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 1 Introduction, fcc, bcc and hcp crystal structures, partial and full dislocations in fcc metals Text book, Office hour, by appointment Department of Materials Science and Engineering 1

Policies and Procedures see 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). Department of Materials Science and Engineering 2

Grading Homework 9% Test 1 30% Test 2 30% Final 30% No grade markup Ask students to think about it, and ask questions No grade markup Department of Materials Science and Engineering 3

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. Department of Materials Science and Engineering 4

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

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?

fcc, bcc and hcp structures Department of Materials Science and Engineering 7

Dislocations Screw Dislocation http://www.youtube.com/watch?v=08a9hNFj22Y Cartepilar http://www.youtube.com/watch?v=iKKxTP6xp74 Dislocation cartoon Department of Materials Science and Engineering 8

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)) Quiz: What is the sense of the dislocation in (a) and (b)? (a) (b) Department of Materials Science and Engineering 9

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 Department of Materials Science and Engineering 10

Dislocations in fcc metals Define slip planes, full dislocations, and partial dislocations Definition of the Thompson tetrahedron Department of Materials Science and Engineering 11

Thompson Tetrahedron Determine the Burgers vector of AB, BC and CD and partial dislocations on ABC plane Define stair-rod dislocation Show some dislocation reactions using the Thompson tetrahedron. Department of Materials Science and Engineering 12

Homework (due in 1 week) Lecture 1: #10 & 13 Department of Materials Science and Engineering 13