Apparent Activation Energy Of Deformation Research Articles

Mechanical properties, deformation behavior and microstructure evolution of Ti-43Al-6Nb-1Mo-1Cr alloys

The high-temperature properties, microstructure evolution and deformation behavior of a novel Ti-43Al-6Nb-1Mo-1Cr(at.%) alloy were studied in this paper. The ultimate tensile strength (UTS) and elongation (δ) were tested at 800, 850, and 900°C, in which the excellent strength 551MPa, 510MPa, and 382MPa, and elongation 2.9%, 12.7%, and 16.8% were exhibited, respectively. Isothermal compression tests were carried out in the temperature range of 1100–1250°C and strain rate range of 0.001–1.0s−1. Based on dynamic materials model (DMM) and phase diagram, dynamic recrystallization mechanism and phase transformation during thermal deformation were clarified and the hot processing maps were calculated. The experimental results show that the values of stress exponent (n) and the apparent deformation activation energy (Q) of Ti-43Al-6Nb-1Mo-1Cr(at.%) were calculated to be 2.77 and 571KJ/mol. Meanwhile, three various types of microstructure evolution in different deformation conditions were discussed and concluded.

Constitutive model and optimal processing parameters of TC17 alloy with a transformed microstructure via kinetic analysis and processing maps

The kinetic and flow stability analysis of TC17 alloy with a transformed microstructure in the hot deformation of α+β region were carried out. The flow behavior in the hot deformation of TC17 alloy was sensitive to the strain, strain rate and deformation temperature. The work hardening effect dominated the flow in the early stage of deformation, and the globularization, dynamic recrystallization (DRX) occurred in the following deformation, in which the apparent activation energy for deformation decreased with the increasing of strain. A strain compensated constitutive model for hot deformation of TC17 alloy was established. The peak efficiency of power dissipation at the strains of 0.3 and 0.5 occurred at 0.01s−1 and 1143K due to α to β transformation and DRX of β phase. However, the peak efficiency of power dissipation at a strain of 0.7 occurred at 0.1s−1 and 1123K due to the globularization of α lamellae and DRX of β phase. The flow instability analysis suggested that the safe processing window should not exceed 0.32s−1. The processing maps of TC17 alloy at different strains were constructed. According to the processing maps and microstructure observation, the optimal processing parameters for globularization of TC17 alloy with a transformed microstructure were at the temperatures ranging from 1043K to 1123K and strain rates ranging from 0.01s−1 to 0.1s−1.

Flow Behavior, Dynamic Recrystallization and Processing Map of Mg-20Pb-1.6Al-0.4B Alloy

Compression tests of the Mg-20Pb-1.6Al-0.4B alloy have been performed in the deformation temperature range of 240-420 °C and the strain rate range of 0.01-10 s−1. Flow behaviors were examined based on stress-strain curves and dynamic recrystallization (DRX) behaviors. The hot deformation apparent activation energy of the alloy is 147.5605 kJ/mol. The developed constitutive equation, which can be used to relate the peak stress to the absolute temperature and strain rate, was obtained. The correlation between the volume fraction of the DRX and strain at 240 °C and 0.01 s−1 was established. Processing maps of the Mg-20Pb-1.6Al-0.4B alloy at different strains are sensitive to strain. According to the processing maps and microstructures, the observed optimum parameters (temperature and strain rate) for hot working of the alloy were 300-420 °C and 0.01-0.06 s−1, respectively.

Hot deformation characteristics and processing map of a phosphorous modified super austenitic stainless steel

Hot deformation behavior of a phosphorous-modified super austenitic stainless steel was studied in the temperature range of 1173–1423K and strain rate range of 0.001–10s−1 employing thermomechanical simulator. The apparent activation energy for deformation in the above processing regime was estimated to be 482kJmol−1. The deformation parameters were modeled using Arrhenius equation and Zener–Hollomon parameter (Z). Peak stress, critical stress for dynamic recrystallization, stress at which flow softening is maximum as well as steady state stress was found to exhibit a linear relationship with ln(Z/A). Strains corresponding to these stresses were also found to exhibit the relation ε=CZAp. Processing maps were developed at different plastic strains employing dynamic materials modeling. Microstructures corresponding to the different processing conditions were characterized employing electron back scatter diffraction. Based on the analysis of microstructure and processing map, the optimum processing domain for hot deformation is identified as strain rate range of 0.01–0.1s−1 and temperature range of 1300–1350K. Although a significant recrystallization was observed following hot deformation in the strain rate ranges of 1–10s−1 and temperature ranges 1373–1423K, this domain was marked as unstable in the processing map.

