Takanobu Shiraishi

Effect of Fluoride Concentration and pH on Corrosion Behavior of Titanium for Dental Use

Titanium is used as a metal for biocompatible materials such as dental implants or restorations because of its excellent chemical stability. However, the corrosion of Ti in the prophylactic fluoride-containing environment can become problematic. To clarify the effects of fluoride concentration and pH on the corrosion behavior of Ti, we conducted anodic polarization and immersion tests in NaF solution of various concentrations and pH values. The concentrations of dissolved Ti in the test solutions were analyzed by inductively coupled plasma mass spectroscopy. There were obvious limits of fluoride concentration and the pH value at which the corrosion behavior of Ti changed. The corrosion of Ti in the solution containing fluoride depended on the concentration of hydrofluoric acid (HF). When the HF concentration in the solution was higher than about 30 ppm, the passivation film of the Ti was destroyed. The results of this study revealed a relation between the fluoride concentrations and pH values at which Ti corrosion occurred and provided data on such corrosion in environments where the fluoride concentration and pH value are known.

Age-hardening of Dental Ag-Pd-Cu-Au Alloys

Age-hardening mechanisms of dental Ag-Pd-Cu-Au alloys were investigated by means of electron microscopic observations. Hardening was due to the precipitation of the L1o type CuPd-ordered platelet in the grain interior and to the discontinuous precipitation at the grain boundary. The characteristics of age-hardening curves were determined by rates of continuous and discontinuous precipitation.

Dental gold alloys with age-hardenability at intraoral temperature

Dental gold alloys with age-hardenability at intraoral temperature were developed. Either 3 or 6 at% Ga, Al, Zn, In, Ni or Pd were added to an equiatomic AuCu alloy and the effect of additives on the low-temperature age-hardenability was examined. Alloys containing Ga, Al or Zn exhibited excellent low-temperature age-hardenability. The hardness of an alloy containing 6 at% Ga or 6 at% Al was doubled in one to ten days by ageing at 37 °C. Pronounced hardening was not demonstrated in an as-cast alloy, but solution treatment for only a few minutes provided enough age-hardenability. The age-hardening rate at low temperature related closely with the melting temperature of the alloy. Experimental gold alloys exhibited electrochemical behaviour similar to that of a commercial Type IV gold alloy. It is therefore concluded that AuCu alloy with added Ga, Al or Zn is expected to have enough age-hardenability and corrosion resistance for clinical use in the oral environment.

Aging Reactions in a Low Gold, White Dental Alloy

Aging behavior of a low gold, white dental alloy was investigated by electrical resistivity measurements, X-ray diffraction, and electron microscopy. It was characterized by both grain interior and grain boundary reactions. The coexistence of AuCu-I and CuPd in dental alloys was first confirmed.

Low temperature ageing in equiatomic CuAu and Cu-Au-Pd ternary alloys

Low temperature ageing at 37° C in an equiatomic CuAu alloy and several Cu-Au-Pd ternary alloys in the CuAu-Cu0.6Pd0.4 pseudobinary system was studied. Age-hardening and a decrease in electrical resistivity were observed in the CuAu alloy and Cu-Au-Pd ternary alloys containing palladium up to 11 at. %. These phenomena were caused by the progress of long-range ordering in the CuAu l type superstructure with the aid of quenched-in excess vacancies. The rate of low temperature ageing decreased with increasing palladium concentration. This was presumed to be due to the increase in migration energy of a quenched-in vacancy with the palladium concentration.

Age hardening in a dental white gold alloy

Abstract Age hardening in a dental white gold alloy was investigated by means of hardness tests, X-ray diffraction, and scanning and transmission electron microscopy observations. Hardening at 300 °C is due to both the precipitation of a metastable face-centred tetragonal (f.c.t.) ordered phase AuCuue5f8I in the grain interior and the formation of fine nodules consisting of an f.c.t. ordered phase AuCuue5f8I, an f.c.c. phase α 1 and a b.c.c. ordered phase CuPd at the grain boundaries. The alloy aged at 500 °C was hardened by the formation of AuCuue5f8I, which was coherent with the matrix, and was softened by the nodular reaction at the grain boundary.

Shape restoration effect associated with order–disorder transformation in equiatomic AuCu and AuCu–Ga alloys

Abstract Shape restoration effects associated with order–disorder transformation in equiatomic AuCu and Au 0.47 Cu 0.47 Ga 0.06 alloys were examined by means of surface roughness measurements of sheet specimens and the change in the distance between tips of U-shaped specimens. The specimen surface became irregular and a macroscopic shape change occurred after an ageing heat treatment. The shape change was attributed to the heterogeneity in the deformation which arose from the stress release by twinning at the free surface. After disordering of the age-hardened specimen, they regained completely to its original shape. When the specimen was aged without any constraint, spontaneous deformation occurred and did not result in any given shape. However, when the external stress was applied during ordering heat treatment, a given shape was obtained by the formation of favorably oriented ordered domains and twin lamellae. It is suggested that once the direction of deformation was decided, growth of the existing variants continued.

Effects of small silver addition to AuCu on the AuCu I ordering process and age-hardening behaviours

Abstract The effects of small silver additions to an equiatomic AuCu on the AuCu I ordering process and age-hardening behaviours at 300 °C were investigated by hardness testing, X-ray powder diffraction, optical microscopy, and transmission electron microscopy. The twinned structure developed at 300 °C in all of the experimental alloys containing silver of up to 6.0 at.%. However, the widening rate of the inter-twin spacing and the growth rate of the antiphase domain size markedly slowed down with the addition of silver to AuCu. The softening rate in the overageing stage also markedly slowed down with increasing the silver content. The strong tendency of retaining a high hardness value in the silver-containing alloys was attributed to the high densities of both twin boundary and antiphase boundary in these alloys. The small silver addition to an equiatomic AuCu tends to stabilize the mechanical properties of the aged alloy. This effect is beneficial in designing and producing a high-duty dental casting gold alloy.

Effects of Au/Cu ratio and gallium content on the low-temperature age-hardening in Au-Cu-Ga alloys

The effects of Au/Cu ratio and gallium content on the low-temperature age-hardening were examined in Au-Cu binary and Au-Cu-Ga ternary alloys by hardness testing, X-ray diffraction and differential thermal analysis. In alloys in which the Au/Cu ratios greatly deviated from the equiatomic one, age-hardenability at 37 °C significantly decreased. Gallium addition lowered the liquidus temperature and increased the age-hardening rate. It is considered that the diffusion rate of the constituents increased with lowering of the liquidus temperature, and that consequently, gallium addition improved the age-hardenability.

Phase transformation and age-hardening of Au-Cu-Pd ternary alloys

Phase transformations in Au-Cu-Pd single-phase alloys Were studied. Age-hardening of the alloys examined vvas attributed to the formation of fine domainu of long-range ordered (LRO) AuCuI type lattice in the interior of the grain. Prolonged ageing caused formation of large LRO domains of single variant at the grain boundary or microtwinning of LRo in the interior of the grain, depending on the ordering rate of the alloy. The electron-atom ratio (e/a) of the alloy and the axial ratio (c/a) of the ordered lattice seemed to playa part in the phase transformation behaviour in this alloy system. The effectiveness of rhodium addition on the grain refining was proved experimentally. These resuRs will be helpful in developing low nobility and high corrosion resistant dental alloys.