Effects of alloying elements on dendritic segregation in iridium alloys

The effects of alloying elements on dendritic segregation in Ir alloys were studied by Auger Electron Spectroscopy (AES). The addition of 10 at.% Nb induces significant segregation of carbon and thorium to dendritic interfaces. The addition of 5 at.% Zr to the Ir alloy leads to the formation of an Ir3Zr intermetallic phase, which results in less dendritic segregation of carbon and thorium. This dendritic segregation appears to be linked to the severe cracking observed in the Ir–Nb alloy, but not in the Ir–Zr alloy, after casting and heat treatment. The mechanism for the dendritic segregation was discussed by consideration of the solidification process and the change of carbon solubility in Ir by Zr addition.