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 2 Twin morphologies in fcc metals: growth twins and deformation twins Text book, Office hour, by appointment Department of Materials Science and Engineering 1

Twin formation by stacking mistakes during growth Department of Materials Science and Engineering 2

Twin formation by the slip of partial dislocations (deformation twins) HW: How about –b1, -b2, - b3 One partial is needed on each slip plane to produce a twin Twin is generated by any or a mixture of b1, b2 and b3 Department of Materials Science and Engineering 3

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 HW: show that the angle between two close packed planes are 70.53° Department of Materials Science and Engineering 4

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

What does an fcc deformation twin look like at low magnification (examples)? Coherent twin boundaries may still look straight, not 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

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