperature on the deformation behavior were characterized by the Zener–Hollomon parameter in a hyperbolic-sine equation, and the constitutive equations were established according to the peak flow stress associated with dynamic recovery, dynamic recrystallization and the dissolution of T1 phases. In the entire strain rate and temperature range, the prediction capabilities of the developed constitutive equation are proved to be feasible and effective with a linear correlation coefficient and an average absolute relative error coefficient of 0.9909 and 6.72%, respectively.

Low-temperature superplastic behavior of beta titanium alloy

The low temperature superplastic deformation behavior of Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe alloy were investigated. Results show that the elongation of the alloy exceeds 100% when deformation at the temperature ranging from of 650°C to 800°C and the initial strain rates ranging from 2.78×10−4s−1 to 8.33×10−3s−1, and the elongation of the alloy increases with the decrease of initial strain rates and the increase of temperature. The true strain–stress curves show that the peak flow stress decreases with the decrease of initial strain rates and the increase of temperatures, and the peak flow stress shows much sensitivity to the initial strain rates as well as the temperatures. During superplastic deformation, the α phase was elongated and the size grows up with the decrease of initial strain rates. The superplastic mechanism of the alloy is grain boundary sliding accommodated by dislocation movement and dynamic recrystallization.

Constitutive modeling for predicting peak stress characteristics during hot deformation of hot isostatically processed nickel-base superalloy

Hot flow behavior of hot isostatically processed experimental nickel-based superalloy is investigated over temperature and strain rate ranging from 1000–1200 °C and 0.001–1 s−1, respectively by carrying out constant true strain rate isothermal compression tests up to true strain of 0.69. True stress–true strain curves corrected for adiabatic temperature rise exhibited rapid strain hardening followed by flow softening behavior irrespective of temperature and strain rate regimes investigated, although anomalous flow behavior is observed at 1200 °C. Variation of peak flow stress with temperature is corroborated to the microstructural changes pertaining to the morphology and relative volume fraction of the phases present. From the experimental results, constitutive model incorporating the effects of strain rate, strain, and temperature is established to describe the hot flow behavior of investigated alloy. Dependence of peak flow stress on strain rate and temperature described by Zener–Hollomon (Z) parameter indicated increase in peak flow stress with Z. Additionally Cingara-Queen equation is employed to predict flow curve up to peak stress. The reliability of developed constitutive models is validated statistically and the results indicate reasonable agreement with experimental findings.

Hot deformation behavior and microstructural evolution of as-homogenized Al–6Mg–0.4Mn–0.25Sc–0.1Zr alloy during compression at elevated temperature

Hot compression tests of homogenized Al–6Mg–0.4Mn–0.25Sc–0.1Zr alloy were carried out on Gleeble-3500 testing system in the temperatures range from 300°C to 450°C and strain rates range from 0.001s−1 to 10s−1. To evaluate the hot workability of this alloy, the processing maps at strains of 0.3 and 0.5 were established on the basis of dynamic material model and Prasad’s instability criterion. The related microstructures were studied by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that flow behavior and microstructural evolution of the alloy are significantly affected by deformation temperature and strain rate. The peak flow stress decreases with increasing deformation temperature, but increases with increasing strain rate. Stable deformation mainly occurs at high temperature with low strain rate due to the softening effect of dynamic recovery and dynamic recrystallization. The volume fraction of new dynamic recrystallized grains increases with increasing temperature and/or decreasing strain rate. These grains are prone to coarsen at elevated deformation temperatures. But thanks to the resistance of Mn-containing particles and nano-scale Al3(Sc,Zr) dispersoids, the coarsening of grains are restrained efficiently. Flow instability mainly occurs at high strain rates because of shear bands, brittle β-phases and initiation of micro-cracks along grain boundaries. According to the processing map at strain of 0.5 and microstructural observation, the optimum processing parameters of the alloy are deformation temperature of 380–430°C with the strain rate between 0.001s−1 and 0.3s−1.

