Crystal Growth of RuAl-base Alloys
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Crystal Growth of RuAl-base Alloys Sebastien Rosset, Rachel E. Cefalu, Louis W. Varner, and David R. Johnson Materials Engineering, Purdue University, West Lafayette, IN 47907-1289
ABSTRACT Different techniques to produce single crystals of RuAl were investigated. Processing from the melt is problematic due to the rapid loss of Al. However, very large grained RuAl specimens can be produced from the solid-state by arc-zone melting. Directional solidification of eutectic alloys is less problematic due to the lower melting temperature. Alloys of RuAl-Mo were found to be eutectic, and the eutectic composition was determined to be RuAl-51 wt% Mo. INTRODUCTION Intermetallics are often considered candidate materials for high temperature structural applications due to the high melting temperature and good oxidation resistance of many compounds. However, problems with brittleness have not been solved for intermetallic systems that are to operate above the use temperature of the superalloys. Ruthenium aluminide, having the CsCl (B2) type crystal structure, is unusual in this respect, as good room temperature toughness has been reported from limited and qualitative tests as first reported by Fleischer et al. [1]. More recently, Wolff et al. [2] have observed extensive room temperature plasticity from compression tests indicating a sufficient number of independent slip systems for polycrystalline deformation. While prior work [1,2] has highlighted the promise of using RuAl for high temperature structural applications, ruthenium aluminide itself is likely to remain an exotic alloy regardless of its mechanical properties due to its great expense. The use in very small quantities may be acceptable such as in a multiphase alloy or as a coating material. Alloys of ruthenium aluminide have good high temperature oxidation resistance as reported by Fleischer et al. [3]. Furthermore, RuAl forms stable equilibria with a number of refractory metals. If multiphase microstructures are designed such that RuAl is the continuous phase, then the weight fraction of Ru can be significantly reduced and such materials may provide a tough coating for the refractory metals. More important, though, is understanding the unusual deformation behavior of this material. Reasons for the good compressive ductility of polycrystalline RuAl are not well understood. The unusual room temperature toughness of this material appears to originate from the multiplicity of slip along the and directions which differs considerably from that of most other B2 compounds in which the common slip directions are either or . However, the observed polycrystalline plasticity in RuAl without grain boundary cracking precludes slip by dislocations alone as the number of independent slip systems is less than that required by the von Mises criterion. Therefore, other slip systems must be operative during room temperature deformation. While slip parallel to would allow for polycrystalline deformation, recent experimental evidence by Lu and Pollock [4] suggests that deformation by both
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