Topic: Deformation Twinning in Hexagonal Materials: Nucleation, Propagation/Growth, and Interactions

Speaker:Dr. Jian Wang, University of Nebraska-Lincoln

Time: 10月21日（周三）14:00-16:00

Venue: Room 403, Shichangxu Building, IMR CAS

Welcome to attend!

Abstract:Twinning which is the dominant deformation mechanisms in hexagonal-close-packed (hcp) metals due to the hard non-basal slipscorresponds to texture evolution and plasticity anisotropy, and exhibit more complicated nucleation, propagation/growth and interaction mechanisms than those in cubic structures. Specifically, twinning is directional in hcp metals. Twin nucleation must precede twin propagation and growth. Twin nucleation in HCP materials is driven by local stress states and accomplished via atomic shuffling mechanisms. After twin nucleation, twin propagation and growth seems more complicated than that in cubic crystals because of the formation of serrated coherent twin boundaries.Twin propagation is driven by long range stress states across grains through the motion of twin boundaries either by gliding of twinning dislocations on the twin plane along the twinning direction or migration of twin boundaries normal to the twin plane via nucleation and gliding of twinning dislocations on the twin plane. Corresponding to crystallography of hcp structure, twin variants on different twin planes nucleate, propagate, and grow during deformation. These twin variants interact and react each other, forming twin-twin junctions that influence twin propagation and growth during loading, as well as de-twinning and nucleation of secondary twins during unloading. For a comprehensive understanding of twin nucleation, propagation and twin-twin interactions, to characterize structures of twin boundaries and twin-twin junctions at atomic level and their influence on mechanical properties at micro/meso-scales is essential and correspondingly highlighted in this lecture. In this talk, I will focus on 10 questions regarding twinning at different scales. 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.

Bio:Dr. Jian Wang isan Associate Professor at Mechanical and Materials Engineering at University of Nebraska-Lincoln. He received his Ph.D in Mechanical Engineering, Rensselaer Polytechnic Institute in 2006. After that, He joined Los Alamos National Laboratory and has been working as Technical Staff Member until Aug. 2015.Currently, his research interests are focused on more quantitative exploring the structure-properties relationships of structural and nanostructured materials. He received several prestigious awards, includingInternational Journal of Plasticity Young Research Award, 2015; TMS MPMD Young Leader Professional Development Award,2013; the LDRD/Early Career Award (2011); andthe 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, and four paper was featured as Journal Covers), two book chapters in Dislocations in Solids and 55+ invited/keynote presentations. He is serving as Editorial Boards for several materials journals.