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

Sang-Hwa Lee,Yong Hoon Kwon,In-Sook Lim,Hyung-Il Kim,Ae-Ri Pyo,Mi-Hyang Cho,Hyo-Joung Seol

doi:10.1007/s13404-011-0021-x

Sang-Hwa Lee, Yong Hoon Kwon + Show 5 more

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https://doi.org/10.1007/s13404-011-0021-x

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Abstract

The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au–25Ag–11.8Cu–5Pd–1.7Zn–0.4Pt–0.1Ir (wt.%) were elucidated by characterizing the age-hardening behaviour, phase transformations, changes in microstructure and changes in element distribution. The fast and apparent increase in hardness at the initial stage of the aging process at 400°C was caused by the nucleation and growth of the metastable Ag–Au-rich phase and the Cu–Au-rich phase by the miscibility limit of Ag and Cu. The transformation of the metastable Ag–Au-rich phase into the stable Ag–Au-rich phase progressed concurrently with the ordering of the Cu–Au-rich phase into the AuCu I phase through the metastable state, which resulted in the subsequent increase in hardness. The further increase in hardness was restrained before complete decomposition of the parent α0 phase due to the initiation of the lamellar-forming grain boundary reaction. The progress of the lamellar-forming grain boundary reaction was not directly connected with the phase transformation of the metastable phases into the final product phases. The heterogeneous expansion of the lamellar structure from the grain boundary caused greater softening than the subsequent further coarsening of the lamellar structure. The lamellar structure was composed of the Ag–Au-rich layer which was Cu-, Pd- and Zn-depleted and the AuCu I layer containing Pd and Zn.

Highlights

Age-hardenability in age-hardenable dental casting gold alloys is usually obtained in Au–Cu and Ag–Cu systems
We investigated a dental casting gold alloy composed of 56Au–25Ag–11.8Cu–5Pd–1.7Zn– 0.4Pt–0.1Ir
The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au– 25Ag–11.8Cu–5Pd–1.7Zn–0.4Pt–0.1Ir were elucidated via hardness test, X-ray diffraction (XRD) study, field emission-scanning electron microscopic (FE-SEM) observation and energy dispersive spectrometer (EDS) analysis

Summary

Introduction

Age-hardenability in age-hardenable dental casting gold alloys is usually obtained in Au–Cu and Ag–Cu systems. In the Au–Cu system, the alloy is hardened by forming the AuCu I ordered structure, where alternate (002) planes are occupied by either all gold or all copper atoms [1]. When the ordered domain grows larger after prolonged aging, the special softening mechanism, that is, twinning occurs to release the lattice distortion [1]. The softening mechanism in the Ag–Cu alloy after prolonged aging is microstructural coarsening to release the lattice distortion in the inter-phase boundaries [2]. In most dental casting gold alloys, both Au–Cu and Ag–Cu systems work due to their complex compositions. The separation into the Ag–Au-rich and Cu– Au-rich phases occurs prior to the ordering into the AuCu I superlattice by the phase separation mechanism such as a spinodal decomposition and a nucleation and growth

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