Nanocomposites and nanomaterials
Peculiarities of the formation of highly dispersed refractory Zr trialuminides in aluminum alloys
A.L. Berezina1, O.A. Molebnyi1, T.O. Monastyrska1, S.L. Polyvoda2
1 G.V. Kurdyumov Institute for Metal Physics of the NAS of Ukraine. 
Vernadsky Blvd. 36, Kyiv-142, 03680, Ukraine. 
E-mail: moleb@imp.kiev.ua
2 Physico-Technical Institute of Metals and Alloys of the NAS of Ukraine. 
Vernadsky Blvd. 34/1, Kyiv-142, 03680, Ukraine.
It is mostly impossible now to improve aluminum alloys properties by alloying with components that have high solubility in solid aluminum. So the creation of new-generation alloys requires a significant increase in the concentration of low-soluble TM, REM in solid aluminum and development of methods of dispersing their refractory trialuminides. 
Al-3%Zr alloy was chosen as a basic to study the patterns of refractory intermetallic compounds dispersion and formation of the supersaturated solid solutions during crystallization. The changes in the structural state of the basic alloy depending on the casting temperature (700-14000С), cooling rate (101-106deg/sec), and the nature of the target alloying were studied. The technology of intensive mixing of the melt and the casting into a chill mold from a temperature of 7000С provided a crystallization rate of 101deg/sec. Al3Zr intermetallides caused by crystallization were observed in the alloy. They form clusters of large particles with a size of up to 10μm and fine particles of size 2-3μm that uniformly distributed in the matrix with a Zr content of 0.12%.
Zr content in the matrix increases from 0.12 to 2.4% already at a crystallization rate of 103deg/sec with overheating of the melt to 1000°C. In other words an anomalously supersaturated solid solution is formed that is capable of dispersion hardening due to the nano-sized Al3Zr hardening particles precipitation. The use of Sc, Er micro-alloying in an amount of 0.05% impacts clustering processes in the liquid-solid region, completely eliminates the formation of large particles of d10μm, and contributes to the refining of primary intermetallic compounds to d1μk with a significant increase in their precipitation density. 
An increase in the crystallization rate is accompanied with thermal diffusion processes, as a result of which the concentration gradient in Zr is established over the depth of the ribbon. The significant difference exists in the density of intermetallides formation on the contact and free sides of the ribbon. The introduction of 3-4% Mg into the alloy increases the melt viscosity, removes thermal diffusion processes, levels the concentration on the free and contact side and increases the degree of supersaturation of the solid solution.