Effect of trace Er on the microstructure and properties of Al–Zn–Mg–Cu–Zr alloys during heat treatments

Highlights

• Microstructures evolution of Al–Zn–Mg–Cu–Zr(-Er) alloys were studied.

• Er formed Al₃(Er,Zr) particles and bulk primary Al₈Cu₄Er in the alloy.

• The recrystallized fraction of the 0.1 wt% Er-containing alloys decreased to 4.2%.

• Er improved strength, IGC resistance and thermostability of Al–Zn–Mg–Zr–Cu alloy.

• Mechanism of tensile fracture and IGC was discussed.

Abstract

The effects of trace Er on the microstructural evolution and mechanical properties of newly designed Al–Zn–Mg–Cu–Zr alloys for drilling pipes were investigated. After homogenization, most of the eutectics dissolved into the matrix and some primary phases remained, including Al₈Cu₄Er, Al₁₂(Mn,Cr), and Al₃Ti. Dispersed nano-sized Al₃(Er,Zr) precipitates were formed in the Er-containing alloys after homogenization. Following extrusion, solution and retrogression re-aging (RRA) treatment, the hardness and yield strength of the 0.10 wt% Er-containing alloy are increased by 14% (25.4 HV) and 12.7% (70 MPa) compared to the alloy without Er. This improvement could be ascribed to the precipitation strengthening of fine Al₃(Er,Zr) particles, which complemented the conventional η' phase precipitation strengthening. The intergranular corrosion resistance of the RRA-treated alloy with the addition of 0.10 wt% Er was improved by 65% due to a lower amount of high angle grain boundaries. The fracture and intergranular corrosion mechanisms in alloys with and without Er addition were linked to the respective grain structures.

Introduction

Al–Zn–Mg–Cu aluminum alloys have been used as important engineering materials in the aerospace field due to their high specific strength, low density and high fracture toughness [[1], [2], [3], [4], [5]]. In recent decades, Al–Zn–Mg–Cu alloys have also been regarded as a potential choice of material for drilling pipes in gas, petroleum and geothermal applications, for both onshore and offshore deep-wells with exposure to high temperatures (100–250 °C) and corrosive environments (H₂S, CO₂ and Cl^‾) [[6], [7], [8], [9]]. Aluminum drilling pipes with both high strength-to-weight ratio and good corrosion resistance can reduce power consumption and improve production efficiency [8]. However, the high strength of the aluminum alloy is often associated with poor corrosion resistance especially in chloride anion solution [10]. For example, Fig. 1 shows that increasing tensile strength (σ_b) is generally associated with increased intergranular corrosion for a range of Al–Zn–Mg–Cu alloys and heat-treatments [1,[10], [11], [12], [13], [14], [15]].

Micro-alloying has been considered an effective method to obtain both high strength and corrosion resistance in Al–Zn–Mg–Cu alloys. The introduction of Mn, Cr, Zr and a few rare earth elements (such as Sc, Er) can lead to the formation of some second phases (e.g. Al₆Mn, Al₃Ti, and AlRE phases) which can inhibit recrystallization and be beneficial to the strength and corrosion resistance [10,[16], [17], [18], [19], [20], [21]].

It has been demonstrated that Sc can improve the recrystallization resistance of Al–Zn–Mg–Cu alloys due to the pinning effect of the L1₂-Al₃Sc particles at grain boundaries and dislocations [[22], [23], [24], [25]]. However, Er could be a less expensive and more effective alternative to Sc [10]. The lattice mismatch between face-centered-cubic FCC-Al matrix and L1₂-Al₃Er is 4%, while that between FCC-Al and L1₂-Al₃Sc is 1.32% [26]. The larger lattice mismatch results in a higher strengthening effect on the alloy at both room and elevated temperatures. In addition, Er has a lower solid solubility than Sc at high temperature in the FCC-Al matrix [[26], [27], [28]], which increases the driving force and the volume fraction of the precipitates. The reported diffusivity values of Er and Sc in aluminium are inconsistent at different temperatures. The diffusivity of Er in α-Al at 300 °C was found to be 4 × 10⁻¹⁹ m²/s, and was higher than that of Sc (0.9 × 10⁻¹⁹ m²/s) [26]. However at 400 °C, the Er diffusivity was 1 × 10⁻¹⁸ m²/s versus 1.98 × 10⁻¹⁷ m²/s for Sc in α-Al [27,28]. The lower diffusion coefficient of Er at higher temperatures might have a number of implications for forming particles during homogenization, in terms of dispersion, size and thermal stability of the particles formed [28]. It is also noteworthy that Er and Zr lead to the formation of core-shelled L1₂-Al₃(Er_xZr_1−x) nano-particles, and the coarsening kinetics of these particles can be inhibited by the Zr-rich shell [27]. The combined addition of Er and Zr inhibits recrystallization and further improves the mechanical properties of the Al–Zn–Mg–Cu alloys.

The major production process of aluminum drill pipes mainly includes casting, homogenization, hot extrusion, solution treatment and aging [29,30]. Characterizing the microstructural evolution during each production process is key to understanding and controlling the final performance of these products. However, most of the previously published research was focused on aluminum sheet, which is not not directly applicable to the practical engineering requirements for oil drill pipes. Moreover, pipe parts require more severe plastic deformation during hot extrusion compared to sheet forming, so the evolution of the microstructure and properties will be different. As reported [16,18,[31], [32], [33]], Al–Mg, Al–Cu, and Al–Zr alloys with the addition of Er show combinations of high strength and reasonable elongation, as well as good recrystallization resistance at elevated temperatures. However, little attention has been paid to the microstructure evolution and properties of the more complex Al–Zn–Mg–Cu–Zr–Er alloys during heat treatments, and even less research has been performed on their application in extruded aluminum pipes [[33], [34], [35]].

Therefore, in this work, the effects of Er on the microstructural evolution at different stages of processing are investigated and linked to the properties required of Al–Zn–Mg–Cu–Zr(-Er) alloys for drill pipes. The results will show that the experimental alloys with an addition of Er can have both higher strength and corrosion resistance (marked as a red five-pointed star in Fig. 1) compared to published data on similar alloys. The mechanisms of tensile fracture and intergranular corrosion in the tested Al–Zn–Mg–Cu–Zr(-Er) alloys will be discussed in detail.