Title page for ETD etd-07232010-232130


Type of Document Dissertation
Author Monsegue, Niven
Author's Email Address nmonsegu@vt.edu
URN etd-07232010-232130
Title Characterizing the Effects of Mechanical Alloying on Solid State Amorphization of Nanoscaled Multilayered Ni-Ti
Degree PhD
Department Materials Science and Engineering
Advisory Committee
Advisor Name Title
Aning, Alexander O. Committee Chair
Kampe, Stephen L. Committee Member
Lu, Guo-Quan Committee Member
Murayama, Mitsuhiro Committee Member
Reynolds, William T. Jr. Committee Member
Schultz, Jeffery P. Committee Member
Keywords
  • Mechanical Alloying
  • Solid State Amorphous Reactions
  • Amorphous Metals
  • Nickel
  • Titanium
  • Solid State Amorphization Model
Date of Defense 2010-07-09
Availability unrestricted
Abstract
Equiatomic composition of Ni and Ti was cryomilled with varying milling times to create Ni-Ti lamella structures with average spacings of 50 nm, 470 nm, and 583 nm in powder particles to vary the interfacial surface area per volume. These surfaces form interfaces for diffusion that are essential for solid state amorphization during low temperature annealing. To compare solid state amorphization in a relatively defect free multilayer system, elemental Ni and Ti were deposited by electron beam physical vapor deposition on titanium plates with comparable spacing as above. Both milled and deposited multilayers were annealed between 225 and 350°C for up to 50 hours.

X-ray diffraction characterization and in situ annealing was conducted on cryomilled and deposited multilayers of Ni-Ti. Based on this characterization, an amorphization model based on the Johnson-Mehl-Avrami nucleation and growth equation has been established to predict the amorphization of both cryomilled and deposited multilayers. Cryomilled powders experienced much larger amorphization rates during annealing than that of deposited multilayer structures, for all layer spacings. This superior amorphization is seen despite the formation of amorphous phase during the milling process; the amount of which increases with increasing milling time. The difference in amorphization rates between cryomilled and deposited multilayers is attributed to excess driving force due to the extensive preexisting defects in the powders caused by cryomilling.

Serial 3D reconstruction of cryomilled Ni-Ti powders was done by scanning electron microscopy and focused ion beam. Through 3D reconstruction it was observed that a random and non-linear lamella structure has been formed in cryomilled powders. Furthermore, lamellar spacing was seen to become smaller with increased milling time while at the same time becoming more homogeneous through the material’s volume. 3D reconstruction of cryomilled Ni-Ti offers a unique insight into the microstructures and surface areas of cryomilled powder particles that has never been accomplished.

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  2h_Cryomilled_NiTi_3D.mp4 509.78 Kb 00:02:21 00:01:12 00:01:03 00:00:31 00:00:02
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  Monsegue_Niven_D_2010_MSEa.pdf 8.73 Mb 00:40:26 00:20:47 00:18:11 00:09:05 00:00:46

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