Simple physically-based constitutive equations for hot deformation of 2024 and 7075 aluminum alloys

The hot working behaviors of 2024 and 7075 aluminum alloys were studied through constitutive analysis based on a physically-based approach which accounts for the dependence of the elastic modulus and the self-diffusion coefficient of aluminum on temperature. It was demonstrated that the lattice self-diffusion activation energy of aluminum (142 kJ/mol) can be used in the Zener–Hollomon parameter's formula as the deformation activation energy and the theoretical exponent of 5 can be set in the modified hyperbolic sine law to describe the peak flow stresses. By consideration of physically-based material's parameters, it was possible to conduct a comparative study on the hot flow stress of 2024 and 7075 aluminum alloys. It was concluded that the used approach in the current work can be considered as a versatile tool in future comparative hot working studies, especially in studies dedicated to alloy development.

Quantification of the strengthening effect of rare earth elements during hot deformation of Mg-Gd-Y-Zr magnesium alloy

The flow stress of Mg-Gd-Y-Zr, Mg-Al-Zn, and Mg-Zn-Zr magnesium alloys during hot deformation were correlated to the Zener–Hollomon parameter through analyses based on the proposed physically-based and apparent approaches. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135kJ/mol) can be set in the hyperbolic sine law to describe the peak flow stresses. As a result, the influence of rare earth elements, gadolinium (Gd) and yttrium (Y), upon the hot working behavior was readily characterized by the proposed approach, which was not possible by the conventional apparent approach. It was shown quantitatively that the rare earth addition exerts a profound effect on the hot strength and hence on the creep resistance.

Constitutive Description of 7075 Aluminum Alloy During Hot Deformation by Apparent and Physically-Based Approaches

Hot flow stress of 7075 aluminum alloy during compressive hot deformation was correlated to the Zener-Hollomon parameter through constitutive analyses based on the apparent approach and the proposed physically-based approach which accounts for the dependence of the Young’s modulus and the self-diffusion coefficient of aluminum on temperature. It was shown that the latter approach not only results in a more reliable constitutive equation, but also significantly simplifies the constitutive analysis, which in turn makes it possible to conduct comparative hot working studies. It was also demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of aluminum (142 kJ/mol) can be set in the hyperbolic sine law to describe the peak flow stresses and the resulting constitutive equation was found to be consistent with that resulted from the proposed physically-based approach.

Plastic deformation in nanoporous aluminum subjected to high-rate uniaxial compression

The mechanical behavior of nanoporous monocrystal aluminum subjected to uniaxial compressive loading at a rate of 2109 s- 1 along [110] crystallographic orientation is studied using molecular dynamics simulations. Subjected to such a loading, nanovoids act as the effective sources of dislocation nucleation and emission, four of the twelve {111}110 slip systems may be activated. With the same strain of 3.8%, dislocation nucleation will occur in both the sample of multiple voids and that with a single void. The configuration of multiple voids decreases the required stress for the onset of dislocation nucleation and emission in comparison with the sample with an isolated void of the same size. Because of the emission of trial partials, the accumulation of dislocation density can be changed into a piecewise linear process by the dislocation density propagation rate dd/d: in the initial stage of plastic deformation we obtain dd/d1.071018 m-2, but this changes to dd/d5.361018 m-2 at higher deformation. The velocity of dislocation is calculated to be subsonic and is a variable value during the plastic deformation. Dislocation loop pairs emit from the same void, glide and approach to each other, leading to the reduction of dislocation velocity. Then one loop of each pair continues to glide to intersect mutually and finally interact with the loops emitted from other voids, causing a strain hardening to reach the peak flow stress of 4.3 GPa. There is a post-yield softening corresponding to the onset of rapid dislocation density proliferation at higher dislocation densities. With the temperature evolution of the sample with multiple voids during plastic deformation, the density of mobile dislocations is calculated to be one magnitude lower than the total dislocation density. There is a decrease of mobile dislocation densities at large strains, showing that the mobile dislocation are diminished by the formation of dislocation forest and junctions. At the onset of their nucleation, the dislocations are all Shockley partials, however, when dislocation intersection happens, the majority are still Shockley partials, while the rest consists of Frank partials, perfect fcc dislocations and other dislocation ingredients. Voids collapse at the strain of 11.8%. No twins are found in the present simulation due to the high stacking-fault energy of aluminum. Prismatic dislocation loop emission is observed in this simulation.