Age‐hardening associated with grain boundary precipitation in a commercial dental gold alloy

Abstract

The aim of this study was to make clear the age-hardening mechanism in a dental high carat gold alloy. For this purpose, age-hardening behaviour of a commercial dental high carat gold alloy, 65.5 wt% Au-14.0 wt% Ag-10.0 wt% Cu-8.9 wt% Pt, was investigated by means of hardness testing, X-ray diffraction study and scanning electron microscopy. Age-hardening was generated by the coherency strain resulting from the transformation of the alpha single phase to the Ag-rich alpha 1 phase and the AuCu I type ordered phase. The coherency strain seemed to be associated with the nucleation of the AuCu ordered structure initially, and then was brought about with the simultaneous formation of the Ag-rich alpha 1 phase and the AuCu I type ordered phase. Hardening was attributed mainly to the very fine coherent precipitates of a lamellar structure composed of the Ag-rich alpha 1 phase and the AuCu I type ordered phase at grain boundaries, and softening, which occurred following prolonged ageing, was due to the coarsening of the fine lamellar structure by releasing the strain at the interfaces of the adjacent lamellae.

Age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental Au–Ag–Cu–Pd–Zn alloy

“…This was the result of initiated lamellar-forming grain boundary reaction. The grain boundary lamellar structure has a much coarser nature compared to the grain interior, which means that the phase boundaries between the metastable Ag-Au-rich a 0 1 phase and the metastable AuCu I′ phase containing lattice strains were reduced significantly as the grain interior was replaced by the lamellar structure, resulting in the softening [17][18][19]. The accumulated lattice strains in the matrix seemed to be the driving force for the lamellar-forming grain boundary reaction.…”

Age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum

“…The microstructural coarsening reduced the interface between the Au-rich 1 matrix and the Pt-rich precipitates, which released the lattice strains between the two phases, resulting in a softening effect [7,11]. Even with a significant decrease in the 2 peak intensity during the period of hardness decrease, the 2 particle-like structure did not show apparent microstructural changes.…”

“…Several solid transformations are the probable mechanism by which the hardening process takes place. The AuCu I ordered phase with a face-centered tetragonal (fct) structure plays an important role in the hardening of conventional dental casting gold alloy which belong to type III or IV [2][3][4][5][6][7]. The ordered phases with a facecentered cubic (fcc) structure play an important role in the hardening of the gold alloy for ceramic-metal restorations [8][9][10][11].…”

Age-hardening by grain interior and grain boundary precipitation in an Au-Ag-Pt-Zn-In alloy for multipurpose dental use

“…It was thought that the early diffusion and then clustering of the In-concentrated phase in the grain interior together with the early diffusion and then ordering of the PtZn phase in the grain boundary introduced the internal strains in the matrix. But, as the microstructural coarsening, that is, mainly lamellar-forming grain boundary reaction progressed, the phase boundaries between the FCC matrix and the FCT PtZn precipitate reduced, and thus the internal strains released, resulting in softening [6,16,17].…”

“…This chemical characterization is the result of Ag and Cu having limited miscibility, while Au is completely soluble with both Ag and Cu at any atomic ratio [1]. Thus, the hardening mechanism is usually attributed by dual mechanisms of phase separation into Ag-rich and Cu-rich, and then ordering which forms the AuCu I ordered phase with a face-centered tetragonal (FCT) structure in the Cu-rich regions [2][3][4][5][6]. These types of dental gold alloys, the Au-Ag-Cu system cannot use as the basis of ceramic-metal restorations due to the relatively low melting temperature and a high thermal coefficient of expansion.…”