Dynamic recrystallization behavior of GH4169G alloy during hot compressive deformation

The microstructure evolutions and nucleation mechanisms of GH4169 G alloy were studied by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The hot compression tests were performed different imposed reductions in the range of true strain from 0.12 to 1.2 at the temperatures of 930 ℃-1050 ℃ with strain rates of 0.01 s⁻¹-1 s⁻¹. It is found that cumulative and local misorientation increase firstly and then decrease when the strain is increased due to the progress of dynamic recrystallization (DRX). The low angle boundaries (LAGBs) rapidly develop to high angle boundaries (HAGBs) at relatively high deformation temperature or the low strain rate. There are three DRX mechanisms observed for GH4169 G alloy during hot deformation. Discontinuous dynamic recrystallization (DDRX) as the dominant mechanism for GH4169 G alloy is characterized by typical necklace structures and bulged-original boundaries. Besides, different deformation bands with dislocation cells formed in deformed matrix at low temperature and large strain, which indicates that continuous dynamic recrystallization (CDRX) contributed to the DRX process. The twin boundaries lost their coherent characteristics and provide sites for nucleation, which also accelerates the nucleation of DRX.     

锻态TB6钛合金β相区压缩变形行为和动态再结晶

使用圆柱形试样在Thermecmaster-Z型热模拟试验机上进行锻态TB6钛合金β相区的热压缩实验(变形温度950~1100℃,应变速率0.001~1 s^-1),研究了合金的高温压缩变形和动态再结晶行为。结果表明,这种合金在β相区的变形激活能为246.7 kJ/mol,其热变形机制是动态再结晶,动态再结晶新晶粒的主要形核机制是弓弯形核。当应变速率为0.01~0.1 s^-1、变形温度为<1000℃时动态再结晶的发展比较充分,变形组织明显细化;当变形温度高于1000℃、应变速率低于0.001 s^-1时,动态再结晶的晶粒明显粗化。在动态再结晶的晶粒尺寸D与Z参数之间存在着相关性,其函数关系为D=6.44×10²·Z^-0.1628。     

Effect of δ-ferrite co-existence on hot deformation and recrystallization of austenite

This work evaluates the effect of co-existence of a large volume fraction of δ-ferrite on the hot deformation and dynamic recrystallization (DRX) of austenite using comparative hot torsion tests on AISI 304 austenitic and 2205 duplex stainless steels. The comparison was performed under similar deformation conditions (i.e. temperature and strain rate) and also under similar Zener-Hollomon, Z, values. The torsion data were combined with electron backscatter diffraction (EBSD) analysis to study the microstructure development. The results imply a considerable difference between DRX mechanisms, austenite grain sizes and also DRX kinetics of two steels. Whereas austenitic stainless steel shows the start of DRX at very low strains and then development of that microstructure based on the necklace structure, the DRX phenomena in the austenite phase of duplex structure does not proceed to a very high fraction. Also, the DRX kinetics in the austenitic steel are much higher than the austenite phase of the duplex steel. The results suggest that at a similar deformation condition the DRX grain size of austenitic steel is almost three times larger than the DRX grains of austenite phase in duplex steel. Similarly, the ratio of DRX grain size in the austenitic to the duplex structure at the same Z values is about 1.5.     

The identification of dynamic recrystallization and constitutive modeling during hot deformation of Ti55511 titanium alloy

In the present paper, an internal-variable identification approach has been proposed to investigate the dynamic recrystallization (DRX) behavior during hot deformation and corresponding constitutive model has been constructed. Isothermal compression experiments of Ti55511 titanium alloy were conducted for verification. Plastic behavior is determined by dislocation evolution in many cases while deforming. The comparison between saturated and DRX critical dislocation density was made to distinguish the occurrence of dynamic recrystallization/recovery (DRV) during hot deformation. The influence of deformation parameters on DRX behavior was illustrated by dislocation evolution map, validated by the power dissipation efficiency distribution. DRX process during hot deformation of Ti55511 alloy tends to occur under moderate temperatures and low strain rates. In addition, a physical-based Arrhenius constitutive formula has been derived for DRX criticality. The strain-rate sensitivity coefficients during hot deformation were fixed as a constant equal to 1/6 and the deformation activation energy was related to the material's self-diffusion activation.     

新型含铝奥氏体耐热合金Fe-20Cr-30Ni-0.6Nb-2Al-Mo的动态再结晶行为

在Gleeble-3500热力模拟试验机上对一种新型奥氏体耐热合金(Fe-20Cr-30Ni-0.6Nb-2Al-Mo)进行单道次热压缩实验,结合OM、EBSD及TEM等表征手段,研究了该合金在950~1 100℃和0.01~1s-1热变形参数下的动态再结晶行为,采用回归法确定了合金的热变形激活能和表观应力指数,并以此构建其高温本构模型。实验结果表明,新型奥氏体耐热合金的应力水平随变形温度的升高而降低,随应变速率的增大而升高;动态再结晶行为更易发生在较高变形温度或较低应变速率下。采用lnθ-ε曲线的三次多项式拟合求解临界再结晶拐点的方法,较准确地预测了合金的动态再结晶临界点。此外,归纳出该合金在动态再结晶过程中的形核机制,主要包括应变诱导晶界迁移、晶粒碎化以及亚晶的合并。     

