等径角挤压铜合金的分节断裂

本研究提供了等径角挤压不同铜合金的局部剪切、坯锭开裂和分节断裂的实验依据。结果表明,尽管很多参数影响局部剪切,但是合金的硬度和分节断裂与其有着直接的关系,而硬度与合金的成分和相组成有关。在室温下,α-黄铜可以成功进行等径角挤压,而α/β黄铜甚至在350°C下都不能成功进行等径角挤压。利用DEFORMTM 软件模拟了开裂和分节断裂,研究不同参数对分节断裂的影响。结果表明,摩擦力和加工速率对获得完美坯锭影响很小,而利用背压可以很好地减小局部剪切、坯锭裂纹、分节断裂和破坏。利用背压能减小流动局部化,当背压由0提高到600 MPa时,可以提高材料流动均匀性并且使坯锭的均匀性提高36.1%。     

Evolution of precipitates of Al-Cu alloy during equal-channel angular pressing at room temperature

The evolution of precipitates and hardness changes in Al-Cu alloys during equal-channel angular pressing（ECAP） at room temperature were investigated by hardness measurement, X-ray diffraction analysis and transmission electron microscopy. The results show that with the increase of the total equivalent strain during ECAP from 0 to 8.4, the hardness of specimens with metastable θ″ phase increases first and decreases in later period. The hardness increases successively in specimens containing metastable θ′ phase and equilibrium θ phase. It is believed that the evolutions of hardness are related to the mechanism of re-dissolution of precipitates. A critical nuclei size concept is provided to express the mechanism of such re-dissolution of three precipitates in Al-Cu alloys.     

Microstructure evolution and mechanical behavior of AZ31 Mg alloy processed by equal-channel angular pressing

AZ31 Mg alloy bar was subjected to 8-pass equal-channel angular pressing(ECAP) at 623 K. Microstructure evolution was observed by optical microscopy(OM) on cross section and X-ray diffraction analysis. The room temperature mechanical properties of the ECAP processed specimens were also investigated. A fine-grained structure with an average sub-grain size of 9 μm is obtained after 7 ECAP passes. XRD analysis indicates that after ECAP, in placing of planes and become the dominant directions that are favourable for grain refinement. ECAP processed AZ31 Mg alloy exhibits significant improvement in elongation but decrease in strength. The elongation of the specimen increases continuously up to 2 passes and then remains stable at further passes. This improvement can be related to the evolution of crystallographic texture and the scattered orientation of the basal plane (0001).     

等径弯曲通道变形制备高性能Cu-Cr合金的组织性能

用两种方式等径弯曲通道变形(equal-channel angular pressing,简称ECAP)制备了的具有等轴晶组织的超细晶Cu-0.4Cr合金,晶粒尺寸为500nm。研究了不同挤压方式、不同挤压道次合金的组织和性能的变化。探讨了不同退火温度对5～8道次材料导电率和硬度的影响。结果表明,经ECAP挤压后的Cu-0.4Cr合金具有很好的综合性能,拉伸强度可达565MPa;硬度和导电率分别为225 HV和66.4%IACS;723K退火1h后材料的导电率和硬度可达80.3%IACS和210.9HV;软化温度可达723K。     

Sulphuric acid corrosion of ultrafine-grained mild steel processed by equal-channel angular pressing

A bulk ultrafine-grained (UFG) mild steel with a ferrite grain size of approximately 200 nm and a dispersed distribution of iron carbide particles was fabricated by equal-channel angular pressing (ECAP) at 400 °C. The corrosion behaviour of the ECAP-processed mild steel and pure iron was investigated in a 0.5 mol/L H₂SO₄ solution. They exhibited a higher corrosion rate and better anodic passivity properties due to the presence of more crystalline defects. As a result of the refinement of the iron carbide particles, the forming ability of a continuous dense passive film was improved.     

Plastic deformation mechanism of ultra-fine-grained AA6063 processed by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is an established method to produce ultra-fine-grained (UFG) materials with extraordinary mechanical properties. However, different methods to characterize the complex microstructure give contradictory results. Therefore an ECAP-processed UFG aluminum alloy AA6063 was characterized by various electron-microscopy-based methods such as backscattered electron contrast, electron backscatter diffraction, transmission electron microscopy and low-voltage scanning transmission electron microscopy. Only a skilful combination of all methods reveals the complete information about the microstructure which is needed to understand the results of the mechanical testing by nanoindentation, tensile tests and strain-rate jump test. The main difference is the amount of low-angle grain boundaries and high-angle grain boundaries which determine hardness, tensile and yield strength and strain-rate sensitivity of ECAP materials produced by different numbers of passes and routes. This is explained by the interaction of dislocations with the different grain boundaries.     

