SEIJI MIURA, MIKI AOKI, YASUHIKO SAEKI, KENJI OHKUBO, YOSHINAO MISHIMA, and TETSUO MOHRI The effect of Zr on the formation of Nb/Nb 5 Si 3 lamellar microstructure by eutectoid decomposition reaction of Nb 3 Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb 3 Si phase into Nb and Nb 5 Si 3 phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb 3 Si phase during solidification.

For the improvement of mechanical properties, especially ductility, of intermetallic compoundbased alloys, microstructural control has been adopted to binary and higher-order systems. Although lamellar structure formed by a eutectic reaction has been attracting broad attention to ductilize alloys with a combination of brittle/ductile or brittle/brittle phases, it should be pointed out that the microstructure such as lamellar size or colony size is hard to be controlled after the solidification process. In that viewpoint, high-temperature solid-solid phase transformation provide significant advantages in handling and large super-cooling for the control of lamellar structure through eutectoid reaction.