OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Introduction Aluminum alloys are widely used in various industries due to its high corrosion resistance, weldability, and low density [1–5]. In particular, in the aerospace industry, Al-Mg alloys, known in foreign literature as 5XXX series of alloys, are one of the most common groups of aluminum alloys [6, 7]. These alloys are in high demand because the addition of magnesium enhances its mechanical properties through solid solution strengthening [8, 9]. The addition of scandium further improves its mechanical properties [10–12]. Scandium has low solubility in supersaturated aluminum solid solution, with a solubility of 0.35 % at equilibrium conditions and 655 °C [13]. However, if the cooling rate is high enough after casting, excess scandium can be dissolved in the aluminum matrix. Heating the alloy between 250 °C and 350°C causes the supersaturated solid solution to decompose and leads to the precipitation of Al3Sc, which has a spherical morphology with a radius ranging from 2 to 20 nm [14–16]. These particles have an L12-type lattice and minimal mismatches with the aluminum matrix, which ensures its coherence. Such nanoparticles provide strengthening, which occurs due to particles intercepting by dislocations. The strengthening eff ect is based on the Orowan mechanism when the nanoparticle sizes range from 1.5 to 4 nm [17–19]. Moreover, scandium is a potent structure modifi er, and its refi nement capability is due to the L12 structure of primary intermetallic compound Al3Sc formed in the liquid phase and the minimal mismatch between crystalline lattice and aluminum solid solution [13, 14]. It is worth noting that the modifying eff ect appears only when the scandium concentration reaches 0.6%, when primary Al3Sc particles begin to form in the liquid [14]. However, as the temperature rises to 400 °C, scandium nanoparticles formed during solid solution decomposition start coagulating and increasing in size. When particle reaches a critical diameter of 30–40 nm, it loses its coherence, and the strengthening eff ect disappears [16]. This is a signifi cant limit for the use of scandium alloy. For example, it reduces the temperature of the homogenization and hot deformation processes, which inevitably aff ects its effi ciency and leads to higher energy costs [20]. Minor zirconium additions are used to improve the thermal stability of Al3Sc-type nanoparticles [21]. Zirconium can form a shell around Al3Sc particles as it is partially soluble in it. This shell inhibits the growth of Al3Sc nanoparticles at elevated temperatures as zirconium has a lower diff usion coeffi cient than scandium [22]. Additionally, zirconium reduces scandium concentration, which is needed to form primary Al3Sc intermetallic compounds in the liquid phase, contributing to the as-cast structure modifi cation [23, 24]. One of the classic aluminum alloys with a high Mg content and Sc and Zr additives, successfully used in industry, is 1570 alloy [25, 26]. However, even with the presence of zirconium, Al3Sc particles still do not have suffi cient thermal stability to retain its size during high-temperature homogenization and further hot deformation [20]. One way to solve this problem is to add hafnium to the 1570 alloy. Hafnium has an even lower diff usion coeffi cient than zirconium [22] and partially dissolves in Al3Sc particles [27], creating thermal stabilizing shells around it [22]. The joint addition of hafnium and zirconium is highly eff ective for thermal stabilization of Al3Sc particles [28, 29]. The eff ect of combined hafnium and zirconium additions on the thermal stabilization of Al3Sc particles has been mainly studied for lean aluminum alloys, but aluminum alloys with a high Mg content have several specifi c features. Firstly, magnesium slightly accelerates the decomposition kinetics of aluminum solid solution supersaturated with scandium [30]. Secondly, it stimulates an increase in the critical size of nanoparticles, after which its coherence is lost [13, 31]. Therefore, studying the hafnium eff ect on Al3Sc particles in commercial aluminum alloys with a high Mg content is of utmost interest. The eff ect of adding 0.5 % hafnium to the 1570 alloy was studied in the as-cast state. It was found that 0.5 % hafnium addition stimulates as-cast structure modifi cation and leads to the complete termination of discontinuous decomposition of aluminum solution supersaturated with scandium [32, 33]. Discontinuous decomposition during ingot cooling down is a negative process when Al3Sc needle-shaped precipitates are formed [34–36]. Such particles are usually semi-coherent to the aluminum matrix and do not contribute signifi cantly to strengthening compared to equiaxed dispersed phases formed during heat treatment. After discontinuous decomposition, the aluminum supersaturated solid solution contains no scandium, which is needed for Al3Sc nanoparticle formation during subsequent thermomechanical treatment [12, 34].
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