Mechanical Properties of Magnesium-Rare Earth Alloy Systems : A Review

Magnesium-rare earth based alloys are increasingly being investigated due to the formation of highly stable strengthening phases, activation of additional deformation modes and improvement in mechanical properties. Several investigations have been done to study the effect of rare earths when they are alloyed to pure magnesium and other Mg alloys. In this review, the mechanical properties of the previously investigated different magnesium-rare earth based binary alloys, ternary alloys and other higher alloys with more than three alloying elements are presented.


Introduction
Magnesium is the sixth most abundant element in the earth's crust and is the lightest of all structural metals with a high specific stiffness.This is one of the prime reasons automobile manufacturers are in a quest to replace denser materials with magnesium (Mg) based materials.However, poor formability (ductility) and secondary processing induced crystallographic asymmetry (texture effects) due to the hexagonal closed pack (HCP) crystal structure represent the major limitations of Mg.It has limited slip systems and the activation of non-basal slip is difficult at room temperature, thereby limiting the ductility [1,2].This limitation is being overcome with the development of new magnesium based alloys [3].Amongst the several common alloying elements, rare earth (RE) addition has given promising results in terms of weakening the texture and improving the deformability of Mg [4,5].Further, the hard eutectic phases formed as a result of RE addition also aid in increasing the strength of the alloy [6].Thus, besides improving the ductility and formability, REs also act as effective strengtheners.Generally, the strengthening of Mg by the addition of RE is believed to be by a solid solution strengthening mechanism and a precipitation hardening mechanism [7].
The Mg-RE based alloys serve a useful purpose in automotive industry as superior light metal-alloys in cast or wrought condition [8,9].Moreover, the Mg-RE alloys also gained prominence in biomedical applications as biodegradable implant materials that aid in healing of the tissues and leaving no implant residues e.g., bone implants, stents [10][11][12].In order to serve such applications, the Mg-RE alloys are to be fabricated economically with minimum alloying compositions and simple processing.The Mg-RE phase diagrams suggest that each RE behaves uniquely when it is added as a dominant alloying element [13].Hence, we study the effects of alloy compositions and processing on microstructure and texture and their effects on the tensile properties.This will in turn help to design alloy systems at the least cost to meet the requirements of the industry.The present article will give an in-detail review of the tensile and compressive properties of different magnesium-rare earth (Mg-RE) alloy systems.

Yttrium
Addition of yttrium (Y) as an alloying element in Mg has been tested and tried by many researchers owing to the fact that there exists a large difference in the atomic radii of Mg (145 pm) and Y (212 pm) which allows strengthening of Mg by both solid solution strengthening and precipitation-strengthening (upon decomposition of the supersaturated solid solution).Zhao et al. [14] reported that Y with max solubility of 4.7 at.% (15.28 wt.%) in Mg can effectively strengthen Mg by solid solution strengthening.Similar observations regarding solution strengthening effects of Y in Mg have been made by Gao et al. [7].In this study, cast Mg-Y alloys containing different Y concentrations ranging between 0.7 wt.% and 6.5 wt.% (below the solubility limit) were investigated.The properties of those Mg-Y alloys after heat treatment at 525 °C for 2-12 h are summarized in Table 1.It can be seen that the addition of Y resulted in the enhancement of strength properties, however, at the expense of ductility.Xuenan et al. [15] investigated Mg-1%Y alloy in as cast and rolled conditions and reported that by rolling, the strength is improved and the elongation is decreased as compared to the as-cast Mg-1Y alloy.The authors also reported that Y has a negative effect on magnesium corrosion properties and Mg-1Y indicated no significant toxicity to osteoblasts and can be considered for biomedical alloy design.Zhou et al. [16] investigated the room temperature mechanical properties of as-extruded Mg-3%Y (extruded at 350 °C) and reported a tensile ductility as high as 33%.It is also interesting that the incremental tensile elongation has occurred without compromising the strength properties (Table 2) and the reason for such mechanical characteristics was reported to be due to the activation of multiple deformation modes.Similar observations of increases in both strength and ductility (Table 2) simultaneously were made by Edassiqi et al. [17] for Mg-2%Y and Sandlobes et al. [18] for Mg-3%Y where both the alloys were hot rolled and annealed.Contrarily, Wu et al. [19] reported increase in elongation to failure from 15% in pure Mg to 30% in Mg-4%Y alloy but this increase in elongation occured at the expense of strength (Table 2).This is reported to be due to the texture effects in case of Mg-2%Y and Mg-4%Y alloys extruded at 420 °C after heat treatment at 480 °C for 12 h.It has also been reported that the addition of Y to Mg as a solute in case of ultra-rapidly solidified and extruded Mg-alloys reduces critically resolved shear stress (CRSS) required to operate the pyramidal slip system.This activation of pyramidal slip system is believed to produce extensive plasticity.Hence, the ductility seems to increase in Mg-10%Y as compared to Mg-5%Y in reference [20].The increase in strength from 5 wt%-10 wt% (Table 2), in this study, is reported to be due to the precipitation of β' phase from supersaturated solid solution [20].

