Influence of minor Sc on microstructure and properties of AA7085 alloy

Abstract

The age-hardening response, mechanical, and corrosion-resistant properties of AA7085 alloys with and without the addition of 0.3 wt.% scandium (Sc) were compared. Using advanced techniques such as aberration-corrected transmission electron microscopy and first-principles calculations, the underlying micromechanisms of Sc microalloying were revealed. Results show that the increase in strength of the AA7085-Sc alloy is mainly attributed to the decreased Al grain size and increased number density of both Al3Sc@Al3(Sc,Zr) core−shell nanoparticles and Sc-containing ηp and GP−ηp nanoprecipitates. Strong strain fields and evident electron transfer from Zr to the neighboring matrix Al atoms exist at the Al3Sc@Al3(Sc,Zr)/Al interface. The Sc doping in GP−ηp and ηp suppresses the GP−ηp → ηp transformation. Modified corrosion resistance of the AA7085-Sc alloy compared with AA7085 alloy is associated with the fine grain boundary precipitates of η phases and narrow precipitation free zone. The reasons of property changes of AA7085 alloy after Sc microalloying are explored based on the multiscale microstructural characterization.

1. Introduction

The 7000 series Al−Zn−Mg−Cu alloys, such as 7085 alloys investigated in the present study, are crucial light-weight structural materials in the aerospace industry (e.g. for aircraft stringers) since they offer a good combination of strength, damage tolerance and corrosion resistance [1]. Their properties mainly result from the contributions of microstructures at different scales, such as Al grain configuration and precipitate types [2−4]. It has been demonstrated that microalloying based on some unique elements like rare-earth Sc has a strong influence on the microstructures and thus is an important method to modulate the alloys’ properties [5−8]. For the microalloyed multicomponent alloys, it is a vital requirement to understand the interaction between different microstructures and microalloying elements, and how these modified microstructures correlate with the resulting macroscopic properties from both experimental and theoretical viewpoints.

Characteristic microstructures of the wrought Al−Zn−Mg−Cu(−Zr) alloys are composed of Al grains decorated with high density second-phase particles. The equiaxed recrystallized Al grains distributed along the deformation direction are formed by solid solution at 420−475 °C for 0.5−2 h [8−11]. Their typical diameters are below 200 μm. Among the secondary-phase particles, the large constituent (primary) particles (e.g. AlCr2 [12], S-Al2CuMg [10,13], Al3Zr [10], η-MgZn2 [11,14] and Mg4Zn7 [15]) in the shape of short rod and irregular geometry are characterized by a size of 0.5−4 μm. These constituent particles are formed by a liquid−solid eutectic reaction and maintained after homogenization, deformation and solid solution. Formation of the Al3Zr constituent particles promotes the heterogeneity and grain refinement of the 7000 series Al alloys during casting. The nanosized Al3Zr phases are precipitated from Al grain interior and grain boundaries during homogenization treatment (~470 °C, 24 h), impeding the grain coarsening of the deformed 7000 series alloys during solid solution treatment [9]. The other fine-scale η-series p