The effect of Nb₃Si precipitates on the deformation of the primary Nb phase in Nb-10 a/o Si in-situ composite

Intermetallic compounds based on refractory metals such as Nb, Mo and W have high melting points (1700"C or greater) and tend to maintain their strength at high temperature (> 1000"C). These properties are extremely attractive for high temperature structural applications such as in jet engines. Unfortunately, a low room temperature fracture toughness and low oxidation resistance impedes direct application of these materials. The toughness of these compounds can be substantially increased by forming In-situ composites consisting of an intermetallic matrix and a dispersed metallic phase. The ductile ligaments of the metallic phase will bridge cracks in the brittle matrix producing an increase in fracture toughness [1]. Since the ductility of the metallic phase contributes toughness to the composite, the mechanical behavior of this phase is extremely important. Composites based on the Nb rich side of the Nb-Si system can be used as a model system to predict the behavior of ductile phase toughened composites [2].

Upon solidification at normal cooling rates, the microstructure of the Nb - 10 a/o Si alloy consists of primary Nb dendrites, containing Nb ₃ Si precipitates in a continuous Nb ₃ Si matrix [3]. The matrix consists of Nb ₃ Si with dispersed eutectic Nb particles. Nb ₃ Si, which is the dark phase in Figure 1, is a metastable phase and is observed at room temperature due to the sluggish nature of the Nb ₃ Si --> Nb ₅ Si ₃ + Nb eutectoid reaction [4]. The solubility of silicon in niobium is at a maximum (≈3.5 a/o) at the eutectic temperature (1920"C) and decreases dramatically with lowering temperature [5]. The dissolved silicon is almost completely rejected by Nb during cooling to form Nb ₃ Si precipitates in the primary Nb phase [3,5]. The presence of stable precipitates has been shown to affect recrystallization of Nb, and reduce the ductility [6]. Previous studies have shown that the Nb phase in Nb - Nb ₅ Si ₃ composite can undergo both ductile and cleavage failure, and brittle failure was-observed in regions with higher silicon content, as detected by EDS [7]. One can, therefore, conclude that the presence of niobium silicide precipitates affects the ductility of the Nb phase. In order to clearly understand the mechanical behavior of ductile phase toughened composites, the deformation behavior of the metallic phase must be investigated. The objective of this study is to determine the effect of the Nb ₃ Si precipitates on the primary Nb phase, and find the mechanisms which limit high temperature deformation and subsequent recovery.