Review on Research and Development of Magnesium Alloys

Acta Metall. Sin.(Engl. Lett.)Vol.21 No.5 pp313-328 Oct. 2008

REVIEW ON RESEARCH AND DEVELOPMENT OF MAGNESIUM ALLOYS Z. Yang1,2)∗ , J.P. Li2,3) , J.X. Zhang1) , G.W. Lorimer3) and J. Robson3) 1) Department of Materials Science and Engineering, Xi an Jiaotong University, Xi an 710049, China 2) Department of Materials Science and Engineering, Xi an Technological University, Xi an 710032, China 3) Manchester Materials Science Centre, University of Manchester, Manchester M 1 7HS, UK Manuscript received 3 November 2007; in revised form 20 February 2008

The current research and development of magnesium alloys is summarized. Several aspects of magnesium alloys are described: cast Mg alloy, wrought Mg alloy, and novel processing. The subjects are discussed individually and recommendations for further study are listed in the final section. KEY WORDS Magnesium alloys; Novel processing; Grain refined

1. Introduction Magnesium alloys have excellent specific strength and stiffness, exceptional dimensional stability, high damping capacity, and high recycle ability[1] . Based on these superior properties and a combinative requirement for reducing environmental burdens by using lightweighted structures, the research and development of magnesium alloys for practical industrial application have overwhelmingly increased worldwide during the past decade[2−5] . Magnesium and its alloys are becoming widely recognized as playing an increasingly important role in automotive, aircraft, and electronic consumer products. Magnesium alloys can be divided into cast magnesium alloys and wrought magnesium ones in terms of difference in processing[6] . Main commercial magnesium alloys include the AZ series (Mg-Al-Zn), AM series (Mg-Al-Mn), AE series (Mg-Al-RE), EZ series (Mg-REZn), ZK series (Mg-Zn-Zr), and WE series (Mg-RE-Zr)[7] . Statistically, more than 90% of the magnesium alloy structural components are produced by casting process, especially by various die-casting processes. In the last decade, the AZ series cast magnesium alloys, especially the AZ91 alloy, have been extensively studied and used for some structural components of automobiles, aircraft, and computers, because of high specific strength and good castability[5−8] . Compared with cast magnesium alloys, wrought magnesium alloys have a more promising perspective of application, which is the reason why research and development of high performance wrought magnesium alloys, novel wrought magnesium processing technology, and producing high quality wrought magnesium alloy products are an important objective. A long time research plan of wrought magnesium alloys was put ∗

Corresponding author. Tel.: +86 29 83208080; Fax: +86 29 83208078. E-mail address: [email protected] (Z. Yang)

· 314 · forward by the international magnesium association (IMA) in 2000[9] . In recent years, the research on wrought magnesium alloys is increasingly attracting the attention of companies and researchers[10−19] . As the desire to further utilize lightweight magnesium alloys in various industrial applications grows, different aspects of magnesium research must be intensified in order to improve various properties of magnesium alloys and enhance their chances of being selected by the product designers. The present article will give a general review of the recent main research and development of magnesium alloys, with emphasis on latest development of high performance magnesium alloys and novel processing technology. 2. Development of High Performance Magnesium Alloys 2.1 New diecastable heat resistant cast Mg alloys 2.1.1 Shortcomings of Mg alloys containing Al Commercial cast magnesium alloys for automotive applications are AZ and AM series alloys (AZ91D, AM50A, and AM60B). These alloys offer an excellent combination of mechanical properties, corrosion resistance, and die-castability. However, they have poor creep resistance above 125◦ C[20] , which makes them inadequate for major powertrain applications. Automatic transmission cases can operate up to 175◦ C, engine blocks up to 200◦ C, and engine pistons, even higher than 300◦ C. Creep resistance is a major requirement for use of magnesium in automotive powertrain components that are currently made of aluminium or cast iron. Creep in pure magnesium and Mg-Al based alloys have been the subject of many studies[21−23] . Both diffusion controlled dislocation climb and grain boundary sliding are reported as creep mechanisms in magnesium alloys, depending upon the alloy system, microstructure, and more importantly, stress and temperature regimes. The poor thermal stability of Mg17 Al12 phase (with a eutectic temperature of 437◦ C) and its discontinuous precipitation can result in substantial grain boundary sliding at elevated temperatures. The accelerated diffusion of aluminium in the magnesium matrix and the self-diffusion of magnesium at elevated temperature also contribute to creep deformation in Mg-Al based alloys. Possible approaches to improving creep resistance in magnesium alloys include[24] : (1) Introducing alloying elements with higher affinity to aluminium to suppress the formation of Mg17 Al12 phase in the microstructure of Mg-Al based alloys. (2) Introducing secondary phase particles at grain boundaries to pin the sliding phenomenon. (3) Introducing a fined dispersion of stable precipitates[25] or a solute, which is strongly attracted to dislocation and diffuses slowly through the matrix[26] . Heat resistance cast magnesium alloy development includes three categories: (1) Alloying of Mg-Al based alloys with Ca (forming AX alloys), with Zn and Ca (forming AZX alloys), with Sr (forming AJ alloys), with Ca and Sr (forming AXJ alloys), with RE (forming AE alloys), with Si and Sb (forming ASS alloys), with Ca and RE (forming ACM alloys and MRI alloys), with Zn, Ca and RE (forming ZACE alloys), with Zn, Sn (forming ASZ alloys); (2) development of a new type of Mg-RE-Zr alloys, including Mg-Y-Nd-Zr (WE) alloys, Mg-Gd-Nd-Zr (GN), Mg-Dy-Nd-Zr (DN) and Mg-Gd-Y-Zr (GW) alloys; (3) development of Mg-RE-Zn (MEZ) alloys. 2.1.2 Alloying of Mg-Al based alloys Ca addition The addition of Ca to Mg-Al based alloys for improved creep resistance

