Masayuki Hasaka

Process of Order-Disorder Transformation in Iron-Aluminum Alloys

The process of order-disorder transformation during slow cooling and isothermal annealing in Fe–Al alloys has been investigated by means of X-ray diffraction. During slow cooling the ordering takes place through the two-phase state at a certain composition. while at other compositions it occurs as a single-phase transformation. The isothermal ordering proceeds by a nucleation and growth of the ordered phase, and under certain circumstances the precipitation of disordered phase takes place gradually in the course of annealing. The disordering into the two-phase state from an ordered single phase proceeds in two steps. The most plausible phase diagram of the Fe–Al system containing the two-phase regions, α+B2 and α+DO3, is proposed. The size of ordered domains is estimated from the widths of superlattice reflections, and its change during the order-disorder transformation is discussed.

Skutterudite structure and thermoelectric property in CefFe8−xCoxSb24 (f = 0–2, x = 0–8)

The skutterudite structure and thermoelectric properties of CefFe8−xCoxSb24 were investigated by means of X-ray diffraction (XRD) experiments, energy-dispersive X-ray (EDX) measurements under a transmission electron microscope, and measurements of the thermoelectromotive force. The filling number of Ce into voids in the skutterudite (f) is variable at each substitution number of Fe by Co (x) and decreases with increasing x. The four-cornered ring of Sb tends to become square more rapidly with increasing f than with increasing x. The lattice parameter increases with increasing f, whereas it decreases with increasing x. The sign of the thermoelectromotive force depends on x for f 1. The type of semiconductor changes at a critical valence electron density of 194/nm3.

Age-hardening behavior in a palladium-base dental porcelain-fused alloy

Abstract Age hardening in a Palladium-base porcelain-fused alloy was investigated by hardness tests, x-ray diffraction, scanning, and transmission electron microscopy. In the early stages of aging, a metastable ordered phase with a face-centered tetragonal structure based on Pd 3 In was formed within the grains. Hardening was due to coherency strain between the metastable structure and the matrix with a face-centered cubic structure. In the later stages, a lamellar structure, consisting of a stable ordered phase with a face-centered tetragonal structure based on Pd 3 In and a stable phase with a face-centered cubic structure, grew from the grain boundaries. Finally, the lamellae covered the whole grain and resulted in softening.

Mössbauer effect of Co- or Mn-doped β-FeSi2

Abstract The Mossbauer effect of 57Fe in transition metal (Co or Mn)-doped β-FeSi2 was measured at room temperature so as to examine the local atomic structure around iron atoms. From the analysis of Mossbauer spectra, it is disclosed that (1) many vacancies exist in site I in undoped β-FeSi2; and (2) Mn atoms occupy I sites in β-FeSi2 whereas Co atoms occupy II sites. Isomer shifts of I sites and II sites in Co- or Mn-doped β-FeSi2 are found to be greater than those in β-FeSi2.

Age-hardening reactions and microstructures of a dental gold alloy with palladium and platinum

The age-hardening reactions and microstructures of a dental casting gold alloy with some palladium and platinum were investigated by means of hardness tests, X-ray diffractometry, electrical resistivity measurements, and scanning and transmission electron microscopy. Ageing reactions during isothermal annealing were completed by two stages. The first stage corresponded to the formation of a metastable CuAu I phase within grains, and the second stage to a cellular reaction at grain boundaries. The former contributed to the hardening and the latter to the softening. An activation energy of 128 kJ mol−1 for hardening was obtained. Two types of cell growth were observed and could be distinguished in terms of the homologous temperature.

The effects of Ti implantation on corrosion and adhesion of TiN coated stainless steel

Abstract Thin titanium nitride (TiN) films of 40 and 70 nm in thickness were deposited on austenitic-type 304 stainless steel substrates by a rf ion plating process, and these specimens were irradiated with 70 kV titanium ions at a fluence of 1 × 10 17 /cm 2 by use of MEVVA IV metallic ion source at room temperature. After that TiN films of 2 μm were deposited by the same method. The results of X-ray photoelectron spectroscopy and Auger electron spectroscopy revealed that implanted titanium penetrated into the substrate and interfacial mixing was verified. The adhesion strength was estimated by a scratch test. It was found that ion implantation can enhance the adhesion strength between the film and the substrate. The corrosion resistance of the specimens was evaluated in aqueous solutions of sulfuric acid by an electrochemical method. Titanium implantation was extremely effective in suppressing the anodic dissolution of stainless steel.

Diffusion of copper, aluminum and boron in nickel

Secondary ion mass spectroscopy (SIMS) is a valuable technique for analyzing the distributions of solutes which diffused from the surface to the interior side of metals. The high resolution of SIMS can determine small diffusion coefficients below 10[sup [minus]12] cm[sup 2]/s observed at low temperature, and also measure the diffusion coefficients of various solutes including aluminum and boron isotopes which are not easily available. By means of a radioactive tracer technique, the diffusion coefficients of various solutes in nickel have been measured only at temperature higher than 0.7T[sub m], where T[sub m] is the melting temperature of nickel. With the benefit of SIMS, the present paper investigates the diffusion coefficients of copper, aluminum and boron in the blocks and the melt-spun ribbons of nickel from 0.7T[sub m] to 0.36T[sub m].

Age-hardening associated with ordering and spinodal decomposition in a AgCu-40 at % Au pseudobinary alloy

The age-hardening mechanism in an AgCu-40 at% Au alloy was studied by means of electrical resistivity measurement, hardness tests, X-ray diffraction and electron microscopy. Two stages of hardening were found by isothermal ageing below 648 K, which was higher than the critical temperature of ordering, Tc=620 K, in the present alloy. The first stage of hardening took place by formation of a modulated structure resulting from spinodal decomposition. Further hardening was brought about by ordering, yielding metastable AuCu I′ and/or AuCu II′ ordered platelets grown from the copper-rich portion of the modulated structure. Transitional ordering which gave rise to a marked hardening of the second stage was found, even though the temperature of below 648 K was higher than the Tc of the present alloy. Drastic softening was also found on disappearance of the transitional ordered phases. Although the modulated structure was observed by ageing at 773 K, there was no age-hardening.

Partially filled skutterudite structure in CefFe8-xNixSb24

Abstract Observation of a Ce–Fe–Ni–Sb system with a partially filled skutterudite structure was performed using high-resolution transmission electron microscope imaging. Multislice high-resolution transmission electron microscopy image simulation was satisfactorily performed using the number of Ce atoms filling in voids and the lattice coordinates of Sb atoms as determined from X-ray measurements. The simulated images depend on these structural parameters.

Mössbauer studies of Cr- or Ni-doped β-FeSi2

Abstract Iron disilicide β-FeSi2 has two iron siites (site I and site II) with an equivalent number of atoms in the unit cell. We estimated the occupation ratio of iron between the two sites when 2 at.% Cr or Ni is doped in β-FeSi2. It is found that the amplitude of site I is greater than that of site II in Ni doped β-FeSi2 whereas the amplitude of site I is smaller than that of site II in Cr-doped β-FeSi2. These results indicate that Ni atoms occupy site II while Cr atoms occupy site I preferentially. The increase in the isomer shift for each sites is observed in Ni-doped β-FeSi2, implying that Ni doping causes enhancement of 3d-screening of 57Fe nuclei.