DOI: 10.1002/adem.201000050 Enhancing Fracture Toughness of Magnesium Alloy by Formation of Low-Angle Grain Boundary Structure** By Hidetoshi Somekawa * , Alok Singh, Tadanobu Inoue and Toshiji Mukai Magnesium alloys are the lightest among all the structural alloys in use, but its fracture toughness, which is one of the mechanical properties for the judgment of reliability and safety requirement, has generally been reported to be lower than that in aluminum alloys. [1,2] One of the reasons of low fracture toughness in magnesium is the low physical properties and the formation of deformation twins. The fracture toughness is influenced by the shear modulus [3,4] and/or the surface energy [5] ; however, the shear modulus in magnesium is lower compared to that of the other metallic materials, [6] and the surface energy of the basal plane, which is the major slip system at the room temperature, is lower than that of the non-basal planes. [7,8] In magnesium and magne- sium alloys, the deformation twins also form easily during plastic deformation because of the lack of slip systems at room temperature. [9,10] However, the amount of movable distance by the deformation twins is shorter than that of the grain boundary by the dislocation slip, due to the limitation in geometry at the twin boundary. Thus, when the strains are accumulated at the twin boundary, the deformation twins and/or the twin boundary become the crack propagation route, and cause a brittle fracture. [11,12] On the other hand, by refining the grain structures, (i) the stress for the formation of deformation twins increases [13–15] and the formation of deformation twins reduces [16] and (ii) not only the basal but also the non-basal slip systems become activated because of the operation of the compatibility stress at the grain boundary. [17] The dominant plastic deformation mechanism changes from the deformation twins to the dislocation slip, [18] and the fracture mechanism changes from a brittle fracture on the account of the deformation twins to a ductile fracture associated with the dimple formation. The fracture toughness also tends to increase with grain refinement in magnesium [19] and magnesium alloys. [20] Some researchers have recently pointed out that the formation of a low-angle grain (or sub-grain) boundary structure is effective for the enhance- ment of ductility in intermetallics [21–23] and/or conventional light structural metals, such as aluminum. [24] Although the grain boundary generally has the role of preventing the dislocation movement, it provides the nucleation site for the micro-void and/or micro-crack formation. On the other hand, the amount of dislocation pile-up at the low-angle grain boundary is lower than that at the high-angle grain boundary, leading to an advantage toward ductility. To date, the authors have been successful in producing a fine-grained magnesium alloy with a high fraction of the low-angle grain boundary, and revealed the strain distribution by FEM simulation corre- sponding to the hardness and microstructure distribution [25] and obtained a high yield strength of over 400 MPa in tension at room temperature [26] ; however, the fracture toughness and its deformation behavior have not been investigated yet. Thus, the effect of the low-angle grain boundary structure on these properties was examined in this study. The present study was conducted by using a commercially extruded AZ31 alloy, and full details are given in the Experimental Section. COMMUNICATION [*] Dr. H. Somekawa, Dr. A. Singh, Dr. T. Mukai Structural Metals Center, National Institute for Materials Science 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan E-mail: somekawa.hidetoshi@nims.go.jp Dr. T. Inoue Exploratory Materials Research Laboratory for Reliability and Safety, National Institute for Materials Science 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan [**] The authors are grateful to Mrs. S. Kuroda, Y. Taniuchi, and K. Nakazato for their materials processing and Ms. M. Isaki for her technical help at the National Institute for Materials Science. This work was supported in part by JSPS Grant-in-Aid for Young Scientists (B) no.21760564 (H. S) and JSPS Gran- t-in-Aid for Scientific Research (B) no. 21360347 (T. M). The effect of the low angle grain boundary structure on the mechanical properties and deformation behavior was investigated using Mg-Al-Zn alloys, which were produced by caliber rolling. The {10–12} deformation twins were formed at the head of the crack-tip during fracture toughness even in the fine-grained structures; however, the present caliber rolled alloys showed high strength and fracture toughness balance that resulted from the high fraction of low-angle grain boundaries, which did not become the origin of the micro-void formation. ADVANCED ENGINEERING MATERIALS 2010, 12, No. 9 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 837