Effect of scandium on the structure and properties of alloy Al-5.5% Zn-2.0% Mg

Abstract

669.715'793Commercial weldable aluminum alloys additionally containing scandium have recently been developed in Russia: thermally nonhardening alloys based on the system AI-Mg (01570, 01523, 01515), and thermally hardening alloys based on the systems Al-Zn-Mg (01970, 01975) and AI-Mg-Li (01421, 01423 In our previous studies [1, 2] it has been shown that alloying aluminum with scandium is promising. This work is a continuation of these studies.Continuous casting was used to prepare ingots 134 mm in diameter from six melts in which the scandium content was varied from 0 to 0.6%. Aluminum A99 and scandium in the form of an A1-2% Sc master alloy were used. Ingots were homogenized and pressed into strips with a cross section of 3 x 100 mm. Some of the pressed strips were cold rolled with = 73% without intermediate annealing into sheet 0.8 mm thick. A study of the macr.o-and microstructure of ingots showed that with an increase in scandium content to 0.4% the average grain size (day) is almost unchanged in an ingot. Thus, in ingots of alloys containing 0% and 0.4% scandium, dav = 120 and 110/zm, respectively. With a further increase in scandium content there is clear grain refinement. In an ingot with 0.6% Sc the day = 22/zm. Here ingots had a clearly defined nondendrititic structure. This nature of change in grain size in an ingot depending on scandium content is in good agreement with the phase equilibrium diagram for A1-Sc. According to work in [3][4][5] scandium and aluminum form a diagram of the eutectic type with limiting solubility. The eutectic point corresponds to alloys containing about 0.5 % Sc. With solidification of hypoeutectic alloys first there is precipitation of crystals in a solid solution of scandium in aluminum and the modifying effect of scandium is not observed. With solidification of hypereutectic alloys the f'trst to solidify is A13Sc particles which are active centers for the generation of new grains of solid solution.In view of this in hypereutectic alloys (containing more than 0.5% Sc) there is marked ref'mement of cast grains up to formation of a nondendritic structure. The results obtained agree qualitatively with data obtained in [6].A study of the structure of pressed strips and rolled sheet showed that introduction and an increase in the content of scandium promotes a sharp increase in the thermal stability of the unrecrystallized structure (Fig. 1). Alloy without scandium after pressing had an entirely recrystallized structure, i.e. the recrystallization temperature was below the pressing temperature (tde f = 350°C). Introduction of only 0.1% Sc increased the recrystallization temperature to 535 °C (degree of recrystallization 50%), 0.2% increased it to 570°C, and 0.4% Sc increased it to 610°C. Scandium has an even stronger effect on the recrystallization temperature of cold-rolled sheet. An increase in scandium content from 0 to 0.6% increases the recrystallization All-Union Institute of Light Alloys.

Developing Sc and Zr containing Al-B C metal matrix composites for high temperature applications /

“…After several years, Russian scientists, Drits [73,80,81], Elagin and Zakharov [82][83][84][85], started to work on the strengthening effects and the optimization of the Sc content in this system. It was reported that within up to ~0.4 wt.%, each 0.1 wt.% Sc addition produced an average increase in the ultimate strength of 50 MPa [84].…”

Microstructure and properties of cast ingots of Al-Zn-Mg-Cu alloys modified with Sc and Zr

“…The synergistic grain refining effect from Sc and Zr added together has been reported in a number of publications (see, for example [4][5][6][7][8][9][10][11] ) and it has been explained by the presence of primary Al 3 (Sc,Zr) particles, which solidify first and contribute to crystallization in these alloys.…”

“…[1,2] The strengths of these alloys generally increase with the concentrations of Zn and Mg; however the alloys become prone to hot cracking during casting or during rapid cooling from a processing temperature when the amount of Zn exceeds 7-8%. Fortunately, the fracturerelated properties of these alloys can considerably be improved by small additions of Sc [3][4][5] , which enables superior combination of higher strength levels and acceptable ductility [6][7][8] .…”

Dynamic Shear Behaviour of Unweldable Aluminum-Scandium (Al-Sc) Alloy

“…[1][2][3] The room temperature strength of AlSc alloys is much higher than that of other Al alloys on an atom-by-atom basis. Furthermore, at high temperature, scandium is notably different from other common aluminum alloying elements 4) in that it produces very fine, thermodynamically-stable, coherent Al 3 Sc precipitates.…”