Influence of temperature and strain rate on hot deformation behavior of Zr50Ti50 alloy in single β field

The deformation behavior and microstructure of the Zr50Ti50 alloy in β phase field were investigated by isothermal compression tests at temperatures ranging from 700 to 850 °C and strain rates ranging from 0.001 to 1 s−1. The flow curves exhibited typical flow softening. The initial discontinuous yielding behavior was observed at higher strain rates, which was not found in other traditional Zr alloys. The apparent deformation activation energy was calculated to be 103 kJ/mol and constitutive equation describing the flow stress as a function of the strain rate and deformation temperature was proposed. The analysis indicated that the hot deformation mechanism was mainly dominated by dynamic recovery. However, dynamic recrystallization was delayed by dynamic recovery. Thereafter, the processing map was calculated to evaluate the efficiency of the forging process at the temperatures and strain rates investigated and to optimize processing parameters of hot deformation. The optimum processing parameters were found to be 830–850 °C and 0.56–1 s−1 for hot the deformation of Zr50Ti50 alloy in the β phase region.

Stress relaxation ageing behaviour and constitutive modelling of a 2219 aluminium alloy under the effect of an electric pulse

In creep age forming (CAF), stress relaxation and the creep process occur during the ageing heat treatment. However, there is a remarkable difference between the deformation and phase transformation activation energies in aluminium alloys. To achieve high-performance and high-precision collaborative manufacturing of large-scale complex panels with high levels of reinforcement, the difference in the energy barriers between the stress relaxation process and the ageing precipitation process needs to be coordinated. An electric pulsed current (EPC) was first introduced into the CAF process for 2219 aluminium. It was found that the EPC can effectively regulate the stress relaxation behaviour during the initial ageing stage. That is, the application of EPC during the first 1 h of the ageing treatment process decreases the stress gradients for different initial stress levels, which makes it possible to reduce the deformation and phase transformation inhomogeneities resulting from stress differences inside a component under a bending load. Meanwhile, the apparent deformation activation energy decreases with the EPC, which decreases the energy barrier difference between the stress relaxation and ageing precipitation from 47 kJ/mol to 18 kJ/mol. Furthermore, the action mechanism for EPC on the stress relaxation process is discussed, and a constitutive model for stress relaxation age forming considering the effect of the EPC is established.

Hot deformation behavior and processing maps of Ti–15Al–12Nb alloy

The isothermal hot compression tests of Ti–15Al–12Nb alloy under wide range of strain rates (0.01–10.00 s−1) and deformation temperatures (950, 1000, 1050, and 1100 °C) were carried out using Gleeble-3500 thermo-simulation machine. A constitutive equation represented as a function of temperature, strain rate and true strain was developed, and the hot deformation apparent activation energy is calculated to be about 453 kJ·mol−1. By employing dynamic material model (DMM), the processing maps of Ti–15Al–12Nb alloy at various strains were established. Maximum efficiency of about 57 % for power dissipation is obtained at high temperature and low strain rate. Owing to the high power dissipation efficiency and excellent processing ability in dynamic recrystallization (DRX) zone for metal material, the optimum processing conditions are selected as the temperature range of 1050–1100 °C and the strain rate range of 0.01–0.10 s−1. Using transmission electron microscopy (TEM) studies, it is found that the dislocation density is directly associated with the value of processing efficiency. It is observed that when the processing efficiency is about 22 %, the dislocation density is reasonably large. The flow instability region occurs at strain rate of 10.00 s−1 with cracks, which should be avoided during hot processing to obtain the required mechanical properties.

