Title page for etd-0110114-123757


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URN etd-0110114-123757
Author Min-chi Yeh
Author's Email Address No Public.
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Department Materials and Optoelectronic Science
Year 2013
Semester 1
Degree Ph.D.
Type of Document
Language zh-TW.Big5 Chinese
Title Short-range order study on Zr based bulk metallic glass
Date of Defense 2014-01-08
Page Count 107
Keyword
  • short-range order structure
  • glass-forming ability
  • droplet-type phase
  • thermodynamic-computation approach
  • metallic glass
  • Abstract To solve the problem of limited deformation in metallic glass, newly ternary or quaternary alloy systems have developed the second phase within an amorphous matrix in recent years. These alloy systems have second-phase particles or nanocrystals in a glassy matrix, phase-separated regions, and variations in short-range order by clustering. The second phase within an amorphous matrix can produce the amorphous/amorphous, amorphous/short-range order or amorphous/crystalline composite structure, depending on its glass-forming ability (GFA). Investigating the possibility for the design of a new type of composite structure (i.e., amorphous/crystalline or amorphous/short-range order composites instead of amorphous/amorphous composites) may be able to improve the ductility of BMG.
    This study presents a newly designed Zr-based alloy (Zr63.36Cu17.22Ni11.47Al7.95) in Zr-Cu-Ni-Al phase separation system based on thermodynamic-computation approach, and describes its microstructure and crystal structure with different cooling rates. It can obtain completely amorphous structure, the amorphous/ short-range order and the amorphous/ crystalline composite structure by fast to slow cooling rate. Although the experimental result was no phase separation phenomenon as expected, but it was surprisingly that forming the dual phase composite structure with short-range order droplet phase and amorphous matrix phase. And it was interesting that the average compositions of the droplet-type phase and amorphous matrix phase were identical within experimental error. It means that no local primary nucleation and growth was occurring during solidification process, so that the compositions of droplet phase are the same as the compositions of matrix amorphous phase.
    In order to understand the same chemical composition but have different image contrast, that we use XRD, EBSD and TEM analysis to understand whether they caused by different degrees ordered structure. The results show that there are three different short-range order structures in droplet-type phase. Therefore, it contributes to the weak crystallization signals and the structure is different from the amorphous matrix phase. Finally, we use the heat treatment in order to understand the relationship between the droplet phase, the matrix phase and equilibrium phase. The results show that the Zr2Ni(Al,Cu)-type phase of cubic structure is a metastable phase and it will occur phase transformation at 686 K in the present study. The Zr2Cu(Al,Ni)-type phase with high temperature stability match the result of phase equilibrium in the Zr-Cu-Ni System at 800℃.
    Advisory Committee
  • Der- shin Gan - chair
  • Liu-wen Chang - co-chair
  • Pou-Yan Shen - co-chair
  • Yung-Chung Chen - co-chair
  • Ker-Chang Hsieh - advisor
  • Files
  • etd-0110114-123757.pdf
  • Indicate in-campus at 3 year and off-campus access at 3 year.
    Date of Submission 2014-02-10

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