· 315 · was reported in a British patent[27] in 1960. Volkswagen[28] attempted the use of Mg-Al-Ca alloys in 1970 s, and claimed an improvement in creep resistance with the addition of 1% of calcium to AZ81 (Mg-8%Al-1%Zn). Recently, a patent[29] by General Motors (GM) disclosed two very important findings in Mg-Al-Ca (AX) alloy system: (1) Casting defects such as cold-shuts, hot-cracking, and die-sticking of AX alloys were very severe when calcium level is about 1%. However, such problems diminished or significantly reduced when Ca levels were about 2%. (2) Additions of about 0.1% strontium to Mg-Al-Ca alloys further improved their creep resistance. Furthermore, Sr additions were also beneficial to the corrosion resistance of the alloys, and could effectively restore the corrosion resistance to the level of AZ91D. Ca-Sr additions Based on the above findings, GM has developed a family of MgAl-Ca-Sr (AXJ) alloys. Developed for engine blocks and transmissions, the AXJ alloys have at least 40% greater tensile and 25% greater compressive creep resistance than AE42, and corrosion resistance as good as AZ91D. These alloys are estimated to have a similar cost to AZ91D and have as good castability[30] . Similar to Mg-Al-Ca system, the development of Mg-Al-Sr system was aimed at replacing RE additions to magnesium with alkaliearth elements[31] . Two experimental alloys, i.e., AJ51x (Mg-5%Al-1.2%Sr) and AJ52x (Mg-5%Al-1.8%Sr), were shown to have elevated temperature tensile properties and creep resistance superior to conventional magnesium die casting alloys[31] . RE-Ca additions In an attempt to reduce the cost of AE42 alloy, a group of Nissan and Ube engineers[32] added Ca (0.25-5.5%) to partially replace the expensive rare earth addition. The resultant Mg-Al-RE-Ca alloys are claimed to have improved creep strength over the AE42 alloy, while retaining comparable tensile and creep properties. Honda[41] disclosed an alloy, referred as ACM522 (Mg-5%Al-2%Ca-2%RE) with better creep resistance than the AE42 alloy and comparable heat and corrosion as a general purpose aluminium alloy A384. However, the alloy is likely to be at least as expensive as the AE42 alloy. More recently, a new Mg-Al-Ca-RE based alloy, code-named MRI153 (Mg-Al-Ca-RE) was patented by Dead Sea Magnesium and Volkswagen[33] . This alloy is claimed to be a creepresistant die casting alloy with capability of long-term operation at temperatures up to 150◦ C under high loads (50–85 MPa). Another patent heat resistant die cast Mg alloy, MRI230 D, was developed DSM Ltd recently. It is claimed this alloy can operate at 190– 200◦ C. Zn-Ca-RE addition Luo and Shinoda[34] reported that Zn addition restores the diecastability of Mg-Al-Ca alloys and the resulting Mg-Zn-Al-Ca alloys offer good high temperature properties. The new alloy development programs based on Mg-Zn-Al-Ca alloy system in Japan are being pursued[35] . As a result, improved high temperature tensile properties and good creep resistant were obtained in Mg-8%Zn-4%Al-0.6%Ca and Mg-6%Zn-3%Al-0.6%Ca alloys containing 1 wt pct–3 wt pct of Ce-rich mischmetal (RE). However, although the Mg-Zn-Al-Ca-RE alloys exhibit high fluidity, die-casting of actual transmission cases was difficult due to the occurrence of hot tearing. Recent research[36] found that low Zn content of about 0.5% does not lead to the crystallization of low melting point eutectics Mg32 (Al,Zn)49 and can eliminate hot tearing. ZACE05613 (Mg-0.5%Zn6%Al-1%Ca-3%RE) and ZACE05411 (Mg-0.5%Zn-4%Al-1%Ca-1%RE) alloys have been developed and ZACE05613 alloy has been successfully used to die-casting complex shape transmission cases. Under the condition of 175◦ C, 100 h and 50 MPa, the alloy exhibits

· 316 · similar creep resistance as ADC12 aluminium alloy that is currently used for powertrain components. The good creep performance of the new diecast ZACE05613 alloy is considered to be due to the presence of Al2 Ca compound at grain boundaries, which controls grain boundary sliding and Al11 RE3 compound that controls dislocation slip within the grains. Sn addition The effect of tin to microstructure and mechanical properties of Mg alloy has been studied by several researchers[37,38] . The effect of Sn addition on tensile properties of Mg-Nd alloys is similar, in general, to that of aluminium[39] . This additive results in an increase of strength at room temperature, especially tensile yield strength, and a decrease of the strength properties at 250◦ C. Simultaneously, plasticity falls at room temperature and becomes significantly more at 250◦ C. Mg alloy patents from Japan[40] include Sn among the alloying elements, beneficial for strength and creep properties. Bakke et al.[41] observed that Mg alloys with high Sn content display tensile yield strength value above that of AZ91D, along with an equivalent UTS. The creep resistance increases with increasing Sn or Sb content. Sb addition The solubility of antimony in magnesium is near zero. Molten magnesium reacts with antimony at the eutectic temperature to form the Mg3 Sb2 phase with high thermal stability. The effect of Sb addition on microstructure and properties of the AZ91 alloy was studied by Yuan et al[42] . The results indicated that small amounts of Sb addition (up to 0.35 wt pct) results in significant increases in yield strength and creep resistance at temperatures up to 200◦ C, but a slight decrease of ductility. The effect of Ce-rich mischmetal and Sb joint additions on as-cast microstructure and mechanical properties of the AZ91 alloy was studied by Yang et al[43] . Substantial grain refining was observed when small amounts of Sb (0.4 wt pct) were added jointly with Ce-rich mischmetal (<1.0 wt pct) and room temperature tensile strength increased to near that of heat treated AZ91 alloy. It is suggested that both the fine grain structure and fine dispersoids phases which formed during solidification by the reaction of Mg-Sb, Al-RE, and even Mg-Al-RE-Sb results in a substantial increase in as-cast room temperature tensile strength. Bi, Ca, Ba, and Co addition Yuan et al.[44] studied the influence of bismuth additions on precipitation in AZ91 and found that Bi additions played various roles including retarding the aging process of AZ91 alloy, suppressing the discontinuous precipitation and accelerating the formation of continuous Mg17 (Al,Bi)12 precipitates. The continuous precipitation compound has semicoherent orientation relationship with matrix and increases the thermal stability. Accordingly, the heat resistance of magnesium alloy at elevated temperatures is improved by the Bi addition. However, addition of Bi to Mg-Nd alloys results in a decrease of the strength properties both at room temperature and at 250◦ C. As Al and Sn, Bi decreases the strengthening effect of Mg-Nd alloys during aging. Its influence on the Mg-Nd alloys can be connected with decrease of the Nd solubility in Mg solid solution[45] . Addition of calcium, lithium, barium, and cobalt results in some increase of tensile yield strength at room temperature with tensile strength remaining practically unchanged. At 250◦ C addition of Ca, Li, Ba, and Co practically does not change both tensile strength and tensile stress strength of Mg-Nd alloys[46] . 2.1.3 MEZ alloys All of the above alloys are based on the Mg-Al system due to its excellent die castability

