Recent advances of high strength Mg-RE alloys: Alloy development, forming and application

Recent advances of high strength Mg-RE alloys: Alloy development, forming and application

Author links open overlay panelYongfeng Li ^a, Ang Zhang ^a, Chuangming Li ^a, Hecong Xie ^a, Bin Jiang ^a, Zhihua Dong ^a, Peipeng Jin ^b, Fusheng Pan ^a

https://doi.org/10.1016/j.jmrt.2023.08.055Get rights and content

Abstract

To further expand the application of magnesium (Mg) alloys, development of the high strength Mg-rare earth (RE) alloys is strongly desired. The strength of the Mg alloys can be greatly improved through adding RE elements. This paper reviews the recent progress of the high-performance Mg-RE alloys, including alloy design, strengthening mechanism, forming processes, and applications. The review aims to provide references for designs, technologies, and applications of the Mg-RE alloys, which will contribute to further advancement of the Mg-RE alloys with excellent properties.

마그네슘(Mg) 합금의 적용을 더욱 확대하기 위해서는 고강도 Mg-희토류(RE) 합금의 개발이 절실히 요구되고 있습니다. Mg 합금의 강도는 RE 원소를 추가함으로써 크게 향상될 수 있습니다. 이 논문에서는 합금 설계, 강화 메커니즘, 성형 공정 및 응용을 포함하여 고성능 Mg-RE 합금의 최근 진행 상황을 검토합니다. 본 리뷰의 목적은 Mg-RE 합금의 설계, 기술 및 응용에 대한 참고 자료를 제공하여 우수한 특성을 지닌 Mg-RE 합금의 발전에 기여할 것입니다.

Keywords

Magnesium alloys; Rare-earth elements; High-performance; Applications

1. Introduction

In the early 19th century, silver-white metal Mg was discovered in experiments due to low density and easy machining. The Mg industry developed rapidly during World War II, and the research focused on engineering and military fields, such as incendiary, illuminating, and military equipment components. It is until the 21st century that Mg alloys attract widespread attention with excellent properties including low density, high conductivity and thermal conductivity, good damping, and electromagnetic shielding. However, further application of Mg alloys faces challenges due to relatively low strength.

To develop high strength Mg alloys, strengthening mechanisms, including fine-grain strengthening, precipitation hardening, dispersion strengthening, and texture hardening, are fully utilized. In the investigation of rare earth (RE) free Mg alloys, Mendis et al. [1] improved the yield strength and tensile strength of the Mg–Zn–Ag–Ca–Zr alloy to 325 MPa and 355 MPa after extrusion and heat treatment by adding Zn, Ag, Ca, and Zr. Kim et al. [2] found that the yield strength and tensile strength of the Mg–Sn–Zn–Al alloy can be increased to 318 MPa and 373 MPa after extrusion, and the strength of the as-extruded Mg–Al–Ca–Mn alloy can be increased to 410 MPa and 420 MPa [3]. The performance improvement can be achieved through bimodal microstructures, nanoscale precipitates, and optimized techniques.

Although RE free Mg alloys account for a large proportion, Mg-RE alloys have special advantages such as higher strength and larger heat resistance. RE elements have high valence, strong interatomic binding force, and slow diffusion rate in Mg matrix. Adding RE to Mg alloys can have many benefits, such as (i) improving high-temperature performance by forming precipitates and intermetallic compounds with high melting point and high stability, (ii) improving fluidity and casting performance by forming low melting point eutectics, (iii) purifying melt, improving microstructure and enhancing hot deformation ability, and (iv) generating stronger precipitation and dispersion strengthening effects. There are also many processing techniques for Mg-RE alloys, such as extrusion, forging and rolling, to improve the strength-ductility balance. Mg-RE alloys, albeit high cost, have been widely used in many key fields such as aerospace and defense industries [4,5].

With the increasing light weighting demand, the use of Mg-RE alloys is expected to continue increasing in the coming days. The strength of Mg-RE alloys is closely related to alloy composition, microstructure, and processing techniques. This article introduces alloy design (such as development of the Mg-RE alloy systems), strengthening mechanism (such as solid solution strengthening and aging hardening), forming processes (such as extrusion, rolling, and forging), and applications (such as aircrafts, weapons, and engines) in sequence. This paper aims to provide a review of high strength Mg-RE alloys by summarizing the progress on the microstructures and forming processes, as well as industrial applications and prospecting future development, which could provide guidance for developing high strength Mg-RE alloys.

2. Mg-RE alloys

RE elements are a general term of 17 metallic elements, and they can be categorized based on element properties, mineral characteristics, and extraction and separation methods. RE elements are usually divided into light RE and heavy RE elements according to element properties. Adding RE to Mg alloys can purify melts, form high-melting-point precipitates and low-melting-point eutectic phases, which generates stronger precipitation strengthening effect and improves high-temperature performance and casting performance. Currently, several Mg-RE alloys have been developed, such as Mg–Y, Mg–Gd, Mg–Nd, Mg–Dy, and Mg–Sm [[6], [7], [8], [9], [10]]. The strength of the Mg-RE casting alloys is mainly improved by the combination of solution and aging treatment. The Mg-RE alloys are heated to a relatively high temperature in which only single phase exists, and then rapid cooling is performed to obtain a supersaturated solid solution of α-Mg since the residual phases can be fully dissolved into the solid solution. Finally, aging is carried out at a lower temperature to obtain metastable or stable precipitates in the α-Mg matrix.

