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| Author(s): |
Leonardo Pratavieira Deo
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD) |
| Defense date: | 2015-12-03 |
| Examining board members: |
Marcelo Falcão de Oliveira;
Conrado Ramos Moreira Afonso;
Waldek Wladimir Bose Filho;
Lauralice de Campos Franceschini Canale;
Claudio Shyinti Kiminami
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| Advisor: | Marcelo Falcão de Oliveira |
| Abstract | |
Since the discovering of amorphous alloys in 1960, the actual causes of why some alloys can be easily formed into glasses while others cannot, are not clearly known, thus there is no universal theory to predict the glass forming ability in metallic systems. In the present work, a selection criterion was applied in the Ni-Nb-Zr system in order to predict the best stoichiometries with high glass forming ability. The actual glass forming ability of alloys were evaluated by the thermal parameter γm and the results have shown a poor correlation with the predictions. This criterion correlates critical cooling rate for glass formation with topological instability of stable crystalline structures; average work function difference and average electron density difference among the constituent elements of the alloy. The thermal parameter depends on the characteristic temperatures of phase transformations which can be easily measured from differential scanning calorimetry curves of metallic glasses. The initial hypotheses to explain the poor correlation between the experimental results and the predictions concerned with the influence of factors not considered in the calculation, such as unknown intermetallic compounds and oxygen contamination. Thus some rapidly quenched alloys were investigated with more accuracy in order to understand the formation of crystalline phases which compete against the glass formation. We characterized the crystalline phases and compared them to crystalline structures found in literature as well as some phase diagrams. The phase diagrams were used as guides in order to understand the crystallization behavior. In addition, the selection criterion also was used to predict the glass forming ability improvement of a Ni-Nb-Zr alloy with minor additions of rare-earth elements Y or Er. It is well known that the minor amount addition of proper rare-earth elements can greatly enhance the glass forming ability of some glass-forming alloys. The actual glass forming ability of the base alloy and rare-earth doped alloys also were evaluated by the thermal parameter γm and the results agree very well with the tendency predicted by the calculation. Bulk amorphous specimens were produced by injection casting. The amorphous nature was analyzed by X-ray diffraction and differential scanning calorimetry. The crystalline phases were analyzed by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and x-ray diffraction. Oxygen contamination was quantified by the inert gas fusion method. (AU) | |
| FAPESP's process: | 11/13221-0 - Study of clusters structures in glass metal compositions determined by a selection criteria based on topology and thermodynamics |
| Grantee: | Leonardo Pratavieira Deo |
| Support Opportunities: | Scholarships in Brazil - Doctorate |