采用沙漏挤压工艺制备超细晶材料

沙漏挤压是一种新的晶粒细化方法,通过挤压过程中产生的大塑性变形和动态再结晶而使晶粒得到细化,本文采用Zn-Al合金对这种工艺进行了初步研究,重点研究了变形量对沙漏挤压效果的影响,给出了显微组织变化和力学性能与超塑性能的变化的初步结果。实验结果表明：采用沙漏挤压能使材料获得等轴超细晶组织,材料性能得到很大的提高,并有助于实现高应变速率超塑性。     

沙漏挤压镦粗复合加工技术

本文研究了一种新的晶粒细化方法 :沙漏挤压工艺。这种工艺是在一定温度下通过对材料进行反复挤压产生大的塑性变形 ,同时进行动态再结晶而使材料得到细化。本文对铸态 Zn- Al合金进行了实验 ,初步研究了沙漏挤压工艺对于材料组织和性能的影响。实验结果表明 :沙漏挤压工艺能够有效的细化晶粒 ,使材料获得均匀的等轴细晶组织和优良的综合性能 ,并有助于实现高应变速率超塑性     

切削法制备超细晶材料研究进展与展望

综述了切削法制备超细晶材料时加工参数和工艺条件对晶粒细化的影响,分析了切削法制备超细晶材料的力学性能、耐腐蚀性能和热稳定性等,探讨了超声振动复合切削法制备超细晶材料的可能性。在超声振动加工中,材料受低应力高速、高频撞击的影响,会发生严重的塑性变形,表面大尺寸的晶粒得到细化,同时超声振动还可以在材料表面形成表面微结构,进一步改善材料性能。因而提出将切削法和超声振动相复合,高效制备具有功能微结构的超细晶材料,为微型零件超细晶材料制备提供新的工艺选择以及理论和技术支撑。     

Severe plastic deformation of commercially pure aluminum using novel equal channel angular expansion extrusion with spherical cavity

Equal channel angular expansion extrusion with spherical cavity (ECAEE-SC) was introduced as a novel severe plastic deformation (SPD) technique, which is capable of imposing large plastic strain and intrinsic back-pressure on the processed billet. The plastic deformation behaviors of commercially pure aluminum during ECAEE-SC process were investigated using finite element analysis DEFORM-3D simulation software. The material flow, the load history, the distribution of effective strain and mean stress in the billet were analyzed in comparison with conventional equal channel angular extrusion (ECAE) process. In addition, single-pass ECAEE-SC was experimentally conducted on commercially pure aluminum at room temperature for validation, and the evolution of microstructure and microhardness of as-processed material was discussed. It was shown that during the process, the material is in the ideal hydrostatic stress state and the load requirement for ECAEE-SC is much more than that for ECAE. After a single-pass ECAEE-SC, an average strain of 3.51 was accumulated in the billet with homogeneous distribution. Moreover, the microstructure was significantly refined and composed of equiaxed ultrafine grains with sub-micron size. Considerable improvement in the average microhardness of aluminum was also found, which was homogenized and increased from HV 36.61 to HV 70.20, denoting 91.75% improvement compared with that of the as-cast billet.     

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic SteelWithout Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide-pearlite steel has been investigated. Particular attention has been given to a quantitative characterization of changes in the ferrite grains. Electron back-scattered diffraction and transmission electron microscopy observations show that the ultrafine ferrite (~388 nm) can be produced through low equivalent strain cold rolling without severe plastic deformation (SPD) and annealing. The average grain size of ferrite depends on the volume fraction, shape and distribution of granular carbides as well as interlamellar spacing of pearlite. A general explanation of granular carbides-assisted grain refinement is that the embedded carbides between natural barrier will significantly facilitate dislocation nucleation during cold rolling. Dislocation reaction occurs more drastically and quickly near these granular carbides. Such reactions promote the formation of high-angle grain boundaries. The formation of ultrafine ferrite grains and subgrains in steel after cold rolling to ε=1.4 strain makes the strength and ductility increased simultaneously compared with ε=0.6 cold-rolled steel. The results suggest a new material design strategy to obtain ultrafine-grained structure via the granular carbides assistance.     

Er、Zr微合金化5083铝合金的超塑性

针对5E83合金(Er、Zr微合金化5083合金),采用超塑性拉伸试验、扫描电镜(SEM)、电子背散射衍射(EBSD)和透射电镜(TEM),探究了Er、Zr微合金元素、晶粒尺寸、变形温度、应变速率对合金超塑性的影响。通过再结晶退火、空冷和水冷的搅拌摩擦加工(FSP),分别获得了晶粒尺寸为7.4、5.2、3.4μm的完全再结晶组织,作为初始状态进行超塑性拉伸。结果表明,初始晶粒尺寸越细小,超塑性伸长率越高。当晶粒尺寸>5μm时,超塑性变形过程晶粒粗化缓慢,细化初始晶粒可显著提高超塑性;而当晶粒尺寸<5μm时,超塑性变形过程晶粒粗化严重,进一步细化初始晶粒对超塑性的提高有限。不同变形温度、应变速率的超塑性拉伸结果显示在变形温度为450～540℃、应变速率为1.67×10^-4～1.67×10^-1 s^-1,超塑性伸长率随变形温度和应变速率的提高呈现先上升后下降再上升的趋势;变形温度为520℃、应变速率为1.67×10^-3 s^-1条件下,水冷FSP态合金获得最大伸长率330%...     

