Aluminium alloys have been the primary material for the structural parts of aircraft for more than 80years because of their well known performance, well established design methods, manufacturing and reliable inspection techniques. Nearly for a decade composites have started to be used more widely in large commercial jet airliners for the fuselage, wing as well as other structural components in place of aluminium alloys due their high specific properties, reduced weight, fatigue performance and corrosion resistance. Although the increased use of composite materials reduced the role of aluminium up to some extent, high strength aluminium alloys remain important in airframe construction. Aluminium is a relatively low cost, light weight metal that can be heat treated and loaded to relatively high level of stresses, and it is one of the most easily produced of the high performance materials, which results in lower manufacturing and maintenance costs. There have been important recent advances in aluminium aircraft alloys that can effectively compete with modern composite materials. This study covers latest developments in enhanced mechanical properties of aluminium alloys, and high performance joining techniques. The mechanical properties on newly developed 2000, 7000 series aluminium alloys and new generation Al i alloys are compared with the traditional aluminium alloys. The advantages and disadvantages of the joining methods, laser beam welding and friction stir welding, are also discussed.

评述了铝锂合金的发展历程和应用以及我国铝锂合金研究所取得的成果。阐述了第3代铝锂合金的成分、微观组织和性能特点,讨论了合金化元素在调控铝锂合金微观组织进而改善性能方面的作用。大量研究表明,这些合金化元素可改变铝锂合金中原有析出相的大小、形状、分布,或刺激新强化相的析出,也可以细化晶粒、控制再结晶和晶粒取向。时效析出过程的本质和动力学在很大程度上取决于合金化元素之间的交互作用和合金原子的团簇化过程。

评述了铝锂合金的发展历程和应用以及我国铝锂合金研究所取得的成果。阐述了第3代铝锂合金的成分、微观组织和性能特点,讨论了合金化元素在调控铝锂合金微观组织进而改善性能方面的作用。大量研究表明,这些合金化元素可改变铝锂合金中原有析出相的大小、形状、分布,或刺激新强化相的析出,也可以细化晶粒、控制再结晶和晶粒取向。时效析出过程的本质和动力学在很大程度上取决于合金化元素之间的交互作用和合金原子的团簇化过程。

采用G1eeble-1500热模拟实验机，对2195铝锂合金变形温度为360～520℃，应变速率为0．001-1．0s^-1的单道次热压缩及变形温度为320℃和360℃，应变速率为0．1s^-1，道次间隔时间30-180s的双道次热压缩的流变应力及静态软化规律进行了模拟研究。通过对幂指数应力函数中系数A和β与应变关系的分析，以及采用平均软化法考虑前一道次变形的残余应变对后一道次变形的影响，建立了2195铝锂合金多道次热变形的流变应力方程。

采用G1eeble-1500热模拟实验机，对2195铝锂合金变形温度为360～520℃，应变速率为0．001-1．0s^-1的单道次热压缩及变形温度为320℃和360℃，应变速率为0．1s^-1，道次间隔时间30-180s的双道次热压缩的流变应力及静态软化规律进行了模拟研究。通过对幂指数应力函数中系数A和β与应变关系的分析，以及采用平均软化法考虑前一道次变形的残余应变对后一道次变形的影响，建立了2195铝锂合金多道次热变形的流变应力方程。

61The experimental evidence for the DPN of σ phase in Al–Cu–Li alloys was found.61The PSN-DRX due to the large Al–Cu–Mn particle was observed.61A novel method was proposed to calculated the stress multiplier α.

在Gleeble 15 0 0热模拟实验机上 ,采用高温等温压缩 ,0 .0 0 1～ 10s-1,变形温度为 36 0～ 5 2 0℃ ,对2 195铝锂合金在高温压缩变形中的流变应力行为进行了研究 ,分析了其高温变形的物理本质。结果表明 :在应变速率为 1s-1(变形温度为 5 2 0℃ )和应变速率为 0 .1、0 .0 1、0 .0 0 1s-1(变形温度为 36 0～ 5 2 0℃ )时 ,2 195铝锂合金热压缩变形流变应力出现了明显的峰值应力 ,表现为连续动态再结晶特征 ;在其它变形条件下存在较为明显的稳态流变特征 ;可采用Zener Hollomon参数的双曲正弦函数来描述 2 195铝锂合金高温变形时的流变应力行为 ;在获得的流变应力σ解析表达式中 ,A、α和n值分别为 2 .5 6 9× 10 17s-1、0 .0 12 4 8MPa-1和 5 .94 ;热变形激活能Q为 2 5 0 .4 5kJ/mol。

