Modeling Deformation Flow Curves and Dynamic Recrystallization of BA-160 Steel During Hot Compression

The hot deformation behavior of a high strength low carbon steel was investigated using hot compression test at the temperature range of 850–1100 °C and under strain rates varying from 0.001 to 1 s^?1. It was found that the flow curves of the steel were typical of dynamic recrystallization at the temperature of 950 °C and above; at tested strain rates lower than 1 s^?1. A very good correlation between the flow stress and Zener–Hollomon parameter was obtained using a hyperbolic sine function. The activation energy of deformation was found to be around 390 kJ mol^?1. The kinetics of dynamic recrystallization of the steel was studied by comparing it with a hypothetical dynamic recovery curve, and the dynamically fraction recrystallized was modeled by the Kolmogorov–Johnson–Mehl–Avrami relation. The Avrami exponent was approximately constant around 1.8, which suggested that the type of nucleation was one of site saturation on grain boundaries and edges.     

Effect of solution treatment on the hot workability of electroslag remelted Ni−Cr−Mo alloy

The effect of solution treatment on the hot workability of electroslag remelted Ni−Cr−Mo alloy was evaluated. Tensile tests were performed in the temperature range of 950 to 1200°C at a strain rate of 6.1/sec. The hot workability of this alloy appeared to be intimately dependent on the amount of precipitates that were identified as the P phase. A solution treatment at 1250°C for 2 hours induced a considerable dissolution of the precipitates. It resulted in a significant improvement in the hot workability (from 40% to 70% in RA value), giving the deformation mechanism for grain boundary sliding at below 1000°C and dynamic recrystallization at above 1100°C. The hot ductility drop at around 1050°C could be attributed to the precipitates reprecipitated especially at the grain boundary before deformation.     

Dynamic Recrystallization Mechanism of Inconel 690 Superalloy During Hot deformation at High Strain Rate

The hot deformation characteristics of Inconel 690 superalloy were investigated on the Gleeble-3800 thermal-mechanical simulator. The testing temperatures were in the range of 1000-1200 °C, the strain rate was 10 s^?1, and the maximum true strain was 0.9. Optical microscopy, transmission electron microscopy, and electron backscatter diffraction techniques were employed to analyze the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). The results show that multiple-cycle discontinuous dynamic recrystallization (DDRX) occurs in the process of hot deformation under the conditions above. DRX grain size decreases with decreasing temperature and increasing strain. DDRX with sub-grains directly transforming into grains is the dominating nucleation mechanism of DRX. And, the nucleation mechanism of bulging of the original grain boundaries can only be considered as an assistant nucleation mechanism of DRX, which mainly occurs in the beginning of the deformation.     

Effects of the Strain Rate and Temperature on the Microstructural Evolution of Twin-Rolled Cast Wrought AZ31B Alloys Sheets

This article investigates the effects of the strain rate and temperature on the microstructural evolution of twin-rolled cast wrought AZ31B sheets. This was achieved through static heating and through tensile test performed at strain rates from 10^?4 to 10^?1 s^?1 and temperatures between room temperature (RT) and 300 °C. While brittle fracture with high stresses and limited elongation was observed at the RT, ductile behavior was obtained at higher temperatures with low strain rates. The strain rate sensitivity and activation energy calculations indicate that grain boundary diffusion and lattice diffusion are the two rate-controlling mechanisms at warm and high temperatures, respectively. An analysis of the evolution of the microstructure provided some indications of the most probable deformation mechanisms in the material: twinning operates at lower temperatures, and dynamic recrystallization dominates at higher temperatures. The static evolution of the microstructure was also studied, proving a gradual static grain growth of the AZ31B with annealing temperature and time.     

Effect of 0.1 wt.% Co on the Hot Deformation and Toughness of Fine-Grained Low-Carbon Steel at Sub-zero Temperatures

The effect of 0.1 wt.% Co on the hot deformation behavior of fine-grained low-carbon microalloyed steel was investigated at temperatures of 850-1200 °C and a strain rate of 5 s^?1. Furthermore, the toughness of the steel with and without Co at sub-zero temperatures was evaluated. The results suggest that the addition of 0.1 wt.% Co increases the flow stress and delays the occurrence of dynamic recrystallization (DRX) at the same deformation temperature and strain. The DRX fraction of steel specimens without and with 0.1 wt.% Co was about 67.4 and 43.9% at 850 °C, respectively. Then, it increased to 100% at 1100 °C. Compared with steel without Co, cementite particles in the tempered sorbite of steel with 0.1 wt.% Co decreased in size but increased in quantity, yield strength increased from 756 to 787 MPa, and Charpy V-notch energy at ? 20 and ? 50 °C improved from 69 and 41 to 102 and 65 J, respectively. The fracture morphology and crack propagation characteristics were consistent with the variation in impact energy.     

