Nanoscale Growth Twins Research Articles

The intrinsic nature of materials failure and the global non-equilibrium energy criterion

Materials failure under some sort of loading is a well-known natural phenomenon, and the reliable prediction of materials failure is the most important key issue for many different kinds of engineering structures based on their safety considerations. In this research, instead of establishing empirical models, the material failure process was modeled as a nonequilibrium process based on the microstructural mechanism. Then, the evolution equations were established and the stability analysis was carried out to obtain the critical conditions for the materials failure. It was found that the material strength was a global property in nature, and the commonly used local criteria based on the most dangerous point failure were not the rational assumption. Based on the idea, some examples were considered, such as the size effect of the material strength, the strength of the polycrystalline metals, the stress-strain relationship of the ultrafine crystalline metal with nanoscale growth twins, the strength of lithium niobite crystal specimens with notches. All of the theoretical predictions gave reasonable results compared with the experimental data.

Thickness effect on fracture behavior of columnar-grained Cu with preferentially oriented nanoscale twins

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Synthesis and microstructure of electrodeposited and sputtered nanotwinned face-centered-cubic metals

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The 1.85 GPa AlSc Bulk Alloy with Abundant Nanoscale Growth Twins**Supported by the Scientific Research Foundation of Xi'an University of Technology under Grant No 101-451115007, the National Basic Research Program of China under Grant No 2007CB613900, the National Natural Science Foundation of China under Grant No 51174161, and the Pivot Innovation Team of Shaanxi Electric Materials and Infiltration Technique under

An in situ ultrahigh-strength ductile Al50Sc50 bulk alloy is produced by the copper mold casting method with a composite microstructure of micron-/submicron-sized grains and nanoscale twins. According to the microstructural investigations, hierarchical nanotwinned lamellar AlSc bundles with embedded micron-/submicron-sized Al2Sc and AlSc2 are observed. The as-cast alloy displays a unique act of ultrahigh strength of ∼1.85 GPa together with pronounced work hardening and a large plasticity of ∼14%. Further microstructural investigations on deformed specimens indicate that abundant hierarchical nanotwinned lamellar AlSc bundles are effective to dissipate localization of shear stress or block dislocations from spreading throughout the alloy and hinder the propagation of microcracks formed by local stress transition.

Effect of Strain Rate on Tensile Ductility and Fracture Behavior of Bulk Nanotwinned Copper

A bulk columnar‐grained copper with preferentially oriented nanoscale growth twins is prepared by means of direct‐current electrodeposition. Tensile tests at different strain rates reveal a significant influence of strain rate on the tensile ductility and fracture behavior. The ductility, especially the post‐necking elongation, reduces dramatically at low strain rates, which is associated with evident intergranular fracture. The results suggest that the grain size as well as the grain boundary microstructural evolution exert a strong influence on the intrinsic fracture mode in the nanotwinned structure.

ELECTRON MICROSCOPY STUDY OF FIVE-FOLD TWINS IN ELECTRODEPOSITED NANO-TWIN Ni

The nanocrystalline Ni thin films with high density nano-scale growth twins were synthesized by direct electrodeposition technology. The five-fold twinning structure in electrodeposited nano-twin Ni was systematically investigated by TEM. The remarkable diffraction pattern and HRTEM images obtained from the cross-section observation demonstrate directly that the electrodeposited nano-twin Ni has five-fold twinning structure with five {111} twinned subcrystals and systematically analyzed the 7.35° intrinsic structural gap. In this work, the 7.35° gap was at least inset in two twin boundaries of the five-fold twin, the twin boundaries which share the 7.35° gap always broaden and was decomposed into other twins, so that, the grain present a irregular shape. cross- sectional TEM micrograph revealed that the electrodeposited nano-twin Ni had columnar grain structure with a strong {110} texture. By means of comprehensive structure characterization, a new space structural model of the five-fold twin was proposed.

Uniting superhardness and damage-tolerance in a nanosandwich-structured Ti–B–N coating

A novel nanosandwich-structured Ti–B–N coating was prepared on a Ti–6Al–4V substrate using a double glow discharge plasma technique. This coating consists of alternating Ti(N,B) nanocrystalline layers, containing nanoscale growth twins, and amorphous TiB2–BN layers. Compared with a monolithic TiB2.06 nanocrystalline coating, the new Ti–B–N coating exhibits a marked increase in both damage resistance and hardness (i.e. ∼55 GPa). The enhanced mechanical properties were found to stem from a combination of nanostructuring and nanotwinning effects.

