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 2 Twin morphologies in fcc metals: growth twins and deformation twins Department of Materials Science and Engineering 1 NC State University

Twin formation by stacking mistakes during growth A B C B A Department of Materials Science and Engineering 2 NC State University

Twin formation by the slip of partial dislocations (deformation twins) One partial is needed on each slip plane to produce a twin Twin is generated by any or a mixture of b 1, b 2 and b 3 Department of Materials Science and Engineering 3 NC State University

What does an fcc twin look like at atomic scale? A twin can be observed along 3 <110> directions on the coherent twin boundary. They should look crystal graphically identical Department of Materials Science and Engineering 4 NC State University

What does an fcc growth twin NORMALLY look like at low magnification? Department of Materials Science and Engineering • Straight, clean twin coherent boundaries • Straight incoherent boundaries • No macoscopic strain in the grains 5 NC State University

What does an fcc deformation twin look like at low magnification (examples)? • Coherent twin boundaries may still look straight, not always • Image looks less clean (often messy) • Macroscopic strain is usually produced, but may not be easily seen at low magnification Newczas Department of Materials Science and Engineering 6 NC State University

Homework (due in 1 week) Lecture 2: 1, Department of Materials Science and Engineering 7 NC State University