Kazuki Sugita

Positron Annihilation Study of Formation of Mg Vacancy in MgO

We have investigated the formation of Mg vacancy induced by ultra-dilute trivalent impurities in MgO by a combination of positron annihilation measurement and theoretical calculations of positron lifetimes. The undoped MgO yields the shortest positron lifetime of 130 ps that is shorter than that of 166 ps previously reported using a single crystal sample. The positron lifetime of the doped samples increases with the increase of the Al or Ga dopant concentration and is saturated at around 170 ps. This result indicates that the previously reported value of 166 ps is ascribed to not the bulk but the vacancy state induced by impurities. The experimental bulk lifetime of 130 ps, which is obtained by employing trapping model, is well reproduced by the theoretical calculation using the semiconductor model. The calculated defect lifetime is about 20 ps longer than the experimental value. This may be due to the lattice relaxation around Mg vacancy associated with the trapping of positrons.

Vacancy trapping by solute atoms during quenching in Cu-based dilute alloys studied by positron annihilation spectroscopy

Frozen-in vacancies and the recovery have been investigated in some Cu-based dilute alloys by using positron annihilation lifetime spectroscopy. Cu-0.5at%Sb, Cu-0.5at%Sn and Cu-0.5at%In dilute bulk alloys were quenched to ice water from 1223 K. A pure-Cu specimen was also quenched from the same temperature. As a result, no frozen-in vacancies have been detected in as-quenched pure-Cu specimen. On the other hand, as-quenched Cu-0.5at%Sb alloy contained frozen-in thermal equilibrium vacancies with concentration of 3 × 10−5. Furthermore, these frozen-in vacancies in Cu-0.5at%Sb alloy were stable until 473 K, and began to migrate at 523 K. Finally, the Cu-Sb alloy were recovered to the fully annealed state at 823 K. This thermal stability clearly implies some interaction exists between a vacancy and Sb atom and due to the interaction, thermal equilibrium vacancies are trapped by Sb atoms during quenching.

Electron irradiation damage and the recovery in a Zr-based bulk amorphous alloy Zr55Cu30Al10Ni5

Zr-based bulk amorphous alloys have been intensively studied because of their superiority in mechanical properties, corrosion resistance and micro-formability, and of great anticipation of their applications. The damage was introduced into a Zr-based bulk amorphous alloy Zr55Cu30Al10Ni5 by electron irradiation below 20K. The thermal recovery process from the irradiation damage was studied by using positron lifetime spectroscopy which sensitively detect open volume defects. The introduced damage was completely recovered at the temperature lower than the glass transition temperature by 200K. On the other hand, the X-ray diffraction pattern shows no crystalline peaks even after the recovery of the irradiation damage. This result is considerably different from the previous reports studied in conventional amorphous alloys.

Strain-Rate Dependence of Vacancy Cluster Size in Hydrogen-Charged Martensitic Steel AISI410 under Tensile Deformation

The strain-rate dependence of vacancy cluster sizes in hydrogen-charged martensitic steel AISI410 under tensile deformation was investigated using positron lifetime spectroscopy. The vacancy-cluster sizes in hydrogen-charged samples tended to increase with decreasing strain rates during the tensile deformations. The vacancy-cluster sizes significantly correlated to the tensile elongations to fracture. It was revealed that the presence of large-sized vacancy-clusters can cause the degradation of mechanical properties and followed by brittle fracture.

The International Workshop on Positron Studies of Defects 2014

The International Workshop on Positron Studies of Defects 2014 (PSD-14) was held in Kyoto, Japan from 14–19 September, 2014. The PSD Workshop brought together positron scientists interested in studying defects to an international platform for presenting and discussing recent results and achievements, including new experimental and theoretical methods in the field. The workshop topics can be characterized as follows: • Positron studies of defects in semiconductors and oxides • Positron studies of defects in metals • New experimental methods and equipment • Theoretical calculations and simulations of momentum distributions, positron lifetimes and other characteristics for defects • Positron studies of defects in combination with complementary methods • Positron beam studies of defects at surfaces, interfaces, in sub-surface regions and thin films • Nanostructures and amorphous materials

Electronic Structure and Bonding in Amorphous Zr67Ni33 and Zr67Cu33

Abstract The electronic structures and the bonding properties in amorphous Zr 67 Ni 33 and Zr 67 Cu 33 alloys have been calculated by the discrete variational (DV)-X α molecular orbital method to investigate the chemical bonding in the amorphous structures. The calculations about Zr 2 Ni and Zr 2 Cu and β -Zr crystals were also performed for comparison. The broad agreement between the DOSs for the amorphous Zr 67 Ni 33 alloy and the Zr 2 Ni crystal implies the structure similarity. The calculated DOSs and the bond overlap population analyses suggest that the amorphous structures are significantly influenced by the tight binding pair (Zr–Ni in the Zr–Ni systems and Cu–Cu in the Zr–Cu systems). The glass-formation abilities reflect the bonding properties in the amorphous and crystal structures.