大塑性变形制备纳米结构金属

细化晶粒是改善材料性能的有效手段，传统的压力加工技术(如轧制、挤压、拉拔和锻造等)可以细化晶粒(微米量级)。纳米结构金属由于具有很小的晶粒尺寸(20-500nm)和独特的缺陷结构，从而表现出优异的物理—力学性能。大塑性变形(SPD)具有将铸态粗晶金属的晶粒细化到纳米量级的巨大潜力，近年来已引起人们的极大关注。介绍了4种大塑性变形制备纳米结构金属的方法、原理、变形特点及应用，分析了纳米结构金属的强度和超塑性变形特征，以及当前研究中存在的主要问题，并对大塑性变形技术的应用前景进行了展望。     

大悬臂挤压模变形机理分析

结合悬臂挤压模具的特点，介绍了恳臂模具受力模型的建立、应力分析、变形分析，提出模具在设计过程中，特别是壁厚的设计中，必须考虑模具的变形以及蝶状补偿量的选择原理。     

镁合金的塑性变形机理及加工工艺

文章综述了镁及镁合金的塑性变形理论，并对镁合金的塑性加工工艺和技术做了较为详细的介绍。     

Severe Plastic Deformation of Difficult-to-Deform Materials at Near-Ambient Temperatures

Plane-strain machining can be used to impart large plastic strains in alloys that are difficult to deform by other severe plastic deformation (SPD) processes. By cutting at low speeds, the heating caused by friction with the tool can be reduced to insignificant levels. The utility of this approach for characterizing microstructure development in SPD is demonstrated using a variety of commercial alloys that exhibit different deformation behaviors and strengthening mechanisms, including CP-titanium, aluminum alloy 6061-T6, nickel-base superalloy IN-718, and pearlitic plain-carbon steel. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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强变形过程中铁镍合金的微观结构演化机制

 采用透射电镜观察了铁镍(Fe 32%Ni)合金在形变温度500 ℃(＜05Tm)、形变速率10-2 s-1的变形条件下多轴锻造变形过程中的微观结构演变。结果表明,低温多轴锻造强变形可明显细化晶粒,细化过程为：首先,位错墙、位错缠绕等结构通过大量位错滑移运动在原始晶粒内形成;其次,不同方向的变形导致不同方向的滑移系开动,从而致使不同方向的位错墙互相交叉,将原始粗晶粒细分成小尺寸的胞块结构,当变形量达到一定程度时,位错墙和位错缠绕结构内的位错开始重新排列,形成小角度晶界,导致亚晶粒形成;由于变形量不断增加强迫大量的位错在亚晶界处积聚、重排,同时不同方向的变形造成亚晶发生转动,位错重新规则排列及亚晶转动使小角度的亚晶界转变为大角度晶界,从而形成细小的新晶粒。     

Processing of Ultrafine-Grained Materials through the Application of Severe Plastic Deformation

Processing through the application of severe plastic deformation (SPD) has become an absorbing tool because it provides the potential for refining the grain size of polycrystalline bulk metals to the submicrometer or even the nanometer level. Several SPD methods are now available, but the more promising procedures seem to be equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). This report examines these procedures with an emphasis on the principles of grain refinement and the mechanical properties developed at high temperatures in materials after processing by ECAP and the hardness homogeneity and microstructural evolution in materials processed by HPT. Moreover, recent observations on the ECAP and HPT processing also are discussed.     

Severe Plastic Deformation Induced Sensitization of Cryo-milled Nanocrystalline Al-7.5 Mg

Transmission electron microscopy (TEM) was employed to investigate the sensitization behavior in nanocrystalline Al-7.5 pct Mg synthesized by cryomilling and hot isostatic pressing. High-resolution TEM reveals the formation of β phase in as-extruded condition as well as in aged condition. Grain boundaries are enriched with Mg during milling as a result of flux of defects, such as vacancy and dislocations that are strongly coupled to Mg atoms, to grain boundaries. It is suggested that sensitization is enhanced due to severe plastic deformation during high energy ball milling.     

正向、反向挤压变形均匀性的定量分析

本文以铝棒材为研究对象,在参考了相关文献的基础上,建立了考虑塑性功、摩擦生热、对流换热和热传导等多种边界条件的热-力耦合温度场的有限元模型。使用MSC-Superform有限元分析软件,选用刚塑性有限元法和平面轴对称单元,通过对2A11铝棒材反向挤压和正向挤压的数值模拟,结合影响挤压效果的各种因素,尝试对铝棒材正、反向挤压的变形均匀性进行定量的分析。     

大型齿圈加工变形与防治措施的研究

通过对烧结混合机大齿圈铣齿后结合缝外缘离缝变形问题的分析研究,查找变形产生的主要原因,采取补救措施,避免了齿圈报废,提出了今后预防齿圈加工变形的有效措施。     

Origin of Significant Grain Refinement in Co-Cr-Mo Alloys Without Severe Plastic Deformation

We have previously reported that ultrafine-grained (UFG) microstructures can be obtained in a Co-29Cr-6Mo (wt pct) alloy by utilizing dynamic recrystallization (DRX) that occurs during conventional hot deformation (Yamanaka et al.: Metall. Mater. Trans. A, 2009, vol. 40A, pp. 1980?94). The present study investigates the novel DRX mechanism of this alloy in detail. The microstructure evolution during hot deformation under relatively high Zener?CHollomon (Z) parameter conditions for which ultrafine grains can develop was systematically investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy. This alloy exhibited a different flow stress behavior and microstructural development from conventional DRX mechanisms. The deformation microstructure contained a large number of stacking faults, which implies that planar dislocation slip is the primary deformation mechanism in the hot deformation of the Co-29Cr-6Mo alloy due to its abnormally low stacking fault energy (SFE) at elevated temperatures. Inhomogeneities in local strain distributions induced by planar slip will enhance grain subdivision by geometrically necessary (GN) dislocation boundaries. Deformation twinning may also contribute to grain refinement. The DRX mechanism operating in the Co-29Cr-6Mo alloy is discussed by considering the relationships between anomalous dislocation structures, flow stress behavior, texture development, and nucleation behavior.     

