Lecture - Chapter 7

Lecture - Chapter 7 - Chapter 7 Dislocations and...

Info iconThis preview shows pages 1–17. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 7 Dislocations and Strengthening Mechanisms
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Why study deformation and strengthening mechanisms? Metal alloys can be made harder, and less ductile by plastic or permanent deformation, via dislocation strengthening mechanisms. A given design application could benefit or suffer from such changes in a given alloys properties, and therefore must be understood and taken into account.
Background image of page 2
Learning objectives Describe how the edge and screw dislocations move under shear deformation, and how plastic deformation occurs. Explain why the strength of a material increases with the decrease in grain size. Explain solid solution strengthening. Explain strain hardening, work hardening or cold work in metals. Explain recrystallization and grain growth in metals.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Basic concepts of dislocations Dislocations are linear defects in metals. There are two types dislocations: edge and screw dislocations. A screw dislocation line is formed when one part of the atomic structure is sheared with respect to the other part. The edge of the distortion is the screw dislocation line (Fig. 4.4a). Edge dislocation line is the end of an extra plane of atoms inserted in the lattice (Fig. 7.1).
Background image of page 4
Fig 4.4(a) A screw dislocation within a crystal
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Fig 7.1 Atomic arrangements that accompany the motion of an edge dislocation as it moves in response to an applied stress. a) The extra half plane of atoms is labeled in a.
Background image of page 6
Fig 7.1 b) The dislocation moves one atomic distance to the right as A links up to the lower portion of plane B, the upper portion of B becomes extra half-plane.
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Fig 7.1 c) A step forms on the surface of the crystal as the extra half-plane exists.
Background image of page 8
Basic concepts of dislocations During plastic or permanent deformation a large number of dislocations move. If the plastic deformation continues then the dislocations move to the surface and a step is formed (Fig. 7.2). If you put a piece of metal in a transmission electron microscope you can see these dislocations lines (Fig. 4.10). Movement of dislocations under shear stress is called slip.
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Basic concepts of dislocations The plane in which dislocations move is called slip plane. The edge dislocation move in the direction of the shear stress or parallel to the shear stress (Fig 7.2 (a)). The screw dislocations move in the direction perpendicular to the shear stress (Fig 7.2(b)). The direction along which the dislocations move is called slip direction. The combination of slip plane and slip direction is called slip system.
Background image of page 10
Fig 7.2 The formation of a step on the surface of a crystal be the motion of (a) an edge dislocation. Slip Plane Slip direction
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Fig 7.2 (b) A screw dislocation Slip direction Slip Plane
Background image of page 12
Fig 4.6 A transmission electron micrograph of a titanium alloy in which dark lines are dislocations.
Background image of page 13

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Fig 7.3 Representation of the analogy between a Caterpillar and dislocation (cont’d).
Background image of page 14
Fig 7.3 (Cont’d)
Background image of page 15

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Characteristics of dislocations Metal pieces are work hardened (plastically
Background image of page 16
Image of page 17
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/03/2010 for the course ME 250-750 taught by Professor Signer during the Summer '10 term at Wichita State.

Page1 / 80

Lecture - Chapter 7 - Chapter 7 Dislocations and...

This preview shows document pages 1 - 17. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online