Influence of Y/Zn Ratio on Secondary Phase Strengthening of Mg-Y-Zn Alloy

Abstract

By adjusting the Y/Zn (wt.%) ratio while maintaining the total mass of Y and Zn unchanged, the evolution of the microstructure and mechanical properties of Mg-Y-Zn alloy castings with different phase types was investigated. The results indicated that the studied alloy exhibited a typical $\mathsf{\alpha }$-Mg dendritic structure, which contained intermetallic compounds such as phase I, phase I + W, phase W + LPSO, and phase LPSO. The phase evolution process of Mg-Y-Zn alloys was analyzed in conjunction with microstructural observations. The strengthening effects on the mechanical properties of five alloys with different compositions revealed that as the Y/Zn ratio increased, secondary phase strengthening gradually became the primary strengthening mechanism, with the contribution order being W + LPSO > LPSO > I + W > I.

1. Introduction

Numerous studies have shown that alloying magnesium alloys with rare-earth elements effectively enhances their strength. Commonly used rare-earth elements include Y, Gd, Nd, Sm, and Ce [1,2,3,4,5]. Ternary Mg-RE-Zn alloys have attracted significant attention due to their superior mechanical properties and unique microstructures [6,7,8]. Extensive research, both domestic and international, has provided theoretical support and technical resources for improving the strength of cast magnesium alloys. The Mg-RE-Zn alloy system has become the primary high-strength cast magnesium alloy due to its abundant strengthening phases and structural units. However, this alloy still suffers from a poor ductility and unstable quality. Due to the stringent requirements for ductility and quality stability in aerospace components and weaponry components, elucidating the toughening mechanism of high-strength cast magnesium alloys and developing microstructural models for both a high strength and toughness have become urgent and critical tasks.

Early studies on Mg-RE-Zn alloys primarily focused on the addition of single rare-earth elements. Among them, Mg-Y-Zn and Mg-Gd-Zn alloys were the most representative. As a result, these two alloys became the foundation for magnesium–rare-earth alloy research, with numerous studies focusing on the transformation of their secondary phases. In Mg-Y-Zn alloys, the LPSO phase forms more readily when the Y content exceeds the Zn content. The high cost of the rare-earth element Y significantly limits the commercial application of Y-containing Mg alloys [9]. Current research indicates a proportional relationship between the phase composition of Mg-Y-Zn alloys and the Y/Zn ratio. Zhang et al. [10] studied how different Y/Zn molar ratios affect the microstructure of Mg-Y-Zn-Zr alloys. Their results showed that with an increasing Y/Zn molar ratio, the secondary phase composition transitioned from the W phase (Mg₃Zn₃Y₂) to the LPSO phase. The LPSO phase formed only when the Y/Zn molar ratio exceeded 0.88. Additionally, Luo et al. [11] established a quantitative relationship between phase composition and the Y/Zn ratio based on phase diagram calculations. Their results showed that, at a Y/Zn molar ratio of approximately 0.164, the phase composition was $\mathsf{\alpha }$-Mg + I (Mg₃Zn₆Y). When the ratio ranged from 0.164 to 0.33, it was $\mathsf{\alpha }$-Mg + I + W. At approximately 0.33, it became $\mathsf{\alpha }$-Mg + W. When the ratio fell between 0.33 and 1.32, the composition changed to $\mathsf{\alpha }$-Mg + W + LPSO, and at approximately 1.32, it was $\mathsf{\alpha }$-Mg + LPSO. Yang et al. [12] systematically analyzed the microstructure of as-cast Mg-Y-Zn alloys with different phase types by varying the Y/Zn ratio. Their results showed that as the Y/Zn ratio increased from 0.25 to 2.3, the secondary phase in the Mg-Y-Zn alloys evolved from phase I to phase I + W, then to the W phase, LPSO + W phase, and finally the LPSO phase.