· 317 · and other physical properties. However, Magnesium Electron Ltd. (MEL), with a long history of developing non-aluminium gravity cast magnesium alloys, has taken a different approach, i.e., to achieve significant improvement in creep performance without aluminium additions. That has resulted in the development of the MEZ alloys (Mg-2.5%RE-0.35%Zn0.3%Mn, Mg-2.5%RE-0.5%Zn) patented by MEL[47] . Good castability to produce automotive castings such as gearboxes, housings, and oil pans using this alloy has also been reported. However, the MEZ alloy remains expensive as it has an even higher content of rare earth elements than the AE42 alloy. 2.1.4 Mg-RE-Zr alloys Currently, the state-of-the-art cast magnesium alloys, with best mechanical properties both at ambient and elevated temperature, are Mg-RE-Zr alloys and their modification alloys. Lorimer and Nie et al.[48−50] have given intensive studies on the precipitation mechanism, microstructure and mechanical properties of Electron WE (Mg-Y-Nd-Zr) series alloys (WE54, WE43 and WE42). The typical mechanical properties of WE43 were reported to be at room temperature TS=265 MPa, TYS=185 MPa, EL=7%–5%, at 300◦ C, TS=150–170 MPa, TYS=110–130 MPa, EL=30%–50%. The alloy was recommended for use up to 300◦ C. Lorimer et al.[51] studied the effect of rare earth elements gadolinium and dysprosium on the microstructure and mechanical properties of Mg-Nd-Zr alloy. Differences have been observed in the precipitation kinetics and precipitate compositions and these have been found to have a significant effect on mechanical properties. Gadolinium additions have been found to give increased yield strength compared to yttrium or dysprosium additions. Two high performance Mg-RE-Zr alloys, GN72 (Mg-7.0%Gd-2.25%Nd-0.6%Zr) and DN72 (Mg-7.0%Dy-2.25%Nd-0.6%Zr), with superior strength to WE42 and WE43 alloys, were developed. In Russia, Mg-RE(Nd, La, Y, mishmetal)-Zn-Zr series alloys, named as ML system alloys, which can also be used to 300◦ C, were developed[46] . ML19 (Mg2%Nd-2%Y–0.6%Zn-0.6%Zr) was claimed to be at 300◦ C TS=150 MPa, TYS=100 MPa, EL=12%. Recently, Magnesium alloys containing rare earth elements Gd and Y such as Mg-10.8Gd-1.2Y-0.5Zn have been developed and extensively investigated. The typical mechanical properties of Mg-10.8Gd-1.2Y-0.5Zn were reported to be at room temperature TS=340 MPa, TYS=280 MPa, EL=7%–5%, at 300◦ C, TS=230–308 MPa, EL=15%– 25%[52] . Table 1 summarizes the main composition, tensile properties, creep performance, castability, and strengthening phase of various cast magnesium alloys for elevated temperature application in comparison with the current workhouse die cast aluminium alloy A380. It is shown that the Mg-RE-Zr alloys (WE54 and ML19) have higher elevated mechanical properties than A380 alloy. The new developed AXJ alloy has a significant improvement over AE42 at both 150◦ C and 175◦ C. The creep strength of the AXJ is 68% higher than that of AE42, but is still 10%–20% less than that of A380. 2.2 Development of wrought Mg alloys Viewing the literatures published both in the conference proceedings and academic journals, it can be seen that the research on wrought magnesium alloys mainly focused on the plastic deformation behavior and its effect of microstructure and properties[53−56] . The deformation behavior, microstructure and tensile properties, fatigue behavior, and superplasticity of AZ31, AZ61, and ZK60 have been extensively studied[57−61] . Using

· 318 · ECAE process, Huang et al.[62] studied microstructures and tensile properties of ZK31 and AZ31 wrought magnesium alloys. They found that both the tensile strength and 0.2% proof stress of the two alloys can be obtained at a low temperature ECAE process, but the elongation is high when processed at high temperature. Itoh et al.[63] studied the recrystallized grain size and tensile properties in cold-rolled and annealed AZ31 alloy affected by rolling direction. They found that grain refinement became more marked when the rolling angle increased. Yield strength and hardness were conformed to increase with decreasing grain size. Microstructural evolution during the hot deformation and the effect of rolling parameters on the deformation behavior of AZ31 magnesium sheet were studied by several researchers[64−66] . Recently many new noble wrought magnesium alloys of rare earth elements Gd and/or Y, Nd, Dy have been well developed and extensively investigated. Compared to commercial magnesium alloys such as WE54, the Mg-Gd-Y, Mg-Gd-Nd, MgGd-Dy system alloys have higher specific strength at both room and elevated temperatures and own good creep resistance[67−69] . Apart from thermal-mechanical processing and heat treatment, high performance wrought magnesium alloys development mainly focused on two areas. One is modification of Table 1 Summary of cast magnesium alloys for elevated temperature applications Alloy AE42 (4Al2RE) AX52 (5Al-2Ca) AX53 (5Al-3Ca) AXJ (5Al-3Ca-0.07Sr) ZAX8506 (8Zn5Al0.6Ca) ACM522 (5Al2Ca2RE)

Tensile properties YS/MPa UTS/MPa Elongation/% 139/RT 226/RT 11/RT 106/177◦ C 135/177◦ C 28/177◦ C 161/RT 228/RT 13/RT 133/175◦ C 171/175◦ C 23/175◦ C 186/RT 250/RT 9/RT 151/175◦ C 196/175◦ C 15/175◦ C 190/RT 238/RT 8/RT 146/175◦ C 196/175◦ C 15/175◦ C

146/RT 117/150◦ C 158/RT 138/150◦ C 132/175◦ C MRI153 170/RT (ALCaRE) 135/150◦ C MRI-230 180/RT (AlCaRE) 150/150◦ C AJ52x(5Al-2Sr) 145/RT 108/150◦ C 103/175◦ C MEZ (2.5RE-0.35Zn) 97/RT 78/150◦ C 73/175◦ C WE54(5.25Y3.5Nd0.5Zr) 205/RT 195/150◦ C 183/200◦ C 175/250◦ C ML19(2Y2Nd0.3Zn0.5Zr) 118/RT 120/200◦ C 110/250◦ C 100/300◦ C A380 155/RT 149/150◦ C 154/175◦ C

219/RT 159/150◦ C 200/RT 175/150◦ C 152/175◦ C 250/RT 190/150◦ C 235/RT 205/150◦ C 202/RT 164/150◦ C 148/175◦ C N/A 285/RT 255/150◦ C 241/200◦ C 230/250◦ C 235/RT 215/200◦ C 200/250◦ C 150/300◦ C 290/RT 255/150◦ C 248/175◦ C

5/RT 11/150◦ C 4/RT 7/150◦ C 9/175◦ C 6/RT 17/150◦ C 5/RT 16/150◦ C 4/RT 14/150◦ C 15/175◦ C 3/RT 8/150◦ C 5/175◦ C 4/RT 5/150◦ C 6.5/200◦ C 9/250◦ C 3/RT 3/200◦ C 7/250◦ C 12/300◦ C 3/RT 6/150◦ C 7/175◦ C

Total creep extension/% 0.33(3MPa/150◦ C/200H) 0.08(34MPa/177◦ C/100H) 0.06(70MPa/175◦ C/100H) 0.26(56MPa/2000◦ C/100H) 0.09(70MPa/175◦ C/100H) 0.28(56MPa/200◦ C/100H) 0.05(83MPa/150◦ C/100H) 0.06(70MPa/175◦ C/100H) 0.20(56MPa/200◦ C/100H) 0.26(35MPa/150◦ C/200H) NA 0.15(50MPa/150◦ C/100H) N/A 0.03(35MPa/150◦ C/200H) 0.09(35MPa/175◦ C/200H) 0.03(50MPa/150◦ C/200H) 0.03(80MPa/175◦ C/200H) N/A

N/A

0.18(35MPa/150◦ C/200H) 0.15(35MPa/175◦ C/200H) 0.08(50MPa/150◦ C/200H)

· 319 · Alloy

Strengthening phase(s) Al4RE

Castability

AE42(4Al2RE)

Creep strength (175◦ C)/MPa 50

AX52(5Al-2Ca)

75

(Mg,Al)2Ca

Good

AX53(5Al-3Ca) AXJ (5Al-3Ca-0.07Sr) ZAX8506 (8Zn5Al0.6Ca) ACM522(5Al2Ca2RE) MRI153 (AlCaRE) MRI-230 (AlCaRE) AJ52x(5Al-2Sr)

74 84

(Mg,Al)2Ca (Mg,Al)2Ca

Good Good

N/A

Low eutectic Mg-Al-Zn-Ca phase Al-Ce-Mg-Ca Al2Ca, Mg17Al12 Al2Ca, Mg17Al12 Mg-Al-Sr

