Tensile properties of high strength cast Mg alloys at room ... · cast Mg alloys According to their...

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Vol.11 No.4 July 2014Special Report CHINA FOUNDRYCelebrating the 10th Anniversray 2004-2014

Tensile properties of high strength cast Mg alloys at room temperature: A review

* Jiang Haiyan

Born in 1970. He got his Ph.D. degree at Harbin Institute of Technology in 1999. Now he is engaged in the research of light alloys at Shanghai Jiao Tong University. His research areas include development of high strength Mg and Al cast alloys and liquid forming technologies.

E-mail: [email protected]

Received: 2014-06-10 Accepted: 2014-07-15

Fu Penghuai, Peng Liming, *Jiang Haiyan, Ding Wenjiang and Zhai ChunquanNational Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, P.R. China

Abstract: As most Mg alloy products are now produced by a casting process, the development of high strength cast Mg alloys can promote their further applications and has already become one of the hot research areas of Mg alloys. The present paper reviews the strengthening mechanisms, tensile properties and modification results of commercial high strength cast Mg alloys; as well as the development of Mg-Gd, Mg-Nd and Mg-Sn based alloys. It concludes that precipitation strengthening is the most important strengthening mechanism in high strength cast Mg alloys, which contributes more than 60% of yield strength in solution & peak-aged (T6) cast Mg alloys. For the yield strength, the alloys follow the sequence of Mg-Gd(Y)-Ag > Mg-Gd(Y)-Zn > Mg-Gd-Y/Sm/Nd > Mg-Y-Nd (WE series) > ZK61 > Mg-Nd > AZ91 > Mg-Sn. Mg-Gd(Y)-Ag based alloys are the strongest cast Mg alloys at present, followed by Mg-Gd(Y)-Zn based alloys. The high yield strengths of Mg-Gd(Y)-Ag and Mg-Gd(Y)-Zn cast alloys are due to the co-precipitation of basal and prismatic meta-stable phases.

Key words: research development; high strength; Mg; cast alloy;

precipitateCLC numbers: TG146.22 Document code: A Article ID: 1672-6421(2014)04-277-10

Magnesium (Mg) alloys are the lightest commonly used structural metals, and have recently received great attention on their applications in the automobile, aircraft,

aerospace, and 3C (computer, communication and consumer electronic products) industries. Compared with Al alloys, the applications of Mg alloys are still limited at present. One of the main reasons is that the tensile properties of the available Mg alloys are still very low. Compared with their wrought counterparts, cast Mg alloys have clear economical advantages for mass production of components due to their shorter processing cycle and assembly costs. Almost 90% of total Mg alloy products involve casting processes [1]. Therefore, improvement of the strengths of cast Mg alloys would definitely promote their applications. Since last century, plenty of research has been carried out to improve the strength of cast Mg alloys. A A Luo [2] reviewed the Mg alloy development for automotive power train applications. N. Hort et al. [3] summarized the applications of different inter-metallics in Mg alloys, while J. F. Nie [4] reviewed the precipitation in most aging hardenable Mg alloys. These reviews mainly focused on the strengthening mechanisms of Mg alloys by different sorts of microstructure. It is a little difficult for readers to get a whole image of the development level of tensile properties in Mg alloys. Hence, the present paper reviews the research development of high strength cast Mg alloys, and emphasizes the tensile properties at room temperature.

The strength of cast Mg alloys usually depends on its chemical composition,

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Vol.11 No.4 July 2014Special ReportCHINA FOUNDRY Celebrating the 10th Anniversray 2004-2014

casting process and heat treatment. The casting process and heat treatment usually give predictable influences on the mechanical properties, therefore, the research activities about high strength cast Mg alloys are mainly focused on the effects of chemical composition (given as weight percent in this paper unless stated otherwise) [2-4]. Therefore, this paper reviews the development of high strength cast Mg alloys with different chemical compositions, and gives a brief introduction to the present strongest cast Mg alloys.

1 Strengthening mechanisms in cast Mg alloys

Strengthening mechanisms of cast Mg alloys include pure Mg strengthening (σMg), secondary phase strengthening (σsp), solid solution strengthening (σss), precipitation strengthening (σppt) and grain boundary strengthening (σgb). Pure Mg strengthening (σMg) comes from the binding of Mg atoms, which is estimated to be about 21 MPa (Sand cast pure Mg) [5].

