Effect of Y addition on microstructure and mechanical properties of cast Mg-Gd-Zn-Zr alloys

ABSTRACT

This research aims to investigate the microstructure evolution and related mechanical properties of Mg-9.5Gd-0.8Zn-0.5Zr alloys with different Y contents (x = 0, 1, 2, and 3wt%). The effects of different heat treatment conditions on these alloys were investigated. The results show that the initial microstructure of the as-cast alloy without Y is composed of α-Mg matrix and Mg₅Gd phase. Additional phases such as Mg₂₄(Gd,Y)₅ and Mg₂₄Y₅ appeared with the increase of Y content. After homogenization treatment, these phases are transformed into high temperature stable 14H-LPSO phase. It is noteworthy that the precipitation of the β′ phases in peak-aged alloys significantly enhances the strength, while the addition of Y enhances the plasticity. The peak-aged Mg-9.5Gd-0.8Zn-0.5Zr-2Y alloy exhibits the best mechanical properties, and the ultimate tensile strength, yield strength and elongation are 313MPa, 211MPa and 6.51%, respectively.

Keywords: Mg-Gd-Zn-Zr (-Y); Age hardening behaviors; Heat treatment; Mechanical properties

1. INTRODUCTION

Magnesium alloys, due to their unique physical and mechanical properties including low density, exceptional shock absorption capabilities, robust electromagnetic shielding abilities, and high specific strength, have extensively penetrated into various industries such as automotive manufacturing, biomedical equipment, aerospace technology, and consumer electronics, becoming indispensable key materials in these fields [¹,²,³]. Nevertheless, the excessively low mechanical properties of magnesium alloys, such as low yield strength and tensile strength, hinder their application in many fields. In response to the aforementioned challenges and demands for improved mechanical properties, researchers have developed a range of high-strength magnesium alloy materials. Adding rare earth elements (RE) to magnesium alloys has become a research hotspot in recent years. Studies have shown that RE elements can improve the microstructure of the alloy and improve the mechanical properties of the alloy [⁴,⁵,⁶,⁷,⁸]. Furthermore, the microstructure of Mg-Gd-Y-Zn-Zr alloys is subject to complex changes during the heat treatment process. These changes are not only affected by the composition of the alloy, but also by the homogenization treatment. The morphology and content of LPSO phase in the alloy are directly controlled by the heat treatment conditions. Furthermore, these heat treatment conditions serve as pivotal factors influencing the mechanical properties of Mg-Gd-Y-Zn-Zr alloys [⁹, ¹⁰]. Consequently, the exceptional mechanical properties exhibited by Mg-RE alloys are predominantly attributed to the presence of β’ precipitates and the second phase within their heat-treated LPSO structure, both of which play decisive roles in enhancing the alloy’s performance [¹¹].

A series of recent studies have indicated that the formation of the LPSO structure has the potential to significantly enhance the plasticity of Mg alloys. A preliminary study on the strengthening of Mg₉₇Zn₁Y₂ magnesium alloy was conducted using the rapid solidification powder metallurgy method. This material exhibits excellent UTS (ultimate tensile strength) above 600MPa, with an EL (elongation at break) of 5%. In Mg-Zn-Y alloys, the 18R-LPSO structure transforms into the 14H-LPSO structure during annealing at 500 °C [¹²]. HOMMA et al. [¹³] reported that the Mg-10Gd-5.7Y-1.6Zn-0.6Zr alloy exhibited a YS of 473MPa, UTS of 542MPa, and EL of 6% after deformation and heat treatment. The primary source of the high strength can be attributed to the formation and precipitation of both the LPSO and β′ phases [¹⁴]. XU et al. [¹⁵] have convincingly shown that the intragranular plate-like LPSO phase within the Mg-Gd-Y-Zn-Zr alloy effectively enhances its YS, whereas the presence of block-like LPSO phase at grain boundaries leads to a detrimental effect on the UTS. Furthermore, the material exhibits inferior ductility. LU et al. [¹⁶] discovered that the block-like LPSO phase in Mg-Zn-Y alloys possesses a superior strength compared to the plate-like LPSO phase. WANG et al. [¹⁷] reported an enhancement in the mechanical properties of the Mg-Gd-Y-Zn-Mn alloy attributed to the dispersion of block-like LPSO phase along its grain boundaries. GAO et al. [¹⁸] studies have shown that the introduction of Y element has a significant enhancing effect on the mechanical properties of Mg-Gd-Y alloys.

