The effect of the cooling rate of dental alloys on their amalgamation properties.

Silver-tin alloys of various compositions (60-80 atomic % Ag) have been prepared from the melt using cooling rates from about 50 degrees C/hr to about 10(6) degrees C/sec. Structural differences due to the different cooling rates were shown by metallographic and x-ray diffraction techniques. From each composition and cooling rate, powders were then prepared and amalgamated. The amalgams were tested for Hg content, dimensional change during setting, and tensile strength. Changes in cooling rates had some effect on the relative amounts of phases present but primarily affected the grain sizes. In general, the faster the cooling rate, the smaller the grain size; the smaller the grain size, the faster and more uniform the amalgamation; the more uniform the amalgamation, the stronger the amalgam. Storage of the alloy powders at room temperatures slowed their amalgamation rates, destroyed their uniformity,and resulted in weaker amalgams. However, no phase changes were observed by x-ray diffraction in the freshly comminuted alloys after aging at room temperature for two months.