Materials and Components Development for Advanced Turbine Systems - Ods Alloy Development

Oxide dispersion strengthened (ODS) alloys have been under intensive development over the past four decades due to their excellent high-temperature creep strength and good oxidation resistance. Nickel-based ODS alloys are the combination of gamma prime (γ’) strengthened superalloy with yttrium oxide evenly distributed throughout the matrix to allow the strength to be maintained to much higher temperatures than conventional superalloys. ODS alloys are being considered for high temperature applications in advanced power plant such as heat exchanger tube, boiler, superheating tube, vane or turbine blade. However, a key factor that limits their wide usage is the high manufacturing cost. In this research, a powder mixing technique, Hosokawa mechano chemical bonding (MCB) technology, is used to produce ODS powder mixtures with the aims of obtaining comparable or enhanced ODS powders with much less manufacturing cost. One of the key ideas of the proposed Hosokawa method is the use of nano-sized oxide particles, which, through the processing, will grow on the surfaces of larger micron-sized master particles and make homogeneous dispersions. In this paper, preliminary results are shown and discussed. Four batches of alloying powder samples, based on composition of Ni-20Cr-5A1-1.5 Y203 and Ni-20Cr-5A1-3W-1.5 Y203 (wt%) and different sizes of master alloy powders (Ni and Cr), have been prepared by the MCB process. The starting powders and MCB-processed powders were subjected to microscopic and spectroscopic characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to examine the microstructure, morphology and mixing homogenization of alloying powders processed by MCB. The results show that during the MCB processes, the particles in alloying powders were subjected to high speed colliding, cold weld and plastic deformation, forming the composite particles hosted by Ni or Cr master elemental powders. A thin film containing the alloyed elements such as Al, Y, O and W was observed to distribute throughout the composite particles. Lowering the size of master powders would improve the homogenization of alloying powder mixture, building up a smooth thin film coated around the composite particles. In addition, the MCB process in this study could manifest the crystalline nano-sized Y2O3 particles as the amorphous film bonded with the host composite particles, resulting in the disappearance of Y2O3 peak in the XRD spectrum. When fabricated with additional high energy ball milling, the resulting alloys are expected to have enhanced mechanical properties for use in advanced turbine applications. ODS alloys are used not only in the form of bulk components but also in the form of coatings. In this research, we attempt to combine the Hosokawa MCB-processed ODS powders with high pressure cold spray coating technique for ODS coatings. This new ODS coating development will be able to deposit high-quality ODS coatings on various solid substrates. In this paper, using the MCB-processed ODS powders and cold spray method, preliminary results of ODS coatings on Inconel 625 are demonstrated, which shows thick ODS coating is possible with high deposition efficiency. Very good interface (coating/substrate) adherence is also observed.