**4. Discussion**

First of all, let us discuss the difference of 2 years in the service life of the compared wires (A50 N8, 10 years and AC50, N2-2, 8 years; A50 N7, 18 years and AC50, N6, 20 years). The results of data analysis have shown that the dependences of the wire parameters on the service life (unit cell parameter *a*(*t*) of Al, Figure 13a; XRD density *ρ*x(*t*), Figure 13b) are linear or close to linear (fraction of aluminum oxide *q*(*t*), Figure 11). Thus, the difference of 2 years between the compared parameters for wires of different types does not play a role because the dependences for the wires of different types differ well from each other, and the values of these parameters for a wire of the same type with a difference of 2 years are close. The same can be said about the time dependences of other parameters (Figures 16 and 19), particularly about the dependence of the deformation characteristics of wires on their service life (Figure 22).

As for the profile dependences (*a*(*T*) and *ρ*x(*T*), Figure 15a,b), then, taking into account that according to determined values of the rate of change of these quantities with time (see Section 3.4), in 2 years they will change only by ~1 · 10−<sup>4</sup> Å and ~2 · 10−<sup>4</sup> g/cm3, respectively. Moreover, the parameter *a* of the Al unit cell will increase, and the density *ρ*x will decrease. As a result, if we scale the values of *a* and *ρ*x of A50 wires (service life of 10 years and 18 years) to the values for AC50 wires (service life of 8 years and 20 years) by substraction/addition of the corresponding quantity for 2 years, then the difference between the dependences *a*(*T*) and *ρ*x(*T*) for different types of wire will change only slightly. All trends remain, and numerical values are virtually unchanged (up to the fourth digit after the decimal point). As an illustration, the SM Figure S3 shows an example of the dependencies of *ρ*x(*T*) Al wires on A50 cables, reduced to the same service life as the

AC50 cable wires. Obviously, this conclusion also applies to the profile dependence of the densitometric density *ρ*dL of NSDL (density defect Δ*ρ*dL/*ρ*dL, Figure 7).

Thus, the analysis of the data obtained indicates that when comparing the Al wires of A50 and AC50 type cables, the difference of 2 years in service life does not significantly affect the comparison results.

As stated in [37], the changes in structure, microstructure, and various physical properties, which are observed for wires in overhead power transmission cables, are probably related to processes occurring during operation. Among these processes, it is worth emphasizing the stretching of wires due to either vibrations or swaying of cables under the action of the variable wind and temperature of the surrounding air atmosphere, the phenomenon of fretting (vibration fatigue due to the friction of the wires in the cable against each other), and the formation of defects of a void nature, especially in the nearsurface layers of the wires. AAAC (A50) type cables are characterized by approximately the same ~50 mm<sup>2</sup> cross-sectional area of their aluminum component as ACSR (AC50) cables (Table 1). The most significant difference in their design is the replacement of an additional central aluminum wire in AAAC (A50) cables with a steel core wire of approximately the same diameter in ACSR (AC50) cables. As can be expected, the presence of a steel core in ACSR (AC50) type cables, which is much stiffer than the Al wires of these cables, will reduce the influence of at least some of the above factors, such as stretching and abrasion of wires, in ACSR (AC50) type cable wires compared to AAAC (A50) cable wires. As a result of using the steel core, changes in the structure, microstructure, and, as a result, in the physical properties of AC50 wires will occur more slowly.

Indeed, a comparative study of wires from A50 (AAAC type) and AC50 (ACSR type) cables, similar in geometric dimensions and structure, with the exception of the material of the central wire (Al wire in A50 (AAAC) cable and steel core wire in AC50 (ACSR), by means of the XRD, EBSD, and densitometry methods in combination with acoustic studies, has shown a noticeable difference in the structure, microstructure, and elastic-plastic properties of these wires.

