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

摘要

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 percent ) 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 percent 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 percent, the microstructures are respectively composed of primary Ir solid solution fcc + peritectic Ir_3Si silicide (nominal Si content: 0-25 mol percent), primary Ir_3Si + eutectoid silicide (Si: 25-33.3 mol percent ), Ir_3Si_2 + eutectoid silicide (Si: 33.3-40 mol percent ) and primary IrSi + Ir_3Si_2 silicide (Si: 40-50 mol percent ). 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 deg 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 (Ir_3Si, Ir_2Si and Ir_3Si_2) are close to 1400 deg C; instead, solid solution hardening on Ir by Si is recommended.