Grain-boundary segregation of impurities in iridium and effects on mechanical properties

Impurity effects were investigated in an Ir alloy of nominal composition Ir–0.3 W–0.006 Th–0.005 Al1. The impurities that were added included Fe, Ni, Cr, Al, and Si, at levels ranging from 50 to 5000 ppm. Of the elements investigated (impurities as well as alloying additions), only Si and Th were found to segregate to the grain boundaries. In alloys doped with low levels of Si (<200 ppm), both Si and Th segregated to the grant boundaries. In addition, Th-containing precipitates were identified in these alloys, but not Si-containing precipitates. When added at higher bulk levels, Si displaced (beneficial) Th from the boundaries. This caused the segregated Th at the boundaries to decrease (to practically zero) when the amount of grain-boundary Si increased (to its saturation level). Silicon also reacted with Th to form Ir–Th–Si precipitates and promoted the formation of a low-melting eutectic on the grain boundaries of the highest Si alloys. As for the other elements, not only did they not segregate to the grain boundaries, but they also did not affect Th segregation. Together, these results explain why Fe, Ni, Cr, Al (and Si at low levels) do not embrittle iridium, whereas Si at high levels causes severe embrittlement.