Ti-6Al-4V合金超塑性变形中的组织演变及变形机制

920℃、应变速率为1×10.3和2×10.4 s.1时,对不同初始晶粒尺寸(2.6、6.5和16.2 μm)的Ti-6Al-4V合金进行超塑性拉伸变形.采用光学显微镜、透射电镜观察变形后的显微组织.结果表明,初始晶粒尺寸的不同对超塑性变形中的组织演变及变形机制有着显著的影响.拉伸变形中晶粒明显粗化,变形诱发晶粒长大是超塑性变形组织的重要特征之一;随着变形程度的增大,应变诱发的晶粒长大显著增大,并且远大于静态长大的增幅.对于细晶粒材料(2.6和6.5 μm),位错运动协调的界面滑动是其变形的主要机制.而对于晶粒较粗的材料(16.2 μm),超塑变形机制是晶界滑动与晶内位错运动的共同作用.随着晶粒尺寸的增大,以晶界滑动为主的变形方式逐渐转向以晶内位错运动为主.     

金属间化合物大晶粒超塑性变形机理

在所研究的Fe3Al，Fe3Si，FeAl，Ni3Al，NiAl和TiAl等金属间化合物中均发现大晶粒超塑性。显微分析表明，超塑性变形过程中晶粒明显细化；电子背散射衍射(EBSD)技术和透射电子显微学(TEM)分析表明，大晶粒金属问化合物超塑变形过程中形成了大量亚晶界网络，且随变形量增大．亚晶界不断吸收晶内滑动位错，使其位向差不断增大，从而逐渐演变成小角度和大角度晶界，即超塑性变形过程中产生了连续动态回复与再结晶(CDRR)。高温塑性变形是通过位错的滑移和攀移进行的，而亚晶界的迁移、滑动和转动起到协调变形的作用，保持了材料在宏观上的超塑性。     

A COMPLEX DEFORMATION MECHANISM FOR SUPERPLASTIC DEFORMATION OF Mg ALLOY

本文选用大晶粒和微细晶粒两种镁合金,利用现代测试手段进行超塑性变形机制的研究.结果表明,在超塑性变形条件下,两种合金都显示以晶界滑动为主的,由扩散蠕变和位错滑移所协调的复合变形机制.作者提出一个包括三种变形机制在内的复合机制模型.在超塑性变形中,下层金属晶粒通过晶界滑动不断涌现到试样表面横向晶界发生宽化及空洞的地方,从而不断增加沿拉伸轴方向上的晶粒数.这是试样在拉伸变形中获得非常大的伸长量的原因.     

镁合金超塑性变形的复合机制

本文选用大晶粒和微细晶粒两种镁合金,利用现代测试手段进行超塑性变形机制的研究.结果表明,在超塑性变形条件下,两种合金都显示以晶界滑动为主的,由扩散蠕变和位错滑移所协调的复合变形机制.作者提出一个包括三种变形机制在内的复合机制模型.在超塑性变形中,下层金属晶粒通过晶界滑动不断涌现到试样表面横向晶界发生宽化及空洞的地方,从而不断增加沿拉伸轴方向上的晶粒数.这是试样在拉伸变形中获得非常大的伸长量的原因.     

粗晶铝合金超塑性变形机理的研究现状

介绍了国内外粗晶铝合金超塑性的基本研究情况。对粗晶铝合金超塑性变形机理的研究进行归类总结，主要包括：扩散蠕变机理、伴随扩散蠕变的晶界滑移机理、位错蠕变/滑移机理、液相协调机理、动态再结晶机理、晶粒群滑移机理和空洞连接协调机制。并针对低成本发展铝合金超塑性研究的迫切性，展望了粗晶材料的应用前景。指出了除大晶粒外，高应变速率和低温条件下的超塑性也是目前铝合金变形研究的方向。     

Mg-Gd-Y-Zr合金热轧板材的粗晶超塑性行为与微结构

对初始晶粒度为66μm的轧制板材在不同温度和不同变形速率下进行超塑性拉伸实验,研究Mg-Gd-Y-Zr合金粗晶热轧板材的超塑性行为与微结构特征。在温度为435℃、应变速率为5×10-4s-1的变形条件下获得的最大伸长率为380%,应变速率敏感系数为0.56。合金的表观变形激活能高于镁的晶界扩散激活能或晶格扩散激活能;合金的超塑性变形机制为晶格扩散控制的位错协调晶界滑动机制。微结构分析结果表明:第二相钉轧晶界,较软的不规则块状的β相承受了部分塑性变形。     

