13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Evaluation of Microstructure Effect on Anisotropic Mechanical Properties of Hot Rolled AZ31 by AE Analysis Kousuke Matsumoto1,*, Takashi Yasutomi1, Manabu Enoki1 1 Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan * Corresponding author: matsumoto@rme.mm.t.u-tokyo.ac.jp Abstract Anisotropic properties of twinning and slip deformation of hot rolled and extruded AZ31 Mg alloy have been reported in our previous researches. AE (Acoustic Emission) measurement has been done during the tensile test of specimen with various angles between tensile direction and rolled or extruded direction. AE energy had good correlation with twinning area and 0.2% proof stress. It has been also reported that twinning stress has higher grain size dependence than the stress required for activation of slip because the Hall-Petch slope for twinning is larger than that for slip. In this research, tensile test specimen were prepared from rolled AZ31 with various angles between tensile and rolling directions. In order to change grain size of hot rolled AZ31 Mg alloy, the specimens were annealed at various temperatures. Then in-situ measurement of AE signals was conducted during the tensile test. The mechanical deformations occurring actually were found by EBSD (Electron Backscatter diffraction) and observation of fracture surface. According to the result, the effect that grain size of hot rolled AZ31 Mg alloy gives to anisotropic mechanical properties during the tensile test was quantitatively evaluated by AE analysis. Keywords Acoustic Emission, AZ31, Annealing, Anisotropy, Twinning 1. Introduction Mg alloy is a light structural metal with high specific strengths; therefore, its alloys are the most attractive materials to be utilized in the automotive and transportation industries. This is due to their lower density compared to that of other structural materials such as steels and aluminum alloys. However, their poor mechanical properties at the room temperature are considered as an important fault. Grain size refinement is a major way to improve mechanical properties. At this point, Hall-Petch relationship predicts that the material strength increases with decreasing grain size. Its hexagonal close-packed structure also characterizes its mechanical properties. In the former research, anisotropic mechanical properties during tensile test of extruded AZ31 Mg alloy is evaluated by AE analysis [1-6]. In this research, the specimens were produced from hot rolled AZ31 Mg alloy with various angles between tensile and rolling directions. These specimen were annealed at various temperatures in order to change their grain sizes. The effect of various grain sizes on mechanical properties was evaluated by AE energy. 2. Experimental procedure 2.1. Material The material used in this study was hot rolled AZ31 sheet (Mg – 3.04 % Al – 1.06 % Zn – 0.38 % Mn) with a thickness of 3 mm bought from Osaka Fuji Corporation. The as-received specimen had an average grain size of 9.8μm. In order to change grain size, the as-received specimens were annealed at 285°C for 30 min and 450°C for 30 min. The grain sizes after annealing were 13.5 and 17.9μm for the first and latter conditions, respectively. To study the anisotropy of mechanical properties, the specimens were prepared with various angles between tensile and rolling directions of 0°, 15°, 30°, 45°, 60°, 75° and 90°, shown in Fig. 1. All tests were carried out at a constant strain rate of 1.67×10-4 /s.
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