In summary, the Y/Zn ratio has a great effect on the composition of the second phase in Mg-Y-Zn alloys, which, in turn, affects the contribution and contribution rate of the second-phase strengthening. However, there are few studies on the contribution and contribution rate of second-phase strengthening in Mg-Y-Zn alloys and the relationship between the contribution rate and the second phase species. Therefore, Mg-Y-Zn alloys with different Y/Zn (wt.%) ratios were prepared in this paper. The effect of second phase evolution on the strengthening mechanism and second-phase strengthening of these alloys was studied by regulating the type of second phase in the alloys.

2. Materials and Methods

Using pure Mg (99.97 wt.%), Mg-30Y (wt.%) master alloy, and pure Zn (99.99 wt.%) as raw materials, five Mg-Y-Zn alloys with different Y/Zn (wt.%) ratios were prepared by melting in a resistance furnace(Chongqing Chengfeng Electric Furnace Factory, Chongqing, China) protected by a mixture of CO₂ and SF₆ gases. First, a crucible containing pure magnesium was placed in the resistance furnace and preheated to 300 °C. After maintaining the temperature for a period of time, the temperature was raised to 500 °C and the mixed protective gas of CO₂ and SF₆ was introduced. After maintaining the temperature for half an hour, the resistance furnace temperature was raised to 720 °C and maintained until the pure magnesium completely melted. Then, pure zinc was added to the melt and maintained at 720 °C for 15 min until it completely melted. After that, the resistance furnace temperature was raised to 750 °C and maintained for 15 min, and then the Mg-30Y master alloy was added to the melt and maintained for 15 min to fully melt it. The resistance furnace temperature was then lowered to 720 °C and maintained for 15 min. Finally, the melt was thoroughly stirred, slagged, and poured into a steel mold that had been preheated to 350 °C. After the melt had fully solidified, the mold was opened and the casting was removed.

The composition of the resulting alloys was measured using an X-ray fluorescence spectrometer (XRF, Rigaku ZSX Primus III+) (Rigaku, Tokyo, Japan), and the results are shown in

Table 1. Composition of Mg-Y-Zn alloy.

Alloy Composition	Y	Zn	Y + Zn (wt.%)	Y/Zn (wt.%)
Mg-1Y-6.5Zn	1.10	6.09	7.19	0.15
Mg-2.5Y-5Zn	2.57	4.35	6.92	0.5
Mg-3.8Y-3.7Zn	4.12	3.23	7.35	1
Mg-4.5Y-3Zn	5.06	2.70	7.76	1.5
Mg-5.5Y-2Zn	5.81	1.76	7.57	2.75

. The phase of the Mg-Y-Zn alloy was determined using an X-ray diffractometer (XRD, Rigaku Ultima IV) (Rigaku, Tokyo, Japan)) with a scanning angle of 20°–80° and a scanning speed of 4°/min. The microstructure test sample was a cube with a side length of 10 mm, taken from the middle edge of the ingot. We polished the side of the sample near the edge of the ingot to a bright finish, corroded it with a 4% nitric acid alcohol solution for about 10 s, then rinsed the etching solution with alcohol and dried it. We observed the microstructure of the alloy under an optical microscope and scanning electron microscope. The microstructure and energy spectrum analyses of the alloy were performed using a ZEISS-Axiolab 5 optical microscope (Carl Zeiss AG (Baden- Württemberg, Germany)), Tescan Vege3LMH (TESCAN Group a.s. (Brno, Czech)), and JEOL JSM-7800F FEG SEM(JEOL(Tokyo, Japan)) scanning electron microscope. The transmission electron microscope test sample was made by taking a thin slice (about 0.4 mm) from the microstructure test sample, sanding it to about 50 μm with sandpaper, and then using ion thinning to obtain a thin area of the sample. Finally, the FEl Talos F200X (Thermo Fisher Scientific (Waltham, MA, USA)) thermal field emission scanning transmission electron microscope was used for observation and testing. The average grain size and volume fraction of the second phase were measured using Image-Pro plus 6.0 software. A microhardness test was performed using an HSX-1000AKY hardness tester (HV) (Shanghai Hao Micro Optoelectronic Technology Co., Ltd. (Shanghai, China)). Each sample was tested at 10 equidistant positions along the diameter direction from beginning to end, and the average microhardness value of these 10 values was taken as the microhardness of the sample. The tensile test was conducted on a new Sansi CMT-5105 testing (New Sansi (Shanghai) Enterprise Development Co., Ltd. (Shanghai, China)) machine at a tensile speed of 1 mm/min at room temperature. Figure 1 is a schematic diagram of the tensile specimen size.

