E Hashimoto

The positron trapping efficiency of dislocations in deformed dilute aluminium alloys

Positron-lifetime measurements have been made on deformed Al-0.01 at.% Mg, Al-0.01 at.% Si and Al-0.01 at.% Cu alloys at room temperature. The specific trapping rate mu d of positrons into dislocations depends strongly on alloying elements. Mg and Cu atoms can reduce mu d to a great extent, while Si atoms are not so effective. These results are closely related to the pinning of dislocations by solute atoms.

Change in the positron-trapping efficiency of dislocations in Al on heating after deformation

Positron-lifetime measurements have been made in order to study how the positron trapping efficiency of dislocations, mu d, in Al (99.9999% purity) changes during isochronal annealing after deformation at room temperature. It has been shown that mu d goes on increasing during the anneal, while the lifetime tau d for trapped positrons decreases at first but than becomes a constant, approximately=210 ps. The highest value of mu d obtained was about 4.5*10-4 m2 s-1, much larger than previous values, (0.066-2)*10-4 m2 s-1. After heating at a higher temperature, it has also been shown that a small applied stress can yield a marked decrease in mu d without any significant change in tau d. These results suggest that mu d depends strongly on dislocation configurations. The increase in mu d during the anneal may be due to a development of long straight dislocation-line segments.

Anisotropy of the size effect in the electrical resistivity of high-purity Al single crystals

To clarify the anisotropy of the dc size effect in Al, measurements have been made of the electrical resistivity of high-purity Al single crystals at 4.2 K. The specimen surfaces were set parallel to each of three crystallographic planes, {100}, {111} and {110}, and the axis orientations were parallel to , and . The main results were the following. (1) The size effect increased in the following order of the surface orientations: {110}, {111}, {100}. (2) For the size effect due to a {110} surface, the Fuchs-Sondheimer theory with gave a good description, while, for the size effect due to {100} and {111} surfaces, substantial disagreement with the theory was suggested for the very thick specimens. (3) For each surface orientation, for sufficiently thin specimens was independent of the axis orientation within the experimental error; i.e. the size effect was independent of the direction of current flow. (4) However, as the specimen thickness increased, strong anisotropy of with respect to the current direction appeared: increased in the following order of directions: . Results (3) and (4) suggest an anisotropy effect of the bulk resistivity in high-purity Al.

Vacancy-type dislocation loops observed at fairly high temperatures in nearly perfect Al crystals

Vacancy-type dislocation loops appeared at 230°C even during a slow-cooling process from 300°C in nearly perfect aluminum crystals when examined by synchrotron radiation topography with white radiation. The temperature of the specimen was cyclicly varied between 300 and 230°C at a cooling rate of 2000°C/h and a heating rate of 1000°C/h. Almost all of these vacancy loops appeared at the same place at each repetition of the cooling process, and some of the loops were formed along the dislocation lines observed in the preceding stage. Formation of these loops cannot be explained by homogeneous nucleation theory. Therefore, it is concluded that the vacancy loops grown at fairly high temperatures were nucleated heterogeneously, and the nucleation site appears to be inclusions in the specimen. Almost all vacancy loops that remained in a nearly perfect aluminum crystal are formed by heterogeneous nucleation during the cooling process.

The size effect on electrical resistivity in single crystals of dilute Al alloys

In order to study how the scattering anisotropy influences the dc size effect, measurements have been made on the orientation dependence of the size effect in dilute Al - Ag and Al - Si alloys at 4.2 K. The solute Ag decreases the relaxation time near the zone boundaries, while Si has a reverse effect. The surface orientations of the specimens were set parallel to the crytallographic planes {100} and {110} in the light of the anisotropic size effect found in pure Al. The axis orientation of the specimens (direction of current flow) was . The results show that the size effect in these specimens can be well described by means of the Fuchs - Sondheimer theory, with the specularity parameter p = 0 and the product of bulk resistivity and bulk mean free path . This is contrary to the theoretical prediction that the size effect in Al is sensitive to the anisotropy of the relaxation time due to the impurities.

Anisotropy of the temperature dependence of electrical resistivity in aluminium single crystals

The anisotropy of the temperature-dependent part of the electrical resistivity has been measured in the range 4.2-80 K on aluminium single crystals with a common surface orientation (110) and specimen axes (100), (111) and (110). The specimens were cut from one single-crystal rod with a residual resistance ratio of about 50 000. The temperature-dependent part of the (100) specimen, rho 100ph(T), is larger than that of the (111) specimen, rho 111ph(T), below about 25 K. Then, rho 100ph(T) becomes smaller than rho 111ph(T) above that temperature. At higher temperatures, the anisotropy gradually disappears. On the other hand, the temperature-dependent part of the (110) specimen, rho 110ph(T), is smaller than rho 111ph(T) at low temperatures and this anisotropy vanishes rapidly at higher temperatures. A mechanism for the anisotropy that arises from the anisotropic distribution of conduction electrons in an electric field applied along (100) is suggested.

Anisotropic dependence of the residual electrical resistivity on the current direction in aluminium single crystals

The detailed dependence of the residual electrical resistivity on the current direction has been measured at 4.2 K in high-purity aluminium single crystals. All the specimens with the same main surface (110) were cut from one single-crystal rod with the residual resistance ratio of about 50000. The bulk residual resistivity rho 0 was obtained from the measured resistivity rho 0 according to the Fuchs-Sondheimer theory with parameters suitable for high-purity aluminium. Large anisotropy of rho 0 with a fine structure has been found with respect to the current direction: a difference between the largest and the smallest values of rho 0 reaches 80% of the latter. This anisotropy of rho 0 shows a strong correlation with the structure of the Fermi surface.

Potentials for the approach of a hydrogen atom to an aluminum metal

Abstract The potential curves when a hydrogen atom is absorbed into aluminum through the lattice gate on the surface are obtained by an ab initio molecular orbital method with cluster models which consist of a hydrogen atom and several aluminum atoms. The calculations are carried out on the (100) and the (111) surface gates. It is found that the electron correlation lowers the potential barrier from the outside to the inside and shifts the inside minimum points to the higher symmetrical site, Td or Oh. Moreover, the results suggest the possibility that a more stable site than the adsorption site on the surface exists at the octahedral site between the surface and the second layer.

Application of High-Resolution Film for Lithography to Synchrotron X-Ray Topography

A high-resolution film for lithography is applied to a detector for synchrotron radiation topography, instead of a nuclear plate. The film shows much better resolution than that of the plate although exposure time an about 500 times longer is required. The size distribution of interstitial loops grown as vacancy sources in a nearly perfect aluminum crystal after a temperature rise is examined from the white beam topograph.

Effects of impurities on the positron trapping efficiency of dislocations in deformed aluminium

Positron lifetime and Doppler-broadening measurements have been made on deformed aluminium ( approximately=99.95- approximately=99.9999% purity) and Al-0.01 at.% Si alloy. It has been found that impurities even in trace amounts can reduce the positron trapping efficiency of dislocations.