The alloy strength can be improved by changing the size, density, shape, and distribution of the precipitation phases, and the crystallographic orientation relationship between the precipitation phases and the α-Mg matrix. During preparation of high strength Mg-RE alloys, the alloy composition and technological processes largely affect the eventual microstructures and thus mechanical properties. In this review, four typical alloy systems are introduced including Mg–Y, Mg–Gd, Mg–Nd, and Mg–Er alloys, since the four elements have larger solubility in the α-Mg matrix and thus higher precipitation hardening ability.

2.1. Mg–Y based alloys

As a heavy RE element, Y has a hexagonal close-packed (hcp) structure and similar atomic radius to Mg. The solubility of Y in Mg is 12.8% at 552 °C, making it highly capable of precipitation hardening. Y can make nucleation easier by reducing the solid-liquid interfacial tension, the nucleation energy, and the critical nucleation radius. By forming rich-Y phase with high melting point, Y can act as heterogeneous nucleation point during solidification, which hinders the growth of grains. Due to high solubility of Y in α-Mg matrix and strong precipitation hardening potential, Mg–Y alloys have attracted extensive research attention [[11], [12], [13]].

To enhance the combined precipitation strengthening and good ductility of the Mg–Y binary alloys, Nd addition to Mg–Y alloys has been widely investigated. The high-strength and heat-resistant Mg alloys WE43 (Mg–4Y-3.3RE (Nd,Gd)-0.5Zr) and WE54 (Mg-5.4Y-2.3Nd-1.6Gd-0.5Zr) are two typical commercial Mg–Y based alloys, which have excellent high-temperature strength and anti-creep properties. The precipitation order of the binary Mg–Y alloys is: SSSS (super-saturated solid solution) → solute clusters → GP zones (single monolayers) → β_S′→ β, where the β_S′ phase exhibits excellent stability at 200 °C aging [14]. The phase diagram of the Mg–Y alloy is shown in Fig. 1, Mg₂₄Y₅, Mg₂Y, and MgY are important intermetallic compounds in Mg–Y alloys, and they play significant roles in precipitation strengthening during aging, in which MgY exhibits the best heat resistance. Besides, the corrosion resistance of Mg–Y alloys increased with the increase of Y content [10,15,16]. To improve the strength, Chen et al. [17] found that adding 0.4 wt% Y to pure Mg could increase the tensile strength at room temperature from 170 MPa to 200 MPa and that at 120 °C from 120 MPa to 190 MPa. Fu et al. [18] indicated that the Mg–4Y-2Nd-1Gd-0.4Zr alloy achieved the yield strength of 196 MPa and the tensile strength of 345 MPa after solution treatment at 525 °C for 8 h and aging treatment at 200 °C for 96 h. After further aging, some β′′ phases were converted into β′ phases with high density and small size, exhibiting better work hardening effect.

LPSO phase is a fundamental microstructure feature of Mg–Y–Zn alloys. When Zn is added to Mg–Y alloy, I (Mg₃Zn₆Y), W (Mg₃Zn₃Y₂), and X (Mg₁₂YZn(LPSO)) phases can nucleate and grow. The Y/Zn ratio affects the precipitation and the content of Mg₃Zn₃Y₂ and LPSO, which in turn affects the alloy performance, as listed in Table 1. In Fig. 2, the regional axis is [11 2¯ 0]_LPSO, and the thermally stable phase 14H LPSO with the stack direction vertical to the [0001]_α has optimal strengthening effect. Precipitation hardening leads to excellent mechanical properties of the Mg–5Zn–5Y-0.6Zr alloy, with the tensile strength of 412 MPa, yield strength of 380 MPa, and elongation of 11.2% [19]. The Mg₃Zn₃Y₂ increases ductility by forming homogeneous dynamic recrystallization (DRXed) microstructures, while the higher volume fraction of hard LPSO ensures strength [20]. Qi et al. [21] found that adding a small number of Mn to the Mg–Y–Zn alloy can increase the yield strength from 302 MPa to 333 MPa and the tensile strength from 380 MPa to 421 MPa by refining grain size.

Table 1. Mechanical properties of Mg–Y based alloys.

LPSO, commonly observed in the Mg–Y–Zn cast alloys, displays finer morphology and smaller size when undergoing precipitation during heat treatment and fracture during deformation process. Fine grain strengthening and dispersion strengthening due to precipitation of high volume fraction LPSO are the basic strengthening mechanisms of extruded or rolled Mg–Y–Zn alloys. Besides, recrystallized grains and nanoscale precipitated phases β′ also play important roles in enhancing the strength.

