Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

Pulse electroplating of copper film: a study of process and microstructure

Copper films with high density of twin boundaries are known for high mechanical strength with little tradeoff in electrical conductivity. To achieve such a high density, twin lamellae and spacing will be on the nanoscale. In the current study, 10 microm copper films were prepared by pulse electrodeposition with different applied pulse peak current densities and pulse on-times. It was found that the deposits microstructure was dependent on the parameters of pulse plating. Higher energy pulses caused stronger self-annealing effect on grain recrystallization and growth, thus leading to enhanced fiber textures, while lower energy pulses gave rise to more random microstructure in the deposits and rougher surface topography. However in the extremes of pulse currents we applied, the twin densities were not as high as those resulted from the medium or relatively high pulse currents. The highest amount of nanoscale twinning was found to form from a proper degree of self-annealing induced grain structure evolution. The driving force behind the self-annealing is discussed.     

Deformation behaviour and shape memory effect of near equi-atomic NiTi alloy

The mechanical shape memory effect associated with the martensitic-type transformation which occurs in polycrystalline Ti-50.3 at. % Ni alloy has been investigated using the techniques of transmission and optical microscopy. Deformation of initially partially transformed material within the recoverable strain range was found to occur by: (1) stress-induced transformation of the most favourably oriented existing martensite variants at the expense of adjacent unfavourably oriented variants and retained high temperature phase (2) stress-induced re-orientation of favourably oriented martensite by utilizing the most favourably oriented twin system, and (3) stress-induced twin-boundary migration within the martensite. The reverse transformation during heating restores the original grain structure of the high-temperature phase in a highly coherent manner. It was concluded that deformation modes limited to those involved in the transformation process and the reversibility of the transformation give rise to the memory effect.     

Influence of homogeneous interfaces on the strength of 500 nm diameter Cu nanopillars

Interfaces play an important role in crystalline plasticity as they affect strength and often serve as obstacles to dislocation motion. Here we investigate effects of grain and nanotwin boundaries on uniaxial strength of 500 nm diameter Cu nanopillars fabricated by e-beam lithography and electroplating. Uniaxial compression experiments reveal that strength is lowered by introducing grain boundaries and significantly rises when twin boundaries are present. Weakening is likely due to the activation of grain-boundary-mediated processes, while impeding dislocation glide can be responsible for strengthening by twin boundaries.     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.     

Fabrication of nanostructured deoxidized low-phosphorous copper processed by three-layer stack accumulative roll-bonding

A nanostructured deoxidized low-phosphorous copper (DLPC) was fabricated by three-layer stack accumulative roll-bonding (ARB) process. The microstructural evolution and the variation of mechanical properties with three-layer stack ARB were investigated in detail. It was found that the microstructure has been evolved from a dislocation cell structure to ultrafine grained structure as the number of ARB cycles increases. In addition, the mean spacing of grain boundaries, which was 45 microm in initial material, reduced to 2.1 microm after 1 cycle, 360 nm after 3 cycles, 250 nm after 5 cycles, then 170 nm after 7 cycles, progressively. The fraction of high-angle grain boundaries after 1-cycle ARB was no more than 0.27, but it increased with the number of ARB cycles, and became surprisingly more than 0.7 after 7-cycle ARB. The tensile strength increased with the number of ARB cycles, and then after 7 cycles it reached about 600 MPa, which is about 2.5 times higher than that of the initial material. Therefore, the three-layer stack ARB is very effective for development of ultrafine grains and high strengthening of DLPC alloy.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.     

Structure–properties’ modification of 70/30 brass by large-load and low-speed friction stir processing

A novel one-step approach named large-load and low-speed friction stir processing was developed to produce an ultrafine-grain structure in 70/30 brass. The material so processed was characterised by a mean grain size of 0.5?µm and high angle boundaries of 91%. Abundant twin boundaries and stacking fault were produced in the grains as well. The grain refinement mechanism is attributed to the combination of discontinuous dynamic recrystallisation (DRX), microshear bands assisted DRX and twinning-induced DRX. A good combination of yield strength and uniform strain was achieved. This study provides a simple and effective method to improve the microstructure and mechanical properties in metals and alloys with low stacking fault energy.     

孪晶片层结构在室温轧制过程中的微观结构演变

研究了一种具有纳米孪晶片层结构的电解沉积铜的微观结构特征及其在室温轧制形变后的微观结构演变.结果表明,电解沉积制备的纯铜样品由柱状晶组成,柱状品内含有平行于样品沉积表面的纳米量级厚度的高密度孪晶片层结构,在孪晶界上缺陷很少,为共格孪晶界.形变后,孪晶片层的微观结构特征与片层厚度密切相关.粗大的孪品片层的形变行为以全位错运动为主,而细小的孪晶片层的形变行为以肖克莱(Shockley)位错在孪晶界上的滑移为主,从而导致几个纳米厚的超细孪晶片层消失.     

Computational Investigation of Effects of Grain Size on Ballistic Performance of Copper

Numerical simulations were conducted to compare ballistic performance and penetration mechanism of copper (Cu) with four representative grain sizes. Ballistic limit velocities for coarse-grained (CG) copper (grain size ≈ 90 µm), regular copper (grain size ≈ 30 µm), fine-grained (FG) copper (grain size ≈ 890 nm), and ultrafine-grained (UG) copper (grain size ≈ 200 nm) were determined for the first time through the simulations. It was found that the copper with reduced grain size would offer higher strength and better ductility, and therefore renders improved ballistic performance than the CG and regular copper. High speed impact and penetration behavior of the FG and UG copper was also compared with the CG coppers strengthened by nanotwinned (NT) regions. The comparison results showed the impact and penetration resistance of UG copper is comparable to the CG copper strengthened by NT regions with the minimum twin spacing. Therefore, besides the NT-strengthened copper, the single phase copper with nanoscale grain size could also be a strong candidate material for better ballistic protection. A computational modeling and simulation framework was proposed for this study, in which Johnson–Cook (JC) constitutive model is used to predict the plastic deformation of Cu; the JC damage model is to capture the penetration and fragmentation behavior of Cu; Bao–Wierzbicki (B-W) failure criterion defines the material's failure mechanisms; and temperature increase during this adiabatic penetration process is given by the Taylor–Quinney method.     