Solid solution strengthening (σss) comes from atoms in the solution, which change the lattice parameters and binding forces of Mg alloys. The solution atoms interact with dislocations and twinning in the Mg alloys, which strengthen the alloys. The interactive intensity is further determined by the size misfit parameter δ [δ = (da/dc)/a] and the modulus misfit parameter η (η = (dG/dc)/G) [5], where c is the atomic solute concentration, a is the lattice parameter and G is the shear modulus. The strengthening effect of solid solution can be expressed as [5]:

σss = ZF G (|δ | + β|η |)3/2 c1/2 (1) or, σss = ZL G (δ 2 + β η 2)2/3 c2/3 (2)

where ZF and ZL are constants and β is between 1/20 and 1/16. The work of Caceres et al. [6] indicates that the exponent of c is 2/3 in AZ91 alloy, and research about the Mg-Nd based alloys [7] and the Mg-Gd-Y-Zr alloys [8] also confirms that the strengthening calculation is more precise when the exponent of c is 2/3.

The precipitation strengthening effect in Mg alloys depends on the size, distribution and volume fraction of the precipitates and on the nature of the interface between the precipitate and the matrix (coherent or incoherent). Dislocations are able to cut fine precipitates but cannot shear through larger incoherent precipitates. If the precipitates cannot be cut by dislocations, the Orowan increment of the critical resolved shear stress (CRSS) produced by the need for dislocations to by-pass these obstacles is given as [5]:

σppt = ZOMGbf1/2R-1 (3)If the precipi ta tes can be cut by dis locat ions, the

strengthening effect can be expressed as [5]:

σppt = ZpMGKp3/2(f R/b)1/2 (4)

where ZO is a constant about 0.6, Zp is a constant, M is the Taylor factor, G is the shear modulus, b is the magnitude of the Burgers vector, f is the particle volume fraction, R is

the average radius of precipitates, and Kp is the parameter depending on the mechanism considered.

As there has been little experimental research about the interaction between dislocations and precipitates in Mg alloys, their interaction patterns are still not confirmed. Assuming the precipitates are shear-resistant (Orowan strengthening), J. F. Nie [9] gave detailed discussions on the strengthening effects of precipitate shape and orientation. By making comparisons of identical volume fractions and number densities of precipitates per unit volume, he concluded that plate-shaped precipitates on prismatic planes of Mg matrix are more effective than spherical precipitates, basal precipitate plates, and [0001]a precipitate rods.

Secondary phase strengthening is also known as dispersion strengthening. Unlike the precipitates, secondary phases are usually incoherent with the matrix, therefore, cannot be cut by dislocations. The strengthening effect can be calculated from the Orowan equation (3) [5].

Grain boundary strengthening effect (σgb) can be expressed through the Hall-Petch relationship as [5]:

σgb = Kd -1/2 (5)

where d is the average grain size and K is the stress intensity factor for plastic yielding. The value of K depends on temperature, texture, compositions and preparation [5]. Considering the results published on the grain size dependence of the yield stress of Mg and Mg alloys, a K value of about 210 MPa·mm1/2 [5] is obtained. For a cast Mg alloy with grains of about 30 μm, assuming K = 210 MPa·m1/2, the contribution from grain boundary strengthening is estimated to be about 38 MPa. As high strength cast Mg alloys are mainly aging hardenable Mg alloys, after solution treatment, their grain size is usually no less than 30 μm. Therefore, the grain boundary strengthening effect is usually less than 38 MPa.

2 Tensile properties of commercial cast Mg alloys

According to their chemical compositions, commercial high strength cast Mg alloys can be classified into Mg-Al, Mg-RE and Mg-Zn based alloys. The typical tensile properties of these cast Mg alloys are listed in Table 1 [10-11]. Cast Mg-Al based alloys are the most widely used Mg alloys due to their good castability and low cost. The typical tensile properties of AZ91 alloy in sand cast T6 condition are 145 MPa in yield strength (YS), 270 MPa in ultimate tensile strength (UTS) and 6% in elongation (see Table 1). The biggest problem for Mg-Al based alloys is their high tendency of shrinkage formation, especially AZ91 alloy. The shrinkages sharply reduce the tensile strength of the AZ91 Mg alloy. Therefore, the actual tensile properties are usually much lower than the typical ones in Table 1, as they often contain shrinkages. Among commercial cast Mg alloys, Mg-RE based alloys have higher YS and the best tensile properties at elevated temperature (150 to 250 °C).

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However, the costs of Mg-RE based alloys are also the highest, which obviously restrict their applications. The typical tensile properties of WE54 alloy in sand cast T6 condition are 185 MPa in YS, 250 MPa in UTS and 2% of elongation (Table 1). Mg-Zn based cast alloys are high strength and high ductility alloys, and their costs are medium. However, the hot tearing

resistance of Mg-Zn based alloys is very poor, which means they can only be cast in sand casting process. Therefore, the application of cast Mg-Zn based alloy is also very limited and most of them are used as wrought Mg alloys. The typical tensile properties of ZK61 cast alloy are 195 MPa in YS, 305 MPa in UTS and 10% of elongation (Table 1).

S - Sand casting; DC - Die casting; T4 - Solution treated; T6 - Solution treated+peak-aged; T5 - cast+peak-aged; HRE - heavy rare earth elements.