3. Results

3.1. Microstructure

Figure 2 presents the XRD patterns of five Mg-Y-Zn alloys. The microstructure of these alloys primarily consists of the following four phases: $\mathsf{\alpha }$-Mg, I phase, W phase, and LPSO phase. The Mg-1Y-6.5Zn alloy contains only the I phase. With an increasing Y/Zn ratio from 0.15 to 0.5, the W phase emerges. In the Mg-2.5Y-5Zn alloy, the I phase decreases significantly compared to the previous composition. When the Y/Zn ratio rises to 1, the W phase further increases. The LPSO phase first appears in the Mg-3.8Y-3.7Zn alloy. As the Y/Zn ratio further increases to 1.5, the LPSO phase becomes more pronounced. In the Mg-4.5Y-3Zn alloy, the W phase decreases sharply compared to the Mg-3.8Y-3.7Zn alloy. At a Y/Zn ratio of 2.75, only the LPSO phase remains in the Mg-5.5Y-2Zn alloy.

Figure 3 presents the as-cast dendritic morphologies and average secondary dendrite arm spacings of the five Mg-Y-Zn alloys. The dendritic structures of all five alloys exhibit a continuous network, with numerous secondary phases dispersed near the grain boundaries. With an increasing Y/Zn ratio, secondary dendrites develop progressively, and Figure 3f indicates a gradual decrease in the average secondary dendrite arm spacing. As the Y content increases, more Y solutes are expelled to the edges of growing dendrites, expanding the undercooled region [13,14]. Meanwhile, Y segregation reduces interfacial tension, nucleation energy, and the critical nucleation radius [15]. Consequently, as the Y/Zn ratio increases from 0.15 to 2.75, the dendrite arm spacing progressively decreases.

Figure 4 presents the SEM morphologies of the five Mg-Y-Zn alloy compounds and the volume fractions of their secondary phases. The morphologies and distributions of phase I, phase W, and the LPSO phase are clearly visible. Apart from Mg, the primary secondary phase morphologies in the five alloys include white fine stripes, networks, and gray lamellar (island-like) structures, forming three distinct types. EDS analysis was performed on these three morphologies, with multiple points sampled from each alloy. The results are presented in

Table 2. The EDS results (at.%) of each point in Figure 4.

Alloy	Position	Element			Possible Phases
		Mg	Zn	Y
Mg-1Y-6.5Zn	A1	99.2	0.77	0.03	α-Mg
	A2	54.0	37.3	8.7	Mg3YZn6 [11]
	A3	36.4	53.5	10.1	Mg3YZn6
Mg-2.5Y-5Zn	B1	99.2	0.6	0.2	α-Mg
	B2	45.6	34.4	20.0	Mg3Y2Zn3 [10]
	B3	41.0	49.3	9.7	Mg3YZn6
Mg-3.8Y-3.7Zn	C1	99.5	0.2	0.3	α-Mg
	C2	53.1	28.9	18.0	Mg3Y2Zn3
	C3	86.9	5.8	7.3	Mg12YZn [12]
Mg-4.5Y-3Zn	D1	99.4	0.2	0.4	α-Mg
	D2	85.7	6.7	7.6	Mg12YZn
	D3	65.5	21.5	13.1	Mg3Y2Zn3
Mg-5.5Y-2Zn	E1	99.2	0.2	0.5	α-Mg
	E2	88.0	4.7	7.3	Mg12YZn
	E3	87.9	4.8	7.3	Mg12YZn

