Mechanical and Electrical Properties of Y-containing Al-Zr Heat-resistant Alloy Produced by Dynamic ECAE Process

ZHOU Tianguo, XIE Haibo, JIANG Zhengyi

(1. School of Materials Science and Engineering, Yangtze Normal University, Chongqing 408100, China; 2. School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, Wollongong, NSW 2522, Australia)

Abstract: The influence of rare earth Y on the microstructure and mechanical properties of Al-Zr alloy produced by dynamic ECAE was studied by OLYMPUS-BX51M optical microscope (OM), S4800 energy disperse spectroscopy (EDS) and SANS CMT5105 electronic universal material testing machine, and the corresponding equivalent conductivity was also investigated by using QJ48 DC electric bridge. The results show that the tensile strength of Al-Zr conductor first increases and then decreases with the increase of the aging time and temperature, and the highest tensile value can be obtained under the aging temperature of 160 °C for 4 h.The ductility and the resistivity of the Al-Zr alloy have inverse proportion to the aging time. The rare earth Y has significantly improved the electrical and mechanical properties of Al-0.3%Zr heat-resistant alloy. In this study,the tensile strength and the elongation of the Al-0.3%Zr-0.2%Y alloy, after aging treatment at 220 °C for 14 h,are about 278.49 MPa and 6.7%, respectively, and the equivalent conductivity is about 59.6 IACS. Hence the synthetical properties of the Y-containing alloy are significantly improved compared with traditional Al-0.3%Zr alloy.

Key words: Al-Zr heat-resistant alloy; ECAE process; equivalent conductivity; aging treatment

1 Introduction

Al-Zr wires have many advantages, such as high heat resistance and large ampacity, resulting from the appropriate amount addition of Zr and Y alloyed elements[1]. The recrystallization temperature, creep and heat resistant properties of the Al-Zr alloy are improved, which could stably work for a long period at 150 ℃ or/and for a short-term at 230 ℃, while the service temperature of conventional aluminum wires was only 70 °C, and at 90 ℃ for a short-term.

The addition of a small amount of Zr can effectively improve the high temperature strength of deformed Al alloys due to the formation of supersaturated Al-Zr precipitates during aging[2].By comparison in an equal specification, the Al-Zr wires, containing Zr and Y, has 40% - 50% increase in ampacity and the tensile strength could be improved to more than 180 MPa[3]. Hence the Zr/Y-modified Al-Zr alloy wires are widely used worldwide.

Additionally, the increase of the Zr content significantly reduces the electrical conductivity of Al-Zr wires. Generally, every 0.1% additional Zr leads to a 4.7% reduction of conductivity of the Al-Zr wires,and low-Zr (lower than 0.12%) heat-resisting wires are produced currently[4]. The sag of heat-resisting aluminum wire is smaller than that of ordinary aluminum conductor steel-reinforced (ACSR) wire under the same span. Furthermore, the sag difference between the heat-resisting aluminum wire and ACSR wire increases with the span. Therefore, the Al-Zr wires are more suitable for the transformation of transmission line capacity in major cities.

In general, equivalent conductivity of XTAl can reach 58% IACS (international annealed copper standard), the tensile strength is between 158 to 183 MPa, continuous service temperature is 230 ℃, and short-term and instantaneous working temperatures are 310 ℃ and 380 ℃, respectively. Similar service parameters of KTAl are of equivalent conductivity of 58% IACS, tensile strength of 218 - 262 MPa,working temperature of 150 ℃, short-term of 180 ℃ and instantaneous of 260 ℃[5]. Currently,dramatic changes of global climate and the need of the freezing disaster facilitates the demand of XTAl with ultra-strength value of 300 MPa[6]. Furthermore,research has been carried out to investigate the alloying additions, such as Er, Yb and Sc, on precipitation behavior, and the results show that the combined additions of Er and Zr can improve the alloy precipitation hardening behavior.

The continuous equal channel angular extrusion(ECAE) dynamic forming process has many advantages in terms of large share deformation, efficient grain refinement, and rapid precipitation of strengthening phases[6,7]. The dynamic ECAE provides a new way for precipitation strengthening of Al-Zr alloy wire. A large number of researches have been conducted on the transformations of Zr from the solid solution to precipitation under the actions of rare earth element(Sc, Er and Y)[8-12].

However, the influence of continuous ECAE process with optimization of rare-earth on the mechanical and electrical properties has not been widely reported. This paper aims to propose a new method for developing high strength and high conductivity Al alloy, and the related research has been conducted on the ECAE optimized process.Furthermore, the microstructure of Al-Zr-Y alloy was also studied in order to find out the mechanism of rare earth on the ECAE synthesis process.

