The Effects of Substitutional Additions on Creep Behavior of Tetragonal and Hexagonal Nb-Silicides
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The Effects of Substitutional Additions on Creep Behavior of Tetragonal and Hexagonal Nb-Silicides B.P. Bewlay1, C.L. Briant2, E.T. Sylven2, and M.R. Jackson1 1 GE Global Research, Schenectady. NY 12301, USA. 2 Division of Engineering, Brown University, Providence. RI 02912, USA.
ABSTRACT Nb-silicide based in-situ composites combine a ductile Nb-based solid solution with highstrength silicides, and they show great promise for aircraft engine applications. The Nb-silicide controls the high-temperature creep behavior of the composite. Previous work has shown that the silicide composition has an important effect on the creep rate, with particular attention on the role of Ti and Hf additions. The aim of the present study is to understand the effects of the substitutional elements on the stability of the silicide phase, ordering in the crystal lattice, including the hP16-tI32 transition, and the creep behavior of the monolithic phases. To pursue this goal monolithic alloys with a range of compositions were prepared and the creep rates were measured at temperatures of 1100-1350oC. The stress sensitivities of the creep rates of the monolithic phases were also determined.
INTRODUCTION Nb-silicide composites combine a ductile Nb phase with high-strength silicides, Laves phases, and T2 niobium borosilicide phases, and they show great promise for future high-temperature structural applications [1-4]. These composites consist of Nb5Si3 and Nb3Si type silicides toughened with a Nb solid solution (substitution of Nb by Ti and Hf is abbreviated by (Nb)). The most recent Nb-silicide based in-situ composites are alloyed with elements such as Cr, Ti, Hf, B and Al. These composites have demonstrated a promising combination of hightemperature strength, creep resistance, and fracture toughness. Alloying elements such as Cr and B improve oxidation resistance, and they stabilize 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 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. The role of substitutional elements on the creep behavior of the tI32, tP32, and hP16 silicides has not been investigated previously. When Nb5Si3 is alloyed with Ti and Hf, the less complex hP16 (Nb)5Si3 structure can also be stabilized [5, 6]. The Laves phases typically have C15, C16, or more complex structures of a hexagonal form. Typically, when the Cr2Nb Laves phased is alloyed with Hf and Ti, higher order hexagonal forms of the Laves phase, such as the C36, are stabilized. The aim of the present study was to determine the creep rates of monolithic intermetallic phases that are used to strengthen Nb-silicide based composites and to develop the constitutive creep laws for these phases. An improved understanding of the creep mechanisms that con
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