Microstructure, Deformation, and Property of Wrought Magnesium Alloys

METALLURGICAL AND MATERIALS TRANSACTIONS 50TH ANNIVERSARY COLLECTION Microstructure, Deformation, and Property of Wrought Magnesium Alloys J.F. NIE, K.S. SHIN, and Z.R. ZENG Pure magnesium (Mg) develops a strong basal texture after conventional processing of hot rolling or extrusion. Consequently, it exhibits anisotropic mechanical properties and is difficult to form at room temperature. Adding appropriate alloying elements can weaken the basal texture or even change it, but the improvement in formability and mechanical properties is still far from expectations. Over the past 20 years, considerable efforts have been made and significant progress has been made on wrought Mg alloys at the fundamental and technological levels. At the fundamental level, textures formed in sheets and extrusions of different alloy compositions and produced under different strain paths or thermomechanical processing conditions are relatively well established, with the assistance of the advanced characterization technique of electron backscatter diffraction. At the technological level, room temperature formability of sheet has been significantly improved, and tension–compression yield asymmetry of extrusion is also remarkably reduced or eliminated. This paper starts with an overview of dislocations, stacking faults and twins, and deformation of single crystals of pure Mg along different orientations and under different loading conditions, followed by a review of microstructure (texture and grain size) and deformation of polycrystalline pure Mg with different textures, grain sizes, and loading conditions. With this information as a base, texture, grain size, and deformation of polycrystalline Mg alloy sheets and extrusions produced under different processing conditions are systematically examined and compared. Remaining and emerging scientific and technology issues are then highlighted and discussed in the context of texture and grain size. The need for better-resolution diffraction and spectroscopy techniques is also discussed in the relationship between texture change and grain boundary solute segregation. https://doi.org/10.1007/s11661-020-05974-z  The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION COMPRISING 2.7 pct of the earth’s crust and being the third most plentiful element dissolved in seawater, magnesium (Mg) is an abundant element. It is readily commercially produced, with a purity exceeding 99.8 pct, from seawater, lake brines, dolomite, magnesite, and other minerals. Its density is 66 pct of aluminum and 25 pct of steel. These unique features make Mg a promising material to substitute steel and aluminum J.F. NIE is with the Department of Materials Science and Engineering, Monash University, Melbourne, VIC 3800, Australia. Contact e-mail: K.S. SHIN is with the Department of Materials Science and Engineering, Seoul National University, 1 Gwannak-ro, Gwannak-gu, Seoul 08826, Republic of Korea. Z.R. ZENG is with the College of Engineering and Computer Science, The Australian National University, Canberra, ACT 2601, Australia. Manuscript submitted April 21, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A alloys for more energy-efficient and environmentally friendly applications. Statistical data indicate that each 100 kilogram reduction in vehicle weight reduces fuel consumption by 0.38 L per 100 km and CO2 emission by 8.7 gram per kilometer.[1] Commercial production of magnesium metal was 277,000 tonnes per annum in 1999, but rose rapidly to approximately 608,000 tonnes in 2009, and reached about 1,100,000 tonnes in 2019, Figure 1. In 2017, a new magnesium production plant was constructed in Qinghai Province China, with an annual production rate of 100,000 tonnes from lake brines. One year later, Magontec’s new magnesium alloy cast house facility started its operation, with an initial annual production rate of 60,000 tonnes of alloy ingots. For the primary magnesium metal produced each year, about 35 pct is used for making magnesium alloys in the form of castings and wrought products. Wrought magnesium products account for only about 1 pct of magnesium consumption, even though they reached 6 pct in 2017 in the USA. The low figure of the wrought magnesium Fig. 1—Primary magnesium metal consumption each year in the period 1999–2019. products is mainly due to low demands from the transportation and construction industries. However, a few significant developments have been made in recent years on the developments of wrought products. In 2014, Korean steel company POSCO and Renault Samsung Motors jointly developed a magnesium sheet to be used for the walls of VIP back seats and the trunks of upgraded SM7 vehicles. In 2015, Porsche selected Mg sheet for the roof of its new model of the 911 GT3 after its tests on Mg, Al, and carbon-fiber-reinforced polymers. In 2018, Nanjing Yunhai Special Metals Co. Ltd and Taiwan Jian Sin Industrial Co. Ltd announced a joint venture to invest one billion Yuan to build a new plant to produce one million forged magnesium wheels each year. With advances of processing and manufacturing technologies and alloy design, it is foreseeable that the global market for wrought magnesium products will expand significantly in the near future. One of the major barriers to the larger usage and wider application of wrought magnesium alloys is their limited formability at room temperature, bulk magnesium is intrinsically difficult to form at this temperature. Therefore, processes such as extrusion, rolling, and press forging must be carried out in the temperature range 300 to 500 C. The productivity of magnesium alloy extrusions is much lower than that of aluminum alloys, and sheet production usually involves more stages of hot rolling. The processing cost is hence higher. Additionally, the extruded magnesium products often have tension–compression yield asymmetry: the compressive yield strength may be only half of the tensile yield strength, and rolled sheet usually has anisotropic formability and mechanical properties along different directions. Such problems have to be solved for any larger usage of wrought magnesium alloys. Deformation modes that are commonly activated in Mg and its alloys include intra-granular slip and twinning and inter-granular grain boundary sliding, Figures 2(a) through (c). Dynamic recrystallization may also occur to assist the plastic deformation, depending on the strain level and the applied temperature, Figure 2(d). The available slip deformation modes are progressively more difficult to activate and this is compounded by a strong basal texture developed during thermomechanical processing. Twinning is highly dependent on orientation and exhausts after all suitably oriented grains have twinned, usually at around a strain of up to 0.08. As a result, in contrast to the substantial formability of aluminum, fracture usually occurs when coarse-grained pure magnesium is cold-rolled by only 20 to 30 pct thickness reduction. The (...truncated)