Most of commercial cast Mg alloys can be strengthened by precipitates, as shown in Table 1. Among Mg-Al based alloys, AM50, AM60 and AS41 Mg alloys have little aging hardenability. Therefore, these alloys can only be used in die casting process, which can produce fine grains under high cooling rate. The fine grains (about 10 μm in size) strengthen the alloy to 125-140 MPa in yield strength. All of the other commercial cast Mg alloys, including AZ91, AZ63, QE22, WE43, WE54, ZC63, ZE41, ZK51, ZK61, can be cast with a lower cooling rate, such as permanent mold casting (PM) and sand casting (S). By being submitted to solution and aging treatment (T6), their tensile properties can be significantly improved compared with the original as-cast condition. Therefore, these cast Mg alloys are usually heat treated, in T6 condition, to obtain higher strength.

3 Modification of commercial cast Mg alloys

Among the commercial cast Mg alloys listed in Table 1, AZ91 alloy is the most widely used commercial cast alloy. In order to enhance its performance, chemical modifications of the AZ91 alloy are widely carried out through alloying with other elements, such as rare earth (RE) elements [misch metal (La, Ce) [12-13], Nd [14-15], Y [14, 16], Ymm (yttrium-rich misch metal)

[17], and Ho [18]], Ca [19], Sb [20-21], Sr [22], Bi [23], Pb [24], B [25], Si [26], Sn [27-28], Cd [29], Ti [30], Sc [31], and even gold [32]. Though some of them can increase the yield strength, and some of them can enhance the ductility, no significant improvement of tensile properties through chemical modification has been achieved at present. The typical tensile properties of modified AZ91 cast alloys are summarized in Table 2. For example, when 2% Ho is added, AZ91 Mg alloy has UTS of 278 MPa with 5.8% of elongation [18]. The tensile properties of cast Mg-9Al-2Sn-0.1Mn alloy [28] are reported to be 154 MPa in YS, 292 MPa in UTS and 5% elongation. At present, the best tensile properties of optimized AZ91 Mg cast alloys are about 150 MPa in yield strength with a ductility of about 5%, which are only comparable with the typical tensile properties of sand cast T6 AZ91 Mg cast alloy (in Table 1).

Because of the poor hot tearing resistance of ZK61 cast alloy, Mg-6Zn based Mg alloys have usually been developed as wrought alloys. In the literature, only a few papers about Mg-6Zn based cast alloys are available [33-36] and the main results are summarized in Table 3. Similar to Mg-9Al based alloys, most alloying modification (Gd, Er, Cu) has little influence on the tensile properties of Mg-6Zn based alloys. The tensile properties are only comparable with, or even poorer than, commercial ZK61 cast alloys (see Table 1). Among the Mg-Zn based cast alloys, at present, the modified Mg-8.0Zn-1.0Al-0.5Cu-0.5Mn alloy [36] has the highest reported tensile

Series Alloy Composition

Condition Tensile Properties

Ref.

(wt.%) UTS (MPa) YS (MPa) El. (%)

S-T6 270 145 6.0 [10] Yes AZ91D Mg-(8.3-9.7)Al-(0.35-1.0)Zn-(0.15-0.50)Mn S-T4 270 90 15.0 [10] DC 230 150 3.0 [10] Mg-Al

AZ63 Mg-(5.3-6.7)Al-(2.5-3.5)Zn-(0.15-0.35)Mn S-T6 232 122 5.5 [10] Yes

S-T4 254 94 10.0 [10] AM50 Mg-(4.4-5.5)Al-(0.26-0.6)Mn DC 210 125 10.0 [10] Little AM60 Mg-(5.5-6.5)Al-(0.13-0.6)Mn DC 225 130 8.0 [10] Little AS41 Mg-(3.5-5.0)Al-(0.2-0.5)Mn-(0.5-1.5)Si DC 215 140 6.0 [10] Little QE22 Mg-(2.0-3.0)Ag-(1.8-2.5)RE-(0.4-1.0)Zr S-T6 260 195 3.0 [10] YesMg-RE WE43 Mg-(4.75-5.5)Y-(2.0-2.5)Nd-(0.4-1.9)HRE-(0.4-1.0)Zr S-T6 250 162 2.0 [10] Yes WE54 Mg-(4.75-5.5)Y-(1.5-2.0)Nd-(1.0-2.0)HRE-(0.4-1.0)Zr S-T6 250 185 2.0 [11] Yes ZC63 Mg-(5.5-6.5)Zn-(2.4-3.0)Cu S-T6 210 125 4.0 [10] Yes

Mg-Zn ZE41 Mg-(3.5-5.0)Zn-(0.75-1.75)RE S-T5 205 140 3.5 [10] Yes

ZK51 Mg-(3.6-5.5)Zn-(0.5-1.0)Zr S-T5 205 140 3.5 [10] Yes ZK61 Mg-(5.5-6.5)Zn-(0.6-1.0)Zr S-T6 310 195 10.0 [10] Yes

Table 1: Tensile properties of commercial high strength Mg cast alloys at room temperature

Precipitate strengthening

ability

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Table 2: Tensile properties of Mg-9Al based cast alloys at room temperature

Table 3: Tensile properties of Mg-6Zn based cast alloys at room temperature

strength at room temperature, with YS of 228 MPa, UTS of 328 MPa and elongation of 16%.

