预处理工艺对双辊铸轧3003铝合金再结晶行为的影响

对双辊铸轧3003铝合金板材进行了3种预处理退火,研究不同预处理工艺下的冷轧板材在380～500℃退火时晶粒组织和再结晶织构的变化规律。结果表明:最优化预处理工艺为610℃/12h+460℃/12h,高温阶段第二相尺寸发生粗化,低温阶段基体中Mn的过饱和固溶度显著降低,两者均有利于提高后续退火时的再结晶形核率。500℃退火时,在粗大第二相的附近产生了粒子诱发形核机制,降低了再结晶织构强度;并且退火时几乎不存在析出,析出相对再结晶形核的抑制作用甚微,从而得到了晶粒细小、织构弱的再结晶组织。     

Effect of electropulsing treatment on static recrystallization behavior of cold-rolled magnesium alloy ZK60 with different reductions

Effect of electropulsing treatment (EPT) on recrystallization behavior of cold-rolled Mg alloy ZK60 strips was investigated. It was found that EPT significantly improved nucleation rate and migration ability of grain boundaries, leading to accelerated recrystallization of the deformed metals at relatively low temperature. After the recrystallization induced by EPT, the average grain size of 20% rolling reduction samples decreased from 113 μm to around 10 μm, meanwhile the typical basal-type texture of the cold-rolled sample was weakened. EPT was normally accompanied with a thermal and an athermal effects. The athermal effect played a dominated role in increasing nucleation rate, while the thermal effect promoted grain growth. A fewer recrystallized grains originated along the grain boundaries in the 10% reduction samples, while most of the recrystallization took place inside the twins in the 20% reduction samples.     

Effects of samarium content on microstructure and mechanical properties of Mg–0.5Zn–0.5Zr alloy

Effects of samarium (Sm) content (0, 2.0, 3.5, 5.0, 6.5 wt%) on microstructure and mechanical properties of Mg–0.5Zn–0.5 Zr alloy under as-cast and as-extruded states were thoroughly investigated. Results indicate that grains of the as-cast alloys are gradually refined as Sm content increases. The dominant intermetallic phase changes from Mg₃Sm to Mg₄₁Sm₅ till Sm content exceeds 5.0 wt%. The dynamically precipitated intermetallic phase during hot-extrusion in all Sm-containing alloys is Mg₃Sm. The intermetallic particles induced by Sm addition could act as heterogeneous nucleation sites for dynamic recrystallization during hot extrusion. They promoted dynamic recrystallization via the particle stimulated nucleation mechanism, and resulted in weakening the basal texture in the as-extruded alloys. Sm addition can significantly enhance the strength of the as-extruded Mg–0.5Zn–0.5 Zr alloy at room temperature, with the optimal dosage of 3.5 wt%. The optimal yield strength (YS) and ultimate tensile strength (UTS) are 368 MPa and 383 MPa, which were enhanced by approximately 23.1% and 20.8% compared with the Sm-free alloy, respectively. Based on microstructural analysis, the dominant strengthening mechanisms are revealed to be grain boundary strengthening and dispersion strengthening.     

Effect of icosahedral phase on the crystallographic texture and mechanical anisotropy of duplex structured Mg–Li alloys

Through comparing the microstructure and mechanical properties of Mg–6%Li and Mg–6%Li–6%Zn–1.2%Y alloys, the difference in their mechanical anisotropy have been deeply investigated. Due to the formation of I-phase (Mg₃Zn₆Y, icosohedral structure), the volume fraction of β-Li phases formed in Mg–6%Li–6%Zn–1.2%Y alloy was two times higher than that in Mg–6%Li alloy even these two alloys having the same Li content. Texture analysis demonstrated that I-phase particle stimulated nucleation of recrystallization (PSN) and the formation of higher volume fraction of β-Li phases can remarkably weaken the basal texture of α-Mg phases, resulting in a very weak mechanical anisotropy. On the contrary, the basal texture of the α-Mg phases in Mg–6%Li alloy was quite strong and obvious mechanical anisotropy between differently oriented samples can be observed.     

