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Materials Science at Oregon State University

Experimental Investigation of Twin-Twin Interaction and its Role on Cyclic Deformation and Fatigue in Magnesium Single Crystal

Date: Thursday, May 7th
Presenter: Dr. Qin Yu, OSU Materials Science


{10-12} tension twin is the most common twinning mode in hexagonal close packed (HCP) magnesium and acts as a major deformation mode to accommodate arbitrary plastic deformation. Under tension along [0001] direction or compression along [10-10] direction, multiple twin variants can simultaneously nucleate, grow, and interact with each other. When the loading direction is reversed, detwinning commences. Both twinning and detwinning processes are affected by interactions among different twin variants. The illumination of twin-twin interaction under cyclic loading is critical to understand fatigue processes in HCP magnesium.
We studied the twin-twin interaction by carrying out strain-controlled cyclic tension-compression experiments on magnesium single crystals in [0001] and [10-10] directions utilizing in situ optical microscopy (OM), ex situ electron backscatter diffraction (EBSD), and post-mortem transmission electron microscopy (TEM) analyses. Experimental characterizations revealed three kinds of twin-twin structures: a quilted-looking twin structures consisting of twins arrested at other twin boundaries, an “apparent crossing” twin structure which links two twins impinged independently on each side of a barrier twin lamella, and a double twin structure that results from secondary twins being nucleated at the twin-twin interface. According to HCP crystallography, twin-twin interactions can be classified into two types: Type I for two twin variants sharing the same 11-20 zone axis and Type II for two twin variants having different zone axes. During twinning growth, one twin does not transmit into the other twin across the twin boundary for Type I twin-twin interactions. For Type II twin-twin interactions, twin transmission is possible under certain loading conditions. In most cases, twin transmission does not occur but twin-twin boundaries form and contain boundary dislocations. Twin-twin boundary plane can be geometrically abstracted as a common interface that bisecting two coherent twin planes, which is verified by both two-dimensional and three-dimensional characterizations in micron scale. Twin-twin boundary dislocations can be inferred by reactions of twinning dislocations associated with two twin variants. For Type I twin-twin interaction, the twin-twin boundary is a low angle pure tilt boundary with the habit plane being either the basal or the prismatic plane. For Type II twin-twin interactions, the twin-twin boundary is a high index crystallographic plane according to the geometric analysis. An “apparent crossing” twin structure is thus a consequence of twin-twin boundary formation. Under reversed loading, detwinning is hindered because of the energetically unfavorable dissociation of twin-twin boundary dislocations. Most interestingly, secondary tension twins is activated at Type II twin-twin boundaries under the reversed loading. During repeated cyclic loading, three twin-twin structures form as consequence of twin-twin interactions, which effectively contributes to the twinning-induced and detwinning-induced cyclic hardening of the material. Microcracks are easily formed at the boundaries of secondary twin fragments due to the incompatible deformation induced during repeated twinning and detwinning processes.

Dr. Qin Yu was born in Shanghai, China. He received his B.Eng. degree in Mechanical Engineering and M.Eng. degree in Solid Mechanics from Shanghai Jiao Tong University. He completed his Ph.D. degree in Mechanical Engineering at University of Nevada, Reno, where he was the major player working on a Department of Energy (DOE) Basic Energy Sciences (BES) sponsored project – “Micro-Mechanisms and Multiscale Modeling of Cyclic Plastic Deformation of Magnesium Single Crystals.” Qin and his advisor Prof. Yanyao Jiang, for the first time, successfully characterized the cyclic stress-strain response and directly observed the evolution of twinning/detwinning process in magnesium single crystal. Because of the leading work on twining/detwinning, Qin was also a graduate research assistant at Material Science and Technology Division, Los Alamos National Laboratory (LANL) during the last two years of his Ph.D. program, being a team member in a DOE BES core project – “Multi-scale Study of the Role of Microstructure in the Deformation Behavior of Hexagonal Materials.” At LANL, Qin carried out detailed material characterization using advanced electron microscopy techniques. His systematic studies illuminated the fundamental structure and micro-mechanism of twin-twin interaction in magnesium. On the completion of his Ph.D. research, Qin’s dedicated efforts on the study of cyclic deformation, fatigue, and the associated mechanisms in magnesium have resulted in over twenty papers published in leading journals. Qin presently joined Prof. Jamie J. Kruzic’s group at Oregon State University working on a NETL sponsored project to develop a new mechanistic models to predict long-term evolution of microstructure and mechanical properties of nickel based superalloys. His research interests include multi-scale exploration of mechanical behavior (in particular cyclic plasticity and creep), failure mecahnism, and the structure-property relationship utilizing in/ex situ mechanical testing and advanced material characterization/analysis techniques. The desired output is to enhance the fundamental understanding of microstructural roles on deformation and fracture and to develop physical mechanism-based constitutive and lifing models.