Materials Science The Relationship Between Hardness and Structure of Pd-Cu-Ga Alloys

Abstract

Most Pd-Cu-Ga alloys on the market have a yield strength of the order of 1000 MPa, which is too high for an adequate adjustment of the margins of a crown and may promote wear of an opposing natural tooth. In the present work, the structure of a commercial Pd - 10 wt% Cu - 9 wt% Ga alloy has been shown by means of TEM to consist of thin (5-25 nm) laths with two ordered fct phases with the composition Pd ₃Ga_xCu_1-x. The c/a ratios in these fct phases are, alternatingly, >1 and <1. The transformation of an fcc structure by short atomic movements into the fct structures occurs rapidly below 500°C, as judged by thermal expansion measurements. Such fct structures are likely to provide substantial resistance to dislocation movement and thus a high yield stress and hardness. By adding 6 wt% Ag to the Pd-Cu-Ga alloy and reducing the Ga and Cu content correspondingly, a predominantly fcc structure occurred with nearly no fct laths. At the same time, the hardness was reduced to approximately half the value for the initial Pd-Cu-Ga alloy.