Instructor: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu Lecture 3

1. MSE 791: Mechanical Properties of Nanostructured MaterialsModule 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 1

2. Dislocation source in coarse-grained metals:Frank-Read source Frank-Read source F = tb https://www.youtube.com/watch?v=5yID78ovcX8 2

3. 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). 3

4. Nanocrystalline grains often are dislocation free Dislocation and microstructure variation with grain size in Ti 4

5. 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

6. Nano Al 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). • Partial dislocations emitted from grain boundaries • 3 deformation twinning mechanisms in nano Al

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

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

9. 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)

10. 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 twinningby partials emitted from grain/twin boundaries Appl. Phys. Lett. 84, 592 (2004).

11. Partials Multiply at Grain Boundary to Form a Twin (nanomaterials) • 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).

12. 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). 12

13. Mechanism for Partial to Multiply on Grain Boundary b  b1 + b2 b2glide out, b1cross slip to the next plane b1b + (-b2) -b2stays on the grain boundary and b repeats the above process Appl. Phys. Lett. 95, 031909 (2009).

14. Mechanism for Partial to Multiply on Grain Boundary b  b1 + b2 b2glide out, b1cross slip to the next plane b1+ b’ + (-b3) -b3stays on the grain boundary b’dissociates b’ b1 + b3 b3glide out, b1cross slip to the next plane b1+ b + (-b2) b2 and b3 are emitted alternatively b2 + b3  b1

15. Homework (due in one week) • Lecture 3: 5, 8 15