Effects of Yb/Zr micro-alloying on microstructure, mechanical properties and corrosion resistance of Al-Zn-Mg-Cu alloy

Abstract

The effects of Yb/Zr micro-alloying on the microstructure, mechanical properties, and corrosion resistance of an Al-Zn-Mg-Cu alloy were systematically investigated. Upon the addition of Yb/Zr to the Al-Zn-Mg-Cu alloy, the grain boundaries were pinned by high-density nanosized Al3(Yb, Zr) precipitates during extrusion deformation, consequently, the average grain size was significantly reduced from 232.7 μm to 3.2 μm. This grain refinement contributed substantially to the improvement in both strength and elongation. The ultimate tensile strength, yield strength, and elongation of the Yb/Zr modified alloy increased to 705.3 MPa, 677.6 MPa, and 8.7%, respectively, representing enhancements of 16.2%, 19.3%, and 112.2% compared to the unmodified alloy. Moreover, the distribution of MgZn2 phases along grain boundaries became more discontinuous in the Yb/Zr modified alloy, which effectively retarded the propagation of intergranular corrosion and improved the corrosion resistance.

1. Introduction

Al-Zn-Mg-Cu alloy has wide applications in aerospace and automotive fields due to its high strength-to-density and excellent fatigue performance [1 −3]. A large amount of metastable MgZn2 (η') phases precipitate during the ageing process, which are the source of ultra-high strength alloy [4, 5]. However, the coarse MgZn2 (η) phases precipitated at the grain boundaries lead to prominent intergranular fracture problems due to stress concentration and the reduction of corrosion resistance [5 −8]. The requirements for the comprehensive performance of Al-Zn-Mg-Cu alloy are increasing with the development of industrial technology. However, the combination of high strength and good corrosion resistance is still an unresolved issue in this field.

In order to improve the comprehensive performance of Al-Zn-Mg-Cu alloy, some researchers have adopted methods to improve heat treatment processes. HUANG et al [9] improved the corrosion resistance through high-temperature pre-precipitation treatment, which increased the Cu content and improved the discontinuity of precipitates at the grain boundaries. CHENG et al [10] coarsen the grain boundary precipitates through a two-stage over-ageing process and also improved the corrosion resistance, while the strength decreased. Therefore, it is difficult to improve simultaneously the corrosion resistance and strength through heat treatment process. It was reported that the grain boundary characteristics can be improved via alloying. Alloy elements such as Sc, Zr, Er, Yb have been confirmed to form Al3X (X=Sc, Zr, Er, Yb) phases, which suppressed the recrystallization effects in Al matrix [11 −15]. Subsequently, it was found that thermally stable Al3(Sc, Zr) particles were distributed uniformly, and the ultimate tensile strength (UTS) improved to 653 MPa [11]. Meanwhile, the Sc/Zr modified Al-Cu-Mg alloy can also reduce intergranular corrosion susceptibility [13]. However, the expensive Sc element has limited its application, which could be replaced by the cost-effective Er and Yb. It has been identified that similar effects can also be achieved by Al3(Er, Zr) particles [14, 15]. It is reported that Yb has a lower solid solubility compared with Sc and Er in the Al matrix at the same temperature. Additionally, Yb has a smaller critical nucleation work and critical nucleation size, which is beneficial to obtaining finer dispersion strengthening phases [16]. However, there is relatively little research on the phase precipitation behavior of Yb/Zr modified Al-Zn-Mg-Cu alloy.

In the present study, the microstructure, mechanical properties, and corrosion resistance of Al-Zn-Mg-Cu and Al-Zn-Mg-Cu-Yb-Zr alloys were systematically explored focusing on the mechanism of the high strength and high corrosion resistance of Yb and Zr modified Al-Zn-Mg-Cu alloy. It provides theoretical and scientific basis for the development of high performance Al-Zn-Mg-Cu alloy.