Deformation and fracture of iridium: microalloying effects

The effects of Ce and Th doping (20-50 wppm) on the mechanical properties of Ir alloys were investigated. At both low (approx = 10 exp -3 s exp -1 ) and high (approx = 10 exp -3 s exp -1 ) strain rates, the Ce + Th doped alloys undergo a transition from brittle intergranular (plus some transgranular) fracture at low temperature to ductile transgranular fracture at elevated temperature. The ductile-brittle transition temperature (DBTT) is approx = 400 K higher at the higher strain rate. Grain size and grain-boundary cohesion both affect the ductility of Ir alloys. Cerium and thorium, when added together, refine grain size more effectively than when Th is added by itself (especially at high temperatures). Their effect on grain-boundary cohesion is similar to that of Th by itself. At low strain rates (approx = 10 exp -3 s exp -1 ), the strength and ductility of alloys doped with Th were similar to those of alloys doped with both Th and Ce. The brittle-ductile transition appears to be related to a change from high work hardening rate at low temperature to low work hardening rate at elevated temperature. (Material: Ir-0.3W.)