单道次ECAP纯铝织构演变的有限元模拟

利用晶体塑性有限元(Crystal Plasticity Finite Element,简称CPFE)子程序和ABAQUS商业软件对多晶体纯铝等径弯曲通道(Equal Channel Angular Pressing,简称ECAP)变形进行了细观三维计算机模拟,获得多晶体纯铝在ECAP变形后各晶粒的取向分布数据,并据此得到晶粒取向的ODF图及极图.通过对结果的分析,初始晶粒取向随机分布的多晶体纯铝在ECAP单道次变形后,靠近模具内角的试样和靠近模具外角的试样由于形变的方式不同而形成了不同的织构形态,靠近模具内角的试样形成剪切织构,靠近模具外角的试样形成扭转织构.因此多晶纯铝在通道夹角Ф=90°、外圆角Ψ=20°模具中的ECAP变形并不是通过理想的纯剪切变形实现的.     

Microstructure and mechanical properties of pure gold processed by equal channel angular pressing

The microstructure and mechanical properties of pure gold were examined after different number of equal channel angular pressing passes. The hardness and 0.2% proof stress from the compression tests were observed to increase only up to the fourth pass whereas the compressive strength at 40% strain kept increasing gradually with increasing the number of passes. Microstructure with low angle grain boundaries and shear bands were observed until the eighth pass and equiaxed grains with high angle grain boundaries were observed only after the twelfth pass.     

Effects of grain size on compressive behaviour in ultrafine grained pure Mg processed by equal channel angular pressing at room temperature

Ultrafine-grained pure magnesium with an average grain size of 0.8 μm was produced by refining coarse-grained (980 μm) ingot by multi-pass equal channel angular pressing (ECAP) at room temperature with the application of a back pressure. The compressive deformation behaviour at room temperature depended on grain size, with deformation twinning and associated work hardening observed in coarse-grained Mg, but absent in the ultrafine grained material as decreasing grain size raised the stress for twinning above that for dislocation slip. The ultrafine grained Mg showed good plasticity with prolonged constant stress after some initial strain hardening.     

EBSP study of the annealing behavior of aluminum deformed by equal channel angular processing

Commercial purity aluminum (99.5%) was fabricated by equal channel angular pressing (ECAP) up to total accumulated strains of approx. 10. The annealing behavior of material deformed to total strains of approx. 1 and 10 was investigated, using heat treatments of 2 h at various temperatures from 100 to 500 °C. The microstructure of the annealed materials was characterized using the electron back-scatter pattern technique. A number of parameters were determined including the distribution and average values of both the boundary spacings and misorientations. For samples deformed to a total strain of 1, annealing resulted in discontinuous recrystallization. For samples deformed to a total strain of 10, annealing resulted in microstructures exhibiting characteristics of both uniform coarsening and, in a number of places, of discontinuous recrystallization. An attempt was made, based on the boundary spacing distributions, to separate these two components. The grain size after annealing was still however small, being just 6.4 μm after 2 h at 300 °C.     

Texture evolution and fraction of favorably oriented fibers in commercially pure aluminum processed to 16 ECAP passes

Texture evolution in 1050 commercial purity aluminum severely deformed by equal channel angular pressing (ECAP) is investigated by electron back scattered diffraction (EBSD). Pole figures and orientation distribution function (ODF) plots are generated for samples processed to 1, 2, 4, 8, 12 and 16 passes. The processing was done using route B_C, in which the samples were rotated by 90° in the same sense between subsequent pressings. Two different sized scans were performed on the flow plane of the processed samples. The orientations constituting the favorably oriented fibers are depicted and crystal orientation maps are generated. The spatial distribution of grains having these orientations are revealed through these maps. The fraction of the main texture fibers for a 5° spread around the specified orientations is experimentally calculated and a quantitative idea on the evolution of texture is presented. The results ascertained that the texture intensity around the main fibers generally weakens with number of ECAP passes.     

