Stability aspects of bulk nanostructured metals and composites

The requirement of thermal stability of nanocrystalline materials can be fulfilled with a composite approach. However, utilizing a two-phase approach includes additional constraints concerning the synthesis and processing, especially of massive nanostructured materials. Here, the potential of deformation processing for synthesizing nanoscale composite structures with uniform microstructures in bulk shape is analyzed for a series of Cu-rich alloys. The results are discussed with respect of governing materials properties that determine the feasibility of a composite approach in combination with severe plastic deformation to obtain massive nano-composite materials with high thermal stability.     

Novel Ti-base nanostructure-dendrite composite with enhanced plasticity

Single-phase nanocrystalline materials undergo inhomogeneous plastic deformation under loading at room temperature, which results in a very limited plastic strain (smaller than 0-3%). The materials therefore display low ductility, leading to catastrophic failure, which severely restricts their application. Here, we present a new in situ-formed nanostructured matrix/ductile dendritic phase composite microstructure for Ti-base alloys, which exhibits up to 14.5% compressive plastic strain at room temperature. The new composite microstructure was synthesized on the basis of the appropriate choice of composition, and by using well-controlled solidification conditions. Deformation occurs partially through dislocation movement in dendrites, and partially through a shear-banding mechanism in the nanostructured matrix. The dendrites act as obstacles restricting the excessive deformation by isolating the highly localized shear bands in small, discrete interdendritic regions, and contribute to the plasticity. We suggest that microscale ductile crystalline phases might therefore be used to toughen nanostructured materials.     

Phase Stability in Nanostructures

Nanostructured materials provide access to tailor‐made materials properties by microstructural design. Excellent mechanical properties such as high strength or wear resistance are often found in nanocrystalline materials. For magnetic materials, the design of nanostructured composites offers advantages if the structural scales match the intrinsic magnetic length scales. In some cases, as in the new nanocrystalline soft magnetic alloys, the combination of amorphous and nanocrystalline phases is necessary to obtain the desired properties. This rises the question of the limiting size for a stable crystalline structure, especially in contact with an amorphous phase. These considerations, which have been of interest for basic research in the context of the microcrystalline model for amorphous materials, are of technical importance for the optimization of nanostructured composites. Recent model experiments about the stability of thin Fe‐based glass forming alloy films are reviewed. A relationship between phase stability, composition, and interface density has been established. The implications of the results for the design of nanostructured alloy systems are discussed.     

Nanoindentation studies on amorphous, nanoquasicrystalline and nanocrystalline Zr8oPt2o and Zr75Pd25 alloys

Nanoindentation studies on rapidly solidified Zr80Pt20 and Zr75Pd25 binary alloys with nanocrystalline, nanoquasicrystalline, and amorphous microstructures are reported. The results indicate that the hardness and elastic modulus are the highest for a mixture of amorphous and nanoquasicrystalline state among the various microstructures studied. Nanoquasicrystalline phase has high hardness and elastic modulus in comparison to amorphous and nanocrystalline phases. The hardness to modulus ratio is close to 0.1 in both the alloys, irrespective of the phase/phase mixture studied indicating that the bonding in these alloys is of covalent nature. In Zr80Pt20, all the phases/phase mixtures have higher hardness and modulus in comparison to similar microstructures in Zr75Pd25 due to higher bond energies caused by more negative heat of mixing in the former case.     

Tensile behavior of bulk nanostructured and ultrafine grained aluminum alloys

In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and low strain hardening were observed in bulk nanostructured and ultrafine-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an influence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation hardening and solid-solution hardening. The large and sudden stress drops in the stress-strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids.     

Novel In Situ Nanostructure-Dendrite Composites in Zr-Base Multicomponent Alloy System

The evolution of a nanostructure-dendrite composite microstructure of two Zr-base alloys solidified through different casting routes is presented. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary phase having dendritic morphology, whereas the residual melt solidifies to a nanostructured/amorphous matrix. The volume fraction and the morphology of the dendritic phase varied with the Nb content and the adopted casting route. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922 Mpa, as well as plastic strain over 15.8% under uniaxial compression testing at room temperature. The high strength of these composites is imparted by the nanostructured matrix, whereas the large plastic strain is a consequence of the retardation of excessive localized shear banding in the matrix by ductile dendrites. The significant work hardening of the composites prior to fracture is attributed to dislocation multiplication in the solid solution-strengthened dendritic phase.     

