**5. Conclusions**

The studies carried out using EDX, EBSD, densitometry, XRD, and acoustic techniques have made it possible to quantitatively estimate the structural, microstructural, and deformation characteristics of Al wires from AAAC (A50) and ACSR (AC50, or AC50/8 in full designation) overhead power line cables after different service life periods from 0 to 20 years.

It has been found that NSDLs with defects of a void nature are formed in Al wires of both types. The integral density *ρ*d, which is measured using the densitometric method, and the X-ray density *ρ*x, which is calculated from the unit cell parameter *a* of the lattice of the Al material in NSDL of wires, decrease according to a law close to the exponential-decay law as the depth *T* from the surface decreases, although more gentle in the case of *ρ*x(*T*) than *ρ*d(*T*). With an increase in the operating time, the *ρ*x and *ρ*d values at depth ~12.5 μm near the wire surface decrease significantly (density defect Δ*ρ*x/*ρ*x ≈ −1.1% after a service life of ~20 years in comparison to Δ*ρ*x/*ρ*x ≈ −0.2% in non-exploited samples and ≈ 0% in the bulk).

The characteristic thicknesses of NSDLs of ~10 μm, ~30 μm–40 μm, and ~100 μm–160 μm for the two types of wires after operation were established, corresponding to ~(80–85)% of the total drop of *ρ*d, ~100% of the total decrease in the *ρ*d and ~50%–70% of the total decrease in *ρ*x, and a ~99% decrease in the X-ray mass density *ρ*x, respectively. The difference in the results obtained by the methods of densitometric and XRD profiling is associated with the different sensitivity of the methods to different effects on the wires during operation. In the case of X-ray density *ρ*x, which is calculated from structural data, the structural state of the Al material of the NSDL of wire will play a major role. The Al lattice of the NSDL of wire expands when approaching the surface, and, accordingly, the X-ray density *ρ*x decreases, which is mainly due to the formation of defects of a void nature in the NSDL of wires under the action of vibrations due to wind, temperature fluctuations of the surrounding atmosphere, and fretting. In the case of densitometric density *ρ*d, which is an integral value, an important role is also played by the contribution of other phases, aluminum oxides in particular, the crystallites of which are formed when the service life increases, which, in turn, leads to the formation of a larger number of void defects near the surface because of the fretting amplification.

The presence of a steel core plays a stabilizing role in changing the structural, microstructural, and deformation characteristics.

In the presence of a steel core in the AC50 type cables, the wires of this cable show a ~0.2% lower drop in both integral and X-ray densities in comparison with the wires of AAAC A50 cables with comparable service life durations. Furthermore, in the presence of a steel core (ACSR-type AC50 cable), the thickness *<sup>T</sup>*layer of that part of the NSDL where the main decrease in X-ray density *ρ*x (~50–70% of the total drop) occurs when approaching the surface is 1–2% less than that in wires from AAAC-type A50 cables without a steel core. For wires from AC50 cables, a smaller value of microstrains is observed, which are formed at depths *T* ≥ 15 μm from the wire surface in NSDL after operation, compared to A50 cables, and it is associated with the steel core effect limiting cable vibrations because of wind.

The degradation rate of the average parameter *a* of the Al cubic unit cell of wires with a service life of 0–20 years and their density *ρ*x, which is calculated from XRD data in NSDL with a thickness of ~35.5 μm, with the service life in the presence of a steel core is noticeably lower (respectively, 1.07(3) · 10−<sup>4</sup> Å/year and −2.13(7) · 10−<sup>4</sup> g/cm3/year for AC50 wires compared to 1.26(4) · 10−<sup>4</sup> Å/year and −2.52(8) · 10−<sup>4</sup> g/cm3/year for A50 wires). As a result, the expansion of the lattice in the NSDL and, accordingly, the decrease in the X-ray density *ρ*x of the NSDL Al wire material from A50 type cables without a steel core occurs faster by ~1 year after a service life of ~10 years and by ~3 years after ~20 years of operation.

At the same time, in comparison with A50 cables, the presence of a steel core in AC50-type cables leads to a significantly smaller change in the deformation characteristics of AC50 wires and, even, to some improvement in their early stages of operation, lasting at least up to 8 years. This smaller change and improvement in the deformation characteristics of AC50 wires is associated with less development of defects in the near-surface layer of up to ~30 μm thick for wires made of AC50 type cables compared to wires made of all-aluminum A50 cables.

**Supplementary Materials:** The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/cryst12091267/s1, Section S1. X-ray diffraction details: Figure S1. Comparison of dependences of the average crystallite size *D*, calculated within the framework of the zero microstrain model (*<sup>ε</sup>*s = 0), on the service life duration *t* for Al-wires of A50 and AC50 type cables of the overhead power lines; Figure S2. WHP and SSP of AC50 wires, without exploitation and after 8 (and 20 years of exploitation.; Figure S3. Distribution of the mass X-ray density *ρ*x(*T*) of the wire Al material along the depth *T* from the surface of the A50 and AC50 wires.

**Author Contributions:** A.A.L., Conceptualization, Investigation (XRD), Methodology, Validation, Project administration, Software, Data curation, Formal analysis, Visualization, Writing—original draft preparation, Writing—review and editing; M.V.N., Conceptualization, Investigation (densitometry, acoustic), Methodology, Validation, Software, Visualization, Writing—original draft preparation; A.I.L., Investigation (SEM, EDX, EBSD), Methodology, Software, Visualization, Writing—original draft preparation; B.K.K., Investigation (acoustic), Methodology, Software, Visualization; A.G.K., Conceptualization, Investigation (acoustic), Methodology, Software; N.D.P., Data curation; Validation, Project administration; A.G.P., Writing—original draft preparation, Conceptualization; R.V.S., Conceptualization, Methodology, Project administration; P.N.B., Supervision, Conceptualization, Methodology, Funding acquisition; M.M.S., Conceptualization, Methodology; Resources; Funding acquisition; A.V.S., Conceptualization, Methodology; Resources; I.A.B., Conceptualization, Methodology; Resources. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Russian Federation public contract No. FSWF-2020-0025 "Technique development and method analysis for ensuring power system object security and competitiveness based on the digital technologies".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The work was carried out using the equipment and software of the Center of Joint Use "Materials Science and Diagnostics in Advanced Technologies" (Ioffe Institute, St. Petersburg, Russia).

**Conflicts of Interest:** The authors declare no conflict of interest.
