Interior dislocation behavior in superplastic deformation of 8090 Al-Li alloy

1lNTRODUCTl0NInteriordislocationslipisoneoftheimpor-tantaccomodationmechanisminsuperplasticde-f....ti..[1~10J,inwhichthegrainsarerefinedbydynamicrecrystallization.Butthesuperplas-ticdeformationprocessleadstodifferentgrainsizesatvariousstagesandcausesachangeingrainboundaryslidingquantity,italsomakestlieaccommodatingmechanismsatvariousstagesshowdifferentfunctions,especiallydislocationslipmechanism.Therefore,itissignificantf0rthisinvestigationtoreplenishthecurrenttheoryofsuperplasticdeformati…     

DYNAMIC RECRYSTALLIZATION IN SUPERPLASTIC DEFORMATION OF TiAl BASED ALLOY

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Effect of minor Sc and Zr on superplasticity of Al-Mg-Mn alloys

The effect of Sc and Zr on the superplastic properties of Al-Mg-Mn alloy sheets was investigated by control experiment. The superplastic properties and the mechanism of superplastic deformation of the two alloys were studied by means of optical microscope, scanning electronic microscope and transmission electron microscope. The elongation to failure of Al-Mg-Mn-Sc-Zr alloy is larger than that of Al-Mg-Mn alloy at the same temperature and initial strain rate. The variation of strain rate sensitivity index is similar to that of elongation to failure. In addition, Al-Mg-Mn-Sc-Zr alloy exhibits higher strain rate superplastic property. The activation energies of the two alloys that are calculated by constitutive equation and linear regression method approach the energy of grain boundary diffusion. The addition of Sc and Zr decreases activation energy and improves the superplastic property of Al-Mg-Mn alloy. The addition of Sc and Zr refines the grain structure greatly. The main mechanism of superplastic deformation of the two alloys is grain boundary sliding accommodated by grain boundary diffusion. The fine grain structure and high density of grain boundary, benefit grain boundary sliding, and dynamic recrystallization brings new fine grain and high angle grain boundary which benefit grain boundary sliding too. Grain boundary diffusion, dislocation motion and dynamic recrystallization harmonize the grain boundary sliding during deformation.     

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.     

5A90铝锂合金电子束焊接头超塑性变形组织演变

对5A90铝锂合金电子束焊接头进行高温拉伸试验,使用光学显微镜观察试样变形过程中的组织演变,并对变形机理进行分析.结果表明,超塑性变形初期,接头超塑性变形机制以扩散导致的晶界迁移为主,焊缝细小等轴晶粒迅速长大.当应变大于100%时,接头中大晶粒开始发生动态再结晶,超塑性变形机制转变为动态再结晶机制.在超塑性变形过程中热影响区平均晶粒尺寸与焊缝平均晶粒尺寸逐渐接近,组织存在耦合均匀化过程.提出采用均匀化系数K来表征焊缝与热影响区的组织均匀化程度,随着变形的进行,K值逐渐升高.     

2091铝锂合金超塑变形的晶内断裂

研究了２０９１铝锂合金超塑变形的断裂行为。扫描电镜观察表明,２０９１铝锂合金超塑变形中存在晶内断裂,光学金相观察表明断裂试样存在较活跃的动态再结晶和较多的大轴径比晶粒,透射电镜观察发现断裂试样内部存在较多的晶内位错。研究表明,由于再结晶缩小了大轴径比晶粒的横截面使其在超塑变形的晶粒转动中受到很大的弯曲应力而造成了晶内断裂。     

Electron backscattered diffraction study on superplastic coarse-grained Fe–27 at.%Al: processing effects

An electron backscattered diffraction technique has been used to investigate detailed crystallographic features of a superplastic coarse-grained Fe–27 at.%Al alloy. Alloy samples studied have been tensile tested to failure at 800°C in air under an initial strain rate of 1×10⁻⁴ s⁻¹. To examine processing effects, the hot isostatic pressing (HIP) has been applied prior to the superplastic deformation. The HIPed sample shows no observable pores in the fracture region while the sample without HIP reveals an elongated pore fracture structure. Nevertheless, HIP is shown to have no beneficial effects on the superplastic elongation, suggestive of the fact that the alleviation of cavity formation alone is insufficient in achieving better superplastic properties. After the superplastic deformation and the refined grains are formed, the presence of numerous small angle subboundaries in the large grain interior indicates the continuous event of recovery and recrystallization that occurs throughout the course of superplastic deformation. The post-deformation annealing yields a classic recrystallized large-grain structure, resulting from the surface-tension-induced boundary migration that reduces the grain surface-area. Conversely, the superplastic deformation of Fe–27 at.%Al involves a strain-induced boundary migration that causes the grain surface-area increase and results in a refined grain structure. The dynamic nature of recovery and recrystallization is therefore confirmed.     