Fair

MEZ(2.5RE-0.35Zn) WE54 (5.25Y3.5.Nd0.5Zr) ML19 (2Y2Nd0.3Zn0.5Zr) A380

74 N/A N/A N/A N/A >150 /200◦ C /0.2% strain N/A 93

Mg12RE Mg12NdY, Mg14Nd2Y, Mg3X, Mg5Gd Mg24Y5, Mg41Nd5, Si, Al2Cu

Good

Fair Good N/A Fair Fair Fair Fair Good

Remarks Expensive Not >175◦ C Excellent combination of properties very promising for powertrains Not suitable at >175◦ C Expensive Suibable up to 150◦ C Suitable up to 190–200◦ C High casting temp. required Expensive High strength at elevated temp. High strength at elevated temp. Workhouse alloy in powertrain

wrought magnesium alloys with alloying elements[46,60−69] and another is rapid solidification plus powder metallurgy (RS/PM) process[46,70−73] . 2.2.1 Effect of alloying elements The alloys additions in wrought magnesium alloys can be divided into three categories[74] : (1) Elements that can improve both strength and ductility of magnesium. Ranging in increasing strength, they are Al, Zn, Ca, Ag, Ce, Ni, Cu, Th. Ranging in increasing ductility, they are Th, Zn, Ag, Ce, Ca, Al, Ni, Cu. (2) Elements that can only improve ductility, but with a little effect on strength of magnesium. Ranging in increasing ductility, they are Cd, Tl, and Li. (3) Elements that decrease the ductility but increase strength of magnesium. In increasing strength, they are Sn, Pb, Bi, and Sb. Zn addition Zinc is a widely used alloying element for commercial Mg alloys. It is used for improvement of strength properties of both cast and wrought alloys. Jaschik et al.[84] studied the mechanical properties of Zn additions to wrought Mg alloys. The results indicated that the yield strength of magnesium alloys containing zinc increased with increasing zinc content. In comparison with the well-known AZ61 alloy, the ZM61 alloy shows a clear increase of the yield strength through increased Zn contents. Investigations have shown that Zn improves the strength properties of Mg-RE alloys, too, but only with certain limitations[46] . According to the data[46] , where hot extruded alloys were studied, addition of 1 wt pct–6 wt pct Zn to the alloy Mg-1.5Nd-0.5Zr (in wt pct) resulted in continuous increase of the strength properties and decrease of plasticity at all test temperatures from ambient to 250◦ C. However, strengthening caused by the Zn addition decreases with rising temperature. The alloy Mg-1.3Nd-5.5Zn-0.5Zr (in wt pct) given a T5 treatment showed the following mechanical properties: at room temperature TS=365–390 MPa, EL=5%–6%, at 250◦ C TS=145–165 MPa, EL=25%–30%. The effect of the Zn additives on the mechan-

· 320 · ical properties of hot extruded Mg-Y alloys was reported by Drits and Rokhlin et al[39] . The results demonstrated that the tensile properties of as extruded Mg-Y alloys at room temperature increase with increasing Zn content. Sufficiently high values of TS and TYS are achieved already at (1.2–1.6)Zn (wt pct), and acceptably high values of elongation are retained. Mg-6.0Zn-0.5Zr (wt pct) is the typical alloy of a good combination of tensile strength and elongation at room temperature[76−78] , Y additives can enhance its mechanical properties at elevated temperature. The microstructure, mechanical properties, deformation behavior of Mg-6.0Zn-0.5Zr-(1-2)Y alloy had been extensively investigated for the past few years[79−82] . The optimum Zn/Y ratio for the formation of two-phase microstructure consisting of α-Mg and I-phase is 5-7, the strength and elongation increases with increasing volume fraction of I-phase[82] . The effect of the Zn additives on the mechanical properties of Mg-2.0Gd-1.2Y-0.2Zr alloys was reported by Honma and Ohkubo et al[83] . The results demonstrated that although the addition of Zn degrades the age-hardening response, it causes the discontinuous precipitation of a 14H-type long-period stacking (LPS) phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. Li addition Lithium is a suitable alloying element, which apart from a density reduction, clearly increases the ductility and impact strength of hexagonal magnesium alloys. When content of lithium in magnesium is above 11%, the cubic body centered lithium solid solution produced stands out through a high ductility. An addition of lithium below 5.5 wt pct, leads to a reduction of c/a axial ratio, which causes the decrease of the critical shearing strain of the prism slippage, and these further sliding systems are more easily activated[46] . Apart from increase in the ductility, addition of lithium can significantly reduce the difference between tensile yield strength and compressive yield strength of magnesium alloys[84] . Mn addition The effect of additional alloying elements on mechanical properties of Mg-RE alloys was intensively studied by Rokhlin et al[46,66−69] . It is found that[57] addition of (0.81–1.90) wt pct Mn to the alloys with 2 wt pct mischmetal resulted in an increase of tensile strength from 195 MPa to 205–230 MPa, tensile yield strength from 120 to 140–195 MPa at room temperature and tensile strength from 50 MPa to 59–69 MPa, tensile yield strength from 29 MPa to 41–51 MPa at 316◦ C. The small increase of strength properties from Mn addition was confirmed for Mg-Nd alloys and Mg-Gd alloys[46] . As in the cases of Mg alloys with mischmetal and neodymium, addition of Mn resulted in increase of strength properties with tensile yield strength increasing more than tensile strength. However, unlike the alloys with mischmetal and Nd, addition of Mn to the alloys with Gd was accompanied by significant decrease of plasticity. According to the phase diagram of the Mg-RE-Mn systems, there are no compounds formed between RE and Mn in Mg-rich alloys. Mn does not change the solubility of the rare earth metals in Mg solid solution. These facts lead to the conclusion that there is actually no interaction between Mn and RE in Mg solid solution and, as a consequence, Mn does not change behavior of Mg-RE alloys during aging. Mg-RE alloys retain their ability to strengthen during solid solution decomposition when Mn is present. However, recent studies on Mg-Sc-Mn alloys indicated that when a small amount of Mn was added into Mg-Sc alloys, creep resistance of the alloy was drastically improved due to formation of Mn2 Sc and other Mnx Scy phases[85] . It is indicated that the role of Mn in various Mg alloys is not fully understood.