. Table 2 indicates that at a relatively low Y/Zn ratio, the alloy exhibits a white fine stripe morphology, with a Zn/Y (at.%) ratio of 5:1 to 6:1, closely matching the reported composition of phase I (Mg₃YZn₆). Compared to the Mg-1Y-6.5Zn alloy, the Mg-2.5Y-5Zn alloy presents a white network morphology, with a Zn/Y (at.%) ratio of approximately 3:2, aligning with the reported composition of phase W (Mg₃Y₂Zn₃). Thus, the white fine stripe and network secondary phases can be preliminarily identified as phase I and phase W, respectively. As the Y/Zn ratio increases, Figure 3c clearly shows that the white fine stripe structure vanishes in the Mg-3.8Y-3.7Zn alloy, while a gray lamellar morphology emerges, with a Zn/Y (at.%) ratio close to one, corresponding to the reported composition of the LPSO phase (Mg₁₂YZn). In the Mg-4.5Y-3Zn alloy, the LPSO phase content increases significantly compared to the previous alloys, and suspected stacking fault (SF) structures [16] become more pronounced. In the Mg-5.5Y-2Zn alloy, the white network structure vanishes completely, and numerous SF structures are dispersed throughout, aligning with the XRD results.

Figure 4f illustrates the volume fractions of the secondary phases in the five Mg-Y-Zn alloys. The Mg-1Y-6.5Zn alloy contains only a minor fraction of the I phase, as observed in Figure 4f. With an increasing Y/Zn ratio, the I phase content in the alloy decreases significantly. In the Mg-2.5Y-5Zn alloy, the I phase content drops to 0.5%, while the W phase emerges. With a further increase in the Y/Zn ratio, the I phase vanishes completely, while the W phase content slightly rises. In the Mg-3.8Y-3.7Zn alloy, the W phase content rises to 5.3%, accompanied by the formation of the LPSO phase. With a further rise in the Y/Zn ratio, the W phase content starts to decline, whereas the LPSO phase content increases significantly. In the Mg-4.5Y-3Zn alloy, the W phase content drops to 0.6%, while the LPSO phase rises to 12.6%, nearly four times that in the Mg-3.8Y-3.7Zn alloy. Finally, at a Y/Zn ratio of 2.75, the W phase vanishes entirely, while the LPSO phase content increases slightly. In the Mg-5.5Y-2Zn alloy, a substantial amount of the LPSO phase remains as the sole secondary phase.

The five as-cast Mg-Y-Zn alloys exhibit the following four distinct phase compositions as the Y/Zn ratio increases: phase I, phase I + W, phase W + LPSO, and phase LPSO. A simplified phase transformation model is illustrated in Figure 5 [12]. By analyzing the as-cast dendritic morphology in Figure 3, it can be inferred that, upon solidification, when the melt temperature drops below the melting point of α-Mg, α-Mg first undergoes L-shaped nucleation followed by an α-Mg reaction. Simultaneously, the Y and Zn atoms in the melt segregate at the liquid–solid interface, reacting with α-Mg to form intermetallic compounds. According to the literature, the melting point of the Mg-Y-Zn intermetallic LPSO phase is higher than that of the W phase and phase I. Therefore, in the Mg-5.5Y-2Zn alloy, the LPSO phase is the first to form via the following reaction: L + α-Mg → LPSO. As the Y/Zn ratio decreases to 1.5, considering that Zn atoms have a significantly higher diffusion coefficient than Y atoms, more Zn atoms segregate in the melt. Consequently, at the LPSO phase transition temperature, the W phase forms and attaches to the LPSO phase in the Mg-4.5Y-3Zn and Mg-3.8Y-3.7Zn alloys through the following reaction: L + α-Mg → W phase [17]. Further decreasing the Y/Zn ratio in the Mg-2.5Y-5Zn alloy leads to the formation of the W phase through a eutectic reaction, consuming more Y atoms and resulting in an uneven solute distribution. Residual melt regions with an elevated Zn content distribute within α-Mg via solute segregation and directly transform into phase I [18]. Finally, when the Y/Zn ratio decreases to 0.15 in the Mg-1Y-6.5Zn alloy, excess Zn atoms accumulate at the W phase/α-Mg matrix interface. Phase I forms either through the peritectic reaction L + α-Mg → I or the eutectic reaction L → α-Mg + I.

