Yasushi Kamimura

Positron lifetime calculations on vacancy clusters and dislocations in Ni and Fe

Abstract Positron lifetime calculations have been performed on vacancy clusters (stacking fault tetrahedra (SFT), vacancy loops), such clusters on dislocation line, interstitial clusters, such clusters on a dislocation line and dislocation line itself in order to investigate the so-called intermediate lifetimes observed in the experiments, namely, positron lifetimes between that at a matrix and that at a single vacancy. Before lifetime calculations, various defects were constructed in the model lattices and were relaxed completely to obtain the stable atomic structure by using N -body potentials. Then positron lifetime calculation was carried out for each defect. It was shown that positron lifetime for a SFT in Ni dependes on its size and becomes smaller with increasing the size. The positron wave function is mainly localized at the corner of a SFT, which gives rather lifetime, e.g., 130 ps for V 28 , but when the cluster size is small, e.g., less than 10 vacancies, it gives a rather longer lifetime, e.g., 177 ps for V 6 because of the wave function localized at the inner space of a cluster. These behaviours are consistent with the experimental results. It was also found that the positron lifetime on a dislocation line and that at a jog are short (113 and 119 ps, respectively for Ni, 117 and 117 ps, respectively for Fe), close to the lifetime at matrix (110 ps for both Ni and Fe) and in these cases trapping potentials for a positron are shallow both for Ni and Fe.

Core structure of a screw dislocation in a diamond-like structure

Abstract The core structure of a screw dislocation in the diamond lattice has been calculated with the Stillinger-Weber potential for the interatomic interaction. The non-dissociated screw dislocation positioned at the centre of a unit hexagon normal to the dislocation line is a stable configuration A. A perfect screw centred on a longer edge of the unit hexagon is also a stable configuration B. Configuration B is more stable and has a lower energy than configuration A. Dissociated configurations of width w = na (a is the repeat distance in the slip direction and n = 1, 2, 3,…) are also stable. The geometrical feature of configuration B explains the cross-slip observed in III-V compounds at low temperatures. The Peierls stress of the perfect screw dislocation is 0.044G, where G is the shear modulus. This value corresponds to the experimental values of III-V compounds deduced from the temperature dependence of the critical shear stress at low temperatures.

Plastic homology of bcc metals

Abstract The temperature dependence of the yield stress T c of four bcc transition metals α-Fe, Nb, Mo and Ta, and an alkali metal K as well, follows a scaling relation of T c/T p against kT/b 3(TpKT)½, where T p is the Peierls stress, b the length of the Burgers vector and K T the line tension factor for a screw dislocation calculated for elastic anisotropy. The same relation seems to apply for the other three bcc transition metals V, Cr and W, suggesting its general applicability at least for low strain rates (about 10−4 s−1). The result implies that the plasticity of bcc metals is characterized by only one non-elastic parameter, the Peierls stress T p.

Temperature dependence of the flow stress of III-V compounds

Abstract An interpretation for the temperature dependence of the plastic flow stress of III–V compounds is presented, invoking only non-dissociated screw dislocations. With the aid of the Stillinger-Weber potential for the interatomic interaction, the Peierls potential of a non-dissociated screw dislocation is deduced in twodimensional space normal to the dislocation line. A saddle-point configuration and the formation energy of a kink pair are calculated in three-dimensional space. The relation obtained between the flow stress T c and temperature T under a constant strain rate describes the experimental T c−T relation of III–V compound well: a strong T dependence at highT c and low T, a stronger T dependence at low Tc and high T, and a plateau-like T c at intermediate T. The hump in the T c−T relation is interpreted as a transition between different paths of the non-dissociated screw dislocation: a planar path and a zigzag path in the (111) plane.

