Special Grain Boundaries in Ultrafine-Grained Tungsten.

Field ion microscopy and computer simulation were used for the study of an atomic structure high-angle grain boundary in hard-drawn ultrafine-grained tungsten wire. These boundaries with special misorientations are beyond the scope of the coincident site lattice model. It was demonstrated that the special non-coincident grain boundaries are the plane-matching boundaries, and rigid-body displacements of adjacent nanograins are normal to the <110> misorientation axis. The vectors of rigid-body translations of grains are described by broad asymmetric statistical distribution. Mathematical modeling showed that special incommensurate boundaries with one grain oriented along the {211} plane have comparatively high cohesive energies. The grain-boundary dislocations ½<110> were revealed and studied at the line of local mismatch of {110} atomic planes of adjacent grains.

A new approach for explanation of specimen rupture under high electric field.

Failure of needle-shaped specimens under the action of high electric fields is one of the most serious problems in many field-ionization devices and methods. It was shown that the length of what was removed during discharge part of specimens in most of the cases was several orders greater than that of an initial radius of the specimen apex. Experimental results obtained in this work indicate that electrical breakdown in field-ion microscopes is initiated by the impact of nanoparticles detached from a point anode in a high electric field. A new approach for explanation of an observed specimen failure in a high electric field was proposed, where the formation and expansion of the cathode plasma play the key role in the mechanical rupture and melting of field-ion tips. The expanding plasma behaves as a dynamic capacitor plate characterized by a high velocity and increasing field strength at the specimen tip. The field rise rate at a constant voltage in a dynamic plasma-to-tip capacitor is of the order of >10(20)V/m/s. The present experimental data along with the ecton model of the vacuum breakdown allow the results to be discussed as a mechanical sample failure under the action of the superhigh positive field pulse induced by the cathode plasma expansion.

Non-kinkwise field evaporation and kink relaxation on stepped W(112) surface.

Low-temperature field evaporation of the [111] steps on a W(211) surface was investigated by field ion microscopy (FIM). The atoms at the (112)[1;1;1] kink site are anomalously stable against field evaporation. This effect results in non-kinkwise field evaporation near the kink of this type. The non-kinkwise field evaporating steps [211]<111> usually produce the reconstructed atomic chain with double-space arranged adatoms. The experimental results reveal atomic relaxation effects at steps and kinks. The normal to surface differential relaxation of the kink-site atoms was estimated by the geometrical method of indirect magnification and by simulation of the FIM image.