Papers by Author: Atsushi Yamamoto

Abstract: Fundamental studies on the thermoelectricity have been mainly done in the pseudo binary systems of Mg2Si – Mg2Ge – Mg2Sn. In recent years, their thermoelectricity is revisited because of light-weight, low initial const and short turning back time in addition to high potential in figureof- merit for ZT approaching to unity or more. Conventional melting and solidification, or, normal PM routes fail in precise, wide-range control of chemical composition and microstructure control. New PM route via bulk mechanical alloying is developed to fabricate the solid solution semiconductive materials with Mg2Si1-xGex and Mg2Si1-ySny for 0 < x, y < 1 and to investigate their thermoelectric materials. Since Mg2Si is n-type and both Mg2Ge and Mg2Sn are p-type, pntransition takes place at the specified range of germanium content, x, and tin content, y. Through optimization of chemical composition, solid-solution type thermoelectric semi-conductive materials are designed both for n- and p-type materials. In addition, appropriate doping condition can be searched in the specified range of x and y.

Abstract: The aluminum alloys AA5052, AA5083 and Al-7%Si (AC4C) were nitrided by electron beam excited plasma (EBEP) technique to improve wear resistance and hardness. The specimens were characterized with respect to the following properties: wear resistance, hardness of nitrided layer (AlN layer), surface and cross sectional microstructures. The friction coefficient of nitrided AA5052 decreased to 0.2 and that of Al-7%Si was improved to 0.1. The AlN layers were relatively uniformly formed on the AA5052 and AC4C alloys with a thickness of 4.5 μm and 2.5 μm during 2hrs and 5.4 ks, respectively. On the surface of nitrided AA5083, a uniform AlN layer was not formed due to the high nitriding temperature. The Si3N4 was found in the nitrided AC4C alloy.

Abstract: The life limit of the standard hardness block of HRC64 made from JIS-SUJ2 steel was evaluated. The HRC64 specimens were annealed at temperatures ranging from 55 to 200
C for about 104 h in maximum. The micro-Vickeres hardness decreased with the annealing time at all temperatures after some incubation periods. Transmission electron microscopy (TEM) showed that the microstructure of the specimen before annealing was composed of primary cementite particles, carbide and martensite matrix. After annealing, fine cementite particles of about few tens of nanometers in size were observed. The decrease in hardness during annealing was caused by the precipitation of the cementite. The life limit keeping the standard hardness value at ambient temperature was estimated to be about 3.5-4 years based on extrapolation of the annealing curves at various temperatures.

Abstract: In this study, the microstructure of two kinds of diamond-like carbon films was studied. These films were produced during the deposition of two kinds of hydrocarbon gas plasma by a newly developed technique called plasma based ion implantation (PBII). Microstructural analysis was performed by means of high resolution transmission electron microscopy (HRTEM). An amorphous DLC film was obtained by using C2H2 plasma. The crystal structure of carbon ( (C 168H ) was generated inside the DLC film when C5H6CH3 plasma was used. The effect of the precursor gases used in the present experiment on the microstructure of the DLC film was also studied.

Abstract: A new technique has been developed for improving corrosion resistance on magnesium alloys. Specimens of AZ31 magnesium alloy were dipped into molten salt of NaBF4 at 723 K for various times, and then cooled, rinsed with water, and dried in air. Corrosion resistance in the surface treated specimens was evaluated by salt immersion test using 1 % NaCl solution as a time for occurring filiform corrosion. On an un-treated AZ31 alloy, the time for starting the filiform corrosion was about 1.2 ks, while on the surface treated specimen, the time was prolonged into about 1300 ks. Moreover, the surface treated specimen showed corrosion resistance in low pH solutions, such as 1 % HNO3 and HCl solutions.

Abstract: In order to improve poor formability in magnesium alloy, grain refinement has been attempted on AZ91E alloy by a thermo-mechanical treatment. Specimens were firstly cold-rolled at 10 %, then solution heat treated at 673 K for 86.4 ks, and hot-rolled at 573 K with about 5 % for four passes, or hot-rolled at 20 % with one pass. The rolled specimens were finally heat treated at 473 to 673 K for 3.6 to 36 ks. Microstructures in the starting material characterized by grain boundary precipitates and aluminum rich regions with about 180 µm in grain size were changed into fine grained microstructures with about 10 to 30 µm in diameter, in which precipitates of Mg17Al12 were uniformly distributed. Although the specimen was prepared by rolling, the (0001) texture was not so remarkable.

Abstract: Grain refinement in AZ31 magnesium alloy has been attempted by hot-rolling and annealing process. Specimens were solution heat treated at 673 K for 36 ks, then hot-rolled at 423 - 773 K with total reduction of 20 - 80 % by multi pass process. The rolled specimens were annealed at 473 - 673 K for 3.6 ks. Grain sizes after the solution heat treatment were about 20 to 150 µm. After hot-rolling at 573 K and annealing at 473 K, grain sizes decreased into about 5 to 10 µm. Suppression of grain growth by pinning due to precipitates was observed by transmission electron microscopic observations.

Abstract: Ion-implantation on high purity iron substrates with nitrogen ions were carried out by using a Cockcroft Walton type accelerator under an accelerating voltage of 150 kV. Hardness measurements on the implanted surfaces showed that hardness effectively increased in the cold rolled specimens by ion-implantation in comparison with in an annealed specimen. Iron nitride, Fe16N2, was formed in the ion-implanted specimens. In the annealed specimen, relatively large particles of Fe₁₆N₂ were formed with low number density, while in the deformed specimens, dislocation substructures due to cold rolling were disappeared by ion-implantation and fine particles were densely formed. Strain field around dislocations induced by deformation provides nucleation sites for Fe₁₆N₂ particles.