Katsuhiro Yasuda

Structure and properties of NbN and TaN films prepared by ion beam assisted deposition

Abstract Niobium nitride NbNx and tantalum nitride TaNx films were deposited on 316L austenitic stainless steel and silicon wafer substrates by evaporation of niobium and tantalum under simultaneous nitrogen ion irradiation, applying an acceleration voltage between 2 and 20 kV. In order to study the influence of the nitrogen content, the atom-to-ion transport ratio Nb, Ta/N was varied. The compositional and structural characterization of the films were carried out using Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. It was observed that the surface morphology strongly depends on the deposition conditions. X-ray diffraction patterns showed that the crystal structure of the films was changed from nitride to solid solution when the transport ratio was increased from 1 to 10, respectively. The crystal structure of the films deposited at Nb, Ta N = 6 and 10 had the feature of a nitrogen supersaturated bcc solid solution. Electron diffraction revealed the coexistence of an amorphous phase and a fine structured nitride phase in these films. The chemical bonding state of metal atoms observed by XPS changed with increasing the transport ratio, whereas the bonding state of nitrogen atoms was independent of the transport ratio. The corrosion behavior was evaluated in an oxygen saturated 5% sulfuric acid solution, using multisweep cyclic voltammetry measurements. The best corrosion protection was observed for the NbN film at an transport ratio of Nb N = 2 and for the TaN solid solution at Ta N = 6 .

Difference in Age-hardening Mechanism in Dental Gold Alloys

TEM and SAED studies were conducted to clarify a difference in age-hardening mechanism with changes of gold content in Au-Cu-Ag ternary alloys, 18K, 16K, and 14K gold alloys, for which the ratio of copper to silver remained at 65 to 35 in weight. Age-hardening in the 18K alloy was attributed to the formation of the AuCu I type ordered platelets on the matrix {100}. In the 16K and 14K alloys, a periodic antiphase domain structure of the AuCu II type ordered phase made a major contribution to age-hardening. An alternating coarse lamellar structure was formed by the mechanism of discontinuous precipitation in the 16K alloy, while a modulated structure by spinodal decomposition was observed in the 14K alloy.

Age-hardening characteristics of a commercial dental gold alloy☆

Abstract The age-hardening characteristics of a commercial dental gold alloy Au-11wt.%Pt-6wt.%Pd-6wt.%Ag-9wt.%Cu were studied by means of resistometric measurements, hardness tests, X-ray diffraction and electron microscopy. The resistometric and hardness studies showed that age hardening occurred in two stages. The first stage arose from the nucleation of the AuCu I type ordered structure and was characterized by a slow growth rate of the ordered platelets. In the second stage, simultaneous ordering and precipitation took place. An alternating lamellar structure composed of the AuCu I type ordered platelets and platinum-rich f.c.c. precipitates was observed in electron micrographs. The age hardening in this stage was considered to arise primarily from ordering and to be brought about with the aid of precipitation. The grain boundary precipitates did not contribute to the age hardening in the alloy we studied.

Spinodal ordering in the equiatomic AuCu alloy

Abstract To identify spinodal ordering in AuCu alloys, electron microscopy and electron and X-ray diffraction examinations were employed on the equiatomic composition of AuCu alloy using direct ageing. The spinodal ordering temperature T′s and the coherent phase boundary T s were determined to be 630 and 648 K respectively. Three distinguishable phase transition mechanisms were found. (1) The ordering proceeds at grain boundaries by a heterogeneous nucleation mechanism after showing an incubation period at a temperature between the transition temperatures T c and T′c. (2) The ordering is induced in the grain interior by a homogeneous nucleation mechanism between T′c and T c. (3) The spinodal ordering is brought about below T s. The microstructure induced by the spinodal ordering is characterized by the tweed structure. The direct-ageing method is effective in distinguishing the ordering processes with or without an incubation period, and in eliminating confusion in microstructures by the unavoidable order...

A coherent phase diagram of the Aux(Ag0.24Cu0.76)1-x section of the AuCuAg ternary system

Abstract The coherent phase diagram of Au x (Ag 0.24 Cu 0.76 ) 1− x was studied by electron microscopy. Various regions were found in the present experimental coherent phase diagram consistent with the theoretical coherent phase diagram obtained by Yamauchi using the cluster variation method: (i) a single-phase region comprising a disordered solid solution of f.c.c. α 0 phase, (ii) a two-phase decomposition region containing copper-rich f.c.c. α 1 and silver-rich f.c.c. α 2 phases, (iii) coexisting regions of a disordered phase and an ordered phase, α 2 + Cu 3 Au ( L 1 2 ), α 2 + AuCuII ( L 1 0- s ) and α 2 + AuCuI ( L 1 0 ), (iv) coexisting regions of two disordered phases and an ordered phase, α 1 + α 2 + Cu 3 Au and α 1 + α 2 + AuCuII , and (v) coexisting regions of a disordered phase and two ordered phases, α 2 + Cu 3 Au + AuCuII and α 2 + AuCuI + AuCuII . Microstructural features of these regions were described in association with phase identifications.

