Hardening of gold-based alloys.

The relationships between composition, heat treatment, hardness, and microstructure have been investigated for a number of commercial dental casting alloys and for several series of experimental gold-silver-copper compositions. The hardnesses and typical microstructures found are reported. The results indicate that at least two distinct hardening mechanisms must occur in these alloys. The ternary alloys outside the two-phasr region of the gold-silver-copper system harden by ordering. The commercial alloys and those within the two-phase region may harden in part by ordering, but some other mechanism in required to explain the observed hardening. This mechanism is not due to the observed grain boundary precipitates. It may be associated with the intragranular needlelike structures seen, but reasons exist to question the sufficiency of that structure as an explantation of the hardening.