Mechanical Behavior and Fabrication of One-dimensional Nanomaterials

For centuries, improvements in structural materials relied heavily on processing, which in turn, dictated resulting microstructure and properties. Materials sciences are entering a revolutionary era, where specific material properties are attained through not only materials but also architecture control of its constituents, often with sub-micron and nanoscale dimensions. One-dimensional homogeneous or heterogeneous micro/nanoscale materials often show the unusual mechanical strength and physical features in electric and optical spectra, and they have broad potential applications in future nano devices. Thus, the study of the structure and properties of micro/nanoscale materials is becoming a burgeoning scientific research field. In parallel, synthesizing such controllable one-dimensional nanoscale materials has attracted extensive attention. In this regard, five articles are included in this issue, three related to mechanical deformation and two to the synthesis. Regarding mechanical deformation in micro/nanoscale materials, Zhiwei Shan at Xi’an Jaiotong University reviews their recent progress on revealing the novel properties of micro/ nanoscaled metal pillars by using in situ transmission electron microscopy (TEM), including facecentered cubic (fcc) single-crystal Ni, body-centered cubic (bcc) single-crystal Mo alloy, hexagonal-closepacked (hcp) a-Ti alloy, and metallic glasses. For instance, ‘‘mechanical annealing’’ was demonstrated in submicron-sized fcc single-crystal nickel pillars fabricated through focused ion beam (FIB). But, mechanical annealing occurs in a bcc single crystal as the dimension is below 200 nm, which was attributed to the high flow stress enhanced by size effect. Strong size effect is also observed in hcp structured Ti alloy. The critical sample size ( 1 lm) below which the plastic flow becomes quite continuous without any major strain burst and the maximum flow stress is observed to saturate at close to the ideal strength of Ti. The second article focuses on deformation mechanism (twinning/slip) dependence on sample size in hcp metals. Quian Yu at the University of California, Berkeley has conducted in situ and ex situ small-scale testing for hcp metals, Mg, Ti, and their alloys. The effect of size on deformation twinning in these systems is remarkable, resulting in a significant change in corresponding mechanical properties. They found that deformation twinning can be restricted by the size effect in certain dimension regimes and materials but also can be promoted by the presence of surfaces at extremely small scales. The third article reviews deformation mechanisms in one-dimensional nanostructures containing homogeneous (i.e., grain and twin boundaries) and heterogeneous (i.e., phase and precipitate-matrix boundaries) interfaces. Julia Greer at the California Institute of Technology has been devoted to investigating mechanical behavior and microstructural evolution in small-scale metallic systems, whose deformation is governed by intricate interactions of defects with interfaces as well as free surfaces, including multiple grain boundaries spanning the sample volume; periodic in-grown twin boundaries; and bi-material samples. This overview sheds light on the relative role of intrinsic versus extrinsic dimension limitations on the deformation mechanisms in nanostructured metals. Although testing one-dimensional samples is usually fabricated by the FIB technique, defects associated with one-dimensional growth are also closely related with the materials mechanical properties. Hanchen Huang at the University of Connecticut has been devoted to developing a solid framework for the growth of one-dimensional crystalline nanostructures. The framework derives from three scientific advancements in crystal growth, with a focus on multiple-layer surface steps: the diffusion barrier of adatoms down multiple-layer surface steps, the formation and stability of multiple-layer surface steps, and the dimension of surface facets that are bounded by competing monolayer and multiple-layer surface steps. As an example of impact, this framework predicts that growth of Al nanorods is not feasible using physical vapor depoJOM, Vol. 64, No. 10, 2012