Structural aspects of AuCu I or AuCu II and a cuboidal block configuration of f.c.c. disordered phase in AuCuPt and AuCuAg pseudobinary alloys

Abstract A crystallographic analysis of the relative orientation of the microstructure of the two-phase (AuCu I or AuCu II) alloy coexisting with cuboidal block configurations of disordered f.c.c. phase was performed by means of transmission electron microscopy (TEM) and high-resolution electron microscopy complemented with selected area electron diffraction (SAED) examinations in (AuCu)1−X−PtX (X = 0.0135, 0.0248, 0.0469 and 0.0692 in atomic ratio), AuCu9 at.% Ag and AuCu14 at.% Ag alloys. In (AuCu)1−X−PtX alloys, a characteristics checkerboard-like microstructure which was composed of cuboidal blocks of disordered f.c.c. enclosed by two parallelogram-shaped orientation variants of AuCu IX and AuCu IY ordered regions without twin relation was observed, although the SAED pattern showed twinning. Evidence for a twin relation was found in small ordered regions observed in the cuboidal blocks unavoidably during quenching from an ageing temperature to ice brine. The cuboidal block was enclosed by four parallelogram-shaped AuCu I regions. The orientation relationship was deduced as (001)AuCu I//{100}α2 and the interface was defined as the {102}AuCu I. The microstructure consisted of the cuboidal block of a silver-rich f.c.c. α2 phase surrounded by three different orientation variants of the AuCu II ordered regions in which two of three variants had twin relation in AuCu9 at.% Ag and AuCu14 at.% Ag alloys. The orientation relationship was deduced as (001)AuCu II//{100}α2 and the interface was defined as the (001)AuCu II. The formation sequence of the checkerboard-like microstructure in (AuCu)1−X−PtX alloys was also analyzed using dark-field images in conventional TEM and SAED observations.

Age-hardening and related phase transformations in dental gold alloys

Gold alloys that can be hardened by thermal ageing play an important role in dentistry. But the ordering and precipitation phenomena responsible for hardening are complex and not fully understood. In this comprehensive survey of the field, the author shows that transmission electron microscopy coupled with electron diffraction measurements is a powerful tool for studying age-hardening mechanisms in dental gold alloys.

Ageing reactions in a high carat gold alloy for dental porcelain bonding

The phase transformation during continuous heating of a high carat gold alloy used for porcelain bonding was investigated by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. Four reaction stages (I, II, III and IV) were found. Stage I corresponded to the formation of a short-range order. A discontinuous precipitation took place in stage II, which contributed to remarkable hardening. Stages III and IV were reactions to the stable phases at each temperature region, and resulted in softening. The activation energies for stages I, II and III are 27.1, 33.8 and 58.2 kcal/mol, respectively.

Three stages of ordering in CuAu

Abstract The ordering behavior during continuous heating after quenching of an equiatomic CuAu alloy was investigated by means of electrical resistivity measurements. The ordering proceeds in three time steps, (stages I, II and III). Stages I and III correspond to ordering due to the migration of quenched-in excess divacancies and single vacancies, respectively. The ordering in stage I can take place at room temperature. Stage II may result in the migration of more mobile defects. The completion of the ordering in the alloy CuAu is established without the need for the migration of equilibrium vacancies.

Age-hardening and the associated phase transformation in an Au-55.2 at% Cu-17.4 at% Ag ternary alloy

To develop a low gold content dental alloy, age-hardening characteristics in an Au-55.2 at% Cu-17.4 at% Ag alloy were studied by means of hardness, electron microscopy and X-ray diffraction examination. Three distinct age-hardening behaviours depending on temperature were found in the alloy, i.e. (i) a dual mechanism of spinodal decomposition and Cu3Au ordering below 673 K, (ii) a single mechanism of spinodal decomposition at 693 K, and (iii) a single mechanism of nucleation and growth of silver-rich precipitate at 773 K. A marked over-ageing was observed by lengthy ageing over the whole range of temperature. The long-period superstructure of Cu3Au(II) was found only in the grain boundary product at temperatures between 623 and 633 K.