CRYSTALLOGRAPHIC ORIENTATION EVOLUTION IN NB_(SS)-NB_5SI_3 EUTECTIC ALLOYS BY EBSD ANALYSES

摘要

The progresses in high temperature materials encourage the development of turbine engine in terms of thrust and efficiency. Ni-based superalloys, which are predominant in elevated temperature application, have limited potential to raise serving temperature. In-situ composites, such as Cr-Cr_3Si, NiAl-Cr and Nb-Nb_5Si_3 eutectic alloys, consisting of a ductile metallic phase and a hard intermetallic phase, are attractive candidates to replace Ni-based superalloys. The microstructure and mechanical properties of these in-situ composites are widely investigated. However, little work is focused on crystallography of in-situ composites, except for preferred growth direction and crystallographic orientation relationship. In this paper, Nb-Si-Mo-based alloys were fabricated by non-consumable arc melting, and then were directionally solidified in an optical floating zone (OFZ) melting furnace. The crystallographic orientation evolutions in Nb-Nb_5Si_3 eutectic alloy are studied by electron back-scattered diffraction (EBSD) analyses. First, the effect of solidification condition on crystallographic orientation is examined. The as-cast alloy displays cellular microstructure. The Nb phase shows different crystallographic orientations in different cells, while the Nb_5Si_3 phase shows similar crystallographic orientation in a number of cells. In directionally solidified alloys, when growth rate is 5mm/h without seed rod rotation, the grain sizes of Nb and Nb5Si3 are both several millimeter. As growth rate rises or seed rod rotates, the grain size of Nb decreases much more drastically than that of Nb_5Si_3. Thus, solidification condition is supposed to influence nucleation of the Nb phase rather than the Nb_5Si_3 phase. Second, the effect of annealing on crystallographic orientation is studied. The Nb_5Si_3 has three allotropic phases. The allotropic phase transformations occur through annealing, during which the Nb5Si3 grain size decreases.