2 Experimental

The Al ingot (purity 99.70 wt%) was melt in a SG2-5-12 resistance furnace until complete dissolution.The Al-10%Zr and Al-10%Y alloy agents were also dropped into the heating furnace at 800 ℃ for preparation of Al-0.3%Zr alloy. After the flux melted,the alloy melt was insulated and boronized at the furnace temperature following exhausting and clearing off the dross in high pure argon atmosphere. Then, the furnace was switched off and the melt was cooled to 710 ℃. 15 min later, the liquid was poured into billet moulds with inner cross-sectional area of 9 × 102mm2.After solidification, the Al-0.3%Zr-Y alloy slices with length of 60 mm were cut out from these alloy billets.The Al-0.3%Zr slices were also produced as contrast in this research. After 450 ℃ homogenization treatments for 2 h, specimens with dimensions of 12 mm × 12 mm were further extruded out from the slices by a self-designed vertical extruder. Al-0.3%Zr alloy with Y addition content 0.1%, 0.2%, 0.3%, 0.4% and 0.5%respectively were prepared by using the above same process.

The specimens were reciprocating extruded in turn for four passes by the B sub C path with the extruded temperature of 180, 185, 220 and 320 ℃from No.1 to fourth pass respectively. The samples were ground, polished, and etched in the Keller reagent for about 30 s in room temperature prior to rinsing by alcohol. The micrographs of the prepared samples were taken on an OLYMPUS-BX51M optical microscope.The energy dispersion spectrums (EDS) were obtained by employing the S4800 scanning electron microscope(EDS).

Table 1 Aging parameters of the Al-Zr and the Al-Zr-Y heatresistant alloy conductors

Tensile tests were conducted to evaluate the mechanical properties of Al-Zr alloy. Multiple sample tensile testing at room temperature was carried out with SANS CMT5105 instrument under a constant speed of 20 mm/min. Specimens used for resistance testing were measured by QJ48 DC electric bridge.Finally, the equivalent conductivity of Al-Zr alloy was obtained by numerical calculation. The best properties of Al-0.3%Zr-0.2%Y alloy were further artificial aged and studied. The aging process is shown in Table 1. The microstructure, tensile strength and electric conductivity was done as the same above step.

3 Results and discussion

3.1 The microstructures and properties of Al-Zr -Y alloy prepared by 4-rollpass dynamics aging ECAE process

The optical microstructure of Al-0.3%Zr alloy with different Y content prepared by 4 roll- pass ECAE are shown in Fig.1. It can be seen that equiaxed grains with elongated of these alloy can be obtained by using reasonable ECAE process with different forming temperature, and their grain size decrease weakly with the Y content increase. The main reason is that the material was seriously sheared by multi-pass ECAE to accelerate the precipitating rate of alloying element Zr and Y form the Al matrix. The secondary recrystallization and grain growth can easy grow in all direction limited by the precipitated phase distributed in grain boundary and inter grain. The growth speed of the grains can be probably restrained by the more precipitated phase with Y, so the grain size decreases lightly with Y content increase.

Fig.1 The optical microstrcuture of Al-0.3%Zr alloy with (a)0.1%Y; (b) 0.2%Y; (c) 0.3%Y and (d) 0.4%Y prepared by 4-roller pass ECAE process

Fig.2 The tensile strength and electrical conductivity of Al-0.3%Zr alloy with different Y addition prepared by 4-roller pass ECAE process

The effect of Y addition on the electric conductivity and tensile strength of Al-Zr alloy prepared by dynamic ECAE process is shown in Fig.2. It can be seen that the equivalent conductivity and tensile strength of Al-0.3%Zr alloy increase first and then decrease with the increase of Y addition. The electrical conductivity of the alloy is the highest about 54.3%IACS with top tensile strength 256.7MPa. When the Y content exceeds 0.2%, the conductivity and tensile strength decreases. The first main reason is that impurities such as Fe and Si react with Y to precipitate rare earth compounds at the grain boundary,which reduces the scattering effect of Fe and Si in solid solution on the electrons adding an appropriate amount of rare earth Y. However, the conductivity of the alloy can be worsened by the excessive addition of Y element by increasing dissolution in the aluminum matrix. Secondly, the matrix purification effect will decrease due to the Y excessive addition, a coarse Al Y equilibrium phase is easier to form, so the conductivity of the alloy will decrease.

The morphology of the coarse impurity phases can be granulated, spheroidized and refined by adding more Y addition. The adverse effects on the mechanical properties can be reduced and the strength of the material can be improved, meanwhile, the density and size of Al3Zr and Al-(Zr, Y) precipitated phase can be enlarged by increasing Y addition. The resistant force to the migration of dislocation, grain boundaries and grain deformation can be effectively increased with a certain strengthening and good thermal stability, so the tensile strength of Al-0.3%Zr alloy with 0.2% Y content is the highest of 256.7MPa. However, the growth up tend of the coarse Al - (Zr, Y) precipitated phase increases with the increase of Y addition during the ECAE process.The coherence with the aluminum matrix lost, the pinning of the grain boundaries weakened, as well as the rare earth compounds segregated seriously[13]. All of these will be harmful to the mechanical properties of Al-0.3%Zr alloy materials, so its tensile strength decreases after adding excessive Y addition.

