等通道转角挤压自增强等规聚丙烯研究

为了研究等通道转角挤压等规聚丙烯的挤压工艺条件、结构与性能之间的关系,采用90°转角模具及挤压路径A、在熔点以下的160℃制备了不同挤压次数的等规聚丙烯样品,利用广角X射线衍射(WAXD)、差示扫描量热分析(DSC)等手段对其结构进行表征.结果表明,随挤压次数增加,等规聚丙烯分子取向明显增加,材料结晶度提高,试样的力学性能提高十分显著,未挤压试样的抗拉强度为17.13 MPa,经过三次挤压后,抗拉强度提高到59.38 MPa.     

Room temperature corrosion properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing

The electrochemical properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing (ECAP) were investigated. The electrochemical properties were evaluated using potentiodynamic tests and electrochemical impedance spectroscopy in corrosion solution of 0.1 M sodium chloride. The electrochemical changes of the sample surface were correlated with microstructure evolution. Material processed by extrusion and subsequently by 8 passes of ECAP shows similar or even inferior corrosion resistance to the extruded material after immersion time up to 96 h. However, corrosion resistance of material after extrusion and ECAP is significantly better than that of the extruded material for immersion time of 168 h. This sudden improvement is caused by different formation and fall off of protective corrosion products. Microstructure after extrusion is inhomogeneous and contains relatively large grains, whereas material after ECAP possesses homogeneous ultrafine-grained (UFG) microstructure. As a result, material after ECAP offers more corrosion nucleation sites, but UFG microstructure causes that only smaller clusters of corrosion products fall off the surface. The easier and faster corrosion protective layer restoration on the surface of UFG material after ECAP leads to enhanced corrosion resistance.     

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.     

Investigation of texture and mechanical properties of copper processed by new route of equal channel angular pressing

The evolution of crystallographic texture and the mechanical properties of copper subjected to severe plastic deformation (SPD) using equal channel angular pressing (ECAP) were investigated. Samples were subjected to ECAP under two different processing routes: B₆₀ and B_C. As the cross sections of the samples were circular, a new route with a rotation angle of 60° in the same direction between consecutive passes was introduced. The material exhibited texture development similar to the simple shear texture in both routes and the most significant changes in texture strength in both processing routes took place after the second pass. Microstructure of ECAP processed samples were investigated using electron backscatter diffraction (EBSD) analysis. Comparison of the EBSD data with optical micrograph of the initial sample confirmed that ECAP process has led to a significant decrease in grain size. Significant increases in hardness and tensile strength were observed after the first pass of ECAP. Variations of tensile strength as a function of the number of passes were related to the dislocation densities and the average boundary spacing.     

Superplasticity of Mg-Zn-Y alloy containing quasicrystal phase processed by equal channel angular pressing

Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10^− 3 to 1.5 × 10^− 4 s^− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10^− 4 s^− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated.     

Annealing twins in high purity aluminium processed by dynamic equal channel angular pressing

Annealing twins are synthesised in high purity aluminium processed by dynamic equal channel angular pressing during annealing. Annealing twins and recrystallised grains encircling the twins have specific crystallographic orientation relationships with the deformed matrix grains: approximately 35°–50°<110> for the twins and 30°–45°<100> for the recrystallised grains. Stored energy during dynamic pressing, and the crystallographic orientation between annealing twins and the deformed matrix, strongly affect twin growth during annealing.     

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.     

Plastic anisotropy in aluminum and copper pre-strained by equal channel angular extrusion

The mechanical response of as-processed equal channel angular extrusion materials is anisotropic, depending on both direction and sense of straining. The stress–strain curves exhibit hardening characteristics different from the usual work hardening responses, e.g., Stages I–IV, expected in annealed fcc metals under monotonic loading. In this work, the anisotropic flow responses of two pure fcc metals, Al and Cu, processed by route Bc are evaluated and compared based on pre-strain level (number of passes), direction of reloading, sense of straining (i.e., compression versus tension), and their propensity to generate subgrain microstructures and to rearrange, should the slip activity change. In most cases, either macroscopic work softening or strain intervals with little to no work hardening are observed. Application of a crystallographically based single-crystal hardening law for strain-path changes [Beyerlein and Tomé, Int. J. Plasticity (2007)] incorporated into a visco-plastic self-consistent (VPSC) model supports the hypothesis that suppression of work hardening is due to reversal or cross effects operating at the grain level.

