Cyclic-Tension Fatigue Behavior in a Rolled AZ31B Magnesium Alloy Studied Using Ultrasonic Shear Waves
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TO achieve a low-carbon society, the wide use of magnesium alloys, one of the most promising light metals for structural applications, will be needed.[1] At present, however, the application of these alloys is mainly limited to cast components such as casings[2] because of their lower plastic deformability and the uncertainness of their fatigue mechanism. Since magnesium exhibits several orders of extraordinary critical resolved shear stresses (CRSSs) for basal and nonbasal slips at room temperature,[3] and it has strong fiber textures that are aligned with the shear-stress direction of the expanding process, the magnesium alloys have strong anisotropy in their mechanical properties.[4,5] In addition to its lopsided slip behavior, twinning deformation is also an important strain-relief mechanism. The deformation mechanism of the alloys strongly depends on their fiber texture. Thus, the deformation mechanism has been investigated actively in previous studies.[6–10] Recent research involving magnesium alloys has indicated that a fine-grain nonbasal glide occurs to maintain the continuity of the strain due to stress concentrations at the boundaries,[11] and that a coarse-grain {10-12} twinning shows reversibility of its deformation in loading HIDEKI YAMAGISHI, Senior Research Engineer, Central Research Institute, Toyama Industrial Technology Center, Takaoka 933-0981, Japan, is Graduate Student, Institute for Materials Research, Tohoku University. Contact e-mail: [email protected]. toyama.jp MIKIO FUKUHARA, Associate Professor, HIROAKI MATSUMOTO, Assistant Professor, and AKIHIKO CHIBA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted August 3, 2009. Article published online June 8, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
and unloading.[12,13] These behaviors provide an insight into appreciating the mechanical properties of the alloys. However, most studies on the microscopic deformation mechanism have only considered plastic deformation, not fatigue behavior. Because a specimen cannot be observed by cutting before the test termination, most fatigue studies are geared toward fracture mechanics in view of stress–strain hysteresis for low cycles[14] or crack propagation for high cycles.[15] Microscopic clarification of the fatigue behavior is expected to increase the amount of magnesium applied to structural uses. Nondestructive monitoring using ultrasonic waves is one promising tool that could be applied to investigate the structural behavior of magnesium. However, the use of ultrasonic waves requires the development of a nondestructive fatigue evaluation method. Two types of dominant waves exist in ultrasonic vibration: longitudinal waves and shear waves (SVs). SVs have not been widely used by engineers because they can only propagate in solids, which have a shear resistance, and acquiring a sufficient echo compared to longitudinal waves is more difficult due to the high damping capacity. However, recent developments in absorption-free cou
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