2.2. Mg–Gd based alloys

The precipitation effect of Gd in Mg alloy is very significant. It can be seen from Fig. 3 that the solubility limit of Gd in Mg is 23.49% at 548 °C, but it is reduced to below 3% at 200 °C, exhibiting good precipitation hardening [29]. In the binary Mg–Gd or ternary Mg-Gd-X alloys (X = Y [30,31], Sm [[32], [33], [34]], Nd [35,36], Zn [37,38]), as shown in Fig. 4, the performance of Mg–Gd or Mg-Gd-X alloys under the aging hardening exhibits advantages over conventional Mg–Al alloys. The aging hardening phase improves mechanical properties by hindering dislocation slip. Mg–Gd based alloys are extensively used in fields such as aerospace due to lightweight feature and good heat resistance.

The precipitation order during aging in Mg–Gd based alloys can be summarized as: SSSS (super-saturated solid solution) → β′′ (Mg₃Gd, hcp, D0₁₉) → β′ (Mg₇Gd, cbco) → β₁ (Mg₃Gd, fcc) → β (Mg₅Gd, fcc) [[41], [42], [43], [44]]. The crystallographic orientation relationship between β′′ (a = 0.64 nm, c = 0.52 nm) and α-Mg is (10 1¯ 0)_β′′//(10 1¯ 0)_α and [10 1¯ 0]_β′′//[10 1¯ 0]_α. In these precipitation phases, as shown in Fig. 5, β′, with nanoscale and high concentration distribution, is a key strengthening phase for the Mg–Gd based alloys in the aging peak. The crystallographic orientation relationship between β' (a = 0.64 nm, b = 2.22 nm, c = 0.52 nm) and α-Mg is (100)_β′//(0001)_α and [001]_β′//[0001]_α. The phenomenon of β' + β" or β' + β₁ co-existence in the aging hardening peak of Mg–Gd based alloys is also reported [[45], [46], [47]]. The aging hardening effect of Mg–Gd based alloys is dependent on the size and the number of precipitates, the plane of precipitates, and the crystallography orientation relationship.

The strength improvement induced by the diamond-shaped precipitates is crucial to the Mg–Gd based alloys, but the limited kinds of precipitates limit further strength enhancement. When adding Ag, as shown in Table 2, the higher strength is obtained [[48], [49], [50]], because γ′′ precipitates appear during the aging [49]. In Fig. 6, the strength is further improved by the combined effect of γ′′ and β′. In the Mg-Gd-Zn alloy, in addition to the precipitation sequence described above in the α-Mg matrix prismatic plane, novel precipitation sequence is observed on the base plane of the α-Mg matrix: SSSS → γ′′ (hcp, Mg₇₀Gd₁₅Zn₁₅) → γ′ (hcp, Mg-GdZn) → γ (hcp, Mg₁₂GdZn) [39]. Besides, LPSO is a crucial precipitation hardening phase in the Mg-RE (RE = Y, Gd, Tb, Dy, Ho, Er, Tm)-X (X = Zn, Cu, Co) alloys [26,51,52]. The LPSO is ordered in both composition and structure, with the constituent atoms in the α-Mg matrix orderly along the c-axis. This results in the appearance of periodic stacks of 10H (ABACBCBCABA), 14H (ABABCACACACBABA), 18R (ABABABCACACABCBCBCA) and 24R (ABABACBCBCBCBACACACACBABA) on the {0001} base plane of the α-Mg matrix [29,53], and 18R-LPSO can be replaced by 14H-LPSO during the heat treatment. As shown in Fig. 7, the crystallographic orientation relationship between 18R and α-Mg are (001)_18R//(0001)_α and [010]_18R//<1 2¯ 10>_α. The 18R crystal structure is an ordered monoclinic system with a = 1.112 nm, b = 1.926 nm, c = 4.689 nm, and β = 83.25° [54]. The 14H crystal structure is an ordered hexagonal structure with a = 1.112 nm and c = 3.647 nm, and it is composed of two structural blocks separated by three (0001)_α planes. The crystallographic orientation relationship between 14H and α-Mg is (0001)_14H//(0001)_α and [0 1¯ 10]_14H//<1 2¯ 10>_α. The 24R and 10H crystal structures can be equivalently formed by four (0001)_α planes and one (0001)_α plane respectively [55].

Table 2. Mechanical properties of Mg–Gd based alloys.

HVDC：high-vacuum die casting.

LPSO can be approximately classified into two categories: block-shaped and needle-shaped. The main reasons for the strengthening of LPSO are: (i) the crystallographic orientation relationship between LPSO and α-Mg matrix [56]; (ii) the torsion deformation of LPSO [57,58]; (iii) the increase of critical shear stress on the base plane [59]; and (iv) promoting the refinement of recrystallized grains [60]. Ozaki et al. [38] found that the tensile strength and the yield strength of Mg-17.4Gd-1.1Zn-0.6Zr after aging were 405 MPa and the 310 MPa, and the strengths at 200 °C were 390 MPa and 280 MPa, displaying excellent high-temperature performance. The tensile strength, yield strength, and elongation of the Mg-1.8Gd-1.8Y-0.7Zn-0.2Zr (mol %) alloy were 520 MPa, 473 MPa, and 8% after the extrusion and aging process. This was achieved by the small precipitates generated at the boundary between the dynamic recrystallized grains through the aging and dynamic precipitation [61], and the strength of this alloy is higher than that of the Mg–Al–Zn and Mg–Al–Mn alloys. The hot extrusion Mg-Gd-Zn alloys show high strength due to the strengthening of highly dispersed 14H-LPSO, with the yield strength of 345 MPa, the tensile strength of 395 MPa, and the elongation of 6.9% [62]. The interlaced β′, dense γ′′ on the <0001>matrix, β' + β" or β' + β₁ co-existence, and LPSO are important factors for improving the performance of Mg–Gd based alloys. Table 2 lists the tensile strength of Mg–Gd based alloys through various casting techniques and heat treatment.