· 321 · Zr addition Zirconium is known as an alloying element, which results in refinement of the grain size of cast Mg and some Mg alloys. Solubility of Zr in molten Mg at temperatures common for melting Mg alloys is 0.8–1.0 (wt pct). Addition of Zr less than 1 wt pct into cast Mg-RE alloys results in a significant increase of tensile strength and plasticity at room temperature and 200◦ C, due to grain refinement. Payne and Bainey[86] studied the effect of Zr addition on the mechanical properties of wrought Mg alloys. They found that in wrought Mg alloys with rare earth metals of the cerium subgroup, additions of Zr did not result in significant improvement of their mechanical properties because of the absence of any significant difference in grain size between alloys with and without Zr after hot working. The addition of Zr to wrought Mg alloys containing rare earth metals is still justified because Zr improves the castability of the alloys, refines as-cast grain size, and facilitates their hot working process. Recently, Mackzie et al.[13] studied the grain fining mechanism of Zr addition to magnesium. They found that the grain refinement of Mg via the addition of Zr is dependant on the formation of Zr particles in the melt, which can undergo a peritectic reaction. This phenomenon is aided by the presence of Fe and Si impurities that act as nucleus or a catalyst for the formation of Zr particles. Sc-Mn-Gd addition Scandium and gadolinium are potential alloying elements for improving the high temperature properties (above 300◦ C) of magnesium alloys[13] because they both have a large solubility in Mg solid solution at high temperature and decrease solid solubility with temperature decreasing. However, in the binary Mg-Sc alloys, MgSc precipitates form very slowly during aging and improve the mechanical properties only slightly because of their incoherent interface. Based on thermodynamic equilibrium calculations, Buch and Schmid-Fetzer et al.[85] added Mn to the binary Mg-Sc alloy and found that Mn2 Sc precipitates form coherently and were very useful for improving creep resistance and hardness. The creep rates of T5 heat-treated new alloys, MgSc15 Mn1 or MgSc6 Mn1 , were up to two orders of magnitude lower than the WE43 alloy at 350◦ C and 30 MPa. More recently, guided by thermodynamic calculations, several vertical sections in the quaternary Mg-Mn-Sc-Gd and Mg-Mn-Sc-Y systems were studied by Grobner et al.[38] in the range of (0–1.5) Mn (wt pct), (0–10) Sc (wt pct), and (0–10) Gd (wt pct) (or Y). New low-scandium Mg-Mn-Gd-Sc and Mg-Mn-Y-Sc alloys, MgGd5 Mn1 Sc0.8 , MgGd5 Mn1 Sc0.3 , and MgY5 Mn1 Sc0.8 , were developed successfully. Most promising is MgGd5 Mn1 Sc0.8 (wt pct) among them. The new quaternary Mg alloys show a creep resistance similar to the ternary high-scandium Mg-Mn-Sc alloys, about 100 times better than the WE43 alloy at 350◦ C and 30 MPa. Apart from Mn2 Sc precipitates, large amounts of secondary precipitates GdMg5 formed in Mg-Gd-Mn-Sc alloys. The beneficial effect of Gd addition on mechanical properties of Mg-Mn-Sc is similar to that in the GN72 alloy[48] . Gd-Y-Zr addition Gadolinium and yttrium are also potential alloying elements for improving the properties at room and elevated temperature (above 300◦ C) of magnesium alloys[49−53] , because they both have a large solubility in Mg solid solution at high temperature and decrease solid solubility with temperature decreasing[68] . About two decades ago, Drits et al.[39] discovered that 6 wt pct Y addition to Mg-10Gd-0.6Mn alloys could increase the room temperature tensile property from 340 MPa to 440 MPa and 300◦ C tensile property from 170 MPa to 230 MPa. Rokhlin et al.[46] developed high performance Mg-Gd-Y alloys, such as Mg-10Gd-5Y-0.5Mn alloy and Mg-10Gd-3Y-0.4Zr alloy, which exhibit higher specific strength at both room and elevated temperatures and better creep

· 322 · resistance than conventional aluminium and magnesium alloys including WE54 alloy. The tensile properties of Mg-10Gd-2Y-0.5Zr alloy at different heat treatments were conducted by He et al.[67] recently and a high rupture tensile strength up to 403 MPa was obtained. Li et al.[69] developed high performance Mg-Gd-Y alloys, such as Mg-12Gd-0.5Zn-0.5Zr alloy, Mg-12Gd-4Y-0.5Zn-0.5Zr alloy, Mg-9Gd-3Y-0.5Zn-0.5Zr alloy, whose tensile strength at room temperature and 300◦ C are more than 450 MPa and 320 MPa respectively[68] . All the studies imply Mg-Gd-Y alloys have a strong potential for use as structural materials in components for aerospace, outer-space applications, weapons and high quality vehicles. The main composition, mechanical properties, and characteristics of high performance commercial wrought magnesium alloys are summarized in Table 2. It is shown that those Mg alloys containing rare earth, Th or Cu, such as WE54, WE43, ZC71, and ML19 have high mechanical properties. Medium strength sheet alloys AZ31 and ZM21 have good Table 2 Summaries of commercial wrought magnesium alloys[46] Major alloying elements Alloy wt pct Zn6.5Cu1.2Mn0.75 ZC71:125mm 10mm Bar 125mm Bar Y5.2Nd2.25HRE1.0Zr0.5WE54 Extr. Y4.0Nd2HRE1Zr0.5 WE43 Extr.

Tensile properties Description TYS/MPa UTS/MPa EL/% 315-335(T6) 340-360(T6) 5-7(T6) High strength, weldable 340-350(T6) 360-375(T6) 4-6(T6) heat treated 180-190(ext)255-275(ext)12-15(ext) 215(T5) 315(T5) 10(T5) High strength at high 190(T6) 275(T6) 10(T6) temperature alloy, weldable 195(T5) 270(T5) 15(T5) High strength, 160(T6) 260(T6) 15(T6) high temperature aerospace alloy Zn2.0Mn1.0 ZM21 160 245 10 Medium strength sheet Extr. (min) /extrusion, fully weldable, good formability Al8.5Zn0.5Mn0.3 AZ80 200(T5) 290(T5) 6(T5) High strength for Forg. forging of simple design Al6.0Zn1.0Mn0.3 AZ61: 0-75mm1) 180 260 8 General medium/high 160 250 7 strength forg/ext, (AZM) 75-150mm2) fully weldable alloy 150 230 8 Medium strength sheet Al3.0Zn1.0Mn0.3 AZ31:0-10mm3) 10-75mm, Extr. 160 245 10 /extrusion, fully weldable, 0.5-6mm, Sheet 120 220-265 10-12 good formability Zn3.25Zr0.6 ZW3: 0-10mm 200 280 8 High strength extrusion (ZK31) 10-100mm, Extr. 225 305 8 /forging, weldable Forg.4) 205 290 7 under good conditions Zn6.0Zr0.8 ZK61 Extr. 210 285 6 High strength Extr. T5 240 305 6 Forg. T5 160 275 4 Th3.2Zr0.7 HK31 Sheet 170(H24) 230(H24) 4(H24) High creep resistance Extr. T5 180 255 4 up to 350◦ C Th2Mn0.8 HM21 Forg. 175(T5) 225(T5) 3(T5) High creep resistance Sheet T8 135 215 6 up to 350◦ C Th0.8Zr0.6Zn 0.6 HZ11 Extr. 120 215 7 High creep resistance Forg. 130 230 6 up to 350◦ C, weldable Li14Al1 Mn0.2 LA141 Sheet 95(T7) 115(T7) 10(T7) Ultra light weight Zn5.5-7Nd1.4-2Cd0.2- MA19 Extr. 345 390 6.5 High strength, 1.0Zr0.5-0.9 Forg. 320 340 8 good heat resistance Note: 1) CYS 130–180 MPa, CUS 370–420 MPa; 2) CYS 115–165 MPa, CUS 340–400 MPa; 3) CYS 200–250 MPa, CUS 385–400 MPa; 4) CYS 165–215 MPa, CUS 370–440 MPa.