在Gleeble 15 0 0热模拟实验机上 ,采用高温等温压缩 ,0 .0 0 1～ 10s-1,变形温度为 36 0～ 5 2 0℃ ,对2 195铝锂合金在高温压缩变形中的流变应力行为进行了研究 ,分析了其高温变形的物理本质。结果表明 :在应变速率为 1s-1(变形温度为 5 2 0℃ )和应变速率为 0 .1、0 .0 1、0 .0 0 1s-1(变形温度为 36 0～ 5 2 0℃ )时 ,2 195铝锂合金热压缩变形流变应力出现了明显的峰值应力 ,表现为连续动态再结晶特征 ;在其它变形条件下存在较为明显的稳态流变特征 ;可采用Zener Hollomon参数的双曲正弦函数来描述 2 195铝锂合金高温变形时的流变应力行为 ;在获得的流变应力σ解析表达式中 ,A、α和n值分别为 2 .5 6 9× 10 17s-1、0 .0 12 4 8MPa-1和 5 .94 ;热变形激活能Q为 2 5 0 .4 5kJ/mol。

The high-temperature deformation behaviour and microstructure evolution of an extruded Al-Cu-Li alloy were investigated by compression tests conducted at various temperatures (613, 673 and 733K) with various strain rates (0.001, 0.01, and 0.1s-1). The results indicated that the deformation activation energy increased from 208.7kJ/mol to 255.7kJ/mol with an increase in strain from 0.1 to 0.7. The electron backscatter diffraction maps indicated that a dynamic recrystallisation occurred during the high-temperature deformation. Two types of recrystallisation mechanisms, grain boundary bulging and a grain boundary transformation from low misorientation to high misorientation, were considered as the mechanisms for controlling the formation of the recrystallised grains. A new dynamic recrystallisation model containing these two mechanisms was proposed to describe the microstructure evolution of the extruded Al-Cu-Li alloy. At the early stage of the deformation, the recrystallised grains were formed by grain boundary bulging along the original grain boundaries. With increasing strain, recrystallised grains were gradually generated in the deformed grains due to the transformation from low angle boundaries to high angle boundaries.

Erratum to: METALLURGICAL AND MATERIALS TRANSACTIONS A, Vol. 40A, No. 4, April 2009, pp. 982–90 DOI: 10.1007/s11661-009-9783-7 In Table02I of the original article, the weight percentages of P and S are incorrect. The correct weight percent of P is 0.002, and the correct weight percent of S is 0.0047.

介绍一种新型大试样平面应变压缩技术。在总结目前模拟技术不足的基础上,提出了大试样平面应变压缩技术的实验方法,设计了整机的结构,阐述了设备的特点与功能。通过工控机对温度、压力以及位移的数据采集,可以实现热轧带钢的不同工艺参数模拟。最后以纯铁为实验,实现了大试样多道次平面应变过程。实验表明:这种大试样平面应变技术能满足材料开发以及工艺优化对组织和力学性能分析的要求。

介绍一种新型大试样平面应变压缩技术。在总结目前模拟技术不足的基础上,提出了大试样平面应变压缩技术的实验方法,设计了整机的结构,阐述了设备的特点与功能。通过工控机对温度、压力以及位移的数据采集,可以实现热轧带钢的不同工艺参数模拟。最后以纯铁为实验,实现了大试样多道次平面应变过程。实验表明:这种大试样平面应变技术能满足材料开发以及工艺优化对组织和力学性能分析的要求。

The classical kinetics models of dynamic recrystallization (DRX) in the form of Avrami function describe the development of DRX process to a large extent; however, because of the characteristics of exponent function, the conventional models cannot exactly exhibit the development speed of DRX process. Based on this analysis, a new kinetics model of DRX was proposed, which represents the ‘slow–rapid–slow’ property of DRX development. According to the new model, the development process of DRX can be divided into three phases: slow-beginning phase, rapid-increasing phase and slow-rising-to-balance phase. Because the turning point between the second phase and the third one corresponds to the inflexion from the faster velocity of DRX development to the slower one, the strain at this moment can be considered as the most appropriate and economic strain that guarantees fine grains and saves energy consumption. Take a typical metal characterized by DRX magnesium alloy AZ31B for instance, the Gleeble-1500 thermomechanical simulation compression tests were conducted together with microscopic examination, according to which the model parameters were determined. Statistics shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.