Cerium
Cerium has almost negligible solid solubitlity in Mg at room temperature and is a eutectic forming solute (α-Mg + Mg12Ce) [21].Previous investigation [21] has shown that the addition of Ce as an alloying element not only produces the dispersion hardening effects but also contributes towards the intergranular percolation strengthening.Mishra et al. [22] investigated the microstructure, texture and mechanical properties of Mg-0.2 wt.%Ce alloy.In this study, the addition of 0.2%Ce to Mg has improved the tensile ductility of Mg from 9.1%-31% (Table 3).The increase in ductility however occurred alongside reduction in 0.2% offset yield strength.The reported mechanical properties were attributed to the favourable crystallographic orientation (rare earth assisted texture randomization) of Mg grains achieved upon dynamic recrystallization during extrusion.The ductility enhancement, in this study, was also accompanied with a little increase in the ultimate tensile strength as compared to pure Mg due to the reduction in the grain size upon Ce addition.Chino et al. [23] claimed that the improvement in ductility due to 0.2%Ce addition as compared to pure Mg is due to the increase in stacking fault energy unlike reduction in c/a ratio that led towards basal/non basal slip activation.Luo et al. [24] reported that the addition of 0.5% Ce to Mg resulted in more surface oxidation during extrusion and reduction in ductility and increase in strength.The authors also suggested that Ce concentration should not be higher than 0.5% in extruded alloys due to the excessive surface oxidation during extrusion.For Mg-1%Ce alloy [25], significant increase in ductility from 2.7%-11.9%was observed after annealing at 350 °C (Table 3).Chia et al. [6] have reported that when Cerium is added as an alloying element to Mg, an intermetallic Mg12Ce forms and the effect of increase in volume fraction of intermetallic is more than the effect of morphology of the intermetallic in increasing the strength and reducing the ductility of the alloy.

Gadolinium
Gadolinium has a solublity of 23.49 wt.% at eutectic temperature [26] in Mg and thus contributes to solid solution strengthening when alloyed with Mg [12].Hort et al. [12] reported that property profile of Mg-Gd alloys is similar to that of the cortical bone and can be adjusted over a wide range.They also reported that these alloys have better elongation to fracture compared to other metallic implant materials like stainless steels, etc. Gao et al. [27] studied the effects of Gd on the solid solution strengthening of Mg alloys.In this study, it was reported that Gd (due to size misfits and valency effects) is an effective solid solution strengthener in Mg as compared to Al and Zn.Peng et al. [28] reported that melt spun Mg-20Gd alloy contained mostly the supersaturated α-Mg solid solution while the as-cast Mg-20Gd alloy comprised of α-Mg + Mg5Gd.In this study, the melt spun alloy exhibited fine grain morphology thus having higher strength when compared to that of the as-cast aloy containing metastable phases (Table 4).Stanford et al. [29] investigated the microstructure-texture-mechanical property relationships in Mg-Gd alloys containing upto 4.5% Gd.They reported that the addition of Gd upto 1% was shown to significantly weaken the recrystallization texture.However, with further addition of Gd, it remained largely unchanged.Similarly, the strength values (Table 5) also increased until 1% Gd addition and remained unchanged thereafter.The authors stated that upon recrystallization annealing, the Gd solute locks the dislocation movement and causes matrix hardening.However, it is reported that such a mechanism did not affect the ducility of the developed Mg-Gd alloys.The same authors in [30] reported that Gd addition to Mg weakens the texture and produces the rare earth texture-component thereby resulting in extended plasticity when tested along the extrusion direction.They further reported that Mg-1.55Gd alloys exhibited high ductility when extruded at 450 °C as compared to 510 °C due to the suppression of formation of RE texture component at higher extrusion temperature.It is also interesting to note that the tensile ductility of Mg-1 wt.%Gd alloy was increased from 4.8%-30% upon annealing between 350 °C and 450 °C [25].The properties of the Mg-Gd alloys available in literature are shown in Table 5.

