Age-hardening behaviour and microstructure of a silver alloy with high Cu content for dental application.

Age-hardening behaviour and the related microstructural changes of a silver alloy with relatively high Cu content were elucidated by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The microstructure of the solution-treated specimen was composed of the Ag-rich matrix, the Cu-rich particle-like structures containing Pd, and the lamellar structure of both phases. By the age-hardening heat-treatment, the Cu element began to precipitate from the Ag-rich matrix by the solubility limit, and the very fine Cu-rich precipitates became coarsened by further aging. The silver alloy with relatively high Cu content showed apparent age-hardenability. The hardness of the solution-treated specimen began to increase and reached a maximum value with increasing aging time, and then the hardness decreased gradually after maintaining the maximum value for short periods of time. The early stage of precipitation of the Cu-rich phase from the Ag-rich matrix seemed to have caused the increase in hardness. The decrease in hardness was attributed to the coarsening of the Cu-rich precipitates in the later stage of the age-hardening process.