Effect of initial microstructure on the processing of titanium using equal channel angular pressing

Equal channel angular pressing (ECAP) is a metal processing technique that is used to produce materials with ultrafine (<1 μm) grain sizes. In this work, the effect of the initial microstructure on ECAP of commercially pure titanium (CP Ti), a material used in many industrial applications, was investigated. To produce different initial microstructures, samples of CP Ti were exposed to different annealing conditions: no annealing (Material 1), annealed at 1033 K for 2 hr (Material 2), or annealed at 1173 K for 4 hr (Material 3). Each material was subjected to one pass of ECAP and the resulting microstructures were analyzed using XRD, SEM, and TEM, and compared to the microstructures before ECAP. It was found that each material developed a unique microstructure after one pass of ECAP, which was attributed to the varying microstructural characteristics before ECAP. Microhardness values before and after ECAP varied with each microstructure.     

Flow properties of an aluminum alloy processed by equal channel angular pressing

Equal Channel Angular Pressing (ECAP) process is an important process for producing ultrafine-grained microstructures in bulk metals and alloys. In the present work, aluminum alloy AA 6063 samples were subjected to ECAP for up to three passes using an ECAP die with a die angle of 105°. The strain imparted to the specimen after three passes was approximately 2.64. Compression testing of the ECAP specimens was carried out to determine the subsequent flow behavior. Two types of compression test specimen orientations, one parallel to the axis of pressed sample and the other at 45° to the axis of the pressed sample, were used for the study. The strain path change (SPC) parameter was used to quantify the strain path change involved in straining by ECAP followed by straining by uniaxial compression. Higher flow strength values were observed in compression in specimens machined at 45º to the axis of the ECAP specimens. Flow softening and anisotropic behavior have been studied with respect to the number of passes and processing routes.     

Three-dimensional numerical analysis and experimental investigation of grain refinement in multi-pass equal channel angular pressing for round-workpieces

Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass ECAP process show that the different processing routes result in the different deformation distributions. The grain in the workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model) and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent with FE analysis results.     

Microstructure and strength of pure Cu with large grains processed by equal channel angular pressing

The pure Cu rods with an initial grain size of 410 μm were treated by using equal channel angular pressing (ECAP). The deformed microstructure and mechanical properties of ECAPed Cu samples were investigated. Special attention was paid on the refinement of grain size and local micromechanics of ECAPed Cu samples. The original coarse grains were refined to 320 μm after 4 passes. The final grains were composed of dislocation cells with a size of 500 nm–3 μm after 5–8 passes. The yield strength reached a saturation value of 368 MPa after 5 passes. The maps of microhardness distribution illustrated the inhomogeneity of local mechanical properties. The dislocation subdivision was the main deformation mode to refine the grain size, while twin fragmentation was restrained by dislocation slips for the reason of large initial grain size. Furthermore, the strengthening of ECAPed Cu was discussed.     

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

Abstract

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y₂O₃ (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.     

Pure copper processed by extrusion preceded equal channel angular pressing

High purity copper (99.99%) was severely plastically deformed by miniaturized extrusion preceded ECAP at 200 °C. A partially recrystallized microstructure was detected by TEM observations due to the relative high processing temperature. Isochronal annealing was performed to investigate the material's behavior by measuring the development of Vickers microhardness. As copper is a metal with a relative low stacking fault energy, recovery does not occur readily and only recrystallization takes place.     

Thermal stability study on two aluminum alloys processed with equal channel angular pressing

Equal channel angular pressing was used to produce sub-micrometer size grain structures in two aluminum alloys (commercially pure 1200 and Al-Mn-Si 3103). ECAP was conducted at room temperature following the process via route C, which involves a 180° angular rotation between passes and strongly affects material microstructure by reversing the shear strain every second pass. This unique characteristic of route C induced limited build-up of new high-angle boundaries, at least for the first three passes. The equal channel angular pressing was extended to six passes for both alloys; three passes by route C throughout the die were sufficient to produce a very fine-structured material for both alloys. High-resolution electron back-scattered diffraction pattern analysis was carried out to measure boundary misorientation within the deformed structures. Measurements of subgrain and grain spacing revealed a more effective microstructure refining effect in the 3103 than the 1200 alloy. Thermal stability of the severely deformed materials was studied at temperatures of 130, 240, 330°C, corresponding to 0.2, 0.35, 0.5 of the melting temperature. The results showed considerable grain growth in both materials solely at temperatures from and above half of the melting one.     

