Ph.D. Theses

Permanent URI for this collection

https://digitalarchive.library.bogazici.edu.tr/handle/123456789/11661

Browse

Browsing Ph.D. Theses by Author "Balıkçı, Ercan."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Experimentally verified numerical simulation of single crystal growth process with a low melt height and an axial vibration
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Sheikhi, Aidin.; Balıkçı, Ercan.
    This Ph.D. dissertation investigates experimental and numerical crystal growth of antimony-doped germanium (Ge-Sb) single crystals. The investigation is a part of the TUBITAK project 212M030. The single crystal growth of Ge-Sb from the melt is investigated by the Vertical Bridgman (VB), Axial Heat Processing (AHP), and Axial Vibrational Control (AVC) techniques. The effects of method dependent growth parameters on the quality of the grown single crystals have been analyzed. To this end, two different pulling rates (10 mm/h and 20 mm/h), different initial melt heights (5 mm, 10 mm, 14 mm, and 58 mm), and three different sets of vibrational parameters (2 mm amplitude and 0.25 Hz frequency, 0.25 mm amplitude and 1 Hz frequency, and 0.25 mm amplitude and 25 Hz frequency) are applied in the growth of seven different crystals. It is observed that the highest single crystal length with the most homogeneous solute redistribution and the least dislocation density are achieved in an AHP crystal which is grown with the lowest pulling rate (10 mm/h). However, it is determined that an appropriate control of the vibration parameters in the AVC technique makes it possible to achieve almost the same crystal quality with doubled growth rate, so the production yield is decreased. Moreover, global and local numerical simulations are performed in order to investigate the effects of the growth parameters on the convective flow patterns. Also, results of the numerical simulations contribute to make better and more reliable interpretations of the experimental observations. The simulation results provide useful information for the experimentalists to investigate the effects of growth parameters on the temperature and solute distribution, flow pattern, and the interface shape. According to the numerical results, it is possible to clarify how the insertion of the baffle, adjusting the melt height, and optimizing the vibrational parameters of the baffle contribute the thermal and the solutal homogenization in the melt, interface stability, and consequently improved crystal quality.
  • Thumbnail Image
    Item
    Integrated computational alloy design, single crystal growth, and characterization of nickel base superalloys
    (Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Montakhabrazlighi, Mehdi.; Balıkçı, Ercan.
    This dissertation is formed by a part of work conducted in a TUBITAK project (112M783). In the dissertation, first alloy design of a third generation superalloy, and then its production as single crystals, heat treatments, and creep tests are investigated. A combined Neural Network (NN) – PHAse COMPutation (PHACOMP) – CALculation of PHAse Diagrams (CALPHAD) method is applied to alloy development of single crystal Ni base superalloys with low density and high creep resistance. PHACOMP method is used for estimation of the volume fraction of the γ’ and a parameter named Md which is an index showing the propensity of the alloys towards formation of the TCP phases. Neural network is used for modeling the density and rupture time by training and testing a network with a set of the known experimental alloy compositions. Modeling results is combined with data obtained from PHACOMP to render very useful scatter plots for the effect of alloying elements; stress, temperature and volume fraction of the γ’ phase on density, rupture strength and formation of TCP phases in the Ni base single crystal superalloys. A third generation alloy (ERBALLOY) was designed and produced by two methods, vertical Bridgman (VB), and vertical Bridgman with a submerged baffle (VBSB). The effect of low melt height on solidification characteristics of the alloys is studied. Evolution of the phases is simulated by CALPHAD bases Thermo-Calc software. The solution and aging heat treatment of the alloys are modeled with Dictra and TC-Prisma software. Characterization of the microstructure is performed by optical, scanning transmission electron microscope (SEM), and electron probe microanalysis (EPMA). The creep behavior of the ERBALLOY is tested at an intermediate and a high temperature and showed reasonable agreement with NN results. The approach used in this study is in line with the Materials Genome Initiative (GMI) and Integrated Computational Materials Engineering (ICME), and it can be applied for designing low density-creep resistant single crystal superalloys for critical parts of the aircraft/gas turbine engines.