WE43 and WE54 are typical Mg-RE based cast alloys. The cost of QE22 (see Table 1) is very high because of the 2% silver addition, which is now nearly not acceptable in practical application. The research on the WE (Mg-Y-Nd) series of cast Mg alloys is mainly focused on the evolution of meta-stable precipitates during the aging process. Typical research work was given by J.F. Nie and B.C. Muddle [37] and C. Antion et al. [38]. Mg-Y-Nd alloys decompose from the solid solution according to the sequence of phases: S.S.S.S.→ β'' (DO19) → β' (cbco) → β1 (fcc) →β (fcc). Apart from the stable β phase, the other three phases are meta-stable. The typical strengthening phases in the WE series alloys are β'' and β' meta-stable precipitates, as shown in Fig. 1 [39]. The β'' phases are thin plates while the β' phases are lens-shaped. Such morphologies

can be observed in nearly all of the Mg-RE alloys. The typical tensile properties of WE43 and WE54 cast Mg alloys are shown in Table 1. The commercial WE series of cast Mg alloys are usually aged at 250 °C under the standard aging process. Aging treatment at a lower temperature (200 °C) can significantly improve the UTS and YS, as shown in Table 4. For permanent mold (PM) cast WE43 Mg alloys [40], which are peak-aged at 200 °C for 96 h, the strength could reach YS 196 MPa and UTS 345 MPa, significantly higher than the typical values (162 MPa in YS and 250 MPa in UTS) in Table 1.

From the above analysis, compared with the typical tensile properties of AZ91 alloy in sand cast T6 condition (see Table 1), the chemical modification of Mg-Al based alloy does not bring an obvious increment of tensile strength (the YS & UTS are increased by < 10 %) (see Table 2). The chemical modification of Mg-Zn based alloy shows good improvement of tensile

Fig. 1: TEM image (a) and SAED pattern (b) of solution-treated Mg-4Y-2Nd-1Gd alloy aged at 200 ºC for 136 h along [0001]a axis zone [39]

Alloy Composition (wt.%) Test condition Tensile properties

Ref. UTS (MPa) YS (MPa) Elongation (%)

AZ91-1Nd Mg-8.13Al-0.74Zn-0.18Mn-1.12Ho PM-as-cast ~240 ~4.2 [15] AZ91-2Ho Mg-8.50Al-0.75Zn-0.22Mn-2.16Ho PM-as-cast 278 5.8 [17]AZ91-0.35Sb Mg-9Al-0.8Zn-0.2Mn-0.35Sb PM-T6 264 4.5 [20] AT92-0.1Mn Mg-9Al-2Sn-0.1Mn PM-T6 292 154 5.0 [27]

PM - Permanent mold casting.

Composition (wt.%) Test condition Tensile properties

Ref. UTS (MPa) YS (MPa) Elongation (%)

Mg-5.81Zn-1.30Gd-0.33Zr PM-T6 276 146 13.2 [33] Mg-5Zn-0.63Er PM-T6 261 124 10.5 [34] Mg-6Zn-0.5Cu-0.6Zr PM-T6 266 185 16.7 [35] Mg-8.0Zn-1.0Al-0.5Cu-0.5Mn PM-T6 328 228 16.0 [36]

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properties. The Mg-8.0Zn-1.0Al-0.5Cu-0.5Mn alloy [36] (see Table 3) is reported to have much higher strength than the typical tensile properties of ZK61 alloy (see Table 1), 33 MPa improvement in YS and 23 MPa improvement in UTS. Using a lower aging temperature (200 °C), the tensile properties of WE series of commercial cast Mg alloys can clearly be improved, and the increment for the WE43 alloy is about 20% in strength.

4 New alloy developmentBesides the modification of cast Mg alloys, some new high strength cast Mg alloys have also been developed, including the Mg-Gd, Mg-Nd and Mg-Sn based alloys.