The methods of EBSD, densitometry, and XRD showed the presence of NSDL (near-surface defect layer, in which most of the defects of the void nature are concentrated) in the wires of cables of both types. A significant change in the Young's modulus, decrement, and stress of the microplastic flow after the removal of the surface layer also indirectly confirmed the existence of defects in the near-surface layer of the Al wire and their significant contribution to the physical properties of the wire.

Analysis of the EBSD maps (Figure 2 and Refs. [10,37]) taken in the center and at a distance of ~150 μm from the edge of the facets of the cross-sections of the wires, which, based on the constructed histograms of grain-boundary misorientation-angle distribution (Figure 6), has made it possible to gain a qualitative conclusion about the difference in the morphology of the near-surface layers and the bulk region in wires of both types after operation. The presence of a steel core in AC50 cables makes the grain misorientation change slightly both in the bulk and in the near-surface layers of AC50 wire over a service life, in contrast to the wire from A50 cables without the stabilizing effect of the steel core, where in the near-surface layers there is a more pronounced tendency to align the crystal lattice of grains along one common direction. The formation of such a textured structure in the near-surface layers of Al wires is probably associated with the combined effect for a long time of the ambient temperature during operation, electric current, and electric voltage, as well as the tensile load, which is noticeably higher in wires of A50 type cables than in AC50 ones due to the absence of a steel core in the A50 cables. Such low-temperature annealing under load contributes to the fact that the grain size in the direction of stretching increases more than in the transverse direction, that is, there is some "pulling" of the grains in the direction of the load [53], as it was observed on the EBSD maps obtained from the longitudinal sections of A50 wires [10].

The application of the densitometry method to the wires after the sequential removal of near-surface layers of a certain thickness by etching has made it possible to quantify the

NSDLs' characteristic thicknesses (Figure 7), amounting to ~10 μm and ~30 μm for wires of both types. In both cases, when approaching the surface, the integral density *ρ*d measured by densitometric technique decreases according to a law close to the exponential-decay law. The greatest drop in *ρ*d is observed in a narrow near-surface layer up to ~10 μm (~80–85% of the total reduction), the smallest value of *ρ*d being near the surface. Taking into account the absence of light chemical elements in sufficiently large quantities according to the results of the EDX microanalysis (Table 3 and Section 3.1), obviously, it indicates the presence of defects of a void nature (nano and micropores, microcracks) in the narrow near-surface layer, the concentration of these defects increasing when approaching the surface. Deeper inside a wire, *ρ*d grows weakly until stabilization at depths from ~10 μm to ~30 μm from the surface, apparently due to a decrease in the number of defects. Such a change in the density and in the number of defects is expected since it is the surface of a wire that is affected by the environment and neighboring wires in the first place.

As noted in Section 3.3, the presence of a steel core leads to the phenomenon that in a narrow (~5 μm thick) near-surface layer of an AC50 wire, the integral density defect Δ*ρ*dL/*ρ*dL (calculated by Formula (3)) is somewhat smaller (by ~0.1–0.2%) in absolute value than in an A50 wire (Figure 7), i.e., the integral density *ρ*dL in this thin layer in the AC50 sample obtained by densitometric measurements of the wire after etching (Formula (2)) is slightly higher than that in the A50 wire. Moreover, the integral density *ρ*d of the entire AC50 wire is ~0.05% higher than that of the A50 wire.

Thus, the density of NSDL in AC50 wire and the integral density of the entire AC50 wire are greater than in A50 wire, which can be caused by various reasons.

First, when the cable is stretched because of sagging and under the influence of the surrounding atmosphere, changes in ambient temperature, and vibrations caused by the wind, the parameter *a* of the unit cell of the wire Al-material increases, as was established by XRD studies (Figure 13a). As a result of stretching the crystal lattice of Al wires after operation, the X-ray density *ρ*x of the wire Al material, calculated from structural data, decreases (see Formula (4) and Figure 13b). Obviously, this also leads to a decrease in the integral density *ρ*dL of NSDL and the integral density *ρ*d of the entire wire, measured by densitometry. The same causes lead to the formation of microvoids (microcracks) with a higher concentration of them near the surface due to the effect of fretting, which leads to a decrease in the *ρ*dL in NSDL and the integral density *ρ*d of the entire wire. In the presence of a steel core, the influence of many of the above listed factors (sagging, vibrations, etc.), leading to a reduction in the integral density of the NSDL and the whole wire, is reduced. As a result, the integral densities *ρ*dL of NSDL in the AC50 wire and the *ρ*d of the entire AC50 wire degrade (decrease) less than in the A50 wire.