Effects of friction stir processing on the microstructure and superplasticity of in situ nano-ZrB2/2024Al composite

In this study, in situ nano-ZrB2/2024Al composites fabricated from 2024Al–K2ZrF6–KBF4 system were processed by friction stir processing (FSP) to achieve superplasticity of the composites. And the effects of particle contents (1wt%, 3wt%, 5wt%), matrix grain size (micron or sub-micron), strain rates (5×10−3s−1, 1×10−2s−1, 2×10−2s−1) and deformation temperatures (400K, 480K, 600K, 700K, 750K) on the superplasticity of the composites were investigated. After the friction stir processing, the coarse grains of the cast composites with matrix grain size of about 80–100μm and nano-ZrB2 reinforcement size of 30–100nm were crushed into small grains about 1μm in size, and the uniformity of the nano-ZrB2 reinforcements was also improved. And under the same superplastic tensile testing condition at the temperature of 750K and strain rate of 5×10−3s−1, the FSP nano 3wt%ZrB2/2024Al composite exhibited an superplastic elongation of 292.5%, while the elongation of the corresponding cast composite was only less than 100%. Meanwhile, the m values of the FSP composites were always higher than the cast composites, especially the FSP composites with 3wt% particles has the m value of 0.5321 i.e., the FSP composites should had better superplastic properties than cast ones. Furthermore, the FSP composites had higher apparent deformation activation energy (Q) than that of the lattice diffusion of pure aluminium, indicating that the deformation mechanisms of the FSP composites should be grain boundary sliding mechanisms.

Dynamic recrystallization behavior of a biomedical Ti–13Nb–13Zr alloy

The dynamic recrystallization (DRX) behavior of a biomedical titanium Ti–13Nb–13Zr alloy has been investigated using the high temperature compression tests under wide range of strain rates (0.001–1/s) and temperatures 900–1050°C. A constitutive equation represented as a function of temperature, strain rate and true strain is developed and the hot deformation apparent activation energy is calculated about 534kJ/mol. By considering the exponential relationship between work-hardening rate (θ) and stress, a new mathematical model was proposed for predicting flow stress up to the critical strain during hot deformation. The mathematical model for predicting flow stress up to the critical strain exhibits better consistency and accuracy. The DRX kinetic equation of Ti–13Nb–13Zr alloy is described as XDRX=1−exp[−0.32(Ɛ−ƐcƐ*)2.3] . The DRX kinetic model was validated by microstructure observation. It was also found that the process of DRX was promoted by decreasing strain rate and increasing deformation temperature. Eventually, the continuous dynamic recrystallization (CDRX) was identified to be the DRX mechanism using transmission electron microscope (TEM).

Deformation behavior and processing maps during isothermal compression of TC21 alloy

In this study, isothermal compression tests were conducted at a Gleeble-1500 simulator at deformation temperatures ranging from 1073 to 1283 K, strain rates ranging from 0.01 to 5.00 s−1, and height reductions ranging from 20 % to 60 %. The flow stress and apparent activation energy for deformation and constitutive equation were used to characterize the deformation behavior of TC21 alloy during the isothermal compression. The processing maps combined microstructure observations were established based on dynamic material model (DMM) over a range of strain rates and temperatures. The results show that an initial yield drop is observed above 1203 K or at higher strain rates ranging from 1.00 to 5.00 s−1, and oscillatory flow curves are presented particularly at a strain rate of 5.00 s−1. Strain has some influence on the apparent activation energy for deformation during the isothermal compression of TC21 alloy. The Q-values and microstructure observation confirm that dynamic recrystallization (DRX) occurs in the β single-phase region. The constitutive equation during the isothermal compression of TC21 alloy is developed using the Zener–Hollomon parameter in the exponent-type equation. The maximum and minimum relative errors between the calculated and the experimental flow stress are 14.1 % and 0.3 %, respectively. The peak efficiency of power dissipation at a strain of 0.7 is about 0.51 occurring at a deformation temperature of 1073 K and strain rate of 0.01 s−1, corresponding to an optimal deformation condition of TC21 alloy.