Activation Energy in Hot Forming and Recrystallization Models for Magnesium Alloy AZ31

Kinetics of the dynamic, as well as postdynamic recrystallization of the wrought magnesium alloy AZ31, was ascertained. Continuous compression tests associated with the study of dynamic recrystallization were realized at temperatures from 523 to 723 K and at the strain rates from 0.001 to 10 s^?1. The activation energy in hot forming was determined as Q = 158 kJ/mol for stress-strain curves of conventional shape, or Q = 146 kJ/mol for stress-strain curves with the concave initial phase affected by twinning. If the Zener-Hollomon parameter Z > 9.1 × 10¹² s^?1 the deformation necessary for the initiation of dynamic recrystallization is almost independent on the forming parameters. Using the results of the stress relaxation tests, equations describing the kinetics of metadynamic recrystallization and the grain size originated in such a way were developed and the effect of individual variables was evaluated.     

Hot Tensile Deformation Characteristics and Processing Map of Extruded AZ80 Mg Alloys

The hot deformation behaviors of extruded AZ80 Mg alloys were investigated using tension tests. True stress-true strain curves were obtained for deformation at temperatures from 250 to 450 °C with the strain rate range from 0.001 to 0.08 s^?1. Optical microscopy analysis was performed to correlate microstructural changes to the flow behaviors. Based on the flow stress, the processing map at a strain of 0.18 was developed using the dynamic materials model theory and can be divided into three zones, including stability zones, change-over region, and instability zones. In stability zones, there are two dynamic recrystallization regions: one region with a peak efficiency of 58% at 350 °C and a strain rate of 0.001 s^?1 called domain I; another region with a peak efficiency of 58% at 400 °C and a strain rate of 0.01 s^?1 taken as domain II. The apparent activation energy for domain I was estimated to be 100.71 kJ/mol, indicating that short-circuit diffusion process is along the grain boundaries and falls at lower temperatures and lower strain rates. A lattice self-diffusion is considered to be rate controlling mechanism with the apparent activation energy estimated as 140.32 kJ/mol at higher temperatures and higher strain rates in domain II. The change-over region is the zone from domain I to domain II, in which the grains abnormally grow. In instability zones, twins, local deformation band, wedge cracking, and matrix cracking were observed, suggesting that these processing parameters for hot tension in this zone are inapplicable.     

Effect of Heat Treatment Parameters on the Microstructure and Properties of Inconel-625 Superalloy

Inconel-625 is a solid solution-strengthened alloy used for long-duration applications at high temperatures and moderate stresses. Different heat treatment cycles (temperatures of 625-1025 °C and time of 2-6 h) have been studied to obtain optimum mechanical properties suitable for a specific application. It has been observed that room temperature strength and, hardness decreased and ductility increased with increase in heat treatment temperature. The rate of change of these properties is found to be moderate for the samples heat-treated up to 850 °C, and thereafter, it increases rapidly. It is attributed to the microstructural changes like dissolution of carbides, recrystallization and grain growth. Microstructures are found to be predominantly single-phase austenitic with the presence of fine alloy carbides. The presence of twins is observed in samples heat-treated at lower temperature, which act as nucleation sites for recrystallization at 775 °C. Beyond 850 °C, the role of carbides present in the matrix is subsided by the coarsening of recrystallized grains and finally at 1025 °C, significant dissolution of carbide results in substantial reduction in strength and increase in ductility. Elongation to an extent of >71% has been obtained in sample heat-treated at 1025 °C indicating excellent tendency for cold workability. Failure of heat-treated specimens is found to be mainly due to carbide particle-matrix decohesion which acts as locations for crack initiation.     

Hot Deformation Characteristics of 13Cr-4Ni Stainless Steel Using Constitutive Equation and Processing Map

Hot compression tests were performed to study the hot deformation characteristics of 13Cr-4Ni stainless steel. The tests were performed in the strain rate range of 0.001-10 s^?1 and temperature range of 900-1100 °C using Gleeble® 3800 simulator. A constitutive equation of Arrhenius type was established based on the experimental data to calculate the different material constants, and average value of apparent activation energy was found to be 444 kJ/mol. Zener-Hollomon parameter, Z, was estimated in order to characterize the flow stress behavior. Power dissipation and instability maps developed on the basis of dynamic materials model for true strain of 0.5 show optimum hot working conditions corresponding to peak efficiency range of about 28-32%. These lie in the temperature range of 950-1025 °C and corresponding strain rate range of 0.001-0.01 s^?1 and in the temperature range of 1050-1100 °C and corresponding strain rate range of 0.01-0.1 s^?1. The flow characteristics in these conditions show dynamic recrystallization behavior. The microstructures are correlated to the different stability domains indicated in the processing map.     