Second, as is known, the maximum acceptable operating temperature of wires of the A50 and AC50 types does not exceed 90 ◦C, with the possible interval of air temperature being from −40 ◦C to +40 ◦C. It is worth recalling that the coefficients of linear thermal expansion are different for aluminum and steel. So, under certain conditions, despite the fact that steel is much stronger than aluminum, the tension of the aluminum part of the wire can noticeably weaken as the temperature rises, whereas the mechanical load on the steel elements of the wire increases slightly. It is possible that this is just a circumstance that provides a greater value of densities *ρ*dL in NSDL and *ρ*d of aluminum wires of an AC50 type cable after operation compared to an A50 cable without a steel core.

Furthermore, taking into account the fact that densities *ρ*d of wire and *ρ*dL of wire NSDL obtained by the densitometric method are integral quantities averaged, respectively, over the densities of all wire or wire NSDL components, the higher density *ρ*dL in NSDL of the Al wires of the AC50 cable compared to the A50 cable, at least at depths up to ~5–10 μm (and, accordingly, a lower value in terms of the absolute magnitude of the negative density defect Δ*ρ*dL/*ρ*dL, see Figure 7) can be due to a larger proportion of Al oxides formed in the NSDL of wires of the AC50 cable (Figure 11), probably as a result of the oxidizing action of the steel core. These aluminum oxides (δ- and/or δ\*-Al2O3) are characterized by a higher nominal calculated XRD density (~3.7 g/cm3) [51,52] compared to Al (~2.7 g/cm3) [50]. Hence, the observed integral density *ρ*dL of NSDL and, consequently, the total integral density *ρ*d of the Al wires of an AC50 type cable will be higher than that of an A50 cable. However, it should be noted that alumina crystallites formed in NSDLs of wires are much denser and harder than aluminum crystallites. It enhances the influence of the fretting effect and, as noted above, leads to the opposite effect of a decrease in density due to the formation of void defects. Since in AC50 wires, probably due to the oxidizing effect of the steel core, the proportion of aluminum oxides is higher compared to A50 wires, the influence of both effects in AC50 wires will arise competitively. On the one hand, due to the greater proportion of aluminum oxides with a higher XRD density than the XRD density of aluminum, integral densities *ρ*d and *ρ*dL increase. On the other hand, due to the greater influence of fretting, more voids are formed and the integral densities *ρ*dand *ρ*dLdecrease.

Profiling the near-surface layer by analyzing XRD reflections with different Bragg angles 2*θ*B corresponding to different X-ray-penetration depths has confirmed the presence of NSDLs in Al wires of both types and made it possible to obtain their quantitative characteristics. All trends indicated by the results of density measurements discussed above are also confirmed. As a result of the smooth expansion of the Al lattice (an increase in the unit cell parameter *a* of the wire Al material) while approaching the sample surface, the X-ray density *ρ*x calculated from the structural data decreases (see Formula (4)). This decrease in *ρ*x with decreasing depth *T* from the surface occurs according to an exponential decay law similar to the dependence of the densitometric density *ρ*dL of the near-surface layer on the thickness *T*etch of the layer removed by etching, although flatter (cf. Figures 7 and 15b).

