ON THE OXIDATION BEHAVIOUR OF Nb-Nb_5Si_3 ALLOYS-EFFECT OF TIN ADDITIONS

摘要

High temperature applications such as turbine engines in the aeronautic industry require new materials able to operate above 1200°C. Indeed, studies [1,2] have reported that an increase of the operating temperature should increase the efficiency of the turbines and consequently decrease the earth gas emission. These materials should have high mechanical properties and be stable in oxidising environments at high temperature [3]. To date, Ni based superalloys are the reference up to 1150°C. Above this temperature these superalloys lose their mechanical properties and become unusable because of the vicinity of their melting point. Metallic systems based on refractory metals are considered to reach higher operating temperatures. The investigations are focused on niobium and molybdenum alloys which seem to be good candidates for replacing or complementing nickel based superalloys. Niobium alloys are considered as a real potential material allowing the use of the refractory alloys in high temperature applications. High mechanical properties have been achieved with the developments of Nb_ss/Nb_5Si_3 based in-situ composites with toughness greater than 20MPa.m1/2 [4] at room temperature. However, their oxidation resistance remains to low to envisage their use at the industrial scale. The main drawback consists in the high reactivity of the niobium solid solution with oxygen that induces catastrophic oxidation around 800°C. At this temperature Nb-Nb5Si3 alloys suffer from the pest phenomenon consisting in the rapid fragmentation of the pieces. For this study, niobium alloys based on the composition: Nb-25Ti-8Hf-2Cr-2Al-16Si (at.%), corresponding to the MASC alloy, are prepared without and with tin additions from 2 to 8at.%. The MASC alloy will be taken as reference for the microstructure and oxidation results. The effect of the addition of tin on the microstructure of niobium alloys is briefly described regarding to the microstructure of the free tin alloy; this part of the work is more detailed in [5]. Then, the work focuses on the characterisation of the oxidation behaviour of these alloys by performing thermogravimetric measurements at 815, 1100 and 1200°C and post-oxidation observations (DRX, SEM, EPMA).