The macroscopic metallographic structures and average grain size statistics of the Mg-Y-Zn alloys are shown in Figure 6. The variation in grain size is more clearly observed in Figure 6f. As the Y/Zn ratio increases, the average grain size of the alloy initially decreases before increasing. The Mg-3.8Y-3.7Zn alloy exhibits the smallest average grain size of 361.8 ± 30 μm, suggesting that the W phase and a small amount of the LPSO phase contribute most effectively to grain refinement. In contrast, the presence of the I phase or an excessive amount of the LPSO phase reduces grain refinement strengthening.

Figure 7 shows the TEM images of the Mg-1Y-6.5Zn and Mg-4.5Y-3Zn alloys. Figure 7a–c show the STEM BF image and element plane distribution of the I phase of the Mg-1Y-6.5Zn alloy, SAED at A, and the local HAADF image in Figure 7b. It can be clearly seen that the black area contains high Y and Zn elements, while the white area is almost all Mg. It can be concluded that the white area is α-Mg, and the diffraction pattern at A is a typical 3-fold shape [12], which is consistent with the regular icosahedral structural characteristics of the I phase. It can be determined that the black area is the I phase. Figure 7d–h show the STEM high-magnification BF image, the local HAADF image in Figure 7d, and Figure 7e the local enlarged HAADF image, the STEM low-magnification BF image, SAED at B and element plane distribution of the Mg-4.5Y-3Zn alloy’s LPSO phase. It can be seen that there are obvious black stripes in the STEM BF image. According to the diffraction pattern at B, the diffraction pattern of the electron beam incident in the direction parallel to <11$\overline{2}$0> mg shows a typical 18R periodic arrangement, which can be preliminarily determined as the 18R-LPSO phase. Further analysis of the local BF image and the enlarged HAADF image (Figure 7d,e) shows that there is an atomic stacking fault of ABABCACABCBCBCAB [19,20,21], which is a typical Long-Period Stacking Ordered structure of the 18R-LPSO phase. Therefore, it can be determined that the black stripes are the 18R-LPSO phase. Figure 7i show the STEM BF image of the W phase of the Mg-4.5Y-3Zn alloy and the SAED at C. By analyzing the distance and angle of diffraction spots in the diffraction pattern with CrystBox software 1.10, it can be seen that this is the diffraction pattern of a face-centered cubic structure formed by the electron beam incident parallel to the axis of the <1$\overline{1}$1> crystal band, so the black area can be determined as the W phase.

3.2. Performance

3.2.1. Microhardness

Figure 8 presents the microhardness of the five Mg-Y-Zn alloys. As the Y/Zn ratio increases, the alloy’s microhardness gradually increases. The microhardness of the alloy primarily depends on the degree of solid solution of each element in the matrix. The solute atoms Y and Zn form a solid solution in the Mg matrix, causing lattice distortion and increasing the lattice distortion energy, which, in turn, enhances the alloy’s deformation resistance. The Y/Zn ratio itself does not directly contribute to this. Table 1 shows that, according to the EDS results, the contents of Y and Zn in the matrix gradually increase and decrease, respectively, as the Y/Zn ratio increases in the alloy. However, Y, with its larger atomic diameter compared to Zn, induces a stronger lattice distortion effect, contributing to the overall increase in the alloy’s microhardness.

