*3.2. Influence of RE Element Addition on the Mechanical Properties of the Extruded ZK60 Alloy*

Figure 6a shows the tensile engineering stress–strain curves for all three alloys. The ultimate tensile strength (UTS) values of the ZK60, ZK60–2Y, and ZK60–2RE alloys were 299, 348, and 337 MPa, while the elongation to failure values were about 30, 21, and 25%, respectively. The formation of the new precipitates and their larger volume fraction due to the addition of Y or Ce-rich elements increased the strength of the two new alloys compared to the ZK60 counterpart. Similar results have been reported for other Mg alloys containing RE elements. For example, according to the research carried out by Sabbaghian et al., the UTS of an extruded Mg–4Zn alloy was enhanced from 301 to 336 MPa after the addition of 1 wt% Gd due to grain boundary hardening, particle strengthening, and texture hardening [27]. Moreover, another study revealed that the UTS of an extruded ZK60 alloy increased from 336 to 378 MPa when 2 wt% Yb was added [15]. In the present case, the addition of Y and Ce-rich RE alloying elements had different strengthening effects. The volume fractions of the secondary phase particles in the ZK60-2Y and the ZK60-2RE alloys were comparable; therefore, their hardening effects could be expected to be similar. On the other hand, the grain size hardening should have been higher for the ZK60–2Y alloy due to its smaller grain size (see Table 1), in accordance with the Hall–Petch relationship [28]. In addition, the strengthening contribution of the crystallographic texture was also higher for sample ZK60-2Y due to the enhanced texture intensity (Figure 5). At the same time, the dislocation density was much lower for ZK60-2Y alloy compared to ZK60-2RE which yielded a reduced dislocation strengthening contribution in the former material. However, the lower dislocation hardening was overwhelmed by the higher grain size and texture strengthening effects, resulting in a higher strength for the ZK60-2Y alloy than for the ZK60-2RE material.

Regarding the ductility, the reduced elongation to failure values for ZK60–2Y and ZK60–2RE alloys compared to the initial alloy was basically caused by the reduced grain size, since for smaller grains, the saturation of the dislocation density usually occurs earlier [22,29]. In addition, an increased amount of precipitates at grain boundaries can cause easier crack nucleation and propagation since these particles may act as stress concentration sites under loading [25,27]. The significance of the weakening effect of precipitates on grain boundary strength was more pronounced in the present ZK60–2Y and ZK60–2RE samples, in which, due to the small grain size, the applied loads were higher than that for the ZK60 alloy. Consequently, the base ZK60 alloy exhibited a greater elongation to failure than the new alloys with RE additives did.

**Figure 6.** (**a**) Tensile engineering stress–strain curves and (**b**) work hardening curves. The lines fitted on the hardening curve in stage III were used for the determination of the values of *θIII* <sup>0</sup> and *σ*<sup>s</sup> for the three studied alloys.

For investigating the WH behavior, the WH rate (*θ*) was determined according to the relationship:

$$
\theta = \frac{d\sigma}{d\varepsilon'} \tag{1}
$$

where *σ* is the true stress and *ε* is the true strain. Figure 6b shows the *θ* versus *σ* − *σ<sup>y</sup>* curve for the three studied alloys where *σ<sup>y</sup>* is the yield strength. The hardening capacity was characterized by the following equation [30]:

$$H\_c = \frac{\sigma\_{\rm UITS} - \sigma\_y}{\sigma\_y}.\tag{2}$$

In Figure 6b, the approximately linear part of the curves is related to stage III which is usually described with the help of the Voce equation, expressed as:

$$\theta = \theta\_0^{III} \left( 1 - \frac{\sigma}{\sigma\_s} \right),$$

where *σ*<sup>s</sup> is the saturation stress. The values of *θIII* <sup>0</sup> and *σ*<sup>s</sup> were obtained from the slope and the intercept of the straight line fitted to the stage III segment of the *θ* versus *σ* − *σ<sup>y</sup>* curve in Figure 6b. For the comparison of the WH behavior of the three alloys, the WH parameters are listed in Table 2. With the addition of 2% RE elements, the *θIII* <sup>0</sup> of the ZK60 alloy increased from about 1016 MPa to the range between 1090 and 1194 MPa, which was due to the stronger initial texture. The hardening capacity (*H*c) for the ZK60 alloy was 0.81, which decreased to 0.34 and 0.47 for the ZK60–2Y and ZK60–2RE alloys, respectively. The change in *σ*<sup>s</sup> was similar to *H*c, and with the addition of RE, the value of *σ*<sup>s</sup> reduced. Grain refinement in the ZK60–2RE alloy and especially in the ZK60–2Y alloy reduced the WH parameters. A similar result was obtained in a former study in which it was also suggested that the increase in the Mn content from 0 to 1.88 wt% in an extruded Mg–1Sn alloy decreased the *H*c, which was attributed to grain refinement [31]. In addition, the uniform plastic deformation stage in the tensile curve was fitted by a power-law constitutive equation:

$$
\sigma = \mathbb{K} \varepsilon^{\mathbb{n}},
\tag{4}
$$

where *K* is the strength coefficient, and *n* is the WH exponent. A higher value of *n* indicates a more pronounced WH. It can be seen in Table 2, that *n* is influenced by the addition of RE, and its value decreased from 0.3 to 0.13 and 0.19 for the ZK60–2Y and ZK60–2RE alloys, respectively. The reduction of *n* can be related to the decrease in ductility of RE-containing alloys, as shown in Figure 6a. The lower *n* value for the RE-containing alloys suggests an early saturation of hardening with the increasing strain which is a typical feature of metallic materials with reduced grain size. Indeed, the addition of RE elements resulted in a significant decrease in grain size as shown in Table 1. Additionally, with the addition of RE, the probability of activation of nonbasal slip systems increased due to the higher stress level in the samples during straining (see Figure 6a) [32]. All of these factors caused the reduction in the WH parameters of the RE-containing alloys compared to the ZK60 alloy. For the ZK60–2Y alloy, which had the smallest grain size, the values of the WH parameters are the lowest among the three studied samples.

**Table 2.** Extrapolated WH limit in stage III (*θIII* <sup>0</sup> ), corresponding saturation stress (σs), hardening capacity (Hc), and work hardening exponent (n) for the three studied alloys.


#### **4. Conclusions**

The influence of the addition of 2 wt% Y and Ce-rich RE elements on the microstructure and mechanical behavior of an extruded ZK60 alloy was investigated. The following conclusions were drawn from the experimental results:


**Author Contributions:** Conceptualization, M.S.; methodology, S.N. and M.S.; validation, M.S. and J.G.; investigation, S.N., A.S., M.S., P.N., K.F. and J.G.; data curation, S.N., A.S. and M.S.; writing original draft preparation, S.N. and J.G.; writing—review and editing, M.S. and K.F.; visualization, S.N., M.S. and K.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the International Visegrad Fund (project V4- Japan Joint Research Program, Ref. JP3936) and the National Research, Development and Innovation Office (Contract No.: 2019-2.1.7-ERA-NET-2021-00030).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

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

#### **References**


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