Mechanical property and microstructural variations for recast low-gold alloy

Using polystyrene plastic patterns meeting the dimensional requirements of ANSI/ADA specification no. 5, tension test specimens were recast multiple times from a popular Type III gold (46%) alloy. The alloy was melted by electrical heating in a graphite crucible and four conditions were compared: new alloy and alloy cast two, three, and four times (n = 4). After casting, age-hardened specimens were loaded at an elongation rate of 0.5 mm/min until failure, and the stress-strain plots were recorded. Values of yield strength, tensile strength, and percentage elongation for the specimen groups were analyzed by one-way analysis of variance, followed by the Student-Newman-Keuls multiple range test. Significant decreases (P < .01) in yield strength and percentage elongation occurred with recasting, although there were no significant differences (P > .05) in tensile strength. Scanning electron microscope examination revealed that the number of casting defects increased with remelting, and that their presence dominates the tensile fracture process. The variation in mechanical properties of the alloy with remelting was attributed to these casting defects.