Structure and mechanical behaviour of interstitial-free steel processed by equal-channel angular pressing

The influence of the number of passes in equal channel angular pressing (ECAP) following route B_C on microstructure and mechanical properties of interstitial-free steel was investigated by means of tensile tests and X-ray texture and diffraction profile analysis. A significant improvement of the mechanical properties was found with increasing the number of ECAP passes. After 8 passes, beside the high strength considerable ductility was observed and at 300 °C the ductility was the same as for the initial sample but with a two-times larger strength. The high strength measured at room temperature was only slightly reduced during annealing at temperatures up to 500 °C.     

Microstructure and tensile properties of ultrafine grained copper processed by equal-channel angular pressing

1. Introduction There has been an interest in the research of spe- cific microstructure and unique mechanical proper- ties in ultrafine-grained (UFG) materials [1]. Equal-channel angular pressing (ECAP) process has been successfully applied to obtain UFG structure in numerous metals and alloys [2-6]. However, there are some deficiencies on copper deformed to large shear strains. Ferrasse et al. [6] argued that intense simple shear promotes dynamic rotation recrystalli- zation during ECAP…     

Mechanical properties at room temperature of an Al-Zn-Mg-Cu alloy processed by equal channel angular pressing

Tensile tests were conducted to evaluate the influence of equal-channel angular pressing (ECAP) on the mechanical properties at room temperature of overaged Al 7075-O alloy. ECAP processing was performed using route B_C at different temperatures and number of passes, i.e. different processing severity conditions. The maximum load (F_max) recorded during the last pass of each ECAP path considered in this study is a very good estimation of the processing severity. The mechanical properties were studied in terms of the balance between tensile strength and ductility. In the processed Al 7075-O alloy, the grain size was reduced down to ∼150 nm. Consequently, tensile testing at room temperature revealed a significant increase in the maximum tensile strength after ECAP with respect to the as start material. In the present study, as the processing severity increases with the number of ECAP passes or with the decrease in processing temperature, there is a consistent trend of increment in ultimate tensile strength with minor decrease in uniform plastic elongation respect to the first ECAP pass at room temperature. This is in contrast to the behaviour after more severe plastic deformation conditions, where an increase in strength together with a strong decrease in elongation would be expected.     

Tensile properties of 2024 Al alloy processed by enhanced solid-solution and equal-channel angular pressing

Equal-channel angular pressing（ECAP） of an enhanced solid-solution treated 2024 Al alloy was successfully performed at room temperature, with an imposed equivalent normal strain of about 0.5. A very high hardness about HV191 and yield strength about 610 MPa （30% higher than those of the unECAPed 2024 Al alloy） in terms of commercial aluminum alloys were observed for the ECAPed 2024 Al alloy. In addition to the strengthening, this process allows the ECAPed 2024 Al alloy have a moderate level of tensile ductility （about 12.7%） and a significant strain hardening capability up to tensile failure. After aged at 373 K for 48 h, the ECAPed alloy increases its hardness （about HV201） and tensile ductility （about 14 %） further. The TEM results show that the ECAPed 2024 AI alloy presents a plate structure （about 50-100 nm） with high density of dislocation and additional thin plate （approximately 〈10 nm= inside. The XRD results show that the ECAP processing decreases the texture and increases the dislocation density of the alloy considerably. The theoretical calculations show that the increase of dislocation density resulting from ECAP processing makes a considerable contribution about 55.2 % for the improvement of yield strength.     

Characterization of creep properties and creep textures in pure aluminum processed by equal-channel angular pressing

High-purity aluminum was processed by equal-channel angular pressing (ECAP) and then tested under creep conditions at 473 K. The results show conventional power-law creep with a stress exponent of n = 5 which is consistent with an intragranular dislocation process involving the glide and climb of dislocations. It is demonstrated that diffusion creep is not important in these tests because the ultrafine grains produced by ECAP are not stable at this temperature. Texture measurements were undertaken using the high-pressure preferred orientation neutron time-of-flight diffractometer and they reveal significant differences in the evolution of texture during creep in pressed and unpressed specimens. These experimental measurements of texture are in excellent agreement with theoretical textures predicted using a visco-plastic self-consistent model that limits deformation to plastic slip. The calculations provide additional confirmation that creep occurs through an intragranular dislocation process.     

Evolution of nanoscale porosity during equal-channel angular pressing of titanium

Small-angle neutron scattering (SANS) analysis and transmission electron microscopy evidence suggest the occurrence of nanoscale porosity in commercial-purity titanium processed by equal-channel angular pressing (ECAP). SANS data were produced at two different facilities (GKSS, Germany; and Los Alamos, USA) and were analysed using three different methods. The results are consistent and yield a conclusive picture of the distribution of the scattering centres, which are believed to be associated with nanoporosity. Back pressure applied during ECAP tends to reduce the average pore size, which also depends on the processing route used. The results of the study strongly suggest that ECAP leaves a footprint in titanium in the form of a population of polydispersed nanovoids, which may play an important role in subsequent processing of the material.     