Incoloy825合金热变形过程中的本构模型和特殊晶界演变

采用原始JC模型、修正JC模型和应变补偿Arrhenius方程，描述了Incoloy825合金在不同温度（950～1150 °C）和应变速率（1~10 s^-1）下经摩擦和温升修正后的应力-应变曲线。结果表明，修正后曲线具有明显的动态再结晶特征。与原始JC模型和修正的JC模型相比，Arrhenius应变补偿模型更适合于描述Incoloy825合金热变形过程中的应力应变行为。温度和应变速率对特殊晶界的演变有显著影响。特殊晶界长度分数与动态再结晶分数呈正相关。与冷轧后退火处理工艺相比，热变形工艺调控的特殊晶界长度分数较低，热变形工艺不适合用来调整特殊晶界分数，其原因是在热变形过程中动态再结晶的大量形核造成较小的晶粒团簇。     

Hot deformation behavior and microstructure evolution of a Mg–Gd–Nd–Y–Zn alloy

The hot deformation behavior of homogenized Mg–6.5Gd–1.3Nd–0.7Y–0.3Zn alloy was investigated during compression at temperatures of 250–400 ℃ and at strain rates ranging from 0.001 to 0.100 s~(-1). Microstructure analyses show that the flow behaviors are associated with the deformation mechanisms. At the lower temperatures(250–300 ℃), deformation twinning is triggered due to the difficult activation of dislocation cross-slip. Dynamic recrystallization(DRX) accompanied by dynamic precipitation occurs at the temperature of 350 ℃ and influences the softening behavior of the flow.DRX that develops extensively at original grain boundaries is the main softening mechanism at the high temperature of 400 ℃ and eventually brings a more homogeneous microstructure than that in other deformation conditions. The volume fraction of the DRXed grains increases with temperature increasing and decreases with strain rate increasing.     

Hot Deformation Behavior of NiTiHf Shape Memory Alloy Under Hot Compression Test

In this study, the hot deformation behavior of Ni₄₉Ti₃₆Hf₁₅ alloy was investigated. Compression tests were carried out at temperatures ranging from 800 to 1100?°C and at the strain rates of 0.001?C1/s. The peak stress decreases with increasing deformation temperature and decreasing strain rate, a behavior which can be described by plotting the Zener-Hollomon parameter as a function of stress. It was realized that dynamic recrystallization (DRX) was responsible for flow softening. Most of the samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall down stress. Microstructure evolution showed that new recrystallized grains formed in the vicinity of grain boundaries. The hyperbolic-sine-type constitutive model of Ni₄₉Ti₃₆Hf₁₅ alloy was obtained to provide basic data for determining reasonable hot-forming process. The activation energy for hot deformation of the Ni₄₉Ti₃₆Hf₁₅ alloy was close to 410?kJ/mol.     

304L不锈钢高温变形组织的演化

采用Gleeble-1500D热/力学模拟试验机对304 L不锈钢铸态及锻态试样进行了热压缩试验研究,工艺参数为：变形温度950℃～1 150℃、变形量0.7,变形速率0.1s-1。结果表明：铸态的峰值应力低于锻态的峰值应力,铸态组织的动态再结晶明显迟于锻态组织;铸态及锻态304L不锈钢流变应力随着温度的升高而降低;随着变形温度的升高,动态再结晶百分数增加,再结晶组织增多并趋于完全。     

Microstructure and temperature monitoring during the hot rolling of AZ31

This study details the microstructural evolution during hot rolling of AZ31 alloy sheet using a pilot-scale rolling mill. The aim is to understand the deformation mechanisms leading to grain refinement under industrial processing conditions and to design and optimize the hot rolling schedule for AZ31 in order to produce sheet with a fine and homogeneous microstructure. The study examined three different hot rolling temperatures, 350, 400, and 450°C, and two rolling speeds, 20 and 50 rpm. A total thickness reduction of 67% was obtained using multiple passes, with reductions of either 15% or 30% per pass. It was found that the microstructure of the AZ31 alloy was sensitive to the rolling temperature, the reduction (i.e., strain) per pass and the rolling speed (i.e., strain rate). The results show that the large cast grain structure is broken down by segmentation of the cast grain through localized deformation in twin bands, where dynamic recrystallization occurs in these bands as well as at the grain boundaries (necklacing).     