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The effect of microstructural inhomogeneity on the growth paths of surface-cracks in copper processed by equal channel angular pressing

The growth behavior of cracks is monitored on specimens of ultrafine grained copper produced by equal channel angular pressing. Temporary retardation of crack growth under low stress amplitudes occurs when the crack length reaches about 0.1 mm, but there is no similar retardation at high stress amplitudes. Dependent on stress amplitude, different crack growth path morphologies develop. Analysis of the fracture surfaces is conducted by scanning electron microscopy, showing planer, granular and striated surfaces. The physical background of growth path and fracture surface formation is discussed by considering crack growth mechanism and microstructural inhomogeneity.     

Characterization of aluminum processed by equal channel angular extrusion: Effect of processing route

Equal channel angular extrusion (ECAE) of aluminum alloys leads to enhanced strength. However, the effects of processing route on the resulting properties were not extensively investigated, and sometimes neglected. In the present work, an aluminum was processed by ECAE via routes B_C and C using a die formed by two channels connected at an angle Ф = 90°. Coherency lengths D calculated from X-ray diffraction peaks were 85 nm after the first passage. For route B_C, it was kept constant when the number of passages N was increased. For route C, an increase of D was observed when increasing N. The remaining densities of dislocations ρ were calculated using a recently published formula. A decrease of ρ was observed during ECAE in route C but not in route B_C. Mechanical tests allowed us to relate the increase of D and the decrease of ρ to an enhanced recovery during ECAE via route C.     

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.     

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 investigation and thermal stability of 99.1% aluminum processed by equal channel angular extrusion

Microstructure evolution of 99.1% aluminum after equal-channel angular extrusion (ECAE) and subsequent heat-treatment was investigated. After deformation the samples were annealed at different temperatures. The deformed and annealed states were characterized by Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), and microhardness tests. It was shown that the observed microstructure changes during subsequent annealing have to be associated with recovery and cells formation. The initial stages of recovery were investigated using weak-beam technique. The microstructure obtained after annealing for 1 h at 100 °C consists of some arrangements of the dislocations into sub-grain boundaries within the wide preexisting grains. Annealing at 300 °C led to the appearance of a duplex microstructure consisting of bands of slightly coarsened grains associated with refined grains. No growth of dislocation cells was observed up to 400 °C. In XRD measurements, the lattice parameter increase with subsequent heating. This indicates a continuous grain growth during annealing. This is due to the important increase of coherency length, D observed parallel to a substantial decrease of rms-strain, ε.     

Grain boundary misorientation and positron annihilation characteristics in steel Eurofer processed by equal channel angular pressing

Electron back-scattered diffraction and positron annihilation lifetime measurements were performed on the ferritic–martensitic 9 %Cr Eurofer steel processed by equal channel angular pressing (ECAP) at 550 °C. The orientation imaging mapping images reveal ECAP-induced changes toward a more stable texture via rotation around 〈111〉 and 〈110〉 axes. These are accompanied by transformation of martensite into ferrite and a remarkable change in the bimodal distribution of grain misorientations with reduction of the fraction of high-angle grain boundaries. The positron annihilation measurements evidence the release of vacancies retained in the ECAP-processed material and their clustering into nanovoids in samples heat treated at T ≥ 500 °C. It is found that tensile strain applied at 600 °C can inhibit this void formation in the ECAP-deformed material for 1 and 2 passes, but not in the case of 4 passes. A model for accounting the results is proposed.     

铸态AZ81镁合金ECAP态组织与性能研究

采用自制的90°模具,经Bc路径在温度为300℃下研究对比了铸态及不同道次的等通道挤压(ECAP)态AZ81镁合金微观组织和力学性能.结果表明ECAP随着挤压道次的增加,AZ81镁合金显微组织和力学性能发生显著变化.当挤压到4道次,平均晶粒尺寸由原来铸态的145um细化为9.6um,拉伸断口韧窝明显增多;抗拉强度从180 MPa提高到306 MPa,延伸率和硬度分别达到15.8%和142HL.分析表明,AZ81镁合金在高温挤压过程中Mg17Al12相粒子被破碎,并部分溶入基体,$-Mg基体与%-Mg17Al12相互相阻碍其晶粒长大,获得细小晶粒组织.     

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.