Ti-6Al-4V合金热压缩过程中的动态再结晶

在变形温度为870~960℃、应变速率为5×10^-4 s^-1~5×10^-2 s^-1的条件下对Ti-6Al-4V合金进行单道次等温压缩实验,测出其应力-应变曲线并建立KM模型、Poliak-Jonas模型和Avrami模型,较为系统地描述了这种合金动态再结晶过程中的流变应力、临界应变量、组织演变动力学等的特征。将动态再结晶组织的转变体积分数引入Prasad功率耗散率模型,得到了Ti-6Al-4V合金动态再结晶过程中能量的变化规律并结合微观组织表征揭示了这种合金的动态再结晶机理。结果表明：随着变形温度的提高和应变速率的降低,Ti-6Al-4V合金的动态再结晶临界应变量减小,组织转变的体积分数增大。发生完全动态再结晶时的功率耗散率大于0.34,形核机制为位错诱导的弓出形核机制。     

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei.The results show that orientation distribution of nuclei is associated with nucleation mechanism.The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation.Differently,discontinuous static recrystallization nuclei formed within the parent grains(TD-45~0 orientation) show random orientations and a variety of misorientation angles but preferred axes <5273> or <5270>.Interestingly,a special oriented nucleation is found.Discontinuous static recrystallization nuclei originated from boundaries of the parent grain(TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70°<1120>(∑18 b).It is speculated that these two special misorientation relationships are related to the dislocation type.     

Effect of homogenization on the microstructure and mechanical properties of the repetitive-upsetting processed AZ91D alloy

The current research investigates the effect of homogenization on the microstructure and mechanical properties of the AZ91 D alloy processed by repetitive upsetting(RU). Results show that during RU processing, the initial large Mg_(17)Al_(12) particles in the as-cast specimen accelerate the dynamic recrystallization(DRX) due to the particle stimulating nucleation(PSN) mechanism. With the progress of RU,the inherent large strain breaks the large second phases into small fragments, which indicates the PSN gradually disappears, while the pinning effect becomes obvious. As for the homogenized specimen, a pre-heat treatment leads to the absence of Mg_(17)Al_(12) particles but a uniform distribution of Al atoms in the Mg alloy. Though the subsequent RU promotes the precipitation of Mg_(17)Al_(12) particles, the relatively small particle size and the uniform distribution are more favorable to act as obstacles for grain growth than contributors to PSN. Finally, a more homogeneous and refined microstructure is obtained in the specimen with prior homogenization than the as-cast one.     

脉冲电流处理对冷轧态GH3030合金再结晶组织性能的影响

目的 探究不同工艺参数下脉冲电流处理对冷轧态GH3030合金静态再结晶组织的影响.方法 利用电子背散射衍射(EBSD),探究脉冲电流处理(EPT)和常规退火处理(CHT)对GH3030合金静态再结晶的影响.探究了两种热处理方式下不同退火温度和时间对冷轧态GH3030合金静态再结晶体积分数、晶粒尺寸以及硬度的影响,计算两种热处理方式下不同工艺参数的静态再结晶动力学方程与激活能.结果 与常规退火处理相比,脉冲电流处理可以快速提高冷轧态GH3030合金的静态再结晶体积分数,并且得到了尺寸更加均匀的晶粒,而脉冲电流处理的合金其硬度值均小于相同条件下常规退火处理的合金.根据静态再结晶动力学方程的结果可知,脉冲电流处理的合金再结晶激活能低于常规退火处理的合金,脉冲电流处理下发生完全再结晶所需时间远远少于常规退火处理下所需时间.结论 脉冲电流处理促进了冷轧GH3030合金静态再结晶行为,并且加速了再结晶晶粒的成核和长大,但脉冲电流处理对改善GH3030合金的硬度效果不明显.     

Influence of particles on recrystallisation textures and microstructures of aluminium alloy 3103

Abstract

The evolution of recrystallisation textures in the commercial aluminium alloy 3103 has been investigated by means of a three-dimensional orientation distribution function (ODF) analysis. The global texture measurements were supplemented by local texture measurements by means of the electron backscattering pattern (EBSP) technique in SEM and optical microscopy, inparticular grain size measurements. The evolution of recrystallisation textures was determined by the competition between particle stimulated nucleation (PSN) and nucleation from cube bands. Precipitated particles were found to have a higher retarding effect on the nucleation from deformation zones around particles than on the nucleation from cube bands. The result of this is a strong cube texture and a large grain size in cases of a strong precipitation reaction. This phenomenon has also been discussed theoretically in terms of a semiquantitative model.     

