Microstructure and texture evolution during high-temperature compression of Al-Mg-Si-Zr-Mn alloy

Dynamic softening behaviors of a promising biomedical Ti-13Nb-13Zr alloy under hot deformation conditions across dual phaseα+βand single phaseβregions were quantitatively characterized by establishing corresponding dynamic recovery (DRV) and dynamic recrystallization (DRX) kinetic models. A series of wide range hot compression tests on a Gleeble-3500 thermo-mechanical physical simulator were implemented under the strain rate range of 0.01-10 s−1and the temperature range of 923-1173 K. The apparent differences of flow stress curves obtained in dual phaseα+βand single phaseβregions were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. Two typical softening mechanisms about DRV and DRX were identified through the variations of a series of stress-strain curves acquired from these compression tests. DRX is the dominant softening mechanism in dual phaseα+βrange, while DRV is the main softening mechanism in single phaseβrange. The DRV kinetic model for single phaseβregion and the DRX kinetic model for dual phaseα+βregion were established respectively. In addition, the microstructures of the compressed specimens were observed validating the softening mechanisms accordingly.

Flow behaviour and microstructure development of forged Ti-25Al-10Nb-3V-1Mo (super α2)

Hot deformation behavior and softening mechanism of Fe–6.5 wt% Si alloy

Flow behavior and microstructures of Al/15% SiCp were investigated by hot compression tests using Gleeble-1500 thermomechanical simulator at temperatures ranging from 440 to 500 °C with strain rates of 0.001-1.0 s−1. The high-temperature deformation behaviors of Al/15% SiCp were analyzed based on the true stress-true strain curves. The results show that the softening mechanism at low strain rate (0.001 s−1) is dynamic recovery, and at high strain rates (0.01, 0.1, and 1 s−1) is dynamic recrystallization (DRX). Based on these experimental data, a set of constitutive equations for Al/15% SiCp are described by the Zener-Hollomon parameter, and the coefficients of equations are found to be functions of strain. The constitutive equations reveal the dependence of flow stress on strains, strain rates, and temperatures. Furthermore, the mean error between the experimental and the calculated flow stress was computed. The result shows that the calculated results from constitutive equations are in good agreement with the experimental results. To demonstrate the potential workability of Al/15% SiCp, the processing map was established. The stable zones and the instability zones in processing map are identified and verified through micrographs. As a result, the optimum strain rates and temperatures for effective hot working of Al/15% SiCp were determined.

Deformation behaviour and microstructural evolution during the hot compression of Ti-5Al4Zr8Mo7V alloy

Microstructure evolution and dynamic softening mechanisms during high-temperature deformation of a precipitate hardening Ni-based superalloy

In this work, a compression experiment of 35CrMo steel is carried out over a wide range of temperatures (1123–1423 K) and strain rates (0.1–10 s−1) to obtain further understandings of the flow behaviors. The results show that the strain hardening effect of 35CrMo steel is stronger than that of dynamic recrystallization at low temperature and high strain rate; on the contrary, the rheological curves show typical dynamic recrystallization characteristics at high temperature and low strain rate. This indicates that the strain hardening and recrystallization behavior of 35CrMo steel is affected by temperature, strain and strain rate, and its true stress-strain curves can be observed typical work hardening and dynamic softening features. A modified Johnson-Cook (JC) model is developed to predict the flow stress of the alloy. The results of the comparison show that the predicted values of the modified JC model are in good agreement with the experimental values.

The flow stress curves of Cu-1.0%Zr-0.15%Cealloy during high temperature deformation were analyzed by using a Gleeble-1500D thermal simulation machine at the hot deformation temperature of 550 C, 650 C, 750 C, 850 C and 900 C, the strain rate of 0.001 s -1 、 0.01 s -1 、0.1 s -1 、1 s -1 、10 s -1 , respectively.The results show that the hot deformation mechanism of the alloy is mainly based on the dynamic recovery under relative lower hot deformation temperature or higher strain rate.The thermal deformation mechanism of the Cu-1.0%Zr-0.15%Cealloy is the dynamic recrystallization under relative higher hot deformation temperature or lower strain rate.The constitutive equation of the alloy was calculated and established respectively on the basis of the relationships among the flow stress, strain rates and deformation temperature.The thermal activation energy of the alloy is about 430.51 kJ/mol.

Hot compression tests were conducted on an as-extruded Mg-2.5Zn-4Y magnesium alloy using a Gleeble thermal simulation machine under various deformation conditions to investigate its hot deformation behavior. The results indicated that, at relatively high strain rates (1 s−1), the alloy exhibited limited dynamic recrystallization, with deformation primarily governed by dynamic recovery. Conversely, at lower strain rates (0.001 s−1 and 0.01 s−1), dynamic recrystallization became the dominant deformation mechanism. Flow stress and peak stress (σ p) decreased with increasing temperature at constant strain rates. By contrast, at constant temperatures, both flow stress and σ p increased with higher strain rates. Furthermore, at a constant temperature, a reduction in strain rate led to a decrease in the fraction of low-angle grain boundaries (LAGBs) and an increase in high-angle grain boundaries (HAGBs). Similarly, increasing the temperature (573–673 K ) at a constant strain rate resulted in a lower fraction of LAGBs, a higher fraction of HAGBs, more complete dynamic recrystallization, and an overall increase in average grain size. A critical strain prediction model for the alloy was also developed. Analysis of the processing map identified optimized processing conditions of 640–673 K and 0.001 s−1. Under these conditions, the alloy demonstrated excellent workability and enhanced mechanical properties.

