Oxygen as impurity in crystal growth of intermetallics

Single crystal growth of intermetallic compounds from the melts is strongly affected by oxygen impurity. In the case of container-free crystal growth techniques there are three main sources for oxygen contamination: (1) oxygen contents in initial chemicals used for alloy synthesis, (2) contamination during alloy preparation, (3) contamination during the crystallization process (e.g. absorption of oxygen traces from the growth atmosphere). All these contamination sources are considered mainly with respect to the synthesis and floating zone crystal growth of rare earth-silicides, -carbides, -borides, and -borocarbides. It was proved that even > 99.9% "pure" metal chemicals supplied may contain up to several at.% of oxygen because the certificate refers mostly to metallic impurities only. Oxygen is either dissolved in the metal or present as oxide particles in the metal matrix. At high temperatures (> 1500 degrees C) silicothermic, carbothermic and borothermic chemical reactions forming volatile oxides can proceed, which can reduce the oxygen concentration in intermetallic melts and growing crystals by 1-2 orders. On the other side, they change the melt composition. Oxygen dissolved in the melt can be incorporated in the crystal matrix and afterwards precipitate as oxide inclusions during its cooling. It can also be accumulated in front of crystallization interface and give rise to constitutional supercooling and cellular growth. Non-dissolved oxide particles can be precipitated in the crystal resulting in nucleation of new grains. (c) 2007 Elsevier B.V. All rights reserved.