晶粒度对690合金热变形特性的影响

利用物理模拟实验方法对具有不同晶粒尺寸的690合金试样进行热压缩变形实验,变形温度范围为1100～1200℃,应变速率分别为0.1,1,10s-1,获得了合金的流变应力数据,并对合金变形后的组织特征进行了分析,建立了包含初始晶粒度参数的本构关系模型。结果表明:晶粒尺寸增大使690合金高温变形时的流变应力增加,发生动态再结晶的临界应变增大,动态再结晶体积分数减小,根据所建立的流变应力本构模型计算出的流变应力值与实验值相近,从而完善了690合金的热变形本构方程。     

Hot Deformation Mechanisms of an As-Extruded TiAl Alloy with Large Amount of Remnant Lamellae

The hot deformation mechanisms of an as-extruded Ti-44Al-5V-1Cr alloy with a large amount of remnant lamellae were investigated by hot compression tests at temperatures of 900-1250 °C and strain rates of 0.001-1 s^?1. The hot processing map of the as-extruded Ti-44Al-5V-1Cr alloy was developed on the basis of dynamic materials modeling and the Prasad criteria. There were four different domains in the hot processing map, according to the efficiency of power dissipation, η. The flow soft and hot deformation mechanisms for different domains were illustrated in the context of microstructural evolution during the process of deformation. As a result, the dynamic recrystallization and superplastic deformation occurred at 1125-1150 °C near 0.001 s^?1, and this region is suitable for superplastic forming. The α phase dynamic recrystallization and dynamic recovery occurred at 1250 °C and 0.1 s^?1. The existence of small amount of the γ and β phases effectively inhibited the growth of α grains.     

Hot deformation and processing maps of an Fe3Al intermetallic alloy

The deformation behavior of an Fe–28Al–5Cr–0.08Zr–0.04B (at.%) intermetallic alloy under hot compression conditions was characterized in the temperature range of 600–1100 °C and strain rate range of 0.001–100 s⁻¹. Processing maps were calculated to evaluate the efficiency of the hot working and to recognize the instability regions of the flow behavior. The investigated alloy possesses the optimum hot-working conditions at 1100 °C and 0.001 s⁻¹, since the material undergoes dynamic recrystallization to produce a fine-grained structure with a high fraction of high-angle boundaries (∼70%). At lower temperature the material exhibited “large grained superplasticity” with a peak efficiency of ∼60% at 1000 °C and 0.001 s⁻¹. These parameters are the optimum ones for superplastic working of that alloy. The occurrence of large grained superplasticity is attributed to the formation of a subgrain structure within the large original grains and higher strain-rate sensitivity. The material also exhibits flow instabilities due to flow localization at lower temperatures (<700 °C) and higher strain rates (>0.1 s⁻¹).     

Hot workability and dynamic recrystallization mechanisms of pure nickel N6

The hot workability and dynamic recrystallization (DRX) mechanisms of pure nickel N6 were systematically investigated using hot compression tests. Based on hot compression data, the constitutive equation of N6 was developed and its reliability was verified. Its hot processing map was constructed, and combined with microstructural observations, a semi-quantitative response relationship between hot deformation parameters and microstructure was established. The DRX process of N6 is a thermally activated process and particularly sensitive to the strain rate. The optimal hot processing parameters for N6 were determined to be 950-1050 °C and 0.1-1 s^-1. Furthermore, it was proven that the dominant nucleation mechanism is discontinuous DRX characterized by grain boundary bulging and twins assisting nucleation, while the continuous DRX characterized by subgrains combined with rotation is an inactive nucleation mechanism.     

Constitutive Modeling of Dynamic Recrystallization Behavior and Processing Map of 38MnVS6 Non-Quenched Steel

The dynamic recrystallization behavior of 38MnVS6 non-quenched steel was investigated by hot compression tests on a Gleeble1500 thermomechanical simulator. True stress-strain curves and deformed specimens were obtained in the temperature range of 850-1200 °C and the strain rate range of 0.01-10 s^?1. By regression analysis of the experimental results, the critical strain model and austenite grain size model for dynamic recrystallization were established as a function of Zener-Hollomon parameter. The dynamic recrystallization kinetic model for 38MnVS6 non-quenched steel was established on the basis of the modified Avrami equation. In addition, based on the dynamic material model, the processing map of the steel was established at the strain of 0.5. It was found that the unstable phenomena of the steel did not appear at the deformation conditions. The processing map exhibited a domain of complete dynamic recrystallization occurring in the temperature range of 950-1200 °C and the strain rate range of 0.01-5 s^?1, which were the optimum parameters for the hot working of the steel.