4.1 Mg-Gd based alloysThe Mg-Gd based alloys are the mostly studied Mg alloys in the last decade and almost all of the absolute high strength Mg alloys come from this alloy series. Table 5 lists the typical tensile properties of high strength Mg-Gd based cast alloys. According to the secondary alloying element, Mg-Gd based alloys can further be divided into Mg-Gd-Y, Mg-Gd-Sm, Mg-Gd-Nd, Mg-Gd(Y)-Zn, and Mg-Gd(Y)-Ag series. In Mg-Gd-Y based cast alloy, Mg-10Gd-2Y-Zr and Mg-10Gd-3Y-Zr alloys show good comprehensive tensile properties, with YS of ~240 MPa, UTS of ~360 MPa and elongation of >4% (see Table 5, and He Shangming’s thesis [8]). The Mg-Gd-Sm and Mg-Gd-Nd series alloys have comparable tensile properties with the Mg-Gd-Y

Table 4: Tensile properties of typical WE43 based cast alloy at room temperature

Table 5: Tensile properties of typical Mg-Gd based cast alloys at room temperature

Series Composition (wt.%) Test condition

Tensile properties Heat treatment

Ref.

UTS (MPa) YS (MPa) El. (%)

Series Composition (wt.%) Test condition

Tensile properties Heat treatment

Ref.

UTS (MPa) YS (MPa) El. (%)

WE43 Mg-4Y-2Nd-1Gd-0.4Zr PM-T6 287 172 11.4 525 °C × 8 h + 250 °C × 17 h

[40] PM-T6 345 196 7.3 525 °C × 8 h + 200 °C × 96 h

Mg-6Gd-3Y-0.4Zr PM-T6 331 198 7.8 500 °C × 8 h + 200 °C × 80 h Mg-8Gd-3Y-0.4Zr PM-T6 362 222 7.6 500 °C × 8 h + 200 °C × 80 h Mg-10Gd-2Y-0.4Zr PM-T6 362 239 4.7 490 °C × 8 h + 225 °C × 16 h Mg-10Gd-3Y-0.4Zr PM-T6 370 241 4.1 500 °C × 8 h + 225 °C × 16 h Mg-12Gd-3Y-0.4Zr PM-T6 328 246 1.1 500 °C × 8 h + 225 °C × 16 h Mg-10.4Gd-3.3Y-0.46Zr PM-T6 348 237 3 500 °C × 6 h + 225 °C × 16 h Mg-10Gd-1Y-Zr PM-T6 215 135 12 525 °C × 10 h + 250 °C × 12 h Mg-10Gd-3Y-Zr PM-T6 235 152 6.5 525 °C × 10 h + 250 °C × 12 h Mg-10Gd-5Y-Zr PM-T6 300 285 ~ 1 525 °C × 10 h + 250 °C × 12 h

Mg-10Gd-3.74Y-0.25Zr PM-T6 325 268 5.1 525 °C × 6 h + 225 °C × 24 h

LF-T6 285 265 4.1 525 °C × 6 h + 225 °C × 24 h Mg-9Gd-4Y-Zr PM-T6 327 279 3.3 520 °C × 8 h + 225 °C × 100 h Mg-6Gd-2Sm-Zr PM-T6 230 108 19.8 500 °C × 4 h + 225 °C × 32 h Mg-8Gd-2Sm-Zr PM-T6 329 195 7.7 500 °C × 5 h + 225 °C × 8 h Mg-10Gd-2Sm-Zr PM-T6 347 237 3.2 500 °C × 6 h + 225 °C × 4 h Mg-6Gd-2Nd-Zr PM-T6 342 182 7.9 500 °C × 6 h + 200 °C × 24 h Mg-8Gd-2Nd-Zr PM-T6 342 200 5 515 °C × 4 h + 225 °C × 12 h Mg-11Gd-2Nd-Zr PM-T6 353 224 3.7 525 °C × 4 h + 250 °C × 2 h Mg-10Gd-3Y-1.2Zn-0.4Zr PM-T6 330 230 3 525 °C × 72 h + 225 °C × 16 h Mg-10Gd-3Y-1.0Zn-0.4Zr PM-T6 364 253 2 500 °C × 10 h + 225 °C × 16 h Mg-3.2Gd-0.5Zn-0.2Zr (at.%) PM-T6 405 278 2.5 500 °C × 12 h + 225 °C×8 h Mg-14Gd-3Y-1.8Zn-0.5Zr PM-T6 366 230 2.8 520 °C × 10 h + 225 °C × 16 h Mg-6.5Gd-2.5Dy-1.8Zn PM-T6 392 295 6.1 510 °C × 10 h + 215 °C × 109 h Mg-2.5Gd-1.0Zn-0.16Zr (at.%) SC-T6 366 260 3.3 500 °C × 10 h + 200 °C × 128 h Mg-3.4Gd-0.5Ag-0.11Zr (at.%) PM-T6 414 293 2.2 490 °C × 10 h + 200 °C × 36 h Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr PM-T6 403 268 4.9 500 °C × 10 h + 200 °C × 32 h

Mg-16Gd-2Ag-0.3Zr PM-T6 423 328

2.6 480 °C × 18 h + 500 °C × 8 h +

200 °C × 32 hLF - Lost foam casting.

