Room Temperature Shear Band Development in Highly Twinned Wrought Magnesium AZ31B Sheet
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INTRODUCTION
THE poor formability of magnesium at room temperature has been attributed to a lack of active slip systems in the HCP crystal structure.[1–4] Additionally, in a rolled AZ31B sheet, there is a strong basal texture present with no easy slip planes that accommodate strain along the c-axis at room temperature.[4–6] The typical approach to overcoming the low formability of AZ31B is to form the material at elevated temperatures usually 523 K (250 °C) or higher.[7,8] Forming at these high temperatures is expensive and difficult to implement in a high volume production environment. Therefore, studying the formability of wrought magnesium at room temperature is important to understanding the failure initiation mechanisms that precede fracture. If an understanding of failure mechanisms can be gained, then it is possible that the starting microstructure or the forming path can be manipulated in order to increase formability. Previous studies on formability and failure mechanisms have been performed on samples that undergo either compression or uniaxial tensile strains, where measurements were taken in a highly deformed or JON SCOTT, formerly Master’s Graduate Student, with Mechanical Engineering, Brigham Young University, Provo, UT 84602, is now Composites Engineer, Infinite Technologies, Inc, Folsom, CA 95630. MICHAEL MILES, Associate Professor, is with Manufacturing Engineering Technology, Brigham Young University. Contact e-mail: [email protected] DAVID FULLWOOD, Associate Professor, and BRENT ADAMS, Professor, are with Mechanical Engineering, Brigham Young University. ALI KHOSRAVANI, formerly Master’s Graduate Student, with Mechanical Engineering, Brigham Young University, is now Ph.D. Student, Materials Science and Engineering, Drexel University, Warminster, PA. RAJA K. MISHRA, Technical Fellow, is with General Motors R&D Center, Warren, MI 48090. Manuscript submitted July 28, 2011. Article published online September 27, 2012 512—VOLUME 44A, JANUARY 2013
necked region. Although compression is vital to the sheet rolling process and uniaxial tension is a good tool for understanding material behavior, biaxial tension and plane strain tension are more typical modes of deformation for sheet stamping. It has been shown that a majority of part failures during sheet forming or stamping occur in plane strain.[9] Since prior work on crack nucleation and failure initiation has depended primarily on uniaxial tension specimens with strains well above 10 pct, the present study focused on failure mechanisms for biaxial and plane strain tension with comparisons to uniaxial tension. The primary mechanism for both through-thickness deformation and failure in AZ31B, in samples strained by either or uniaxial tension, is thought compression to be 1011 compression twinning.[1,10] The strong basal texture in AZ31B means that in-plane tensile deformation of wrought sheet causes compression along the c-axis, which is accommodated by compression twinning. At high strains, compression twins can also be a contributing factor in fracture
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