Improving Hydro-formability of a Ferritic Stainless Steel Tube Through Severe Plastic Deformation

The aim of this work is to improve the hydro-formability of a 409L ferritic stainless tube through a treatment involving severe plastic deformation followed by annealing. For this purpose, the variation in hydro-formability is determined in terms of parameters like the circumferential R-value and the circumferential elongation after application of the abovementioned treatment. In addition, the deformation-induced surface roughening of the tube, called ridging, is measured through tension tests before and after the treatment. Moreover, the evolution in tube texture caused by the treatment is studied to reveal the relationship between the hydro-formability parameters and the texture. The results show that the treatment dramatically improves the hydro-formability through a marked modification in texture. For example, the emergence of the {110}〈100〉 texture component through the present treatment causes a noticeable increase in the circumferential elongation and the circumferential R-value of the tube while decreasing the degree of ridging.     

Permanent Strength of Metals: A Case Study on FCC Metals Processed by Severe Plastic Deformation

Metallurgical and Materials Transactions A - The present study emphasizes the importance of the direct evaluation of the athermal strength of structural materials, which is the time-independent...     

Static and Cyclic Crack Growth Behavior of Ultrafine-Grained Al Produced by Different Severe Plastic Deformation Methods

Fracture-mechanics experiments were carried out on ultrafine-grained (UFG) samples of aluminum and two Al alloys to obtain the fracture behavior under static and cyclic loading. The UFG materials investigated show crack resistance behavior under static loading, which was confirmed by ductile fracture surfaces. Under cyclic load, the crack growth rate was described well by the ESACRACK model. The crack propagation results show no influence of the type of the severe plastic deformation method in the Paris region but more effect in the threshold region. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.     

Activation Enthalpies of Deformation-Induced Lattice Defects in Severe Plastic Deformation Nanometals Measured by Differential Scanning Calorimetry

Samples of 99.99 pct pure copper and nickel of 99.998 pct purity were deformed by high-pressure torsion (HPT) at different hydrostatic pressures, to different shear strains. Activation enthalpies (Q) were determined by differential scanning calorimetry (DSC) using Kissinger’s method. For the one annealing peak found in HPT Cu, Q amounts to Q = 0.78 to 0.48 eV depending on the shear strain applied. In the case of Ni, the activation enthalpies of the two annealing peaks were determined as Q = 0.65 eV and Q = 0.95 eV, respectively, with no obvious dependence on shear strain, although this has been indicated by the annealing peak temperatures. Applying defect specific analyses of the annealing peaks, it turned out that the larger Q value represents the annihilation of dislocations and agglomerates, while the smaller one reflects the annihilation of single or double vacancies. Concerning the strain dependence of the larger Q, two possible explanations have been discussed: (1) the annihilation of dislocations assisted by the strain-dependent density of vacancy agglomerates and (2) the annihilation of dislocations enhanced by a strain-dependent level of long-range internal stresses. Because of closer correlations of Q with external and internal stresses at very high shear strains, explanation (2) has been favored.     

Tube Twist Pressing (TTP) as a New Severe Plastic Deformation Method

Tube twist pressing (TTP) as a new severe plastic deformation method for processing tubular parts was presented. The commercially pure aluminum tubes successfully were processed by TTP method. Microstructural examination by XRD analysis of the processed tubes revealed the formation of fine grains in the average size of 1.1 μm after four TTP passes. Also, the obtained results of mechanical tests showed a notable increase in microhardness, yield and ultimate strengths. The capabilities of TTP method were verified via comparison of the obtained results with the results of other SPD processes. To further investigate the TTP method, FE modeling was carried out using the Abaqus/Explicit to study the macroscopic deformation and microstructural evolution (the evolution of dislocation density and grain size) during TTP via continuous dynamic recrystallization. In the FE model, the strain hardening behavior of the material was related to microstructure quantities based on the micromechanical constitutive model. The FEM simulated grain refinement behavior was consistent with the experimentally obtained results.     

Research on Natural Aging Behavior of High Purity Ag Sheets After Severe Plastic Deformation

High purity silver (Ag) powder was melted in vacuum at ~10^?3?Pa and in an atmosphere with charcoal dust as a protective agent. Both samples were subjected to accumulative rolling with 95% thickness reduction and naturally aged for 1?year. The mechanical properties and microstructural studies of as melted and processed samples were evaluated. Results showed that after natural ageing for a year the sample melted in vacuum was responsible for total recrystallization and the sample melted in atmospheric conditions showed slight recovery. It is suggested that grain growth and rotation of Ag is hampered by some oxygen interstitial solute and a few of Ag₂O particles which can keep the orientation of Ag grains stable and obviously resist natural aging and softening.