Role of Small Addition of Sc and Zr in Clustering and Precipitation Phenomena Induced in AA7075

Abstract

A detailed characterization of phase transformations in the heat-treated commercial 7075 aluminum alloys without/with low Sc–Zr addition was carried out. Mechanical and electrical properties, thermal and corrosion behavior were compared to the microstructure development. The eutectic phase consists of four parts: MgZn2 phase, Al2CuMg phase (S-phase), Al2Zn3Mg3 phase (T-phase), and primary λ-Al(Mn,Fe,Si) phase. Strengthening during non-isothermal (isochronal) annealing is caused by a combination of formation of the GP zones, η’-phase, T-phase and co-presence of the primary and secondary Al3(Sc,Zr)-phase particles. Positive influence on corrosion properties is owing to the addition of Sc–Zr. Positron annihilation showed an evolution of solute Zn,Mg-(co-)clusters into (precursors of) the GP zones in the course of natural ageing. The concentration of the (co-)clusters is slightly negatively affected by the low Sc–Zr addition. A combination of both precipitation sequences of the Al–Zn–Mg–Cu-based system was observed. The apparent activation energy values for dissolution/formation of the clusters/GP zones and for formation of the metastable η’-phase, stable T-phase and stable η-phase were calculated.

Effects of B Addition on the Microstructure and Microhardness of Melt-Spun Al-7075 Alloy

“…In addition to the precipitation hardening, other different mechanisms such as cold deformation [22][23][24], grain boundary hardening [6,25], semi-solid casting [7, 26,27] can also contribute to the mechanical strength. Along with improving the mechanical strength with the aforementioned mechanisms, various researchers are working on adding various impurity elements and/or using new production methods such as rapid solidification techniques in order to modify the microstructure of the Al alloys [9,15,[35][36][37]16,[28][29][30][31][32][33][34]. Various impurity elements as an inoculant agent are used to provide grain refinement and obtain a microstructure having homogeneous distribution and minimum segregation in the Al alloys [38].…”

“…They concluded that Al3(Sc, Zr) intermetallic particles, which promote grain refinement in the microstructure with the Sc and the Zr addition, play a role in heterogeneous nucleation [34]. In recent years, the rapid solidification techniques are also of great interest owing to their ability to modify the microstructure of the alloys significantly and furthermore to achieve superior mechanical properties with their ability to form amorphous microstructures [37]. Thanks to the rapid solidification processes, the grain refinement can be effectively triggered by obtaining high-density nuclei in the microstructure of the alloys [4,42,43].…”

“…High solidification rates increase solute concentrations, by suppressing microhardness of melt-spun Al-7075 alloy their atomic motion [33]. When existing studies in the literature are examined, it is striking that the melt spinning method is used in the production of various aluminum-based alloys [4,37,[45][46][47][48][49]. The melt spinning process offers a number of beneficial aspects for the Al alloys, such as increased dispersoid fraction due to higher solute atom content [15,50], finer grain size [50], shorter distance between adjacent precipitate particles and dislocations during plastic deformation [51], slightly improved precipitate nucleation caused by deformation-induced faults [52].…”

Flow Stress Modelling and 3D Processing Maps of Al4.5Zn4.5Mg1Cu0.12Zr Alloy with Different Scandium Contents

“…A Zn/Mg ratio of ~1.0 or less [6] and a (Zn+Cu)/Mg ratio below 1.5 [7] provides a better combination of mechanical, corrosion and technological properties. Moreover, it is well known that minor additions of transition metals, such as Sc and Zr, to Al-Zn-Mg system-based alloys can improve mechanical and corrosion resistance properties [8][9][10][11][12]. Coherent Al 3 (Sc,Zr) dispersoids formed during ageing from an aluminium solid solution (after rapid solidification) with an L1 2 -structure refine the microstructure and increase the recrystallisation temperature; therefore, the microstructure remains nonrecrystallised and mechanical properties remain strong up to elevated temperatures [13][14][15][16][17].…”

Mechanical and electrical properties of cast Al–Er–Zr alloy

“…Further addition of a trace amount of Zr leads to formation of ternary L12-Al3(Sc,Zr) precipitates. These ternary particles have both thermodynamic and kinetic tendencies to form a core-shell structure [3][4][5][6]. The resulting Zr-rich shell can retard the fast diffusing Sc and therefore impede the coarsening of the precipitates [3,5,7].…”