Abstract
Materials processed by additive manufacturing often exhibit a very fine-scaled microstructures due to high cooling rates in the process. In this study, single-layer surface electron beam melting is used to create very high cooling rates similar to additive manufacturing processes to investigate the resulting microstructure. In the case of Nb-Si-Cr in-situ composites, a nano-scaled eutectic microstructure is beneficial for improving the mechanical and oxidational properties. Fast solidification results in the formation of supersaturated phases of Nbss and Cr2Nb with phase diameters down to 10 nm as well as in the stabilization of the metastable Nb9(Cr,Si)5 phase at room temperature. After processing with different solidification rates, the decomposition of the Nb9(Cr,Si)5 phase has been studied in detail with atom probe microscopy. The stabilization of mixed silicide phases by electron beam melting shows a new pathway for improving hardness and enhancing oxidation resistance of nanostructured eutectic in-situ composites, by which the inherent weaknesses of Nb-Si-Cr can be overcome without further alloying elements.Graphical
Highlights
IN order to build more efficient engines and gas turbines, high-temperature structural materials with increased maximum service temperature are required
There is a controversial discussion about the existence of an equilibrium state of Nb9CrxSiy phases that are stable at room temperature,[4,13,15,26,27] especially due to the coexistence of this phase with an unknown mixed silicide phase with an Nb content of about 54 at. pct.[4,10]
This study aims towards the creation of a nanostructured and aligned eutectic Nb-10.9Si-28.4Cr in-situ composite like it has been done previously for NiAl-Cr(Mo) in-situ composites.[30]
Summary
IN order to build more efficient engines and gas turbines, high-temperature structural materials with increased maximum service temperature are required. The microstructure is formed by solidification of a hypoeutectic alloy with a eutectic phase transition (L fi (Nb) + Nb3Si) at 1920 °C followed by a eutectoid reaction (Nb3Si fi (Nb) + Nb5Si3) at 1770 °C.[2,3] The high melting point above 2400°C and their light weight with a density of approximately 7 g/cm are beneficial for the application at high temperatures.[1,4,5,6] the low fracture toughness of Nb5Si3 of around 3 MPam1/2 at room temperature and poor oxidation resistance of the Nb-rich solid solution limit their application.[7,8,9] Many researchers have investigated the influence of different alloying elements like Ti, Cr, Al, Hf, Mo, W, and V in ternary, quaternary, or even more complex alloys to improve these properties.[1,2,4,8,10,11,12,13,14,15,16] The addition of Cr to the Nb-Si system is a promising approach, leading to a ternary eutectic at Nb-10.9Si-28.4Cr
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