Tension–compression asymmetry in severely deformed pure copper

In this work, we investigate the tension–compression asymmetry in the flow response of pure copper, severely deformed by a single pass of equal channel angular extrusion (ECAE), and tested afterwards in tension and compression along three orthogonal directions. The tension and compression responses differ in flow stress, hardening rate and transient behavior. The asymmetry in tension and compression responses depends on the direction of straining. Predictions from a microstructurally based hardening law implemented in a viscoplastic self-consistent polycrystal model demonstrate that the tension–compression asymmetry and its anisotropy arise not only from crystallographic texture but also from the directional substructure induced by the severe pre-straining. Slip activity differs in each grain and in each axial test, depending on its crystallographic orientation, orientation relationship with the previously generated substructure and pre-strain history. Asymmetry occurs because tension and compression represent two different types of strain path changes. Consequently, macroscopic deformation is a reflection of the type of strain path change represented by the post-ECAE axial test.     

连续等径角挤压制备超细晶铜

介绍一种制备超细晶材料的新技术———连续等径角挤压,它将连续挤压技术应用于制备超细晶材料的等径角挤压工艺,解决了传统等径角挤压不能制备大尺寸超细晶材料的问题,该技术对超细晶材料的推广应用具有重要意义。通过DEFORM3D对铜的连续等径角挤压过程的数值模拟,得到变形过程中金属的流动、应变场和温度场情况,并对不同变形速度和摩擦条件下的变形过程进行比较,得到优化的工艺参数,并以此参数进行实验。实验结果表明,连续等径角挤压后铜的硬度明显上升,连续等径角挤压3道次后,硬度趋于饱和;连续等径角挤压12道次后,铜的平均晶粒尺寸为400 nm。     

高应变速率下AZ31镁合金的各向异性及拉压不对称性

采用分离式霍普金森拉杆及压杆装置,研究挤压态AZ31镁合金高速变形下的各向异性及拉压不对称性,并从微观变形机制的角度探讨具有强烈初始基面织构的挤压态镁合金各向异性及拉压不对称性产生的原因。结果表明:在高速变形条件下,依据加载方向及应力状态挤压态AZ31镁合金的拉伸行为表现出很强的各向异性,但压缩行为的各向异性不明显;在挤压方向表现出很强的拉压不对称性,而在垂直于挤压方向的拉压不对称性很低。挤压态AZ31镁合金宏观上的各向异性及拉压不对称性是由于不同的微观变形机制所引起的。沿挤压方向拉伸的主要变形机制为柱面滑移,沿垂直于挤压方向拉伸及压缩的主要变形机制为锥面滑移;沿挤压方向压缩时初始变形机制为拉伸孪晶,当变形量为0.08(8%)左右时由于孪晶消耗殆尽,变形变而以滑移的方式进行。     

120°模具室温ECAP制备工业纯钛的热压缩变形行为

在Gleeble-1500热模拟机上对室温120°模具等径弯曲通道变形(ECAP)制备的平均晶粒尺寸为200nm的工业纯钛(CP-Ti)进行等温变速压缩实验,研究超细晶(UFG)工业纯钛在变形温度为298～673K和应变速率为10-3～100s-1条件下的流变行为。利用透射电子显微镜分析超细晶工业纯钛在不同变形条件下的组织演化规律。结果表明:流变应力在变形初期随应变的增加而增大,出现峰值后逐渐趋于平稳;峰值应力随温度的升高而减小,随应变速率的增大而增大;随变形温度的升高和应变速率的降低,应变速率敏感性指数m增加,晶粒粗化,亚晶尺寸增大,再结晶晶粒数量逐渐增加;超细晶工业纯钛热压缩变形的主要软化机制随变形温度的升高和应变速率的降低由动态回复逐步转变为动态再结晶。     

异步叠轧辅助退火制备超细晶铜材的形成机制及组织控制(英文)