TB17钛合金β相区动态再结晶行为及转变机理EI北大核心CSCD

通过Gleeble-3800热压缩模拟试验机对TB17钛合金β相区进行热压缩实验,研究该合金β相区的动态再结晶行为及转变机理。结果表明:TB17钛合金在β相区变形时会发生动态回复(DRV)与动态再结晶(DRX)。不同应变速率下存在两种动态再结晶形核位置,低应变速率下主要在晶粒内部形核,高应变速率下主要在晶界附近形核。通过EBSD和TEM分析可知,在低应变速率下发生连续动态再结晶(CDRX),其发生的主要形式为亚晶合并转动。高应变速率下发生不连续动态再结晶(DDRX),发生的主要形式为晶界剪切伴随着亚晶转动。尽管两种动态再结晶的转变方式不同,其本质都是通过位错的增殖、滑移和胞状结构演化形成新的动态再结晶晶粒。     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

Dynamic recrystallization behavior of a typical nickel-based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980 °C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener–Hollumon parameter.     

The rate of dynamic recrystallization in 17-4 PH stainless steel

The hot working behavior of 17-4 PH stainless steel (AISI 630) was studied by hot compression test at temperatures of 950–1150 °C with strain rates of 0.001–10 s⁻¹. The progress of dynamic recrystallization (DRX) was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the reciprocal of the time corresponding to the DRX fraction of 0.5% or 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. The Zener-Hollomon parameter (Z) was found to be inappropriate for analysis of DRX kinetics. Therefore, the dynamic recrystallization rate parameter (DRXRP) was introduced for this purpose. The DRXRP may be determined readily from the Avrami analysis and can precisely predict the rate of DRX at hot working conditions.     

Dynamic recrystallization behavior of Ti–5Al–5Mo–5V–1Cr–1Fe alloy

Dynamic recrystallization process was considered as an important method to fabricate titanium workpieces with desired properties. The objective of this study was to investigate dynamic recrystallization behavior of Ti–5Al–5Mo–5V–1Cr–1Fe alloy through isothermal compression experiments. The volume fraction of dynamic recrystallization was quantified with the net softening effect by dynamic recrystallization (DRX). The saturated stress during hot deformation process was analyzed based on dislocation evolution influenced by working-hardening and dynamic recovery. The linear relationship between the saturated stress and peak stress has been obtained. The dependence of DRX process on deformation parameters has been discussed in detail and a model based on Avrami kinetics has been proposed to track DRX process with strain. A constitutive model incorporating DRX process has been proposed to describe the flow curves at large strains.     

Dynamic and metadynamic recrystallization of Hastelloy X superalloy

Hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹ were performed on Hastelloy X superalloy in order to investigate the kinetics of hot deformation. An Arrhenius-type equation was used to characterize the dependence of the flow stress on deformation temperature and strain rate. The results showed that dynamic recrystallization (DRX) as well as metadynamic recrystallization (MDRX) occurred during hot working. A novel technique has been developed for calculating the DRX kinetics parameters on the basis of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isothermal transformation rate equations. The variation of grain size in the DRX and MDRX regimes correlated with the standard Zener–Hollomon parameter.     

Effect of stacking fault energy on the dynamic recrystallization during hot working of FCC metals: A study using processing maps

The influence of stacking fault energy (SFE) on the mechanism of dynamic recrystallization (DRX) during hot deformation of FCC metals is examined in the light of results from the power dissipation maps. The DRX domain for high SFE metals like Al and Ni occurred at homologous temperature below 0·7 and strain rates of 0·001 s⁻¹ while for low SFE metals like Cu and Pb the corresponding values are higher than 0·8 and 100 s⁻¹. The peak efficiencies of power dissipation are 50% and below 40% respectively. A simple model which considers the rate of interface formation (nucleation) involving dislocation generation and simultaneous recovery and the rate of interface migration (growth) occurring with the reduction in interface energy as the driving force, has been proposed to account for the effect of SFE on DRX. The calculations reveal that in high SFE metals, interface migration controls DRX while the interface formation is the controlling factor in low SFE metals. In the latter case, the occurrence of flow softening and oscillations could be accounted for by this model.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Dynamic Recrystallization of a Cr-Ni-Mo-Cu-Ti-V Precipitation Hardenable Stainless Steel

In this research, the dynamic recrystallization （DRX） behavior of an as-cast precipitation hardenable （PH） stainless steel was investigated by conducting hot compression tests at temperatures between 950-1150℃ and under strain rates of 0.001-1 s^-1. The flow stress curves show that the DRX is responsible for flow softening during hot compression. The effects of temperature and strain rate on the strain and stress corresponding to peak point （εp and σp） of flow curve were analyzed individually. It is realized that, they increase with strain rate and decrease with temperature. The relationship between Zener-Hollomon parameter （Z） and εp was investigated and the equation of εp=4.3×10^-4^0.14 was proposed. The strain for the maximum rate of DRX （εmax） was determined under different deformation conditions. Therefore, it is realized that it increases with Z parameter and vise versa. On the basis of obtained results, the equation of εmax=9.5 × 10^-4Z0.12 was proposed.     

