Y.B. Wang

Deformation-induced grain rotation and growth in nanocrystalline Ni

Nanobeam electron diffraction and a series of dark field images techniques were used to investigate the deformation mechanisms of nanocrystalline (nc) Ni in response to in situ tensile deformation under transmission electron microscopy (TEM). The experiments exhibit the complete processes of individual grain rotation and neighboring grain rotation/growth. Deformation-induced grain rotation and growth as one of plastic deformation mechanisms in nc materials was revealed. At the same time, these results were confirmed further by ex situ TEM observation on deformed sample and were also better understood by physical deformation model.

In situ observation of twin boundary migration in copper with nanoscale twins during tensile deformation

In situ transmission electron microscopy observations are reported of the dynamic process of twin boundary migration in Cu with nanoscale twins. The experiment provides the first direct evidence of twin boundary migration via Shockley partial dislocation emission from the twin boundary/grain boundary intersections, and reveals that such migration is the dominant deformation mechanism in the initial stage of plastic straining. The behaviour is discussed in comparison with molecular dynamics simulations and in terms of the unique characteristics of the sample microstructure.

Mechanism of grain growth during severe plastic deformation of a nanocrystalline Ni–Fe alloy

Deformation induced grain growth has been widely reported in nanocrystalline materials. However, the grain growth mechanism remains an open question. This study applies high-pressure torsion to severely deform bulk nanocrystalline Ni-20 wt % Fe disks and uses transmission electron microscopy to characterize the grain growth process. Our results provide solid evidence suggesting that high pressure torsion induced grain growth is achieved primarily via grain rotation for grains much smaller than 100 nm. Dislocations are mainly seen at small-angle subgrain boundaries during the grain growth process but are seen everywhere in grains after the grains have grown large.

Super Deformability and Young’s Modulus of GaAs Nanowires

A significant size effect on the mechanical properties of GaAs nanowires (NWs) is reported. A remarkable elastic strain of approximate to 11% for NWs with diameters of 50-150 nm and obvious plastic deformation in NWs with diameters <= 25 nm are revealed. The Youngs modulus of the NWs can be more than double that of bulk GaAs.

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.

A visualization of shear strain in processing by high-pressure torsion

Optical microscopy was used to examine the shear strain imposed in duplex stainless steel disks during processing by high-pressure torsion (HPT). The results show a double-swirl pattern emerges in the early stages of HPT and the two centres of the swirl move towards the centre of the disk with increasing revolutions. Local shear vortices also develop with increasing numbers of revolutions. At 20 revolutions, there is a uniform shear strain pattern throughout the disk and no local shear vortices.

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

Dislocation density evolution during high pressure torsion of a nanocrystalline Ni-Fe alloy

High-pressure torsion (HPT) induced dislocation density evolution in a nanocrystalline Ni-20 wt %Fe alloy was investigated using x-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is fundamentally different from that in coarse-grained materials. The HPT process initially reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle subgrain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the subgrain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials.

Nanocrystalline β-Ti alloy with high hardness, low Young's modulus and excellent in vitro biocompatibility for biomedical applications

High strength, low Youngs modulus and good biocompatibility are desirable but difficult to simultaneously achieve in metallic implant materials for load bearing applications, and these impose significant challenges in material design. Here we report that a nano-grained β-Ti alloy prepared by high-pressure torsion exhibits remarkable mechanical and biological properties. The hardness and modulus of the nano-grained Ti alloy were respectively 23% higher and 34% lower than those of its coarse-grained counterpart. Fibroblast cell attachment and proliferation were enhanced, demonstrating good in vitro biocompatibility of the nano-grained Ti alloy, consistent with demonstrated increased nano-roughness on the nano-grained Ti alloy. Results suggest that the nano-grained β-Ti alloy may have significant application as an implant material in dental and orthopedic applications.

Self-healing of fractured GaAs nanowires

In-situ deformation experiments were carried out in a transmission electron microscope to investigate the structural response of single crystal GaAs nanowires (NWs) under compression. A repeatable self-healing process was discovered in which a partially fractured GaAs NW restored its original single crystal structure immediately after an external compressive force was removed. Possible mechanisms of the self-healing process are discussed.