2.3. Mg–Nd based alloys

The solubility of Nd in Mg alloy is 3.6%, indicating good precipitation hardening. Adding appropriate Nd to Mg alloy can effectively refine grain size and improve mechanical properties [29]. Besides, the rich-Nd has a lower corrosion rate by improving the performance of surface films [70]. The precipitation sequence in Mg–Nd alloys is: SSSS (super-saturated solid solution) → GP zones (zig-zag shape d = 0.37 nm) → β′′ (Mg₃Nd, hcp, D0₁₉, a = 0.64 nm, c = 0.52 nm, hexagonal prism) → β′ (Mg₇Nd, orthorhombic, a = 0.64 nm, b = 1.14 nm, c = 0.52 nm, lenticular shape) → β₁ (Mg₃Nd, fcc, a = 0.74 nm) → β (Mg₁₂Nd, tetragonal, I4/mmm, a = 1.03 nm, c = 0.59 nm) → β_e (Mg₄₁Nd₅, tetragonal, I4/mmm, a = 1.14 nm, c = 1.04 nm) [71]. As shown in Fig. 8, there are many second phases such as MgNd, Mg₂Nd, Mg₃Nd, and Mg₄₁Nd₅ in the Mg–Nd system, and these phases can hinder dislocation slip [72]. By adding Zn to Mg–Nd alloys, the strength of the Mg–Nd alloys can be greatly improved through solid solution strengthening. The addition of Zn can promote the element diffusion rate and benefit heat treatment processes. The precipitation sequence in the Mg-Nd-Zn alloy is: SSSS (super-saturated solid solution) → GP zones (hcp, a = 0.556 nm)→ γ′′ (Mg₅(Nd, Zn), hcp, a = 0.55 nm)→ γ (fcc, Mg₃(Nd, Zn), a = 0.70 nm) [71]. As shown in Fig. 9, which exhibits good precipitation hardening [73]. Zhao et al. [74] found that adding 0.5 wt% Nd to the ZK20 (Mg-2.11Zn-0.021Zr) alloy reduced the grain size from 24 μm to 4.6 μm, and the tensile strength reached 237 MPa. The microstructure refinement led to a 50% increase in the elongation (from 21.8% to 32.7%). In Fig. 10, Li et al. [75] found that the cyclic deformation of the Mg-3.73Nd-0.56Zr alloy mainly depended on dislocation slips (between base and non-base or ) and twins, and the tensile strength of the as-cast Mg-3.73Nd-0.56Zr alloy reached 167 MPa at 150 °C. Drozdenko et al. [76] found that the number of Mg₃Nd precipitates in Mg-8.3Nd-1.4Zn could inhibit the formation of dynamic recrystallization grains, and the yield strength reached 397 MPa by high density precipitates.

RE solutes are easier to form localized clusters that provide ordered hardening, which ensures stable strengthening effect at high temperatures [74,77]. RE atoms are larger in size than Mg, which can enhance local ordering by hardening atomic bonds and affecting local electronic density. The Nd atomic radius is 0.18214 nm, which is larger than that of Gd (0.18013 nm) and Y (0.18012 nm) [78], and thus Nd can produce stronger local ordering hardening. Co-existing Nd and Zn can form Nd–Zn dimers to exert the drag effect, which is helpful for strain hardening and high temperature strength improvement [79]. Besides, the formation of dense Nd₂O₃ layer can effectively resist stress corrosion cracking. Table 3 lists the tensile strength of Mg–Nd based alloys strengthened through different casting techniques and heat treatment.

Table 3. Mechanical properties of Mg–Nd based alloys.

2.4. Mg–Er based alloys

Er is a heavy RE element with HCP structure. The maximum solubility of Er in Mg is 32.6%, and the solubility decreases with decreasing temperature, indicating strong solution hardening effect. Er has a low electronegativity (1.24) and has a higher tendency to form stable compounds to strengthen the Mg matrix by combining with the elements with higher electronegativity such as Al and Zn. As shown in Fig. 11, the Mg–Er alloys have precipitates such as MgEr₂, MgEr, Mg₂Er, and Mg₂₄Er₅. The MgEr₂ and MgEr phases are formed at 1255 °C and 830 °C, respectively, and the forming process occurs in a wider decomposition range, and the two phases diffuse to the rich-Mg region at 670 °C. Mg₂Er forms at 670 °C, and the phase diagram shows that the solubility range is narrow. The peritectic Mg₂₄Er₅ forms at 600 °C, and the lattice constant slightly decreases with the increase of Mg content [83]. Table 4 shows the specific lattice parameters of each phase.

Table 4. Lattice parameter data of Mg–Er alloys.

