Microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity

1. Introduction

Scandium in aluminium alloys is the most effective additive to achieve a large strengthening effect during the annealing of as-cast samples [1,2]. The formation of Al₃Sc nanosized precipitates after annealing ensures preservation of the non-recrystallization structure in the high-temperature range after annealing the deformed alloys [1,[3], [4], [5], [6], [7]]. Zirconium improves the thermal stability of Al-Sc alloys due to the formation of Al₃(Sc_xZr_y) dispersoids [1,2]. Al-Zr-Sc is the most popular system to fabricate electrical conductors owing to the high strength, thermal stability, and good electrical conductivity of its alloys [8,9]. An Al-0.35Sc-0.2 Zr alloy exhibited a tensile strength of 210 MPa, elongation of 7.6%, and electrical conductivity of 34.9 MS/m (60.2% IACS) [8]. A low-scandium-content Al-0.06Sc-0.23 Zr alloy exhibited a tensile strength of 194 MPa and electrical conductivity of 61% IACS (35.4 MS/m) [9].

Yttrium provides the same effect in Al-Zr and Al-Mg alloys [[10], [11], [12], [13], [14], [15], [16], [17]]. Yttrium significantly accelerates the precipitation kinetics [[10], [11], [12], [13]], to thereby improve the thermal stability of the alloys during annealing at 250-370 °C and eliminate the negative effects of iron and silicon impurities [14,15]. Al₃(Zr,Y) dispersoids increase the recrystallization temperature and exhibit a higher coarsening resistance than Al₃Zr precipitates [10,11].

The aim of this study is to develop a novel composition for electrical conductors based on the Al-Y-Sc system and investigate the evolution of the microstructure, mechanical properties, and electrical conductivity of the alloys after casting and rolling.

2. Material and methods

Al-0.2Y-0.05Sc (AlYSc005) and Al-0.2Y-0.2Sc (AlYSc02) alloys were melted in a resistance furnace from pure Al (99.99%), Al-2Sc, and Al-9Y master alloys. The melt was poured into a water-cooled copper mould. The ingots were 40 mm in width, 20 mm in thickness, and 120 mm in height. The cooling rate was approximately 15 K/s. Heat treatment was performed using Nabertherm and Snol furnaces with an accuracy of ±2 °C. The ingots were rolled from 20 to 10 mm at 370 °C and from 10 to 1 mm at room temperature.

The liquidus and solidus temperatures were determined by differential scanning calorimetry (DSC; Labsys Setaram). Light microscopy (LM) using a Neophot-30 and an image analysis system with Axiovert 200MMAT and Axiovision 4.5 software were used for the qualitative and quantitative analyses of the microstructure, respectively. The grain structure after rolling was investigated by LM under polarized light. The microstructure was revealed by anodizing (15-25 V, 0-5 °C) using Barker’s reagent (46 ml of HBF₄, 7 g of HBO₃, and 970 ml of H₂O). The microstructure as well as chemical and phase compositions of the alloys were examined by scanning electron microscopy (SEM; TESCAN VEGA 3LMH) in conjunction with energy-dispersive X-ray spectroscopy (EDS; XMAX-80). The structure of the foils was investigated by transmission electron microscopy (TEM; JEM 2100) at 200 kV. The specimens were prepared using the A2 electrolyte on Struers Tenupol-5 equipment.

The electrical conductivity was measured on samples sized of 1 mm × 70 mm × 5 mm using the ‘double bridge’ method with an INSTEK GOM-802 mO m.

The hardness was measured using Vickers hardness equipment with a load of 5 kg. The tensile tests were performed using a Zwick/Roell Z250 testing machine. The strain rate was 3 × 10^-3 s^-1. The standard deviation from the mean value was within ±(1-4) MPa of the measured value. The specimens for the tensile tests were cut out from 1 mm rolled sheets with a gage length of 20 mm and width of 6 mm. Three specimens were tested per point.

3. Results and discussion

3.1. Microstructure and phase composition

Aluminium solid solution and Y-rich phases (according to the ternary phase diagram in Fig. 1 for the Al₃Y phase [18]) are present in the as-cast state in the investigated alloys (Fig. 2(a) and (b)). Al₃Y phase particles are located on the grain boundaries and form a round-shaped eutectic structure in the dendritic cell boundaries (inset in Fig. 2(a)). The size of the Al₃Y particles is approximately 100-200 nm. Scandium is homogenously distributed in the aluminium solid solution (Fig. 2(a) and (b)). A solidus temperature of 645 °C was determined by DSC for the investigated alloys (Fig. 3). The AlYSc005 and AlYSc02 alloys were homogenised at 635 °C for 24 h and quenched in water. The Al₃Y phase completely dissolved during homogenization treatment (Fig. 2(c) and (d)).

3.2. Annealing of as-cast and homogenised ingots

The hardness values for as-cast AlYSc005 and AlYSc02 are 23 HV and 26 HV, respectively. The hardness decreased to 20 HV and 19 HV, respectively, after homogenization treatment. Al₃Y particles with 100-200 nm in size contribute more to the hardness of the material than the dissolved Y in the aluminium solid solution. The as-cast and quenched ingots were annealed at 370, 400, and 440 °C for different time periods. Increase in the annealing temperature from 370 to 440 °C accelerated the attainment of the hardness peak from 2 to 0.5 h (Fig. 4(a) and (c)), and softening after reaching the peak (Fig. 4(a) and (c)). The highest hardness values were achieved after annealing the investigated alloys at 370 and 400 °C in both conditions. The hardness increase after annealing was approximately 5-10 HV and 15-23 HV in the AlYSc005 and AlYSc02 alloys, respectively.