Cryogenic equal channel angular pressing of commercially pure titanium: microstructure and properties

Cylindrical samples of CP Titanium (Grade 2) were deformed by one, two and three passes of equal channel angular pressing (ECAP) each at temperatures 77, 300 and 575 K, respectively. The microstructure of samples processed at 77 K shows retardation of recrystallisation, high density of dislocations and deformation twins, diffuse and obscure grain boundaries compare to microstructure of samples processed at room and high temperature, where recrystallised ultrafine equiaxed grains are observed. Mechanical properties for all structural states of Ti were studied by microhardness measurements at 300 K and uniaxial compression at temperatures 300, 170, 77 and 4.2 K. Higher levels of ECAP deformation (more passes of ECAP) lead to higher values of strength and hardness at all studied temperatures. Decrease of ECAP temperature leads to increase of strength characteristics in all cases. Influence of ECAP and compression temperatures on possible changes of deformation mechanism are discussed.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Microstructure and mechanical properties of ultrafine grain ZK60 alloy processed by equal channel angular pressing

The microstructure evolution and tensile properties of ZK60 magnesium alloy after equal channel angular pressing (ECAP) have been investigated. The results show that the two-step ECAP process is more effective in grain refinement than the single-step ECAP process due to the lower deformation temperature, a mean grain size of ~0.8 μm was obtained after two-step ECAP process at 513 K for four passes and 453 K for four passes. The EBSD examination reveals that ZK60 alloy after two-step ECAP process exhibits a more homogeneous grain size and misorientation distribution than single-step ECAP process. Both alloys after ECAP process present similar strong {0002} texture. The tensile strength of two-step ECAP alloy has also been improved compared with the single-step ECAP alloy. The strengthening effect was mainly ascribed to grain refinement.     

Effect of texture evolution on tensile properties of Al–Ni eutectic alloy fabricated by equal channel angular pressing

Abstract

The present study investigated in detail the effect of texture evolution on the mechanical properties of an Al–5·7 wt-%Ni eutectic alloy, which was subjected to severe plastic deformation by the equal channel angular pressing (ECAP) technique. The ECAP procedure was carried out using two strain introduction methods, route B_C and route A, at a temperature of 298 K and a pressing rate of 0·33 mm s^?1. The as pressed microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that the Al–Ni eutectic alloy specimens after ECAP processing by route B_C and route A methods had very different microstructures, which strongly affected the tensile properties of the specimens. It was demonstrated that after ECAP processing by route B_C, fine Al₃Ni particles of ～300 nm were homogeneously dispersed in the aluminium matrix, and the specimens showed no clear anisotropy in tensile properties. After ECAP processing by route A, however, eutectic textures containing α-Al and Al₃Ni fibrous dispersoids had a highly anisotropic distribution and were demonstrated to have significantly anisotropic tensile properties. Based on the experimental results, the fracture mechanism during tensile testing of the Al–Ni eutectic alloy using different strain induction methods is discussed.     

连续等通道转角挤压对Al-Ti-C合金组织与性能的影响EI北大核心CSCD

采用连续等通道转角挤压工艺,以连续的方式对Al-Ti-C合金进行多道次挤压,通过观察微观组织演化,探讨晶粒细化机理和力学性能变化。结果表明:连续等通道转角挤压工艺可有效细化Al-Ti-C合金微观组织,晶粒尺寸减小至1μm左右,形变诱导是变形过程中最主要的晶粒细化机制;高密度位错堆积引起Al基体和TiAl_(3)界面的裂纹以及TiAl_(3)内部的空洞产生,裂纹进一步扩展贯穿整个TiAl_(3)颗粒,最终导致第二相TiAl_(3)组织的细化,同时细小的第二相TiAl_(3)组织的钉扎机制和剪切机制促进了Al基体细化;连续等通道转角挤压1道次后,合金硬度提升最明显,与原始态相比提高59.2%;之后随挤压道次的增加,硬度提升的趋势变缓,合金塑性下降,韧性提高。     

等径角挤压对Al-Cu-Mg-Ag合金组织性能的影响

为研究大塑形变形对耐热铝合金的作用,采用铸冶金工艺制备了新型的Al-Cu-Mg-Ag耐热铝合金,通过显微组织观察、差热分析及硬度测试等方法,研究了等径角挤压对耐热铝合金显微组织与力学性能的影响.结果表明:通过对挤压态的Al-Cu-Mg-Ag耐热铝合金在固溶淬火后时效前进行等径角挤压变形,可获得晶粒尺寸低于2μm的块体超...