Effect of the eutectic Al-(Ce,La) phase morphology on microstructure, mechanical properties, electrical conductivity and heat resistance of Al-4.5(Ce,La) alloy after SPD and subsequent annealing
Graphical abstract
Introduction
Pure aluminium is commonly used as a material for conductors in the electrical industry due to its relatively high electrical conductivity, low weight and high corrosion resistance [[1], [2], [3]]. Another important factor is the lower cost of aluminium compared to copper. However, the mechanical strength of pure aluminium is quite low, so limiting its application. Alloying of pure aluminium leads to an increase in its mechanical strength via solid solution and/or precipitation hardening. However, the electrical conductivity of aluminium alloys is typically lower than the conductivity of pure aluminium due to electron scattering on solute atoms and precipitates [[4], [5], [6]]. The most significant factor causing a decrease in the electrical conductivity is solute atoms [[7], [8], [9]].
Al-based immiscible systems, such as Al-RE, show promise for use as electrical conductors as these alloying elements have almost no negative effect on the electrical conductivity. Moreover, intermetallic phases, dissipated as uniformly distributed small particles, can significantly increase the mechanical strength and thermal stability of the alloy [5,10,11]. Cast Al-RE alloys are stronger than pure aluminium [7] but are not too strong to be used for advanced electrical conductor applications. RE stands for rare-earth and is a common term for the 15 lanthanide elements including Sc and Y. Al alloys with Sc and Y have been the subject of research for a considerable time, which can't be said about such elements, as La, Ce, Nd, Pr, Sm and other lanthanides [12]. Being the commonest elements, amongst the others, La and Ce were chosen as the best candidates for this investigation.
Al-RE is one of the least investigated systems for electrical conductors, although a few attempts have been made to produce an alloy with uniform structure and fine grains using different casting techniques [13,14]. There are also a number of investigations dedicated to rare-earth additions to Al-Zr alloys [[15], [16], [17]], but, currently, no special attention seems to have been payed to this system. Alloying of Al with RE leads to the formation of an AlxRE (phase type depends on the alloy composition) eutectic, whose presence significantly reduces the aluminium matrix grain size and improves the thermal stability of the alloy up to temperatures of about 250 °C, while also improving the alloy's mechanical strength and retaining a high level of electrical conductivity. The lightweight Al-based materials, with such a complexity of properties, may be of great value for the aerospace industry or the electrical one, being a material for overhead powerlines working in a high-temperature environment.
Recent investigations of Al-8.5RE alloy show that the formation of an ultrafine-grain or nanoscale grain structure, leads to further strengthening of the alloy without any significant loss of conductivity related to the low influence of grain boundaries on electron scattering [11]. It was shown that high-pressure torsion at room temperature (HPT at RT) leads to the refinement of grains and intermetallic particles, down to 136 nm and 44 nm, respectively. Both high mechanical strength (553 MPa) and electrical conductivity (from 49.5 to 52.4% IACS, where IACS – International Annealed Copper Standard) were reached after HPT at RT and subsequent annealing at 280 °C [7]. The HPT processed alloy was thermally stable, as no grain growth was observed, even after annealing at 400 °C for one hour. Once again, this proves the potential of Al-RE alloys and the necessity for their further research.
The high cost of RE additions is an incentive to decrease its amount in aluminium alloys. It was shown [18] that the most optimal concentration of La and Ce in aluminium alloys lies in the range of 3.5–4.5 wt%, so the alloy with a concentration of 4.5 wt% RE was used for this work. Moreover, Mogucheva et al. [13] showed that annealing at 500–600 °C changes the morphology of the intermetallic phase in Al-8%RE alloy, increasing its ductility and decreasing its mechanical strength. Such a phase morphology transformation may potentially alter the behaviour of the microstructural change during HPT, so a preliminary heat treatment of the investigated alloy was carried out.
Section snippets
Materials and experimental methods
Al-RE alloy of 4.5 (2.9 La and 1.6 Ce) wt.% concentration was produced by casting. HPT was performed at RT for 20 revolutions under a pressure of 6 GPa and speed of 1 rpm. The number of revolutions was chosen based on works in the literature [7,18,19], where it was shown that the same strain level leads to saturation of physical properties. Besides, such high deformation is required to shear the intermetallic particles and redistribute them within the aluminium matrix. Specimens, after HPT, had
Microstructural evolution of alloy during HPT and subsequent annealing
Microscopy and XRD analysis results of the alloy, in all conditions, are presented in Fig. 1. It can be seen that the microstructure of the cast alloy consists of an aluminium phase and an intermetallic plate-shaped phase (Fig. 1a,c). Plates of intermetallic phase, after ST, transform into spherical particles (Fig. 1b). This shape transformation is explained by the capillary effect – diffusion along phase boundary, since volume diffusion of La and Ce, in Al, is inhibited [13]. It is likely that
Ce and La atoms diffusion activity in Al during HPT
In sections 2 Materials and experimental methods, 3 Experimental results it was demonstrated that a supersaturated solid solution of Ce and La in aluminium forms. In order to find out the driving diffusion mechanism, the following calculations were made.
As the diffusion of these alloying elements is inhibited, the dissolving of Ce and La might be deformation-activated, i.e. by the high concentration of structural defects, such as dislocations and vacancies. It is known that during HPT an
Conclusions
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This research shows that a relatively small addition of RE alloying elements (∼4.5%) is sufficient to obtain a good combination of mechanical strength (500 MPa) and electrical conductivity (52.2% IACS) using high pressure torsion at room temperature followed by annealing at 280 °C;
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It was shown that ST (550 °C for 3 h) before HPT allows a reduction in the annealing temperature, after deformation, to 230 °C to obtain the best combination of strength (430 MPa) and conductivity (55.9% IACS);
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Acknowledgements
This work was partly financially supported by the Russian Science Foundation within the project N 17-19-01311.
R. Lapovok acknowledges the Marie-Curie Fellowship within the EU Framework Program for Research and Innovation ‘Horizon 2020’.
Deakin University's Advanced Characterisation Facility is acknowledged for use of Electron Microscopy instruments and assistance from Dr Pavel Cizek. The authors are grateful to Robin Taylor for useful discussion.
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