Ultrahigh-temperature Nb_(ss)/Nb_5Si_3 fully-lamellar microstructure developed by directional solidification in OFZ furnace

摘要

Directional solidification in an optical floating zone furnace was employed to develop a fully-lamellar Nb_(ss)/ Nb_5Si_3 structure for ingots having a composition of Nb-17Si-10Mo-3Al. It was determined that β-Nb_5Si_3 is a faceted phase through the estimation of the entropy of melting and that the Nb_(ss)/β-Nb_5Si_3 eutectic has a non-faceted/faceted structure, in which the microstructure is thought to be sensitive to growth conditions such as growth rate and rod-rotation speed. In the absence of the rotation speed of the seed rod, a fully-lamellar Nb_(ss)/β-Nb_5Si_3 structure was obtained for growth rates below f 0 mm/h. The relationship between lamellar spacing and growth rate was in agreement with the Jackson—Hunt theory with λ~2R = 170 μm~2 mm/h in this alloy. Electron backscatter diffraction analysis on both Nb-17Si-10Mo-3Al alloy and Nb-18Si—10Mo alloys revealed that the crystallographic orientation relationship in the as-grown Nb_(ss)/β-Nb_5Si_3 lamellae was {112}_(Nb_(ss))//{100}_(β-Nb_5Si_3).and (113)_(Nb_(ss)//[001]_(β-Nb_5Si_3). No preferred crystallographic orientation relationship was identified between either β-Nb_5Si_3 or Nb_(ss) and α-Nb_5Si_3, but the Nb_(ss)/α-Nb_5Si_3 lamellar structure exhibited good stability at high temperature. It was inferred that one β-Nb_5Si_3 may nucleate with several Nb_(ss) nucleation supposedly at the interface between the liquid and β-Nb_5Si_3.