Microstructural properties of Al-containing refractory high entropy alloys for high temperature applicationsWednesday (26.09.2018) 16:30 - 16:45 S1/01 - A5 Part of:
High entropy alloys within the Ta-Nb-Mo-Cr-Ti-Al system are expected to possess promising properties, such as high creep and oxidation resistance as well as outstanding specific mechanical properties due to comparably low density. However, the arc-melted and subsequently homogenized alloys exhibit a lack of ductility in a temperature range between room temperature up to 400 to 600 °C depending on the alloy composition. To reveal the cause for this observed intrinsic brittleness, we present a comprehensive microstructural investigation of a series of Ta-Nb-Mo-Cr-Ti-Al derivatives obtained by complementary characterization techniques. Backscattered electron imaging, energy dispersive X-ray spectroscopy and atom probe tomography were used to proof the uniform element distribution after homogenization. X-ray diffraction and transmission electron microscopy indicate a B2 ordered crystal structure from a disorder-order phase transformation during cooling revealed by the appearance of thermal antiphase domain boundaries. An approach to determine the site occupation of the multiple principal elements on the lattice sites of the ordered crystal structure is proposed. Thus, we estimate the extent of ordering and its contribution to the room temperature brittleness. Alloy development strategies to increase the ductility are suggested and evaluated in terms of mechanical properties characterization.