Ultrahigh-temperature Nbss/Nb5Si3 fully-lamellar microstructure developed by directional solidification in OFZ furnace

Directional solidification in an optical floating zone furnace was employed to develop a fully-lamellar Nbss/Nb5Si3 structure for ingots having a composition of Nb–17Si–10Mo–3Al. It was determined that β-Nb5Si3 is a faceted phase through the estimation of the entropy of melting and that the Nbss/β-Nb5Si3 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 Nbss/β-Nb5Si3 structure was obtained for growth rates below 10 mm/h. The relationship between lamellar spacing and growth rate was in agreement with the Jackson–Hunt theory with λ2R = 170 μm2 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 Nbss/β-Nb5Si3 lamellae was {112}Nbss//{100}β-Nb5Si3.and〈113〉Nbss//[001]β-Nb5Si3. No preferred crystallographic orientation relationship was identified between either β-Nb5Si3 or Nbss and α-Nb5Si3, but the Nbss/α-Nb5Si3 lamellar structure exhibited good stability at high temperature. It was inferred that one β-Nb5Si3 may nucleate with several Nbss nucleation supposedly at the interface between the liquid and β-Nb5Si3.

Typical microstructure of as-grown alloys for different RSR with a growth rate of 5 mm/h: (e)(f) 10–0 r/min; (e) longitudinal section; (f) transverse section. Figure optionsDownload as PowerPoint slideValues of the lamellar spacing (λ) against the reciprocal of square root of growth rate (R−1/2).Research highlights
► A fully-lamellar Nbss/Nb5Si3 structure was obtained only when the seed rod was not rotated.
► The relationship between the lamellar spacing and the growth rate was found to be in agreement with the Jackson-Hunt theory.
► The crystallographic OR between the as-grown Nbss and β-Nb5Si3 lamellae is {112}Nbss//{100}β-Nb5Si3.and〈113〉Nbss//[001]β-Nb5Si3.
► No preferred OR was identified between either β-Nb5Si3 or Nbss and α-Nb5Si3.
►  The Nbss/α-Nb5Si3 lamellar structure was found to possess good stability at high temperature.