· 323 · formability and weldability. Mg-Li alloy LA141 is the most lightly wrought Mg alloy but with the lowest tensile strength. 2.2.2 Rapid solidified Mg alloys Rapid solidification is quite an effective way of enhancing the strength properties of metals and it was also applied to Mg alloys including those with various rare earth metals. Very recently, Kawamaura et al.[70,71] found that a nanocrystalline Mg-1 mol pct Zn2 mol pct Y bulk alloy sample prepared by warm extrusion of rapidly solidified powders indicates high 0.2% proof strength of about 600 MPa and sufficient elongation of about 5% at room temperature. Abe et al.[72] investigated the microstructure of the bulk alloy sample in details, using conventional HRTEM, atomic-resolution high-angle annular dark STEM with Z-contrast and EDX spectroscopy with a sub-nanometer electron probe. As a result, they showed that the microstructure is composed of grains of about 50–200 nm in diameter, which is divided into two types: hcp-Mg solid solution grain and fine-lamellar grains. The latter grains consist of a novel long-period ordered structure with hexagonal lattice of 6H-type (ABCBCB). 3. Thermomechanical Processed Grain Refined Magnesium Alloys 3.1 Tensile properties of grain refined magnesium alloys Magnesium alloys are expected to be useful materials for minimizing global environmental problems, but wider applications of the alloys have not been achieved due to their inferior cold workability caused by their plastic anisotropy. Generally, it is said that the cold workability of magnesium results from its limited slip systems at ambient temperature. Recently, however, there are some instances where the ductility is significantly improved even at room temperature in magnesium alloys grain refined by a severe plastic deformation (SPD) such as repetitive equal channel angular extrusion (ECAE)[13−19] , rolling, and annealing. ECAE processed magnesium alloys have a fine-grained structure and superior tensile properties and superplasticity trend. In recent years, ECAE processes were used to process several wrought and cast magnesium alloys such as AZ31, ZK60, and AZ91 alloys. Koike et al.[87] investigated the deformed microstructure and superplasticity of AZ31 and ZK60 alloys in details and found the occurrence of dislocation cross slip from basal plane to non-basal plane, dynamic recovery within twins and in the untwined matrix, and grain boundary sliding even at room temperature. They concluded that the activities of nonbasal dislocation slip systems and dynamic recovery were responsible for the large tensile ductility in the ECAE processed AZ31 and ZK60 Mg alloys. These results suggested that the poor ductility of magnesium relates to microstructural conditions, and not necessarily to intrinsic nature of the material. Itoh et al.[2] studied recrystallized grain size in coldrolled and annealed AZ31 wrought magnesium alloys affected by rolling direction, and found that cold-rolled and annealing post hot-rolling or extrusion is an convenient effective grain refining method. Grain refinement became more marked when the rolling angle increased. Yield strength and hardness were confirmed to increase with decreasing grain size. Both tensile strength and elongation increased with rolling angle increasing from 0 degree to 45 degrees. Wang et al.[88] studied the isothermal hot compression behavior of AZ31B alloy at 300–450◦ C and found that fine grain dynamic recrystallized microstructure of extruded AZ31B alloy between 10–20 μm can be obtained at a lower temperature of

· 324 · 300–350◦ C. There is a large amount of research[90,91] about the correlation between grain size and mechanical properties, particularly 0.2% proof stress of magnesium alloys based on HallPetch relationship. In magnesium, the critical resolved shear stress for non-basal slip plane is about one hundred times as large as that for basal slip, which gives rise to anisotropy of mechanical properties. The mechanical properties of wrought magnesium alloys are strongly affected by crystallograghic texture. In the condition of having a similar texture but different grain size, the dependence of 0.2% proof stress and grain size corresponds well with Hall-Petch relationship. When basal slip plane is parallel to the extrusion direction, the effect of grain size on tensile strength becomes less important. For instance, although the specimen of ECAE-processed AZ31 alloy at 300◦ C has larger grain size than that processed at 250◦ C, the 0.2% proof stress of the former is larger because the latter specimens have a texture in which the basal plane is inclined at 45 degrees to the extrusion direction, whereas, the former specimens have a basal plane which is parallel to the extrusion direction, which is parallel to the tensile direction. Therefore, crystallographic orientation has a profound effect on the 0.2% proof stress in magnesium alloys, and grain size has a small effect[25] . Kamado et al.[36] found that AZ61 alloy specimen subjected to 4-pass ECAE processing at 200◦ C, produces an average grain size of <1 μm and two textures in which some of the basal planes are included at 45 degrees, whereas, others are parallel to the extrusion direction, indicating that both high strength and ductility can be obtained. The tensile properties are superior to those of the fully heat-treated 6061 alloy sheet. The results suggest that fine-grained AZ61alloy is applicable to the automobile body, considering only the tensile properties. 3.2 Superplasticity of grain refined Mg alloys Various plastic deformation processes, such as rolling, extrusion, and ECAE have been applied to magnesium alloys to obtain superplasticity. The key of getting superplasticity in an alloy is to obtain uniform fine grained structures. Previous study[46] has indicated that superplasticity occurs in many kinds of magnesium alloys at low temperatures or under high strain rates. Watanabe et al.[91] experimentally investigated the possibility of a combination of high strain rate superplasticity and low temperature superplasticity using extremely fine grained magnesium alloy. Based on the relationship between effective diffusion compensated strain rate and the reciprocal grain size for superplastic flow, they estimated that the size of <0.4 μm is required for occurrence of high strain rate superplasticity at low temperatures. They reported that high strain rate superplasticity in powder metallurgy processed ZK61 alloy specimen was obtained even at a relatively low temperature of 473 K, which corresponds to half the absolute melting point of the alloy. Similar results were obtained in the (α+β) two phases Mg-Li-Zn alloy ECAE-processed at 50◦ C. Superplasticity occurs at 150◦ C, which is below half of melting point of the alloy, under a relative high strain rate of 1×10−3 s−1 with fracture elongation of 391%[62] . Such a specimen after tensile test contains fine grains due to dynamic recrystallization and the precipitation of β phase along the grain boundaries and at triple points in recrystallized α phase. This microstructure change enhances grain boundary sliding (GBS), resulting in the occurrence of low temperature superplasticity at a relatively high strain rates. Using extrusion and the ECAE process (EX-ECAE), high strain rate superplasticity of Mg-9%Al,

· 325 · Mg-7.2%Al and AZ61 alloys was achieved under a low temperature of 437 K by Matsubara et al[92] . It is shown that the introduction of extrusion before ECAE is important in order to refine the grain size and succeed through ECAE process instead of breaking the sample. 4. Development of Novel Processing 4.1 Semi-solid processing The semi-solid metal (SSM) processing is an emerging new technology for near netshape production of engineering components, in which alloys are processed in the temperature range where the liquid and solid phases coexist. The semi-solid slurry with a non-dendritic microstructure (thixotropic structure) exhibits distinct rheological behavior, namely, thixotropy and pseudo-plasticity. These rheological properties make the SSM processing an unique and effective process for near net-shape product and property enhancement. The MHD (magneto hydrodynamic stirring) technique induced during partial solidification and the SIMA (strain induced melt activated) and RAP (recrystallization and partial remelting) processes[82] before partial remelting comprise the dominant methods currently used to produce semi-solid billets. This thixotropic structure can be produced either during partial solidification or during partial remelting. Semi-solid processing can be divided into rheocasting and thixoforming. Recently, Collot[93] developed a new magnesium alloy thixoforming process called THIXOMAGTM, adaptable to a conventional cold pressure die casting machine. This process has been used to produce AZ91 and AM50 alloys products. Thixocasting AZ91 and AM50 structures exhibits a better Young s modulus, tensile strength, and elongation with T4 and T6 heat treatment. The rheodiecasting process, combining twin screw stirring is a new development of the rheocasting process. However, this process is expensive due to a high cost of the rheodiecasting machine[93] . 4.2 Continuous casting More recently, Motegi et al.[94] developed a continuous caster equipped with an inclined cooling plate in order to produce raw materials for semi solid casting and for plastic working. In this method, nucleation occurs on the surface of the inclined cooling plate, and the nuclei are separated from the surface by the melt flow and then flow into the tundish. Consequently, large amounts of nuclei continuously form on the surface of inclined cooling plate and a grain size of less than 50 μm is obtained in the as-cast condition. When such an ingot is reheated to the semi-solid temperature solid particles are round, resulting in good fluidity during semi-solid casting. In Australia, Australian Magnesium Corporation (AMC) and the cooperative research centre for Cast Metals Manufacturing (CASTmm) alliance is developing a horizontal direct chill (HDC) process to continuous casting magnesium alloys ingot and/or billet[72,73] . The HDC process offers better protection of molten metal from oxidation and thus metal loss on remelting can be reduced. The process also results in more controllable surface finish with a much lower propensity for defect formation and less cracks. Considerable effort has been put into mathematical modeling of heat and fluid flow in the process to assist in mould design and optimization of other process parameters. Systems development and testing with pilot-scale laboratory trial has taken place at CSRRO s Queensland Centre for Advanced Technologies. Large-scale trials have been conducted at AMC s demonstration