Lanthanum
Lanthanum has limited solid solubility in Mg and has a very high eutectic temperature of 612 °C.Due to its poor solid solubility in Mg, Mg-La alloys do not undergo age hardening [31].Chia et al. [6] reported that Mg-La eutectic is lamellar and with the increase in La content, strength (Table 6) is seen to increase due to the Mg12La intermetallic.This was observed with the reduction in ductility.The have also reported [6] that La is largely believed to be an effective grain refiner as well as an effective texture modifier in Mg alloys .In a study by Stanford et al. [30], it was reported that Mg-La alloys extruded at 450 °C exhibited new texture component that is similar to "Rare Earth Texture" along 1121 direction parallel to the extrusion direction.However, the development of such texture components was suppressed in those Mg-La alloys extruded at higher temperatures thus resulting in lowering of the ductility (Table 6).

Erbium
Erbium is one of the rare earths that is well soluble in Mg.Wu et al. [32] investigated the strengthening of the extrusion texture component <0 0 0 1> parallel to the direction of extrusion occured with the addition of Er.This developed to RE texture after complete recrystallization in Mg-6Er.The tensile ductility (Table 7) of the Mg-Er alloys was reported to be due to the reduction in the c/a ratio and development of texture that led to the activation of different modes of plastic deformation.A high tensile ductility of about 30% was observed in Mg-6Er alloy.In Mg-3.6%Er [33], the authors reported that ageing at 200 °C led to the dynamic strain ageing that gave rise to the serrated flow behaviour.

Neodymium
Neodymium has the highest solid solubility in Mg and lowest eutectic temperature of about 552 °C and shows best response to age hardening when added to Mg due to its high solid solubility [6].Chia et al. [6] reported that unlike Ce and La, Nd forms Mg3Nd phase which is a very hard phase and Metals 2015, 5 not Mg12Nd phase.They have attributed the same for better strength of Nd containing Mg alloys as compared to that of Ce and La containing Mg alloys.In contrast, Jingli et al. [34] reported that the microstructure of Mg-Nd alloys consists of dendritic α-Mg and divorced eutectic Mg12Nd.The improvement in strength with increase in Nd content was attributed to both solid solution hardening and precipitation hardening.Seitz et al. [35] studied Mg-2 Nd alloys in different extruded and heat treated conditions and reported that the high elongation ratios combined with the low yield strength (Table 8) and low degradation of the Mg-2Nd alloys make them promising for resorbable stent applications and comparable to the conventional WE 43 alloys.

Dysprosium
Dysprosium has a high solid solubility in Mg and the melting point of the intermetallic Mg24Dy5 is 560 °C [36].It is expected to improve the room temperature mechanical properties of Mg by solid solution strengthening and precipitation hardening as the solubility decreases drastically with decrease in temperature.The ductility decreases with increase in Dy content and is very poor at room temperature [37,38].The authors [37] reported that of all the alloys, Mg-10Dy can be developd further for biomedical applications due to its mechanical and corrosion properties.They also suggested in reference [38] that ageing treatment at 200 °C can be selected for applicability of the Mg-Dy alloys as bone fictures as low ducility and high strength are required.The mechanical properties of various Mg-Dy binary alloys are shown in Table 9.

Ternary Systems
Several compositions are tried where rare earths are major as well as minor alloying elements in order to investigate the influence of the rare earths on the properties of the Mg based alloy.In this section, properties of such investigated Mg-RE based ternary alloys containing magnesium, rare earth element and another alloying element i.e., Mg-X-RE (X = RE, Al, Zn, Zr, Sn, Mn, Cu) are discussed.

Mg-RE Ternary Systems
The Mg-10Y-2.5Smalloy was investigated by Zhang et al. [39] and it was reported that Mg24Y5 phase was distributed in α-Mg matrix uniformly and no phases contained Sm.The tensile properties, in the study, are attributed to the solid solution strengthening effect of Sm and strengthening by Mg24Y5.In the Mg-8.3Gd-1.9Eralloy, the strength increased with ageing due to the β' phase that has formed by precipitation mechanism.Gavras et al. [40] investigated Mg-La-(Y,Gd,Nd) systems and reported that the tensile strength of Mg-2.5La-3.6Ndsystem is highest of all the investigated Mg-La-(Y,Gd,Nd) alloys with a value of 195 MPa.It was reported to be due to the higher amount of intermetallics in the eutectic that settled at the grain boundaries.The tensile properties of the above discussed alloys are shown in Table 10.

Mg-Al-RE Ternary Systems
Luo et al. [24] reported that the addition of 0.2 or 0.5% Ce to Mg-3Al did not show significant improvement in tensile properties due to the affinity of Ce for Al thus forming Al11Ce3 in Mg-Al-Ce ternary alloys resulting in the properties as shown in Table 11.

