Effect of equal-channel angular pressing and aging on corrosion behavior of ZK60 Mg alloy

Commercial ZK60 Mg alloy was processed by multi-pass equal-channel angular pressing（ECAP） and subsequent aging to investigate the effect of grain refinement and second-phase redistribution on its corrosion behavior. Electrochemical tests show that the fine-grained samples after more ECAP passes have higher corrosion current densities（Jcorr） in the polarization curves, lower charge-transfer resistance（Rt） values in the EIS plots. The severe plastic deformation decreases the alloy corrosion resistance besides the well-known strengthening and toughening. Scanning Kelvin probe（SKP） measurement shows that the anodic and cathode sites are homogeneously distributed on the surface of the fine-grained alloy, which inhibits localized corrosion. The SKP potential, having linear relationship with the corrosion potential（φcorr）, decreases with increasing the ECAP pass. Furthermore, the post-ECAP aging can slightly improve the corrosion resistance of the fine-grained ZK60 Mg alloy and enhance the comprehensive performances, due to the stress relief and uniform distribution of second-phase particles.

新型医用β钛合金研究的发展现状及加工方法

简述了生物医用β钛合金的发展历程,从元素体系上将其分为Ti-Mo系、Ti-Zr系、Ti-Ta系和Ti-Nb系,归纳了目前报道的典型β钛合金的研究现状,重点介绍了Ti-Nb系β钛合金的发展状况,总结了生物医用β钛合金的塑性加工和热处理工艺,提出了目前研制的β钛合金存在的问题,并对钛合金的发展趋势进行了展望。分析结果表明:根据市场及社会需求发展,生物医用钛合金的应用前景乐观,开展新型β钛合金的研发和临床实践十分必要。

Grain refinement and mechanical properties of pure aluminum processed by accumulative extrusion bonding

Ultrafine-grained aluminum processed by a new severe plastic deformation technique, accumulative extrusion bonding (AEB), was investigated. Microstructural characterization indicated good interfacial bonding and an average grain size of ~440 nm was obtained after six passes. Tensile testing revealed that the strength reached the maximum value of 195 MPa and the total elongation exceeded 16% after five passes. The hardness was also significantly improved and almost reached saturation after the first pass. SEM fractography of AEB-processed specimens after tensile test showed that failure mode was shear ductile fracture with elongated shallow dimples. Comparison with conventional accumulative roll bonding indicates that this new AEB technique is more effective in refining grain and improving mechanical properties of the specimens.

剧烈塑性变形对块体纳米金属材料结构和力学性能的影响

综述了剧烈塑性变形引起的块体纳米金属材料的结构和力学性能演变．以电化学沉积法制备的fcc结构纳米晶Ni-20％Fe（质量分数）合金为研究对象，通过对其进行不同应变量的高压扭转实验，系统分析了变形引起的结构和力学性能演变．结构表征结果表明：（1）变形引发纳米晶Ni-Fe合金晶粒旋转，实现晶粒长大．同时，晶粒长大过程伴随着位错密度、孪晶密度的演变；（2）存在一个最有利于变形孪晶生成的晶粒尺寸范围（45～100nm），在这个晶粒尺寸范围之外，去孪晶起主导作用使原有的生长孪晶或变形孪晶消失；（3）位错密度是影响位错与孪晶反应的新的影响因素．当发生孪晶的晶粒内位错密度低时，位错可完全穿过孪晶界，部分穿过孪晶界，或被孪晶界吸收；发生孪晶的晶粒内位错密度高时，大量位错缠绕并堆积在孪晶界附近、形成应力集中，破坏孪晶界原有的共格性．为释放局部应力，将从孪晶界的另一侧发射不全位错形成层错和二次孪晶；（4）在塑性变形导致的晶粒长大过程中，原先偏聚于消失了的晶界上的C和S沿残留晶界扩散并继续偏聚于晶界上．结构与力学性能关系结果表明：随着应变量的增加，应变强化、应变软化交替出现．位错密度对硬度的演变起主导作用，其它结构演变（如孪晶密度的变化和晶粒尺寸变化）对硬度的演变起次要作用．

异构金属材料及其塑性变形与应变硬化

金属材料异构(heterostructure)是将具有显著流变应力差异的软硬相间区域作为基元进行有序构筑而成的微观组织,是旨在提高应变硬化能力和拉伸塑性的微结构设计策略,迄今应用于各种金属结构材料并获得了强度与塑性/韧性等力学性能的优异匹配。异构策略的出发点是其特征的力学响应,即塑性变形时在异构基元界面形成的应变梯度,异构的特征应力应变响应是力学迟滞环。相比均质结构中主导的林位错塑性和林硬化,异构为了协调界面应变梯度而产生几何必需位错,新增了基于几何必需位错的异质塑性变形并引起额外的应变硬化与额外的强化。本文综述了近期异构金属材料的研究进展,首先定义了异构中基元并据此把异构分类为基元异构、亚基元异构以及复合异构,随后分析并讨论了异构塑性变形时界面和位错等微结构演化,以及异质塑性变形、应变硬化和强化行为,最后展望了异构提升宏观力学性能匹配的潜力。

异构金属材料专刊前言

异构金属材料概念的提出迄今不到十年。异构是指一种微结构,其标志性特征是组织结构包括晶粒、晶体缺陷、相组成等,在三维空间的跨尺度多层级有序构筑,这或多或少地是从竹子和贝壳等自然界具有优异综合强韧性能的生物材料中得到了启发。

Plastic accommodation during tensile deformation of gradient structure

Gradient structure(GS)possesses a typical trans-scale grain hierarchy with varying internal plastic stability,and the mutual plastic accommodation plays a crucial role in its superior strength-ductility combination.Using the in-situ synchrotron X-ray diffraction(XRD)during tensile loading,we measured lattice strains sequentially from the nanostructured(NS)surface layer to the central coarsegrained(CG)layer to elucidate when and how plastic accommodation occurs and evolves within the GS,along with their roles in plastic deformation and strain hardening.Throughout the tensile deformation,two types of plastic incompatibility occur in the GS.One is an extended elastoplastic transition due to layer-by-layer yielding.The other is strain localization and softening in the NS layer,in contrast with the stable plastic deformation in the CG layer.Plastic accommodation thus occurs concurrently and manifests as both an inter-layer and intra-layer change of stress state throughout tensile deformation.This produces different micromechanical responses between layers.Specifically,the NS layer initially experiences strain hardening followed by an elastoplastic deformation.The hetero-deformation induced hardening,along with forest hardening,facilitates a sustainable tensile strain in the NS layer,comparable to that in the CG layer.

Inter-zone constraint modifies the stress-strain response of the constituent layer in gradient structure

How an individual constituent zone behaves during the deformation of a heterostructured metallic material is a fundamental issue for understanding heterostructure deformation, but it remains a challenge to experimentally observe it. Here we report a study on the stress-strain behavior of the nanostructured gradient layer(NGL) in an integrated gradient specimen that consists of a coarse-grained(CG)central layer sandwiched between two NGLs. Constraint from the CG central layer led to the formation of dense and dispersed stable strain bands(SBs) in the NGL, which regained dislocation hardening after initial recovery and grain coarsening. Consequently, the NGL exhibited a transient plateau of flow stress after yielding, and then regained extra strain hardening to achieve excellent uniform elongation. These unique behaviors are dramatically different from those of a freestanding NGL, indicating a fundamentally different deformation principle that is intrinsic to heterostructures, i.e.,inter-zone constraint modifies the constitutive behavior of constituent zones.