MSE 791 Mechanical Properties of Nanostructured Materials Module

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 Department of Materials Science and Engineering 1 NC State University

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: www. mse. ncsu. edu/research/zhu/papers/Meta l/PMS-TWIN%20 in%20 NC%20 Mater. pdf Department of Materials Science and Engineering 2 NC State University

Grading • • Homework Test 1 Test 2 Final 9% 30% 30% No grade markup Department of Materials Science and Engineering 3 NC State University

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 NC State University

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 Valiev, Alexander, Zhu & Lowe, J. Mater. Res. , 17 (2002) 5. Our work Youssef, Scattergood, Murty, Horton, Koch, APL, 87, 091904 (2005) NC State University

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? NC State University

fcc, bcc and hcp structures hcp http: //www. youtube. com/watch? v=49 qzox. ACHf. E fcc Department of Materials Science and Engineering 7 NC State University

Dislocations Screw Dislocation http: //www. youtube. com/watch? v=08 a 9 h. NFj 22 Y Cartepilar http: //www. youtube. com/watch? v=i. KKx. TP 6 xp 74 Dislocation cartoon Department of Materials Science and Engineering 8 NC State University

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) Department of Materials Science and Engineering (b) 9 NC State University

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 NC State University

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

Thompson Tetrahedron Determine the Burgers vector of AB, BC and CD and partial dislocations on ABC plane Define stair-rod dislocation Department of Materials Science and Engineering 12 NC State University

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