Effect of temperature on the formation of stable and metastable aluminide phases in the Al-Zr-Nb alloys

Abstract

The paper considers formation conditions for stable and metastable aluminides Al n (Zr 1-x Nb x ) obtained in the crystallization of superheated Al—Zr—Nb melts. At the close zirconium content in the alloys of 0.23—0.25 at.%, the niobium content varied from 0.05 to 0.21 at.%. Alloys were prepared in a resistance furnace at 1230 °C in an argon atmosphere in graphite crucibles. Alloys were cast into a bronze mold where the cooling rate was estimated at 200 °C per second. The structural features, distribution pattern, morphology, the composition of the matrix, dendritic cells and aluminides, as well as the structural type of intermetallics in the investigated alloys were studied using scanning electron and optical microscopy, X -ray phase analysis, X -ray diffraction analysis, electron microprobe analysis including inductively coupled plasma atomic emission spectroscopy. It was shown that the growth forms of stable aluminides (D0 23 structural type) change from the faceted to dendritic one, and primary metastable aluminides ( L 1 2 structural type) begin to form during the Al—Zr—Nb melt overheating at 360—365 °C above the liquidus temperature. Only metastable aluminides with both polyhedral and dendritic growth forms are formed in the alloys when overheated by 390 —395 °C and higher. The near-peritectic niobium composition of the Al—Zr—Nb alloy (at the zirconium content more than seven times higher than the peritectic one) are crucial factors in the formation of a large fraction of metastable Al n Zr aluminides having a cubic L 1 2 structure. It was shown that, in accordance with isomorphism rules, Nb replaces Zr at equivalent positions of the crystal lattice of aluminides. The intensity of isomorphism of the Al 4 (Zr 0.79 Nb 0.21 ) intermetallics formed increases with the increasing melt overheating temperature.