# Role of Multi-Scale Microstructure in the Degradation of AI Wire for Power Transmission
## 1 Introduction  
Au, Ag, Cu, and Al are four kinds of excellent conductive materials, among which Al--depending on its excellent comprehensive performance--is widely used on the overhead transmission line as the outer layer of Al conductor steel reinforced (ACSR) [1, 2, 3, 4, 5]. In recent years, more and more Al alloy conductors were developed to replace the expensive copper to prepare Al-Fe cable and to replace the steel core to strand Al conductor steel reinforced (ACSR) and all Al alloy conductor (AAAC) to acquire higher energy efficiency [6, 7].  
As conductive materials, strength and electrical conductivity are two important parameters closely related to safety and energy-saving against various loads and conductor resistance, respectively [8, 9]. Wind load, ice load and dead weight are some common loads that the conductor must be borne during the service. Therefore, stable mechanical properties are vital for the power transmission. However, due to the resistance of the conductor itself, heat will inevitably be generated in the processof power transmission, which inevitably influences the reliability of the overhead conductor [10]. Degradation of Al conductor is defined as the loss of mechanical properties caused by heat during the long-term service, which depends on the evolution of multi-scale microstructure. Moreover, in long-term service, not only the mechanical properties are changing gradually, but also the electrical conductivity, so the strength-electrical conductivity relation will also be significantly influenced. In the previous studies, some experiments were carried out to simulate the degradation of Al conductor, and the strength loss of Al wire is attributed to the recovery of defects and the growth of grains [11]. Furthermore, the degradation of Al-Fe alloy conductor was also investigated in our previous studies and the result shows that the strength loss of Al-Fe wire (AFW) can be attributed to the recovery of defects, the growth of grain and precipitates [12]. However, the effect of alloying elements on the degradation rate of mechanical properties during the degradation of Al and Al alloy wires is little investigated, which is closely related to the anti-degradation design of microstructure. Additionally, the strength-electrical conductivity relations of the cold-drawn commercially pure Al wire (CPAW) and the cold-drawn AFW were systematically discussed [13; 14]. Many researchers focus on the strength and the electrical conductivity of the deformed pure Al and Al alloys. [PERSON] et al. [15] manufactured a kind of Al-Mg-Si alloy with high strength and high electrical conductivity by high-pressure torsion (HPT) at 180 \({}^{\circ}\)C. [PERSON] et al. [16] focused on the strength and electrical conductivity of commercially pure Al processed by accumulative roll bonding (ARB). [PERSON] et al. [17] investigated the trade-off relation between the strength and the electrical conductivity of an Al-Fe alloy produced by HPT and aging. However, the strength-electrical conductivity relation for the CPAW and the AFW during the degradation has never been studied.  
In this paper, the influence of the multi-scale microstructure of the Al wire on the degradation behavior of the mechanical properties were compared, especially the strength degradation rate of the strength under different temperatures, to reveal the degradation mechanism and principles for the design of Al wire with anti-degradation microstructure.