In the Mg-Gd-Y-Zn-Zr alloy system, the addition of Y element significantly affects the formation of long-period stacking ordered (LPSO) phase and the mechanical properties of the alloy. However, previous studies have primarily concentrated on the total content of rare earth (RE) elements, with limited examination into the specific effects of Y, leading to an unclear understanding of the precise contribution of Y content to alloy performance. Through meticulously designed heat treatment protocols, the distribution, volume fraction, morphology, and size of reinforcing phases within the alloy can be effectively modulated, thereby leading to a substantial enhancement in mechanical properties. Notably, current research in this field remains insufficient in elucidating the roles of Y in grain refinement and the promotion of LPSO phase formation in Mg-RE alloys. Consequently, this study endeavors to systematically investigate the microstructural evolution and mechanical property variation of Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys (with x = 0, 1, 2, and 3 wt%) under as-cast, homogenized, and peak-aged conditions. The objectives are to provide a theoretical foundation and experimental reference for further optimizing alloy performance.

2. MATERIALS AND METHODS

Mg-9.5Gd-0.8Zn-0.5Zr-xY(x = 0, 1, 2, 3wt%)alloys were cast from commercially pure Mg (99.90 at%), pure Zn (99.99 at%), pure Zr powder (99.99 wt%), Mg-30%Gd (wt%), and Mg-30%Y (wt%) master alloys. Based on the different Y content, the prepared samples were labeled as 0Y, 1Y, 2Y, and 3Y, respectively. Table 1 lists the actual chemical compositions of these alloys in detail.

Table 1
The actual composition of Mg-9.5Gd-0.8Zn-0.5Zr-xY (wt%) alloy.

ALLOYS	NOMINAL COMPOSITIONS	CHEMICAL COMPOSITIONS
No.		Mg	Gd	Zn	Zr	Y
0	Mg-9.5Gd-0.8Zn-0.5Zr	Bal.	9.45	0.79	0.53	/
1Y	Mg-9.5Gd-0.8Zn-0.5Zr-1Y	Bal.	9.52	0.81	0.47	0.98
2Y	Mg-9.5Gd-0.8Zn-0.5Zr-2Y	Bal.	9.52	0.78	0.54	2.05
3Y	Mg-9.5Gd-0.8Zn-0.5Zr-3Y	Bal.	9.49	0.82	0.56	3.12

During the alloy fabrication process, raw materials were initially heated in an argon-saturated atmosphere using a medium-frequency induction furnace. The castings were then rapidly quenched in saltwater for prompt cooling, aiding initial solidification and microstructure control. To refine grain sizes, enhance homogeneity, and optimize properties, homogenization treatment was applied by holding the ingots at 500°C for 8 hours. Following this, the ingots were swiftly quenched in 25°C water to fix the high-temperature microstructure. Subsequently, the homogenized alloy sheets underwent controlled aging treatment at 220°C to modulate age-hardening behavior.

The heat treatment instrument was the SX2-12-16A box resistance furnace. The microstructure was analyzed by the JSM-6610 scanning electron microscopy (SEM). The phases were identified by the AL-2700B X-ray diffractometer (XRD). The elemental composition of each micro-region was observed and analyzed using an Energy Dispersive Spectrometer (EDS). Vickers hardness tests were conducted on a DHV-1000ZCCD machine with a load of 1.96 N and a holding time of 15 s, with each sample being measured at least six times. Tensile mechanical properties were tested using a WDW-100D precision universal testing machine, with a tensile strain rate of 0.2 mm/min. The ultimate tensile strength (UTS), yield strength (YS, with an offset of 0.2%), and elongation were calculated as the average values from repeated tests on at least three samples.

3. RESULTS AND DISCUSSION

3.1. Initial microstructure

Figure 1 shows the SEM micrographs of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys. All the micrographs reveal a dendritic microstructure. The dendritic arm spacings of alloys 0Y and 3Y are relatively small, estimated at 8–26μm. In contrast, alloys 1Y and 2Y have larger dendritic arm spacings, estimated at 12–44μm. This can be attributed to compositional undercooling, as indicated by the large primary dendrite arms marked with red arrows. All alloy microstructures contain two phases, the bright phase is distributed in an island shape, and the gray phase is accompanied by it.

Figure 1
SEM images of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys: (a) Alloy 0Y (b) Alloy 1Y (c) Alloy2Y (d) Alloy 3Y.

Figure 2 displays an enlarged SEM image of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloy, while Figure 3 presents the microstructural EDS image of the same alloy along with the corresponding test results. The elemental composition at the detection points depicted in Figure 3 is tabulated in Table 2. Through a conjunctive analysis with the XRD patterns shown in Figure 4, it can be deduced that the island phase, gray phase, and particle phase within the alloy correspond to Mg₂₄Y₅, Mg₂₄(Gd,Y)₅, and Mg₅Gd, respectively. As the amount of Y element increases, there is a slight increase in the particle Mg₅Gd phase.It is generally believed that it is difficult to form LPSO phase in as-cast Mg-Gd-Zn alloys [¹⁹,²⁰,²¹], which is consistent with the experimental results.

