基于流动应力曲线的AZ80镁合金动态再结晶动力学表征

热物理模拟获得了铸态AZ80镁合金热压缩真应力-应变曲线,以此作为计算动态再结晶体积分数演变的底层数据.通过求解流动应力的双曲正弦表征模型,获得了本构模型与动态再结晶激活能等重要参数.分析峰值应力出现之前与之后的硬化曲线,识别了真应力-应变曲线所隐含的关键应变点:临界应变,峰值应变及最大软化速率应变.进一步引入表征晶体...     

Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.     

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1 s⁻¹ at 400 °C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200 s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500 s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.     

Prediction of the critical conditions for initiation of dynamic recrystallization

Both the critical stress and strain for initiation of dynamic recrystallization (DRX) were determined using: (1) the strain hardening rate versus stress curve, (2) the natural logarithm of strain hardening rate versus strain curve, and (3) the constitutive equations. In order to perform these analyses, the behavior of a 17-4 PH stainless steel during hot compression test was investigated at temperatures of 950–1150 °C and strain rates of 0.001–10 s⁻¹. The first and second methods were found to be the best ones for determining the critical stress and strain, respectively. The Cingara constitutive equation was also used to model the flow curves up to the peak point and subsequently was used for predicting the critical strain. In summary, for 17-4 PH stainless steel, the DRX was found to start when the normalized stress and strain reach to the values of 0.89 and 0.47, respectively.     

Constitutive modeling and prediction of hot deformation flow stress under dynamic recrystallization conditions

Simple modeling approaches based on the Hollomon equation, the Johnson–Cook equation, and the Arrhenius constitutive equation with strain-dependent material’s constants were used for modeling and prediction of flow stress for the single-peak dynamic recrystallization (DRX) flow curves of a stainless steel alloy. It was shown that the representation of a master normalized stress–normalized strain flow curve by simple constitutive analysis is successful in modeling of high temperature flow curves, in which the coupled effect of temperature and strain rate in the form of the Zener–Hollomon parameter is considered through incorporation of the peak stress and the peak strain into the formula. Moreover, the Johnson–Cook equation failed to appropriately predict the hot flow stress, which was ascribed to its inability in representation of both strain hardening and work softening stages and also to its completely uncoupled nature, i.e. dealing separately with the strain, strain rate, and temperature effects. It was also shown that the change in the microstructure of the material at a given strain for different deformation conditions during high-temperature deformation is responsible for the failure of the conventional strain compensation approach that is based on the Arrhenius equation. Subsequently, a simplified approach was proposed, in which by correct implementation of the hyperbolic sine law, significantly better consistency with the experiments were obtained. Moreover, good prediction abilities were achieved by implementation of a proposed physically-based approach for strain compensation, which accounts for the dependence of Young’s modulus and the self-diffusion coefficient on temperature and sets the theoretical values in Garofalo’s type constitutive equation based on the operating deformation mechanism. It was concluded that for flow stress modeling by the strain compensation techniques, the deformation activation energy should not be considered as a function of strain.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31

Texture development in magnesium alloy AZ31 was studied by uniaxial compression tests at temperatures, strain rates and final strains ranging from 573 to 773 K, 1.0 × 10⁻³ to 5.0 × 10⁻⁵ s⁻¹ and −0.2 to −1.5, respectively. Fiber texture was formed in all of the deformation conditions. The main component of the texture varied depending on deformation conditions; it appeared about 33–38° away from the basal pole after the deformation at higher temperatures and lower strain rates. This can be attributed to the increased activity of the secondary pyramidal slip system. With a decrease in temperatures and an increase in strain rate, the tilting angle of the main component (compression plane) from the basal pole decreased down to about 20°. Construction of a basal fiber texture was detected after deformations at the lowest temperature and high strain rates.     

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.     

Determination of the critical conditions for the initiation of dynamic recrystallization in boron microalloyed steels

From the present research, the critical conditions associated with the onset of dynamic recrystallization (DRX) of hot deformed boron microalloyed steels were precisely determined based on changes in the strain hardening rate (θ) as a function of the flow stress. For this purpose, a low carbon steel microalloyed with four different amounts of boron (29, 49, 62 and 105 ppm) was deformed by uniaxial hot-compression tests at high temperature (950, 1000, 1050 and 1100 °C) and constant true strain rate (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Results indicate that the critical conditions for the initiation of dynamic recrystallization depend on the temperature and strain rate. In addition, both critical stress σ_c, and critical strain ?_c, were noticed to decrease as boron content increased. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. The critical ratios σ_c/σ_p and ?_c/?_p for all boron microalloyed steels remain fairly constant (≈0.82 and ≈0.53, respectively), such values are in agreement with those commonly reported for Al-killed, C-Mn, Nb, Nb-Ti, high carbon and stainless steels.     

