Instructor: Yuntian Zhu MSE 791: Mechanical Properties of Nanostructured Materials Module 3: Fundamental Physics and Materials Design Instructor: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 3 Difference in the deformation of coarse-grained and nano grains Text book, Office hour, by appointment Department of Materials Science and Engineering 1

Dislocation source in coarse-grained metals: Frank-Read source F = tb Dislocation velocity should be similar under the same stress https://www.youtube.com/watch?v=5yID78ovcX8 Department of Materials Science and Engineering 2

Twinning mechanisms in CG metal: Pole mechanism Due to the screw dislocation, the partial moves to the next slip plane after rotating around the screw for 360o Other mechanisms: 3 layer twinning: Mahajan & Chen, Acta Metall. 21, 1353 (1973). Prismatic Glide mechanism: Venables, Phil. Mag. 6, 379 (1961) Double pole mechanism:Niewczas & Saada, Phil. Mag. 82, 167(2002). Department of Materials Science and Engineering 3

Nanocrystalline grains often are dislocation free Define Nanocrystalline and nanostructured mater. Dislocation and microstructure variation with grain size in Ti Department of Materials Science and Engineering 4

What Do We Know about Deformation Physics in Nanomaterials? Grain rotation and grain boundary sliding at finest grain size (e.g. <10nm) Molecular Dynamics simulations Swygenhoven et al, Phys. Rev. B, 60, 22 (1999). Schiotz et al, Nature, 391, 561 (1998). Yamakov et al, Nature Materials, 3, 43 (2004). Experimental Evidence Shan et al, Science, 305, 654 (2004). Ke et al, Nanostructured Materials, 5, 689 (1995). Liao et al, Appl. Phys. Lett. 88, 021909 (2006 ), our own work Note: the critical grain may change from metal to metal.

Molecular Dynamics simulations At a few tens nanometers, partial dislocation emission and deformation twinning dominate Molecular Dynamics simulations Swygenhoven, Science, 296, 66 (2002). Schiotz et al, Nature, 391, 561 (2002) Yamakov et al, Nature Materials, 1, 43 (2002). Nano Al Partial dislocations emitted from grain boundaries 3 deformation twinning mechanisms in nano Al

Our experimental work provides first and direct experimental evidence to support the MD simulation results in nanomaterials Appl. Phys. Lett. 84, 592 (2004) Partials emission from grain boundaries and twinning Nano Cu Twinning mechanisms 1: homogeneous twinning inside a nano Al grain Appl. Phys. Lett. 83, 632 (2003)

Twinning mechanisms 2 in nanomateials: twinning by partials from grain boundaries Twin formed by successive partial dislocation emission from a grain boundary Chen et al, Science, 300, 1275 (2003). Nano-Al film/indented Appl. Phys. Lett. 83, 5062 (2003). Cryo-milled nano-Al powder

Twinning mechanisms 3 in nanomaterials: Grain boundary splitting/migration MD simulation Yamakov et al, Nature Materials, 1, 45 (2002). Appl. Phys. Lett. 83, 5062 (2003)

Fivefold Twins in nanomaterials Five-fold twins were usually grown in nano-particles obtained by vapor deposition, electrodeposition, etc HPTed Cu (RT) This five-fold twin was formed via a sequential twinning by partials emitted from grain/twin boundaries Appl. Phys. Lett. 84, 592 (2004).

Partials Multiply at Grain Boundary to Form a Twin (nanomaterials) Most twins are formed this way One partial is needed on each plane to form a twin It is statistically impossible to have a partial on each plane Zhu, Wu, Liao, et al, APL, . 95, 031909 (2009).

Partial dislocations on the grain boundary GPFE curves are not applicable to nanomaterials with non-equilibrium grain boundaries Non-equilibrium grain boundaries Wu, Zhu, APL, 89, 03122 (2006). Our work Huang, Zhu, Jiang, Lowe, Acta Mat, 49, 1497 (2001). Department of Materials Science and Engineering 12

Mechanism for Partial to Multiply on Grain Boundary b  b1 + b2 b2 glide out, b1 cross slip to the next plane b1  b + (-b2) -b2 stays on the grain boundary and b repeats the above process Appl. Phys. Lett. 95, 031909 (2009).

Mechanism for Partial to Multiply on Grain Boundary b  b1 + b2 b2 glide out, b1 cross slip to the next plane b1  + b’ + (-b3) -b3 stays on the grain boundary b’ dissociates b’  b1 + b3 b3 glide out, b1 cross slip to the next plane b1  + b + (-b2) b2 and b3 are emitted alternatively b2 + b3  b1

Homework (due in one week) Lecture 3: 5, 8 Department of Materials Science and Engineering 15