High resolution transmission electron microscopy characterization of fcc → 9R transformation in nanocrystalline palladium films due to hydriding

Sputtered nanocrystalline palladium thin films with nanoscale growth twins have been subjected to hydriding cycles. The evolution of the twin boundaries has been investigated using high resolution transmission electron microscopy. Surprisingly, the ∑3{112} incoherent twin boundaries dissociate after hydriding into two phase boundaries bounding a 9R phase. This phase which corresponds to single stacking faults located every three {111} planes in the fcc Pd structure was not expected because of the high stacking fault energy of Pd. This observation is connected to the influence of the Hydrogen on the stacking fault energy of palladium and the high compressive stresses building up during hydriding.

Growth and properties of coherent twinning superlattice nanowires

Although coherent twin boundaries require little energy to form in nanoscale single crystals, their influence on properties can be dramatic. In recent years, some important steps forward have been made in understanding and controlling twinning processes at the nanoscale, making possible the fabrication of nanoengineered twinning superlattices in crystalline nanowires. These advances have opened new possibilities for properties and functionalities at the atomic and quantum scales by modulating twin densities. This article presents a brief overview of recent theoretical and experimental progress in growth mechanisms and promising properties of coherent twinning superlattice nanowires with special emphasis toward cubic systems in semiconductor and metallic materials. In particular, we show how nanoscale growth twins can considerably enhance bandgap engineering and mechanical behaviour in quasi-one-dimensional materials. Opportunities for future research in this emerging area are also discussed.

EFFECT OF TEMPERATURE ON MICROSTRUCTURE AND NANOINDENTATION MECHANICAL PROPERTIES OF ELECTRODEPOSITED NANO-TWINNED Ni

The nanocrystalline nickel thin films with high density nano-scale growth twins were synthesized by means of pulse electrodeposition technology. Three samples were deposited at different bath temperatures of 30, 50 and 80 , by keeping all the other parameters as constant, such as electrolyte, pH value, current density and on-time and off-time period. The effect of temperature on deposition rate, sample texture, grain size, and twin boundary length and twin lamella thickness were systematically analyzed by SEM, XRD and TEM techniques. The microscopical feature of twin boundary were investigated by HRTEM. The results show that the nano-twinned nickel films deposited at the rates ranging from 20 nm/s to 30 nm/s have a preferred growth plane of (220) when de- posited at 30 and 50 , but changes to (200) when the temperature increases to 80 . With increasing

Numerical simulation on size effect of copper with nano-scale twins

To understand the tensile deformation of electro-deposited Cu with nano-scale twins, a numerical study was carried out based on a conventional theory of mechanism-based strain gradient plasticity (CMSG). The concept of twin lamella strengthening zone was used in terms of the cohesive interface model to simulate grain-boundary sliding and separation. The model included a number of material parameters, such as grain size, elastic modulus, plastic strain hardening exponent, initial yield stress, as well as twin lamellar distribution, which may contribute to size effects of twin layers in Cu polycrystalline. The results provide information to understand the mechanical behaviors of Cu with nano-scale growth twins.

Simulation of Size Effects in Material with Growth Twins

A comprehensive numerical study of tensile deformation on electro-deposited Cu with nano-scale growth twins is presented using a conventional theory of mechanism-based strain gradient plasticity (CMSG). Systematic research on the size effects of growth twins in Cu polycrystalline is carried out. The roles of many material parameters which affect the size effects, such as grain sizes, Young’s modulus, twin/matrix lamellar structures aspect ratio and volume fraction, as well as the plastic strain hardening exponent, are presented.

Repulsive force of twin boundary on curved dislocations and its role on the yielding of twinned nanowires

The effects of twin size and sample diameter on yield stress and surface dislocation emission in twinned metal nanowires deformed uniformly were studied using classical dislocation theory and the concept of image force from twin boundaries. This theoretical study is shown to quantitatively capture the linear increase in yield stress as twin size decreases in periodically twinned Au nanowires predicted by atomistic simulations. The implication of this model as a yield criterion for realistic metal nanostructures with nanoscale growth twins is discussed.

<i>In Situ</i> Transmission Electron Microscopy Investigation of the Deformation Behavior of Cu with Nanoscale Twins

This paper reviews our recent studies on the effect of twin boundary (TB) on the deformation behavior in Cu with nanoscale growth twins. In situ straining transmission electron microscopy investigations on TB migration, TBs and twin ends acting as dislocation emission sources, and the interactions between dislocations and TBs are highlighted. Results provide some useful understanding of why Cu with nanoscale twins leads to a combination of ultrahigh strength and high ductility.

