Enhancing Ductility in Nanostructured Materials: Challenges and Solutions

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 1 Introduction, fcc, bcc and hcp crystal structures, partial and full dislocations in fcc metals 1

2. Policies and Proceduressee the Syllabus for more details • Attendance • Attendance expected (0.2% penalty for missing a quiz) • All HW are due in 1 week. 75% of the credit for late HW Reading (textbook) Material: “Deformation Twinning in Nanocrystalline Materials,”Prog. Mater. Sci . 57, 1-62 (2012). 2

3. Grading • Homework 9% • Test 1 30% • Test 2 30% • Final 30% No grade markup 3

4. Teaching style • Active student participation in class • A dice will be rolled to determine who participates • 0.2% will be deducted if you are not present in the class to answer a quiz; • 0.2% will be given if you give the right answer • 0.2% will be given to a volunteer who gives the right answer • No credit will be given or deducted if you give a wrong answer. 4

5. Strength and ductility of materials • A material can be strong or ductile, but rarely both at the same time • Good ductility is desired to prevent catastrophic failure Nano-Ti Nano-Cu Youssef, Scattergood, Murty, Horton, Koch, APL, 87, 091904 (2005) Valiev, Alexander, Zhu & Lowe, J. Mater. Res., 17 (2002) 5. Our work

6. Only a few nanostructured materials show good ductility The yielding strength is normalized by the yield strength of a material’s coarse-grained counterpart Nanostructured materials have much higher strength than their coarse-grained counterparts Koch, Scripta Mater. 49 (2003) 657 Zhu & Liao, Nature Mat., 3 (2004) 351. Issue: How do we obtain high ductility in nanostructured materials?

7. fcc, bcc and hcp structures hcp http://www.youtube.com/watch?v=49qzoxACHfE fcc 7

8. Dislocations Screw Dislocation http://www.youtube.com/watch?v=08a9hNFj22Y Cartepilar http://www.youtube.com/watch?v=iKKxTP6xp74 Dislocation cartoon 8

9. Burgers Vector • Decide on the sense (vector) of dislocation line (arbitrary) • Use “right-hand screw” to draw the Burgers circuit around the dislocation line • Draw a Burgers circuit with equal length on each side; b = vector from the starting point to ending point (see (a)) • Or: Draw a closed Burgers circuit and then the circuit in a perfect crystal, b = vector from the ending point to starting point in the perfect crystal circuit (see (b)) (a) (b) 9

10. The Burgers Vectors are conserved • Each dislocation line can only have 1 Burger vector • At a dislocation node where several dislocations meet, the sum of Burgers vectors of dislocations going to the node equals the sum of Burgers vectors of dislocations going out from the node • A dislocation line cannot end inside a perfect crystal 10

11. Dislocations in fcc metals Define slip planes, full dislocations, and partial dislocations Definition of the Thompson tetrahedron 11

12. Thompson Tetrahedron Determine the Burgers vector of AB, BC and CD and partial dislocations on ABC plane Define stair-rod dislocation 12

13. Homework (due in 1 week) Lecture 1: #10 & 13 13