NC State University Department of Materials Science and Engineering 1

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

NC State University Department of Materials Science and Engineering 2

Policies and Procedures see the Syllabus for more details

•  Attendance –  Attendance expected (0.2% penalty for missing a quiz)

Reading (textbook) Material: “Deformation Twinning in Nanocrystalline Materials,”Prog. Mater. Sci . 57, 1-62 (2012).

•  All HW are due in 1 week

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Grading

•  Homework 9% •  Test 1 30% •  Test 2 30% •  Final 30%

No grade markup

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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.

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Quiz Coordinate

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Cai, Shaobo, Fang, Xiaomeng, Ledfor, John Chris Lim, Mina Mahbooba, Zaynab M Rohrbaugh, Nathaniel W 1 2 3 4 5 6

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• A material can be strong or ductile, but rarely both at the same time

Nano-Ti

Nano-Cu

Valiev, Alexander, Zhu & Lowe, J. Mater. Res., 17 (2002) 5. Our work

• Good ductility is desired to prevent catastrophic failure

Youssef, Scattergood, Murty, Horton, Koch, APL, 87, 091904 (2005)

Strength and ductility of materials

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Only a few nanostructured materials show good ductility

The yielding strength is normalized by the yield strength of a material’s coarse-grained counterpart !

Issue: How do we obtain high ductility in nanostructured materials? !

Nanostructured materials have much higher strength than their coarse-grained counterparts!

Koch, Scripta Mater. 49 (2003) 657 !Zhu & Liao, Nature Mat., 3 (2004) 351.!

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fcc, bcc and hcp structures

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hcp

http://www.youtube.com/watch?v=49qzoxACHfE fcc

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Dislocations

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

Screw Dislocation

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Burgers Vector

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•  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)

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The Burgers Vectors are conserved

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•  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

•  A dislocation line cannot end inside a perfect crystal

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Dislocations in fcc metals

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Define slip planes, full dislocations, and partial dislocations Definition of the Thompson tetrahedron

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Thompson Tetrahedron

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

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Homework (due in 1 week)

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Lecture 1: #10 & 13