Nanocomposites and nanomaterials
Use of multistage aging treatments to increase the thermal stability of rapidly quenched alloys of Al-Mg-Sc-Zr system alloyed with Hf and Cr
A.L. Berezina, O.A. Molebnyi, T.O. Monastyrska, V.M. Shyvaniuk

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
Experimental alloys of the AlMgScZr system additionally alloyed with Hf and Cr, were obtained by the melt-spinning method in the form of rapidly quenched ribbons. Structural studies of the alloys were carried out by metallographic, X-ray and electron-microscopic methods. In order to determine heat-resistance of the alloys, the decomposition degree of the experimental alloys was studied using the resistometric method and the method the microhardness measuring after various heat treatments. 
For all rapidly quenched ribbons of AlMgScZr-Hf-Cr alloys, the change in the temperature coefficient of electrical resistance Т=1/R0·dR/dT was investigated during continuous heating at the rate of 30/min in the range from room temperature to 5500С. The curves Т=F(T) were obtained in the initial state, after long-term natural aging (n.a.) during 1 year and after following heat treatments: n.a. + 3000С×10 hours; n.a. + 3000С×10 hours  + 4000С×2 hours; n.a. + 3000С×10 hours  + 4000С×10 hours.
The temperature coefficient of electrical resistance Т when there are no any phase transformations in an alloy is known as а constant. During the aging, the formation of phases is accompanied by depletion of the solid solution and by the appearance of minima on the curve Т=F(T), whose position and depth allow us to determine decomposition degree and the temperature intervals of aging of supersaturated solid solutions.
Two-stage heat treatment of ribbons undergone 3000С×10 hours + 4000С×10 hours heat treatment makes possible to increase the hardness of AlMgScZr-Hf-Cr alloys up to 50% in comparison with isothermal aging at 4000С for 10 hours. Additional alloying with Cr not only increases the strength properties of the alloys, but also increases their thermal stability. If Hf or Cr is added to the alloys of the AlMgScZr system, the temperature range of phase transformations at alloying by Hf does not shift, but mainly widen to higher temperatures, and Cr effect results to their shift on almost 70°C toward higher temperatures.