Effect of Temperature on the Formation of Stable and Metastable Aluminide Phases in Al‒Zr‒Nb Alloys

Abstract

The conditions of the formation of stable and metastable aluminides Aln(Zr1 – xNbx) formed during the crystallization of superheated melts of the Al‒Zr‒Nb system are considered. Upon a close zirconium content in the alloys of 0.23–0.25 at %, the niobium content varies from 0.05 to 0.21 at %. Alloys are prepared in a resistance furnace at a temperature of 1230°C in an argon atmosphere in graphite crucibles. The alloys are cast into a bronze mold; the cooling rate is estimated as 200°C/s. The structural features, distribution, and morphology; the composition of matrix, dendritic cells, and aluminides; and the structural type of aluminides in the alloys are studied using scanning electron and optical microscopy, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy. It is shown that the growth forms of stable aluminides (D023 structural type) change from faceted to dendritic and primary metastable aluminides (L12 structural type) begin to form; in the process of the Al‒Zr‒Nb melt, overheating was 360–365°C above the liquidus temperature. Exclusively metastable aluminides with both polyhedral and dendritic growth forms are formed in the Al‒Zr‒Nb alloys when the overheating of their melt is 390–395°C (as well as at higher overheating) above the liquidus temperature. The near-peritectic niobium composition of the Al‒Zr‒Nb alloy and the zirconium content (more than seven times higher than peritectic) are crucial factors in the formation of a large fraction of metastable AlnZr aluminides having a cubic L12 structure. It is shown that, in accordance with the rules of isomorphism, Nb replaces Zr at equivalent positions of the aluminides crystal lattice. The intensity of isomorphism of the formed Al4(Zr0.79Nb0.21) aluminides increases with the increase in temperature of the melt overheating.