Slip and Twinning, Dislocations, Edge Dislocations and Screw Dislocations

SLIP:
A slip involves the sliding of blocks of crystal over one another along different crystallographic planes known as slip planes. 

TWINNING:
In twinning, the portion of crystals takes up an orientation related to the orientation of the rest of the untwined lattice in a symmetrical and definite way.

1. Slip and Twinning Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.) Assistant Professor, Department of Mechanical Engineering, Mookambigai College of Engineering, Pudukkottai-622502, Tamil Nadu, India. Email:[email protected] Dr. R.Narayanasamy , B.E., M.Tech ., M.Engg ., Ph.D., (D.Sc.) Retired Professor (HAG), Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu, India. Email: [email protected]

2. SLIP: A slip involves the sliding of blocks of crystal over one another along different crystallographic planes known as slip planes.  TWINNING: In twinning, the portion of crystals takes up an orientation related to the orientation of the rest of the untwined lattice in a symmetrical and definite way. Definition

3. Plastic deformation occurs when dislocations move through the crystal lattice, causing a change in the shape of the material. Unlike elastic deformation, where the material returns to its original shape after the applied stress is removed, plastic deformation results in a lasting change in shape.

4. Twinning, Shear Deformation and Martensite Formation

5. Slip takes place along the close packed planes and close packed directions. When slip is difficult to take place twinning occurs.

6. Slip Twinning Occurs in discrete multiples of atomic spacing The movement of atoms is lesser in atomic spacing The orientation of the crystal above and below the slip plane is the same after deformation as before. Orientation difference takes places across the twin plane Occurs over a wide plane Every atomic plane is involved Slip occurs when shearing stress on the slip plane in the slip direction strikes a threshold value known as the critical resolved shear stress. No critical resolved shear stress for twinning. Takes places in several milliseconds Takes place in few microseconds Slip lines are present in even or odd numbers Twin lines occur in pair Difference Between Slip and Twinning

7. Figures After Slip and Twinning

8. Shape changes arising from dislocation glide and deformation twinning (a shear transformation)

9. The figure schematically depicts the motion of atoms during (a) dislocation glide and (b) deformation twinning, stimulated in both cases by an applied tensile stress. Slip tends to take place by repeated passage of dislocations on specific slip planes. For a single crystal, in the early stages (as depicted here), the slip takes place on a single system (slip plane and slip direction), with little or no interference between individual dislocations. The schematic does not show individual dislocations, but illustrates the displacement of parts of the lattice, relative to other parts, created by their passage. For deformation twinning, on the other hand, the simultaneous, rapid displacement of a large number of neighbouring atoms, all in the same direction, also creates a (shear) displacement of the lattice, although in this case the sheared region has a different orientation from that of the parent crystal. In fact, the structure is a mirror image of the parent, reflected across the twin plane. Contd.

10. Plastic Deformation Stress-strain curve. Credit: Adapted from Figure 7.10(a),  Callister & Rethwisch 5e.

11. What is plastic deformation? Distinguish between slip and twin mechanism of plastic deformation.

12. Deformation Twinning in Single Crystals

13. An example of a conservative dislocation motion is motion of an edge dislocation (arrows show the applied shear stress) [47]

14. 1 Schematic diagrams of A) an edge and B) screw dislocation in an atomic lattice. 

15. FIG. 2. A step on a crystal face arising from a screw dislocation which gives a growth front for extension of the crystal as a Riemann surface generated by the spiral.

16. Influence of the lattice structure on ductility (BCC metals)

17. Influence of the lattice structure on ductility (FCC metals)

18. Crystal Plasticity with Multiple Slip

19. Crystal structures and slip systems of binary TiAl phases. (a) Hexagonal a 2 (Ti 3 Al) phase and (b) tetragonal c( TiAl ) phase.

20. Figure 3. Different slip systems of (a) hcp crystal structure and (b–d) bcc slip systems for Ti6Al4V alloy.

21. Schematic representation of different slip systems for  hcp  crystals:

22. References: Authors of Technical articles and Scopus Journals are Acknowledged. Thank You