Development of Nb-Si-Ti-Al-Cr-X alloys for high temperature aeroengine applications

Advanced intermetallic materials, in particular the refractory Nb-based silicides, possess a good combination of properties like high strength at elevated temperatures, high stiffness, low density, etc., which make them potential candidate materials for high temperature applications. However, materials based on the binary Nb-Si composition exhibit low room temperature ductility and poor oxidation at elevated temperature. A multi element approach, which produces these silicides in equilibrium with the soft β-matrix, appears to provide a solution to the aforementioned problems. The goal of the present work is to develop new alloys that have the potential to meet the property requirements for applications between 800-1000 deg C. The present work is about the microstructures, oxidation behavior and mechanical properties of as-cast and heat treated Nb-Si-Ti-Cr-Al based multi component system. The microstructural investigation has revealed the presence of four distinct phases, namely, the matrix β phase, the Cr2(Nb,Ti) Laves phase and Nb- and Ti- 5-3 silicides, in these alloys. The morphological distribution, structure and chemical composition of these phases have been investigated. The β phase is observed to display B2 ordering. A strong solute partitioning tendency among the various elements has been noticed. Dissolution and re-precipitation of the Laves phase, which has been encountered during various heat treatments, has been studied in detail and characterized in terms of elemental distribution in the phase and mechanistic aspects. The results reveal that additional strengthening of the β matrix can be achieved by precipitation of the Laves phase. Mechanical properties of these alloys have also been evaluated which show interesting results. In addition, materials have been thermo-mechanically processed and microstructural changes evaluated. Oxidation experiments at two different temperatures (650 deg C and 980 deg C) under cyclic as well as static conditions have been carried out. The thickness of the oxide layer has been measured as a function of time at these temperatures. The chemical composition and in depth profile analysis of the oxide layer have shown presence of different oxides scales. Cross sectional examination under electron microscope of oxide samples has revealed the presence of cracks in the oxide layer and deep penetration of oxygen in to the matrix phase. Elemental mapping of these oxides have clearly revealed that many elements, like Cr, Si do not constitute substantially into the oxide layer. Based on this study, two new alloys are being tried to explore any possibility for commercial applications. (author)