通过拉伸力学性能测试和断口形貌分析,研究了2195铝锂合金冷轧薄板的各向异性随时效时间变化的规律。结果发现:在150℃时效条件下,峰值时效前,随时效时间的增加,合金的各向异性程度逐渐下降,过时效时合金的各向异性比峰值时效时有所增加;延伸率各向异性大于强度各向异性。150℃/24h时效时,与轧向成0°和45°方向试样的断裂机理不同,存在断裂方式的各向异性。最后对该合金各向异性的主要形成机理进行了初步探讨。

通过拉伸力学性能测试和断口形貌分析,研究了2195铝锂合金冷轧薄板的各向异性随时效时间变化的规律。结果发现:在150℃时效条件下,峰值时效前,随时效时间的增加,合金的各向异性程度逐渐下降,过时效时合金的各向异性比峰值时效时有所增加;延伸率各向异性大于强度各向异性。150℃/24h时效时,与轧向成0°和45°方向试样的断裂机理不同,存在断裂方式的各向异性。最后对该合金各向异性的主要形成机理进行了初步探讨。

After a description of the evolution of Al-Li alloys, this paper first reviews the subject of anisotropy in mechanical properties of Al-Li wrought products. In plane, through thickness and axisymmetric flow anisotropies present in Al-Li wrought products are defined. The root causes for these anisotropies are discussed. Pros and cons arising from anisotropic products are then discussed in the context of designing and manufacturing for space and aerospace structures. Numerous attempts aimed to reduce anisotropy in rolled and extruded products are summarized. It is concluded that `isotropic' Al-Li products can be manufactured via the selection of appropriate process, composition and processing parameters

The evolution of the new microstructures produced by two types of dynamic recrystallization is reviewed, including those brought about by severe plastic deformation (SPD). The microstructural changes taking place under these conditions and the associated mechanical behaviors are described. During the conventional discontinuous dynamic recrystallization (dDRX) that takes place at elevated temperatures, the new grains evolve by nucleation and growth in materials with low to medium stacking fault energies (SFE). On the other hand, new ultrafine grains can be produced in any material irrespective of the SFE by means of SPD at relatively low temperatures. These result from the gradual transformation of the dislocation sub-boundaries produced at low strains into ultrafine grains with high angle boundaries at large strains. This process, termed in situ or continuous dynamic recrystallization (cDRX), is still not perfectly understood. This is because many SPD methods provide data concerning the microstructural changes that take place but little information regarding the flow stress behavior. By contrast, multi-directional forging (MDF) provides both types of data concurrently. Recent studies of the deformation behavior of metals and alloys under SPD conditions, carried out using MDF as well as other SPD methods, are synthesized and the links between the microstructural and mechanical observations are examined carefully. Some models for grain formation under SPD conditions are discussed. Next, the post-dynamic recrystallization behavior, i.e. that of annealing after both dDRX and cDRX, is described. The differing annealing behaviors result from the differences in the natures of the deformed microstructures. Finally, an integrated recrystallization model for these phenomena, i.e. dynamic and static recrystallization of both the continuous and discontinuous types, is presented and discussed.

Hot deformation behavior and microstructure evolution rule of as-extruded 7075 aluminum alloy have been investigated based on the thermal compression experiment and quantitative metallographic test. It was found that the dynamic recovery was the main softening mechanism, along with the dynamic recrystallization in the local area within the deformation temperature range of 300-450 degrees C and strain rate range of 0.01-10 s(-1). Deforming below 350 degrees C, only dynamic recovery occurred with a typical subgrain structure, above 350 degrees C, dynamic recrystallization happened in local area. The dynamic recrystallized grain size increased with the increase of temperature, and decreased as strain rate increasing, showing typical characteristics of continuous dynamic recrystallization. At a higher deformation temperature and a lower strain rate, geometric dynamic recrystallizetion occurred. A larger strain resulted in more dynamic recrystallization. The second phase particles in the alloy would hinder the slip of grain boundaries, while would promote continuous dynamic recrystallization. (c) 2014 Elsevier Inc. All rights reserved.

The hot deformation behavior of Al-Cu-Li alloy was studied under plane strain compression in the temperature range of 400–50002°C and strain rate of 0.01–1002s 61021 . The related microstructure was studied by optical microscopy, electron back-scattered diffraction and transmission electron microscope. The results showed that the stress decreased significantly with the increase of temperature and decrease of strain rate. However, stress increase was found after peak stress at strain rate of 0.0102s 61021 and deformation temperature 440–50002°C. Constitutive equation based on the hyperbolic sine equation was established and the apparent activation energy of plane strain compression was estimated to be 226.702KJ/mol. Precipitates decreased and finally disappeared with decreasing Z value, having an impact on the softening mechanism. Plate-shaped particles and precipitate zones restrained the proceeding of dynamic recovery but increased store energy for discontinuous dynamically recrystallization nuclei. Therefore, recrystallization nuclei more obviously occurred in the sample with higher ln Z value than the one with lower ln Z value at close ln Z value. The microstructure of all deformed specimens was composed of elongated grains and new quasi-equiaxed grains. However, the basic deformation mechanism was dynamic recovery. The mechanism of recrystallization nuclei transformed discontinuous dynamic recrystallization into continuous dynamic recrystallization with the decreasing Z values.