Microstructure and Stress-Rupture Life of Polycrystal, Directionally Solidified, and Single Crystal Castings of Nickel-Based IN 939 Superalloy

A comparative investigation of microstructural and mechanical properties (stress-rupture life) in conventionally cast, directionally solidified, and single crystal IN 939 superalloy has been undertaken. Directional castings possess only a few columnar grains, all oriented in the <100> crystallographic direction, whereas only one grain is present in a single crystal. Single crystals are characterized by the highest values of stress-rupture life, much higher than those of directionally solidified and, especially, polycrystal castings, which is accounted for by the absence of grain boundaries.     

Microscale 3D Printing of Nanotwinned Copper

Nanotwinned (nt)‐metals exhibit superior mechanical and electrical properties compared to their coarse‐grained and nanograined counterparts. nt‐metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (additive manufacturing) of nt‐metals. Microscale 3D printing of nt‐Cu is demonstrated with high density of coherent twin boundaries using a new room temperature process based on localized PED (L‐PED). The 3D printed nt‐Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps. The L‐PED process enables direct 3D printing of layer‐by‐layer and complex 3D microscale nt‐Cu structures, which may find applications for fabrication of metamaterials, sensors, plasmonics, and micro/nanoelectromechanical systems.     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

The new trends in fabrication of bulk nanostructured materials by SPD processing

During the past decade, fabrication of bulk nanostructured metals and alloys using severe plastic deformation (SPD) has been evolving as a rapidly advancing direction of nanomaterials science and technology aimed at developing materials with new mechanical and functional properties for advanced applications. The principle of these developments is based on grain refinement down to the nanoscale level via various SPD techniques. This paper is focused on investigation and development of new SPD processing routes enabling fabrication of fully dense bulk nanostructured metals and alloys with a grain size of 40–50 nm and smaller, namely, SPD-consolidation of powders, including nanostructured ones, as well as SPD-induced nanocrystallization of amorphous alloys. We also consider microstructural features of SPD-processed materials that are responsible for enhancement of their properties.     

Structural dependence of gold deposition by nanoplating in polycrystalline copper

In the present work, the gold-nanoplating technique is used to monitor differences in the electrochemical activity of different types of grain boundaries in high-purity copper. Gold-nanoplating is based on the electrochemical displacement of gold, which is deposited as particles from an aqueous solution on the polycrystalline copper surface. The complementary use of electron backscatter diffraction for revealing microstructural features, field emission scanning electron microscopy for imaging, and energy-dispersive X-ray analysis for quantification of the deposited gold makes it possible to detect differences in the grain boundary activity for different types of grain boundaries. In this way, it becomes possible to distinguish special from random boundaries in an efficient way. Also quantitative experimental results on grain boundary activity are produced, which correlate strongly with literature predictions on grain boundary energy.     

Deformed metals and alloys with a structural scale from 5 nm to 100 nm

The processing, structure and properties of deformed metals and alloys with a structural scale from the micrometer to the nanometer dimensions has been the subject of a recent viewpoint set [1]. The present paper will focus on deformed metals and alloys with a structural scale from 5 nm to 100 nm, concentrating on materials processed by high pressure torsion (HPT), surface mechanical attrition treatment (SMAT) and sliding. A detailed microstructural characterization has been followed by an analysis of the relationship between structural features and processing parameters. In this analysis, some general approaches have been applied for example scaling of the evolution of the boundary spacing. This analysis is the basis for a brief discussion of the relationship between the microstructural parameters and the strength.     

Tensile ductility and deformation mechanisms of a nanotwinned 316L austenitic stainless steel

A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.     

Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

This research explores the prospect of fabricating a face-centered cubic(fcc) Ni-base alloy cladding(Inconel 690) on an fcc Fe-base alloy(316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing(AM) utilizing electron-beam powder bed fusion(EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes[100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel.In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2–3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm~2, and a homogeneous distribution of Cr_(23)C_6 precipitates. Precipitates were formed within the grains(with ~3 μm interparticle spacing),but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainlesssteel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685?C for50 h precipitated Cr_(23)C_6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle(and low energy) subgrain boundaries. In contrast, Cr_(23)C_6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

Modeling the effect of microstructural features on the nucleation of creep cavities

A crystal plasticity based finite element model has been applied to study the deformation of metals at the microstructural length scale, in order to determine the effect of various microstructural features on the nucleation of creep cavities. The deformation model captures the non-uniform distributions of the equivalent plastic strain and the hydrostatic stress within the different grains of the microstructure when subjected to cyclic loading conditions. The influence of various microstructural features such as grain boundaries, triple junctions, and second-phase particles, on the strain and stress fields is examined through the simulations. The results indicate that the various microstructural parameters, such as grain orientation, presence of the precipitates and their shape, and alignment of the boundaries with respect to the loading direction influence the strain and stress distributions, and therefore, the conditions that favor the nucleation and growth of creep cavities.