Mechanical Behaviour and Structure Instability of Al_3Ti Alloy

Trialuminide alloys of elements such as Ti. Nb or Zr are of particular interest as materials for high temperature usage because their density is very low and specific strength and elastic rnoduli are then very high. This report concentrates on recent work on Al3Ti alloys which have been alloyed with ternary elements such that the higher symmetry ordered cubic structure is obtained, leading to somewhat easier operation of deformation mechan isms and hence improved ductility and toughness.Fine details of the crystal structure of cubic trialuminides are considered here and it is shown that the materials generally possess some remnant tetragonal chemical ordering which can affect their me chanical behaviour. In addition the compositional range over which a stable single phase is retained is shown to be extremely small, such that in most cases the materials examined show some form of microstructural instability. These instabilities affect the mechanical behaviour of the materials, for exarnple producing general strengthening. leading to precipitation hardening du ring hig h temperature testing, and causing age hardening instabilities during high temperature static or dynamic testing.Such structural instabifity feads to significant modifications at superdislocations, affecting both the dislocation cores and their associated APB's. Failure for these cubic materials still occurs at very small plastic strains and seems to be determined by difficulties of superdislocation creation near a propagating crack rather than by problems of suitable dislocation configuration and mobility. Possible ways to enhance ductility and toughness by alloying and microstructural modification will be discussed.     

Structural nanocrystalline materials: an overview

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given.     

A nanostructural design to produce high ductility of high volume fraction SiCp/Al composites with enhanced strength

High ductility and increased strength of SiC_p/Al composites are highly desirable for their applications in complicated components. However, high ductility and high strength are mutually exclusive in high volume fraction SiC_p/Al composites. Here, we report a novel nanostructuring strategy that achieves SiC_p/Al–Sc–Zr composites with superior maximum tensile strain and enhanced tensile strength. The new strategy is based on combination of grain refinement down to ultra-fine scale with nanometric particles inside the grain through adding distinctive elements (Sc, Zr) and refining nucleation centers to nanoscale under the action of high volume fraction reinforcement during the fabrication process. The nanostructured SiC_p/Al–Sc–Zr composites had an increase of ∼300% in maximum tensile strain and a 21% increase in tensile strength. This thought provides a new sight into enhancement of both strength and ductility of particle reinforcement metal matrix composites.     

Cover Picture: Recent Developments in Nanostructured Materials (Adv. Eng. Mater. 11/2005)

The cover shows transmission electron micrographs of typical microstructures in as‐deposited copper samples. More about recent developments in nanostructured materials research can be found in the review paper by C. Suryanarayana on page 983. Nanostructured materials with extremely fine grain sizes, typically less than 100 nm, have been shown to exhibit extremely high strength and hardness, increased diffusivity, useful sintering characteristics, and other unusual properties. Significant developments have taken place in recent years in trying to achieve high strength and ductility at the same time. New and potential applications are also being sought after. Several commercial ventures have also started (estimated to be around 1000 in different parts of the world). This overview will present the recent developments in our understanding of the mechanical behavior and molecular dynamics simulation studies, highlight the present unresolved issues, and discuss some of the promising applications for these novel and exciting materials.     

High-strength Zr-based bulk amorphous alloys containing nanocrystalline and nanoquasicrystalline particles

Abstract

It was recently found that the addition of special elements leading to the deviation from the three empirical rules for the achievement of high glass-forming ability causes new mixed structures consisting of the amorphous phase containing nanoscale compound or quasicrystal particles in Zr–Al–Ni–Cu–M (M ? Ag, Pd, Au, Pt or Nb) bulk alloys prepared by the copper mold casting and squeeze casting methods. In addition, the mechanical strength and ductility of the nonequilibrium phase bulk alloys are significantly improved by the formation of the nanostructures as compared with the corresponding amorphous single phase alloys. The composition ranges, formation factors, preparation processes, unique microstructures and improved mechanical properties of the nanocrystalline and nanoquasicrystalline Zr-based bulk alloys are reviewed on the basis of our recent results reported over the last two years. The success of synthesizing the novel nonequilibrium, high-strength bulk alloys with good mechanical properties is significant for the future progress of basic science and engineering. © 2000 Published by Elsevier Science Ltd.     

