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

The conditions of the formation of stable and metastable aluminides Al n (Zr 1 – x Nb x ) 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 ( D 0 23 structural type) change from faceted to dendritic and primary metastable aluminides ( L 1 2 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 Al n Zr aluminides having a cubic L 1 2 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 Al 4 (Zr 0.79 Nb 0.21 ) aluminides increases with the increase in temperature of the melt overheating.