Mg-Zn-RE Ternary Systems
Luo et al. [42] investigated the Mg-Zn-Ce alloys and reported that the addition of Zn to Mg-0.2Ce alloys has improved the strength significantly with a slight reduction in ductility and attributed it to the independent effect of solid solution strengthening by Zn.The authors also reported that Zn does not react with Ce thereby producing random texture in extrusion.Le et al. [43] investigated Mg-2Zn-0.4REalloys and reported that the highest strength was seen in Ce containing alloy and highest ductility in Y containing alloy.Wu et al. [44] reported that the excellent ductility of the rolled Mg-Gd-Zn alloy sheets is due to the texture weakening effects of Gd.The effects of ultrasonic treatment was studied on Mg-5Zn-2Er alloys [45] and it was reported that the improved mechanical properties are a result of cavitation and acoustic streaming during ultrasonic treatment.The properties of cast Mg-5Zn-0.63Eralloy were studied under heat treated and peak aged conditions [46].In this study, it was reported that the properties were high in aged condition due to the presence of rod-like MgZn2 particles.Wang et al. [47] suggested that the texture of as-extruded Mg-xZn-xEr was weakened by the recrystallization via particle stimulated nucleation (PSN).Srinivasan et al. [48] investigated Mg-Gd-Zn alloys and reported that the Mg-10Gd-xZn alloys (x = 2, 6) exhibited good strength due to the 14H-type LPSO phases present in the matrix while those alloys containing lower Gd exhibited good ductility due to the lower fraction of LPSO.Singh et al. [49] attributed the high strength of the Mg-Zn-Y alloy to the nano-quasi-crystalline phase that has formed during extrusion.The properties of all the alloys that are discussed/investigated are listed in Table 12.

Mg-Zr-RE Ternary Systems
Huang et al. [4] reported the effect of multi-micro alloying of RE on the ductility of Mg alloys.Different rare earths were studied in the ternary system and the best ductility was observed with Gd addition to Mg-0.5Zr.Investigations on Mg-0.6Zr-8Gd were done in [53] and the properties were attributed to the precipitate β-Mg5RE (Gd/Er) and dispersed β'-Mg15RE3 (Gd/Er) metastable phase.The properties are shown in Table 13.

Mg-Sn-RE Ternary Systems
The properties of Mg-Sn-RE system have been investigated by Zhao et al. [54].In this study, Y was used as the rare earth element and it was reported that when Y is 1.5%, MgSnY phase forms and with increase of Y to 3%, MgSnY+Sn3Y5 phases form and at 3.5%Y, Sn3Y5 phase forms.The combined effect of intermetallics in Mg-3%Y is responsible for the higher properties as shown in Table 14.Wang et al. [55] reported that Mg-8.23Sn-2Nd exhibited the best tensile properties.This was related to the microstructure as α-Mg, Mg2Sn and Mg-Sn-Nd phases were present in the microstructure of the alloys and the strength was attributed to the change in morphology of the Mg-Sn-Nd phase and size of the Mg2Sn phase.The properties of the different Mg-Sn-RE alloys that are investigated are shown in Table 14.

Other Ternary Systems
The addition of Er to Mg-1.8Mn resulted in the increase in ductility and this was attributed to the resistance to recrystallize and retard the grain growth with the addition of Er [56].The highest tensile properties were found in alloy containing 0.7% Er.Du et al. [57] reported that 18R LPSO phase is formed in Mg-10Er-2Cu alloy that resulted in the properties as shown in Table 15.

Higher Alloy Systems
In this section, magnesium rare earth alloys containing more than three elements are discussed.The alloying elements whose effect is studied include REs, Al, Li, Zr, Zn, Sn, Mn, Cu, Ca and V.

Mg-RE Higher Alloy Systems
Zhang et al. [58] reported the properties of WE43 degradable biomaterial in extruded conditions as reported in Table 16.In reference [59], the authors reported the properties of Mg-4Y-3.2RE at room temperature and reported that ageing improved the strength of the alloy significantly.Su et al. [60] reported the properties of peak aged WE43 alloy as shown in Table 16.Mukai et al. [61] reported the properties of the WE43 alloys in annealed, extruded and aged conditions and reported that the extruded alloy exhibits superior properties in terms of strength and ductility due to the fine grained microstructure and transition from intergranular to transgranular fracture with grain refinement.Panigrahi et al. [62] studied the effects of forging and ageing on the mechanical properties of WE43 alloys and reported that the improvement in strength is due to the combined effect of grain refinement, work hardening and precipitation strengthening.The improvement in ductility is also reported to be due to the limited intergranular fracture and activation of non-basal slip prior to twinning.

