Development of niobium based intermetallic alloys doped with silicon for low pressure turbine applications

Niobium based refractory intermetallic alloys are considered as having great potentials for high temperature applications based on a good balance of high temperature strength and low-temperature damage tolerance. Moreover this family of alloys also exhibits a high melting point and a low density; hence they are good candidates for low pressure turbine blade applications over the temperature range of 800 – 1000 °C. The aim of this study is to investigate the chemical compositions changes and the Si addition effects on the microstructures in Nb-based alloys. These alloys based on Nb-Ti-Al (42.5 to 55%at Nb, 10 to 15%at Al) were doped with a small amount of Si (0.5 to 2%at) to improve high temperature strength by keeping an acceptable ductility at room temperature.

The samples were prepared by arc-melting starting from pure elements (99.99%).The microsegregations and solidification paths were investigated by using Energy-Dispersive X-ray spectroscopy -mapping on the as-cast and aged materials. The alloys were homogenized and two aging treatments were performed at the target temperatures: 800°C and 1000°C for 168h to investigate the stability of the microstructure at service temperature. The chemical compositions changes, Si addition and heat treatments effects on the microstructures were investigated by using X- Ray Diffraction technique for phase identification. The different transition temperatures were investigated by Differential Thermal Analysis and compared to those calculated by using ThermoCalc®. Scanning electron microscopy techniques have been used to assess the microstructure. Finally the influences on the mechanical properties have been investigated by microhardness.

The results of this study indicate alloy chemical compositions required to avoid the formation of the d-Nb₃Al and α₂-Ti₃Al phases, known to be very brittle at room temperature. The influence of the Si addition on the mechanical properties and on phase transformation in the Nb-Ti-Al system after aging will be presented and discussed.