High temperature deformation behavior of Nb–1 wt.%Zr alloy

The hot deformation behavior of Nb–1wt.%Zr alloy was studied using uniaxial compression tests carried out in vacuum to a true strain of 0.6 in the temperature range of 900 to 1700°C and the strain rate range of 3×10−3 to 10s−1. The optimum regime of hot workability of Nb–1Zr alloy was determined from the strain rate sensitivity (m) contour plots. A high m of about 0.2 was obtained in the temperature and strain rate range of 1200–1500°C and 10−3 to 10−1s−1 and 1600–1700°C and 10−1 to 1s−1. Microstructure of the deformed samples showed features of dynamic recrystallization within the high strain rate sensitivity domain. Compared to the study on Nb–1Zr–0.1C alloy, Nb–1Zr showed a lower flow stress and an optimum hot working domain at lower temperatures. In the 1500 to 1700°C range the apparent activation energy of deformation for Nb–1Zr was 259kJmol−1, the stress exponent 5, and the activation volume about 200 to 700 b3.

The research on the constitutive modeling and hot working characteristics of as-cast V–5Cr–5Ti alloy during hot deformation

Isothermal compression tests of as-cast V–5Cr–5Ti vanadium alloy are conducted in the deformation temperature ranging from 1150 to 1400 °C with an interval of 50 °C, strain rate ranging from 0.001 to 1 s−1 and height reductions of 55% on a computer controlled thermal simulation machine. The hot deformation behavior of as-cast is characterized based on the analysis of the stress–strain behaviors, the constitutive equations and process map for obtaining optimum processing parameters and achieving desired microstructure during hot working. A constitutive equation by which the flow stress was expressed as a function of strain rate and deformation temperature was established and the apparent activation energy of deformation was calculated to be 428.480 kJ/mol. The processing maps are constructed at the true strain of 0.5 and 0.7 based on dynamic materials model (DMM) to delineate the safe and unsafe regions in the axes of temperature and strain rate. The area of unsafe region increases with the increasing of strain. These instability domains exhibited localized flow and cracking along grain boundaries which should be taken care of during hot processing. The recommended domain is in the condition of the temperature range of 1250–1400 °C and strain rate range of 0.001–0.02 s−1. And in this state the main soften mechanism is dynamic recovery. It is worth noting that when as-cast V–5Cr–5Ti alloys deformed in the recommended deformation zone, the continuous dynamic recrystallization occurred, which display the feature of serrate grain boundaries and the equiaxed grains in the grain boundaries. Microstructure observation of deformed specimens validated the applicability of the processing map at obtaining the optimum processing parameters of as-cast V–5Cr–5Ti alloy.

High Temperature Behavior of Isothermally Compressed M50 Steel

The isothermal compression of M50 steel is carried out on a Gleeble-3500 thermo-mechanical simulator in temperature range of 1 223–1 423 K and strain rates range of 10–70 s−1. The results show that the carbides play a significant role in the flow behavior and microstructure evolution during isothermal compression of M50 steel. The average apparent activation energy for deformation in isothermal compression of M50 steel is (281.1±42.6) kJ·mol−1 at the strains of 0.4–0.8. The dynamic recrystallization of austenite grains occurs in isothermal compression of M50 steel at 1 363 K and 1 393 K, enhancing with the increase of strain rate and/or strain. The volume fraction of the carbides decreases with the increase of deformation temperature during isothermal compression of M50 steel and the fine carbides inhibit the dynamic recrystallization of austenite grain. With the occurrence of dynamic recrystallization, the austenite grains are refined, leading to a minor increase in the flow stress and apparent activation energy for deformation in isothermal compression of M50 steel. The austenite grains begin to coarsen at 1 423 K and dynamic recrystallization is limited. Hot working of M50 steel should not be performed above 1 393 K in order to achieve good workability.

Effect of the alpha grain size on the deformation behavior during isothermal compression of Ti–6Al–4V alloy

Abstract The effects of alpha grain size on the flow stress, the apparent activation energy for deformation (Q) and the processing maps of Ti–6Al–4V with an equiaxed microstructure are thoroughly investigated using isothermal compression tests, and detailed explanation is given based on the microstructure observation and quantitative analysis. The shapes of flow curves are dependent on the microstructure characteristic of the alloy before deformation and during the deformation process. The flow stress increases with increasing equiaxed alpha phase, but decreases with increasing alpha grain size. The Q-values for d r1 and d r2 are smaller than those for d r3 and d r4 , respectively, which is possibly attributed to that Ti–6Al–4V alloy for d r1 and d r2 which exhibits a very strong grain-boundary sliding (GBS) mode besides dominant dislocation glide/climb mechanism. The local efficiency maxima and unstable regions in processing maps change with the alpha grain size, which implies that proper hot-working domains should be modified in different grain size range so as to meet the precision forging process.

