| Co-Investigator(Kenkyū-buntansha) |
GUSTAAF VAN アントワープ大学, 超高圧電子顕微鏡研究室, 教授
JOSEPH VAN L アントワープ大学, 超高圧電子顕微鏡研究室, 教授
SEVERIN AMEL アントワープ大学, 超高圧電子顕微鏡研究室, 教授
NAKAGAWA Masaharu Assistant Lecturer, Department of Dental Materials Engineering, Faculty of Denti, 歯学部, 助手 (80172279)
UDOH Koh-ichi Assistant Lecturer, Department of Dental Materials Science, Nagasaki University, 歯学部, 助手 (60145266)
AMELINCKX Severin Professor, Laboratory of Electron Microscopy for Materials Research, University
VAN LANDUYT Joseph Professor, Laboratory of Electron Microscopy for Materials Research, University
VAN TENDELOO Gustaaf Professor, Laboratory of Electron Microscopy for Materials Research, University
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| Research Abstract |
The eventual purpose of the present research project was the tailoring of new type dental alloys based on designing principle which was constructed with elucidating the correlationship between mechanical, electrochemical, biological factors and microstructural features of the alloy crystal, because these factors were governed greatly by solid interface such as grain boundaries, stacking faults, twin boundaries, and antiphase domain boundaries. The followings were concrete descriptions of the research project. (1) Construction of the coherent phase diagram of AuCu-Ag and (Ag_<0.5> Cu_<0.5>)_x-Au_<1-x> sections combining a theoretical results and available experimental data. (2) High-resolution electron microscopic observations of the long period antiphase domain boundary structure in AuCu-Ag pseudobinary alloys. (3)Structural features of twin boundary in AuCu-3at. %Ag alloy. (4) High-resolution electron microscopic study of age-hardening mechanisms and the associated phade transformatio
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ns in commercial dental gold alloys. Works done were as follows: (1) Three distinguishable mode of phase transformations were found in the AuCu-Ag pseudobinary system, i.e., (i) the AuCu ordering was brought about solely from disordered solid solution of FCC in structure ( _0 phase) at a composition region of outside of the miscibility gap. It was found that a temperature region of coexistence of the _0 disordered and AuCu II ordered phases, a single phase region of AuCu II ordered phase, a region of coexistence of two ordered phases AuCu i and AuCu II, and a single phase region of AuCu I were formed in aucu-3at. %Ag alloy under the critical temperature for ordering at 653K, (ii) dual mechanism of ordering and two-phase decomposition was induced by ageing in a composition region between the miscibility gap and a spinodal lucus. The phase transformations were governed by a nucleation and growth process in this composition range, i.e., (a) a two-phase decomposition region containing copper-rich FCC _1 and silver-rich FCC _2 phases, (b) a coexisting region of two disordered phases and an ordered phase, _1+ _2+AuCu II, (c) a coexisting region of an ordered phase and a disordered phase, AuCu II + _2, (d) coexisting region of two ordered phases and a disordered phase, AuCu I + AuCu II + _2, and (e) a coexisting region of an ordered phase and a disordered phase, AuCu I + _2, and (iii) dual mechanism of ordering and the spinodal decomposition was generated by ageing in a composition region of inside of the spinodal lucus. A rate of the process of the phase transformation was higher in this composition range than in a composition range located between the miscibility gap and the spinodal lucus. Phases coexisting were essentially analogous to this range except for differences in microstructures.
Although the differences in microstructures induced by ageing were the most inportant and great interest, the temperatures and composition dependence of the phase changes have not been determined exactly yet. (2) High-resoultion electron microscopic observations were performed to clarify a configuration of the long period antiphase domain boundary, structural characteristics of interfaces between different phases, and wtin boundary in AuCu-3at.%Ag, AuCu-6at.%Ag, AuC1-9at.% and AuCu-14at.%Ag alloys which were aged at various temperatures and periods. The microstructure induced by ageing was quite different from each other depending on silver content in the alloys, i.e., the AuCu ordering was brought about solely from disordered solid soulution in a composition range of outside of the miscibilith gap, the phase transformation was governed by a nucleation and growth mechanism in a composition range between the miscibility gap and a spinodal lucus and the AuCu ordering was concurrent with the spinodal decomposition in a composition range of inside of the spinodal lucus. The characteristic mosaic structure which cosisted of the AuCu II ordered phase and a silver-rich disordered FCC _2 phase was found in high-resolution electron micrographs obtained from the AuCu-9at.%Ag alloy aged at 614K for 100ks. (3) During the course of a systematic study to determine the coherent phase diagrams of Au-Cu-Ag ternary system, it was necessary to identify a disordered FCC phase coexisting with the AuCu II long period superstructure, as was expected from the phase rule. High-resolution electronmicroscopy was employed to distinct the disordered FCC phase and the AuCu II phase in th AuCu-3at.%Ag alloy aged at 643K for 100ks. It was found that a triangle shaped area was formed in the vicinity of the tip of the twin platelets. The triangle shaped area showed a considerable amount of ealstic strain and was thought to be the disordered FCC phase from geometric features of atomic configurations. (4) Age-hardening characteristics of a commercial dental gold alloy, Au-5wt.%Pt-3wt.%Pd-6wt.%Ag-15wt.%Cu-3wt.%Zn, were studied by means of resistometric measurements, hardness tests, X-ray diffraction and electron microscopy. At lower ageing temperatures the hardening was due to the nucleation of the AuCu I type ordered structure and was characterized by a slow growth rate of the ordered platelets. An alternating coarse AuCu I ordered platelets abutted on each other was formed by ageing at higher temperatures. No strain field was observed at the interfaces between the AuCu I ordered platelets and the surrounding matrix in this temperature range. Less
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