Effect of Mg addition on microstructural, mechanical and environmental properties of Nb-Si eutectic composite

The paper reports the effect of addition of small amount of Mg on the mechanical and oxidation properties of Nb-Nb3Si eutectic composites in Nb-Si system under the condition of suction casting. Mg addition increases the volume fraction of primary dendrites of Nb solid solution. This phase contains significant amount of strengthening precipitates. Two different precipitates are identified. The large plate shaped precipitates are that of hcp phase, while fine coherent precipitates have the structure similar to recently identified δ-Nb11Si2 phase. The Mg addition improves both the strength and ductility of the composite at room temperature (∼1.4 GPa and ∼5% engineering strain) as well as at 700 °C(∼1.2 GPa and ∼7% engineering strain). The presence of Mg results in a complex barrier layer which significantly increases the oxidation resistance up to a temperature of at least 1000 °C.