Hot deformation characteristics of forged and β-quenched Zr-1.0Sn-0.3Nb-0.3Fe-0.1Cr (N18 alloy) in the temperature range 625–950°C and in the strain rate range 0.005–5 s−1 have been studied by uniaxial compression testing of Gleeble 3500. For this study, the approach of processing maps has been adopted and their interpretation done using the Dynamic Materials Model (DMM). Based on a series of true stress-true strain curves on various temperatures and strain rates, the flow stress has been summarized and both the strain rate sensitivity index (m) and deformation activation energy (Q) have been calculated by the constitutive equations that flow stress and the relationship of Z parameter and flow stress have been established subsequently. Furthermore, the efficiency of power dissipation (⬜) given by [2m/(m+1)] and improved by Murty has been plotted as a function of temperature and strain rate to obtain different processing maps at different true strain rates ranging from 0.1–0.7. Subsequently, the microstructures of the specimens after compression testing were characterized by electron channeling contrast (ECC) imaging techniques used an FEI Nova 400 field emission gun scanning electron microscopy (FEG-SEM). The results showed that: (i) The hyperbolic sine constitutive equation can describe the flow stress behavior of zirconium alloy, and the deformation activation energy and flow stress equation were calculated under the different temperature stages which insists that the deformation mechanism is not dynamic recovery. (ii) The hot processing maps and its validation were analyzed, which indicated that the DMM theory was reliable and could be adopted as useful tool for optimizing hot workability of Zr. The optimum parameters for extrusion and hammer forging were revealed on the processing maps of 830–950°C, 0.048–2.141 s−1 and 916–950°C, 2.465–5 s−1. (iii) The microstructure of the ingot exhibits a typical lamellar Widmanstatten structure. Under the different strain rates, the grains formed by dynamic recrystallization existed normally in the central zone of the compression samples while the no uniformity of grain size increased with the increasing of strain rate. Meanwhile, due to the dynamic recrystallization as a thermal activation process, the grains size and uniformity increased with the increasing of temperature. In brief, microstructure analysis showed that continuous dynamic recrystallization and geometric dynamic recrystallization operated concurrently during the isothermal compressive deformation.

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

The flow behavior and associated microstructural changes of wrought 6069 Al alloy deformed in tension were analyzed in this work. Tensile tests were conducted on an extruded tube with a thickness of 1.6 mm in the temperature range of 300–500 oC, with initial strain rates from 0.001 to 0.1 s-1. The true stress–true strain curves exhibited a peak stress at a critical strain. The overall level of the flow curve increased when the strain rate was increased and/or the temperature was decreased. The flow curves exhibited a typical flow behavior with dynamic softening and showed that the softening degree after reaching the peak stress was dependent on the deformation conditions. This could be related to the softening mechanism. The main softening mechanism of the alloy was dynamic recovery (DRV) at low temperatures; dynamic recrystallization (DRX) occurred as deformed at high temperatures.

Isothermal compression testing was carried out on newly developed low-cost (α + β) Ti-4.5Al-1 V-3Fe alloy with lamellar initial microstructure using a Gleeble 3500 thermomechanical simulator. The tests were performed under different conditions of strain rate (0.001, 0.01, 0.1, 1 and 10 s −1), deformation temperature (750, 800, 850, 900 and 950 °C) and a constant total strain of 0.6. Stress-strain analysis, constitutive constant calculations, processing maps and microstructural validation were used to understand the hot working behaviour of the alloys and the underlying softening mechanisms. The results show that the deformation behaviour was significantly influenced by the deformation parameters. Two main softening mechanisms, which have been reported in existing commercial alloys with a fully lamellar structure, also controlled the deformation behaviour of the Ti-4.5Al-1 V-3Fe alloy. Lath bending and rotation caused flow softening at low temperatures and high strain rates, while dynamic globularisation led to flow softening at the higher temperatures and low strain rates. The optimum condition for hot working of the alloy in the safe deformation region was found at ~890–905 °C and 0.003–0.01 s −1. The region of instability identified at ~875–930 °C/0.15–0.4 s −1 should be avoided during hot working to prevent flow localisation, shear cracks, cavitation and other instabilities that may arise.

The hot deformation behavior and microstructural evolution of SA508‐ІV steel are studied through hot compressive tests in the temperature range of 950–1250 °C, and in the strain rate range of 0.001–1 s−1. The hot deformation activation energy and the stress exponent are calculated to be 328.73 KJ mol−1 and 3.29 by the regression analysis of sine hyperbolic function. On this basis, the constitutive Equations are developed. Moreover, the relationship between work hardening and dynamic softening is explained, and a two‐stage constitutive model is established to predict the flow stress of SA508‐ІV steel. The predicted and experimental values show a good consistency. Based on the conventional dynamic recrystallization kinetics model, the volume fraction of the dynamic recrystallization is also estimated, and the results show that the maximum fraction is close to 100% under the higher temperature and lower strain rate. Furthermore, the microstructural evolutions at different deformation conditions are also analyzed, and the DRX grain size has an exponential increase with increasing deformation temperature and decreasing strain rate.

There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above Ae3 in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above Ae3 once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation. Key words: low-carbon steel, hot deformation, dynamic recrystallization, dynamic transformation, flow stress analysis