Plastic flow characteristics of ultrafine grained low carbon steel during tensile deformation

In order to explain steady-state plastic deformation, i.e. the absence of strain hardening in ultrafine grained low carbon steel during tensile deformation, steel of different ferrite grain sizes was prepared by intense plastic straining followed by static annealing and then tensile-tested at room temperature. A comparison between the ferrite grain size of ultrafine grained steel and the dislocation cell size of coarse grained steel formed during tensile deformation revealed that uniform dislocation distribution with high density and cell formation were unlikely to occur in this ultrafine grained steel. This is ascribed to the fact that the ultrafine grain size is comparable to or smaller than the cell size at the corresponding stress level. In addition, from a consideration of dynamic recovery, it was found that the characteristic time for trapped lattice dislocations to spread into the grain boundaries was so fast that the accumulation of lattice dislocation causing strain hardening could not occur under this ultrafine grain size condition. Therefore, the extremely low strain hardening rate of ultrafine grained low carbon steel during tensile deformation is attributed to the combined effects of the two main factors described above.     

Microstructural evolution and plastic flow behavior of As-equal channel angular pressed 5083 Al alloy in low temperature superplasticity regime

In this study, the plastic flow behavior of ultrafine grained 5083 Al alloy fabricated by severe plastic deformation was examined in conjunction with microstructural evolution during deformation in the low temperature superplasticity regime. The present investigation was aimed at providing a better understanding of the nature of the low temperature superplasticity of ultrafine grained metallic materials. For this purpose, an ultrafine grained structure was introduced into the commercial 5083 Al alloy by equal channel angular pressing. A series of tensile tests was performed on the as-equal channel angular pressed samples at the initial strain rates of 10^?5–10^?2 sec^?1 and temperatures of 498–548 K, belonging to the low temperature superplasticity regime. The relationship between the true stress and true strain rate showed a sigmoidal behavior in a double logarithmic plot. The superplastic elongation was obtained within the limited intermediate strain range of 10^?4–10^?3 sec^?1 at 523 and 548 K. The microstructural examination and analysis of plastic flow curves revealed that low temperature superplasticity of the present alloy was attributed to dynamic recrystallization. In addition, necking instability during low temperature superplastic deformation of the alloy was discussed by applying Harts necking instability criterion.     

SEVERE PLASTIC DEFORMATION TECHNIQUES

1.~onTheinterestindevelopmentoftiltwhnegrainsizeincreasedinrecentyearsforlowtemperaturehighstrainratesuperplasticityofaluminumalloysandsuperstrongsteelswithoutlossoftoughness.Theultimatendcrostmctureinaspecimenisdetendnedlargelybytheflowpatternsduringstrain.Whenlargestrainisused(morethan3),multipleflowpatternsusuallyoccurexceptfortorsion.Thesemtiltipleflowpatternsareobtainedbychangingthedirectionofdeformation.Normalplanestrainhasasingledirectionofdeformationandistypicallylimitedtoamaximumstrai…     

Using ECAP to achieve grain refinement, precipitate fragmentation and high strain rate superplasticity in a spray-cast aluminum alloy

An aluminum 7034 alloy, produced by spray casting and with an initial grain size of ˜2.1 μm, was processed by equal-channel angular pressing (ECAP) at 473 K to produce an ultrafine grain size of ˜0.3 μm. It is shown that the rod-like MgZn₂ precipitates present in the as-received alloy are broken into very small spherical particles during ECAP and these particles become distributed reasonably uniformly throughout the material. The presence of these fine MgZn₂ particles, combined with a distribution of fine Al₃Zr precipitates, is very effective in restricting grain growth so that submicrometer grains are retained at elevated temperatures up to at least ˜670 K. Tensile testing of the pressed material revealed high elongations to failure, including elongations of >1000% when testing at a temperature of 673 K at initial strain rates at and above 10⁻² s⁻¹. These results confirm the occurrence of high strain rate superplasticity in the spray-cast alloy.     