3.2.2. Mechanical Properties

Figure 9 and Figure 10 present the tensile curves and mechanical properties of the five Mg-Y-Zn alloys. The tensile strength of the alloy first increases and then decreases as the Y/Zn ratio increases, reaching its maximum at a Y/Zn ratio of 1. The tensile strength, yield strength, and elongation are 179.6 ± 4 MPa, 104.3 ± 2 MPa, and 5.1 ± 0.3%, respectively. This suggests that variations in the Y/Zn ratio influence factors such as the type, size, and distribution of microstructures, which, in turn, affect the alloy’s overall mechanical properties. Microstructural analysis reveals that, as the Y/Zn ratio increases, the alloy’s microstructure transitions from the initial I phase to the W phase, and eventually to the LPSO phase. This implies that when the W phase and LPSO phase coexist in optimal proportions, they maximize the enhancement of the alloy’s overall mechanical properties. However, it cannot be concluded that the I phase does not contribute to the alloy’s strength. Its contribution to the alloy’s strength may be lower than that of the other two secondary phases. Therefore, a statistical analysis of the contribution rate of second-phase strengthening to yield strength across various strengthening mechanisms is necessary to evaluate the impacts of these three secondary phases on this contribution rate.

The force–displacement curve, obtained from the raw tensile test data in Figure 11, is integrated in Equation (1) [22], as follows:

$$\mathrm{S}=\int Fxdx$$

(1)

In Equation (1), S represents the area under the curve, F denotes the tensile force, and x signifies displacement. The area obtained can indirectly reflect the alloy’s toughness. The Mg-4.5Y-3Zn alloy exhibits the highest strength–ductility product of 5.49 J. Although the tensile strength of this alloy is lower than that of the Mg-3.8Y-3.7Zn alloy, the Mg-4.5Y-3Zn alloy exhibits a superior toughness and comprehensive mechanical properties when both strength and ductility are considered. The strength–ductility product of the Mg-3.8Y-3.7Zn alloy is 4.67 J, showing a significant difference from that of the Mg-4.5Y-3Zn alloy. Therefore, the ratio of the LPSO phase to the W phase significantly impacts the alloy’s toughness.

4. Discussion

The mechanical properties of as-cast alloys typically result from grain refinement strengthening (${\sigma }_{gs}$), solution strengthening (${\mathsf{\sigma }}_{SS}$), and second-phase strengthening (${\sigma }_{sps}$) [23,24,25,26,27].

Table 3. Contributions of three strengthening mechanisms in Mg-Y-Zn alloys.

Alloy	$\mathbf{Fine} \mathbf{Grain} \mathbf{Strengthening} ({\mathsf{\sigma }}_{\mathbf{g}\mathbf{s}}$)		$\mathbf{Solution} \mathbf{Strengthening} ({\mathsf{\sigma }}_{\mathbf{S}\mathbf{S}}$)			$\mathbf{Second}-\mathbf{Phase} \mathbf{Strengthening} ({\mathsf{\sigma }}_{\mathbf{s}\mathbf{p}\mathbf{s}}$)
	dgs/μm	${\mathsf{\sigma }}_{\mathbf{g}\mathbf{s}}$/MPa	CY/at.%	CZn/at.%	${\mathsf{\sigma }}_{\mathbf{S}\mathbf{S}}$/MPa	${\mathsf{\sigma }}_{\mathbf{s}\mathbf{p}\mathbf{s}}$/MPa
Mg-1Y-6.5Zn	483.0 ± 30	21.0	0.03	0.7	34.6	2.8
Mg-2.5Y-5Zn	465.4 ± 30	21.2	0.10	0.6	35.0	14.1
Mg-3.8Y-3.7Zn	361.8 ± 30	22.6	0.30	0.4	38.8	42.9
Mg-4.5Y-3Zn	425.0 ± 30	21.7	0.40	0.3	40.6	43.3
Mg-5.5Y-2Zn	456.2 ± 30	21.3	0.50	0.2	42.3	40.9

presents the contributions of the three strengthening mechanisms in each of the five Mg-Y-Zn alloys. As shown in the table, as the Y/Zn (wt.%) ratio increases, the d_gs (average grain size) of the alloy first decreases and then increases. The d_gs of the Mg-3.8Y-3.7Zn alloy is the smallest, indicating the most pronounced pinning effect of the second phase on the grain boundary in this alloy. Using the Hall–Petch formula, the contribution of grain refinement strengthening to the yield strength is calculated as follows [23,24]:

$${\sigma }_{gs}={\sigma }_0+\mathrm{k}{{\mathrm{d}}_{\mathrm{g}\mathrm{s}}}^{-{\displaystyle \frac{1}{2}}}$$