Multi-pass large strain rolling of as-cast AZ31 magnesium alloy

As-cast AZ31 magnesium alloy subjected to multi-pass large strain rolling was investigated. A successive rolling process up to three passes was carried out at 370°C with a pass reduction of 30%. Deformation microstructure characteristics prove that the dynamic recrystallisation (DRX) mode changed with the increase of rolling passes. In the first pass, DRX related to twinning played a dominant role. But in the third pass, DRX grains mainly appeared around the pre-existing grain boundaries. The ultimate strength and elongation of rolled sheets after three passes rolling are enhanced by 37 and 39%, respectively, compared to the as-cast alloy. Meanwhile, the tensile fracture mode was ductile fracture which was different from the ductile–brittle fracture of as-cast.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

Influences of tensile pre-strain and bending pre-deflection on bending and tensile behaviors of an extruded AZ31B magnesium alloy

This paper focused on the influences of tensile pre-strain and bending pre-deflection on the three point bending and uniaxial tensile properties of an extruded AZ31B magnesium alloy. The influences of pre-strain/deflection on bending/tensile curves could be divided into three stages. The results show that: (1) In the elastic stage, considering the variation of specimen’s cross sectional area, the pre-strain/deflection did not affect the measured elastic modulus obtained from both bending and tensile tests. (2) In the transition hardening stage, the specimen presented obvious hardening behaviors on basis of the pre-strain/deflection, the phenomenon was mainly caused by the strain hardening effects produced from previous uniaxial tensile and bending processes. (3) In the large plastic deformation stage/necking stage, as the accumulation of plastic deformations caused by pre-strain/deflection were significant, the specimen’s ability to resist plastic deformation was weakened. Specially, as the tensile pre-strain increased, the bending load decrement rate gradually decreased, and as the bending pre-deflection increased, both the tensile strength and elongation sharply decreased, the accumulated irreversible plastic work promoted the damage process of the magnesium alloy. The influences of tensile pre-strain on the bending behaviors of the magnesium alloy were also analyzed via finite element method.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Anisotropic and tensile flow behaviors of Mg alloy AZ31B thin sheet in H24 condition at elevated temperatures

In this work, a series of experiments was performed to explore the effects of anisotropy, strain rate, and temperature on microstructure change and associated mechanical response of a rolled AZ31B-H24 Mg alloy sheet under tension. Tensile tests were carried out on specimens in the 0, 45, and 90° to the rolling direction, using initial strain rates in the range of 4 × 10⁻³ to 1 × 10⁻¹ s⁻¹ at temperatures of 250 and 370 °C. Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. Resultant microstructures, such as degree of dynamic recrystallization, grain growth, and shape of the grain, were associated with temperature, strain rate, and tensile loading direction. The initial texture influenced the variations in changes in microstructure and mechanical properties upon testing in different directions. The specimens upon testing in the 45° to the rolling direction yielded higher m-value, lower strength, and greater elongation to failure under all test conditions.     

30%Cr-Cu复合材料热变形及动态再结晶临界条件

采用放电等离子烧结法(SPS)制备出30%Cr-Cu复合材料,对其致密度、硬度和导电率等相关性能进行测试,并观察分析该复合材料的显微组织。利用Gleeble-1500D型热模拟试验机在变形温度650~950℃、应变速率0.001~10s-1、变形量60%的条件下对30%Cr-Cu复合材料进行热模拟压缩试验。对热压缩试验得到的真应力-应变数据进行拟合、计算和分析,构建该复合材料的本构方程,同时得到材料的加工硬化率θ,利用材料的lnθ-ε曲线出现有拐点和-(lnθ)/ε-ε曲线对应有最小值这一判据,分析该复合材料的动态再结晶临界条件。结果表明:30%Cr-Cu复合材料的真应力-应变曲线主要以动态再结晶软化机制为特征,峰值应力随应变速率的增加和温度的降低而升高;该复合材料的lnθ-ε曲线出现拐点,-(lnθ)/ε-ε曲线对应有最小值,该最小值所对应的应变为临界应变εc,且εc随变形温度的升高和应变速率降低而减小,εc与Zener-Hollomon参数Z的函数关系为εc=2.38×10-3 Z0.1396。     

