Strengthening Mechanisms The ability of a metal to plastically - - PowerPoint PPT Presentation

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Strengthening Mechanisms The ability of a metal to plastically - - PowerPoint PPT Presentation

May 13, 2023 143 likes •262 views

Kasetsart University 213211: Strengthening Strengthening Mechanisms The ability of a metal to plastically deform depends on the ability of dislocations to move. Restricting or hindering dislocation motions renders a material harder and

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

135

Strengthening Mechanisms

  • 1. Grain Size Reduction
  • 2. Solid-Solution Strengthening
  • 3. Precipitation Hardening
  • 4. Strain Hardening

The ability of a metal to plastically deform depends on the ability of dislocations to move. Restricting or hindering dislocation motions renders a material harder and stronger.

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

136

Grain Size Reduction

  • Grain boundaries are

barriers to slip.

  • Barrier "strength"

increases with misorientation.

  • Smaller grain size:

more barriers to slip.

  • Hall-Petch Equation:

grain boundary slip plane grain A g r a i n B

σyield = σo + kyd−1/2

Note: not valid for both very large grain and extremely fine grain materials.

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

137

Example: Grain Size Strengthening

  • 70wt%Cu-30wt%Zn brass alloy

σyield = σo + k yd−1/2

  • Data:

[grain size (mm)]-0.5 σyield(MPa)

50 100 150 200 4 8 12 16 10-1 10-2 5x10-3

grain size, d (mm)

1 ky

0.75mm

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

138

Solid Solution

  • Impurity atoms distort the lattice & generate stress.
  • Stress can produce a barrier to dislocation motion.
  • Smaller substitutional

impurity

  • Larger substitutional

impurity

Impurity generates local shear at A and B that opposes disl motion to the right. Impurity generates local shear at C and D that opposes disl motion to the right.

C D A B

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

139

Example: Solid Solution Strengthening

Nickel in Copper

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SLIDE 6

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

140

Precipitation Strengthening

  • Hard precipitates are difficult to shear.

Ex: Ceramics in metals (SiC in Iron or Aluminum).

Large shear stress needed to move dislocation toward precipitate and shear it. Side View Top View Slipped part of slip plane Unslipped part of slip plane

S

Dislocation “advances” but precipitates act as “pinning” sites with spacing S.

precipitate

  • Result:

σy ~ 1 S

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

141

Strain Hardening (Cold Working)

  • Room temperature deformation.
  • Common forming operations change the cross sectional area:

%CW = Ao − Ad Ao x100

Ao Ad force die blank force

  • Forging
  • Drawing

tensile force Ao Ad die die

  • Extrusion

ram

billet container container

force die holder die Ao Ad

extrusion

  • Rolling

roll Ao Ad roll

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Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

142

Impact of Cold Work

Stress % cold work Strain

  • Yield strength (σy) increases
  • Tensile strength (TS) increases
  • Ductility (%EL;%RA) decreases
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SLIDE 9

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Strengthening

143

Effect of Heating after %CW