Mg-Gd-Y

[8]

[41][42][42][42][43][43][44]

[45]

[46]

[47][41][48][49][50][51][52][53]

[54]

Mg-Gd-Sm

Mg-Gd-Nd

Mg-Gd(Y)-Zn

Mg-Gd(Y)-Ag

[8]

[8][8]

[8]

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series alloys. For example, Mg-10Gd-2Sm-Zr alloy [45] in PM-T6 condition has YS of 237 MPa, UTS of 347 MPa and elongation of 3.2 %. The Mg-11Gd-2Nd-Zr alloy [46] in PM-T6 condition has YS of 224 MPa, UTS of 353 MPa and elongation of 3.7 %. When Zn element is added to Mg-Gd(Y) based alloys, the tensile properties can be improved further, especially the YS. For example, the YS of semi-continuous cast (SC) Mg-2.5Gd-1.0Zn-0.16Zr (at.%) [51] in T6 condition is as high as 260 MPa. Mg-3.2Gd-0.5Zn-0.2Zr (at.%) alloy [48] has YS of 278 MPa, with UTS higher than 400 MPa. At present, the strongest cast Mg alloys come from the Mg-Gd(Y)-Ag based alloys. As shown in Table 5, the tensile properties of Mg-3.4Gd-0.5Ag-0.11Zr (at.%) cast alloy [52] in PM-T6 condition are 293 MPa in YS, 414 MPa in UTS and 2.2% in elongation. The Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr cast alloy [53] also in PM-T6 condition gets YS of 268 MPa, UTS of 405 MPa and elongation of 4.9%. Recently, Mg-16Gd-2Ag-0.3Zr cast alloy is reported to have better strengths of YS 328 MPa and UTS 423 MPa, with an elongation of 2.6% [54].

The Mg-Gd(Y)-Zn and Mg-Gd(Y)-Ag based alloys have higher strength is due to their co-precipitates, β' meta-stable

phase on prismatic planes and γ" meta-stable phase on basal planes, which effectively contribute to the strength of the alloys. In the Mg-Gd-Y, Mg-Gd-Sm and Mg-Gd-Nd based alloys, the main strengthening phase is β' precipitate with cbco structure on the habit plane of {1ī00}α. Figure 2(a) shows the typical image of β' precipitate in Mg-Y alloy [56], and Fig. 2(b) reveals the typical features of β' precipitates in Mg-Gd alloy [56]. The β' precipitates of Mg-Gd-Y [8], Mg-Gd-Nd [46], and Mg-Gd-Sm [45] based alloys in Table 5 have similar appearance to Fig. 2(b). When Zn or Ag is added, basal precipitate plates are formed on the basal plane of the matrix with high number density. The typical precipitation morphologies of Mg-Gd-Ag based alloys are shown in Fig. 3 [54]. The γ" precipitates lie on (0001)α planes and β' phases locate at (1ī00)α planes. The basal plates remain in the peak-aged condition and co-exist with the β' phases on the prismatic plane that is subsequently formed. The strengthening effects of co-existing basal γ" precipitates and prismatic β' phases in Mg-Gd(Y)-Zn and Mg-Gd(Y)-Ag alloys are higher than the single prismatic β' phases in Mg-Gd-Y, Mg-Gd-Sm and Mg-Gd-Nd alloys. Thus, the co-existance of basal and prismatic precipitates leads to higher yield strength.

Fig. 2: HAADF-STEM images showing β' precipitates in Mg-2.32at.%Y (a) and Mg-2.66at.%Gd (b) alloys. The electron beam is parallel to [0001]α [55]

Fig. 3: Bright field (BF) TEM micrograph (a) and their corresponding SAER pattern (b) of Mg-16Gd-2Ag-0.3Zr alloy aged at 200 ºC for 32 h. The electron beam is parallel to [2

-110]α [54]

(a) (b)

(a)

γ"

γ"

β'

(b)

β'

283


Fig. 4: HAADF-STEM images showing microstructures of Mg-0.5at%Nd alloy taken with the [0001]α incidence [58]: 200 ºC for 10 h (a), 170 ºC for 100 h (b)

4.2 Mg-Nd based alloysMg alloys with Nd addition exhibit a strong age-hardening response and improvement in tensile properties. The maximum solubility of Nd in solid Mg is 3.6wt.% at the eutectic temperature of 545 °C, and Mg-Nd based alloys can get relatively high strength with low RE addition [56]. Table 6 lists the typical tensile properties of Mg-Nd-Zn based alloys [56]. In PM-T6 condition, Mg-2.75Nd-0.2Zn-Zr alloy shows typical tensile properties of 155 MPa in YS, 305 MPa in UTS and 11% of elongation, which are better than the commercial AZ91 Mg alloy. A recent study [57] confirmed that 0.2%Y addition can

significantly improve the hardening effect and tensile properties after ageing at 200 °C for up to 14 h, and in particular, enhance the YS in the temperature range of 200 – 350 °C. Typical images of the main strengthening phases in Mg-Nd based alloy are shown in Fig. 4 [58]. Generally, there are two kinds of meta-stable precipitates, β" phase with DO19 structure (thin) and β' phase with cbco structure [lens-shape, indicated by arrows in Fig. 4(a) and rectangle in Fig. 4(b)]. The absolute strength values of Mg-Nd based cast alloys are obviously lower than those of Mg-Gd and WE series alloys.

