Eri Miura

Ordering behaviors and age-hardening in experimental AuCu-Zn pseudobinary alloys for dental applications

Age-hardening mechanisms and related ordering behaviors of the experimental (AuCu)(1-x)Zn(x) alloys with x < or = 0.2 were investigated for dental applications. The addition of Zn to equiatomic AuCu greatly increased the age-hardening rate and delayed overaging. It was suggested that the quenched-in excess vacancies were greatly related to the age-hardening rate in the AuCu-Zn pseudobinary alloys. In these alloys, the hardness became maximum during the very initial stage of ordering, and with the development of ordered phase, the hardness began to decrease. Transmission electron microscopy revealed that the age-hardening of AuCu-Zn pseudobinary alloys is caused by lattice distortion that occurred during the very early stage of atomic ordering. The addition of Zn to AuCu effectively increased the density of antiphase boundaries per unit volume of the AuCu II superstructure. This is suggested to be the main cause for the retardation of the overaging in the alloys containing Zn of 5 at% or more. This pronounced effect of Zn addition to AuCu alloy on its age-hardening characteristics may be advantageous for obtaining stable mechanical properties of dental casting gold alloys.

Optical Properties of Au-Pt and Au-Pt-In Alloys

The effects of platinum addition to gold and indium addition to a gold-platinum alloy on their optical properties were investigated using a computer-controlled spectrophotometer. Spectral reflectance data from a polished sample for the incident CIE standard illuminant D65 was collected as a function of the wavelength at 10 nm intervals from 360 to 740 nm. Three coordinates,L* (lightness),a* (red-green),b* (yellow-blue), in the CIE 1976L*a*b* colour space, were determined to quantify the colour of experimental binary Au-Pt and ternary Au-Pt-In alloys. p ]The pronounced step near 520 nm (approximately 2.4 eV) in the spectral reflectance-wavelength curve for pure gold, which is responsible for the rich yellow colour of gold, became less pronounced with the addition of platinum. The decoloring effect of platinum was found to be due to this phenomenon. The addition of indium of up to ca 4 at% to a Au-10 at% Pt alloy increased both chromaticity indices, ie,a* andb* values, giving a gold tinge to the alloy: this effect was brought about by both the slight increase in reflectance in the long-wavelength range and the slight decrease in reflectance in the short-wavelength range of the visible spectrum.

Deformation behavior of Mo5Si3 single crystal at high temperatures

Abstract Compressive deformation behavior of D 8 m -type Mo 5 Si 3 single crystals was investigated at temperatures between 1473 and 1723 K in an argon atmosphere. Four compression axes, namely [001], near-[111], near-[101] and [100] were chosen. Plasticity occurred at and above 1573 K, whereas at 1473 K the crystals failed by brittle fracture before yielding. After high-temperature yielding, all crystals except the [001] crystal exhibited a large yield drop, followed by an apparent steady state flow. Slip traces of {110} and other relatively low index planes were observed on crystal surfaces. Dislocations were characterized on (001) slip plane by TEM, and 〈110〉(001) slip was identified. Considering the constant-stress flow behavior as a steady-state creep process, it was estimated that a stress component is about 6 and an apparent activation energy of deformation is approximately 490 kJ mol −1 .

Effects of Zn Addition to AuCu on Age-hardening Behaviors at Intraoral Temperature

The effect of Zn addition to AuCu on the age-hardening rate at the intraoral temperature was investigated to find out the proper condition for high age-hardening rate. The increase in hardness of Zn-added alloys during aging at 37 °C was due to the atomic ordering. With an increase in Zn concentration, hardness of a sample under the as-quenched condition decreased, but the age-hardening rate obviously increased. When Zn content was fixed, a higher solution treatment temperature was more effective for the age-hardening at 37 °C. It was suggested that the formation energy of a vacancy considerably decreased with an increase in Zn content. It is reasonable to consider that the amount of quenched-in excess vacancies are markedly increased with an increase in Zn content when the solution treatment temperature was fixed. By transmission electron microscopic observations, it was revealed that the formation of the AuCu II superstructure contributed to the age-hardening at 37 °C in the high zinc content alloy.

Two types of checkerboard-like microstructures in Au–Cu–Pd ternary alloys

Abstract The phase transformation process in Au–Cu–Pd ternary alloys with the compositions within or near the two-phase region (Au 3 Cu+AuCu I) was studied by means of transmission electron microscopy and hardness testing. Two types of characteristic checkerboard-like microstructures were observed: (1) the coexistence of the Au 3 Cu and AuCu I ordered phases, and (2) the coexistence of the AuCu I ordered phase and disordered phase with face-centered cubic structure. Different age-hardening behaviors were recognized: The high hardness value in the former was maintained for prolonged aging periods, and that produced in the latter was maintained to a certain period followed by over-aging.