Deformation and recrystallization behavior of a coarse-grain, nickel-base superalloy Udimet720Li ingot material

The deformation and recrystallization behavior were studied using isothermal hot compression experiments for a cast-and-homogenized Udimet720Li material deformed in the temperature range of 1070 to 1190°C with strain rates of 0.01 to 0.5s−1 and total strain of 1.21. Flow behavior was well modeled and the variation of the apparent activation energy for deformation with strain rate and temperature were discussed. The distinctions of the dynamic recrystallization (DRX) behavior for the subsolvus and supersolvus deformation were significantly related to the presence of γ′ precipitates. The relationship between γ′ precipitates and deformation conditions and the interaction between γ′ precipitates and grain boundaries (GBs) were presented. Multiple mechanisms of DRX nucleation were discussed in consideration of subgrain rotation, twinning and γ′ precipitates. Particular attention was paid to the role of intragranular DRX nucleation and continuous dynamic recrystallization (CDRX) nucleation.

Hot deformation behavior of TC11/Ti–22Al–25Nb dual-alloy in isothermal compression

The high-temperature flow behavior of TC11/Ti–22Al–25Nb electron beam (EB) weldments was investigated by the isothermal compression tests at the temperature of 900–1060 °C and the strain rate of 0.001–10 s−1. Based on the experimental data, the constitutive equation that describes the flow stress as a function of strain rate and deformation temperature is obtained. The apparent activation energy of deformation is calculated, which decreases with increasing the strain and the value is 334 kJ/mol at strain of 0.90. The efficiency of power dissipation η changes obviously with the variation of deformation conditions. Under the strain rates of 0.01, 0.1 and 1 s−1, the value of η increases with increasing the true strain for different deformation temperatures. While the value of η decreases with increasing the strain under the strain rates of 0.001 and 10 s−1. The optimum processing condition is (topi=1060 °C, opi=0.1 s−1) with the peak efficiency of 0.51. Under this deformation, dynamic recrystallization (DRX) is observed obviously in the microstructure of welding zone. Under the condition of 1060 °C and 0.001 s−1, the deformation mechanism is dominated by dynamic recovery (DRV) and the value of η decreases sharply (η=0.02). The flow instability is predicted to occur since the instability parameter ξ() becomes negative. The hot working process can be carried out safely in the domain with the strain rate of 0.001–0.6 s−1 and the temperature of 900–1060 °C.

Effect of Cu concentration on the semi-solid deformation behavior and microstructure of Ti–Cu alloy

The semi-solid compressive deformation behavior of Ti–Cu alloys was investigated by Gleeble-3500 hot simulator at the deformation temperatures ranging from 1273 to 1473 K with strain rates ranging from 5×10−3 to 5×10−1 s−1. The relationship between Cu concentration and flow stress was analyzed, and the deformation apparent activation energy was also calculated. The results show that Cu concentration has significant influence on the flows’ behavior of Ti–Cu alloys, especially at high semi-solid deformation temperatures. The Ti–14Cu exhibits the highest flow stress at 1273 and 1373 K, Ti–2.5Cu alloy exhibits the highest flow stress at 1473 K, and Ti–7Cu alloy shows the lowest flow stress at all tested temperatures, which corresponds to liquid fraction caused by varied Cu concentration and the deformation temperature. The difference in microstructure suggests that the shape and distribution of Ti2Cu precipitates are significantly affected by Cu concentration. The increase in Cu concentration leads to the growth and precipitation of acicular Ti2Cu along grain boundaries at high semi-solid deformation temperatures. The deformation apparent activation energy of Ti–14Cu alloy significantly decreases from solid deformation to semi-solid deformation owing to the change in main deformation mechanism from plastic deformation of solid particles to solid particles’ slippage and rotation of grain boundaries.

