Super-formable pure magnesium at room temperature

ARTICLE DOI: 10.1038/s41467-017-01330-9 OPEN Super-formable pure magnesium at room temperature Zhuoran Zeng1, Jian-Feng Nie1, Shi-Wei Xu2, Chris H. J. Davies3 & Nick Birbilis 1 Magnesium, the lightest structural metal, is difficult to form at room temperature due to an insufficient number of deformation modes imposed by its hexagonal structure and a strong texture developed during thermomechanical processes. Although appropriate alloying additions can weaken the texture, formability improvement is limited because alloying additions do not fundamentally alter deformation modes. Here we show that magnesium can become super-formable at room temperature without alloying. Despite possessing a strong texture, magnesium can be cold rolled to a strain at least eight times that possible in conventional processing. The resultant cold-rolled sheet can be further formed without cracking due to grain size reduction to the order of one micron and inter-granular mechanisms becoming dominant, rather than the usual slip and twinning. These findings provide a pathway for developing highly formable products from magnesium and other hexagonal metals that are traditionally difficult to form at room temperature. 1 Department of Materials Science and Engineering, Monash University, Melbourne, Vic 3800, Australia. 2 Automotive Steel Research Institute, Research Institute (R&D Centre), Baoshan Iron & Steel Co., Ltd, Shanghai 201900, China. 3 Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Vic 3800, Australia. Correspondence and requests for materials should be addressed to J.-F.N. (email: ) or to N.B. (email: ) NATURE COMMUNICATIONS | 8: 972 | DOI: 10.1038/s41467-017-01330-9 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01330-9 Results Super-formability of magnesium in cold compression or rolling. Polycrystalline pure magnesium becomes super-formable at room temperature after it is extruded at or below 80 °C. Specimens extruded in the temperature range 150–400 °C have poor formability at room temperature. They fracture when compressed by 20–30% reduction in height, consistent with previous studies17, 18. However, specimens extruded at or below 80 °C do not fracture during compression at room temperature and a strain rate of 10−3 s-1 (Fig. 1a, b). In contrast to the high work hardening of the specimens extruded at higher temperatures, these super-formable specimens exhibit no work hardening after yielding. Instead, the true stress decreases gradually with strain, implying minimal twinning and dislocation slip during the compression. Such behaviour is associated with grain boundary sliding19 and/or dynamic recrystallization20, 21 in magnesium alloys tested at elevated temperature. Photos in Fig. 1b show the compression test results of the specimens extruded at 80 and 400 ° C. While the 400 °C extruded specimen fractures after ~20% height reduction, the 80 °C extruded specimen can be compressed 2 True stress (MPa) a 300 b Extrusion temperature 25 °C 65 °C 80 °C 150 °C 200 °C 250 °C 300 °C 200 100 0 True stress (MPa) M agnesium is a widely available metal. Comprising 2.7% of the earth’s crust it is readily commercially produced from seawater and from its ore with a purity that can exceed 99.8%. It has a density that is 66% of aluminium and 25% of steel. These unique features make magnesium a promising candidate for substituting steel and aluminium alloys for more energy efficient and environmentally friendly applications1, 2. For example, each 100 kg reduction in vehicle weight reduces fuel consumption by 0.38 litre per 100 kilometre and CO2 emission by 8.7 g per kilometre3. One major barrier to the wide use of magnesium products is their limited formability: magnesium itself is intrinsically difficult to form at room temperature. Deformation modes that are commonly activated in magnesium are slip on the basal plane, prismatic planes and pyramidal planes, as well as twinning4. The available slip deformation modes are progressively more difficult to activate and this is compounded by a strong basal texture developed during thermomechanical processing4, 5. Twinning is highly dependent on orientation and exhausts after all suitably oriented grains have twinned, usually at around a strain of up to 0.086. As a result, in contrast to the substantial formability of aluminium in cold rolling (recalling that aluminium beverage can walls are around 100 μm thick and aluminium foil is an everyday item), fracture usually occurs when polycrystalline pure magnesium is cold rolled by only ~30% thickness reduction7. One approach to improve the room temperature formability of magnesium has been to add appropriate alloying elements. Alloying additions can reduce the stress required to activate more deformation modes and/or weaken the basal texture to allow easier plastic deformation8–13. While such efforts have achieved some success in terms of formability improvement, they have not developed any magnesium products that are super-formable at room temperature. Reducing grain sizes to the micron scale by severe plastic deformation processes, such as equal channel angular extrusion (ECAE) or high pressure torsion, can activate grain boundary sliding to improve ductility14, 15, but such samples fracture after about 0.2 compression strain16. Furthermore, magnesium products produced by severe plastic deformation processes are too small to be used industrially (i.e., in the automotive industry) and cannot be upscaled. In this work, we report a breakthrough in the design and development of formable magnesium–polycrystalline pure magnesium can be tailored to be super-formable at room temperature by conventional processes. 0 0.1 300 0.2 0.3 True strain 0.4 Extruded at 400 °C 0.5 Manual stop 200 100 Extruded at 80 °C Extruded at 400 °C 0 Extruded at 80 °C 0 20 40 60 80 Height reduction (%) Fig. 1 Cold compression of extruded specimens. a Room temperature compressive true stress-strain curves of specimens extruded a in the temperature range 25–300 °C. b Room temperature compression of specimens extruded at 80 and 400 °C. Photo insets show the specimens before and after compression test. Scale bars in photo insets indicate 5 mm from 10 to 1.5 mm without fracture. Further height reduction from 1.5 mm is possible if the compression test continues. As a further demonstration of the super-formability of the extruded polycrystalline pure magnesium, specimens extruded at 80 °C were also rolled at room temperature without any intermediate annealing. Their thickness was reduced continuously from 3 to 1 mm without any edge cracking. The 1 mm-thick sheet was further cold rolled to 0.5 mm, and even 0.12 mm (96% total thickness reduction equating to a true strain of 3.2). The resultant 0.12 mm-thick strip was cut into two pieces that were shaped into letters “m” and “g”, as shown in Fig. 2a. This result is in distinct co (...truncated)