Significant enhancement of the strength-to-resistivity ratio by nanotwins in epitaxial Cu films

Epitaxial nanotwinned Cu films, with an average twin spacing ranging from 7 to 16 nm, exhibit a high ratio of strength-to-electrical resistivity, ∼400 MPa(μΩ cm)−1. The hardness of these Cu films approaches 2.8 GPa, and their electrical resistivities are comparable to that of oxygen-free high-conductivity Cu. Compared to high-angle grain boundaries, coherent twin interfaces possess inherently high resistance to the transmission of single dislocations, and yet an order of magnitude lower electron scattering coefficient, determined to be 1.5–5×10−7 μΩ cm2 at room temperature. Analytical studies as well as experimental results show that, in polycrystalline Cu, grain refinement leads to a maximum of the strength-to-resistivity ratio, ∼250 MPa(μΩ cm)−1, when grain size is comparable to the mean-free path of electrons. However, in twinned Cu, such a ratio increases continuously with decreasing twin spacing down to a few nanometers. Hence nanoscale growth twins are more effective to achieve a higher strength-to-resistivity ratio than high-angle grain boundaries.

Atomic-scale in situ observation of lattice dislocations passing through twin boundaries

In situ transmission electron microscopy tensile experiments were carried out to investigate lattice dislocation and twin boundary (TB) interaction in Cu with nanoscale growth twins. Results show that extended dislocations form inside thick twin lamellas and slip toward TBs. The extended dislocations shrink, combine, and redissociate when they pass through TBs, leaving behind detwinning partial dislocations at the TBs. The mechanism of the observed dislocation/TB interactions and the effect of the mechanism on mechanical properties are discussed.

Mechanical behavior of an electrodeposited nanostructured Cu with a mixture of nanocrystalline grains and nanoscale growth twins in submicrometer grains

A nanostructured Cu with a mixture of nanocrystalline grains and nanoscale growth twins in submicrometer grains was synthesized by electrodeposition, and its mechanical behavior was tested on MTS-810 system at different strain rates and room temperature. It exhibited high yield stress of 506–717 MPa and good ductility of 6.2%–9.1%. Two stage strain rate sensitivity (m value), different fracture directions, and different morphologies of deformed and fracture surfaces at high strain rates and low strain rates were observed in the nanostructured Cu, which demonstrated the deformation mechanism transition from dislocation deformation at higher strain rates to both dislocation deformation dominated in larger grains and grain boundary diffusion and grain boundary sliding dominated in small grains at low strain rates.

Twinning in nanocrystalline metals

Twinning is a commonly observed deformation mode in lowsymmetry crystals such as hexagonal-close packed metals, but not in high-symmetry metals that possess many possible slip systems with lower critical resolved shear stress (CRSS) than the twinning stress. However, if the CRSS for slip can be increased to a level greater than the twinning stress, then twinning may be favored as a deformation mechanism even in cubic materials. Nanostructured metals present such a scenario. Following the observation of deformation twinning in nano-crystalline metals, even in high stacking fault energy metals such as aluminum, recently there has been signifi cant interest in studying twinning as a deformation mode in nanocrystalline face-centered cubic (fcc) metals. Another important breakthrough is the fabrication of fcc metals with nanotwinned structures using methods such as electro-deposition or sputter deposition. Nanoscale growth twins have also been observed in nanowires. Nanotwinned fcc metals made via sputtering can have twin boundary spacing as small as 2–4 nm and preferential alignment of {111} twin boundaries normal to the growth direction. These as-grown nanotwinned structures exhibit unusual properties as compared to nanocrystalline metals. Typically, in the equiaxed grain nanocrystalline metals with high-angle grain boundaries the increased strength is accompanied by a loss in ductility, thermal stability, and electrical conductivity. Twinning in Nanocrystalline Metals

Nanoscale growth twins in sputtered metal films

This article reviews recent studies on the mechanical properties of sputtered copper and 330 stainless-steel films with {111} nanoscale growth twins preferentially oriented perpendicular to growth direction. The mechanisms of formation of growth twins during sputtering, unusually high strengths, and excellent thermal stability of nanotwinned structures are highlighted.

The effect of twin plane spacing on the deformation of copper containing a high density of growth twins

In-situ tensile straining in a transmission electron microscope was used to investigate the role of twin plane spacing on the deformation and fracture mechanism of pure copper containing a high density of nanoscale growth twins. Real-time and post-mortem observations clearly reveal that twin plane spacing plays a key role in determining the operative deformation mechanism and therefore the subsequent crack propagation path. The deformation mechanism transition, which results from changes in the twin plane spacing, has implications for interpreting the unusual mechanical behavior of the copper with a high density of nanoscale growth twins.