Tuning ideal tensile strengths and intrinsic ductility of bcc refractory alloys.

An important theoretical ductility criterion for group V and VI metal-based refractory alloys in body-centered cubic (bcc) lattices is the mechanical failure mode of their perfect crystals under tension along the weakest direction [100]. Pure Mo and W fail by cleavage and are deemed intrinsically brittle. However, first-principles calculations show that alloying with group IV or V transition metals can transform these materials into ones that display intrinsically ductile behavior, failing in shear under [100] tension. Remarkably, this transition can be understood as an electron filling effect with the intrinsically ductile response the manifestation of a Jahn-Teller distortion.

Nanoscale structural engineering via phase segregation: Au-Ge system.

A tunable structural engineering of nanowires based on template-assisted alloying and phase segregation processes is demonstrated. The Au-Ge system, which has a low eutectic temperature and negligible solid solubility (<10(-3) atom %) of Au in Ge at low temperatures, is utilized. Depending on the Au concentration of the initial nanowires, final structures ranging from nearly periodic nanodisk patterns to core/shell and fully alloyed nanowires are produced. The formation mechanisms are discussed in detail and characterized by in situ transmission electron microscopy and energy-dispersive spectrometry analyses. Electrical measurements illustrate the metallic and semiconducting characteristics of the fully alloyed and alternating Au/Ge nanodisk structures, respectively.

Embedded binary eutectic alloy nanostructures: a new class of phase change materials.

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles.

Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the sphere's diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation.

Effect of solute atoms on dislocation motion in Mg: an electronic structure perspective.

Solution strengthening is a well-known approach to tailoring the mechanical properties of structural alloys. Ultimately, the properties of the dislocation/solute interaction are rooted in the electronic structure of the alloy. Accordingly, we compute the electronic structure associated with, and the energy barriers to dislocation cross-slip. The energy barriers so obtained can be used in the development of multiscale models for dislocation mediated plasticity. The computed electronic structure can be used to identify substitutional solutes likely to interact strongly with the dislocation. Using the example of a-type screw dislocations in Mg, we compute accurately the Peierls barrier to prismatic plane slip and argue that Y, Ca, Ti, and Zr should interact strongly with the studied dislocation, and thereby decrease the dislocation slip anisotropy in the alloy.

Metallurgy. Origin of dramatic oxygen solute strengthening effect in titanium.

Structural alloys are often strengthened through the addition of solute atoms. However, given that solute atoms interact weakly with the elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. By using transmission electron microscopy and nanomechanical characterization, we report that the intense hardening effect of dilute oxygen solutes in pure α-Ti is due to the interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations reveal that distortion of the interstitial sites at the screw dislocation core creates a very strong but short-range repulsion for oxygen that is consistent with experimental observations. These results establish a highly effective mechanism for strengthening by interstitial solutes.

Statistical approach to the unfolding of mechanically stressed biopolymers.

The mechanical properties of structural biomaterials are determined by features at many length scales. One example is the presence of an organic adhesive in nacre. The organic material contains biopolymers with discrete domains that may unfold as the polymer is extended. A statistical model for the mechanical response of a single biopolymer with these characteristics is introduced and studied. The response to tensile strain under displacement-controlled and load-controlled conditions is examined. Under the assumption of irreversible unfolding, analytical expressions for the load at first unfolding were derived, and a transition in behavior was observed for fast and slow loading. For titin, under displacement controlled extension, this transition occurs at about 5.25 pm/s.

Theory of nanocluster size distributions from ion beam synthesis.

Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict the existence of a steady-state shape for the cluster-size distribution that depends only on a characteristic length determined by the effective diffusion coefficient, the ion solubility, and the volumetric ion flux. The average cluster size in the steady-state regime is determined by the implanted species or matrix interface energy.

"Ideal" engineering alloys.

A newly discovered group of alloys, called Gum Metals, approaches ideal strength in bulk form, exhibits significant plastic deformation prior to failure, and shows no indications of conventional-dislocation activity. Two conditions must be met for a material to exhibit this "ideal" behavior: (1) the stress required to trigger conventional-dislocation plasticity in the material must exceed its ideal strength, and (2) the material must be intrinsically ductile when stressed to ideal strength. Gum Metals satisfy both criteria, explaining their remarkable mechanical properties.

Large melting-point hysteresis of Ge nanocrystals embedded in SiO2.

The melting behavior of Ge nanocrystals embedded within SiO2 is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (+/-17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.

Metal-induced assembly of a semiconductor island lattice: Ge truncated pyramids on Au-patterned Si.

We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of submicron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering.

Distortion and segregation in a dislocation core region at atomic resolution.

The structure of an isolated, Ga terminated, 30 degree partial dislocation in GaAs:Be is determined by high resolution transmission electron microscopes and focal series reconstruction. The positions of atomic columns in the core region are measured to an accuracy of better than 10 pm. A quantitative comparison of the structure predicted by an ab initio electronic structure total energy calculation to the experiment indicates that theory and experiment agree to within 20 pm. Further analysis shows the deviations between theory and experiment appear to be systematic. Electron energy loss spectroscopy establishes that defects segregate to the core region, thus accounting for the systematic deviations.

Adatom transport on strained Cu(001): surface crowdions.

Surface strain is often suggested as a means to control the self-assembled growth of nanostructures. Strain affects both the kinetics of nucleation and the free energies of formation of the desired nanostructure. It is demonstrated here that diffusion on some strained surfaces may be mediated by newly identified adatom transport mechanism: the formation and motion of a surface crowdion.

Structure and energy of the 90 degrees partial dislocation in diamond: a combined Ab initio and elasticity theory analysis.

The core structure and stability of the 90 degrees partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10-20 A, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis.