Instructor: Yuntian Zhu. Lecture 3

Transcription

1 MSE 791: Mechanical Properties of Nanostructured Materials Module 3: Fundamental Physics and Materials Design Instructor: Yuntian Zhu Office: 308 RBII Ph: Lecture 3 Difference in the deformation of coarse-grained and nano grains Department of Materials Science and Engineering 1

2 Dislocation source in coarse-grained metals: Frank-Read source Frank-Read source F = τ!b Department of Materials Science and Engineering 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 360 o 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

4 Nanocrystalline grains often are dislocation free Dislocation and microstructure variation with grain size in Ti Department of Materials Science and Engineering 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, (2006 ), our own work

6 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

7 Our experimental work provides first and direct experimental evidence to support the MD simulation results in nanomaterials 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)

8 Twinning mechanisms 2 in nanomateials: twinning by partials from grain boundaries 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).

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 HPTed Cu (RT) Five-fold twins were usually grown in nano-particles obtained by vapor deposition, electrodeposition, etc This five-fold twin was formed via a sequential twinning by 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, (2009).

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

13 Mechanism for Partial to Multiply on Grain Boundary b è b 1 + b 2 b 2 glide out, b 1 cross slip to the next plane b 1 è b + (-b 2 ) -b 2 stays on the grain boundary and b repeats the above process Appl. Phys. Lett. 95, (2009).

14 Mechanism for Partial to Multiply on Grain Boundary b è b 1 + b 2 b 2 glide out, b 1 cross slip to the next plane b 1 è + b + (-b 3 ) -b 3 stays on the grain boundary b dissociates b è b 1 + b 3 b 3 glide out, b 1 cross slip to the next plane b 2 and b 3 are emitted alternatively b 1 è + b + (-b 2 ) b 2 + b 3 " b 1

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