Mechanical Engineering PhD Thesis Defense by Benay Uzer

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KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

PhD THESIS DEFENSE BY BENAY UZER

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Title: Investigating the Microstructural and Mechanical Properties of the Novel Metallic Materials Used in Interdisciplinary Fields and Constructing a Relationship with the Biocompatibility in Biomedical Applications

Speaker: Benay Uzer

Time: July 19, 2017, 15:00

Place: Eng 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Demircan Canadinc (Advisor, Koc University)

Assoc. Prof. B. Erdem Alaca (Koc University)

Asst. Prof. Alper Uzun (Koç University)

Assoc. Prof. Hakan Usta (Abdullah Gül University)

Asst. Prof. İlknur Eruçar Fındıkçı (Özyeğin University)

Abstract:

In this dissertation novel metallic materials utilized in interdisciplinary applications have been analyzed by constructing a relationship between their microstructural and mechanical properties. For this aim the initial tests were carried out to understand the effect of microstructural evolution on the cell response on the potential implant materials which were deformed up to different strain levels. Microscopy analyses showed that the brain tumor cells exhibited the best attachment and viability on the sample with the greatest plastic deformation where a significant slip activity was prevalent, accompanied by considerable slip-twin interactions. This study showed that by manipulating the microstructure of the implant material improved cell response and consequently a more successful treatment can be obtained. This study was continued with another cell type in order to understand whether different cells would all react in the same way or not. Conversely, fibroblast cells did not exhibit enhanced cell response with the increased plastic deformation, which might stem from the failure of the adhesion of ECM proteins, or incompatibility with the cell size or surface roughness. This study pointed out the importance of selecting a unique implant material specific for the body location and the tissue that the metal will be in contact with.

Biocompatibility of the metallic materials was further analyzed in order to thoroughly understand the parameters affecting it. Specifically the immersion experiments of Nickel Titanium (NiTi) orthodontic archwires through the incorporation of the realistic mechanical conditions into ex situ experiments showed that toxic Ni ion release increased due to stress corrosion cracking (SCC). This study was followed by investigating the dissolution-reformation cycle of the protective oxide layer on the archwires. The results showed that Ni ion release enhanced owing to the dissolution of the oxide layer and with the reformation the net ion contribution has decreased significantly. These findings showed the need for incorporating the actual chemical and mechanical condition into the ex situ experiments and thorough analysis of the protective oxide layer in order to correctly evaluate the biocompatibility of the biomaterials.

After thoroughly evaluating the biocompatibility of the metallic implants further experiments were carried out to better understand the microstructure and mechanical properties of the materials which exhibit complex slip and twinning activity upon loading. With this aim both polycrystalline and single crystal metals were tested in order to better understand the activation of the plastic deformation mechanisms with and without the influence of the grain boundary interactions. These studies showed the enhanced mechanical properties owing to the impediment of slip by the twins and formation of nanotwins within the primary twins. The mutual interaction of these mechanisms has strengthened the material, thus materials were able to accommodate the great amount of deformation.

Overall the findings of this dissertation show that a thorough analysis of microstructural analysis is needed to better understand the mechanical properties of the metallic materials. Moreover, for the biocompatibility evaluation of the metallic implants the incorporation of microstructural and mechanical properties was pointed out to be utmost importance.