Microstructural evolution of a Ni-Co based superalloy during hot compression at γ'sub-/super-solvus temperatures

The effects of strain rate on the microstructural evolution and deformation mechanism of a Ni-Co based superalloy were investigated by isothermal compression tests performed at γ'sub-solvus(1090℃)andγ'super-solvus temperatures(1150℃)with a wide strain rate range from 0.001 to 10 s-1 under a true strain of 0.693.Electron backscatter diffraction(EBSD),electron channeling contrast imaging(ECCI)and transmission electron microscope(TEM)techniques were used to characterize the microstructures.The results revealed that the dynamic recrystallization(DRX)volume fraction increased and stored energy of the γ matrix grains decreased with increasing the strain rate during γ'sub-solvus temperature defor-mation,while the opposite phenomena were observed during γ'super-solvus temperature deformation.The comprehensive effect of initial grain size,primary y'phase,twins and adiabatic temperature rise led to these results.The primary γ'particles undergone the deformation behavior within itself and obviously accelerated the DRX of the matrix.The microstructural evolution proved that discontinuous dynamic recrystallization(DDRX)was the dominant mechanism during the hot deformation carried out at both γ'sub-solvus and γ'super-solvus temperatures.Primary γ'particles obviously accelerated the nucleation step and retarded the growth step of DDRX during γ'sub-solvus temperature deformation.Besides,the acceleration effect of primary γ'particles on DDRX increased with the increase of strain rate.Continuous dynamic recrystallization(CDRX)was confirmed to be an assistant mechanism during γ'super-solvus temperature deformation and was promoted with the increase of strain rate.     

TB6钛合金β区变形的动态再结晶动力学

使用圆柱形TB6钛合金试样在Thermecmaster-Z型热模拟试验机上进行热模拟压缩实验(变形温度为825~1100℃,应变速率为0.001~1 s^-1)。对采集的流变数据进行加工硬化率处理,确定动态再结晶体积分数,研究了TB6钛合金β区变形的动态再结晶动力学。结果表明,流变应力随着变形温度的降低或应变速率的提高而增大,流变曲线呈现出动态再结晶类型的特征。随着应变速率的降低和变形温度的提高,动态再结晶的体积分数和晶粒尺寸增大。在变形温度高于950℃、应变速率低于0.001 s^-1条件下,动态再结晶的晶粒严重粗化。动态再结晶动力学曲线经历缓慢增加—快速增加—缓慢增加三个阶段,呈现出典型的“S”型特征。确定了动态再结晶的体积分数达到50%时的应变,建立了TB6钛合金的动态再结晶动力学模型。     

新型奥氏体耐热钢CHDG-A的动态再结晶行为及其动力学模型

进行新型奥氏体耐热钢(CHDG-A)的热压缩实验,研究了在900~1100℃、应变速率为0.01-10 s^-1条件下这种钢的热变形特征。结果表明:随着变形温度的提高或应变速率的降低这种钢的流变应力显著降低。基于Arrhenius模型构建了这种材料的本构方程,得到CHDG-A热变形激活能Q为515.618 kJ/mol。微观组织分析结果表明,动态再结晶(DRX)是该材料在实验热变形条件下最主要的软化方式,DRX形核主要通过晶界弓出,变形温度的升高和应变速率降低均有利于再结晶形核。基于真应力-应变曲线求得动态再结晶用Z参数表示的峰值和临界值(σ_p、ε_p、σ_c和ε_c),并确定了ε_c/ε_p,σ_c/σ_p的比值分别为0.52和0.98。同时,还基于Avrami方程建立了CHDG-A的DRX动力学模型。     

Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300–550 °C and 0.001–1 s^–1, respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg₂Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300–350 °C and at strain rate 1 s^–1.     

Finite element modelling on microstructure evolution during multi-pass hot compression for AZ31 alloys using incremental method

Based on the principle of piecewise linearization, the incremental forms of microstructure evolution models were integrated into the thermo-mechanical coupled finite element(FE) model to simulate nonlinear microstructure evolution during multi-pass hot deformation. This is an unsteady-state deformation where dynamic recrystallization(DRX), meta-dynamic recrystallization(MDRX), static recrystallization(SRX) and grain growth(GG) take place during hot deformation or deformation interval. The distributions of deformation and microstructure for cylindrical AZ31 sample during single-pass and double-pass hot compressions were quantitatively calculated and compared with the metallographic observation. It is shown that both the deformation and microstructure are non-uniformly distributed due to the presence of friction between the die and the flat end of sample. The average grain size and its standard deviation under the double-pass hot compression are slightly smaller than those under single-pass compression.The simulated average grain sizes agree well with the experiments, which validates that the developed FE model on the basis of incremental forms of microstructure evolution models is reasonable.