Crystallography and microstructural studies of phase transformations in the Dy 2 O 3 system
- PDF / 526,504 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 183 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Crystallography and microstructural studies of phase transformations in the Dy2 O3 system Youn Joong Kima) and Waltraud M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 19 August 1997; accepted 2 January 1998)
The crystallography, microstructures, and phase transformation mechanisms in dysprosia (Dy2 O3 ) have been studied. The lattice parameters of B and C phases were refined by x-ray diffraction (XRD). The modulated structures and decomposed structures in the CaO-doped samples were characterized by transmission electron spectroscopy (TEM). A new twin was observed in the modulated B phase. Contrary to the previous studies, the B to C transformation was induced by grinding. The A to B transformation was considered to be ferroelastic and the spontaneous strain was calculated. The major driving force for the B (monoclinic) to C (cubic) transformation is suggested to be the release of lattice strains and cation charge repulsions in the B structure, which is analogous to the b (monoclinic) to g (orthorhombic) transformation in Ca2 SiO4 . This transformation can be displacive, if some conditions are provided to overcome the bonding energy of the interlayer oxygens in the B structure.
I. INTRODUCTION
Toughening of ceramics by phase transformation methods are usually limited by temperature since the toughening agents are unstable at high temperature and sometimes pretransformed before applying stress. In addition, chemical reactivities are so high at higher temperatures that the control of microstructures and microchemistry is much more difficult than at lower temperatures. Lanthanide sesquioxides (Ln2 O3 , or rare earth oxides) are remarkably stable materials at high temperatures with respect to chemical reaction and phase stability. As shown in Fig. 1,1,2 the monoclinic (B) to cubic (C) phase transformation of dysprosia (Dy2 O3 ) occurs ,1850 ±C on cooling, which is much higher than the comparable, tetragonal (t) ! monoclinic (m) transformation temperature of zirconia at ,950 ±C on cooling. The lanthanum series also has a unique nature in its linear variation of atomic radii between the elements, such that the retention of stable phases and the control of phase transformation temperatures may be achieved, i.e., by solid solution formation between elements. Kriven3 reviewed the crystallographic aspects of these potential transformation tougheners. Five different phases have been identified in the pure lanthanide sesquioxide systems, in which three phases (A, B, and C) occur more commonly. Figure 1 displays their stability fields with respect to cation species and temperatures at one atmosphere. With increasing atomic a)
Current address: Korea Basic Science Institute, Yusung P.O. Box 41, Taejeon 305-600, Republic of Korea.
2920
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 10, Oct 1998
Downloaded: 13 Mar 2015
number, the ionic radius of the cation is decreased and the st
Data Loading...