Mg-Al-RE Higher Alloy Systems
Rzychoń et al. [63], in their study on Mg-Al-RE alloys, reported that when RE/Al ratio is >0.5, no Mg17Al12 phase forms.The Mg17Al12 phase has lower melting temperature compared to the other Al11RE3, Al2RE phases and thus when the ratio between RE and Al is maintained at an optimum level, the thermal stability of Mg alloys can be improved.In reference [64], the authors investigated the microstructure and mechanical properties of Mg-10Gd-3Y-0.8Alalloys and reported that the microstructure of cast Mg-10Gd-3Y alloy was refined with the addition of 0.8%Al and when solution treated at 520 °C for 6 h + 550 °C for 7 h, the ductility improved from 5%-13%.Further, they also discussed the effect of ageing on the properties of the alloy.The strength increased due to the precipitation strengthening and solute dissolution of the intermetallic particles in the solute.Zhang et al. [65] investigated the properties of Mg-3.0Al-1.8Ce-0.3Y-0.2Mnalloy and found that it exhibited high structural stability and strength due to the presence of dendrite boundaries with Al11(Ce,Y)3 intermetallics.The strength enhancement, in this study, is also thought to be due to the solid solution strengthening effects of Y. Similar strengthening mechanism was reported in reference [66] by the same authors wherein Ce was mostly present in the Al11RE3 and Y was observed to exist in Al2RE.Rzychoń et al. [67] investigated the properties of AE44 alloys and reported that the high pressure die cast alloys exhibited better properties when compared to the sand cast alloys.They attributed the same to the low solidification rates in the sand cast alloys that led to the unfavorable morphology of the intermetallics Al11RE3 (needle shaped) and Al2RE (polyhedral) and coarse grained structure.Zhang et al. [68] also reported that when La substitutes Ce rich misch metal, α-Mg and Al11La3 phases are observed instead of Al11RE3 and Al2RE.It was reported that La containing alloy exhibits better properties due to the stability of Al11La3 phase in Mg-4Al-4RE-0.4Mnalloy.The same author also reported in [69] that with an increase in Ce content, the strength was improved and was attributed to the acicular morphology of the main strengthening phase, Al11Ce3.A similar trend was observed with La, Pr and Nd in [70][71][72].Wang et al. [73] also observed that Mg-5Al-0.3Mn-1.5Cealloy exhibited the best tensile strength due to the presence of optimized content of Al11Ce3 + β-Mg17Al12.Chen et al. [74] reported that the addition of Nd led to the formation of higher melting Al-Nd intermetallic and also improved the room temperature strength of the Mg-6Al-2Ca-xNd (x = 1,2) alloys.Wu et al. [75] reported that the addition of REs and Ca to AZ91 alloys led to the improvement in strength as well as corrosion resistance due to the presence of Al2Ca phases.The properties of the above discussed alloys are given in Table 17.

Mg-Li-RE Higher Alloy System
Krausse et al. [76] investigated the biodegradation behavior of LAE442 alloys and reported that the alloys have sufficient initial strength to be used in weight bearing applications in bones.In reference [77], the authors characterized Mg-1.21Li-1.12Ca-1Yalloy and reported that the alloy exhibited better tensile properties in extruded state due to the refinement of microstructure.The authors also indicated that the corrosion resistance of extruded alloy is better than the as-cast alloy due to the delay of the initiation of the corrosion pits.Tao et al. [78] investigated the structural and mechanical properties of Mg-Li-Al-Zn-xRE alloys containing varying RE content between 0.2% and 1%.In this study, it was reported that the microstructure of the alloys mainly comprised of α-phase, β-phase, Mg17Al12 phase, Mg64.5Li34.3Al0.9Zn0.3 and A12Zn2La intermetallic compounds.They also reported that besides reducing the laminar spacing of the matrix, RE also acted as an effective grain refiner.The improvement of strength at both room temperature and high temperature was attributed to the formation of A12Zn2La strengthening phase.Zhou et al. [79] studied the Mg-Li-Al-RE alloys and reported that the high properties of the alloys are due to the addition of Al and rare earths that result in grain refinement, solid solution strengthening and dispersion strengthening.They also reported the applicability of the alloys for cardiovascular stent materials due to the good corrosion resistance and good cytotoxicity test results.Wang et al. [80] studied the Mg-8Li-1Al-1Ce alloy in as-cast and extruded condition and reported that the α(Mg) phase and β(Li) phase are refined after extrusion and the long rod-like Al2Ce present in the as-cast state becomes short and rod-like after extrusion which is responsible for the properties as shown in Table 18.

