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。     

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。     

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.     

镁合金变形的晶体塑性有限元分析

晶体塑性理论已被广泛应用于现有的有限元分析中,从微观角度来模拟和预测晶体材料的宏观各向异性力学行为及塑性变形过程中织构的演化与发展。现有的晶体塑性理论框架核心主要基于由滑移引起的塑性变形机制,在预测由滑移和孪晶引起塑性变形的材料力学响应方面还不够完善。本文以具有密排六方(HCP)结构的变形镁合金塑性变形过程为例,综述了以滑移和孪晶为核心的晶体塑性理论的理论研究和应用现状,重点评述了现有孪晶的数值实现方法,并预测了相应理论的发展方向。     

Achieving Exceptional Grain Refinement through Severe Plastic Deformation: New Approaches for Improving the Processing Technology

The processes of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are now established for the fabrication of ultrafine-grained metals having superior properties by comparison with their coarse-grained counterparts. This article examines the recent developments designed to improve the processing technology of ECAP and HPT and to establish these techniques as viable procedures for use in industrial applications. Based on these developments, it is reasonable to anticipate these processing procedures will experience increasing use in the fabrication of commercial products.     

大塑性变形对纯铜力学性能的影响

室温下采用锻压对纯铜进行了大变形加工，对变形后的纯铜样品进行了拉伸和硬度测试，并采用扫描电镜对拉伸试样的断口进行了分析。研究结果表明：经过大变形后，由于纯铜的晶粒得以细化，抗拉强度、屈服强度、硬度和断裂延伸率都显著增加。     

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.     

Evolution of Texture and Deformation Mechanisms During Repeated Deformation and Heat Treating Cycles of U-6Nb

The evolution of the crystallographic texture and lattice strain of uranium 6-weight percent niobium alloy samples are tracked during multiple deformation and heat treating cycles in an effort to understand and control the mechanical properties of the material following thermo-mechanical processing. The heavily twinned microstructure and low-symmetry crystal structure of U-6Nb result in multiple sequential active deformation mechanisms associated with distinctive deformation textures in strain ranges from 0-0.15 true strain. It is found that heating into the high-temperature γ-phase erases much of the texture formed during deformation at room temperature in the α′′-phase and resets the active deformation mechanisms. Through a small number of deformation/heat treat cycles to moderate strains, i.e., ~ 0.13 per cycle, the flow strength of the material is recovered to its original value. However, on the fourth such cycle, a reduction of strength is observed and the sample failed.

     

Evolution of Microstructure and Texture During Deformation and Recrystallization of Heavily Rolled Cu-Cu Multilayer

A Cu-Cu multilayer processed by accumulative roll bonding was deformed to large strains and further annealed. The texture of the deformed Cu-Cu multilayer differs from the conventional fcc rolling textures in terms of higher fractions of Bs and RD-rotated cube components, compared with the volume fraction of Cu component. The elongated grain shape significantly affects the deformation characteristics. Characteristic microstructural features of both continuous dynamic recrystallization and discontinuous dynamic recrystallization were observed in the microtexture measurements. X-ray texture measurements of annealing of heavily deformed multilayer demonstrate constrained recrystallization and resulted in a bimodal grain size distribution in the annealed material at higher strains. The presence of cube- and BR-oriented grains in the deformed material confirms the oriented nucleation as the major influence on texture change during recrystallization. Persistence of cube component throughout the deformation is attributed to dynamic recrystallization. Evolution of RD-rotated cube is attributed to the deformation of cube components that evolve from dynamic recrystallization. The relaxation of strain components leads to Bs at larger strains. Further, the Bs component is found to recover rather than recrystallize during deformation. The presence of predominantly Cu and Bs orientations surrounding the interface layer suggests constrained annealing behavior.     

Evolution of Texture and Microstructure in Commercially Pure Titanium with Change in Strain Path During Rolling

The evolution of microstructure and texture in commercially pure titanium has been studied as a function of strain path during rolling using experimental techniques and viscoplastic self-consistent simulations. Four different strain paths, namely unidirectional rolling, two-step cross rolling, multistep cross rolling, and reverse rolling, have been employed to decipher the effect of strain path change on the evolution of deformation texture and microstructure. The cross-rolled samples show higher hardness with lower microstrain and intragranular misorientation compared to the unidirectional rolled sample as determined from X-ray diffraction and electron backscatter diffraction, respectively. The higher hardness of the cross-rolled samples is attributed to orientation hardening due to the near basal texture. Viscoplastic self-consistent simulations are able to successfully predict the texture evolution of the differently rolled samples. Simulation results indicate the higher contribution of basal slip in the formation of near basal texture and as well as lower intragranular misorientation in the cross-rolled samples.     