B₀(GPa): bulk modulus, B': pressure derivative of bulk modulus,ΔH (kJ/mol-atoms): enthalpy of formation.

Adding high-oxidation-affinity RE to Mg alloys can improve oxidation resistance. Wu et al. [84] found that the double Er₂O₃ layers, which are composed of dense fine-crystalline layers and coarse columnar layers, play crucial roles in the anti-oxidation ability of the Mg-8.1Er alloy. Zhang et al. [85] found that the adding of Er promoted the formation of Al₈Mn₄Er, reduced the content of Fe, and decreased the electrochemical corrosion tendency, which resulted in a more passivation coating with higher density. In the extruded Mg-14.4Er-1.4Zn-0.3Zr alloy, the anti-corrosion ability of the alloy is improved, e.g., the corrosion rate at 3.5 wt% NaCl is 1.11 mm y⁻¹, due to formation of a quasi-passivation corrosion film by the release of Er³⁺ after corrosion of the rich solution layer fault on the base plane [86]. Besides, Er can inhibit dynamic recrystallization behavior, refine grain, increase strengthening phase particles, and enhance mechanical properties (especially the improvement of elongation).

3. Forming processes of Mg-RE alloys

The forming processes of the Mg-RE based alloys can be mainly divided into liquid forming, semi-solid forming, and solid forming. Liquid forming mainly includes gravity casting, low pressure sand casting, high-pressure casting, etc. Semi-solid forming includes thixo-forming and rheo-forming. Solid forming, also known as plastic forming, includes extrusion, rolling, forging, etc.

3.1. Liquid forming

Mg-RE alloy castings are usually formed by gravity casting (GS) with steel molds or sand molds. To reduce additional energy consumption and material costs during the cutting and surface processing of the castings, the pursuit of net forming or near net forming promotes the rapid development of precision liquid forming technologies, e.g., low pressure sand casting (LPSC) and high pressure die casting (HPDC).

3.1.1. Gravity casting

GS is a traditional casting that has been developed for 6000 years [90]. GS can be divided into sand mold casting and metal mold casting. During sand mold GS process, proper inhibitors and suitable gating system should be determined to reduce turbulence when the molten metal fills the mold cavity. This technology has the disadvantages of lower precision and poor heat dissipation. As a comparison, the metal mold has good heat conductivity, resulting in high cooling rates and fine grain structures [91]. The metal mold can be repeatedly used, but it is usually used to form the parts with non-complex shapes due to difficulty of removing from the mold. However, the poor air permeability in the metal mold might cause misrun, crack, and pore defects. So, it is necessary to design a reasonable casting system to avoid these defects [92]. Okayasu et al. [93] found that the tensile strength and yield strength in the as-cast Mg₉₇Y₂Zn₁ (at%) alloy are 184 MPa and 139 MPa, which is better than AZ91 (Mg-8.5Al-0.5Zn-0.15Mn) and AM60 (Mg-5.8Al-0.32Mn). Zhang et al. [94] found that the different cooling rates caused changes in the microstructures, resulting in the increase of the tensile strength from 136 MPa to 169 MPa and the increase of the yield strength from 81 MPa to 107 MPa in the as-cast Mg-Nd-Zn-Zr alloy. However, the strength and plasticity of GS Mg-RE alloys are generally poor due to oxidation inclusions, shrinkage porosities, and coarse grains.

3.1.2. Low pressure sand casting

LPSC technology uses gas pressure to press molten melt into the mold, and the melt solidifies under the gas pressure. LPSC can obtain the parts with good surface quality, good dimensional accuracy, and fewer shrinkage micro-porosity due to smooth filling and sequential solidification characteristics [95]. The LPSC technological parameters, such as filling time, hold pressure time, mold temperature, and pouring temperature, largely affect the solidification microstructures, heat treatment methods, and mechanical properties of the Mg-RE alloys [96]. The Mg-10Gd-3Y-0.5Zr alloy formed by LPSC exhibits excellent high-temperature performance, and the tensile strength and yield strength are 368 MPa and 255 MPa after solid solution and aging treatment in room temperature [97]. Similar conclusion was found by Liu et al. [98] The casting defects such as porosity, defect bands, oxide film, etc. can be significantly reduced in LPSC. Particularly, LPSC is a suitable technology for producing Mg-RE parts with thin walls (2–5 mm) and complex structures [99].

3.1.3. High pressure die casting

The application of HPDC in Mg alloys is becoming increasingly popular, especially in the automotive industry [100]. HPDC can produce large-sized, thin-walled, and complex shapes parts. It has significant advantages in production efficiency, production costs, and complete connectivity of the parts [101]. As shown in Fig. 12, the boss structure in HPDC Mg alloys can be designed with various reinforcement structures, making it possible to use threaded forming fasteners in cast holes without the need for drilling [102]. The Mg-8Gd-1Dy-0.3Zn alloy by HPDC has the outstanding mechanical properties due to the small grain boundaries, stable skin region and dispersed precipitates [103]. The grain boundary strengthening and the thermal stability of precipitates reduce the creep rates of the HPDC Mg–La-RE (Nd, Y or Gd) alloy [104].