The flow behavior, microstructure evolution and softening mechanism of an AA7085 aluminum alloy are investigated by isothermal hot compression tests at 450°C with the strain rates of 0.001s611and 0.1s611. Optical microscopy (OM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) are used to characterize microstructure evolution during deformation. The results reveal that the flow stress curves exhibit a single peak and then two types of flow stress curves are found in the present study. Microstructure characterization reveals that dynamic recovery (DRV) and recrystallization (DRX) occur in the both conditions. The microstructure discrepancy becomes significant with increasing strain rate after a certain strain level (ε≥0.3). In the case of 0.001s611, the dynamic recovery is thought to precede continuous dynamic recrystallization. As the strain rate increased, the absence of dynamic recovery provides enough stored energy for discontinuous dynamic recrystallization (DDRX), resulting in a retardation in the occurrence of continuous dynamic recrystallization (CDRX).

61Processing map is established for an as-extruded AA2099 Al-Li alloy.61Stored energy calculation is performed to discriminate restoration mechanisms.61DDRX is confirmed to occur in specimens with low Zenner-Holloman parameters.61Power dissipation efficiency is demonstrated to relate to the restoration mechanisms.

对Al-Cu-Li合金进行温度300~500℃、应变速率0.001~10s^-1的等温热压缩,分析合金的流变行为：结合TEM和EBSD研究合金热变形过程中的组织演变。结果表明：合金流变曲线分为3个阶段：加工硬化阶段、过渡阶段和稳态变形阶段;变形温度越高,流变应力达到动态平衡所需应变量越小。基于应变硬化率（θ）与流变应力（σ）之间的关系,确定动态再结晶的临界应变（ε_c）;不同热变形条件下的临界应变（ε_c）与峰值应变（ε_p）之比为0.30342~0.92828;临界应力（σ_c）与峰值应变（σ_p）之比为0.88492~0.99782。引入最大软化率应变（ε^＊）和中间变量Z/A,建立ε_c和ε~＊与Z/A的关系表达式。构建Al-Cu-Li合金动态再结晶动力学模型,模型表明,温度越高或应变速率越低,越有利于促进动态再结晶分数的增加;显微组织分析结果与模型预测规律一致。Al-Cu-Li合金动态再结晶形核机制主要为晶界突出形核机制、亚晶合并长大机制以及粒子促进形核机制,随温度升高和应变速率的降低,晶内亚晶合并长大机制得到加强。

对Al-Cu-Li合金进行温度300~500℃、应变速率0.001~10s^-1的等温热压缩,分析合金的流变行为：结合TEM和EBSD研究合金热变形过程中的组织演变。结果表明：合金流变曲线分为3个阶段：加工硬化阶段、过渡阶段和稳态变形阶段;变形温度越高,流变应力达到动态平衡所需应变量越小。基于应变硬化率（θ）与流变应力（σ）之间的关系,确定动态再结晶的临界应变（ε_c）;不同热变形条件下的临界应变（ε_c）与峰值应变（ε_p）之比为0.30342~0.92828;临界应力（σ_c）与峰值应变（σ_p）之比为0.88492~0.99782。引入最大软化率应变（ε^＊）和中间变量Z/A,建立ε_c和ε~＊与Z/A的关系表达式。构建Al-Cu-Li合金动态再结晶动力学模型,模型表明,温度越高或应变速率越低,越有利于促进动态再结晶分数的增加;显微组织分析结果与模型预测规律一致。Al-Cu-Li合金动态再结晶形核机制主要为晶界突出形核机制、亚晶合并长大机制以及粒子促进形核机制,随温度升高和应变速率的降低,晶内亚晶合并长大机制得到加强。

通过等温压缩实验,系统研究热变形参数(变形温度、应变速率及应变量)对7085铝合金热变形组织演变的影响。结果表明:升高变形温度以及降低应变速率,均有利于7085铝合金的动态再结晶发生,导致变形后的7085铝合金位错密度降低,再结晶晶粒尺寸增大;随着应变量的增加,变形后的合金位错密度降低,动态再结晶体积分数增大。采用线性回归方法建立包括峰值应变方程、临界应变方程、动态再结晶动力学方程以及动态再结晶晶粒尺寸方程的7085铝合金动态再结晶模型。