Promotion of texture weakening in magnesium by alloying and thermomechanical processing: (I) alloying

The recrystallization and texture evolution of four Mg–Zn–Ce sheets with a warm-rolled microstructure obtained through two stages that can be characterised as rough rolling and finish rolling were investigated at different stages of post-rolling annealing. On annealing, the same regions of the microstructure, located by hardness indentations, were examined and tracked by electron backscatter diffraction (EBSD). Furthermore, intragranular misorientation axes (IGMA) analysis was used to investigate the associated deformation mechanisms in the as-deformed material. By combining these two methods, the development of the recrystallization microstructure was investigated and important aspects, such as preferential nucleation sites, correlation between activated deformation mechanism and initial orientation of the recrystallized grains, were studied. The results showed that the Mg–1Zn–1Ce alloy, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and the most homogenous distribution of shear banding/twinning. The IGMA analysis also showed that in Mg–1Zn–1Ce, other types of dislocations rather than basal 〈a〉 were activated; in particular, prismatic 〈a〉 type was activated during deformation. Therefore, the weakening of recrystallization texture during rolling resulting from the addition of RE elements was linked with a change in dynamic recrystallization (DRX) behaviour. Since the Mg–1Zn–1Ce alloy corresponds to the highest level of Ce in solid solution, the observed texture weakening was possibly due to decreasing grain boundary mobility as a result of solute partitioning of RE elements to dislocations and grain boundaries.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

Texture evolution and probability distribution of Goss orientation in magnetostrictive Fe–Ga alloy sheets

Texture evolution and the distribution of Goss orientation in polycrystalline Fe–Ga alloy were investigated as a series of rolling and subsequent annealing processes were used to develop highly textured rolled sheet. A dramatic change from the random nature of the as-rolled and primary recrystallized texture is observed when careful control of atmosphere and temperature during anneal leads to development of a sharp Goss orientation over up to 98 % of the surface of a sample during secondary recrystallization. In this work, grain boundary properties in local areas surrounding Goss grains are investigated and the evolution of Goss orientation is traced through the different stages of alloy processing using electron backscatter diffraction analysis. To evaluate the evolution of grains with Goss orientation, {011} grains are selected and separated from other texture components at each processing step and statistical analysis used to correlate the structural inheritance chain of Goss-oriented grains. The four processing stages considered are the alloy after hot rolling, the as-rolled alloy (i.e., after subsequent warm and cold rolling), the alloy after an initial anneal during which primary recrystallization occurs, and the alloy after final anneals in which secondary recrystallization with abnormal grain growth occurs. Analysis of Goss grain orientation probability distribution functions after primary and secondary recrystallization convincingly demonstrates that the orientation of the abnormally grown Goss texture that develops during secondary recrystallization is determined by the orientation of Goss components that develop during the primary recrystallization stage of alloy processing.     

ZK60镁合金高温动态再结晶行为的研究

采用Gleeble-1500热模拟试验机进行压缩实验,研究了ZK60镁合金在变形温度为473～723K、应变速率为0.001～1s~(-1)范围内变形过程中的组织演变.分析了变形程度、变形温度、变形速率对其动态再结晶行为的影响,探讨了其动态再结晶的形核机制.结果表明:ZK60合金高温塑性变形时的主要软化机制为动态再结晶,变形温度623K,应变量超过0.24时,在原晶界处出现大量的动态再结晶晶粒,并形成易延展的剪切区.变形温度是影响ZK60合金动态再结晶晶粒尺寸的主要因素,变形温度高于623K时,动态再结晶晶粒超过25μm.ZK60合金动态再结晶晶核在晶界弓弯处形成,随着应变量增加,出现亚晶界合并长大,长条状亚晶快速长大以及在剪切带变形区形核等.     

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.     

The effect of particle content,particle distribution and cold deformation on the recrystallization of low oxide Al-Al2O3 products

The recrystallization of dispersion-strengthened Al-Al₂O₃ products containing 0.6 and 1.2 wt % Al₂O₃ was followed by optical and transmission electron microscopy and by hardness measurements. The recrystallization was retarded compared to aluminium and the important structural parameters were the oxide content (proportional to the reciprocal particle spacing) and the distribution of oxide particles either as a uniform distribution or as a regular three-dimensional network. From the microstructures after cold work and after recovery it is suggested that particle-retarded recrystallization may be caused by pinning, during the recovery stage, of sub-boundaries and of individual dislocations. The hypothesis of retardation of recrystallization as due to particle-enhanced homogenization of dislocation structures during deformation is not supported by the microstructural observations. In the product containing 0.6 wt% Al₂O₃ the recrystallization was markedly retarded after 50% cold reduction, whereas the retardation was small after 80 and 90% reduction. An increase in the degree of cold deformation may reduce the critical size of the recrystallization nuclei, and thus the retarding effect of particles during nucleation may be reduced or disappear. It is therefore suggested that the degree of cold deformation and the particle spacing may be interdependent parameters when determining the recrystallization behaviour of dispersion-strengthened products.     

