Effects of Cr and Hf additions on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-3Al-2B-xHf-yCr ((x, y)=(0, 0), (0, 5), (4, 0) and (4, 5)) (at%) were prepared by vacuum non-consumable arc melting. The effects of Cr and Hf additions on the phase selection, microstructure, room-temperature fracture toughness and oxidation resistance at 1250 degrees C of the alloys have been investigated. The results show that all the four alloys are comprised of primary suicides and Nbss dendrites together with one or two types of eutectic. The Cr addition does not change the crystal structures of 5-3 suicides but imposes a significant influence on the amount, size and morphology of the constituent phases. The Hf addition suppresses the formation of alpha(Nb,X)(5)Si-3 but promotes the formation of gamma(Nb,X)(5)Si-3. The combined additions of Cr and Hf also cause the formation of Cr-2(Nb,X) containing three-phase eutectic. The room-temperature fracture toughness of the alloys decreases with Cr addition while increases with Hf addition. The oxidation-resistant performance of the alloys has been ameliorated by Cr addition, while it has been significantly degraded by Hf addition in the absence of Cr. Furthermore, the formation mechanism of the scale has been illustrated. (C) 2015 Elsevier B.V. All rights reserved.