Vacancy-assisted hardening in nanostructured metals

Bulk metals deformed by severe plastic deformation have some unusual phenomenon such as hardening by low temperature annealing. Commercial purity aluminum processed by equal channel angular pressing has been used to investigate the deformation mechanism during further annealing of the severed plastic deformed materials. It has been found that vacancy-type defects induced by severe plastic deformation are responsible for some unusual phenomena, such as hardening by annealing and softening by slight deformation in nanostructured metals. It is anticipated that the effective control of vacancy-type defects during SPD and consequent annealing process will be combined to attain an optimum combination of strength and ductility in nanostructured metals.     

Ductility of nanocrystalline materials with a bimodal grain structure

A simple model describing the tensile deformation behavior of a nanostructured material with a combined nano- and microcrystalline grain structure has been developed. The nanocrystalline matrix ensures a high mechanical strength, while the microcrystalline grains provide an increase in the ductility of such materials. The proposed model of tensile straining of the material with bimodal grain structure predicts that the regime of uniform straining until neck formation must depend on the dimensions and volume fractions of the nano- and microcrystalline grains. It is shown that a growth in the ultimate uniform strain is related to an increase in the ratio of the yield stress to the coefficient of strain hardening and in the parameter of plastic strain distribution between the nano- and microcrystalline components.     

Unusual super-ductility at room temperature in an ultrafine-grained aluminum alloy

Processing by severe plastic deformation (SPD) typically increases the strength of metals and alloys drastically by decreasing their grain size into the submicrometer or nanometer range but the ductility of such materials remains typically low. This report describes the first demonstration that it is possible to increase the room temperature ductility of aluminum-based alloys processed by SPD and to attain elongations to failure of >150% while retaining the enhanced strength. This unique combination of properties is due to the occurrence of grain boundary sliding at room temperature. The sliding was obviously achieved by introducing a grain boundary wetting of the aluminum/aluminum grain boundaries.     

Tailoring nanostructured, graded, and particle-reinforced Al laminates by accumulative roll bonding

Accumulative roll bonding (ARB) is a very attractive process for processing large sheets to achieve ultrafine-grained microstructure and high strength. Commercial purity Al and many Al alloys from the 5xxx and the precipitation strengthened 6xxx alloy series have been successfully processed by the ARB process into an ultrafine-grained state and superior ductility have been achieved for some materials like technical purity Al. It has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. This allows optimizing the properties based on two or more materials/alloys. For example, to achieve high corrosion resistance and good visual surface properties it is interesting to produce a composite of two different Al alloys, where for example a high strength alloy of the 5xxx series is used as the core material and a 6xxx series alloy as the clad material. It has been shown that such a composite achieves more or less the same strength as the core material although 50% of the composite consists of the significant softer clad alloy. Furthermore, it has been found, that the serrated yielding which typically appears in 5xxx series alloys and limits applications as outer skin materials completely disappears. Moreover, the ARB process allows many other attractive ways to design new composites and graded material structures with unique properties by the introduction of particles, fibres and sheets. Strengthening with nanoparticles for example is a very attractive way to improve the properties and accelerate the grain refining used in the severe plastic deformation process. With an addition of only 0.1 vol.-% Al2O3 nanoparticles a significantly accelerated grain refinement has been found which reduces the number of ARB passes necessary to achieve the maximum in strength. The paper provides a short review on recent developments in the field of ARB processing for producing multicomponent ultrafine-grained sheet materials with tailored properties.     