Ti40阻燃合金粗晶超塑性变形行为及机理

借助OM、TEM研究了高温条件下Ti40阻燃合金的粗晶超塑性变形行为及机理。结果表明:在920℃下,应变速率为5×10-5～1×10-2s-1的Ti40合金表现出良好的超塑性行为,拉伸延伸率均超过250%,应变速率敏感指数m大于0.3。超塑变形后,粗大的等轴组织细化。TEM分析表明,在变形过程中,位错运动形成亚晶界,亚晶界通过吸收滑移位错形成小角度晶界甚至大角度晶界。Ti40合金的粗晶超塑性是由动态回复和再结晶共同作用的结果。     

钛合金针状焊缝组织的超塑性变形机理

钛合金激光焊缝是非理想的针状组织,对其超塑性变形机理的研究可进一步推进钛合金LBW/SPF技术的实际应用,也对材料成形机理的发展具有一定意义.研究结果表明,TC4钛合金激光焊接试样具有良好的超塑性变形能力,在变形过程中,焊缝发生两次重要的组织转变,即针状组织片层化和片层组织等轴化的组织转变.片层组织在应力作用下,通过断裂、解体和等轴化的过程而转变成等轴晶粒.片层断裂的主要机制是动态再结晶和应力挤压作用;片层解体的主要机制是晶界滑动和转动作用,接头组织的塑性流动机理为晶粒滚动和晶界滑动机理.     

AZ31镁合金板材超塑性气胀成形研究

研究了AZ31镁合金板材不同工艺条件下的气胀成形性能。实验表明,胀形高度随温度的升高而增大,且应变速率敏感指数值均大于0．3。在673 K,0．7 MPa下胀形25 min所得的胀形件胀形高度达23．34 mm,高径比为0．67。由金相及SEM电镜观察可知,在胀形件的顶端晶界处聚集了大量空洞。通过动态再结晶,晶粒得到了很大细化。并且随变形程度的增大,晶粒细化更明显。AZ31镁合金板材的超塑性胀形主要由晶界滑移控制,动态再结晶则为重要的辅助机制。     

AZ31镁合金板材超塑性气胀成形研究

研究了AZ31镁合金板材不同工艺条件下的气胀成形性能。实验表明，胀形高度随温度的升高而增大，且应变速率敏感指数值均大于0.3。在673K,0．7MPa下胀形25min所得的胀形件胀形高度达23．34mm，高径比为0．67。由金相及SEM电镜观察可知，在胀形件的顶端晶界处聚集了大量空洞。通过动态再结晶，晶粒得到了很大细化。并且随变形程度的增大，晶粒细化更明显。AZ31镁合金板材的超塑性胀形主要由晶界滑移控制，动态再结晶则为重要的辅助机制。     

拉伸条件对Al—Li—Cu—Mg—Zr合金超塑变形行为的影响

研究了拉伸条件对温轧态Al-Li-Cu-Mg-Zr合金超塑变形行为的影响.结果表明:该合金在变形初期发生了界面取向差逐渐增大,从亚晶组织向再结晶组织变化的形变促使连续再结晶过程;应变速率越高,再结晶速度越快,再结晶晶粒越细;经高应变速率的第一阶段拉伸变形后,形成的细晶组织具有高的应变速率敏感性,同时在低应变速率的第二阶段拉伸变形中发生晶粒长大而具有高的应变硬化效果,两者的综合作用是两段速率拉伸获得高延伸率的根本原因。     

EFFECT OF TENSION CONDITIONS ON SUPERPLASTIC BEHAVIOUR OF AN Al-Li-Cu-Mg-Zr ALLOY

A much higher elongation of a warm rolled superplastic Al-Li-Cu-Mg-Zr alloy was madeunder two-stage strain rate tests comparing with the single ones.During initial stage ofdeformation a deformation-induced continuous recrystallization which converted a subgrainstructure into a recrystallized grain structure by a continuous increase in boundarymisorientations had occurred.The higher the strain rate,the faster the continuousrecrystallization and the finer the recrystallized grains.The fine recrystallized grain structureformed during the first stage deformation is the essential condition for the material to havehigh strain rate hardening and strain hardening during the low second stage superplasticdeformation.The combination of strain rate hardening and strain hardening is the reason whythe higher elongation may be obtained during two-stage superplastic deformation of the alloy.     