对原始晶粒大小为60～100μm的铜材进行六道次大变形异步叠轧并辅助退火处理,获得晶粒大小为200nm的超细晶铜材,研究超细晶铜材的微观组织结构和性能。结果表明:六道次大变形异步叠轧后的超细晶铜材组织中存在大量的亚结构,也存在特定织构C组分,其强度和显微硬度高但伸长率和电导性下降。经220°C、35min退火处理后,亚结构消失,晶界由大角度晶界组成,织构由多种织构组分组成,也出现部分孪晶。与六道次大变形异步叠轧的超细晶铜材相比,经220°C、35min退火处理的超细晶铜材的抗拉强度和屈服强度略有下降,但伸长率和导电性明显提高。     

Plastic flow anisotropy of pure zirconium after severe plastic deformation at room temperature

The present work investigates the plastic flow anisotropy of zirconium following one pass of equal-channel angular extrusion (ECAE) at room temperature in a 90° die. Samples were oriented with their strong basal pole texture either aligned with or perpendicular to the extrusion direction prior to processing, which leads to two significantly different starting textures. After ECAE, the samples were compressed to 30% along one of three sample directions. A visco-plastic self-consistent polycrystal model was used to determine the mechanisms responsible for the observed anisotropy. Using the same single-crystal material parameters, the model reasonably predicts texture evolution and the post-compression stress–strain response. We found that basal slip is crucial in reproducing the experimental results, and twinning makes an important contribution. These findings are noteworthy because basal slip is not active in zirconium at room temperature, and grain size reductions associated with ECAE are expected to suppress deformation twinning.     

Studying grain fragmentation in ECAE by simulating simple shear

A crystal plasticity based finite element model is developed to simulate grain fragmentation during equal-channel angular extrusion (ECAE). Route C multiple passes are approximated by repetitive planar simple shears. Results are presented in terms of plastic strain distribution, texture and grain size evolution. Numerical results are found to be in qualitative agreement with experimental observations associated with the grain refinement in ECAE.     

120°模具室温ECAP制备工业纯钛的压缩变形本构关系

在Gleeble-1500热模拟机上对120°模具室温Bc方式ECAP变形8道次制备的平均晶粒尺寸约为200 nm的工业纯钛进行等温变速压缩实验,研究超细晶工业纯钛在变形温度为298~673 K和应变速率为1×10-4~1×100s-1条件下的流变应力行为。结果表明:变形温度和应变速率均对流变应力具有显著影响,峰值应力随变形温度的升高和应变速率的降低而降低;流变应力在变形初期随应变的增加而增大,出现峰值后逐渐趋于平稳,呈现稳态流变特征。采用双曲正弦模型确定了超细晶工业纯钛的变形激活能Q=104.46 kJ/mol和应力指数n=23,建立了相应的变形本构关系。     

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.     

Microstructure–mechanical property relationships in ultrafine-grained NbZr

The present paper reports on the microstructure–mechanical property relationships in an ultrafine-grained (UFG) niobium–1 wt.% zirconium (NbZr) alloy, a potential biomedical material, severe plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Monotonic tensile and low-cycle fatigue (LCF) experiments were carried out on the NbZr samples processed along ECAE routes 8B_C and 16E, along with extensive microstructure analysis. The important finding is that the NbZr alloy processed along ECAE routes that lead to a higher volume fraction of high-angle grain boundaries (HAGBs) exhibits a stable cyclic deformation response in the LCF regime. This stands in good agreement with prior studies on other materials, such as UFG interstitial-free steel, in which the stable fatigue behavior was associated with the dominance of HAGBs. The current results provide a venue for utilizing the UFG NbZr alloy in biomedical applications that require a combination of long-term durability, high strength and very good biocompatibility, where the latter is not altered by ECAE processing. Furthermore, for the first time, we present guidelines for optimizing processing parameters that define the microstructure–cyclic stability relationship in UFG alloys.     

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.     