· 326 · plant. In the United Kingdom, the cooperative research on the influence of direct chill cast parameters on structure and properties of magnesium alloys have been carried out by Magnesium Electron Ltd. and University of Manchester/UMIST and Birmingham University and a good result has been obtained[13] . 5. Summary and Suggestions The current status of main research fields in magnesium alloys is presented. The research efforts are yielding positive results and it is expected that various industries will begin to apply some of the newly developed magnesium alloys and processes to new products. On the other hand, however, much research study still should be undertaken. In comparison with cast magnesium alloys, the understanding of wrought magnesium alloys, especially, the effect of alloying elements and dispersoid particles on microstructure and mechanical properties of wrought magnesium alloys is poorly understood. The wrought magnesium alloy market is a key area of growth for magnesium producers. A better understanding of composition-microstructure-processing-properties relationship of wrought magnesium alloys is of great importance. The control of the grain structure in wrought magnesium products is critical to achieving the required levels of strength, ductility and toughness, as well as limiting mechanical anisotropy. More than 50 years of intensive effort has led to a good understanding of how to control grain structures in aluminium alloys. In contrast there is a lack of equivalent knowledge for magnesium alloys. A particularly important aspect that has yet to receive much attention in magnesium alloy systems is the interaction between second phase particles and grain structure. Wrought aluminium alloys usually contain elements specifically added to control the grain structure though the precipitation of dispersoid particles. These dispersoids are small, high stable particles, formed from elements with low solubility in matrix. Unlike the age hardening elements they are not dissolved during thermomechanical processing. The dispersoids exert a pinning force on grain and subgrain boundaries. Acknowledgements—The authors acknowledge the support from the Chinese Foundation Research Projection, Magnesium Elektron Ltd. and the Manchester Materials Science Center of University of Manchester. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

R.L. Edgar, Magnesium Alloys and their Application (K.U. Kainer Pub., France, 2000) p.3. D.V. Hai, S. Itoh, S. Kamado and Y. Kojima, Adv. Mater. Res. 11-12 (2006) 413. S. Schumanm and H. Friendich, Mater. Sci. Forum 419-422 (2003) 51. L.L. Rokhlin, Magnesium Alloys Containing Rare Earth Metals-Structure and Properties (Taylor & Francis Pub., Moscow, 2003) p.83. B.L. Mordike and T. Ebert, Mater. Sci. Eng. A302 (2001) 37. I.J. Polmear, Mater. Sci. Technol. 10 (1994) 1. R.W. Cahn, C.X. Shi and J. Ke, Structure and Properties of Nonferrous Alloys (Beijing, Science Press, Beijing, 1999). P. Cavaliere and P.P. de Marco, Mater. Charact. 3 (2007) 226. E. Aghion and B. Bronfin, Mater. Sci. Forum 350 (2000) 19. M. Kettner, F. Pravdic, W. Fragner and K.U. Kainer, Magn. Technol. 2006 (2006) 133. D. Letzig, J. Swiostek, J. Bohlen, J. Goken and K.U. Kainer, JOM 56 (2004) 32. A.A. Luo and A.K. Sachdev, Metall. Mater. Trans. 38A (2007) 1184.

· 327 · [13] L.W.F. Mackzie, F.J. Humphrey and G.W. Lorimer, Proc. 6th Int. Conf. Magnesium Alloys and Their Applications (K.U. Kainer Pub., Florida, 2003) p.158. [14] R. Gardinger and P. Stolfig, Magnesium Technology 2003 (TMS, Warrendale, PA, 2003) p.231. [15] F.A. Slooff, J. Zhou, J. Duszczyk and L. Katgerman, Scr. Mater. 57 (2007) 759. [16] L. Wu, A. Jain, D.W. Brown, G.M. Stoica, S.R. Agnew, B. Clausen, D.E. Fielden and P.K. Liaw, Acta. Mater. 56 (2008) 688. [17] G. Ben-Hamu, D. Eliezer and K.S. Shin, Mater. Sci. Eng. A447 (2007) 35. [18] F.O. Riemelmoser, M. Kuhlein, H. Kilian, M. Kettner, A.C. Hanzi and P.J. Uggowitzer, Adv. Eng. Mater. 9 (2007) 799. [19] H. Watari, T. Haga, R. Paisern, N. Koga and K. Davey, 10th Int. Conf. on the Mechanical Behaviour of Materials 345-346 (2007) 165. [20] C. Suman, SAE Technical Paper No.910416 (Warrendale, PA, Society of Automotive Engineers, 1991). [21] A.A. Luo, Magnesium Technology 2004 (TMS, Warrendale, PA, 2004) p.329. [22] P. Lyon, J.F. King and Nuttall, Proc. 3rd Int. Magnesium Conf (G.W. Lorimer Pub., London, 1996) p.99. [23] B. Bronfin and E. Aghion, Magnesium Technology 2001 (J.N. Hryn Pub., Warrendale, 2001) p.127. [24] A.A. Luo, Mater. Sci. Forum 419-422 (2003) 57. [25] A. Yamashita, Z. Horita and T.G. Langdon, Mater. Sci. Eng. A300 (2001) 142. [26] S.R. Agnew and K.C. Liu, Magnesium Technology 2000 (TMS, Warrendale, PA, 2000) p.285. [27] B. Bronfin, E. Aghion, F. von Buch, S. Schumann and M. Katzir, US Patent, No.7041179 (May 9, 2006). [28] F. Hollrigl-Rosta and E. Just, Light Metals Age 8 (1980) 22. [29] B.R. Powell, V. Rezhets and A.A. Luo, US Patent No. 6,264,763 (July 24, 2001). [30] A.A. Luo, Int. Mater. Rev. 1 (2004) 13. [31] M.O. Pekguleryuz and E. Barai, Magnesium Technology 2001 (TMS, Warrendale, PA, 2001) p.119. [32] K. Sakamoto, Y. Yamanoto and N. Sakate, European Patent EP 0 799 901 A1, (Aug. 10, 1997). [33] B. Brofin and E. Aghion, US Patent No. 6,139,651 (Oct. 31, 2000). [34] A.A. Luo and T. Shinoda, SAE Technical Paper No.980086 (Sep.10, 2004). [35] I.A. Anyanwu, Mater. Sci. Forum 350-351 (2000) 73. [36] S. Kamado, 4th Pacific Rim Int. Conf. On Advanced Materials and Processing (PRICM4) (S. Hanado and Z. Zhong Pub., JIM, Japan, 2001) p.1175. [37] T.T. Sasaki, K. Oh-ishi, T. Ohkubo and K. Hono, Scr. Mater. 55 (2006) 251. [38] J. Grobner and R. Schmid-Fezter, J. Alloy. Compd. 320 (2000) 296. [39] M.E. Drits, Z.A. Sviderskaya, L.L. Rokhlin, N.I. Nikitina, Metall. Sci. Heat Treat. 21 (1979) 887. [40] K. Aikawa and K. Taketani, US Patent No.5304260 (Apr. 19, 1994). [41] P. Bakke, K. Pettersen and H. Westengen, Magnesium, Proc. 6th Int. Conf. on Magnesium Alloys and their Applications (K.U. Kainer Pub., Stuttgart, 2003) p.140. [42] G. Yuan, Y. Sun and W. Ding, Scr. Mater. 43 (2000) 1009. [43] Z. Yang, J.P. Li, G.H. Li and J.M. Yang, Mater. Sci. Forum 488-489 (2005) 219. [44] G. Yuan and Y. Sun, J Shanghai Jiaotong Univ. 3 (2001) 451. [45] M.E. Drits, Z.A. Sviderskaya and L.L. Rokhlin, Izv. Vyss. Uchebn, Zaved Tsvetn. Metall. 3 (1962) 117. [46] L.L. Rokhlin, Magnesium Alloys Containing Rare Earth Metals-Structure and Properties (Taylor & Francis Pub., Moscow, 2003) p.197. [47] J.F. King, Magnesium Alloys and Their Applications (B.L. Mordike and F. Hehmann Pub., Frankfuet, 1998) p.37. [48] D.H. Ping, K. Hono and J.F. Nie, Scr. Mater. 48 (2003) 1017. [49] P.J. Apps, H. Karimzadeh, J.F. King and G.W. Lorimer, Scr. Mater. 48 (2003) 475. [50] J.F. Nie and B.C. Muddle, Acta Mater. 48 (2000) 1691 [51] G.W. Lorimer, P.J. Apps, H. Karimzadeh and J.F. King, Scr. Mater. 48 (2000) 53. [52] Z. Yang, J.P. Li and Y.C. Guo, Mater. Sci. Eng. A485 (2008) 478. [53] M. Kettner, F. Pravdic, W. Fragner and K.U. Kainer, Magnesium Technology 2006 (TMS, Warrendale, PA) p.133. [54] C.J. Bettles and M.A. Gibson, JOM 57 (2005) 46. [55] C. Davies and M. Barnett, JOM 56 (2004) 22.