Hot deformation behavior, dynamic recrystallization, and physically-based constitutive modeling of plain carbon steels

The high-temperature deformation behaviors of low and medium carbon steels with respectively 0.06 and 0.5wt% C were investigated under strain rate and temperature ranges of 10−4–10−1s−1 and of 900–1100°C. Three types of dynamic recrystallization (DRX) flow behaviors were identified, namely single peak, multiple transient steady state (MTSS), and cyclic behaviors. The normalized critical stress (and strain) for the low and medium carbon steels were about 0.846 (0.531) and 0.879 (0.537), respectively. For both steels, the apparent deformation activation energy and the power of the hyperbolic sine law were found to be near the lattice self-diffusion activation energy of austenite (270 kJ/mol) and 4.5, respectively. As a result, it was concluded that the flow stress of plain carbon steels in hot deformation is mainly controlled by dislocation climb during their intragranular motion, and based on physically-based constitutive analysis, it was found that carbon has a slight effect on the hot flow stress of plain carbon steels. The significance of the approach used in this work was shown to be its reliance on the theoretical analysis based on the deformation mechanisms, which makes the comparison more reliable.

Hot deformation behavior of an aluminum-matrix hybrid nanocomposite fabricated by friction stir processing

A fine-grained aluminum-matrix hybrid nanocomposite reinforced with TiO2, MgO and Al3Ti nanoparticles was prepared via reactive friction stir processing (FSP) of an Al–Mg sheet with pre-placed TiO2 particles (50nm; 3.1vol%). The microstructure of the hybrid nanocomposite comprises high-angle grain boundaries (~90%) with an average size of 2µm and hard inclusions with sizes in the range of 30–50nm. Evaluation of the hot deformation behavior of the nanocomposite by uniaxial tensile testing at different temperatures (300–450°C) and strain rates (0.001–0.1s−1) shows that the deformation apparent activation energy of the nanocomposite is 137kJmol−1 at ≤300°C. The values of the activation energy for the Al–Mg alloy before and after FSP at this temperature range are about 105 and 135kJmol−1, respectively. This observation highlights the role of ultrafine hard particles and the structural changes induced by FSP on the deformation process. At the higher temperatures, the deformation activation energy for the aluminum alloy without and with the reinforcing particles is 303 and 456kJmol−1, respectively. Detailed microstructural analysis by electron back scattered diffraction and transmission electron microscopy suggests that dynamic recrystallization is responsible for the deformation behavior at the elevated temperatures. Meanwhile, the presence of the hard nanoparticles operates as a grain growth inhibitor improving the thermal stability of the fine-grained aluminum alloy.

Investigation on the hot deformation behavior of powder metallurgy TiAl-based alloy using 3D processing map

The deformation behavior of the powder metallurgy Ti–47Al–2Nb–2Cr alloy in hot working process was researched by the isothermal compression tests performed in the deformation temperature range of 950–1200°C and the strain rate range of 0.001–0.1s−1 with true strain of 0.8. The constitutive equation which presented the flow stress as a function of the strain rate and deformation temperature was established for hot deformation of this alloy. The hot deformation apparent activation energy was calculated to be 337.501kJ·mol−1. The 3D processing maps were developed on the basis of experimental data and dynamic material model theory for the identification of the instability regions and optimization of hot processing parameters. It was found that the dynamic recrystallization of powder metallurgy Ti–47Al–2Nb–2Cr alloy occurs at two domains, one occurs in the temperature range of 1040–1060°C and strain rates of 0.001–0.0015s−1, with peak efficiency of power dissipation of 55.0% occurring at about 1050°C/0.001s−1, the other occurs in the temperature range of 1180–1200°C and strain rate of 0.003–0.01s−1 with peak power dissipation efficiency of 48.8% occurring at about 1200°C/0.001s−1, which suggested that these domains can be considered as the optimum processing window for hot working of this alloy. Meanwhile, the flow instability domain was found to occur at strain rates higher than 0.01s−1 with the microstructural observation of the grain boundary cracks, which has to be kept away during hot processing in pursuit of achieving the desired mechanical properties.