SiCp增强2024铝基复合材料超塑性的研究

对搅拌铸造法制备的SiCp/2024Al复合材料超塑性的预处理、力学行为、微观结构及变形机制进行了研究,结果表明,合适的强烈塑性变形是改善复合材料组织进而提高超塑性的有效方法:经小挤压、热轧和冷轧后,在温度为823K、初始应变速率为 1.1×10-3 s-1的拉伸变形条件下,超塑延伸率为405%,超塑变形机制为晶粒的适度长大、动态连续再结晶和适当的微量液相共同协调的晶界滑动:液相不是该复合材料展现超塑性的必要条件。     

MB26合金的静态与动态再结晶

研究了热挤压态ＭＢ２６镁合金的静态与动态再结晶过程。结果表明该合金静态再结晶很难完成,而动态再结晶却能在短时间内获得等轴细晶组织。其再结晶粒尺寸比静态再结晶的均匀细小。再结晶为该合金的超塑变形提高了组织条件。     

2214铝合金超塑性变形机制

温轧态２２１４铝合金在超塑性变形过程中，由于动态回复和动态再结晶的作用，使晶内位错密度在一定程度上保持平衡。超塑性变形的主要机制为晶界滑动；晶内位错滑移和扩散蠕变作为重要的协调机制，促进了晶界滑动的顺利进行。该合金的超塑性变形机制符合位错协调晶界滑动模型。     

Deformation Mechanisms of Ti-6Al-4V During Tensile Behavior at Low Strain Rate

A superplastic Ti-6Al-4V grade has been deformed at a strain rate of 5 × 10⁻⁴ s⁻¹ and at temperatures up to 1050 °C. Structural mechanisms like grain boundary sliding, dynamic recrystallization, and dynamic grain growth, occurring during deformation, have been investigated and mechanical properties such as flow stress, strain hardening, and strain at rupture have been determined. Dynamic recrystallization (DRX) brings on a decrease in the grain size. This could be of great interest because a smaller grain size allows a decrease in temperature for superplastic forming. For DRX, the driving force present in the deformed microstructure must be high enough. This means the temperature must be sufficiently low to ensure storing of enough dislocation energy but must also be high enough to provide the activation energy needed for DRX and to allow superplastic deformation. The best compromise for the temperature was found to be situated at about 800 °C; this is quite a bit lower than the 925 °C referenced in the literature as the optimum for the superplastic deformation. At this medium temperature the engineering strain that could be reached exceeds 400%, a value high enough to ensure the industrial production of complex parts by the way of the superplastic forming. Microstructural, EBSD, and mechanical investigations were used to describe the observed mechanisms, some of which are concurrent. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

THE SUPERPLASTICITY AND DEFORMATION MECHANISM OF ULTRALIGHT BINARY Mg-8wt%Li ALLOY

1.IntroductionMagnesium--lithiumalloysarethelightestalloysamongthenonpoisonousmetalsandalloystll.Becauseoftheirlowdellsityandhighspecificstiffness,theyhavethepotentialforuseinaerospacesarmoredvehicle,automobileandelectricindustry.InordertoturnMg--Lialloysintospecificcomponentswithminimumamountofmachiningandjoining,itisdesirablethatMg-Lialloysexhibitsuperplasticity.UPtonow,superfinegraillscanbeobtainedinmagnesium-lithiumalloysbythefollowingways'(1)staticannealingafterthermomechanicaltreatment…     

工业态AZ31镁合金的超塑性变形行为

研究了工业态AZ31镁合金在温度 6 2 3～ 72 3K和应变速率 1× 10 -5～ 1× 10 -3 s-1范围内的超塑性变形行为。结果表明 ,工业态AZ31镁合金表现出良好的超塑性 ,其最高断裂延伸率达到 314%,应变速率敏感指数达 0 .4。显微组织观察和断口分析表明 ,工业态AZ31镁合金超塑变形主要由晶界滑动机制所控制 ,同时 ,动态再结晶也是合金超塑变形的一种协同机制。     

5A90Al-Li合金超塑性变形过程中的显微组织及微观织构演变

通过采用电子背散射衍射技术研究具有扁平状组织的5A90 Al-Li合金板材在变形条件475℃、8×10^-4S^-1时的显微组织演变。结果表明,在进行超塑性变形前,其显微组织表现为扁平状组织,具有大量的小角度晶界,且初始板材具有明显的黄铜织构{110}（112）。在变形过程中发生了晶粒的长大、晶粒形貌的改变、晶粒取向差的增大以及织构的弱化,而且大角度晶界的含量在流动应力到达最大值后开始逐渐增加。同时,对不同阶段的相关变形机制进行了讨论。超塑性变形初始阶段的主要变形机制为位错运动,随着变形的进行而开始发生动态再结晶,晶粒旋转作为晶界滑移的主要协调机制。在超塑性拉伸的最后大应变阶段,晶界滑移是主要的变形机制。