Based on the dependences *ρ*x(*T*) obtained from the analysis of XRD reflections corresponding to different X-ray penetration depths (i.e., different depths from the wire surface), two characteristic thicknesses of NSDLs of the wires were estimated. One of the characteristic thicknesses obtained from XRD studies (Section 3.4) is close to the characteristic NSDL thickness of ~30 μm obtained from densitometric measurements. In particular, the characteristic thickness *<sup>T</sup>*layer, corresponding to the density *<sup>ρ</sup>*x*<sup>T</sup>*layer ~99.6% of the density *<sup>ρ</sup>*xbulk in the bulk of wire, is *<sup>T</sup>*layer = 36.4–39.1 μm for A50 wires and *<sup>T</sup>*layer = 35.9–38.2 μm for AC50 wires of different service life lengths (Figure 16).

Thus, the presence of a steel core (in ACSR (AC50) cable) results in a lower *<sup>T</sup>*layer, i.e., in a smaller thickness of that part of the NSDL where the main drop of the X-ray density *ρ*x occurs (~50–70% of the total decrease from *<sup>ρ</sup>*xbulk) when approaching the surface. Although the second characteristic thickness *<sup>T</sup>*layersat, which corresponds to the density *<sup>ρ</sup>*xsat ~99.99% of *<sup>ρ</sup>*xbulk and thus to the entire thickness of the layer from the surface, where almost the entire observed decrease in the X-ray mass density *ρ*x (~99%) occurs, is practically the same for A50 and AC50 type wires after 18–20 years of operation (115 μm–119 μm) and ~1.5 times more after 8–10 years of service for AC50 wire (with steel core) than for A50 wire without steel core (160 μm vs. 96 μm, respectively). At the same time, in wires without operation, the value of *<sup>T</sup>*layersat of the total thickness of NSDL in AC50 wire was, on the contrary, ~2.5 times less compared to A50 wire (respectively, ~56 μm vs. ~22 μm, see Figure 16). It is possible, however, that such a non-smooth (irregular) dependence of the total thickness of *<sup>T</sup>*layersat of NSDL in AC50 wires on the cable-operation duration *t*, in contrast to the almost linear increase in *<sup>T</sup>*layersat for A50 wires, is not due to the presence of a steel core in AC50 wires, but to the peculiarities of manufacturing the cables under study.

A noticeable difference in the amplitude of changes in the unit cell parameter *a* of the wire Al material and, thus, in both the calculated X-ray density *ρ*x and density defect Δ*ρ*x/*ρ*x depending on the depth *T* from the surface (Figure 15a,b) is another peculiarity obtained from the analysis of XRD reflections from wires, which indicates a notable effect of the steel core in cables AC50 and A50 of the same cross section of ~50 mm<sup>2</sup> of the Al component of the cables. For unused wires and at depths *T* ≈ 25 μm–36 μm from the wire surface after operation for both types of cables, the approximation dependences *ρ*x(*T*) practically coincide. On approaching the surface, the approximation curves *ρ*x(*T*) begin to diverge for operated wires of different types, and the closer to the surface they get, the greated. Moreover, in the presence of a steel core, the decrease in *ρ*x(*T*) and in Δ*ρ*x/*ρ*x

is smaller. For example, near the surface ( *T* ≈ 12 μm) of wires from cables of the AC50 and A50 types, respectively, after 8 years–10 years of operation, the density defect Δ*ρ*x/*ρ*x decreases in absolute value by ≈9 and ≈14 times compared to the value established for the depth from the surface *<sup>T</sup>*layer. After 18 years–20 years of operation, this difference is ≈21 and ≈23 times for AC50 and A50 wires, respectively.

Thus, in the presence of a steel core in the cable after operation in overhead power lines, the decrease in the X-ray density *ρ*x in the NSDLs of the cable wires when approaching the surface from the depth of the wire occurs more gently. This flatter course of *ρ*x(*T*) in NSDLs of AC50 wires indicates that the degradation ("aging") of the AC50 wires from cables with steel cores is slower. The quantitative characteristics of wire degradation were obtained by analyzing the wire parameters averaged over all observed reflections, i.e., over the NSDL with a thickness of ~35.5 μm (Figure 13a,b). In the presence of a steel core (ACSR (AC50) cable), the rate of expansion of the crystal lattice and, thus, that of decrease in the X-ray density *ρ*x of the Al material of wires in NSDL with a thickness of ~35.5 μm is ~1.2 times lower ( −2.13(7) · 10−<sup>4</sup> g/cm3/year for AC50 wires in comparison to −2.52(8) · 10−<sup>4</sup> g/cm3/year for A50 ones, see Section 3.4). As a result, the delay in the degradation of the lattice and *ρ*x of the Al material of the AC50-type wires ranges from ~1 year after a service life of ~10 years up to ~3 years after ~20 years of operation.