Mg-Zr-RE Higher Alloy Systems
For Mg-10Gd-3Y-0.4Zr, the mechanism of strengthening is similar to as discussed in the case of Mg-10Gd-3Y-0.8Al[64].Mg-xY-1.5LPC-0.4Zr(x = 0, 2, 4, 6) (LPC = 85% La, 8% Ce, 7% Pr) alloys were investigated in [82].In this study, the authors showed that the tensile properties of the Mg-Zr-RE alloys improved with an increase in Y content and it was attributed to the distribution of the cubic β-Mg24Y5 precipitate phases and prismatic β′ phases in Mg matrix.Mohri et al. [83] investigated the Mg-4Y-3Re-0.5Zralloy in different conditions and reported that the material extruded at 400 °C exhibited the best properties due to the presence of fine spherical precipitates in the grains.Zhang et al. [84] reported that the solution treatment and ageing treatment can enhance the strength of the alloy.The authors also reported that the in vitro degradation rate of the alloy increases by solution treatment and decreases by aging due to coarse microstructure and relief of internal stresses in the precipitation phase, respectively.Zhang et al. [58] reported that Mg-0.44Zr-3.09Nd,0.22Zn (JDBM) alloy exhibited superior mechanical properties (due to finer grain size) as well as biocorrosion properties as compared to WE43 and AZ31 alloys and thus is a promising degradable biomaterial.Su et al. [60] reported the age hardening behavior and mechanical properties of the Mg-4Y-2.4Nd-0.2Zn-0.4Zralloy and suggested that the presence of fine β'' precipitates in the matrix result in the high mechanical properties in the peak aged condition.Zhang et al. [85] reported that the cyclic extrusion resulted in better mechanical properties and bio corrosion of the Mg-2.73Nd-0.16Zn-0.45Zralloy.In reference [86], the authors studied the mechanical properties and biocorrosion behavior of the Mg-Nd-Zr-Zn alloys at different extrusion ratios.With extrusion ratio 8, the alloys exhibited high strength and moderate elongation while with extrusion ratio 25, the alloys exhibited high elongation and moderate strength.Thus, the authors suggested the optimal properties of the alloy when the alloy undergoes dynamic recrystallization and the growth is suppressed.The authors also indicated that the corrosion properties and cytotoxicity of the alloy meet the requirement of the cell toxicity and hence this makes it a potential biomaterial.Zhang et al. [87] reported the effect of double extrusion on the improved mechanical properties and improved corrosion resistance of the biodegradable Mg-Nd-Zn-Zr alloy due to the grain refinement.Kielbus et al. [88] investigated Mg-4Y-3RE and Mg-3Nd-1Gd alloys and reported that Mg-4Y-3RE alloys exhibit higher tensile and creep properties due to the presence of higher stable Y containing phases in Mg-4Y-3RE alloys.Zheng et al. [89] investigated the effect of thermomechanical treatment on Mg-6Gd-2Nd-0.5Zrand reported that cold deformation increase from 5%-10% accelerated the age hardening response of the alloy and improved the strength.They also reported that hot extrusion and ageing lead to the very high tensile properties of the alloy as reported in Table 19.Xiao et al. [90] investigated the Mg-10Gd-3Y-0.5Zrand reported that the Friction Stir processing (FSP) led to grain refinement and dissolution of the eutectic Mg5(Gd,Y) thereby improving the ductility.Ageing treatments done after the FSP led to the improvement in the strengths.Similarly, Li et al. [91] studied the effects of FSP on WE43 alloy and reported that the improvement in mechanical properties is due to the refinement of grains in the alloy's microstructure.He et al. [92] studied WE 93 alloy and it was observed that the extruded and aged alloy exhibited the best combination of tensile properties.This was reported to be due to the Mg24Y5 phase that is completely dissolved in Mg12(MM) (MM = Misch Metal) which is present around the grain boundaries.In [93], the authors reported that the rolling process has effectively reduced the grain size and improved the mechanical properties.The properties of the different alloys discussed above are reported in Table 19.Mg-3.99Y-3.81Nd-0.53Zr-260 8 As-Cast + Frictrion stir processed at  * indicates the compressive properties of the same alloy.