TEM images of the AlYSc02 alloy annealed at 400 °C for 1 and 5 h are presented in Fig. 5, Fig. 6. L1₂ precipitates nucleated homogenously into the aluminium matrix and heterogeneously on the dislocations. Al₃Sc dispersoids typically nucleate homogenously [19,20]. The L1₂ precipitate is Al₃(Sc_xY_y) phase in the investigated alloy. The average size of the L1₂ precipitates is 11 ± 2 nm after annealing for 1 h at 400 °C in both conditions (Fig. 5(a) and (b), Fig. 6(a) and (b)). The size of the precipitates increased to 25-30 nm after annealing for 5 h (Fig. 5(d) and (e), Fig. 6(d) and (e)), which led to a decrease in the hardness of the material (Fig. 4(b)).

3.3. Recrystallization behaviour

AlYSc005 and AlYSc02 alloys exhibit a higher hardness after annealing the as-cast ingots. The as-cast ingots were rolled and annealed at different temperatures, and the results for the hardness and microstructure evaluations are presented in Fig. 7. Significant softening occurred after annealing for 1 h at 450 °C. However, the microstructure of the AlYSc005 was recrystallized, while the AlYSc02 alloy presented deformed grains. The recrystallization temperatures of the AlYSc005 and AlYSc02 alloys were in the range of 350-450 °C and 450-550 °C, respectively (Fig. 7(a) and (b)). Annealing at temperatures less than 350 °C for different time periods resulted in a low decrease in the hardness after 0.5 h. Increasing the duration to 7 h did not change the hardness values, due to fixing of the dislocations and grain boundaries by L1₂ precipitates and eutectic Al₃Y-phase particles. (Fig. 7(c)).

3.4. Tensile properties and electrical conductivity

The tensile tests results and typical tensile stress-strain curves are presented in Table 1 and Fig. 8. The yield strength (YS) values are 146 MPa and 186 MPa in the as-deformed state of the AlYSc005 and AlYSc02 alloys, respectively. The specimens annealed at 200 and 300 °C for 1, 5, and 7 h were also tested. The YS of the AlYSc005 alloy decreased by 10-15 MPa and elongation increased up to 5%. The AlYSc02 alloy demonstrated higher thermal stability of the YS due to the high Sc content and high fraction of the L1₂ precipitates.

The electrical conductivity values of the studied alloys compared with Al (99.99%) and 1350 conductor alloy [21] are presented in Table 2. The 1350 alloy is typically used for wires, stranded conductors, bus conductors, and transformer strips [21]. AlYSc02 alloy has a slightly lower electrical conductivity than AlYSc005 due to the higher content of alloying elements. Increase in the annealing temperature up to 300 °C results in an increase in the electrical conductivity due to decrease in the dislocation density after annealing of the deformed sheets. The highest electrical conductivity of 61.5%-62.4% IACS was achieved after annealing at 300 °C for 5 and 7 h; for the same duration, that of the commercial 1350 alloy is 61% IACS. Moreover, the maximum typical YS of the 1350 alloy is 110 MPa [21]. The YS values of the AlYSc005 and AlYSc02 alloys after annealing at 300 °C were 127-128 MPa and 177-183 MPa, respectively (Table 1). The high-scandium-content Al-0.35Sc-0.2 Zr alloy exhibited the same level of tensile strength, but a lower electrical conductivity of 60.2% IACS and low plasticity of 7.6% [8], compared with the 61.5% IACS and 15% Elongation. for the investigated AlYSc02 alloy.

4. Conclusions

(1)The evolution of the microstructure, mechanical properties, and electrical conductivity of novel Al-0.2Y-0.05Sc (AlYSc005) and Al-0.2Y-0.2Sc (AlYSc02) alloys were investigated. Aluminium solid solution and eutectic Al₃Y-phase particles sized 100-200 nm were detected in the as-cast investigated alloys.

(2)Annealing of the as cast alloys resulted in a higher hardness than homogenized alloys in the temperature range of 370-440 °C due to eutectic Al₃Y-phase particles sized 100-200 nm. L1₂ precipitates of the Al₃(Sc_xY_y) phase nucleated homogenously within the aluminium matrix and heterogeneously on the dislocations during annealing at 400 °C. Increase in the annealing time from 1 to 5 h at 400 °C led to the growth of L1₂ dispersoids from 11 ± 2 nm to 25-30 nm. Consequently, the hardness decreased by 15 HV.

(3)The recrystallization temperature range for the AlYSc005 and AlYSc02 alloys was 350-450 and 450-550 °C, respectively. The hardness and tensile properties slightly decreased, but did not change after annealing the rolled alloys for up to 7 h at 200 and 300 °C due to fixing of the dislocations and grain boundaries by L1₂ precipitates and eutectic Al₃Y-phase particles.

(4)The good combination of strength, plasticity, and electrical conductivity of the investigated AlYSc02 alloy makes it a promising material for electrical conductors: YS = 177-183 MPa, UTS = 199-202 MPa, Elongation = 15.2%-15.8%, and 60.8%-61.5% IACS.

Acknowledgment

This work was supported financially by the Russian Science Foundation (No.17-79-10256).

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