Qiuhong Lu

Strain hardening and large tensile elongation in ultrahigh-strength nano-twinned copper

A high density of growth twins in pure Cu imparts high yield strength while preserving the capacity for efficient dislocation storage, leading to high strain hardening rates at high flow stresses, especially at 77 K. Uniform tensile deformation is stabilized to large plastic strains, resulting in an ultrahigh tensile strength of similar to1 GPa together with an elongation to failure of similar to30%

Microstructural fingerprints of phase transitions in shock-loaded iron

The complex structural transformation in crystals under static pressure or shock loading has been a subject of long-standing interest to materials scientists and physicists. The polymorphic transformation is of particular importance for iron (Fe), due to its technological and sociological significance in the development of human civilization, as well as its prominent presence in the earths core. The martensitic transformation α→ε (bcc→hcp) in iron under shock-loading, due to its reversible and transient nature, requires non-trivial detective work to uncover its occurrence. Here we reveal refined microstructural fingerprints, needle-like colonies and three sets of {112}<111> twins with a threefold symmetry, with tell-tale features that are indicative of two sequential martensitic transformations in the reversible α→ε phase transition, even though no ε is retained in the post-shock samples. The signature orientation relationships are consistent with previously-proposed transformation mechanisms, and the unique microstructural fingerprints enable a quantitative assessment of the volume fraction transformed.

Structural Change during Cold Rolling of Electrodeposited Copper

Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20˚ of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.

History-independent cyclic response of nanotwinned metals

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated ‘necklace’ dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

Length scale effect on the deformation microstructures of grown-in twins in copper

Electrodeposited copper samples composed of columnar grains subdivided by alternating twin/matrix (T/M) lamellae have been cold rolled to 30–85% reduction in thickness. The thickness of the T/M lamellae varies over a wide range from a few nanometres to about 1 μm. The deformation microstructure has been characterized systematically. In thin T/M lamellae (below 50–100 nm) the deformation behaviours differ significantly from that of thick T/M lamellae, as the dislocation activity is concentrated at the T/M boundaries illustrated by the observations of stacking faults and Shockley partial dislocations. In thick T/M lamellae (100–1000 nm), the deformation microstructure is related to the grain orientation as also observed previously in deformed single crystals and polycrystals with a grain size at the micrometre scale. The experiment therefore suggests that the universal structural characteristics of deformation microstructure can be extended one order of magnitude from about 5 μm to the sub-micrometre scale (about 0.5 μm).

Orientation and length scale effects on dislocation structure in highly oriented nanotwinned Cu

Highly oriented nanotwinned Cu has been compressed to 6% strain in directions 90°, 0° and 45° with respect to the twin boundaries of the almost parallel twins. In the 90° and 0° compressed samples Mode I and Mode II dislocations and their interactions with twin boundaries dominate the deformation of twin/matrix (T/M) lamellae with thickness less than 500 nm. In 45° compressed samples, Mode III dislocations, especially partial dislocations moving along the twin boundaries, dominate the deformation of fine T/M lamellae with thickness less than 100 nm, while dislocations from slip Modes I, II and III are identified in T/M lamellae more than 100 nm thick, where these dislocations extensively interact in the T/M lamellae with thicknesses more than 200 nm. Dislocation cells are observed in a twin lamella with a thickness of about 500 nm.