• PDF / 1,297,106 Bytes
• 14 Pages / 593.972 x 792 pts Page_size
• 21 Downloads / 210 Views

DOWNLOAD

REPORT

---

INTRODUCTION

MECHANICAL twin deformation mechanisms are important for many non-cubic metallic materials, as they provide additional modes of deformation that facilitate the arbitrary shape changes required for formability. Since the seminal review of Christian and Mahajan,[1] there has been sustained interest in mechanical twinning. The most commonly observed twin in hexagonal metals such as Mg, Zr, and Ti is the {10 12}h10 1 1i extension T1 twin[2–7] illustrated in Figure 1. The formation of a twin results in movement of atoms in the positive c direction where the c-axis of the crystal is rotated 85 deg about the hai axis common to both the twin and the parent. This figure shows that THOMAS R. BIELER, Professor, is with Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226. Contact e-mail: [email protected] LEYUN WANG, formerly Graduate Student with Chemical Engineering and Materials Science, Michigan State University, is now Postdoc Associate with Nuclear Engineering, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439. ARMAND J. BEAUDOIN, Professor, is with Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Mechanical Engineering Building, 1206 W. Green St., Urbana, IL 61801. PETER KENESEI, Beamline Scientist, is with X-ray Science Division, Argonne National Laboratory, and also with Advanced Photon Source, Argonne National Laboratory. ULRICH LIENERT, Manager, is with Ru¨ntgen-A˚ngstro¨m-Cluster, Deutsches Elektronen-Synchrotron, Notkestraße 85, 22607 Hamburg, Germany. Manuscript submitted March 21, 2013. Article published online November 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

when the c-axis of the grain is aligned with the tensile axis, the Schmid factor for twinning is very high. In some hexagonal metals, such as Mg alloys, twinning can even serve as the primary deformation mechanism for plastic deformation, especially when the majority of the grains have orientations that do not favor basal or prism slip, e.g., with uniaxial deformation along their crystal c-axis, which is often called a hard orientation.[6,8–11] Mechanical twinning is also an important deformation mechanism in titanium alloys, and perhaps more so than has been traditionally realized. Even in fine-grain Ti-6Al-4V, where mechanical twinning has not been considered a dominant deformation mechanism, recent work has shown that some grains completely reorient by mechanical twinning at modest strains.[12,13] Extensive statistical analysis of twin nucleation in hexagonal metals such as Zr and Mg has shown that twin activation does not necessarily follow the traditional Schmid law, as in many cases the twin variant that forms is not the one with the highest Schmid factor[14–16] The fact that the local stress state in a grain is often more complex than the global stress state may account for this, and consequently Schmid factors calculated using the global stress state (e.g., uniaxial tension or compression) may not accurately represent the true r