挤压铸造铝基复合材料的高应变速率超塑性

用挤压铸造、挤压比仅为１０∶１的挤压以及进一步的轧制成功地制备了具有高应变速率超塑性行为的βＳｉＣ晶须增强ＬＹ１２复合材料。该复合材料晶粒细小,约为２μｍ;在温度为８０３Ｋ和初始应变速率为１．１×１０－１ｓ－１时,延伸率达３５０％,应变速率敏感系数ｍ值约为０．３５;超塑性变形的主要机制是细小晶粒的晶界滑动,适当的微量液相有利于该复合材料的高应变速率超塑性。     

Flow stress anisotropy and Bauschinger effect in ultrafine grained copper

The effect of strain path and magnitude on the flow stress anisotropy and Bauschinger effect (BE) in ultrafine grained (UFG) copper was investigated. The material billets were deformed via multipass equal channel angular extrusion (ECAE) following several deformation routes. The monotonic stress–strain responses under tension and compression and forward compression/reverse tension response along three perpendicular directions were determined in each billet. It was observed that, in certain cases, the strong tension/compression asymmetry was in favor of tension as opposed to what has so far been reported for UFG materials, and an increase in the number of ECAE passes caused a decrease in yield strength along certain sample directions. Finally, the BE was found to be more pronounced for the lower number of passes. It was shown that crystallographic texture and grain size differences cannot be the only factors responsible for these unexpected observations. Grain morphology and grain boundary character are argued to be additional parameters that have to be taken into account. How these factors affect tension/compression asymmetry, flow anisotropy and BE in UFG copper and how they can help elucidating the observations are discussed.     

等径通道挤压制备超细晶金属过程中后压力影响的有限元分析

使用有限元方法模拟在等径通道挤压过程中,后压力对材料塑性变形的影响,并对多道次挤压试验结果进行分析比较.结果表明:施加后压力可以有效提高材料每道次挤压的塑性变形程度和分布均匀性.在多道次挤压过程中,施加后压力可以大幅度降低晶粒最终细化尺寸,降低挤压温度来减小温度对晶粒细化效果的影响.     

Low temperature superplasticity of a fine-grained ZK60 magnesium alloy processed by equal-channel-angular extrusion

Fine-grained ZK60 magnesium alloy with the grain size of 1.4 μm was processed by equal-channel-angular extrusion. The material exhibited low temperature superplasticity. The normalized plot suggested that the present material had equilibrium grain boundaries at the superplastic temperature in contrast to typical materials processed by severe plastic deformation.     

等通道球形转角挤压过程中工业纯铝的微观组织演变与力学性能

突破传统ECAP变形全过程通道等截面思路,提出一种耦合剪切应变和正应变于一体的新型等通道球形转角挤压(equal channel angular extrusion with spherical cavity,ECAE-SC)工艺。在自行研制的模具上对工业纯铝进行室温单道次ECAE-SC挤压实验,采用OM、EBSD和TEM等技术手段,研究了ECAE-SC变形过程中工业纯铝微观组织的演变规律,并测试了变形后试样的显微硬度。结果表明,在ECAE-SC工艺剧烈简单剪切变形诱导下,工业纯铝仅需1道次挤压变形即可获得等轴、细小、均匀的超细晶组织,平均晶粒尺寸约为400 nm;工业纯铝室温ECAE-SC变形以位错滑移为主并伴有不完全连续动态再结晶,其微观组织经历了剪切带→位错胞→小角度亚晶→大角度等轴晶粒等动态演化过程。1道次ECAE-SC变形后,工业纯铝组织以{110}001高斯织构为主,同时存在部分{111}112铜型织构;材料显微硬度值大幅提升,由初始289.4 MPa提高到565.3 MPa,增幅高达95.33%,且分布均匀性良好。     

Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.     

大型阀体锻造的一种省力成形新工艺

针对电站锅炉用大型截止阀阀体的制造 ,提出了“剪切挤压”新工艺 ,并利用模拟实验和工艺实验的方法对剪挤工艺进行研究 ,通过 DN1 0 0大型 1 2 Cr Mo V合金钢截止阀阀体在较低吨位压力机上成功地锻造 ,进一步验证此工艺的可行性和优越性。另外 ,此工艺还可用于大型闸阀阀体、三通管等枝杈类零件的制造。     

Ultragrain refinement of plain low carbon steel by cold-rolling and annealing of martensite

Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.     

Microstructure and crystallographic texture of an ultrafine grained C–Mn steel and their evolution during warm deformation and annealing

The evolution of microstructure and texture of a 0.2%C–Mn steel during large strain warm deformation and subsequent annealing has been investigated. The process of grain subdivision during warm deformation is essential for the formation of ultrafine grains in such a material. The study reveals that pronounced recovery instead of primary recrystallization is required to obtain a large fraction of high-angle grain boundaries (HAGBs) as a prerequisite for the development of ultrafine grains in the course of warm deformation. The prevalence of primary recrystallization instead of recovery is not generally beneficial in this context since it reduces significantly the dislocation density and removes the substructure which is important for the gradual formation of subgrains and, finally, of ultrafine grains which are surrounded by HAGBs. Consistently, the texture of the ultrafine grained 0.2%C–Mn steel observed after large strain warm deformation and subsequent annealing, consists primarily of the α-(〈1 1 0〉∥RD) texture fiber which indicates strong recovery. The γ-(〈1 1 1〉∥ND) texture fiber which is typical of recrystallized rolled ferritic steels does not appear. The process occurring during the deformation and subsequent annealing can, therefore, be interpreted as a pronounced recovery process during which new grains are created without preceding nucleation.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.