Generally, HPDC parts cannot be further strengthened through solid solution and aging treatment because the melts during the die-casting can cause high-speed turbulence, leading to disorderly flow and large amount of entrapped gas. The entrapped gas shrinks and deforms, leaving on the surface or inside, resulting in a higher number of micro-pores in the die-cast parts [105,106]. Once undergoing heat treatment, the cracks can originate from micro-pores during tensile deformation, and the surface of the part will blister, which deteriorates material properties. By using high-vacuum assisted high-pressure die-casting (HV-HPDC) technology, the micro-porosity rate of Mg alloys can be greatly reduced, effectively avoiding the formation of micro-pores, and the properties can be improved through aging treatment [107,108], which expands the application range of Mg alloys. Li et al. [69] found the performance was greatly improved due to the aging hardening in the HV-HPDC Mg-6Gd-3Y-0.5Zr alloy, and the yield strength, tensile strength, and elongation reached 173 MPa, 248 MPa, and 17.5%, respectively. The addition of RE (such as La and Ce) can enhance the high-temperature performance and anti-creep properties [[109], [110], [111], [112]] due to forming high-temperature stable phases, but the hot tearing or sticking defects might occur when excessive RE is added. Similar conclusion was reported by Nie et al. [113], as shown in Fig. 13, the nanoscale precipitates increased the yield strength by ∼34 MPa (∼26%) after aging. However, the HV-HPDC has the disadvantages of the relatively high cost and low productivity. Besides, the external solidified cells (ESCs) and the skinning effect are difficult to avoid. The ESCs will increase the unmeasurable failure of the castings, and the skinning effect will lead to segregation of RE and deteriorate mechanical performances [39].

3.2. Semi-solid forming

Semi-solid forming refers to the process utilizing the good flow-ability of semi-solid metals in the two-phase transformation from solid to liquid or from liquid to solid. Semi-solid forming is a novel and advanced technology. Compared with traditional liquid forming, semi-solid forming has the advantages of the lower forming temperature, longer mold life, improved production conditions, refined grains, fewer porosities, and denser microstructures. Semi-solid melts have thixotropic and rheology properties, and thus semi-solid forming includes thixo-forming and rheo-forming. Fig. 14 is a schematic diagram of the semi-solid forming.

3.2.1. Thixo-forming

Thixo-forming is used to process the preheated materials that is in the semi-solid temperature range by traditional forging (or casting) techniques. In thixo-forming, after heated to semi-solid state with globular microstructures, the alloy block is pressed into the mold by punch. The fine globular particles can be “frozen” at room temperature, improving the mechanical properties through grain boundary strengthening [114,115]. Wu et al. [116] prepared Mg-10Gd-3Y-0.5Zr by isothermal treatment and found that the equiaxed microstructures could refine the initial particles, homogenize the grains, and shorten the heating time. Meng et al. [117] obtained uniform and isotropy globular microstructures by reheating extruded Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr to 580 °C at a rate of 20 °C/s and holding isothermally for 20 s in a rotary electric resistance furnace, as shown in Fig. 15. Xie et al. [118] found Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr could be formed without defects or cracks at 580 °C by thixo-forming through different temperatures and forming loads tests. Although Mg-RE alloys can form uniform globular α-Mg structures through heating and isothermal treatments, large amount of work (e.g., how to reduce high product cost and how to control oxidation skin defect) still needs to be done in thixo-forming.

3.2.2. Rheo-forming

To reduce the high cost of preparing the semi-solid metal in thixo-forming and further improve the quality of castings, the rheo-forming technology of the semi-solid slurry is developed. The materials utilization rate of rheo-forming is close to that of HPDC, higher than isothermal stamping and superplastic forging. Rheo-forming can manufacture complex and large-sized parts, and reduce the tonnage and energy consumption of the forming equipment. Fig. 16 shows the diagram of the rheological squeezing casting (RSC) process. Fang et al. [119] prepared the Mg-3RE-6Zn-1.4Y alloy (RE = 65% Ce + 35% La) by rheological squeezing casting and found with the increase of pressure, the grain size of α-Mg became finer, the solid solubility in matrix increased, and the rich-RE-intermetallic compound became uniformly distributed along the grain boundary. When the pressure was increased to 200 MPa, the yield strength, the tensile strength, and the elongation were 110 MPa, 180 MPa, and 8.6%, respectively, which were improved by 17.0%, 19.2%, and 81.7% than those under no pressure. Similar conclusions were also found by Chen et al. [120], in which the nucleation rate and dislocation density of Nd and Zn in the α-Mg matrix increased with pressure in the Mg-3Nd-0.2Zn-0.4Zr alloy. The yield strength, the tensile strength, and the elongation after solid solution aging treatment reached 165 MPa, 309 MPa, and 5.7%, respectively. However, entrapped defects [121] and inter-granular segregation [122] can worsen the mechanical properties during the rheo-forming. Designing and optimizing the alloy materials to reduce and eliminate the forming defects are the primary challenges in the rheo-forming Mg-RE alloys.

