Effect of trace Er on the microstructure and properties of Al–Zn–Mg–Cu–Zr alloys during heat treatments
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 (σb) 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(ErxZr1−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.
Section snippets
Experimental procedures
The actual compositions of the investigated alloys are shown in Table 1 and denoted by A, B and C, for Er contents of 0, 0.06 and 0.10 wt% respectively. The processing steps used for the alloys are shown schematically in Fig. 2. The experimental materials in this study were industrial sized products provided by Southwest Aluminum Co. Ltd., China. The Φ250 mm-ingots were produced by vertical direct chill (VDC) casting, with a nominal melt temperature of 730 °C and a casting speed of 80 mm/min.
Initial microstructure
Fig. 3 shows that the as-cast microstructures of the three experimental alloys are typical for VDC casting and high alloy content, i.e. predominantly rounded dendrites and coarse grains. To distinguish the grain size clearly in the OM images, the typical grains in each alloy are marked with yellow and red colors in Fig. 3(a)–(c). Significant coring, caused by segregation of elements, is evident in each alloy but this sub-structure is coarser and more easily recognized in the non-Er containing
Initial microstructure
The initial grain size of as-cast and homogenized Al–Zn–Mg–Cu–Zr(-Er) alloys increased with Er content. Besides η-MgZn2 and S–Al2CuMg phases, there are also blocky, Er-containing constituent intermetallics in alloys B and C, which are most likely Al8Cu4Er [32]. The Er-containing particles are located on grain boundaries (Fig. 4) and are co-located with Al12(Mn,Cr) [42] and Al3Ti [43] intermetallic phases, according to the EDS and XRD analyses.
The grain size in the alloys will depend on the
Conclusion
The influence of trace Er (0.06–0.10 wt%) on the microstructure and properties of a new type of Al–Zn–Mg–Cu–Zr alloy have been investigated. The following conclusions can be drawn from this study:
- (1)
The number density and volume fraction of Al3(Er,Zr) precipitates following homogenization are both improved with the increase of Er content. As Er content increases from 0.06 wt% to 0.10 wt%, the volume fraction increases from 0.283% to 0.475% with only a small increase in particle size.
- (2)
The
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Yichang Wang: Investigation, Writing - original draft. Xiaodong Wu: Conceptualization, Supervision, Writing - review & editing. Lingfei Cao: Supervision, Funding acquisition, Writing - review & editing. Xin Tong: Investigation. Malcolm J. Couper: Writing - review & editing. Qing Liu: Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2016YFB0300901), the National Natural Science Foundation of China (51871033, 51421001), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2018002), and the Opening Project of State Key Laboratory for Advanced Metals and Materials (2020-ZD02).
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