3.2 The morphology observation and precipitates analysis

The optical morphology of Al-0.3%Zr-0.2%Y alloy formed by ECAE extrusion is shown in Fig. 3.It can be seen that the microstructure is much more refined and uniformed with the increase of ECAE extruding passes. The grain size decreases from 50 μm (Fig.3(a)) to about 20 μm (Fig.3(b)) during all the four passes, which indicates that the increase of ECAE passes is effective for the refinement and the structural uniformity of Al-0.3%Zr-0.2%Y alloy. During all the four passes, the specimens are mostly composed of fine and equiaxed grains.

As in present study, the deformation bands are cut into numerous fine sub-grains during ECAE due to the increase of the dislocation density. Moreover, the fine crystal grains are formed during the dynamic courses of the decomposition and the recombination of subgrains. Hence the original coarse grains are induced to transform to tiny structure gradually. However, in the deformation conduction of 220 °C, recrystallization occurs and this leads to microstructure coarseness obviously. Hence based on preliminary research, four passes are determined during ECAE process and large amount of equiaxed grains are obtained and the optical morphology is shown in Fig.1(d).

Fig.3 The morphology of Al-0.3%Zr-0.2%Y heat-resistant alloy during (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes

Fig.4(a) shows the cross-sectional microstructure of Al-0.3%Zr and corresponding added elements contents of the second phases measured by EDS spectrum. It can be seen that the volume fraction of precipitates is quite small. Ruling out the interference of Al matrix, the mole fraction ratio of Al and Zr element is about 3:1. The precipitated content of Al-0.3%Zr-0.2%Y alloy is shown in Fig.2(b). The mole fraction ratio of Al, Zr and Y of the second phases is about 2:2:1. Therefore, Al (Zr, Y) compounds makes up the main component of the second phases. In general, the solid solubility of Y element is relatively low in alloy leading to the increase of precipitation tendency of the strengthening phases[14-16]. Firstly, the fine Al3Y phases appear in forming process of the metal rob, which acts as the nucleation core of Zr-containing phases.Secondly, during four passes, the Al (Zr, Y) phases are crushed by strong shearing action during ECAE process. This process contributed to rapid precipitation of ZrY from the aluminum matrix. After aging process,under joint action of residual strain energy and thermal activation energy of ECAE process, large amount of Al3Y phases stemmed from the microstructure of Al-0.3%Zr-0.2%Y alloy. Moreover, Al3Zr precipitates phases are generated by the reactions of Zr element and Al matrix.

Fig.4 Microstructures of (a) Al-0.3%Zr as well as (b) Al-0.3%Zr-0.2%Y, and the EDS spectrums of corresponding second phase

Fig.5 Diagram of the UTS values of the investigated materials, including (a) Al-0.3%Zr and (b) Al-0.3%Zr-0.2%Y

Fig.6 Diagram of the breaking elongation of the investigated materials, including (a) Al-0.3%Zr and (b) Al-0.3%Zr-0.2%Y

3.3 The mechanical and electrical properties

The relationship between the mechanical properties of the Al-0.3%Zr-0/0.2%Y alloy and aging process is shown in Figs.5 and 6. It can be seen that under certain aging temperature, the ultimate tensile strength (UTS) of the samples increases firstly and then decreases with the increase of aging time. In addition,the UTS values are in linear decrease approximately with the increase of aging temperature. In contrast, the UTS values of Al-0.3%Zr-0.2%Y alloy are improved.

After aging treatment for 4 h, the maximum UTS was obtained. As shown in Fig.3(a), the UTS maximum value of the Al-0.3%Zr alloy is 275.7 MPa under the aging temperature 160 ℃. With the increase of aging time, the UTS decreased gradually. Al-0.3%Zr-0.2%Y alloy is the same, and the maximum UTS value of 281.3 MPa is obtained during 160 ℃ for 4 h.Additionally, when the aging temperature increases up to 220 ℃, the UTS values descend correspondingly.

The changes mainly stem from the unique shear deformation of continuous ECAE forming processes.The shear deformation could promote alloying elements precipitates rapidly, which provides basis for the growth of Al3Zr phases.

With a shorter aging time, the size of Al3Zr strengthening phases was small. Hence the damping effect on the movement of dislocation was lower. As the aging time increases, the strengthening phases constantly grows and hinders the movement of dislocation. Therefore, the peak UTS values appear.With the longer aging time, the coarse precipitate gathered and the density decreased[17], leading to lower UTS values.