(2)

In Equation (2), for magnesium alloys, ${\sigma }_0$ (friction stress) = 11 MPa and k (Hall–Petch slope) = 220 MPa·${\left(\mathsf{\mu }\mathrm{m}\right)}^{-{\displaystyle \frac{1}{2}}}$ [24]. Thus, the ${\sigma }_{gs}$ maximum value for the Mg-3.8Y-3.7Zn alloy is 22.6 MPa. As shown in Table 2, with a gradual increase in Y content and a decrease in Zn content, C_Y and C_Zn (the solid solubility of Y and Zn in the magnesium matrix) also gradually increase and decrease, respectively. Hence, based on the relationship between solid solution strengthening and the solid solubility of Y and Zn elements [28], we obtain the following:

$${\sigma }_{SS}=({k_Y}^{{\displaystyle \frac{1}{n}}}{C}_Y+{k_{Zn}}^{{\displaystyle \frac{1}{n}}}{C}_{Zn}{)}^{\mathrm{n}}$$

(3)

In Equation (3), n = 2/3, k_Y (strengthening rate) = 1249 MPa·${(\mathrm{a}\mathrm{t}.\%)}^{-{\displaystyle \frac{2}{3}}}$, and k_Zn (strengthening rate) = 905 MPa·${(\mathrm{a}\mathrm{t}.\%)}^{-{\displaystyle \frac{2}{3}}}$ [29,30]. From this, the ${\sigma }_{SS}$ maximum value for the Mg-5.5Y-2Zn alloy can be calculated as 42.3 MPa. According to the relevant literature, the Orowan strengthening theory [31] primarily addresses the precipitation strengthening of prismatic second phases, such as those in alloys after aging, deformation, and other treatments. However, it is not applicable for calculating the contribution of second-phase strengthening in as-cast alloys. Therefore, there is no specific calculation formula for second-phase strengthening in as-cast alloys. Hence, only grain refinement strengthening, solid solution strengthening, and second-phase strengthening are considered. The contribution of second-phase strengthening to yield strength can be expressed as follows [32]:

$${\sigma }_{sps}={\sigma }_s-{\sigma }_{gs}-{\sigma }_{SS}$$

(4)

By subtracting the contributions of grain refinement strengthening and solid solution strengthening from the alloy’s yield strength, we can determine the contribution of second-phase strengthening to the yield strength. According to the table, the ${\sigma }_{sps}$ maximum contribution in the Mg-4.5Y-3Zn alloy is 43.3 MPa, indicating that the presence of a certain amount of the W phase is beneficial for improving the alloy’s strength.

Figure 12 shows the contribution proportions of the three strengthening mechanisms to the yield strength of Mg-Y-Zn alloys. As shown in the figure, as the Y/Zn ratio increases, the contribution of grain refinement strengthening to the alloy’s strength gradually decreases, while the contribution of solid solution strengthening first decreases and then increases. The ${\sigma }_{SS}$ contribution rate of the Mg-3.8Y-3.7Zn alloy reaches the lowest at 37%. The contribution rate of second-phase strengthening first increases and then decreases with an increase in the Y/Zn ratio. When the Y/Zn ratio is 1 or 1.5, that is, for the Mg-3.8Y-3.7Zn and Mg-4.5Y-3Zn alloys, their ${\sigma }_{sps}$ contribution rates reach the maximum at 41%, nearly half.

