TEM analysis of tweed structure in high-palladium dental alloys.

Transmission electron microscopy and convergent-beam electron diffraction were used to study three high-palladium dental alloys: an as-cast Pd-Ga alloy, a cast Pd-Cu-Ga alloy aged at room temperature for over 5 years, and a cast Pd-Cu-Ga alloy annealed at 1023 K for 2 h and slowly cooled to room temperature. Bands containing a tweed structure in the Pd-Ga alloy were 120degrees twins with (1 1 0 mirror planes. Within these bands the alloy had a face-centered tetragonal structure with a c/a ratio of 1.03. The aged Pd-Cu-Ga alloy contained a larger amount of tweed structure than the original as-cast alloy, along with randomly oriented bands and thin lath-like regions. The annealed Pd-Cu-Ga alloy contained rectangular particles of a secondary phase that may be Pd2Ga, Pd5Ga2 or Pd13Ga5. The microtwinned band structure relieves the strain energy arising from transformation of the f.c.c. palladium solid solution on cooling. Copyright 1999 Kluwer Academic Publishers