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Chapter 8: . Deformation and Strengthening Mechanisms of Materials. Team 1 CHEN 313. Add Figure(s). Summary. Summary 8.2.

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Presentation Transcript
summary 8 2
Summary 8.2
  • In the 1930s It was theorized that the that the discrepancy in in mechanical strengths was due to the different types of linear crystalline defect which is known as “dislocation.”
  • Only after the 1950 were the ability to see with the electron microscopes possible, making it possible to visualize dislocations.
summary 8 3
Summary 8.3

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  • The motion of dislocations due to an applied shear stress causes plastic deformation; this process is termed “slip”. There are three types of dislocations: edge, screw, and mixed.
summary 8 4

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Summary 8.4
  • Lattice Strain-Compressive, shear, and tensile stresses in the lattice are formed as a dislocation travels through the lattice.
  • Stress fields around dislocations include compressive and tension.
  • When dislocations are in the same plane, they can either repel each other if they have the same sign, or destroy each other if they have opposite signs.
  • Deformations cause the number of dislocations to increase.
summary 8 5
Summary 8.5
  • The combination of both the preferred slip plane and directions forms the slip system.
  • Slip plane is generally taken as the closest packed plane in the system.
  • Slip direction is the direction on the slip plane with the highest linear density.
summary 8 6
Summary 8.6
  • Slip in single crystals:
  • When a shape of a single crystal changes under a tensile stress, plastic deformation will occur by slip on parallel crystal planes.
  • During the deformation sections of the crystal slide to one another, causing the geometry of the single crystal to change as shown in the following diagram.

Summary 8.7:

Because the grain boundaries are arranged randomly the direction of the slips would be in different directions, and the dislocation would occur towards the highest shear stress.

When the material is deformed by plastic deformation the grain boundaries is maintained and the grains itself are constrained in a shape depending on the grains that are next to it.

summary 8 8
Summary 8.8
  • Under particular conditions, two or more intergrown crystals are formed in a symmetrical fashion. Twinning is important because the crystallographic reorientation makes the slip process more favorable.

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  • What is strengthening of a metal? Strengthening of a metal is the ability of a metal to deform plastically due to the motion of a large number of dislocations.
  • Three types of mechanisms to strengthen a metal:
    • Strengthening by grain size reduction
    • Solid-solution strengthening
    • Strain hardening
summary 8 9

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Summary 8.9
  • Dislocation motion is restricted by a larger area of grain boundaries (finer grains)
    • Slower cooling results in courser grains
    • Grain size may be regulated by the rate of solidification from the liquid phase and by plastic deformation
    • Hall-Petch equation: sy = sy + ky/
    • Yield strength increases with decreasing grain size

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summary 8 10

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Summary 8.10

Interstitial Atoms

Substitutional Atoms

  • Impurity atoms may be substitutional or interstitial. Substitutional atoms that are larger than the host atoms will impose a compressive stress on the surrounding lattice. Conversely interstitial atoms that are smaller than the host atoms will cause tensile stresses in the lattice.
  • The stress fields associated with impurity atoms interact with the stress fields associated with dislocations and restrict their movement.
summary 8 11
Summary 8.11

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  • It is the increase in the strength and hardness of a metal due to a mechanical deformation in the metal's microstructure.
  • Any material with a reasonably high melting point such as metals and alloys can be strengthened in this fashion



Earlier theoretical strengths of perfect crystals were computed from early materials that were studied


  • It was theorized that the that the discrepancy in in mechanical strengths was due to the different types of linear crystalline defect which is known as “dislocation”

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Electron microscope was used to visualize the dislocation which then proved it’s existence.


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