Thesis Type: Postgraduate
Institution Of The Thesis: Middle East Technical University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Turkey
Approval Date: 2018
Thesis Language: English
Student: ALİCAN TUNCAY ALPKAYA
Principal Supervisor (For Co-Supervisor Theses): Sezer Özerinç
Co-Supervisor: Yunus Eren KalayAbstract:
Nanolayered metals are materials composed of alternating metallic layers with layer thicknesses on the order of 100 nm or smaller. These nanostructured materials exhibit higher yield strength, superior wear resistance, and better thermal stability compared to most conventional alloys. These outstanding mechanical properties make them promising for many fields including transportation, aerospace and energy industries. The relationship between the microstructure and mechanical properties of these materials should be well understood for being able to utilize them in applications. There have been many studies in the literature investigating the structure-property relationship for pure nanocrystalline metallic layers. However, the effects of alloying additions on the mechanical properties of these nanostructures are not well understood. This dissertation aims to investigate this topic through the detailed characterization of Cu/Nb nanolayers. Pure Cu / pure Nb nanolayers and alloyed Cu90Nb10 / pure Nb nanolayers were prepared in the form of thin films by magnetron sputtering on silicon substrates with layer thicknesses varying in the range 5 nm – 100 nm.The films with a total thickness of 1 μm were characterized for their microstructure using X-ray diffraction and electron microscopy. Mechanical properties were determined by nanoindentation. Alloying additions improved the strength of Cu/Nb significantly for all layer thicknesses considered. The highest hardness of 6.98 GPa was obtained at a layer thickness of 5 nm, which is stronger than the hardest Cu/Nb nanolayer produced to date. High levels of strengthening cannot be explained by the conventional strengthening mechanisms, but it is in agreement with the recent molecular dynamics predictions of strengthening in alloyed nanocrystalline copper. This suggests that the strengthening is due to the reduction of the grain boundary energy of copper layers upon alloying. The findings provide a new path for further enhancing the strength of nanostructured materials and tailor their properties towards applications.