通过等温压缩实验,系统研究热变形参数(变形温度、应变速率及应变量)对7085铝合金热变形组织演变的影响。结果表明:升高变形温度以及降低应变速率,均有利于7085铝合金的动态再结晶发生,导致变形后的7085铝合金位错密度降低,再结晶晶粒尺寸增大;随着应变量的增加,变形后的合金位错密度降低,动态再结晶体积分数增大。采用线性回归方法建立包括峰值应变方程、临界应变方程、动态再结晶动力学方程以及动态再结晶晶粒尺寸方程的7085铝合金动态再结晶模型。

The hot deformation behavior of an Al-Cu-Li alloy was studied by isothermal compression in the temperature range of 573-773 K with strain rates of 0.01-10 s(-1) on a Gleeble-3500 thermo-mechanical simulator. The microstructural evolution during hot deformation was investigated by optical microscope (OM) and transmission electrical microscope (TEM). The results show that the flow stress increases with decreasing deformation temperature and increasing strain rate. Processing maps at the strains of 0.3-0.8 are obtained by isothermal compression data and exhibit two flow instability domains (573-648 K at 0.014-1.5 s(-1) and 698-773 K at 0.3-10 s(-1)) and two peak efficiency domains (723-773 K at 0.01-0.1 s(-1) and 573-623 K at 0.01-0.03 s(-1)). The strain has a significant effect on the peak efficiencies, and the peak efficiencies increase almost linearly as the strain increases. Dynamic recrystallization takes places at high temperatures (673-773 K) and low strain rates (0.01-0.1 s(-1)). The second phases precipitating dynamically during hot deformation pin dislocations and boundaries of subgrains and restrain the occurrence of dynamic recrystallization, resulting in a higher temperature for dynamic recrystallization. According to the processing maps and microstructural observation, the optimized processing condition of hot deformation for Al-Cu-Li alloy is at 723-773 K and 0.01-0.1 s(-1). Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

61Three types of dynamic recrystallization processes occurring during hot deformation are reviewed.61The mechanisms of these three types of dynamic recrystallization processes are discussed in detail.61Physically based numerical models for all the three dynamic recrystallization process are reviewed.61Topics for further investigation on dynamic recrystallization are recommended.

Serrated-flow characteristics have been studied in binary Al Li-alloys with different compositions. Load-elongation curves seem divided into two sections of which the first is weakly, the second heavily, serrated. The initial weak serrations, which diminish with ageing, arise probably from the interaction of dislocations with lithium atoms in solid solution. It is proposed that the larger serrations which appear later on the load-elongation curve (if the elongation to necking is large enough to allow them to develop), and which increase in size with progressive ageing, are connected with shearable -precipitates. The effects of zirconium and magnesium additions to Al Li-alloys on serrations have also been investigated. These elements do not change the basic mechanisms of serrated flow. However, the apparent activation energies as calculated for the alloy containing magnesium from the onset strains do not make much sense, because two elements contribute now to serrated flow, and the situation becomes complex.

Twenty-six flow curves determined under dynamic recrystallization (DRX) conditions on 11 different steels were analyzed. The double-differentiation method was used to define the critical strain ε c for initiation of DRX. The “athermal” hardening parameter h and the dynamic recovery parameter r were evaluated from the work-hardening behavior prior to ε c. The saturation stresses σ sat pertaining to the unrecrystallized regions as well as the associated work-hardening curves, σ recov, were derived in this way. The net softening attributable to DRX was then defined as the difference between the σ recov and experimental curves. Avrami plots were prepared from all 26 sets of data. The time exponents were all in the neighborhood of 3.0; however, the strain rate and temperature dependences differed considerably from their static values. The dependence of the time of half-softening, t 50, on strain rate and temperature is used to predict the DRX curves expected under industrial conditions.

An experiment has been designed to check a previously proposed equivalence of the effects of changes in strain rate and in temperature upon the stress‐strain relation in metals. It is found that this equivalence is valid for the typical steels investigated. The behavior of these steels at very high rates of deformation may, therefore, be obtained by tests at moderate rates of deformation performed at low temperatures. The results of such tests are described. Aside from changing the isothermal stress‐strain relation, an increase of strain rate tends to change the conditions from isothermal to adiabatic. It is found that at low temperatures, the adiabatic stress‐strain relation in the plastic range is radically different from the isothermal, having an initial negative rather than a positive slope. This initial negative slope renders unstable homogeneous plastic deformation.