超细晶铜在退火与高温变形条件下微观结构的不稳定性研究

通过差示扫描量热仪(DSC)和显微硬度测试研究了等通道转角挤压(ECAP)制备的超细晶铜在退火条件下的热稳定性和硬度变化, 同时利用扫描电镜电子通道衬度(SEM--ECC)技术和透射电镜(TEM)研究了超细晶铜在室温到300 ℃的温度范围内分别在单向压缩和循环变形后的微观结构变化. 结果表明: 超细晶铜即使在低于再结晶温度退火条件下也会以缓慢渐进的方式发生逐步的再结晶和晶粒粗化, 该结构软化过程通过DSC随退火时间的响应曲线探测不到. 高温压缩下晶粒的粗化行为与应变速率有关, 应变速率越大, 粗化的局部化越明显; 应变速率越小, 更多的晶粒发生整体粗化. 高温循环加载促使晶粒粗化发生得更为显著、均匀, 在粗化的晶粒内可观察到一些典型的位错组态, 如墙结构和胞结构等. 另外, 利用最大晶粒尺度(D_max)与平均晶粒尺度(D_aver)的比值V定量讨论了不同高温变形情况下晶粒粗化的不均匀性.     

Modelling texture development during equal channel angular extrusion of aluminium

The deformation textures that develop in aluminium during ECAE (without rotation of the billet) have been investigated experimentally and modelled using the FC-Taylor approach, for two different die angles (90 and 120°), by using actual deformation histories measured from scribed marker grids. This has shown that the deformation during ECAE can best be described in terms of streamline coordinates and involves a simple shear parallel to the streamline, which becomes aligned with the final extrusion direction, and a plane strain tension and compression component that develops as the material enters and leaves the dies deformation zone. The textures observed were similar to those found following torsion straining and had the main components {001}〈110〉 and {112}〈110〉 along a B partial fibre. However, in the case of ECAE, the positions of maximum intensity were rotated by ~15–20° about the transverse direction (TD). Similar textures were seen for even and odd numbers of extrusion passes, suggesting that the TD rotation is not caused by alignment of the fibre direction with the die’s ‘shear plane’, as has been previously reported. In contrast, texture simulations showed that this rotation occurs as a consequence of the additional plane strain compression component in ECAE deformation.     

等径弯曲通道变形制备超细晶铜的热稳定性

室温下采用等径弯曲通道变形(Equal Channel Angular Pressing,ECAP)C方式进行了纯铜(99.95%)12道次挤压变形。通过等温和等时退火,研究ECAP变形后铜的退火行为,并研究了等径弯曲通道变形和退火后纯铜的显微硬度和显微结构变化。分析了ECAP应变量、退火时间和退火温度对超细晶铜的再结晶行为、抗软化性能的影响。结果表明:ECAP变形后的超细晶铜在退火过程中,表现出不连续再结晶现象;ECAP降低了铜的热稳定性,变形道次越高再结晶温度越低。退火后稳态晶粒尺寸随变形道次的增加而细化,硬度值随变形道次的增加而增大,回归分析表明,晶粒尺寸与硬度之间的关系符合Hall-Petch公式。     

AZ31镁合金包辛格效应研究

采用拉、压循环试验测试了AZ31镁合金的包辛格效应(BE),并研究了BE的机制。测试结果表明:压缩预变形后反向拉伸出现明显的BE,而拉伸预变形后反向压缩出现反包辛格效应(RBE);且包辛格效应比反包辛格效应明显。循环拉、压加载过程中的显微组织和晶体取向演化研究结果表明,出现包辛格效应是由于预压缩时改变晶粒取向与反向拉伸时去孪生效应共同作用的结果;预拉伸变形虽然不改变晶粒取向,但使轴比c/a值降低,使反向压缩时发生孪生更加困难,从而导致反包辛格效应。     

Improved fatigue properties of ultrafine-grained copper under cyclic torsion loading

