Nanocrystallization in marginal glass forming alloys

Demirtaş, Tuba
The marginal glass-forming alloys have attracted much attention due to unique products of devitrification with a very high number density of nuclei up to 10^23 m^-3. Among these alloy systems, utmost interest is given to Al-RE and Al-TM-RE alloys with excellent lightweight mechanical (fracture strength close to 1 GPa) and chemical properties attributed to the presence of an extremely high density of nanocrystals embedded in an amorphous matrix. Classical nucleation theory fails in explaining this abnormal nucleation behavior, several other mechanisms have been proposed; however, there is still no agreement on the exact nucleation mechanism. Al-Tb system was investigated in liquid and solid amorphous states with a collective study of ab-initio MD and RMC simulations and state of art X-rays and e-beam techniques. Regions of pure Al clusters in the solid and liquid states were detected with the sizes extending up to 1-2 nm length. Al clusters interconnecting regions lead to formation of RE rich MRO structure which gave rise to the pre-peak in S(Q)-Q data in liquid and solid states. Specimens having MRO were crystallized within a controlled atmosphere and temperature and investigated using a combined study of TEM, HRTEM, SEM, XRD and DSC. HRTEM investigations and JMA results indicated different mechanism of nucleation. Therefore the kinetics of highly populated nuclei formation was found too complicated to be explained by well-known JMA approach. Mechanical tests were applied to determine the effects of morphology and populations of nanocrystals embedded in amorphous matrix. The tensile tests and the subsequent fracture surface analysis indicated brittle type of failure and the formation of shear bands, respectively. Relatively high hardness and tensile strength were detected by nanocrystallization.


Angle of graph energy - A spectral measure of resemblance of isomeric molecules
Gutman, I; Türker, Burhan Lemi (2003-11-01)
A method, elaborated earlier by one of the present authors, for measuring the structural resemblance of isomeric alternant conjugated hydrocarbons, based on a graph-spectral quantity theta, called the angle of total pi-electron energy approach has been extended now to arbitrary molecules. Some general properties of theta have been established.
Local chemical and topological order in Al-Tb and its role in controlling nanocrystal formation
Kalay, Yunus Eren; HWANG, Jinwoo; VOYLES, P. M.; KRAMER, M. J. (2012-02-01)
How the chemical and topological short- to medium-range order develops in Al-Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al90Tb10. Reverse Monte Carlo simulations and V...
Empirical Comparison of Random and Periodic Surface Light-Trapping Structures for Ultrathin Silicon Photovoltaics
BRANHAM, Matthew; HSU, WeiCsun; Yerci, Selçuk; LOOMİS, James; BORİSKİNA, Svetlana; HOARD, Brittany; HAN, Sang Eon; EBONG, Abasifreke; CHEN, Gang (2016-06-01)
Pyramidal light-trapping structures of a range of length scales — and both periodic and random arrangements — are shown to yield similarly high absorption in thin film crystalline silicon photovoltaics. Through the combination of results from experiment and simulation, the trade-off between absorption effectiveness and ease of fabrication of various pyramidal light-trapping structures is investigated for application in thin-film crystalline silicon solar cells.
Local atomic structure in al-re marginal metallic glasses
Övün, Mert; Kalay, Yunus Eren; Department of Metallurgical and Materials Engineering (2016)
Partial devitrification of the Al-RE marginal metallic glasses results in anomalous nucleation rate of nanocrystals conflicting with the classical nucleation theory by several orders of magnitude. One of the theoretical approaches explaining this phenomenon is the medium-range order (MRO) structures within the range of 1-2 nm that may be present both in the quenched amorphous state and its liquid precursor. In this work, these structures have been investigated conducting both experimental methods such as hi...
Rheological characterization of polyethylene glycol based TiO2 nanofluids
YAPICI, KERİM; KEKLİKCİOĞLU ÇAKMAK, NEŞE; Ilhan, Naciye; Uludağ, Yusuf (2014-11-01)
Rheological characterization of TiO2 nanoparticle dispersions in polyethylene glycol (PEG 200) is presented over 1-10 wt% particle mass fraction range in terms of shear viscosity, thixotropy and linear viscoelasticity. A stress controlled rheometer fitted by a cone-and-plate system was employed for the rheological measurements between -10 degrees C and 40 degrees C. The non-linear viscoelastic experiments revealed that TiO2-PEG 200 nanofluid exhibits a shear thinning behavior when particle mass fraction exc...
Citation Formats
T. Demirtaş, “Nanocrystallization in marginal glass forming alloys,” M.S. - Master of Science, Middle East Technical University, 2013.