粗晶Ti40合金超塑性变形时的动态软化行为研究

采用单向拉伸试验对粗晶Ti40合金进行了超塑性能测试,并结合TEM和EBSD分析技术研究了该合金超塑性变形过程中的动态软化行为及机制。结果表明：粗晶Ti40合金在所选实验条件下具有良好的超塑性能并在840oC、1×10-3s-1条件下获得最大延伸率436%;基于形变Z因子和断裂延伸率并结合微观组织分析可将变形条件划分为无超塑性、动态回复、动态再结晶三个区域;分别基于Sellars模型和KM方程建立了Ti40合金超塑性变形的动态再结晶临界应变模型和位错密度演变模型;粗晶Ti40合金超塑性变形过程中的动态回复以位错运动—位错胞—多边形化—形成亚晶的机制为主;动态再结晶机制主要为亚晶持续转动导致大角度晶界形成的连续动态再结晶。     

EBSD Study on the Superplastic Deformation of Mg-7.0Al-0.4Zn Magnesium Alloy

采用EBSD分析技术研究了Mg-7.0Al-0.4Zn镁合金超塑性变形机制。结果表明,超塑拉伸变形主要是通过晶界滑动和晶内塑性滑移协调完成的。变形初期,随着变形量的增大,{0002}//ED的织构明显增强,晶内滑移起主要协调变形作用。变形中后期孪生开动,接近断裂时,晶内滑移基本消失,孪生成为主要的协调变形机制,但孪生的贡献较小。     

纳米孪晶金属塑性变形机制

本文综述了纳米孪晶金属材料的塑性变形机制.通过分析纳米孪晶二维结构变形时可启动的滑移位错类型,揭示纳米孪晶金属塑性变形的3种位错机制,即位错塞积并穿过孪晶界机制,Shockley不全位错诱导孪晶界迁移机制以及贯穿位错在孪晶片层内受限滑移机制.通过改变加载方向与孪晶界面的相对取向可实现这3类位错机制的可控转变.     

晶粒尺寸及取向对AZ31合金板材室温拉伸塑性变形与断裂机制的影响

对AZ31镁合金轧制态板材分别在473~673 K温度范围退火1 h以获得不同初始显微组织。通过金相显微镜、背散射电子衍射分析(EBSD)和力学试验机,研究晶粒尺寸和取向分布对合金板材室温单向静拉伸过程塑性变形和断裂机制的影响。结果表明,晶界取向角呈连续分布有利于晶粒协调塑性变形。随着晶粒尺寸的降低,晶界对室温塑性变形的贡献增大,晶界与位错滑移和孪生的交互作用增强。对于取向角均呈连续分布的473 K和573K退火态板材,平均晶粒尺寸分别为3.6μm和9.5μm,塑性变形主导机制由晶界滑动和位错滑移转变为晶界滑动、位错滑移和孪生,断裂机制由微孔聚集型转变为微孔聚集型和解理型混合型断裂方式。673 K退火态板材的平均晶粒尺寸达22.9μm,塑性变形主导机制为位错滑移和孪生。此时,由于晶界取向角呈离散分布,晶粒协调塑性变形能力差,断裂机制转变为解理断裂。     

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.     

The large-scale deformation of polycrystalline aggregates: cooperative grain-boundary sliding

The mechanisms and processes of large-scale deformation of polycrystalline materials are examined using model aggregates comprising circular or hexagonal bars. It is emphasized that cooperative grain-boundary sliding is essential to allow large-scale deformation, contradicting the conventional models for the superplastic deformation of polycrystalline materials which proceed with the sliding process via switching respective grains. Theoretical considerations of the effects of grain size and shape, grain-boundary phase, and of the dimension of test specimen on the cooperative grain-boundary sliding are made.     

Superplastic behavior of friction-stir welded Al–Mg–Sc–Zr alloy in ultrafine-grained condition

The present work was undertaken to improve superplastic ductility of friction-stir welded joints of ultrafine-grained (UFG) Al–Mg–Sc–Zr alloy. In order to suppress the undesirable abnormal grain growth, which typically occurs in the heavily deformed base material, the UFG material was produced at elevated temperature. It was suggested that the new processing route could reduce dislocation density in the UFG structure and thus enhance its thermal stability. It was found, however, that the new approach resulted in a relatively high fraction of low-angle boundaries which, in turn, retarded grain-boundary sliding during subsequent superplastic tests. Therefore, despite the successful inhibition of the abnormal grain growth in the base-material zone, the superplastic deformation was still preferentially concentrated in the fully-recrystallized stir zone of the material. As a result, the maximal elongation-to-failure did not exceed 700%.     

