Enhancement of mechanical and electrical properties of Al-RE alloys by optimizing rare-earth concentration and thermo-mechanical treatment
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. Another important factor is the lower cost of aluminium compared to copper [1,2]. However, the mechanical strength of aluminium is low, which restricts its use and encourages the use of aluminium alloys, where the alloying of pure aluminium leads to an increase in mechanical strength via solid solution and/or precipitation hardening. However, introducing other elements into pure aluminium lowers its electrical conductivity to electron scattering caused by solute atoms, dislocations, grain boundaries and precipitates. The most significant effect on decrease of electrical conductivity results from solute atoms [[3], [4], [5]].
Therefore, the Al-based immiscible systems, such as Al-RE or Al-Fe, seem to be promising materials for high conductivity conductors as these alloying elements have zero solubility in Al and thus have an insignificant effect on electrical conductivity [[6], [7], [8], [9], [10]]. Moreover, intermetallic phases, precipitated as small particles and uniformly distributed throughout the alloy, may significantly increase its mechanical strength and thermal stability [11]. However, the immiscible element compound concentration should be controlled as an excessive amount might result in a loss of electrical conductivity as was shown for Al-Fe alloys [[12], [13], [14], [15], [16], [17]].
Despite that cast Al-RE alloys being stronger than pure aluminium [1], they still have insufficient strength due to large grains and large particles of intermetallic phase. Therefore, forming is necessary to improve the mechanical properties of alloys due to grain refinement and breaking and redistribution of second phase particles. Recent investigations have shown that formation of ultrafine-grained or nanoscale grain structure by severe plastic deformation methods leads to strengthening of alloy without significant loss of conductivity, which is explained by low effect of grain boundaries on electron scattering [4,5,18,19].
Alloying of Al with La and Ce leads to forming of Al(La,Ce) eutectic, whose presence significantly reduces aluminium matrix grain size and improves thermal stability of alloy up to temperatures of about 250 °C. It was shown in our latest investigation that HPT of Al-8.5RE alloy at room temperature leads to grain and intermetallic particles refinement down to 136 nm and 44 nm, respectively. Both high mechanical strength (ultimate tensile strength of 495 MPa) and electrical conductivity (from 49.5 to 52.4 %IACS) were obtained after HPT at RT and subsequent annealing at 280 °C [18]. It was also shown that grain growth was significant only after annealing at 400 °C, which permits use of this alloy as a conductor at elevated temperatures below 350 °C as specified in IEC 62004 [20].
Despite the obvious advantages of Al-RE alloys, they are the least investigated Al-based alloys with only a few papers published on the topic [21,22]. There has been no systematic investigation of the effect of RE concentration on strength and electrical conductivity, and optimization of these alloys has not been performed.
In this paper, Al-RE alloys with three different RE concentrations were used to define the composition providing optimal combination of mechanical and electrical properties. Severe plastic deformation via HPT was used for grain refinement and intermetallic particle redistribution. Thermal stability of these alloys was investigated and discussed in section 3.2.
Section snippets
Materials and experimental methods
Three Al-RE alloys with different concentrations of rare-earth elements (La + Ce), namely 2.5 (0.9% La and 1.6% Ce), 4.5 (2.9% La and 1.6% Ce) and 8.5 (3.1% La and 5.4% Ce) wt. %, were produced by casting.
High-Pressure Torsion (HPT) was performed using constrained anvils at room temperature (RT) under pressure of 6 GPa–20 revolutions. Part of the samples after deformation was annealed at 230 °C, 280 °C and 400 °C in a Nabertherm B 180 atm furnace for 1 h. Microstructural, mechanical and
Mechanical and electrical properties of alloys after HPT at RT and annealing
Electrical conductivity and mechanical properties of Al-RE alloys after HPT and subsequent annealing are presented in Table 1. Mechanical properties of all three alloys after HPT are significantly higher than for pure aluminium. Alloy containing 2.5 wt% RE has strength of 297 MPa and ductility of 18.5%, which is 10% lower than for pure aluminium. Increasing RE content to 4.5 wt% leads to a significant increase in strength (580 MPa), and a very slight decrease in ductility down to 17.3% compared
Modeling of structural mechanism contributions
Assuming that contributions from different strengthening mechanisms act independently, the total strength of the nanostructured Al-RE alloys σtheor can be estimated asWhere σGB is strengthening due to the grain size, σd is strengthening due to the dislocation density, σOr is strengthening due to the particles according to Orowan mechanism, as the size of precipitates is relatively large and dislocations will preferably bypass them [26]; and σSS is strengthening due to the
Conclusions
Al-RE (La + Ce) alloy with total Ce and La concentration of 2.5, 4.5 and 8.5 wt % was studied as a potential material for high strength conductors. Formation of ultrafine grained structure due to severe plastic deformation by high-pressure torsion at room temperature and precipitation of Al11(Ce,La)3 intermetallic phase substantially increase the alloy's mechanical strength and thermal stability, but reduce its electrical conductivity. It was confirmed that increasing RE concentration has a
Acknowledgements
M.Yu. Murashkin and R.Z. Valiev would like to thank the Ministry of Education and Science of the Russian Federation for financial support from the Framework of Increase Competitiveness Program of NUST ‘‘MISIS″, implemented by a governmental decree dated 16th of March 2013, No 211. Deakin University's Advanced Characterisation Facility is acknowledged for use of Electron Microscopy instruments. The authors are grateful to Robin Taylor for useful discussion.
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