Characterization of second-phase plates in a Gd₅Ge₃ intermetallic compound

Rare-earth compounds based on the stoichiometry R₅(Si(x)Ge(1-x))₄ (R = rare-earth elements) exhibit many unusual features, including possessing R₅(Si(x)Ge(1-x))3 thin plates which always precipitate from the matrix despite efforts to suppress their formation. In an effort to better understand the unique relationship between these two intermetallic alloy systems, the bulk microstructure of the compound Gd₅Ge₃ was examined using scanning (SEM) and transmission electron microscopy (TEM) and optical microscopy. Surprisingly, SEM examination revealed a series of thin plates present in the Gd₅Ge₃ matrix similar to what is seen in Gd₅Ge₄. TEM observation revealed that a role reversal had occurred, with the thin plates possessing the orthorhombic structure and composition of Gd₅Ge₄. The orientation relationship between Gd₅Ge₄ thin plates and the Gd₅Ge₃ matrix was determined to be [1 0 1 0 ](1 2 1 1)(m)||[0 1 0](1 0 2)(p), the same relationship reported for Gd₅Ge₃ plates precipitating from a Gd₅Ge₄ matrix. However, by exchanging the respective roles of the phases as regards matrix vs. precipitate, the total number of precipitation variants seen can be increased from two to six. The persistence with which these two intermetallic systems co-exist is truly unique. However, understanding exactly the kinetic and thermodynamic conditions that lead to their unique relationship is hampered by the high formation temperatures at which the observed reaction occurs.