Mechanical Engineering MS Thesis Defense by Mehmet Emre Çetinkaya

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

MS THESIS DEFENSE BY MEHMET EMRE ÇETİNKAYA

Title: Microstructural Characterization of Al-Ag-Cu Eutectic System

Speaker: Mehmet Emre Çetinkaya

Time: September 10th, 2018, 2:00 PM

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Research Asst. Prof. Melis Şerefoğlu Kaya (Advisor, Koç University)

Assoc. Prof. Murat Sözer (Koç University)

Assoc. Prof. Derya Dışpınar (Istanbul University)

Abstract:

Characterization of microstructure is vital to be able to control properties of materials. Solidification is the main manufacturing technique 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 an industrially important class of alloy and offering a variety in microstructural features. Moreover, well-known phase diagram makes understanding of dynamics of solidification better. 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, highly controlled directional solidification experiments are performed by using Bridgman technique on Al-Ag-Cu ternary system at invariant composition. Experiments are performed in the velocity range of 0.1 µm/s to 4.0 µm/s at a constant thermal gradient of 15 K/mm. The selected velocities and thermal gradients are sufficient to form planar solid/liquid interface which is extremely important for the eutectic spacing studies. The microstructures formed at various velocities are quantitatively characterized in terms of morphology and microstructure using discovered method, Feret technique. In addition, phase fraction and area analyses are utilized for investigation on growth length. The stable pattern for the eutectic system is determined as chain-like structure at specific velocity range. Jackson-Hunt constant is calculated by using different solidification velocities. Microstructural changes over the growth length is investigated and formation of paw structure and faceted structure are examined. Additionally, different size samples are used to determine convection effect during growth. Similarly, wall effect on solidification is studied at specific growth lengths for the comparison. As a result, it is aimed to characterize Al-Ag-Cu eutectic system in diverse aspects by testing the effect of solidification velocity, growth length and sample size.