Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing

doi:10.1016/j.jmst.2018.12.023

摘要/Abstract

Abstract:

Developing heat-resistant conductors with high strength and high electrical conductivity is a key issue in the electrical conductor industries, as the ever-increasing power transmission poses higher requirement on the thermal stability of electrical conductor wires. Dilute Al-Sc-Zr alloys are considered as promising candidates due to the excellent heat resistance and high electrical conductivity, but the low strength always limits their application on electrical wires. Yet, few efforts on process design have been made in dilute Al-Sc-Zr alloys to enhance the strength. Here, various kinds of processing paths via combination of cold drawing, ageing and/or annealing were conducted to improve the strength and electrical conductivity of a dilute Al-Sc-Zr alloy. Results show that enhanced strength and electrical conductivity were obtained after cold drawing + ageing or pre-ageing + cold drawing + annealing treatments processes. Optimal properties (194 MPa in ultimate tensile strength and 61% IACS in electrical conductivity) were obtained through cold drawing followed by ageing. Microstructure evolution which affects strength and electrical conductivity was systematically investigated using TEM and 3DAP. The enhanced strength was mainly attributed to the suitable interactions between strain strengthening and precipitation strengthening. The enhancement in electrical conductivity was caused by precipitation of solute atoms and recovery of defects. These results provide foundations for the processing design of Al-Sc-Zr conducting wires with good properties and push forward their potential application in heat resistant conductor industries.

Key words: Al-Sc-Zr alloy, Cold drawing, Ageing, Mechanical properties, Electrical conductivity

. [J]. 材料科学与技术, 2019, 35(6): 962-971.

Li Liu, Jian-Tang Jiang, Bo Zhang, Wen-Zhu Shao, Liang Zhen. Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing[J]. J. Mater. Sci. Technol., 2019, 35(6): 962-971.

图/表 12

Fig. 1. Metallographic photographs of the hot extruded Al-0.06Sc-0.23 Zr alloy: (a) ED-TD plane, (b) TD-ND plane, (c) ED-ND plane. ED, TD and ND stand for extrusion direction, traverse direction and normal direction, respectively.

Fig. 2. Bright-field TEM micrographs of the hot extruded Al-0.06Sc-0.23 Zr alloy: (a) fine recrystallized grains, (b) Al3(Sc,Zr) precipitates inside grain as indicated by red arrows.

Fig. 3. Microhardness and electrical conductivity evolution of Al-0.06Sc-0.23 Zr as a function of ageing time under different ageing treatments: (a), (c) one-stage ageing at 300 °C, (b), (d) two-stage ageing (first ageing at 300 °C for 25 h, then ageing at 400 °C for different times), the data for the first ageing stage is marked with red circle in (a) and (c).

Fig. 4. The mechanical properties of Al-0.06Sc-0.23 Zr at different ageing times during the secondary stage ageing at 400 °C after ageing 25 h at 300 °C.

Table 1 The strengths and electrical conductivity of Al-0.06Sc-0.23 Zr under different processes.

Treatments	Yield strength (MPa)	Ultimate tensile strength (MPa)	Electrical conductivity (% IACS)
As-quench	26.1	43.9	51.5
CD	130.7	135.2	50.3
Peak ageing	125.5	154.2	61.3
CD-A	167.6	194.3	61.4
A-CD	184.2	199.4	57.6

Fig. 5. Mechanical strengths and electrical conductivity of Al-0.06Sc-0.23 Zr (with A-CD path) annealed at different temperatures for 5 h.

Fig. 6. TEM micrographs of Al-0.06Sc-0.23 Zr: (a) under one-stage ageing at 300 °C for 25 h, (c) under two-stage ageing at 300 °C for 25 h, then at 400 °C for 50 h, (b) and (d) the SAED patterns taken at [011] direction of Al matrix for (a) and (c), respectively; (e) HRTEM of Al3(Sc,Zr) precipitate taken at [011] direction of Al matrix after two-stage ageing, (f) its corresponding FFT image.

Fig. 7. (a) and (b) three-dimensional atom reconstruction of Al-0.06Sc-0.23 Zr after ageing at 300 °C for 25 h, displaying (a) Sc atoms, (b) Zr atoms, (c) proxigram of Al, Sc and Zr concentration in precipitates.

Fig. 8. Three-dimensional atom reconstruction of Al-0.06Sc-0.23 Zr under two-stage ageing (300 °C/25 h + 400 °C/50 h): (a) Sc atoms are displayed in red, (b) Zr atoms are in blue, (c) both Sc and Zr atoms are displayed and Al atoms are omitted for clarity, (d) proxigram of Al, Sc and Zr concentration in Al3(Sc,Zr) precipitates.

Fig. 9. Bright-field TEM micrographs of Al-0.06Sc-0.23 Zr under different processing paths: (a), (b) CD, (d), (e) CD-A path, recrystallized grains are marked with blue dashed lines in (d) and precipitates on dislocations are indicated by blue arrows in (e), (c), (f) the SAED patterns taken from [011] of Al matrix for CD and CD-A processed samples, respectively.

Fig. 10. Bright-field TEM micrographs of the A-CD processed Al-0.06Sc-0.23 Zr alloy under different treatments: (a), (d) before annealing, the drawing direction is indicated by red arrows, (b), (e) anneal at 250 °C for 5 h, (c), (f) anneal at 400 °C for 5 h, some recrystallized grains are marked by blue dashed lines in (c) and the pinning effect of precipitates are indicated by red arrows in (f).

Fig. 11. Electrical conductivity of Al-0.06Sc-0.23 Zr under various processes is plotted vs. their corresponding ultimate tensile strengths.

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