The method of calculating *ρ*x (namely, from the unit cell volume of the Al material in NSDLs of wires (Formula (4)) directly indicates that the cables are less stretched because of vibrations due to wind and, possibly, temperature fluctuations of the surrounding atmosphere, which are the main reasons for the higher density *ρ*x in NSDL of Al wires from AC50 cables compared to A50 ones. Due to the stabilizing effect of the steel core, the wires of an AC50 cable are less affected by vibrations and stretches. As a result, the Al lattice of the wire material of AC50 cable expands less, and accordingly, the X-ray density *ρ*x estimated from XRD structural data decreases less compared to A50 wires.

The ~10% lower rate of amplification of the effects of the preferential orientation of XRD reflections attributed to the Al material of the wires of the AC50-type cable during operation with an increase in service life from 8 to 20 years compared to those of the A50 one with an increase in service life from 10 to 18 years (Table 4 and Figure 10) can probably also be associated with the stabilizing effect of the steel core in AC50 wires and, thus, limitation of the cable stretch during vibrations because of the wind influence. This observation is also consistent with the result of the EBSD analysis (Section 3.2), that in the near-surface layer of AC50 wires there is a less pronounced tendency to align the crystal lattice of grains along one common direction than for A50 wires.

The lower stretch of the AC50 cable during service due to the stabilizing effect of a steel core in comparison to the A50 cable without a steel core is probably also a reason for the lower value of average microstrain *ε*s formed in the Al crystallites of NSDL of wires of AC50 cables after service in comparison with that of A50 cables (Figure 14b). Moreover, apparently, the same reason is associated with a lower value of microstrains *<sup>ε</sup>*ssat observed at depths *T* below the wire surface in AC50 wires after operation compared to A50 ones (Figure 19). In this case, directly at a depth from the surface down to *T* ≈ 12.5 μm, the wires of both types are apparently relaxed (*<sup>ε</sup>*s = 0), which seems to be the natural state of the surface of the wires.

One can see from Figure 15b for AC50 wires and from [10] for A50 samples that the value of the X-ray density *<sup>ρ</sup>*xbulk in the bulk of the wire at depths *T* of 200 μm and more (and, accordingly, the value *<sup>ρ</sup>*xsat = 0.9999 · *<sup>ρ</sup>*xbulk), which are estimated by extrapolation from the approximation curves *ρ*x(*T*) for unexploited samples A50 and AC50, are close to each other. Quantitative estimation by approximating curves *ρ*x(*T*) gives *<sup>ρ</sup>*xbulk = 2.6987(2)g/cm<sup>3</sup> and 2.6970(2) g/cm<sup>3</sup> for A50 and AC50, respectively. These *<sup>ρ</sup>*xbulk values also agree satisfactorily with the average density values in NSDL of wires *ρ*x = 2.6973(2) g/cm<sup>3</sup> and 2.6972(2) g/cm<sup>3</sup> (Table 4). Therefore, the X-ray densities *ρ*x estimated for NSDLs of wires and their bulk agree satisfactorily for the two types of zero-service-life wires. These estimated values of *ρ*x are larger than the tabular calculated X-ray density of Al material at a temperature of 312.3 K, which is close to the temperature of XRD measurements in this work (*ρ*x = 2.6964 g/cm<sup>3</sup> according to PDF-2 card 01-071-4008). As discussed in Section 3.4, this discrepancy may be because of the inclusion of a few Si and Fe atoms in the Al structure, which are present in the composition of wires according to EDX (Table 3, Figure 1a,b and Ref. [37]).