In this study, fatigue behaviors of pure copper with different grain sizes are investigated under cyclic tension–compression and torsion loadings. The fatigue responses of ultrafine-grained (UFG) Cu subjected to equal-channel angular pressing (ECAP) are compared and contrasted with those of coarse-grained (CG) and cold-rolled (CR) Cu. It is found from the S–N curves under the two different loading modes that, in the high-cycle fatigue (HCF) range, the fatigue strength of Cu does not exhibit strong dependence on the grain size under cyclic tension–compression loading, whereas the fatigue strength of UFG Cu is greatly improved over those of CG and CR Cu under cyclic torsion loading. Under cyclic tension–compression loading, the fatigue strength exponent decreases with the refinement of grain size; however, under cyclic torsion loading, with decreasing grain size, its fatigue strength exponent shows the opposite trend and goes up. To explain the phenomena above, the relations between the fatigue strength exponent and fatigue strength coefficient are discussed. Based on the two main stages of fatigue failure (crack initiation and propagation stages), the influences of grain size on fatigue strength exponent and fatigue strength in the HCF range under the two fatigue modes are comprehensively analyzed.     

Mechanical behavior and microstructural evolution of a Mg AZ31 sheet at dynamic strain rates

The mechanical behavior of an AZ31 Mg sheet has been investigated at high strain rate (10³ s^?1) and compared with that observed at low rates (10^?3 s^?1). Dynamic tests were carried out using a Hopkinson bar at temperatures between 25 and 400 °C. Tensile tests were carried out along the rolling and transverse directions and compression tests along the rolling and the normal directions in both strain rate ranges. The tension–compression yield asymmetry as well as the yield and flow stress in-plane and out-of-plane anisotropies were investigated. The microstructure of the initial and tested samples was examined by electron backscatter diffraction. The dynamic mechanical behavior is characterized by the following observations. At high temperatures the yield asymmetry and the yield anisotropies remain present and twinning is highly active. The rate of decrease in the critical resolved shear stress of non-basal systems with temperature is smaller than at quasi-static rates. Rotational recrystallization mechanisms are activated.     

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.     

Machinability of ultrafine-grained copper using tungsten carbide and polycrystalline diamond tools

Equal channel angular extrusion (ECAE) is an effective process to produce bulk ultrafine-grained (UFG) materials from regular coarse-grained materials. Such ECAE-processed materials typically excel in strength, wear resistance, ductility, and high strain-rate superplasticity, with promising applications in lightweight transportation and medical industries. Precision machining work is generally indispensable for further applications after bulk materials are produced by ECAE. To effectively and efficiently machine such ECAE-processed materials for further broad applications, machining issues such as machinability and tool material selection should be considered. This study was undertaken to investigate the machinability of ECAE-processed pure copper using both tungsten carbide (WC) and polycrystalline diamond (PCD) cutting tools in order to facilitate broad applications of ECAE-processed UFG coppers. It is found that despite its higher cost, PCD is favored to machine UFG copper based on this study since it has better wear resistance, gives lower cutting forces, yields a better workpiece surface finish, and results in no smearing on the workpiece. In machining UFG copper, depth of cut notching was observed as the wear pattern and abrasion as the wear mechanism for the WC tool, while flank wear was observed as the wear pattern and diffusion as the wear mechanism for the PCD tool.     

Effect of temperature on mechanical behavior of AZ31 magnesium alloy

Strain rate sensitivity and tension/compression asymmetry of AZ31 magnesium alloy at different temperatures and strainrates were investigated.Both of mechanical behaviors are temperature dependent.Strain rate sensitivity increases with increasingtemperature.Thermally activated slip is the source of strain rate sensitivity.At the temperature below or near 373 K,strain ratesensitivity is very little.Tension/compression asymmetry in yielding decreases with increasing temperature.Twinning is the reasonof tension/compression asymmetry.At the temperature above or near 573 K,the material shows little tension/compressionasymmetry of the flow stress.