Additionally, the UTS values are decreased with the increase of the aging temperature, which mainly stems from the increase of alloying atoms diffusion.The Al3Zr, as the strengthening phase, hinders the dislocation movements and the corresponding capacity diminishes (Fig.5(a)). Hence the UTS values have presented a downward trend. When rare earth element Y was applying, Al3Y phase that acts as another important strengthening phase was obtained, resulting in the increase of the mechanical properties of Al-0.3%Zr-0.2%Y alloy (Fig.5(b)).

Fig.6 shows an inversely proportional relationship between breaking elongation of alloy and aging parameters. It can be seen that as the aging time extended, the breaking elongations of Al-0.3%Zr alloy decrease from 13% to 10%, correspondingly(Fig.6(a)). Besides, the elongation values reduce gradually with the aging temperature. Moreover, the breaking elongation of Al-0.3%Zr-0.2%Y alloy has the similar changing trend with both the aging time and temperature, and is shown in Fig.4(b). The values change from about 9.5% to 6%, individually.

Fig.7 Diagram of the equivalent conductivity of the investigated materials which contains (a) Al-0.3%Zr and (b) Al-0.3%Zr-0.2%Y

The he extension of aging time and the elevation of aging temperature can lead to the growth of the second phase. Theoretically, the front interface of precipitates transforms from simi-coherent initially to incoherent phase boundaries finally[18,19], which results in poor deformation compatibility of grains.Furthermore, the non-conjugation during aging process has been improved due to the application of rare earth element Y. Hence, the breaking elongation is in a faster downward trend.

Fig.7 shows a positive relationship between equivalent conductivity of the Al-0.3%Zr -0/0.2%Y alloy and the aging parameters. It can be seen that the equivalent conductivity first increased rapidly and then maintained stably with the increase of aging time.With the aging time extension, the size of precipitates has been enhanced, and the density decreases correspondingly. Therefore, equivalent resistivity increases in macro level. Meanwhile the increase of the average electrical thermal motion provoked the lattice defects to disappear, which are favor for reducing the scattering of the electron motion. Generally, the equivalent conductivity increases with the increase of the aging temperature (Fig.7).

In contrast, the equivalent conductivity of Al-0.3%Zr alloy can reach the range of 57.5 IACS to 57.7 IACS under aging treatment of 220 ℃ for 14 h.While under the same aging process, the equivalent conductivity values of Al-0.3%Zr-0.2%Y alloy are about 59.8 IACS. EDS spectrum shows that the Sicontaining phases widely exist in Al-0.3%Zr-0.2%Y alloy. The Si element, as impurity element, has interaction with Y-compound in front of solid-liquid interface, hindering the diffusion of Si atoms. Hence,the YSi and YSi2 phases were formed near grain boundaries. In general, the Si-containing compounds in Al matrix greatly improve the resistance of metallic conductor[20,21]. Therefore, the conductivity properties of Al-0.3% Zr-0.2%Y alloy conductor were much better, as shown in Fig.7(b).

In terms of comprehensive mechanical and electrical properties of conductive materials, the capacity-expanded conductor in society is considered to be the most effective way to save the electric power.During aging treatment of 220 ℃ for 14 h, a preferable property with higher mechanical and electrical properties, was obtained and is considered to be the Al-0.3%Zr-0.2%Y alloy.

4 Conclusions

a) The continuous ECAE forming process has outstanding advantages of unique large shear deformation on microstructure of Al alloy. During four passes, large amount of fine dendrites with the size of 20 μm were obtained. It is found that no matter whether the Al-Zr alloy containing 0.2%Y element or not, the microstructure mostly consists of these fine and equixed grains.

b) During four passes, the temperature of the ECAE mould was increased up to 260 ℃, which can promote the precipitation of Zr from Al-Zr alloy rapidly and completely. After four different aging processes,the Al3Zr strengthening phases were formed. Besides,adding a small amount of Y element is in favor of improving the comprehensive mechanical properties of Al-0.3%Zr-0.2%Y alloys. During aging treatment of 220 ℃ for 14 h, the tensile strength of the Al-Zr alloys applying Y element or not are about 273 and 280 MPa,respectively. Furthermore, the corresponding ultimate elongations are both 5 % approximately.

c) During four passes, the electric-conductivity first increases with the increase of aging time, and then remains the same after 4 h of aging treatment.After 220 ℃ aging treatment for 14 h, the electricconductivity of Al-0.3%Zr alloy can reach 57.7 IACS,and the corresponding conductivity of Al-0.3%Zr-0.2%Y alloy is lower than 60 IACS. Additionally, the electric-conductivity is reduced after lowering the aging temperature.