3.3. Solid forming

Solid forming is the manufacturing technology that the desired shape, size, and mechanical properties of billets or parts are obtained by the plastic deformation under external forces. Solid forming requires external force and the formed material has plasticity. The principal forming methods include extrusion, rolling, forging, drawing, and stamping. The overall properties of cast Mg alloys are usually poor due to certain microstructure defects, and the cast Mg alloys are generally used in parts without requiring higher mechanical properties (such as steering control arms, etc.). The Mg-RE alloy parts fabricated by the solid forming can effectively reduce and eliminate most defects during casting and improve mechanical properties by reducing grain size [123,124]. Extrusion, rolling, and forging are three common solid forming techniques, and this section provides a review of them.

3.3.1. Extrusion forming

Mg alloy parts such as pipes, bars, profiles, strips, etc. are mainly manufactured by extrusion. The extrusion process can refine grain size, improve strength, and gain good surface quality. The extrusion forming of Mg alloys can be divided into warm extrusion and hot extrusion, and the extrusion temperature is typically between 300 and 450 °C. In the hot extrusion, the mechanical properties can be largely improved by refining the grain size [125]. Besides, the hot extrusion can also change the texture [126] and promote the generation of dynamic recrystallized grains and un-recrystallized grains [127]. The texture has an important effect on the plasticity of Mg-RE alloys. Guo et al. [128] found that after applying 7% pre-deformation along the extrusion direction, the rotation of the texture in the base direction increased the yield strength of the Mg-15Gd-0.5Zr by 103 MPa. Fig. 17 shows the EBSD analysis of the extruded Mg₉₇Zn₁Y₂ alloy. Yamasaki et al. [129] found that after extrusion, the microstructure can be divided into three regions: small DRXed α-Mg grains with weak texture, large un-DRXed α-Mg grains with strong texture, and fiber-like LPSO grains. The small DRXed grains with weak texture are beneficial for the ductility, while the large un-DRXed grains with strong texture are mainly helpful for improving the strength. After extrusion, the grain refinement caused by recrystallization increased the yield strength, the tensile strength, and the elongation of the Mg-11Gd–1Zn alloy from 159 MPa, 196 MPa, and 3.0% to 191 MPa, 335 MPa, and 9.0%, respectively [130]. Nie et al. [131] found that the yield strength, the tensile strength, and the elongation of the extruded Mg-14Gd-0.5Zr alloy were 190 MPa, 290 MPa, and 19.5% respectively. Although the extruded Mg alloy has excellent performance, there are some disadvantages, such as slow extrusion speed, high deformation resistance, and anisotropy of mechanical properties caused by texture after extrusion.

3.3.2. Rolled forming

Mg alloy strips and sheets are generally manufactured by rolled forming. Rolling can improve the mechanical properties of the Mg alloys by changing the microstructures such as refining grain size. Rolling is an important process to produce high performance Mg alloy sheets, and the investigation of the short process, high efficiency, and low-cost rolling technology has long been the research focus. Rolling process parameters are crucial to the properties of Mg alloys [132]. The alternating change in the direction of tensile and compressive stresses during rolling process can generate finer recrystallized grains and weaker base texture [133]. In Fig. 18, Yao et al. [134] found that the deformation mechanism of the hot rolling Mg-8Gd-2Y-0.35Zr alloy in the low strain region was the tension twins and prismatic slip, and in the high strain region was the tension twins, dynamic recrystallization and multiple slip mechanisms. The microstructures of the Mg-8Gd-3Y-0.4Zr alloy which are composed of ultrafine dynamic recrystallized grains become homogeneous after rolling [135]. Furthermore, in Fig. 19, the hard-plate accumulative roll bonding can generate homorganic fine grain structure and tight interface bonding, while decreases warping and edge cracks caused by severe plastic deformation [136]. However, Mg-RE alloys have limited sliding systems, and the strong basal texture is easily formed during the rolling process, resulting in lower plasticity and easier occurrence of edge cracking [132]. Therefore, developing cost-effective and high-efficient rolled forming technologies to manufacture high performance Mg alloy strips and sheets are hot research topic in recent years.

3.3.3. Forging forming

Forging is a common metal processing technique, and Mg alloys forging products are often used in important load-bearing structural parts. Hot forging can improve the poor processing ability caused by the hcp structure of Mg [137]. Temperature, strain rate, friction, pre-form shape, and material properties all have important impacts on the forging process [138]. The lower the forging temperature, the higher the precision, but the lower the formability [139]. As the forging temperature increases, the mechanical properties of the edge region are greatly affected, which further affects the casting properties [140]. Fig. 20 shows the effects of temperature and strain rate on the volume fraction of recrystallized grains and grain size [141]. In the forged Mg-8.59Gd-3.85Y-1.14Zn-0.49Zr alloy, the effect of the deformation temperature on the grain refinement is more notable than the strain rate due to the significant difference in the number of β phases at different temperatures [142]. Wang et al. [143] found that the yield strength, tensile strength and elongation are 292 MPa, 384 MPa and 9.0% in the Mg-6.2Gd-3.7Y-0.9Zn-0.3Zr-0.3Ag alloy by forging. Precision forging, developed from traditional forging, can produce high-precision and complex-shaped forgings, while also increases the bearing capacity of the forged parts [144]. Compared to the HPDC and semi-solid forming, the precision forging has the advantages of high production efficiency, high yield rate, and low cost.

