Institute of Metallurgy of Ural Branch of Russian Academy of Sciences, Ekaterinburg, Russia:

E. A. Popova, Senior Researcher of Laboratory of Physical Chemistry of Metallurgical Melts, e-mail: po.elvira@gmail.com
P. V. Kotenkov, Researcher of Laboratory of Physical Chemistry of Metallurgical Melts, e-mail: p.kotenkoff@yandex.ru
A. B. Shubin, Head of Laboratory of Physical Chemistry of Metallurgical Melts, e-mail: fortran@list.ru

The aim of our investigation is producing of the new master alloys based on Al – Hf – Ti system including meta-stable aluminides with the cubic lattice of L1₂ structure type identical to the aluminum matrix. The preparing of ligature alloys was carried out in the carbon resistance furnace. Melts overheated on about 100–400 degrees higher than liquidus were cast into a bronze mold. The crystallization speed was about 10^{3 o}C/s. High speed of crystallization of Al-transition metal melts leads to the appearing of internal tensions and then to the formation of primary metastable aluminides which are thermodynamically non-stable phases. Under the studied conditions we can observe different aluminide growth forms: cuboid, petal, cross-like and dendritic. The internal structure of primary crystals shows gradually increasing of titanium and hafnium concentration from the edge of aluminide crystal to its center. The Ti and Hf content in the formed Al_n(Hf_хTi_1–х) meta-stable aluminides is proportional to their content in alloys in spite of the partial solubility of addition metals in the α-Al dendritic cells. The 2.3 times increasing of hafnium content in alloy leads to the similar increasing of Hf fraction x in the aluminides (about 2.1 times, from 0.31 to 0.66). At the same time the lattice parameter mismatch between the aluminide lattice and aluminum matrix lattice significantly decreases. The master alloy with such nucleating phase will provide a greater effect of grain refinement. In the crystallization of superheated melts of the Al – Hf – Ti system, the degree of supersaturation of the aluminum solid solution by hafnium is almost twice as high as that for the binary Al – Hf system and with less overheating. This indicates the possibility of obtaining a larger volume fraction of secondary Al₃(Hf_хTi_1–х) aluminides upon doping the dispersively hardened aluminum alloys with small additives of Hf + Ti.

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