Creep Studies of Monolithic Phases in Nb-Silicide Based In-Situ Composites
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Creep Studies of Monolithic Phases in Nb-Silicide Based In-Situ Composites B.P. Bewlay1, C.L. Briant2, E.T. Sylven2, M.R. Jackson1 and G. Xiao2 1GE Corporate Research and Development, Schenectady, NY 12301, USA 2Division of Engineering, Brown University, Providence, RI 02912, USA ABSTRACT Nb-silicide composites combine a ductile Nb-based solid solution with high-strength silicides, and they show great promise for aircraft engine applications. Previous work has shown that the silicide composition has an important effect on the creep rate. If the Nb:(Hf+Ti) ratio is reduced below ~1.5, the creep rate increases significantly. This observation could be related to the type of silicide present in the material. To understand the effect of each phase on the composite creep resistance, the creep rates of selected monolithic phases were determined. To pursue this goal, monolithic alloys with compositions similar to the Nb-based solid solution and to the silicide phases, Laves, and T2 phases, were prepared. The creep rates were measured under compression at 1100 and 1200oC. The stress sensitivities of the creep rates of the monolithic phases were also determined. These results allow quantification of the load bearing capability of the individual phases in the Nb-silicide based in-situ composites.
INTRODUCTION Nb-silicide based in-situ composites are being explored for structural applications at very high temperatures [1-4]. These composites consist of Nb5Si3 and Nb3Si type silicides toughened with a Nb solid solution (abbreviated by (Nb) in the present paper). More recent Nb-silicide based in-situ composites are highly alloyed with elements such as Cr, Ti, Hf, B and Al. These in-situ composites have demonstrated a promising combination of high-temperature strength, creep resistance, and room-temperature fracture toughness [1-3]. With the appropriate combination of alloying elements it is possible to achieve the required balance of room temperature toughness and high temperature creep resistance. Alloying elements such as Cr and B have beneficial effects on oxidation resistance, stabilizing Laves phases and T2 niobium borosilicide phases, respectively. The Nb5Si3 and Nb3Si have the tI32 and tP32 ordered tetragonal structures with 32 atoms per unit cell. The unit cells also possess large lattice parameters; the large Burgers vectors and complex dislocation cores associated with these structures would suggest that dislocation creep makes only a small contribution to creep deformation in these silicides. When Nb5Si3 is alloyed with Ti and Hf, the less complex hP16 structure can also be stabilized [1, 6]. The Laves phases typically have C14, C15, or more complex structures of a hexagonal form [3]. The present study was performed to determine the creep rates of monolithic intermetallic phases and to develop the constitutive creep laws for these phases. These creep laws are also required to perform predictive modeling of more complex two-phase and multi-phase systems [7]. The monolithic phases described in this paper were produced b
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