Mg-Zn-RE Higher Alloy Systems
Xu et al. [94] reported that the as cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zralloy consisted of α-Mg grains surrounded by Mg3(Gd,Y) eutectic compounds while the as-homogenized alloy consisted of 18R and 14 H type LPSO phases which was attributed to the higher strength of as-homogenized alloy.The same authors reported in [95] that the Mg-8.2Gd-3.8Y-1.0Zn-0.4Zralloy exhibited better properties when it is rolled with 96% reduction.Xu et al. [96] reported that upon ageing, the microstructure consisted of β' phase in the α-Mg grains and LPSO and Mg-Gd-Y containing phases at grain boundaries.Freeney et al. [97] reported the effects of Friction stir processing and ageing on the grain refinement and breakage and dissolution of second phase particles that resulted in the increase in the strength of the aged alloy.Yang et al. [98] attributed the high strength of the GWZ930 (Mg-9Gd-3Y-0.5Zn-0.5Zr)alloy mainly to precipitation strengthening and slightly to grain boundary strengthening.Al-Samman et al. [99] investigated sheet texture modification in ZEK100 alloys containing rare earths Ce, Nd, La, Gd and Ce rich Misch metal and reported that the Gd containing alloy has the highest tensile ductility of 30% and a very low tensile yield strength.This was because the Ce, La, Nd and Misch metal containing alloys were said to depict a common rare earth texture while Gd containing alloy revealed a different type of rare earth texture.In Mg-Zn-Mn based alloys, Stulikova et al. [100] reported that the as-cast MgY4Zn1Mn1 alloy contained 18R LPSO which were responsible for the high strength.For MgCe4Zn1Mn1 alloy, it was reported that Mg12Ce phase is present along with small particles of Zn and Mg which pins the dislocations thereby increasing the thermal stability of the alloy.Dobron et al. [101] investigated the effect of variation of extrusion speeds on the mechanical properties of ZEK100 alloys and reported that the increase in extrusion speed leads to the recrystallized and homogenized microstructure without much effect on the texture.Garcia et al. [102] investigated ZEK 100 alloys (with RE = Ce rich Misch metal) which are considered important biomaterials and reported the tensile and compressive properties with respect to the extrusion speeds.The addition of Ce rich misch metal led to the grain refinement leading to very high tensile and compressive properties and also inhibited grain growth due to the presence of intermetallic particles distributed in the matrix.With the addition of Ca to Mg-Zn-RE based alloys, Kamrani et al. [103] studied that the microstructure consists of Mg-Ca besides the Mg-RE precipitates that are responsible for the strength.The tension-compression asymmetry was studied and analyzed to be due to the grain size, texture, precipitates that are present at the grain boundaries and inside the grains.Zhang et al. [104] studied the effects of Er and reported that Er played a major role in enhancing the ductility as well as solid solution strengthening effects.Zhang et al. [105] also reported that the addition of Er has caused the interactions between dislocations and solute and thereby caused yield point phenomenon.Yu et al. [106] investigated Mg-Zn-Zr alloys and reported that with the addition of Gd to the alloys, quasicrystal I-phases (Mg-Zn-Gd ternary phase) are formed along the grain boundaries and increased with increasing the Gd content.In a similar study, Xiao et al. [107] reported that the Al2REZn2 phases are formed with increase in RE content.They also reported in [108] that after ageing, precipitates such as Mg24Y5, W-phase and I-phase were formed and affected the tensile properties of Mg-alloys as shown in Table 20.Langelier et al. [50] reported that the combined micro alloying of Ce and Ca results in the formation of Mg6Ca2Zn3 particles and MgZnCe T-phase in annealed alloys that limit the texture effects due to their large size and coarse distribution.The improved properties of the alloys are due to the grain boundary pinning by Mg6Ca2Zn3 precipitates.Li et al. [109] reported that the addition of Nd to Mg-5Zn-0.6Zrled to the change in the morphologies of the phases.The continuous intergranular phases in Mg-Zn-Nd-Zr led to the significant deterioration in the strength and ductility in the as-cast alloys.Yu et al. [110] studied the effect of extrusion speed on the mechanical properties and reported that the texture intensity decreased with the increase in extrusion speed thereby improving the tensile properties due to the increased fraction of unDRXed grains.Xu et al. [111] reported the properties of forged ZK60-Y alloys as shown in Table 20.Wang et al. [112] studied the effects of addition of RE to Mg-8Zn-4Al alloy and reported that a new quaternary Mg3Al4Zn2RE phase is formed and the microstructure is refined with increase in the RE due to the crystal multiplication mechanism and prevention of the grain growth by the quarternary phase that result in the tensile properties as shown in Table 20.* indicates the compressive properties of the same alloy.

Mg-Sn-RE Higher Alloy System
Cheng et al. [115] reported that peak ageing occurred faster with the addition of Ce to the Mg-5Sn-4Zn alloy.They showed that the microstructure consisted of Sn-Ce precipitates in the aged alloy in addition to the α-Mg phase and Mg2Sn phase.It was also reported that with the addition of Ce, the Sn-Ce phase predominates reducing the Mg2Sn phase thereby affecting the properties as shown in Table 21.