Table 6: Tensile properties of typical Mg-Nd-Zn based cast alloys at room temperature

Composition (wt.%) Test condition Tensile Properties

Heat treatment

Ref. UTS (MPa) YS (MPa) El.(%)

Mg-2.75Nd-0.4Zr PM-T6 280 153 7.3 Mg-2.75Nd-0.4Zr-0.2Zn PM-T6 305 155 11.3 Mg-2.75Nd-0.4Zr-0.5Zn PM-T6 313 162 10.9 Mg-2.75Nd-0.4Zr-1.0Zn PM-T6 271 170 6.5 Mg-1.25Nd-0.4Zr-0.2Zn PM-T6 258 110 14.1 [56]Mg-1.75Nd-0.4Zr-0.2Zn PM-T6 278 126 14.0 Mg-2.25Nd-0.4Zr-0.2Zn PM-T6 298 139 13.6 Mg-3.00Nd-0.4Zr-0.2Zn PM-T6 318 165 11.0 Mg-3.25Nd-0.4Zr-0.2Zn PM-T6 301 175 6.1

540 °C×10 h + 200 °C×16 h

4.3 Mg-Sn based alloysFrom the above description, it is clear that most of the high strength cast Mg alloys contain RE elements, such as Mg-Gd based, WE series, and Mg-Nd based alloys. Though these alloys have good tensile properties, their costs are relatively high because of RE addition, which seriously limits their applications. Therefore, it is necessary to explore promising age-hardenable Mg alloys that are RE-free. Mg-Sn based alloys seem to be the possible alternatives. The Mg-Sn system has a relatively high Sn solubility limit of 14.48% at 561 °C and a low solubility at ambient temperature. In addition, Mg2Sn precipitates have a high melting temperature

of 770 °C. Thus, Mg-Sn based alloys are expected to have better high temperature tensile properties than Mg-Al based alloys. There has been plenty of research work carried out on the development of Mg-Sn based alloy, most of which is about the observation of precipitates and the hardening behavior. Table 7 summaries some typical tensile properties of Mg-Sn based cast alloys [59-62]. It can be seen that the Mg-Sn based alloys only have comparable tensile properties with Mg-Al based alloys (see Table 1): the strength is lower while the ductility is higher. Before application, the tensile properties of the Mg-Sn based cast Mg alloys has to be improved further.

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Table 7: Tensile properties of optimized Mg-Sn based cast alloys at room temperature

Table 8: Strengthening contributions in Mg-10Gd-2Y-0.5Zr alloy at room temperature [63]

Table 9: Strengthening contributions in Mg-3Nd-0.2Zn-0.4Zr alloy at room temperature [64]

Strengthening contribution

Cast-T4 Cast-T6 Extruded-T5 MPa % MPa % MPa %

Pure Mg strength 21 16 21 9 21 6 Solid solution strengthening 75 58 38 16 38 11Grain boundary strengthening 34 26 34 14 68-135 21-41 Precipitation strengthening 0 0 146 61 136 41 Experimental yield strength 130 100 239 100 331 100

Strengthening contribution

As-cast Solution-treated Peak-aged at 200 ºC Aged at 250 ºC for 10 h MPa % MPa % MPa % MPa %

Pure Mg strength 21 28 21 25 21 15 21 15 Second phase strengthening 21 28 0 0 0 0 0 0 Solid solution strengthening 0 0 30 35 0 0 0 0Grain boundary strengthening 34 44 34 40 34 24 34 24 Precipitation strengthening 0 0 0 0 82-87 61 85 61 Experimental yield strength 76 100 85 100 137-142 100 140 100

5 DiscussionAs mentioned above, the strengthening mechanisms of cast Mg alloys include pure Mg strengthening (σMg), secondary phase strengthening (σsp), solid solution strengthening (σss), precipitation strengthening (σppt) and grain boundary strengthening (σgb). S. M. He et al. [63] and P. Fu et al. [64] estimated the strengthening contributions of different strengthening mechanisms in cast Mg-10Gd-2Y-Zr and Mg-3Nd-0.2Zn-Zr alloys, and the results are shown in Table 8 and Table 9, respectively. It is shown that in the T6 condition, the precipitate strengthening is the largest contributor to the yield strength in high strength cast Mg alloys. The absolute YS strength contribution of precipitation is about 82 MPa in Mg-3Nd-0.2Zn-Zr alloy (see Table 9) and 146 MPa in Mg-10Gd-2Y-Zr alloy (see Table 8), which represents 61% of the YS.

