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Formation and Crystallization Kinetics of Nd-based Metallic Glasses and Crystallization of Zr-based Bulk Metallic Glasses under High Magnetic Field

Author: ChenJun
Tutor: ZhuangYanZuo
School: Northeastern University
Course: Materials Science
Keywords: Metallic glasses formation glass forming ability crystallization activation energy high magnetic field
CLC: TG139.8
Type: Master's thesis
Year: 2009
Downloads: 42
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Abstract


Metallic glass has attracted wide interest due to its special physical, chemical and mechanical properties. It has been demonstrated that compositions of alloys have significant effect on the glass forming ability, and high magnetic fields have remarkable influence on the formation and crystallization of metallic glasses. One purpose of the work is to prepare Nd-based metallic glasses, and to investigate the glass forming ability and the crystallization kinetics of the Nd-based metallic glasses. Another objective of the research is to investigate the effect of high magnetic field on the crystallization of two Zr-based bulk metallic glasses.Nd60Al10Co30-xNix (x=0,5,10,15,20 at.%) amorphous ribbons have been prepared by melt-spinning. The glass forming abilities of the alloys have been evaluated by the reduced glass transition temperature and the width of the supercooled liquid region, respectively. It has been found that the Nd60Al10Co15Ni15 alloy has the strongest glass forming ability among the alloys investigated. Meanwhile, the Nd60Al10Co15Ni15 bulk metallic glass with 5 mm in diameter and 40 mm in length has been successfully fabricated by copper mould suck-casting.Non-iosthermal DSC has been used to study crystallization kinetics of Nd60Al10Co30, Nd60Al10Co25Ni5 and Nd60Al10Co20Ni10 amorphous ribbons and Nd60Al10Co15Ni15 bulk metallic glass. It has been found that the characteristic temperatures shift to higher temperature with the heating rate increasing, meaning that the crystallization process has obvious kinetic effect. The apparent activation energy has been obtained using Kissinger equation. The relationships between characteristic temperatures and the heating rate have been discussed using Kissinger equation, Losocka equation and VFT equation. Though they all illustrate the relationship well when the heating rate is in the range of 5-80 K/min, VFT equation is considered as a best choice for the alloys investigated.The continuous heating transformation diagrams of the Nd60Al10Co30, Nd60Al10Co25Ni5 and Nd60Al10Co20Ni10 amorphous ribbons and the Nd60Al10Co15Ni15 bulk metallic glass has been studied. It has been found that the VFT equation is more precise in describing the continuous heating transformation diagrams of the Nd-based metallic glasses investigated.The isothermal crystallization behavior of Zr41Ti14Cu12.5Ni5Be22.5 and Zr46.75Ti8.2sCu7.5Ni10Be27.5 amorphous alloys under high magnetic field has been investigated using X-ray diffraction. It has been found that the high magnetic field enhances or retards the precipitation of certain crystallization phases. Either in the Zr4iTi14Cu12.5Ni5Be22.5 or in the Zr46.75Ti8.25Cu7.5NiioBe27.5 bulk metallic glass, the precipited phases are strongly dependent on the magnetic field applied. In the Zr41Ti14Cu12.5Ni5Be22.5 metallic glass, the precipation of the crystallized phase Be2Zr is enhanced by the high magnetic field. While in the Zr46.75Ti8.25Cu7.5NiioBe27.5 bulk metallic glass, the precipitation of the Zr2Ni, ZrCu2, and NiTio.6Zro.4 depend not only on the intensity of the magnetic field, but also on the relative direction of the magnetic field and the sample surface. It has been found that the high magnetic field parallel to the sample surface enhance the precipitation of the phases mentioned above, while the magnetic field perpendicular to the sample surface retards the formation of the phases. The results indicate that the high magnetic field affects the crystalzation process of the Zr-based metallic glasses. It has been also found that a twist structure has been formed in the crystallized Zr46.75Ti8.25Cu7.5NiioBe27.5 alloy. The appearance of the twist structure depends on the annealing temperature and magnetic field. The possible formation mechanism has been discussed in the thesis.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Amorphous alloy
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