Microstructural changes in grain interior and grain boundary by formation of metastable and stable phases related to age
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The hardness changes caused by formation of the metastable and stable phases were examined and correlated with the microstructural changes in grain interior and grain boundary during aging at 350 °C to clarify the age-hardening and softening mechanism of a low-gold Au-Cu-Ag-Pd dental alloy. Aging in this context refers to the time-delay that occurs wherein such alloys are kept at elevated temperatures for periods upto many hours to allow precipitation or ordering to take place. During the period of increasing hardness, the matrix was separated into the Ag-rich a1 and AuCu I phases through the metastable phases, forming block-like structure. The apparent hardening was attributed primarily to lattice strain due to the tetragonality of AuCu I9 [the primer (9) here indicates a metastable phase; likewise (I) and (I9) indicate stable AuCu I and metastable AuCu I9 phases, respectively] and AuCu I phases along the c-axis, secondarily to the coherency or semicoherency strain between the metastable a19 and AuCu I9 phases and between the a0 and AuCu I phases along the a-axis. The apparent softening was caused primarily by growth and coarsening of the lamellar structure in the grain boundaries, secondarily by coarsening of the block-like structure in the grain interior.
I. INTRODUCTION
Dental casting gold alloys for crown, bridge and removable partial denture, require high physical and mechanical properties to withstand the considerable occlusive forces. Such properties can be obtained by proper heat treatment accompanied by a phase transformation.1,2 The product phases formed by a phase transformation through heat treatment have different lattice parameters. The gap in the lattice parameter determines whether the interphase interfaces will be coherent, semicoherent or incoherent. Incoherency strain, which forms in an incoherent interface between the matrix and the precipitates, has a strong hardening effect, but also increases brittleness, lowering ductility.3 Recent studies have shown that the introduction of coherent phase boundaries has a substantial strengthening effect while maintaining acceptable levels of ductility.4 The formation of coherent or semicoherent interfaces becomes possible in dental casting gold alloys by forming metastable phases with a minor gap in the lattice parameter.5 Dental casting gold alloys contain various elements to achieve the required mechanical properties. Therefore, the alloy composition can become different from equiatomic gold (Au)-(copper) Cu, which can form the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.77 J. Mater. Res., Vol. 28, No. 9, May 14, 2013
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AuCu I ordered phase, the traditional age-hardening mechanism. Consequently, the age-hardening mechanism of dental casting gold alloys manifests in several forms such as ordering,6–9 precipitation,10,11 spinodal decomposition6,8,12 and a combination of spinodal decomposition and ordering.8 Therefore, in dental casting gold alloy
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