The precipitation strengthening is also reported to be the largest contributor in cast Mg-8.5Gd-2.3Y-1.8Ag-Zr alloy [53]. The YS of cast Mg-16Gd-2Ag-Zr alloy [54] is 162 MPa in T4 condition and 328 MPa in T6 condition. If the contributions of other strengthening mechanisms are evaluated as those of cast Mg-10Gd-2Y-Zr alloy, the contribution of precipitation in cast Mg-16Gd-2Ag-Zr alloy is about 235 MPa (328 -21 -38 -34 = 235, 21/38/34 are from Table 8). It is the highest precipitation strengthening value reported in cast Mg alloys up to date, which makes about 70% (235/328) of the total yield strength. Therefore, the precipitation strengthening is the main strengthening mechanism (> 60%) in high strength cast Mg alloys, and improving the strengthening effect of the precipitate is the main method in enhancing the tensile properties of the alloys.

Composition (wt.%) Test condition Tensile properties

Heat treatment

Ref. UTS (MPa) YS (MPa) El. (%)

Mg-5Sn-5Al-1Zn PM ~220 ~85 ~16 As-cast [59]Mg-6Sn-5Al-2Si PM 180 80 16.5 As-cast [60]Mg-3Sn-1Mn PM-T6 167 137 4.2 420 °C × 24 h + 250 °C × 16 h [61]Mg-7Sn-4Al-2Zn PM-T6 171 127 6.0 430 °C × 10 h + 200 °C × 8 h [62]Mg-7Sn-4Al-2Zn-3Sr PM-T6 207 167 8.6 430 °C × 10 h + 200 °C × 8 h [62]

Based on the yield strength of different alloy series in Table 1 to Table 7, it can be concluded that the strength sequence of the alloy series is: Mg-Gd(Y)-Ag > Mg-Gd(Y)-Zn > Mg-Gd-Y/Sm/Nd > Mg-Y-Nd (WE series) > ZK61 > Mg-Nd > AZ91 > Mg-Sn. The tensile properties of AZ91 and Mg-Sn based alloys are lower, probably because their strengthening phases are relatively larger and stable precipitates (Mg17Al12 and Mg2Sn phases). In contrast the main strengthening phases of other alloy series, including Mg-Gd(Y)-Ag, Mg-Gd(Y)-Zn,

Mg-Gd-Y/Sm/Nd, Mg-Y-Nd (WE series), ZK61, and Mg-Nd based alloys, are all meta-stable phases, which are smaller in size and higher in density. These meta-stable precipitates lead to a higher strengthening effect. Comparing Mg-Gd(Y)-Ag and Mg-Gd(Y)-Zn based alloys with Mg-Gd-Y/Sm/Nd based alloy, it can be found that the addition of Ag and Zn elements can introduce a new kind of precipitate on basal planes, which can further improve the yield strength. Therefore, concerning the precipitation strengthening in cast Mg alloys, it seems

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that the strength contribution of meta-stable phases is usually higher than that of stable phases, and the co-precipitates give a higher strength contribution than the single precipitates.

6 ConclusionsBased on the tensile properties at room temperature, the present paper reviews the development of Mg-Al, Mg-Zn, Mg-RE and Mg-Sn based high strength cast Mg alloys, including the currently commercial ones and the new ones. The following conclusions can be drawn.

(1) Precipitation strengthening is the most important strengthening mechanism in high strength cast Mg alloys, contributing more than 60% of yield strength in the solution & peak-aged condition (T6). The absolute strengthening effect of precipitates in yield strength is from ~ 82 MPa (Mg-3Nd-0.2Zn-Zr alloy) to ~ 235 MPa (Mg-16Gd-2Ag-Zr alloy).

(2) Based on the yield strength, the strength sequence of the alloy series is: Mg-Gd(Y)-Ag > Mg-Gd(Y)-Zn > Mg-Gd-Y/Sm/Nd > Mg-Y-Nd (WE series) > ZK61 > Mg-Nd > AZ91 > Mg-Sn.

(3) Mg-Gd(Y)-Ag based alloys are the strongest cast Mg alloys at present and Mg-Gd(Y)-Zn based alloys are the second ones. The high tensile properties of these two kinds of cast Mg alloys are due to the co-precipitation of basal and prismatic meta-stable phases.

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This work was financially supported by the National Natural Science Foundation of China (51201103 & 51304135), the Specialized Research Fund for the Doctoral Program of Higher Education (20110073120008), the New Century Excellent Talents in University of Ministry of Education of China (NCET-11-0329), the Program of Shanghai Subject Chief of Engineering (14XD1425000) and the Assembly Pre-research Project (51312030706).

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