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 Nb_5Si_3 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 Nb_5Si_3 grain size decreases.