For wires of both types, when service life increases from 0 to 18–20 years, a decrease in both the X-ray density *ρ*x of the Al material averaged over NSDL of 35.5 μm thick (i.e., averaged over the near-surface layer with a thickness of about 1st characteristic thickness *<sup>T</sup>*layer of NSDL from the wire surface, see Figure 13b) and the integral density *ρ*d of entire wire (Figure 8) obtained from densitometry measurements is observed. Moreover, for A50 wire from the AAAC type cable, the average X-ray density defect Δ*ρ*x/*ρ*x (NSDL characteristic) and the defect of the integral (densitometric) density of the entire wire increase in absolute value from ~0% for unused wire to practically the same value ≈ −0.17% after 18 years of operation. In the case of AC50 wires from the ACSR cable, the average defect of the X-ray density of NSDL wire is also ~0% for unused wire and Δ*ρ*x/*ρ*x = −0.162(1)% after 20 years of operation, i.e., somewhat less in absolute value than for A50 wire with a comparable service life. Thus, although the tendency to decrease the density *ρ*x of the NSDL with a thickness of *<sup>T</sup>*layer ~35.5 μm is the same for both types of cables, for AC50 wire the decrease is less than for A50 wire, which once again emphasizes the stabilizing effect of the steel core in AC50 cables.

However, as one can see from the inset to Figure 15b for AC50 wires and Ref. [10] for A50 wires, unlike the average *ρ*x in NSDL of a thickness from the surface of about *<sup>T</sup>*layer ~35.5 μm, the XRD density *<sup>ρ</sup>*xsat = 0.9999 · *<sup>ρ</sup>*xbulk at depths *T* = *<sup>T</sup>*layersat (~100 μm–160 μm for wires after operation ~10–20 years) does not decrease but increases with an increase in service life from 0 to ~20 years. Figure 23 shows the obtained values of the density *<sup>ρ</sup>*xbulk and the unit cell parameter *a*bulk of the Al material in the bulk, recalculated from *<sup>ρ</sup>*xbulk using Formula (4) depending on the service life *t*. For both kinds of wires, the initial value (*t* = 0 years of operation) of *<sup>ρ</sup>*xbulk is close to the tabular value *ρ*x of pure Al powder at a temperature approximately equal to the temperature of XRD measurements in this work. As can be seen, the dependences *<sup>ρ</sup>*xbulk(*t*) for wires of both types qualitatively resemble the dependences of the fraction *q*(*t*) of the Al2O3 phases in the wires (Figure 11). For wires of the A50 cables, the dependences *<sup>ρ</sup>*xbulk(*t*) and *q*(*t*) are close to linear in the interval from 0 to 18 years, with the density *<sup>ρ</sup>*xbulk increasing from the initial value by ≈0.4% to *<sup>ρ</sup>*xbulk = 2.7092(2) g/cm<sup>3</sup> after 18 years of service. In the presence of a steel core (the AC50 cables), *<sup>ρ</sup>*xbulk increases from the initial value to almost the same value *<sup>ρ</sup>*xbulk = 2.7091(2) g/cm<sup>3</sup> after 8 years of service, slightly decreasing to *<sup>ρ</sup>*xbulk = 2.7085(2) g/cm<sup>3</sup> after 20 years of operation.

**Figure 23.** Dependences of the densities *<sup>ρ</sup>x*bulk and unit cell parameters *a*bulk in the bulk of the wires from the cables of AAAC (A50) and ACSR (AC50) types on the service life *t* of the cables.

