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.


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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.
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Ionic conductivity of microporous titanosilicate ETS-10 and ion-exchanged Mn+-ETS-10 (where, Mn+ = Li+, Na+, Mg2+, Zn2+, Ca2+) thin films prepared by secondary growth method
Galioglu, Sezin; Çam, İbrahim; Akata Kurç, Burcu (2017-09-15)
Impedance spectroscopy was used to investigate the long-range ionic conductivity of the microporous, titanosilicate (Na,K)-ETS-10 and ion-exchanged Mn+-ETS-10 (where, Mn+ = Li+, Na+, Mg2+, Zn2+, Ca2+) thin films prepared by secondary growth method. To figure out the effect of grain boundary on ionic conduction, as-synthesized (Na,K)-ETS-10 films possessing different thicknesses of columnar grain structure (i.e., films prepared via 4h-, 6h-, 8h-, and 10h-growth) were tested. The conductivities of the films w...
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...
Structural and dynamical evolution of nanocrystals
Yıldırım, Can; Kalay, Yunus Eren; Department of Metallurgical and Materials Engineering (2013)
Among all metallic glasses, marginal glass-forming alloys exhibit unique devitrification features due to their primary crystallization products having intriguing nucleation number density values up to 1024m-3. Remarkable materials properties such as high strength, corrosion resistance and appealing magnetic properties along with the inconsistency of classical nucleation theory predictions with the experimentally observed number density of nanocrystals have recently attracted much of an attention. A full agr...
Citation Formats
T. Demirtaş, “Nanocrystallization in marginal glass forming alloys,” M.S. - Master of Science, Middle East Technical University, 2013.