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Microstructural evaluation of CMSX-4 superalloy single crystal castings of various geometry

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The turbine blades and combustion chamber belong to critical parts of aircraft jet engines. They work under extreme conditions: very aggressive corrosion environment, high mechanical and thermal stresses. Therefore the creep resistance is very important criterion for selection of critical parts materials and methods of their production [1]. Advances in turbine blade manufacturing were mainly caused by implementation of directional crystallization in casting process. This method enabled obtaining columnar grains in castings, elongated in the main axis direction. This kind of microstructure with no transverse grain boundaries provides higher creep resistance of castings. It was found that much better properties characterize single crystal castings. Casting process of single crystal blades is analogous to casting process of blades having columnar grains. The difference is in the beginning of crystallization process - only one grain properly oriented is selected for further growth [2÷5]. The main parameters controlling single crystal casting quality are temperature gradient and withdrawal rate of mould (vw). Too high cooling rate causes crystallization of grains in front of interface between liquid and solid phases. Whereas too slow mould removal from the heating zone leads to excessive microstructural segregation and defects formation in castings. It is considered that higher temperature gradient (in the accepted range) contributes to improvement of casting quality. The most often applied values of temperature gradient are in the range of 3÷6.5°C/mm and withdrawal rate of about 5 mm/min [6÷12]. Microstructure of most nickel superalloys is mainly composed of gamma (γ) and gamma prime (γ′) phases. The γ′ phase (Ni3(Al, Ti)) - primary strengthening phase - is coherent with the matrix - γ phase. The close match in matrix lattice parameter (~0÷1%) combined with the chemical compatibility allows the ^[...]

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