Hot-stage transmission electron microscopy study of phase transformations in hexacelsian (BaAl 2 Si 2 O 8 )
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Phase transformations in synthetic hexacelsian were investigated by hot-stage transmission electron microscopy. A second phase transformation from an orthorhombic to hexagonal structure was identified in the synthetic hexacelsian at approximately 700 °C. The hexacelsian was found to exhibit a sequence of phase transformations on heating of hexagonal (P63/mcm)–orthorhombic (Immm)–hexagonal (P6/mmm). Antiphase domain boundaries, which were observed in P63/mcm and Immm phases, were absent in the P6/mmm phase. Crystal symmetries of the three phases were determined by convergent beam electron diffraction, and space group symmetries were derived by comparison of experimental selected area electron diffraction patterns with computer-simulated patterns.
I. INTRODUCTION
Recently, BaAl2Si2O8 was identified as one of the materials with displacive phase transformation, which show potential for use as transformation weakeners.1,2 Experiments with composites of BaAl2Si2O8 and alumina revealed a new mechanism for achieving debonding in composites. This mechanism, called “reconstructive transformation toughening,” uses a volume reducing, reconstructive phase transformation to generate tensile stresses at the interfaces in composites. Excellent debonding behavior was observed in layered BaAl2Si2O8/Al2O3 composites tested at room temperature and at 850 °C. The most significant advantage of this new mechanism is that it is relatively insensitive to changes in temperature. The phase transformation in hexacelsian BaAl2Si2O8 at approximately 300 °C has an associated volume change of 0.43% with a decrease in volume on cooling.1,2 It is this transformation which is of interest for shear-induced, transformation weakening.1 It has been known for many years that there are four polymorphs of BaAl2Si2O8: celsian; paracelsian; ␣- and -hexacelsian. Celsian and paracelsian occur naturally as minerals. Celsian is the thermodynamically stable phase under ambient conditions. It is a framework aluminosilicate and a member of the feldspar family.3 Paracelsian is a less common mineral and is believed to be metastable at all temperatures under ambient pressure. The structure consists of chains of tetrahedra similar to those found in
a)
Now at Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong. b) Now at Siemens Westinghouse, Pittsburgh, PA 15235-5098. J. Mater. Res., Vol. 17, No. 6, Jun 2002
the feldspar structures, but in paracelsian they are linked together in a different manner.4 On the other hand, hexacelsian is the stable high-temperature phase of BaAl2Si2O8 existing under equilibrium conditions between 1590 °C and the melting point, 1760 °C. However, hexacelsian persists metastably when cooled below 1590 °C.5–7 Hexacelsian is structurally quite different from celsian and paracelsian. It is composed of double sheets of Al2Si2O8 with the barium cations located between the sheets.8 The presence of two polymorphs of hexacelsian, ␣- and -hexacelsian, was first observed by Take´uchi usi
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