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Synthesis and devitrification of high glass-forming ability bulk metallic glasses.

Huang, Hong. (2007) Synthesis and devitrification of high glass-forming ability bulk metallic glasses. Doctoral thesis, University of Surrey (United Kingdom)..

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In this thesis, literature on the production, microstructures and properties of bulk metallic glasses (BMG) has been reviewed with particular reference to glass forming ability (GFA) and alloys of the Fe-Zr-B and Zr-based BMG systems. The experimental procedures used in the research are presented and the results for the amorphous Fe80Zr12B8 ribbon and the Zr57Ti5Al10Cu20Ni8, Zr57Nb5Al10Cu20Ni8, Zr53Nb2Al8Cu30Ni7 BMGs are given and discussed. Wedge-shaped ingots of the Zr-based BMGs were produced by casting in air or water cooled copper moulds. The Zr-based BMGs were fully amorphous up to 2 mm thickness of the wedge. In the Fe80Zr12B8 alloy and the Zr-based BMGs the glass transformation and crystallization temperatures increased with increasing temperature increase rate in the DSC. This shows that atom mobility dominates the onset of crystallization. Using XRD and TEM, the crystallization of the Fe80Zr12B8 amorphous alloy was studied from room temperature to 1273 K. The phase evolution path during continuous heating was: Amorphous → amorphous + alpha-Fe → amorphous + alpha-Fe + Fe23Zr6 (Fe3Zr) → amorphous + alpha-Fe + Fe23Zr6 (Fe3Zr) + Fe2Zr → amorphous + alpha-Fe + Fe23Zr6 (Fe3Zr) + Fe2Zr + Fe3B. In the air-cooled Zr57Ti5Al10Cu20Ni8 alloy the A1 rich Zr3Al2 and the bcc Al1.5(Cu+Ni)3(Zr+Ti)5 phases were quenched in the amorphous matrix and as the temperature decreased nanocrystalline Zr3Al2 phase nucleated. It is suggested that A1 is critical regarding GFA. The main crystalline phase that contained Zr, Ti, Al, Cu and Ni was the tetragonal (Zr, Ti)2(Cu, Al, Ni) phase, which is isomorphous with the Zr2Cu phase. Compared with the air-cooled alloy, the water-cooled amoiphous Zr57Ti5Al10Cu20Ni8 alloy has a larger supercooled liquid region and the same reduced glass transition temperature. This would suggest that fine crystallites were probably present in the air-cooled amorphous Zr57Ti5Al10Cu20Ni8 alloy. Thermal stability and GFA increased with the substitution of Ti with Nb in Zr57Nb5Al10Cu20Ni8 and the increase of the Cu/Nb ratio in Zr53Nb2Al8Cu30Ni7. Increase of the Cu/Nb ratio in Zr-based BMGs improved the GFA criteria, the thermal stability, hardness and eliminated crack formation. It is suggested that the Zr-Ti/Nb-Al-Cu-Ni BMGs with high glass forming ability can be obtained over a wide composition range Zr = 50-60 at.%, Ti or Nb = 2-5 at.%, A1 ≤ 8 at.%, Cu+Ni ≥ 30 at.%. On the basis of the results of this study on the formation of the primary crystallization phase, it is suggested that atomic packing close to the amorphous phase, high melting temperature and strong bonding among component elements lead to the ease of formation of the primary phase. In terms of reliability and applicability, good glass formers should capture the following features: low Ti, high Tg and high (Tx-Tg). Good indicators are made of ratios and sums of ratios that reflect these features. New GFA indicators are proposed namely: Tx/Tg + Tx/T1, Tx/(T1-Tg) + Tx/Tg, Tx/Tg + Tg/T1 +Tx/T1, (Tx-Tg)/T1 + Tx/T1. Casting at specific cooling rate or annealing at temperature between the glass transition temperature (Tg) and onset crystallization temperature (Tx), especially annealing near Tx for enough time, leads to the formation of nanoparticles in an amorphous matrix.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors :
Huang, Hong.
Date : 2007
Contributors :
Depositing User : EPrints Services
Date Deposited : 09 Nov 2017 12:10
Last Modified : 16 Mar 2018 20:15

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