Solid-Solution Strengthening by Oxygen in Nb–Ta and Nb–Mo Single Crystals

We have investigated the high-temperature solid-solution strengthening by oxygen in Nb, Nb-Ta and Nb-Mo single crystals by compression tests with constant strain rate. Nb, Nb-20Ta, Nb-40Ta, Nb-20Mo, and Nb-40Mo single crystals with various O content were grown by use of the electron-beam floating zone melting method. It is found that Nb and Nb-Ta single crystals are strengthened by oxygen at temperatures below 1100 K, while Nb-20Mo single crystals are strengthened even at 1473 K. The critical resolved shear stress (c.r.s.s.) of Nb-Ta single crystals at 298 K increases linearly with increasing O content, irrespective of Ta content. Furthermore, the increment of c.r.s.s. of Nb-Ta single crystals is larger than that of Nb. The c.r.s.s. of Nb-Mo single crystals is higher than that of Nb for all temperatures tested, and the c.r.s.s. of Nb-20Mo single crystals also increases with increasing O content. These results indicate that the interstitial-substitutional (i-s) interaction between O and Ta or Mo affects the mechanical properties in the Nb-based alloys at high temperature.

Internal-friction study of the interstitial–substitutional effect on the deformation behaviour of Nb–O, Nb–Ta–O and Nb–Mo–O single crystals

To investigate the interstitial-substitutional interaction in dislocations, the effect of 0 on the temperature, frequency and amplitude dependence of the internal friction Q −1 in Nb–O, Nb-20 mol% Ta–O and Nb-20 mol% Mo–O single crystals has been studied (f = 1.55–8.2 Hz) in the temperature range from 298 to 1473 K. In our previous study, Nb–Mo and Nb–Ta single crystals were found to be strengthened by solute O. It was also suggested that the interstitial–substitutional interaction in dislocations contributes to the increase in their critical resolved shear stress (CRSS). In this study, Snoek-type relaxation peaks due to O are observed between 500 and 700 K in all the single crystals. The Snoek peak of Nb-20mol% Mo–O consisting of several peaks is analysed. The activation energy of the Snoek peak in Nb-20mol% Mo–O is higher than that of Nb–O. These results are attributable to the existence of the interstitial–substitutional complexes. The amplitude dependence of Q −1 at intermediate and high temperatures decreases as the O content increases. Moreover, the breakaway stress of Nb-20 mol% Mo–O still has a high value at 1200K and does not decrease much even at 1473 K. This suggests that the formation of Mo–O complexes reduces the dislocation mobility at high temperatures. From the results, the effect of the interstitial-substitutional interaction on the CRSS was discussed.

Partial phase diagram for the AuCu–Zn pseudobinary system

Abstract Zinc addition to AuCu was found to have significant effects on the structural change of AuCu. A partial phase diagram for the AuCu–Zn pseudobinary system was constructed based on electrical resistivity measurements and X-ray diffraction study. The addition of Zn to AuCu stabilized the AuCu II superstructure and extremely enlarged the AuCu II single-phase region in the Au–Cu–Zn system. The order–disorder transition temperature was slightly lowered with the addition of Zn to AuCu but the AuCu II↔AuCu I transition temperature was greatly lowered. The antiphase domain size of the AuCu II superstructure markedly decreased with increasing Zn content. The lattice parameter a decreased and c increased with Zn addition, thus the axial ratio, c / a , considerably increased.

CHARACTERISTIC MICROSTRUCTURES ASSOCIATED WITH ORDER-DISORDER TRANSITION IN Au-Cu-Pd TERNARY ALLOYS

Department of Dental Materials Engineering, Faculty of Dentistry,Kyushu University, Fukuoka 812-8582, Japan(Received February 24, 2000)(Accepted in revised form March 28, 2000)Keywords: Transmission electron microscopy; Gold alloy; OrderingIntroductionBinary Au-Cu alloys undergo order-disorder transition in some compositional regions. At lowertemperatures, they form a cubic L1

Coherent phase diagram of Au-Cu-Pd ternary system near and within the two-phase region of Au3Cu and AuCu I ordered phases

Abstract Phase identification and analysis of microstructural configuration were done by transmission electron microscopy to determine the coherent phase diagram of the Au–Cu–Pd ternary system near and within the two-phase region of the Au 3 Cu and AuCu I phases. Six distinguishable regions were found in the present experimental phase diagram: a single-phase region of the disordered solid solution, α; a single-phase region of the Au 3 Cu ordered phase; a single-phase region of the AuCu I ordered phase; a region of the coexisting Au 3 Cu and AuCu I ordered phases; a region of the coexisting Au 3 Cu and α phases; and a region of the coexisting AuCu I and α phases.