Figure 2
Magnified SEM images of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys: (a) Alloy 0Y (b) Alloy 1Y (c) Alloy 2Y (d) Alloy 3Y.

Figure 3
Microstructure of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloy EDS pattern.

Table 2
EDS analysis of as-cast alloy phases at each point in Figure 2.

POINT	CHEMICAL COMPOSITION(at%)					POSSIBLE COMPOUND
	MG	GD	ZN	ZR	Y
(a)A	75.82	17.62	5.85	0.72	/	α-Mg
(b)A	82.56	12.85	3.23	0.56	0.23	Mg24(Gd,Y)5
(c)A	88.48	8.37	1.71	0.41	1.03	Mg24(Gd,Y)5
(d)B	78.03	12.75	7.92	0.61	0.34	Mg24Y5
(d)C	74.75	18.56	4.15	0.82	1.72	Mg5Gd

Figure 4
XRD patterns of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys.

3.2. Microstructural evolution via homogenization

Figure 5 shows the SEM micrographs of the Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys subjected to homogenization at 500°C for 8h, respectively. It can therefore be inferred that all Mg₂₄(Gd,Y)₅ constituents from the eutectic phase were fully dissolved within the α-Mg matrix. However, within the remaining three alloys, a notable presence of regionally distinct gray phases can be unambiguously identified, with a particular emphasis on their preferential localization at grain boundaries. After homogenization treatment, the grain size increases significantly and the volume fraction of eutectic compounds decreases significantly, especially in 0Y and 1Y alloys. Multiple large phases are scattered non-continuously along the grain boundaries, evident in Figure 5a and b. Besides the primary bulk phase, smaller lamellar structures can be observed at the grain boundaries, arranged in parallel within the crystal lattice, as illustrated in Figure 5c and d. Furthermore, in the 1Y and 3Y alloys, a rectangular cuboid phase emerges, alongside the bulk and lamellar phases. An optimal quantity of cuboid phase can enhance the alloy’s strength; however, an excessive amount of cuboid phase may result in stress accumulation in proximity to the grain boundary, ultimately diminishing the material mechanical properties [²²]. Previous studies have shown that in the homogenized Mg-Gd-Zn-Zr-Y alloy, both the bulk phase and the lamellar phase are composed of 14H-LPSO structure [²³,²⁴,²⁵]. This indicates that Mg₂₄Y₅, Mg₂₄(Gd, Y)₅ and Mg₅Gd in the as-cast alloy are completely transformed into 14H-LPSO phase after homogenization treatment. This phase transformation is the key to improve the properties of the alloy. Obviously, as the Y content rises from 0% to 2wt%, there is a noticeable increase in the volume fraction of the 14H-LPSO phase. This trend suggests the remarkable high-temperature stability of the 14H-LPSO phase.

Figure 5
SEM images of the as-homogenized Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys: (a) alloy 0Y (b) alloy 1Y (c) alloy 2Y (d) alloy 3Y.

3.3. Mechanical properties of the as-cast and as-homogenized alloys

Figure 6 shows the mechanical properties of the as-cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys. The mechanical properties of the as-cast alloy 2Y were shown to be superior to those of the other three as-cast alloys, with UTS, YS, and EL values of 259MPa, 174MPa, and 11.79%, respectively. For the as-cast alloy 3Y, its mechanical properties are the worst, with UTS, YS and elongation of 197MPa, 143MPa and 5.87%, respectively. This is attributed to the existence of a rectangular cuboid phases in the alloy 3Y, which leads to stress concentration and brittle fracture of the alloy.

Figure 6
Tensile properties of the cast Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys.

Figure 7 presents the improved mechanical properties of Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys following homogenization treatment at 500°C for 8 hours. The enhancement is a result of the combined effects of solid solution strengthening by Gd and Y elements, as well as the formation of the newly generated 14H-LPSO phase after homogenization treatment. Notably, the alloy 2Y demonstrates the optimal mechanical performance post- homogenization, featuring an ultimate tensile strength of 283MPa, a yield strength of 202MPa, and an elongation of 4.93%. It is also noteworthy that the alloy 0Y, devoid of Y, shows a substantial increase in mechanical properties after homogenization, highlighting the strengthening influence of the Gd solid solution and blocky LPSO phases.

Figure 7
Tensile properties of the as-homogenized Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys.

