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Home  >  Journal list  >  MATERIALS TRANSACTIONS  >  Vol.42  No.12 (2001)  >  pp.2553-2558

MATERIALS TRANSACTIONS
<<Previous article Vol.42  No.12 (2001)   pp.2553 - 2558 Next article>>

Nanometer-Size Polycrystallization in bcc→hcp+C15 Structural Change of a Ti-30 mol%Cr Alloy

Akihiko Hirata1), Makoto Tanimura2) and Yasumasa Koyama1)
1) Kagami Memorial Laboratory for Materials Science and Technology and Department of Materials Science and Engineering, Waseda University
2) Research Department, Nissan ARC Ltd.


The crystallographic features of a bcc→hcp+C15 structural change in a Ti–30 mol%Cr alloy have been examined by transmission electron microscopy. When the alloy was annealed at 873 K below the eutectoid temperature, the structural change occurred in the following five steps: bcc→bcc+Zone I→bcc+Zone II→bcc+Zone II+hcp→bcc+hcp+C15→hcp+C15. The interesting features of the change are that the diffuse ω state is present in the metastable bcc matrix, and that in the fourth step the C15 grains with a size of about 2 nm appear in the bcc matrix around the hcp region. As a result of these extremely small C15 grains, the final microstructure, which was obtained from the 5-h-and 100-h-annealed samples, consists of the hcp and C15 grains with an average size of about 100 nm. On the basis of these results, the physical origin of the nm-size polycrystallization in the appearance of the C15 phase is discussed in relation to both the presence of diffuse ω state in the metastable bcc matrix and the formation of the complex coordinated polyhedra in the C15 structure.


Keyword:
titanium chromium alloy, transmission electron microscopy, structural change, complex coordinated polyhedra, polycrystallization

Received: July 23, 2001
Accepted: October 03, 2001 , Published online: September 06, 2005
Copyright (c) 2005 The Japan Institute of Metals

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