A Study of Plastic Deformation Behavior of AA1050 Aluminum Alloy during Pure Shear Extrusion with Back Pressure

In the present study, the effects of back pressure on the filling fraction of die and the effective strain distribution throughout severely deformed material during pure shear extrusion, a novel severe plastic deformation process, are investigated by finite element analysis. A pure shear extrusion process found in the literature is employed and the predicted forming load is compared with experiments. A good agreement is observed between the results of the simulation with Coulomb friction of 0.12 and experiments. Various back pressures are applied to plunger at the exit channel of the die, and their influence on the filling fraction of the die and the effective strain in severely deformed billets are studied, indicating that the homogeneity of the effective strain on the cross-section of the deformed billet is decreased slightly. It is also found that the filling fraction of the die exit channel as well as average strain on the cross-section of the billet are increased.     

Texture and Grain Boundary Network Evolution of Laser Powder Bed Fusion Processed Pure Ni During Post-printing Annealing

Metallurgical and Materials Transactions A - Commercially pure Ni was additively manufactured using laser powder bed fusion. The printed specimens were then subjected to annealing treatments in a...     

Microstructure and Texture Evolution during Single- and Multiple-Pass Friction Stir Processing of Heat-Treatable Aluminum Alloy 2024

Microstructure and crystallographic texture evolution during single- and multiple-pass friction stir processing (FSP) of an age-hardenable aluminum alloy 2024 (Al-Cu-Mg) was investigated. Multiple-pass experiments were carried out using two different processing strategies, multi-pass FSP, and multi-track FSP. Effect of a post-FSP heat treatment above and below the solutionizing temperature of the alloy was also studied. FSP experiments were carried out using an optimal set of parameters. Characterization tools used in the study include scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), electron probe micro-analyser (EPMA), and X-ray diffraction (XRD). Microstructural features indicate the occurrence of particle stimulated nucleation (PSN) assisted dynamic recrystallization (DRX) as the dominant microstructural evolution mechanism in the nugget zone. Geometrical coalescence occurred, leading to the formation of some larger grains in the nugget zone. Heterogenous micro-texture distribution was observed in the nugget zone with the bulk textures consisting of FCC shear texture components dominated by A ₁*/A ₂* and C. Microstructure and texture in the nugget zone remained stable after both routes of multiple-pass processing, demonstrating the possibility of FSP to produce bulk volume of fine-grained materials. Post-FSP heat treatment indicated the stability of microstructure and texture up to 723 K (450 °C) owing to relatively lower strain energies retained after FSP.

     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

Flow Behavior and Microstructural Evolution of 7A85 High-Strength Aluminum Alloy During Hot Deformation

Metallurgical and Materials Transactions A - Hot deformation behavior of 7A85 high-strength aluminum alloy was investigated at 593 K to 713 K (320 °C to...     

Evolution of Texture from a Single Crystal Ti-6Al-4V Substrate During Electron Beam Directed Energy Deposition

Additive manufacturing of Ti-6Al-4V commonly produces 〈001〉 _β -fiber textures aligned with the build direction. We have performed wire-feed electron beam directed energy deposition on the {112} _β plane of a single prior β-grain. The build initially grew epitaxially from the substrate with the preferred 〈001〉 growth direction significantly angled away from the build direction. However, continued layer deposition drove the formation of a 〈001〉 _β -fiber texture aligned with the build direction and the direction of the strongest thermal gradient.     

新型AlTiC中间合金对工业纯铝的细化研究

采用铝热还原法工艺制备AlTiC中间合金,通过X射线及扫描电镜(SEM)等分析了AlTiC中间合金的成分、组成和形貌.利用自行研制的AlTiC中间合金对工业纯铝的细化进行研究,借助于光学显微镜(OM)等手段对细化效果进行了分析对比,并初步探讨了其细化效果的机理.试验结果表明,与国产AlTiB中间合金相比,AlTiC中间合金对工业纯铝的细化效果较优.     

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.