3.4. Age hardening behaviors

Figure 8 shows the age hardening behaviors of the as-homogenized Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys aged at 220°C. Upon the addition of 2% Y element, the alloy exhibited a pronounced age-hardening response, reaching a hardness of 116HV within a duration of 34 to 52 hours. After a duration of 22 hours, the homogenized alloy containing 3% Y attained a peak age-hardening hardness value of 113HV. Conversely, the alloy with 0% Y content reached its peak hardness of 106HV after 48 hours. For the as-homogenized alloy, the reason for the increase in hardness during aging treatment at 220°C is the precipitation of a large number of β’ precipitates. The fine β’ phase combines with α-Mg to form a semi-coherent organizational structure. This unique structure effectively inhibits basal plane slip phenomena, ultimately leading to a significant enhancement in the tensile strength of the material [²⁶, ²⁷].

Figure 8
Age hardening behaviors of the as-homogenized Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys during aging at 220°C.

3.5. Mechanical properties of peak-aged alloys

After homogenization and subsequent peak aging treatment, the tensile mechanical properties of the Mg-9.5Gd-0.8Zn-0.5Zr-xY alloy were tested at ambient temperature, and the results are presented in Figure 9. With the exception of elongation, the mechanical properties of the Mg-9.5Gd - 0.8 Zn-0.5Zr-xY alloy underwent significant enhancement following peak aging treatment. Notably, the UTS and YS of the peak-aged alloy 1Y exhibited substantial increases, reaching respective values of 304MPa and 205MPa. The peak-aged alloy 2Y distinguishes itself with its outstanding mechanical properties. In this state, alloy 2Y exhibits remarkable mechanical characteristics. Specifically, the ultimate tensile strength (UTS) and yield strength (YS) reach 313MPa and 211MPa respectively, which are significantly higher than those of similar materials. Furthermore, the elongation of alloy 2Y reaches 6.51 %, indicating that the material has a good ability to withstand deformation before fracture. The combination of these performance indicators not only demonstrates the stability of alloy 2Y under high loads but also reveals its potential value in practical applications, especially in engineering fields that require high strength and good toughness. However, it is noteworthy that despite the high Y content in the 3Y alloy, its response to aging hardening is relatively small. Specifically, the UTS, YS, and elongation of this alloy are only 290MPa, 180MPa, and 4.65%, respectively, indicating that its mechanical properties are somewhat limited. After undergoing peak aging treatment, a significant amount of β′ phase precipitates in the 3Y alloy. This precipitation process has a notable impact on the overall mechanical properties of the alloy, effectively enhancing its structural stability and improving its resistance to deformation.

Figure 9
Tensile properties of the peak-aged Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys.

4. CONCLUSIONS

• [1]

  The microstructure of the as-cast alloy without Y is mainly composed of α-Mg and Mg₅Gd phases. With the increase of Y content, Mg₂₄(Gd,Y)₅ and Mg₂₄Y₅ phases were formed in the alloy. These microstructural changes are expected to further affect the mechanical properties of the alloy.

• [2]

  After homogenization treatment, Mg₂₄Y₅, Mg₂₄(Gd,Y)₅, and Mg₅Gd phases were transformed into 14H-LPSO phase. The volume fraction of 14H-LPSO phase with good high temperature stability increases with the increase of Y element.

• [3]

  Compared with the as-cast alloy, the mechanical properties of the alloy after homogenization treatment are significantly improved. The homogenized alloy containing 2wt.% Y with a small amount of lamellar 14H-LPSO phase exhibits the best mechanical properties, with UTS and YS of 283MPa and 202MPa, respectively.

• [4]

  The Mg-9.5Gd-0.8Zn-0.5Zr-xY alloys after homogenization treatment show obvious aging hardening effect. In the peak-aged state, the precipitation of the β′ phase effectively reduces the softening effect caused by basal plane slip, thus enhancing the alloy’s strength. The peak-aged alloy containing 2wt.% Y exhibits the highest mechanical performance, with UTS of 313MPa, YS of 211MPa, and elongation of 6.51%.

5. ACKNOWLEDGMENTS

This work is supported by the Hunan Provincial Natural Science Foundation of China (2022JJ50172, 2022JJ50215), Hunan Provincial Department of Education Higher Education Teaching Reform Research Project (HNJG-2022-1018), Science and Technology Innovation Guidance Project of Shaoyang (2022GX4073), Research Project on Degree and Graduate Teaching Reform at Shaoyang University (2022JGSY003).

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Publication Dates

• Publication in this collection
  13 Dec 2024
• Date of issue
  2024

History

• Received
  10 Sept 2024
• Accepted
  01 Nov 2024