4. Applications of Mg-RE alloys

There has been enormous increase in global energy demand with the growth of the world's population and urbanization [145]. Fully utilizing the resource and lightweight advantages of Mg alloys has great significance for achieving energy conservation, emission reduction, carbon neutrality, and carbon peaking strategies [[146], [147], [148], [149]].

Mg-RE alloys develop rapidly and are widely applied in fields such as aerospace, biomedicine, and national defense [150]. WE43 and Mg-Nd-Zn-Zr alloy have been investigated and applied as orthopedic implants (scaffolds and screws). The Mg–Y-Nd-Zr and Mg-3Nd-1Gd-0.4Zn-0.5Zr alloy are firstly accepted for applications in jet engines and military aircrafts, and the WE43C alloy can also be used as aircraft seats, integrated reduction gearboxes and other structural components [100,151]. The use of Mg-3Nd-1Gd-0.4Zn-0.5Zr alloy can make military helicopters obtain fast speed at higher temperature [151]. In Fig. 21, Mg-10Gd-3Y-0.5Zr alloy has been enrolled in certain type of weapons, such as the light missiles and radar components [39]. In Fig. 22, Mg-RE alloys also play an irreplaceable role in automotive manufacturing parts, such as V6 engine blocks manufactured by the Mg-2.85Nd-0.18Zn-0.45Zr alloy [152]. High-performance Mg-RE alloys can be applied in (i) aerospace and missile (such as Mg-Gd-Y series), (ii) automotive engine casing, transmission housing, engine cylinder head and other components (such as Mg–Y-Nd-Zr series), and (iii) it is expected to be used as artificial bone graft materials, cardiovascular stent materials, and to replace existing metal implant materials (such as Mg-Nd-Zn-Zr series).

5. Summary and outlook

This paper reviews the research of high-strength Mg-RE alloys in recent years, including alloy properties, forming techniques, and applications. The following conclusions can be drawn.

• (1)Four Mg-RE alloy systems with strong aging hardening effects are reviewed. Improvement of the overall performance is closely related to the size, shape, crystallographic orientation, and precipitates distribution on basal and prismatic planes. The aging strengthening phases β′ and β′′ with specific crystallographic orientation relationship to the matrix have significant effects on the high-strength Mg-RE alloys. The precipitation of γ′, γ′′, and LPSO with large aspect ratio can improve the comprehensive properties of the Mg-RE based alloys.
• (2)Reducing the grain size of the α-Mg matrix can be one of the primary focuses in the design of Mg-RE alloys, because the grain refinement is crucial for developing high-strength Mg-RE alloys. Grain refinement can be achieved by adding grain refiners during the melting process or through deformation processes such as extrusion, rolling, and forging. LPSO can significantly improve the strength and can also be a key point for the development of the high strength Mg-RE alloys.
• (3)High strength Mg-RE alloys with different applications require different processing methods. HV-HPDC is important in the production of large-scale parts. However, the external solidified cells and the skinning effect are difficult to avoid during HPDC. Extrusion forming can produce parts with fine grain size and excellent combined properties, but large deformation resistance and anisotropic mechanical properties can impair the material properties. Precision forging can form high precision and complex-shaped parts, however it has the disadvantages of serve mold accuracy requirement, complex forming processes, and high cost.

With the development of automobile, aerospace, biomedicine and electronics, the demand for Mg-RE alloy parts has been increasing. Mg-RE alloys can improve alloy strength and satisfy the current application requirements in certain extent. However, the development of high-performance Mg-RE alloys still requires further efforts, and the following aspects can be prospected.

• (1)The strengthening effect of single RE is not appealing, and the strengthening mechanism of multiple REs needs to be explored. Particularly, how to develop low RE-containing but high-performance alloys requires further investigation, since REs are nonrenewable strategic resources with high cost.
• (2)The strength-ductility synergy improvement needs more attention due to few slip systems in Mg alloys with hcp structure. The alloy design theory based on thermo-kinetics principles, e.g., “solid solution strengthening and ductilizing”, can be further utilized to optimize the strength-ductility balance.
• (3)The alloy performance deteriorates with the temperature. Developing high-strength and heat-resistant Mg alloy deserves more investigation on elements, contents, phases, and structures.
• (4)The greenhouse gases generated during the production process of Mg-RE alloys may counteract the reduced emission caused by lightweight. Developing the recycling technologies of Mg-RE alloys are crucial for improving lightweight efficiency.
• (5)The sustainability, availability, and criticality of materials are crucial for improving the life cycle dimension of Mg-RE alloys. A comprehensive optimization is required in areas including mining of raw materials, smelting technology, alloy design, use of cast parts, and recycling and reuse.
• (6)The application and promotion of Mg-RE alloys in large-scale parts face challenges in terms of gating system optimization, alloy performance improvement (e.g., fluidity), melt protection (e.g., refining and purification), and defect control (e.g., entrapped gas, hot tearing and oxidation).

Developing short-process, energy-efficient, and material-saving preparation and processing technologies have a long way to go. Continuous innovative technologies and equipment are expected to optimize the design and forming of Mg-RE alloy parts and to promote the application of Mg-RE alloys in more fields.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.