Effect of Pd or Au addition on age-hardening in AgMn-based alloys

Objective: The objective of this research was to characterize the age-hardening behavior of AgMn alloys modified with Au or Pd. These alloys are being studied as possible alternatives to Type III dental alloys. Methods: The age-hardening reactions in Ag-37 at% Mn alloys with 5 at% Au or Pd were investigated by optical microscopy, electrical resistivity, X-ray diffraction and hardness tests. Results: Optical microscopy showed no grain boundary precipitates formed after overaging. The maximum hardness reached by aging in all alloys is about 260 (Hv) and identical to that of traditional AgPdCu dental alloys. The precipitation reactions were retarded with the addition of 5 at% Au or Pd. X-ray diffraction studies suggest a fine precipitate, α-Mn, is dispersed within the grain interior. The activation energy for the precipitation reaction is 114 kJ/mol for the AgMn and AgMnAu alloys. This value is about one-half of the activation energy for volume diffusion of manganese in AgMn alloys. Significance: The age-hardenable AgMn-based alloys modified with Au or Pd can achieve hardness values comparable to those of traditional AgPdCu alloys. Provided the in vivo corrosion resistance of these alloys is adequate, they show promise as a Cu-free alternative material for cast restorations.