变形镁合金的研究进展

镁合金由于具有质量轻、比强度和比刚度高以及良好的铸造性能等特点，在理论研究和实际应用上引起了人们极大的关注。近年来，世界各国纷纷致力于镁合金的研究开发。本文综述了国内外主要的变形镁合金材料的基本特性、力学性能和应用领域，介绍了目前变形镁合金材料的研究现状和进展，以及制备高性能变形镁合金材料的新工艺，探讨了镁合金的合金化原理和主要合金元素在变形镁舍金中的作用。     

具有大过冷液相温度区间的Cu基大块非晶合金的制备和机械性能

利用铜模铸造方法制备了具有大过冷液相温度区间的Cu-Zr-Ti-Ni系高强度Cu基大块非晶合金,对于Cu55Zr55Ti15Ni5合金,最大直径达5mm.过冷液相区温度范围ΔTx达45.48～70.98 K.Cu基玻璃合金棒表现出非常高的机械性能和明显的塑性,对于Cu50Zr25Ti15Ni10、Cu55Zr25Ti15Ni5和Cu54Zr22Ti18Ni6合金,压缩断裂强度分别达2155MPa、2026MPa和1904MPa,维氏硬度分别达674、678和685.加入Co元素扩大了CuZr-Ti-Ni系合金的ΔTx,Cu50Zr22Ti18Ni6Co4合金的ΔTx高达74.5K.     

Deep drawing behaviour of ultrafine grained copper: modelling and experiment

Ultrafine grained materials produced by severe plastic deformation methods possess attractive mechanical properties such as high strength compared with traditional coarse grained counterparts and reasonable ductility. Between existing severe plastic deformation methods the Equal Channel Angular Pressing is the most promising for future industrial applications and can produce a variety of ultrafine grained microstructures in materials depending on route, temperature and number of passes during processing. Driven by a rising trend of miniaturisation of parts these materials are promising candidates for microforming processes. Considering that bi-axial deformation of sheet (foil) is the major operation in microforming, the investigation of the influence of the number of ECAP passes on the bi-axial ductility in micro deep drawing test has been examined by experiments and FE simulation in this study. The experiments have showed that high force was required for drawing of the samples processed by ECAP compare to coarse grained materials. The limit drawing ratio of ultrafine grained samples was in the range of 1.9–2.0 with ECAP pass number changing from 1 to 16, while a higher value of 2.2 was obtained for coarse grained copper. However, the notable decrease in tensile ductility with increase in strength was not as pronounced for bi-axial ductility. The FE simulation using standard isotropic hardening model and von Mises yielding criterion confirmed these findings.     

Nano structure of rapidly quenched Cu–(Zr or Hf) – Ti alloys and their devitrification process

The structure and primary devitrification process of the melt-spun Cu₆₀(Zr or Hf)₃₀Ti₁₀ alloys were investigated. It was confirmed that the compositional segregation in the diameter range of 5–10 nm exists in the as-quenched state. The nanocrystalline particles with cubic structure are observed in the glassy matrix in thehigh-resolution transmission electron microscopy images, of which size is corresponding to the scale of compositional segregation. Small-angle X-ray scattering measurement also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. The nanocrystalline region has high Cu content. In contrast, Zr or Hf and Ti elements are enriched in the glassy region. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of bcc CuZr phase as a primary crystallization phase proceeds in the glassy phase remaining the nanocrystalline phase in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu₈Hf₃ phase at the initial crystallization stage in the Cu–Hf–Ti alloy. These differences of crystallization process are consistent with the results of thermodynamic and kinetic analyses of the transformation mode.     

On the intrinsic ductility of electrodeposited nanocrystalline metals

While nanocrystalline materials hold promise for structural applications in which increased strength is beneficial, their adoption has been hindered by concerns over the achievable ductility, resulting largely from considerable data scatter in the literature. A statistically significant set of 147 electrodeposited nanocrystalline tensile specimens was used to investigate this topic, and it was found that while necking elongation obeys similar processing quality and geometrical dependencies as conventional engineering metals, the intrinsic ductility as measured by uniform plastic strain was unexpectedly independent of microstructure over the grain size range of 10–80 nm. This indicates that the underlying physical processes of grain boundary-mediated damage formation are strain-oriented phenomena that can be defined by a critical plastic strain regardless of the strength of the material as a whole.