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

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

A grain-boundary diffusion model of dynamic grain growth during superplastic deformation

Dynamic grain growth during superplastic deformation is modelled on the basis of a grain-boundary diffusion mechanism. On the grain boundary where a static and a dynamic potential difference coexist, matter transport along the boundary is assumed to contribute to dynamic grain growth through depositing the matter on the grain surface located opposite to the direction of grain-boundary migration. The amount of the diffusive matter during deformation is calculated for an aggregate of spherical grains and is converted to the increment of mean boundary migration velocity. The obtained relationship between the strain rate and the dynamic grain growth rate is shown to be independent of deformation mechanisms, provided that the grain growth is controlled by grain-boundary diffusion. The strain dependence, strain-rate dependence and temperature dependence of grain growth predicted from this model are consistent with those observed in superplastic ZrO₂-dispersed Al₂O₃.     

Effects of grain growth on grain-boundary diffusion creep by molecular-dynamics simulation

Molecular-dynamics simulations are used to elucidate the effects of grain growth on grain-boundary diffusion creep and grain-boundary sliding during high-temperature deformation of a nanocrystalline Pd model microstructure. The initial microstructure consists of a 25-grain polycrystal with an average grain size of about 15 nm and a columnar grain shape. Prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble creep accompanied by grain-boundary sliding. While grain growth is generally known to decrease the creep rate due to the increase of the average grain size, the results obtained in this study reveal an enhanced creep rate at the onset of the grain growth, when rapid grain-boundary migration occurs. The enhanced creep rate is shown to arise from topological changes during the initial growth phases, which enhance both the stress-induced grain-boundary diffusive fluxes and grain-boundary sliding. Dislocations generated as a result of grain-rotation-induced grain coalescence and grain-boundary decomposition in the vicinity of certain triple junctions also contribute to the deformation.     

Tension-Compression Asymmetry Under Superplastic Flow in Magnesium Alloys

Superplastic magnesium alloys prepared by ingot metallurgy and powder metallurgy were processed and characterized. By performing uniaxial tension and compression tests of the extruded alloys along the longitudinal direction, it was found that both alloys were highly symmetric at low-strain rates within the superplastic regime. However, near the maximum strain rate within the superplastic regime, the symmetric flow disappeared. Specifically, the flow stress in early deformation under tension was slightly lower than that under compression, and the strain hardening under tension was higher than that under compression. The asymmetry was explained using the hypothesis that grain-boundary sliding under tension is easier than under compression. As indirect evidence for easier grain-boundary sliding under tension, it was shown that the coarsened intergranular precipitates tended to agglomerate on grain boundaries experiencing a tensile stress.     

High-strain-rate superplasticity in oxide ceramics:a trial of microstructural design based on creep-cavitation mechanisms

From existing knowledge about high-temperature cavitation mechanisms,necessary conditions were discussed for the suppression of cavitation failure during superplastic deformation in ceramic materials.The discussion,where special attention was placed on the relaxation of stress concentrations during grain-boundary sliding and cavity nucleation and growth,leaded to a conclusion that cavitation failure could be retarded by the simultaneous controlling of the initial grain size,the number of residual defects,diffusivity,dynamic grain growth and the homogeneity of microstructure.On the basis of this conclusion,high-strain-rate superplasticity (defined as superplasticity at a strain rate higher than 0.01 s-1) could be intentionally attained in some oxide ceramic materials.This was shown in tetragonal zirconia and composites consisting of zirconia,α-alumina and a spinel phase.     

Crossover from grain boundary sliding to rotational deformation in nanocrystalline materials

A theoretical model is suggested which describes cooperative action of grain boundary (GB) sliding and rotational deformation in mechanically loaded nanocrystalline materials. Focuses are placed on the crossover from GB sliding to rotational deformation occurring at triple junctions of GBs. In the framework of the model, gliding GB dislocations at triple junctions of GBs split into dislocations that climb along the adjacent boundaries. The splitting processes repeatedly occurring at triple junctions give rise to climb of GB dislocation walls that carry rotational deformation accompanied by crystal lattice rotation in grains of nanocrystalline materials. The role of GB sliding, rotational deformation and conventional dislocation slip in high-strain-rate superplastic flow in nanocrystalline materials is discussed.