This contradiction can probably be qualitatively explained as follows. The integral density *ρ*d obtained in densitometric investigations is a characteristic of the entire wire volume (NSDL and bulk, including all crystalline and non-crystalline phases), which is measured experimentally. On the contrary, the X-ray densities *<sup>ρ</sup>*xbulk in the bulk of the wires are estimated from the approximation dependences of the X-ray densities *ρ*x(*T*) down to large depths *T* ~200 μm from the surface, where the experimental values of *ρ*x are calculated from the experimental values of the unit cell parameter *a* of the Al material in the NSDL of wires at depths *T* from ≈12.5 μm to ≈35.5 μm. Along with a possible partial overestimation of the value of *<sup>ρ</sup>*xbulk due to experimental errors in determining the Bragg angles 2*θ*B of the observed Al reflections and, accordingly, the individual values of the X-ray density *ρx* corresponding to these reflections, another physical reason for the increase in the X-ray density of *<sup>ρ</sup>*xbulk in the volume (in the bulk) of wires can be considered. The increase in the density *<sup>ρ</sup>*xbulk in the bulk of the wires corresponds to the compression of the lattice in the bulk and correlates with the increase in the proportion of aluminum oxides δ- and/or δ\*-Al2O3 (Figure 11), with higher mass densities ~3.7 g/cm<sup>3</sup> than ~2.7 g/cm<sup>3</sup> of aluminum. Presumably, the formed aluminum oxide crystallites compress the Al lattice of the wire material, the values of *<sup>ρ</sup>*xbulk increasing consequently.

Structural and microstructural data obtained by the methods of EBSD, XRD, and densitometry fully explain the changes in elastic and microplastic properties during the operation of wires (Figure 22). The decrease in the elastic modulus *E* for the A50 for the entire 18-year period of operation and after 8 years of service for the AC50 is most likely due to a decrease in the integral (*ρ*d) and XRD (*ρ*x) density of samples detected by densitometry and XRD methods. A slight increase in the *E* modulus for the AC50 from 0 to 8 years of operation can be explained by taking into account the data on the proportion of aluminum oxides shown in Figure 11. For AC50, after 8 years of service, the volume fraction of Al2O3 oxides in the wire NSDL is almost three times greater than for A50. According to the literature data, the elastic modulus for Al2O3 is *E* = 247–380 GPa compared to *E* ~70 GPa for Al, therefore, the formation of such a layer contributes to the increase in the elastic modulus of the AC50 wire. With an increase in the service life of up to 20 years, it is likely that the formation of defects of a hollow nature (nano and micropores, etc.) in Al wires is decisive for changing the *E* modulus and leads to its decrease in AC50 wires. In addition, the alignment of the crystal structure of grains in the near-surface layer along one direction and their elongation in the direction of stretching [10] and the crystallographic texture of crystallites along [011], which is enhanced for both types of wires during their operation, can also affect the modulus of elasticity [54].

In the behavior of the amplitude-independent decrement *δi*, depending on the operating time *t* (Figure 22), it can be noted that for AC50, the decrement *δ*i changes slightly, while for A50, its changes are substantial. The observed change for the A50 is most likely due to more intensive plastic deformation processes caused by a greater load resulting from the absence of a steel core. This is also indirectly confirmed by the results of EBSD for longitudinal sections of A50 wires, according to which the process of changing the shape of the grains to a more elongated one in the direction of the load action takes place in the surface layer [10]. At the same time, the non-monotonic nature of the change in *δ*i(*t*) for A50 correlates with the dependence of the sizes of the crystallites *D*(*t*) (Table 4 and Figure 14a). These microstructural changes are associated, among other things, with a change in the dislocation structure of the material, which determines the changes in the decrement *δ*i. The same structural changes also affect the change in the microplastic stress *σ*s(*t*) (Figure 22), which demonstrates the positive effect of the steel core in overhead power line cables, leading to the preservation of a high level of *σ*s in AC50 wires with a service life of up to 20 years for the studied samples.

Thus, EBSD, densitometry, and XRD methods have shown that the steel core in AC50 type cables leads to a slower change in NSDL of Al wires from cables of this type compared to wires from A50-type cables (without steel core). Remarkably, acoustic studies demonstrated less change and even improvement in the deformation characteristics of AC50 wires compared to A50 wires, which correlates with the results of EBSD, densitometry, and XRD.
