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Materials Frontier No.170

Title:Deformation Twinning in Hexagonal Materials: Nucleation, Propagation/Growth,and Interactions
Speaker:Dr.Jian Wang.
Venue:Room 308,Material Building A
Invitor:Yao Shen
    Dr. Jian Wang is an Associate Professor at Mechanical and Materials Engineering at University of Nebraska-Lincoln. He received his Ph.D in Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA, in 2006. After that, He joined Los Alamos National Laboratory and has been working as Technical Staff Member for 9 years. Currently, his research interests are focused on more quantitative exploring the structure-properties relationships of structural and nanostructured materials. He was awarded International Journal of Plasticity Young Resaech Award, 2015; TMS MPMD Young Leader Professional Development Award, 2013; the LDRD/Early Career Award (2011); and the LANL Distinguished Postdoctoral Performance Award in 2009. He was leading two DoE BES Core programs with focus on (1) Deformation Physics of Ultra-fine Materials and (2) Multiscale Constitute Laws for HCP materials; and two LDRD-ER Award (2013), Los Alamos National Laboratory, USA. He has ~170 peer-reviewed publications (~4000 citations and H-index = 38, 25 papers selected as 25 Hottest Articles in Materials Science), two book chapters in Dislocations in Solids and 55+ invited/keynote presentations. He is serving as Editorial Boards for several materials journals.
    Twinning and de-twinning, which are the dominant deformation mechanisms in hexagonal-close-packed (hcp) metals due to the hard non-basal slips, correspond to texture evolution, plasticity anisotropy, and cracking, and exhibit more complex nucleation, propagation/growth and interaction mechanisms than those in cubic structures (FCC and BCC). Specifically, twinning and de-twinning are directional in hcp metals. Twin nucleation, a necessary first stage of twinning, seems to involve pure shuffling mechanisms and cause a near 90-degree reorientation with respect to the matrix. After twin nucleation, twin propagation and growth seems more complicated than that in cubic crystals because of the formation of serrated coherent twin boundaries. Due to multiple twin variants that are activated under a given stress, a twin unlikely crosses through another twin, forming twin-twin junctions. These junctions further influence twinning and detwinning, and result formation of secondary twins. In this talk, I will focus on the fundamental understanding of twinning mechanisms and multiscale modeling of twinning behaviors in hcp metals. The fundamental mechanisms, including pure-shuffle, shear-shuffle and climb shuffle, will be discussed in association with twinning and de-twinning. Several boundaries associated with twinning/detwinning will be discussed based on in situ HRTEM and atomistic simulations.

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