Change in creep rate and microstructure in an aged, low‐copper amalgam

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Abstract
Low‐copper amalgams aged at 37°C, 50°C, 60°C, 70°C, and 80°C for periods of 7 and 30 days were examined using a scanning electron microscope and an x‐ray diffractometer. Gamma‐one Ag‐Hg grain size and γ2 Sn‐Hg volume fraction, and surface area were determined by quantitative metallography. At 60°C and below, γ1 was the predominant matrix phase. Little β1 Ag‐Hg was found. In this temperature range, both γ1 grain size and ADA creep were found to obey an Arrhenius law. A linear relationship was found between the log of the mean ADA creep and the mean γ1 grain size. Above 60°C, the γ1 phases and the majority of the γ Ag‐Sn particles disappeared. In their place, β1 appeared. Gamma‐two volume fraction increased during aging at temperatures over 60°C, but a coarsening of γ2, reflected in a decrease in γ2 surface area per unit volume, also took place. Aging for 30 days or less resulted in decreases in ADA creep. At 60°C and below, the decrease was caused by increases in γ1 grain size. Above 60°C, the decrease in ADA creep was caused by several factors, one of which was the appearance of large β1 grains.

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