Publication » On the mechanism of hot deformation.

通过Gleeble-3500热模拟机在350～500℃温度范围对2195铝锂合金进行平面应变压缩实验,应变速率为0.01～1 s-1,变形量60％.研究合金在高温下的流变行为并通过背散射电子衍射(EBSD)和X射线衍射(XRD)揭示了微观组织的演变规律.结果表明:流变应力随应变速率的增加而升高,而随着温度的升高流变应力降低,并且达到峰值应力和稳态流变时所需的应变也减少,建立含有双曲正弦关系的本构方程并得到合金的变形激活能Q为278.208 kJ/mol;随着Zener-Hollomon参数值的增加,合金的流变集中增加,合金的软化机制主要是动态回复,此外还有动态再结晶形成,动态再结晶主要为不连续动态再结晶,而连续动态再结晶和几何动态再结晶均容易在较低Z值和较高Z值下形成;变形组织中含有大量的轧制织构,变形晶粒主要为S{123}〈634 〉 、Brass{011}〈211〉 、Copper{112}〈111〉 和Goss{011}〈100〉这4种取向.

通过Gleeble-3500热模拟机在350～500℃温度范围对2195铝锂合金进行平面应变压缩实验,应变速率为0.01～1 s-1,变形量60％.研究合金在高温下的流变行为并通过背散射电子衍射(EBSD)和X射线衍射(XRD)揭示了微观组织的演变规律.结果表明:流变应力随应变速率的增加而升高,而随着温度的升高流变应力降低,并且达到峰值应力和稳态流变时所需的应变也减少,建立含有双曲正弦关系的本构方程并得到合金的变形激活能Q为278.208 kJ/mol;随着Zener-Hollomon参数值的增加,合金的流变集中增加,合金的软化机制主要是动态回复,此外还有动态再结晶形成,动态再结晶主要为不连续动态再结晶,而连续动态再结晶和几何动态再结晶均容易在较低Z值和较高Z值下形成;变形组织中含有大量的轧制织构,变形晶粒主要为S{123}〈634 〉 、Brass{011}〈211〉 、Copper{112}〈111〉 和Goss{011}〈100〉这4种取向.

, the kinetics model for Mg alloy AZ31B characterized by DRX for instance was built and parameters were determined. Microscopic examination shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.Then combined with grain size of DRX model, the kinetic model built under steady state conditions was rewritten as superimposed step form to apply in the prediction of grain size under unsteady state conditions. The simulated data accord with the experimental results by means of quantitative metallography, which verified the rationality of the superimposed prediction method.

, the kinetics model for Mg alloy AZ31B characterized by DRX for instance was built and parameters were determined. Microscopic examination shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.Then combined with grain size of DRX model, the kinetic model built under steady state conditions was rewritten as superimposed step form to apply in the prediction of grain size under unsteady state conditions. The simulated data accord with the experimental results by means of quantitative metallography, which verified the rationality of the superimposed prediction method.

AA5052 was subjected to severe plastic deformation using the equal channel angular pressing (ECAP) technique. The microstructure, quantified using electron backscattered diffraction, showed that ECAP refined the grain size, increased the fraction of high angle boundaries (from 0.46 to 0.75) and increased total boundary area per unit volume of AA5052. ECAP increased the flow stress from about 325 to 425MPa and strain rate sensitivity from 0.005 to 0.012 and reduced strain hardening exponent from 0.34 to 0.25. Shear localization in the ECAP as well as unpressed AA5052 was studied using hat-shaped samples deformed at both quasi-static strain rates in a screw-driven machine and dynamic strain rates in a split-Hopkinson bar. The shear band formation in the hat-shaped sample was narrow and well defined for the ECAP condition as compared to the unpressed condition where the shear band was broad and diffuse, implying an increased propensity for shear localization as compared to the unpressed condition. The microstructure within the shear band of the ECAP condition sample showed that the boundary area per unit volume decreased as compared to that away from the shear band.

Discontinuous dynamic recrystallization (involving nucleation and grain growth) is rarely observed in metals with high stacking fault energies, such as aluminium. In this metal, two other types of recrystallization have been observed: continuous dynamic recrystallization (CDRX, i.e. the transformation of subgrains into grains); and geometric dynamic recrystallization (due to the evolution of the initial grains). The main purpose of this work was to bring clearly into evidence and to better characterize CDRX. Uniaxial compression tests were carried out at 0.7 T m and 10 2 s 1 on three types of polycrystalline aluminium: a pure aluminium (1199), a commercial purity aluminium (1200) and an Al-2.5wt.%Mg alloy (5052), and also on single crystals of pure aluminium. In addition, 1200 aluminium specimens were strained in torsion. The deformed microstructures were investigated at various strains using X-ray diffraction, optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Observations of the single crystalline samples confirm that subgrain boundaries can effectively transform into grain boundaries, especially when the initial orientation is unstable. In the case of polycrystalline specimens, after separating the effects of the initial and new grain boundaries, it turns out that CDRX operates faster in the 1200 aluminium compared to the two other grades. Moreover, it appears that the strain path does not alter noticeably the CDRX kinetics.

