Nanoscale Growth Twins Research Articles

Dynamical dislocation emission processes from twin boundaries

In situ tensile straining transmission electron microscopy investigation of electrodeposited copper with high density of nanoscale growth twins reveals that twin boundaries (TBs) can serve as dislocation sources. Atomic steps at TBs formed by sessile Frank partial dislocations are beneficial for TBs serving as dislocation sources. The underlying mechanism that includes dislocation reactions with TBs is discussed.

Thermal stability of sputtered Cu films with nanoscale growth twins

We have investigated the thermal stability of sputter-deposited Cu thin films with a high density of nanoscale growth twins by using high-vacuum annealing up to 800 °C for 1 h. Average twin lamella thickness gradually increased from approximately 4 nm for as-deposited films to slightly less than 20 nm after annealing at 800 °C. The average columnar grain size, on the other hand, rapidly increased from approximately 50 to 500 nm. In spite of an order of magnitude increase in grain size, the annealed films retained a high hardness of 2.2 GPa, reduced from 3.5 GPa in the as-deposited state. The high hardness of the annealed films is interpreted in terms of the thermally stable nanotwinned structures. This study shows that nanostructures with a layered arrangement of low-angle coherent twin boundaries may exhibit better thermal stability than monolithic nanocrystals with high-angle grain boundaries.

Electrical resistivity of ultrafine-grained copper with nanoscale growth twins

We have investigated electrical resistivities of high-purity ultrafine-grained Cu containing different concentrations of nanoscale growth twins, but having identical grain size. The samples were synthesized by pulsed electrodeposition, wherein the density of twins was varied systematically by adjusting the processing parameters. The electrical resistivity of the Cu specimen with a twin spacing of 15nm at room temperature (RT) is 1.75μΩcm (the conductivity is about 97% IACS), which is comparable to that of coarse-grained (CG) pure Cu specimen. A reduction in twin density for the same grain size (with twin lamellar spacings of 35 and 90nm, respectively) results in an increment in electrical resistivity from 1.75to2.12μΩcm. However, the temperature coefficient of resistivity at RT for these Cu specimens is insensitive to the twin spacing and shows a consistent value of ∼3.78×10−3∕K, which is slightly smaller than that of CG Cu (3.98×10−3∕K). The increased electrical resistivities of the Cu samples were ascribed dominantly to the intrinsic grain boundary (GB) scattering, while the GB defects and GB energy would decrease with increasing twin density. Transmission electron microscope observations revealed the GB configuration difference from the Cu samples with various twin densities. Plastic deformation would induce an apparent increase in the resistivity. The higher of the twin density, the higher increment of RT resistivity was detected in the Cu specimens subjected to 40% rolling strain. Both the deviated twin boundaries and strained GBs may give rise to an increase in the resistivity.

Work hardening of ultrafine-grained copper with nanoscale twins

Work hardening behaviors of electrodeposited ultrafine-grained Cu with nanoscale growth twins are investigated by means of uniaxial continuous tensile and loading-unloading tests. A high density of nanoscale twins leads to a significant enhancement in flow strength and work hardening rate, while the work hardening coefficient (n) does not show any obvious variation with the twin density. These effects were analyzed in terms of the post-deformation microstructures. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents a brief overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure Cu with nanoscale growth twins. Sustained research efforts have established a group of nanocrystalline and nanostructured metals that exhibit a combination of high strength and considerable ductility in tension. Accompanying the gradually deepening understanding of the deformation mechanisms and their relative importance, quantitative and mechanisms-based constitutive models that can realistically capture experimentally measured and grain-size-dependent stress–strain behavior, strain-rate sensitivity and even ductility limit are becoming available. Some outstanding issues and future opportunities are listed and discussed.

Strength, strain-rate sensitivity and ductility of copper with nanoscale twins

We present a comprehensive computational analysis of the deformation of ultrafine crystalline pure Cu with nanoscale growth twins. This physically motivated model benefits from our experimental studies of the effects of the density of coherent nanotwins on the plastic deformation characteristics of Cu, and from post-deformation transmission electron microscopy investigations of dislocation structures in the twinned metal. The analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating the twin boundary as an internal interface and allowing special slip geometry arrangements that involve soft and hard modes of deformation. This model correctly predicts the experimentally observed trends of the effects of twin density on flow strength, rate sensitivity of plastic flow and ductility, in addition to matching many of the quantitative details of plastic deformation reasonably well. The computational simulations also provide critical mechanistic insights into why the metal with nanoscale twins can provide the same level of yield strength, hardness and strain rate sensitivity as a nanostructured counterpart without twins (but of grain size comparable to the twin spacing of the twinned Cu). The analysis also offers some useful understanding of why the nanotwinned Cu with high strength does not lead to diminished ductility with structural refinement involving twins, whereas nanostructured Cu normally causes the ductility to be compromised at the expense of strength upon grain refinement.

Strain rate sensitivity of Cu with nanoscale twins

The strain-rate sensitivity of ultrafine-crystalline Cu with different concentrations of nanoscale growth twins is investigated using tensile strain rate jump tests. Higher twin density leads to enhanced rate sensitivity, which decreases mildly with increasing strain rate and strain. Mechanisms underlying these effects are explored through post-deformation transmission electron microscopy.

High-strength sputter-deposited Cu foils with preferred orientation of nanoscale growth twins

Bulk Cu foils have been synthesized via magnetron sputtering with an average twin spacing of 5nm. Twin interfaces are of {111} type and normal to the growth direction. Growth twins with such high twin density and preferred orientation have never been observed in elemental metals. These Cu foils exhibited tensile strengths of 1.2GPa, a factor of 3 higher than that reported earlier for nanocrystalline Cu, average uniform elongation of 1%–2%, and ductile dimple fracture surfaces. This work provides a route for the synthesis of ultrahigh-strength, ductile pure metals via control of twin spacing and twin orientation in vapor-deposited materials.

Thermal stability of sputter-deposited 330 austenitic stainless-steel thin films with nanoscale growth twins

We have explored the thermal stability of nanoscale growth twins in sputter-deposited 330 stainless-steel (SS) films by vacuum annealing up to 500°C. In spite of an average twin spacing of only 4nm in the as-deposited films, no detectable variation in the twin spacing or orientation of twin interfaces was observed after annealing. An increase in the average columnar grain size was observed after annealing. The hardness of 330 SS films increases after annealing, from 7GPa for as-deposited films to around 8GPa for annealed films, while the electrical resistivity decreases slightly after annealing. The changes in mechanical and electrical properties after annealing are interpreted in terms of the corresponding changes in the residual stress and microstructure of the films.

Tensile properties of copper with nano-scale twins

An electro-deposited Cu sample with a high density of nano-scale growth twins shows an ultrahigh tensile strength (∼1 GPa) with a considerable plastic strain (>13%). Both the strength and the ductility increase with a decreasing twin lamellae thickness. The yield strength values follow the empirical Hall–Petch relationship for conventional polycrystalline Cu.

Ultrahigh strength and high electrical conductivity in copper.

Methods used to strengthen metals generally also cause a pronounced decrease in electrical conductivity, so that a tradeoff must be made between conductivity and mechanical strength. We synthesized pure copper samples with a high density of nanoscale growth twins. They showed a tensile strength about 10 times higher than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The ultrahigh strength originates from the effective blockage of dislocation motion by numerous coherent twin boundaries that possess an extremely low electrical resistivity, which is not the case for other types of grain boundaries.