· 328 · [56] D. Letzig, J. Swiostek, J. Bohlen and K.U. Kainer, Magnes. Technol. 67 (2005) 55. [57] A.A. Luo and A.K. Sachdev, Metall. Mater. Trans. 6A (2007) 1184. [58] R. Gonzalez-Martinez, J. G¨ oken, D. Letzig, J. Timmerberg, K. Steinhoff and K.U. Kainer, Acta Metall. Sin. 20 (2007) 235. [59] P. Skubisz, J. Sinczak and S.J. Bednarek, Mater. Process. Technol. 177 (2006) 210. [60] J.S. Bohlen, B. Jacek, L. Heinz-Gunter, K. Dietmar and U. Karl, Magnesium Technology 2006 (TMS, Warrendale, PA, 2006) p.213. [61] A. Galiyev, R. Kaibyshev and G. Gottetein, Acta Mater. 49 (2001) 1199. [62] Z.W. Huang and Y. Yoshida, Mater. Sci. Forum 419-422 (2003) 243. [63] G. Itoh, Y. Iseno and Y. Mothashi, Mater. Sci. Forum 419-422 (2003) 355. [64] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki and H. Beladi, Mater. Sci. Eng. A456 (2006) 52. [65] K. Mueller and S. Mueller, J. Mater. Process Technol. 187-188 (2006) 775. [66] C. Yasumasa, I. Hajime and M. Mamoru, Mater. Sci. Eng. A466 (2007) 90. [67] S.M. He, X.Q. Zeng and L.M. Peng, J. Alloy. Compd. 427 (2007) 316. [68] Z. Yang, J.P. Li and Y.C. Guo, Mater. Sci. Eng. A454-455 (2007) 274. [69] J.P. Li, Z. Yang and T. Liu, Scr. Mater. 56 (2007) 137. [70] Y. Kawamura, K. Hayashi and A. Inoue, Mater. Trans. 42 (2001) 1301. [71] A. Inoue, Y. Kawamura and M. Matsushita, J. Mater. Res. 16 (2001) 1894. [72] E. Abe, Y. Kawamura, K. Hayashi and A. Inoue, Acta Mater. 50 (2002) 3845. [73] H. Jones, Mater. Sci. Eng. A304-306 (2001) 11. [74] A. Luo and M.O. Pekguleryue, J. Mater. Sci 29 (1994) 5259. [75] U. Noster and B. Scholtes, Mater. Sci. Forum 419-422 (2003) 103. [76] C.Y. Wang, X.J. Wang, H. Chang, K. Wu and M.Y. Zheng, Mater. Sci. Eng. A464 (2007) 52. [77] K. Yu, W.X. Li and R.C. Wang, Chin. J. Nonferrous Met. 17 (2007) 188. [78] H. Somekawa and T.Mukai, Mater. Trans. 47 (2006) 995. [79] Z.D. Zhao, W.W. Shan and S.J. Luo, Mater. Sci. Technol. 15 (2007) 59 (in Chinese). [80] W.W. Shan and S.J. Luo, Mater. Sci. Eng. A465 (2007) 247. [81] S.J. Luo and W.W. Shan, Trans. Nonferrous Met. Soc. China 17 (2007) 974. [82] J.Y. Lee, D.H. Kim, H.K. Lim and D.H. Kim, Mater. Lett. 59 (2005) 3801. [83] T. Honma, T. Ohkubo, S. Kamado and K. Hono, Acta Mater. 55 (2007) 4137. [84] F.W. Bach, M. Schaper and C. Jaschik, Mater. Sci. Forum 419-422 (2003) 1037. [85] F. von Buch, J. Lietzau, B.L. Mordike, A. Pisch and R. Schmid-Fetzer, Mater. Sci. Eng. A263 (1999) 1. [86] R.J.M. Payne and N. Bailey, J. Inst. Met. 88 (1960) 417. [87] T. Kobayashi and J. Koike, Mater. Sci. Forum 419-422 (2003) 231 [88] L. Wang, G. Huang, Y. Fan and G. Huang, Chin. J. Nonferrous Met. 13 (2003) 594. [89] R.W. Armstrong, Acta Metall. 16 (1968) 347. [90] T. Mukai and M. Yamanoi, Mater. Trans. 42 (2001) 1177. [91] H. Watanabe and T. Mukai, Scr. Mater. 41 (1999) 209. [92] K. Matsubara and Y. Miyahara, Mater. Sci. Forum 419 (2003) 497. [93] J. Collot, Mater. Sci. Forum 419-422 (2003) 617. [94] T. Motegi, E. Yano and N. Wada, Mater. Sci. Forum 419-422 (2003) 605.