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

Martin Vlach,Volkmar-Dirk Neubert,Miroslav Cieslar,Tomáš Kekule,Lucia Bajtošová,Sebastian Zikmund,Michal Leibner,Eva Cernoskova,Petr Kutalek,Veronika Kodetova,Vladimir Sima,Volkmar Neubert,Hana Kudrnová,František Lukáč,Jakub Cizek

doi:10.3390/met11010008

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.

Highlights

The heat-treated commercial aluminum 7xxx series (Al–Zn–Mg–Cu-based) alloys extensively appear in various applications, including metalworking, automotive, aircraft, and space engineering thanks to their precipitation-hardening reaction and light weight [1,2,3,4,5]
In view of results obtained by X-ray diffraction (XRD), it can be said that the content of primary Mn,Fe,Si-containing particles observed by transmission electron microscopy (TEM) may be relatively small and inhomogeneous
Parallel to Positron annihilation spectroscopy (PAS) observations in the Al–Mg–Si-based [49,50,51] and Al–Zn–Mg–Cu-based alloys [19,20] one can conclude that immediately after high temperature treatment the studied alloys contain vacancies associated with single and/or multiple Zn- or Mg-solutes and/or Zn,Mg(-co)-clusters developed during quenching

Summary

Introduction

The heat-treated commercial aluminum 7xxx series (Al–Zn–Mg–Cu-based) alloys extensively appear in various applications, including metalworking, automotive, aircraft, and space engineering thanks to their precipitation-hardening reaction and light weight [1,2,3,4,5]. The Sc and Zr content can help to refine and to affect the microstructure and the recrystallization temperature and the materials in a non-recrystallized state improve their mechanical properties and resistance to corrosion [1,15,40,41,42] It has recently been found in the commercial Al alloys with low Sc–Zr addition that the inter-metallic particles of primary Al3(Sc,Zr) phase have a multi-layered structure with many different shapes (dimension about 1–3 μm) and they appear inside (in the center of) the grains in principle [15,20]. Due to the fact that this is a study of a cast heat-treated (“pseudo-homogenised”) aluminum 7075 (-Sc–Zr)-based alloy (i.e., without thermo-mechanical processing such as hot rolling or cold rolling), the research results are relevant to the use of the alloys as shape castings or perhaps more significantly for additive manufacturing the solidification conditions would be very different

Materials and Methods

Results and Discussion

Natural Ageing of the Heat-Treated Alloys

Phase Development during Isochronal Annealing

Conclusions