Other Higher Alloy Systems
The combined addition of Er/Al by microalloying has led to improved mechanical properties due to the reduced grain size and homogenous microstructure after particle-stimulated nucleation assisted dynamic recrystallization [56].Du et al. [57] reported that the addition of V resulted in improvement in morphology of 18R LPSO thereby resulting in improved properties as compared to Mg-10Er-2Cu stated in Table 15.The properties are as shown in Table 22.Hence, the rare earths have an immense effect on the properties of the Mg alloys but the cost of most of the rare earths is higher compared to the conventional alloying elements due to the scarcity in their availability.

Conclusions
In this review, the mechanical properties of various magnesium-rare earth alloys processed under different conditions, investigated by various researchers, are reviewed, and the reasons for their mechanical behavior are studied.The tensile properties of the investigated binary alloys which include Mg-Y, Mg-Ce, Mg-Gd, Mg-La, Mg-Er, Mg-Nd, and Mg-Dy systems are reported.The report also includes the tensile and compressive properties of Mg-RE ternary system containing two different rare earths as alloying elements and Mg-Zn-RE, Mg-Zr-RE, Mg-Sn-RE, and other ternary systems are reported.Finally, the properties of Mg-Al-RE, Mg-Li-RE, Mg-Zr-RE, Mg-Zn-RE, Mg-Sn-RE and other higher alloy systems containing three or more alloying elements are also briefly studied.
Rare earths are widely added to Mg in the form of Misch Metals which are unspecified blends of RE due to the low cost of Misch metals.However, considering the microstructure and mechanical properties of different Mg-RE systems, each one behaves differently from the others.So, it would be necessary to indicate the actual type and composition of RE addition in order to attribute the effect of RE addition to the properties.Further, in biodegradable Mg-RE alloys, each RE element has unique toxicity level and self-degradation period and therefore the use of Misch metals would make the alloy design complex.
Of all the rare earth elements, Y and Ce are being researched widely in combination with other alloying elements due to their significant influence on mechanical properties and texture effects.Besides having good tensile properties, some of the alloys like Mg-Y, Mg-Gd, Mg-Dy, Mg-Nd, WE43, LAE442, ZEK100, JDBM, etc., have good biodegradability and the properties of these biodegradable materials are also presented.
The research on Mg-RE systems conducted so far revealed that with regard to specific Mg-RE binary alloy systems, Mg-Y alloys exhibited the best strength while the Mg-Er alloys exhibited the best ductility.In ternary alloys, Mg-Zn-RE system exhibited the highest strength and ductility.Similarly among the higher alloy systems, Mg-Zn-RE based higher alloy system containing three or more alloying elements exhibited best tensile strength and ductility levels.
Overall, the best combination of both strength and ductility was observed in Mg-Y alloys in binary systems, Mg-Zn-RE alloys in ternary systems and Mg-Zn-RE based alloys in higher alloy systems.
Owing to the high cost of rare earths, it is not economical to use rare earths in high concentrations.Hence, it is suggested that further research be done by micro alloying (<1%) to offset the cost of the rare earths without compromising the properties.

Table 2 .
Tensile properties of Mg-Y binary alloys after secondary processing.

Table 3 .
Tensile properties of Mg-Ce binary alloys.

Table 4 .
Tensile properties of Mg-Gd binary alloys.

Table 5 .
Tensile properties of Mg-Gd binary alloys after secondary processing.

Table 6 .
Tensile properties of Mg-La binary alloys.

Table 7 .
Tensile properties of Mg-Er binary alloys.

Table 8 .
Tensile properties of Mg-Nd binary alloys.
* indicates the compressive properties of the same alloy.

Table 9 .
Tensile properties of Mg-Dy binary alloys.

Table 10 .
Tensile properties of Mg-RE ternary alloys.
* indicates the compressive properties of the same alloy.
* indicates the compressive properties of the same alloy.

Table 14 .
Tensile properties of Mg-Sn-RE ternary alloys.

Table 15 .
Tensile properties of other ternary alloys.

Table 16 .
Tensile properties of Mg-RE based higher alloys.

Table 17 .
Tensile properties of Mg-Al-RE based higher alloys.
* indicates the compressive properties of the same alloy.

Table 18 .
Tensile properties of Mg-Li-RE based higher alloys.

Table 19 .
Tensile properties of Mg-Zr-RE based higher alloys.

Table 21 .
Tensile properties of Mg-Sn-RE based higher alloys.

Table 22 .
Tensile properties of other higher alloys.