Dispersion of Nano-Sized Oxide Particles in Ni-based Alloy

High temperature materials with a good resistance to oxidation or corrosion are crucial for the Generation IV nuclear system such as VHTR (Very High Temperature Reactor). While several commercial Ni-based high temperature alloys have been among the candidate ones for the IHX (intermediate heat exchanger) or HGD (hot gas duct) of VHTR, they disclose their insufficiency for the application to the anticipated outlet temperature range such as 900 deg. C or above. Unless the operating temperature is significantly lowered and the performance of the system drops in consequence, a breakthrough in the candidate materials for the key components shall be necessary for the realization of the system. Feasibility for the oxide dispersion strengthening (ODS) of Ni-based alloys was studied by fabrication of alloy 617-base ODS alloy. Alloy 617 powders were mechanically alloyed (MA) with 0.3 wt% Y₂O₃ powders of around 30 nm in diameter using a horizontal ball mill (ZoZ CM 08). After degassing and canning procedure, MA powders were hot extruded. Extruded alloy samples were tensile tested at high temperatures as well as at room temperature. Also high temperature creep rupture tests were conducted at 950 deg. C in air under 20 and 25 MPa, respectively. The behavior of oxide particles within the Ni-based alloy matrix was investigated by mechanical alloying of Ni- Cr-Fe ternary alloy powders and pure Ni powders with oxide particles, respectively. Oxide such as Y₂O₃, Cr₂O₃, HfO₂, ZrO₂, TiO₂, La₂O₃ were chosen from the thermodynamic point of view as the first stable candidates from more than twenty oxides, nitrides and carbides. The behavior of oxide particle within the MA powder matrix along with the temperature was characterized by using high temperature XRD (X-ray Diffractometer) and (HR)TEM (High Resolution Transmission Electron Microscope). Creep rupture tests of the ODS alloy samples at 950 deg. C showed one order of magnitude lower creep rates than those of the corresponding alloy. (authors)