The concept of geometric dynamic recrystallization (GDX) originated in 1980s with work on elevated-temperature deformation aluminum to large strains. In this case, substantial grain refinement occurs through a process of grain elongation and thinning leading to a dramatic increase in grain boundary area. The grain boundaries become serrated as a result of subgrain (low angle) boundary formation. Pinching off and annihilation of high-angle grain boundaries occurs as the original grains thin to about twice the subgrain diameter to and a “steady-state” structure. This concept has since been carefully verified in pure Al, as well as Al–Mg alloys deforming in the three-power regime. Large strain deformation of Al single crystals is also consistent with the concept. Also, data in the literature on large strain deformation of a bcc iron alloy are consistent with GDX. Recent experiments on α-zirconium show that GDX applies to this hcp metal. Thus, it appears that GDX is a general phenomenon that can lead to grain refinement in the absence of any discontinuous dynamic recrystallization (DRX) or continuous dynamic recrystallization (CDX). A discussion of continuous dynamic recrystallization and geometric necessary boundaries in relation to GDX will also be discussed. This may be particularly relevant to severe plastic deformation such as rolling and equal-channel angular pressing where dramatic increases in the number of high-angle boundaries are observed.

ABSTRACT Geometric dynamic recrystallization (GDRX) is a process in which a refined and nearly equiaxed grain structure is formed, because grain boundaries which have become serrated during formation of subgrains in the course of hot deformation recombine as serrations pinch off or as the grains thin down. GDRX was first found in aluminium and more recently in AlMg solid solution. In the present work the question was addressed whether GDRX occurs also in an industrial Al alloy (5083) containing particles. Specimens were deformed in torsion from 473 to 773 K at equivalent (surface) strain rates between 10613 and 4 s611 to strains up to 3.6. Under these conditions the egg tray model predicts that GDRX will occur. This is indeed found from observations of the grain structure with light and electron microscopy. The results indicate that GDRX occurs not only by recombination of opposite boundaries of the thinned grain but also by pinching off of serrations. The size of the GDRX grains is about two to three times the subgrain size. The close similarity to Al5Mg means that the particles in the alloy do not prevent the small-scale grain boundary migration which is necessary to form the serrations. Static recrystallization after hot deformation destroys the DRX structure, if the specimen is not cooled fast enough.

An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains, nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains as they spiral around the torsion axis. No evidence was found for discontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei. It is concluded that dynamic recovery in the solute drag regime gives rise to geometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains.

A physically based model for nucleation during discontinuous dynamic recrystallization (DDRX) has been developed and is coupled with polyphase plasticity and grain growth models to predict the macroscopic stress and grain size evolution during straining. The nucleation model is based on a recent description for static recrystallization and considers the dynamically evolving substructure size. Model predictions are compared with literature results on DDRX in pure Cu as a function of initial grain size, deformation temperature and strain rate. The characteristic DRX features such as single to multiple peak stress transitions, convergence towards a steady-state stress and grain size, and a power-law relationship between the stress and grain size are quantitatively reproduced by the model. The critical conditions for the onset of nucleation in the model are shown to compare well with Gottstein et al. experimentally determined critical stress criteria.

Although hot working had been defined as deformation above the recrystallization temperature (determined after cold working), it was only about 1965 that dynamic recrystallization (DRX) was confirmed to be occurring during the deformation; two decades were required to clarify the similarities to, and the differences from, static recrystallization. In classical discontinuous DRX in Cu, Ni, and -Fe, successive necklaces of new grains cause work softening; however in steady-state, the nuclei are uniformly distributed as reestablished dislocation structure limits growth. In high recovery metals at high strains, the grain boundary (GB) serrations meet across the elongated thinned grains thus pinching them off into almost equiaxed grains containing a substructure, thus geometric DRX.