According to the volume fraction of each second phase in the five alloys shown in Figure 3f, it can be observed that the Mg-4.5Y-3Zn alloy has the highest total volume fraction of second phases, and its ${\sigma }_{sps}$ contribution rate is also the highest. This indicates that there is a significant relationship between the total volume fraction of second phases and ${\sigma }_{sps}$, but it is not solely dependent on it. Therefore, it is necessary to separately study the contributions of the three types of second phases to the alloy’s strength. Based on the variation patterns of each alloy ${\sigma }_{sps}$ shown in Figure 12, combined with the analysis of the volume fractions of various second phases in Figure 4f, assuming that the volume fraction of each second phase has a linear impact on yield strength, the following equation system can be formulated to calculate the yield strength contribution per unit volume fraction of the three types of second phases:

$$\left\{\begin{array}{c}2.7x_1=2.8\\ 0.5x_2+4.8y_1=14.1\\ 5.3y_2+3.4z_1=42.9\\ 0.6y_2+12.6z_1=43.3\\ 13.1z_2=40.9\end{array}\right. \text{→} \left\{\begin{array}{c}{x}_1=x_2=1.04\\ y_1=2.83\\ y_2=6.08\\ z_1=3.15\\ z_2=3.12\end{array}\right.$$

Among them, x, y, and z represent the yield strength contributions (MPa/%) of the I, W, and LPSO second phase unit volume fractions, respectively. The subscripts indicate different scenarios for the three second phases in different microstructure types. It is found that the yield strength contributions of the I and LPSO phase unit volume fractions change minimally with changes in microstructure types, while the W phase exhibits significant changes. When coexisting with the LPSO phase, the yield strength contribution of its unit volume fraction is significantly higher than that when coexisting with the I phase, indicating that the strengthening effect of the W phase varies when coexisting with different phases. When coexisting with the LPSO phase, the strengthening effect of the W phase is very obvious, with its yield strength contribution per unit volume fraction reaching 6.08 MPa/%, which is more than twice that of the I phase and even exceeds that of the LPSO phase, indicating that the LPSO phase optimizes the strengthening mechanism of the W phase. The main reason is that the W phase is significantly finer and more evenly distributed when it coexists with LPSO than when it coexists with the I phase, which greatly improves its role in blocking dislocations. Overall, the W phase contributes the most, followed by the LPSO phase, and the I phase contributes the least. However, the presence of the LPSO phase is beneficial for the W phase to exert its strengthening effect, and the ratio between these two phases seems to have a significant impact on this effect. Therefore, further research is warranted in the future.

5. Conclusions

1. As the Y/Zn ratio increases from 0.15 to 2.75, the microstructure of the Mg-Y-Zn alloy undergoes significant changes, with its intermetallic compounds transitioning sequentially from I → I + W→W + LPSO → LPSO.

2. As the Y/Zn ratio increases, the dominant strengthening mechanism in the five Mg-Y-Zn alloys changes accordingly. When Y/Zn ≥ 1, the proportion of second-phase strengthening is the highest. Based on the contribution of second-phase strengthening to the alloy’s yield strength, the contribution order of each second phase is as follows: W + LPSO > LPSO > I + W > I. The strengthening contribution of the W phase is dependent on the type of microstructure. Further research is needed in this regard. When coexisting with the LPSO phase, its yield strength contribution per unit volume fraction is the highest, exceeding that of the LPSO and I phases. However, in the combination of W + LPSO phases, there seems to be a certain ratio between the two phases that optimizes the second-phase strengthening effect. Further research is needed in this regard.

3. The Mg-4.5Y-3Zn alloy shows the largest integral area under the force–displacement curve, reaching 5.49 J, which indicates its superior toughness. Although its tensile strength is lower than that of the Mg-3.8Y-3.7Zn alloy, its overall mechanical properties are superior, with a relatively small difference in tensile strength compared to the Mg-3.8Y-3.7Zn alloy. Therefore, the optimal Y/Zn ratio is 1.5.

4. When the combined mass of Y and Zn remains constant, the lower density of the LPSO phase compared to the I and W phases, along with the easier formation of the LPSO phase at higher Y/Zn ratios, leads to a higher Y content, resulting in more secondary phases and a relatively greater proportion of secondary-phase strengthening.