Tin Addition to Improve the Oxidation Behaviour of NbSs/Nb5Si3 Based In Situ Composite

This work focuses on the effect of tin additions (2, 5 and 8%) to the MASC alloy (Nb-25Ti-8Hf-2Cr-2Al-18Si) on the microstructure and the oxidation behaviour at 815°C in air. The alloys are mainly constituted of a niobium solid solution plus the () Nb5Si3 silicides. For the higher Sn additions (5 and 8%), a fourth phase is evidenced: it is enriched in Sn and has a crystal structure close to Nb3Sn. The oxidation resistance of these alloys is clearly improved by tin additions: the oxygen inward diffusion is hindered and consequently the fragmentation of the silicides is avoided. Cracks in silicides are no longer observed for the MASC containing 8%Sn. This effect is not attributed to a better efficiency of the oxide scales but rather to the reduction of the niobium solid solution fraction with tin additions.

Effect of Isothermal Forging on Microstructures and Mechanical Properties of Nb-Si In Situ Composite

Nb-Si based in-situ composites have great potential for the application of high temperature structure components. In this paper, the influence of microstructure on the compression behavior of Nb-Si in-situ composite forged at high temperature was studied. The alloy with nominal composition of Nb-12Si-24Ti-4Cr-4Al-2Hf was consumable arc-melted, and then isothermal forged at 960°C. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were used to characterize the microstructure, composition and phases. The compressive behaviors at 1250°C were carried out by Gleeble thermo-mechanical simulator. The results showed that the microstructures were inhomogenous in the different sections of the ingot, and no evident directional texture formed, and the main phases were niobium solid solution, Nb5Si3 and Ti5Si3. However, no macro-elements segregation in the ingot was observed. The compression strength was in the range of 140-360MPa. BSE observation showed that irregular-shaped Nb5Si3 and Ti5Si3 phases distributed in Nb solid solution and the size of Nb5Si3 in three tested samples was 10μm. Large size of eutectoid texture existed in the sample with strength of ~140MPa. On the contrary, in the sample with higher strength of 360MPa, eutectoid structures were hardly detected. The results suggested that the strength decreased gradually with size increase of eutectoid structure.

High Temperature Properties of Refractory Intermetallics

On the basis of creep strength, ultimate tensile strength and oxidation resistance, seven intermetallic compounds with melting temperatures above 1600°C have been selected as possible candidate materials for high temperature structural applications in advanced aero-turbines. These compounds, Nb3Al, Cr3Si, Co2Nb, Cr2Nb, MoSi2, Mo5Si3 and Nb2Al, have been evaluated and their properties reported herein. All seven of the compounds displayed excellent creep resistance at 1200°C with Mo5Si3 and Nb2Al being the strongest. Nb3Al, with the precipitation of the niobium solid solution displayed the greatest low temperature toughness. The greatest ultimate tensile strengths were observed for Co2Nb and MoSi2, while MoSi2 was by far the most oxidation resistant.