Al-Ag-Cu ötektik sisteminin mikroyapısal karakterizasyonu

Characterization of microstructure is vital to be able to control properties of materials. Solidification is one of the major process for the formation of microstructure. For eutectic solidification microstructures, while extensive studies were performed on binary systems, microstructure evolution in multiphase systems are still needed to be understood. Al-Cu-Ag system is a good candidate to study microstructure evolution in multiphase materials due to both being model alloy and offering a variety in microstructural features. Moreover, well-established phase diagram and thermo-physical properties make it easier to understand the dynamics of solidification. However, existence of three phases in Al-Cu-Ag system, namely (Al), Al2Cu, Ag2Al, orientation relationships, and anisotropic crystal/crystal interphases lead to very complex microstructures. In this work, Al-Ag-Cu samples at the nonvariant composition are directionally solidified by highly controlled Bridgman type setup. Experiments are performed in the velocity range of 0.1 µm/s to 4.0 µm/s and at a constant thermal gradient of 15 K/mm. The thermal gradient and the purity of the alloy are sufficient to form planar S/L interface which is extremely important for the eutectic microstructure studies. The structures formed at various growth velocities are quantitatively characterized in terms of morphology and microstructure using newly-established method which involves Feret technique. In addition, phase fraction and area are measured as a function of growth length. The mostly observed pattern for the Al-Ag-Cu eutectic system is determined to be chain-like structure. Microstructural changes over the growth length is investigated and formation of paw structure and faceted structure are examined. λ2V fitting on the eutectic spacing data taken from microstructures grown within the velocity range shows that the microstructure follows the Jackson & Hunt scaling law. Additionally, different size samples are used to determine the effect of convection on the microstructure. As a result, three phase Al-Ag-Cu eutectic system is characterized in terms of solidification velocity, growth length and sample size.