Dispersoid Formation and Recrystallization Behavior in an Al-Mg-Si-Mn Alloy

The nucleation and precipitation of Mn-containing dispersoids in an Al-Mg-Si-Mn alloy (6082) have been studied by optical microscopy,EPMA (electron probe microanalysis) and TEM (transmission electron microscopy).The influence of Mn-containing dispersoids on the recrystallization behavior was also investigated.The size and distribution of dispersoids were strongly affected by both the homogenization process and the alloying element distribution formed in the direct chill cast procedure.The Mn-containing dispersoids were observed to nucleate preferentially on the β-Mg 2 Si phase and to be aligned along the 100 direction of the matrix.After cold deformation,the morphology of dispersoids greatly influences the recrystallization and grain growth behavior in the annealing process.     

硬铝 LY12的动态再结晶诱发超塑性

研究了供应状态的变形铝合金 LY 12Y 高温变形时的动态再结晶诱发超塑性效应。未经任何预处理的供应状态 LY 12Y 板材可以在较宽的温度及应变速率范围内产生超塑性效应,最大延伸率δ_(max)=243%。分析表明,该效应是由变形初期的动态再结晶诱发产生;诱发过程可分为三个阶段。对第二相粒子在过程的作用及动态再结晶的形核方式进行了探讨。     

AlCuMgFeNiSc（Zr）系铝合金再结晶过程与退火行为的研究

在2618铝合金的基础上,添加Sc,Zr,降低Cu量,配制实验合金,采用金相,X－射线衍射,透射电镜,硬度测量等手段,研究了实验合金冷轧板材经不同温度退火组织性能的变化,探讨了Sc对实验合金再结晶过程与退火行为的影响规律,结果表明,Al3(Ac,Zr)质点钉扎位错,稳定亚结构,迟滞以亚晶聚合与长大为主的再结晶形核,同时也阻碍晶粒长大,因此,2618铝合金中添加Sc,Zr合金化可以提高再结晶温度;并细化再结晶组织,但Sc,Zr不宜过量,否则不利于稳定未再结晶组织,降低Cu量,虽然合金硬度下降,但是可能防止W相生成,要增添Sc,Zr含量,进一步推迟再结晶过程,提高再结晶温度。     

Stircasting of transparent organic alloys: thixotropy and rosette formation

The fluid flow of a stircast slurry is very complex due to its thixotropic behaviour, i.e. at a fixed fraction of solid the viscosity is shear-rate and time dependent. The fraction of solid in its turn depends on the temperature gradient and the cooling rate in the stir caster. In order to understand the effects of the variables upon final alloy composition, particle size distribution and fluid flow, it is necessary to simultaneously perform viscosimetric measurements and microscopic observations. Metals do not allow this, but transparent organic compounds do. Therefore we used 98% pure succinodinitrile as a model alloy to study nucleation and solidification during stirring. Our microscopic observations of solidifying succinodinitrile in a stirred cell showed that it formed the same spherical or rosette-shaped particles as aluminium alloys. Our viscosimetric studies showed that partially solidified succinodinitrile behaves thixotropically.     

定向凝固Fe-6.5wt%Si合金热压缩变形行为研究

采用Gleeble 1500热模拟机对定向凝固Fe-6.5%(质量分数)Si合金进行热压缩变形研究。结果表明,Fe-6.5%(质量分数)Si合金柱状晶组织热形变过程中很难发生动态再结晶,以动态回复为主,易产生严重的变形带;在无序相区间,在柱状晶晶界处发生动态再结晶形核,但长大受到限制,仅局限于晶界附近。与锻态等轴晶组织相比,有序相区间,两种组织形变行为基本一致,受温度影响剧烈,在无序相区间,随温度升高,等轴晶组织的形变激活能略有升高,平均值约为294kJ/mol,而柱状晶组织的形变激活能逐渐降低,平均值约为260kJ/mol。