Influence of Texture on Mechanical Behavior of Friction-Stir-Processed Magnesium Alloy
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Magnesium and its alloys have limited room-temperature plasticity. The only slip system operative during ambient temperature deformation is basal slip 20 ; which is strongly favored compared to f0002g 11 nonbasal dislocation slip. As basal slip is limited only to basal planes, to accommodate internal strains that set up at grain boundaries when inelastic strains are not compatible, twinning plays an important role during low-temperature deformation.[1] There are two deformation twin modes for Mg; c-axis tension twin 10 12 10 11 accommodating extension along the c-axis and c-axis compression twin 1011 1012 accommodating compression along the c-axis. The low ductility and workability of magnesium alloys can be increased by activating the nonbasal
slip system, i.e., prismatic slip systems 1010 1120 and pyramidal slip 11 22 11 23 : This can be accomplished in three ways: by increasing the deformation temperature, by decreasing the axial (c/a) ratio of magnesium, or by microstructural control.[2] Microstructural control and refinement can be achieved by various reported G. BHARGAVA and W. YUAN, Graduate Students, S.S. WEBB, Undergraduate Student, and R.S. MISHRA, Curators’ Professor of Materials Science and Engineering, are with the Center for Friction Stir Processing, Department of Material Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409. Contact e-mail: [email protected] Manuscript submitted June 24, 2009. Article published online October 29, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
techniques such as equal channel angular extrusion (ECAE),[3] differential speed rolling,[4] and thermomechanical processing.[5] Grain refinement can significantly enhance mechanical properties such as fracture toughness and ductility in magnesium alloys.[6,7] A transmission electron microscopic study reported by Koike et al.[8] on ECAE-processed AZ31 confirmed that the improved strength and ductility in ECAEprocessed fine-grained material was due to the activation of nonbasal slip systems. Friction stir processing (FSP) has been effectively shown to produce fine grain microstructure with improved ductility.[9] Recently, Chang et al.[10] achieved a grain size of ~100 nm by multiple pass FSP on AZ31 alloy. Hence, FSP could be a potential tool to improve the room-temperature formability of magnesium alloys by grain refinement. In addition, FSP can provide fine-grained microstructure with unique texture for understanding the microstructure-texture-deformation mechanism relationships in magnesium alloys. In the present study, microstructure and mechanical properties of friction-stirprocessed AZ31 were characterized in the longitudinal direction (LD) and transverse direction (TD). The difference of tensile properties in two orthogonal directions is discussed in the context of the FSPinduced texture. Rolled plates of magnesium alloy AZ31B of thickness 6.5 mm were used for the present study. The initial average grain size of parent material determined by the linear intercept method was
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