The isothermal deformation compression tests of AA7085 aluminum alloy were performed on Gleeble-1500 system in the temperature range from 250°C to 450°C and at strain rate range from 0.01s611 to 10s611. The microstructure of samples was observed using optical microscopy (OM) and transmission electron microscopy (TEM) techniques. The results show that the peak stress levels decreased with the increase of deformation temperatures or the decrease of strain rate, which can be represented by the Zener–Hollomon parameter in the exponent-type equation with the hot deformation activation energy of 249.11KJ/mol. Dynamic recrystallization more obviously occurred in the sample with higher Z value than in the sample with lower Z value. Dynamic recrystallization is sensitively dependent on the deformation temperature.

The flow behavior of 7050 aluminum alloy was investigated by means of isothermal compression tests. Isothermal compression of 7050 aluminum alloy was carried out on a Gleeble-1500 thermal simulation machine at the deformation temperatures ranging from 593K to 743K, the strain rates ranging from 0.01s611 to 20.0s611, and the height reductions of 30%, 50% and 70%. The characteristics of stress–strain curves are determined by the interaction of work hardening, dynamic recovery and dynamic recrystallization. The flow stress decreases with the increasing of deformation temperature and the decreasing of strain rate. The relationship between microstructure and processing parameters was analyzed. The constitutive equations for characterizing the flow behavior during the whole deformation process had been established, based on the experimental results and the kinetic analysis. The average relative error between the calculated and the experimental flow stress is 5.73%, which indicates that the constitutive equations can be used to predict the flow behavior of 7050 aluminum alloy accurately during high temperature deformation.

The hot deformation behavior of Al–6.2Zn–0.70Mg–0.3Mn–0.17Zr alloy was studied using hot compression tests over deformation temperature range of 623–773K and strain rate of 0.01–20s611. The flow stress behavior and microstructural evolution were observed during the hot deformation process. The results show the flow stresses and microstructure evolution are sensitive to deformation parameters. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by the Zener–Hollomon parameter Z in the hyperbolic sine equation with the hot deformation activation energy of 178.85KJ/mol. The dynamic flow softening for the alloy is accounted for dynamic recovery and dynamic recrystallization. Under conditions of high lnZ values, only dynamic recovery occurred, and the main soften mechanism transformed from dynamic recovery to dynamic recrystallization at low lnZ values. With decreasing the value of lnZ parameter, the size of recrystallization grain becomes larger. According to the TEM evolution, it can easily observed that high densities of the fine and nano-scaled Al3Zr particles have precipitate in the aluminum matrix and grain boundary, which can effectively inhibit the dynamic recrystallization of experiment alloy. Based on dynamic material model and Prasad’ instability criterion, the processing maps for the alloy are built at true strains of 0.3 and 0.5. The processing map at the strain of 0.5 exhibits the optimum processing conditions are in deformation temperature range from 703K to 773K and strain rate range from 0.03s611 to 0.32s611 with the maximum efficiency of 33%.

对2197铝锂合金进行形变热处理,采用HREM高分辨电镜观察合金欠时效和峰时效两个阶段的显微组织,研究T1相析出长大行为及其界面特征。结果表明:形变热处理2197合金在欠时效阶段主要存在δ′、θ′和T1相,T1相尺寸约为40 nm;峰时效阶段合金中δ′和θ′数量减少,主要析出相为T1相,其尺寸为50~150 nm,T1相可通过δ′相的溶解而长大。在欠时效和峰时效两阶段,T1相末端与基体共格,其他区域与基体非共格。T1相为六方晶系,与基体存在惯析关系:(0001)T1//(111)A1、[1010]T1//[110]A1,惯析面为{111}A1。

对2197铝锂合金进行形变热处理,采用HREM高分辨电镜观察合金欠时效和峰时效两个阶段的显微组织,研究T1相析出长大行为及其界面特征。结果表明:形变热处理2197合金在欠时效阶段主要存在δ′、θ′和T1相,T1相尺寸约为40 nm;峰时效阶段合金中δ′和θ′数量减少,主要析出相为T1相,其尺寸为50~150 nm,T1相可通过δ′相的溶解而长大。在欠时效和峰时效两阶段,T1相末端与基体共格,其他区域与基体非共格。T1相为六方晶系,与基体存在惯析关系:(0001)T1//(111)A1、[1010]T1//[110]A1,惯析面为{111}A1。

The influence of large ( > 1 m diameter) particles on the recrystallization behaviour of magnesium alloys deformed in plane strain compression has been studied. Deformation zones characterized by a misorientation gradient were observed surrounding large particles, which were similar to those seen in other alloys where particle-stimulated nucleation (PSN) of recrystallization occurs. During annealing, new grains formed by PSN, but only at a small fraction of the coarse particles. Recrystallization was instead dominated by growth of existing dynamically recrystallized grains and nucleation of new grains from the deformed grain mantle region.