Ti-6Al-2Zr-1Mo-1V合金热变形激活动态再结晶的临界条件识别及表征

热压缩实验获得Ti-6Al-2Zr-1Mo-1V合金在温度1073～1323K,应变速率0.01～10s-1条件下的真应力-应变曲线,以此作为识别及表征动态再结晶临界条件的底层数据。对比分析流变应力曲线发现高温、低应变速率下动态回复型软化态势显著;低温、高应变速率下动态再结晶型软化态势显著。引入材料加工硬化率θ,结合θ-σ曲线拐点判据识别了流变应力曲线隐含表征激活动态再结晶的特征参量:临界应变、临界应力。采用含动态再结晶激活能Q的Arrhenius方程求得α、β、n1、n2等材料常数并获得该合金动态再结晶激活能对应变速率及温度的响应图。进一步引入表征动态再结晶临界条件的临界应变模型,获得了临界应变与各热力参数之间的数学关系,验证表明该临界模型预测精度最大为12.9%。     

变形镁合金AZ80的腐蚀疲劳机理

根据挤压镁合金AZ80人工时效热处理(T5-177℃,16 h)前后分别在空气和NaCl介质中的疲劳寿命,研究了变形镁合金的腐蚀疲劳机理以及β相在腐蚀疲劳中的作用.结果表明:时效可导致AZ80组织β相体积分数增加、拉伸强度和硬度提高,可明显地提高在低应力水平下的腐蚀疲劳寿命.在空气中,疲劳裂纹萌生于表层和亚表面中的夹杂物;而在腐蚀介质中,腐蚀疲劳微裂纹萌生于试样表面的腐蚀坑,点蚀坑萌生于与β相相邻的α相.疲劳断口可见河流花样、二次裂纹、韧窝,具有解理特征.阳极溶解是挤压镁合金AZ80的腐蚀疲劳机制.     

Rapid evaluation of fatigue limit in AZ31B magnesium alloy using acoustic emission

Acoustic emission (AE) was used in a fatigue experiment to characterise AE signals and to rapidly determine the fatigue limit of AZ31B magnesium (Mg) alloy. The AE signals during fatigue were characterised according to waveforms and frequency. Meanwhile, the energy dissipation in the process of fatigue, which was represented by the accumulative AE energy, can be used to determine the fatigue limit. Based on the AE parameters, the fatigue limit was 97?MPa, with an 8% error value when compared with the results obtained by the conventional S–N curve method. This model only requires the accumulated energy of the signals during strain hardening. Therefore, the fatigue limit can be determined rapidly.     

Influence of undissolved second-phase particles on dynamic recrystallization behavior of Mg-7Sn-1Al-1Zn alloy during low-and high-temperature extrusions

This study investigates the effects of fine and coarse undissolved particles in a billet of the Mg-7Sn-1Al-1Zn (TAZ711) alloy on the dynamic recrystallization (DRX) behavior during hot extrusion at low and high temperatures and the resultant microstructure and mechanical properties of the alloy.To this end,partially homogenized (PH) and fully homogenized (FH) billets are extruded at temperatures of 250 and 450 ℃.The PH billet contains fine and coarse undissolved Mg2Sn particles in the interdendritic region and along the grain boundaries,respectively.The fine particles (＜1 μm in size) retard DRX during extrusion at 250 ℃ via the Zener pinning effect,and this retardation causes a decrease in the area fraction of dynamically recrystallized (DRXed) grains of the extruded alloy.In addition,the inhomogeneous distribution of fine particles in the PH billet leads to the formation of a bimodal DRXed grain structure with excessively grown grains in particle-scarce regions.In contrast,in the FH billet,numerous nanosized Mg2Sn precipitates are formed throughout the material during extrusion at 250 ℃,which,in turn,leads to the formation of small,uniform DRXed grains by the grain-boundary pinning effect of the precipitates.When the PH billet is extruded at the high temperature of 450 ℃,the retardation effect of the fine particles on DRX is weakened by their dissolution in the α-Mg matrix and the increased extent of thermally activated grain-boundary migration.In contrast,the coarse Mg2Sn particles in the billet promote DRX during extrusion through the particle-stimulated nucleation phenomenon,which results in an increase in the area fraction of DRXed grains.At both low and high extrusion temperatures,the extruded material fabricated using the PH billet,which contains both fine and coarse undissolved particles,has a lower tensile strength than that fabricated using the FH billet,which is virtually devoid of second-phase particles.This lower strength of the former is attributed mainly to the larger grains and/or absence of nanosized M2Sn precipitates in it.