Dislocation-related optical absorption in plastically deformed GaN

Optical absorption spectra have been measured for plastically deformed GaN to investigate effects of dislocations. The interband absorption edge has been observed to shift noticeably to lower photon energy by deformation, which has been analyzed based on a model of the Franz-Keldysh effect by the electric fields associated with charged dislocations. This model has satisfactorily reproduced the observed absorption spectra. In a lower energy region, a decrease in free-carrier absorption by deformation has been observed, which is partly attributable to the decrease in carrier concentration by carrier trapping at dislocation states.

Direct observation of carrier depletion around a dislocation in GaP by scanning spreading resistance microscopy

The local electrical resistivities in deformed n-GaP have been measured by scanning spreading resistance microscopy (SSRM). The SSRM images show chainlike alignments of spots with high resistivity along the slip direction. These spots can be attributed to carrier-depletion around a charged dislocation. From the observed spot size, the line charge density of the dislocations has been estimated to be 0.4–0.9e/b, where b denotes the magnitude of the Burgers vector. The estimated value of line charge density has been discussed in relation with the dislocation core structure.

Mechanical properties of dual-phase alloys of B2 and icosahedral phases in the Al–Cu–Fe system

Intermetallic alloys with B2 structure and dual-phase alloys of the B2 phase and an icosahedral (i-) phase have been produced, and their mechanical properties have been investigated. Vickers hardness measurements and compression experiments have shown that the dual-phase alloys have considerably higher strength than that of the B2 single-phase alloy up to a high temperature of around 750 K. The dual-phase alloys have also exhibited much better ductility than i-single-phase alloys. One possibility to be pointed out is that we can further design a high-strength and good-ductility alloy by tuning the volume fraction, size and morphology of the i-phase precipitates.

Electrical conduction along dislocations in plastically deformed GaN

Electrical conduction along dislocations in plastically deformed n-GaN single crystals has been investigated by scanning spread resistance microscopy (SSRM). In the SSRM images, many conductive spots have been observed, which correspond to electrical conduction along the dislocations introduced by deformation. Here, the introduced dislocations are b=(a/3) edge dislocations parallel to the [0001] direction. The current values at the spots normalized to the background current value are larger than 100. Previous works have shown that grown-in edge dislocations in GaN are nonconductive. The high conductivity of the deformation-introduced edge dislocations in the present work suggests that the conductivity depends sensitively on the dislocation core structure.

Dissolution patterns caused by etching of low-indexed surfaces of Al-Pd-Mn icosahedral and Al-Cu-Co decagonal quasicrystals

Abstract Dissolution patterns caused by anodic etching of an Al–Pd–Mn icosahedral quasicrystal and chemical etching of an Al–Cu–Co decagonal quasicrystal were examined by optical and electron microscopy as well as by atomic force microscopy. Appropriate etching treatments of these quasicrystals, i.e. the anodic etching of the Al–Pd–Mn quasicrystal in a solution of CH 3 OH–HNO 3 , and the chemical etching of the Al–Cu–Co quasicrystal in a solution of HF–HNO 3 –H 2 O resulted in occurrence of dissolution patterns composed of a number of well-facetted micropits (and hillocks for the Al–Cu–Co alloy as well) on low-indexed surfaces of the specimens. The morphological features of these etch-figures are discussed in relation to the internal structural symmetries.

Transmission Electron Microscopy and Atomic Force Microscopy Studies of an Al–Pd–Mn Thin Film Prepared by a Combined Technique of Vacuum Deposition and Thermal Annealing

A thin-film sample of Al–Pd–Mn ternary alloy has been produced by a vacuum deposition technique using dual evaporation sources, followed by thermal annealing. The microstructure of the film has been examined by transmission electron microscopy (TEM) and by atomic force microscopy (AFM). The TEM observations reveal that the film has a granular texture of highly ordered icosahedral phase islands embedded in the matrix consisting mainly of multiple-twinned crystals exhibiting a pseudo-tenfold symmetry. It is shown by AFM that the icosahedral islands form protuberant structures with the average height of approximately 50 nm.