Phase and microstructural evolution of Ir-Si binary alloys with fcc/silicide structure

To search hardening approach or new probable phases benefiting to high temperature behavior of Ir-based superalloys, Ir alloyed with Si was employed. Investigations on phase and microstructural evolution of a series of Ir-xSi (x=2.5, 5, 15, 20, 30, 36 and 45 mol%) binary alloys were carried out by XRD, EPMA and SEM analysis. A schematic plot of the Ir-Si binary diagram with the nominal Si content ranging from 0 to 50 mol% was primarily drafted. Room temperature mechanical properties, the Vicker hardness and Young's modulus, of bulk material or each kind of phases were also measured. Researches reveal that with Si addition up to 50 mol%, the microstructures are respectively composed of primary Ir solid solution fcc+peritectic Ir₃Si silicide (nominal Si content: 0-25 mol%), primary Ir₃Si+eutectoid silicide (Si: 25-33.3 mol%), Ir₃Si₂+eutectoid silicide (Si: 33.3-40 mol%) and primary IrSi+Ir₃Si₂ silicide (Si: 40-50 mol%). With plastic characteristic, the fcc phase has the low Vickers hardness and Young's modulus, while both of the silicides are high and the silicides behave brittle. For the high temperature applications over 1400 °C, Ir-based alloys with Si dropping must avoid the appearance of any kind of Ir/Si silicides in microstructure because the melting points of silicides (Ir₃Si, Ir₂Si and Ir₃Si₂) are close to 1400 °C; instead, solid solution hardening on Ir by Si is recommended.