Application of ultrasonic methods to determine elastic anisotropy of polycrystalline copper processed by equal-channel angular pressing

Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and four passes of equal-channel angular pressing (ECAP) is investigated by means of ultrasonic methods. For each material, Young’s and shear moduli in the principal processing directions are evaluated and the symmetry and orientation of the anisotropy are identified. The relation between the determined symmetry and the processing mechanisms is discussed. It is shown that the material after one and two passes of ECAP exhibits a measurable anisotropy, while the material after the fourth pass behaves isotropically. Within the discussion, it is shown that the origin of the observed anisotropy may be attributed to the spatial arrangement of grain boundaries rather than to the crystallographic texture. In the light of this conclusion, the obtained results correspond well with optical and transmission electron microscopy observations of the microstructures of ECAPed materials documented in the literature.     

等通道转角挤压制备超细晶Mg15Al双相合金组织与性能

对高铝双相合金Mg15Al在553K以Bc路线进行了不同道次的等通道挤压(ECAP),获得了超细晶高铝镁合金。通过OM,SEM,TEM分析了ECAP前后合金的微观组织结构及断口形貌,并测试了不同挤压道次后合金的硬度和室温拉伸性能,分析了ECAP细化晶粒机理及其性能改善原因。结果表明,随挤压道次增加,累计形变增强,网状硬脆相β-Mg17Al12破碎,合金晶粒显著细化,但对单相区和两相混合区细化效果不同。在α、β两相共存区内,4道次ECAP后形成100nm～200nm的细晶粒;在α单相区,4道次ECAP后晶粒为1μm以下,且在初晶α-Mg内析出弥散细小的β相,起到细晶强化和弥散强化作用。8道次ECAP后,晶粒略有长大。ECAP使合金的硬度、抗拉强度和延伸率同时得到提高,尤其是4道次ECAP后,硬度提高了32.04%,抗拉强度σb从150MPa提高到269.3MPa,延伸率δ由0.05%提高到7.4%;8道次ECAP后,硬度、抗拉强度略有下降,延伸率略有上升。SEM断口观察显示ECAP使合金拉伸断口形貌由铸态的解理断裂特征转变为延性韧窝断裂特征。     

3D finite element method analysis of deformation and temperature rise during equal-channel angular pressing

The material flow, temperature rise of the billet and pressing load during equal-channel angular process （ECAP） were studied by using 3D finite element method for Cu at different comer angle of mold, interfacial friction coefficient between the billet and the mold. As comer angle increases, the magnitude of shear deformation decreases and the strain difference between upper and lower part of the material becomes more apparent. The pressing load and peak temperature rise of the billet become low as the interracial friction coefficient decreases. The effects of the comer angle of mold on the temperature rise of the billet can be ignored, but the effects of the friction coefficient between the mold and the billet must be taken into account. For pressing load, the effect of the friction coefficient is larger than that of the comer angle. A good agreement between the simulated and measured material flow is obtained.     

Obtaining a nanostructure in titanium by equal-channel angular pressing

Conclusions 1. We have developed a technological setup for isothermal equal-channel angular pressing (ECAP) for hard-to-deform materials that serve at up to 500°C and specific loads on the punch of up to 2000 MPa. 2. We established the process parameters that provide fabrication of defectless preforms from titanium of grade VT1-0 with a nanostructure and obtained specimens 100 mm long and 20 mm in diameter. 3. We used the method of grids to study the strain state of preforms obtained by the ECAP method and established the great influence of the geometry of the channels, in particular, of the back angle of intersection of the pressing channels, on the uniformity of the distribution of shear strain in the plane of flow of the material. 4. Rotation of a preform of titanium VT1-0 after each cycle of ECAP by 90° about the longitudinal axis provides a homogeneous structure with grains 0.2 – 0.5 μm in size after eight treatment cycles. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 27 – 31, September, 2000.     

Grain refinement mechanism during equal-channel angular pressing of a low-carbon steel

The grain refinement mechanism during equal-channel angular pressing of a plain low-carbon steel was explored by a careful analysis of the slip systems operating at each pass of repetitive pressing. The steel was subjected to one to eight passes of pressing, in which a single passage yielded an effective strain of ∼1, at 623 K. At the initial stage of pressing, submicrometer-order ferrite grains enclosed by serrated and low-angled boundaries were formed. Transmission electron microscopy examination revealed that these boundaries resulted from interaction between the slip systems that are typical in body-centered cubic structures. Further pressings mainly resulted in rotation of ultrafine subgrains rather than grain refinement, providing the formation of high-angle grain boundaries. Since the serrated boundaries restrict dislocation movement, the rotation of subgrains with the serrated boundaries is more favorable for accommodating further deformation than intragranular strain, and therefore boundaries become high-angled.