Nanomachining of nanocrystalline nickel by focused ion beam

Nanocrystalline and sub-microcrystalline samples of nickel have been machined by a focused Ga⁺ ion beam (30?keV and 187?pA) at doses of 8.92?×?10¹⁶?–?2.68?×?10¹⁸ ions/cm² and their surface topography was investigated by atomic force microscopy (AFM). Values of the root-mean-square (RMS) roughness increase with increasing ion dose. The surfaces of the nanocrystalline Ni were smoother than those of the sub-microcrystalline Ni, indicating that smoothing due to diffusion for the former works more effectively than that for the latter.     

Flux-driven nucleation at interfaces during reactive diffusion

Nucleation of intermetallic compounds and of voids at interfaces during reactive diffusion is treated with account of influence of the flux divergence in the nucleation regions of the real space as an additional term for drift in the size space (in Fokker–Planck equation for nucleation). Such approach enables the construction of effective Gibbs nucleation barrier which may (in the broad region of parameters) increase to infinity meaning the full suppression of nucleation, or, by the contrast, decrease assisting the nucleation. The introduced effective nucleation barriers depend on kinetic factors – on the ratio of diffusivities in nucleating and in neighboring phases. Thus, the competition of stable and metastable phases is reconsidered, as well as nucleation of Kirkendall/Frenkel voids at the interfaces.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

Structural evolution during crystal nucleation in supercooled liquids with icosahedral short-range order

The structural evolution during crystal nucleation in supercooled Lennard–Jones liquids at a supercooling of 0.3T _m (T _m is the melting temperature) has been studied by molecular dynamic simulations. The icosahedral clusters are observed to compete with crystalline clusters in space, and rearrange before crystal nucleation. Both the stable face-centered-cubic and hexagonal-close-packed (hcp) structures and metastable body-centered-cubic (bcc) structures nucleate simultaneously, resulting in the formation of an incomplete bcc meso-layer in the nuclei. The nuclei form twinned crystal with five-fold axes through a successive twinning process bounded by planes with hcp atoms.     

Deformation twinning of Bi–Sb solid alloy formed under a strong gravitational field

We performed an ultracentrifuge experiment on Bi–Sb alloy. Deformation twins with misorientations of about 90° were observed in the low-gravitational region where grain refinement had not occurred. The twins were thicker than the conventional deformation twins and their thickness was proportional to the gravitational field. We found that the minimum gravitational field required for grain refinement was 0.17 × 10⁶ G at 240°C for periods <10 h.     

On the ultra-high-strain rate shock deformation in copper single crystals: multiscale dislocation dynamics simulations

Multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper single crystals that have undergone shock loading at high strain rates ranging from 1?×?10⁶ to 1?×?10¹⁰?s^?1. Plasticity mechanisms associated with both the activation of pre-existing dislocation sources and homogeneous nucleation of glide loops are considered. Our results show that there is a threshold strain rate of 10⁸?s^?1 at which the deformation mechanism changes from source activation to homogeneous nucleation. It is also illustrated that the pressure dependence on strain rate follows a one-fourth power law up to 10⁸?s^?1 beyond which the relationship assumes a one-half power law. The MDDP computations are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models.