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Materials Moments:

Abdelrhman A. – Paper

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Principles of Materials

Are everywhere

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Section 5.1-5.3, 5.5-5.6
Diffusion

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YouTube: Diffusion

posted by smcblackburn

0:16-1:10

View the rest of this video if you have trouble understanding the concept of diffusion.

Diffusion

  • Gases
  • Liquids
  • Solids

Diffusion

Connect 2 gas tanks: Ar & He

  • Assume equal T & P
  • Gasses diffusereach uniform comp.
  • 2 gases: Ar & He
  • 2 liquids: water & alcohol
  • Diffuse until uniform composition

Diffusion

  • 2 gases: Ar & He
  • 2 liquids: water & alcohol
  • 2 solids: Blocks of Cu & Ni
  • Heat at elevated T (below Tm)
  • Leave for days
  • Eventually reaches uniform composition

Diffusion

Rate

  • 2 gases: Ar & He Fastest

  • 2 liquids: water & alcohol

  • 2 solids: Blocks of Cu & Ni Slowest

Diffusion

In all cases, diffusion rate increases as T increases.

Fig. 5.3

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Vacancy

Diffusion

Interstitial

Diffusion

Diffusion Mechanisms

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Fig. 5.3

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Vacancy

Diffusion

Interstitial

Diffusion

Diffusion Mechanisms

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Interdiffusion

Example: Cu, Ni

Vacancy diffusion or interstitial diffusion?

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Metallic xl Structures

Face-Centered Cubic (FCC)

Cu, Al, Ag, Au, Pb, Ni, Pt

Body-Centered Cubic (BCC)

Na, Fe, Cr, Mo, W

Hexagonal Close-Packed (HCP)

Ti, Zn, Cd, Co, Mg

Example: Cu, Ni

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Thermocouples and resistors whose resistance is stable across changes in temperature contain the 55% copper-45% nickel alloy (constantan).

Monel metal is a nickel-copper alloy, containing a minimum of 63% nickel.

Cupronickel or copper-nickel or "cupernickel" is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese.

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Interdiffusion

Vacancy diffusion

Example: Cu, Ni

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Initially

See Figs. 5.1 and 5.2

Interdiffusion

After some time

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http://www.yale.edu/yaleche/chemeng/eia/eia.htm
Eric I. Altman

“Vacancy diffusion in a layer of adsorbed Br atoms on Cu(100)”

What’s Cu(100)?

f03_05_pg112

Interdiffusion

  • Vacancy diffusion
  • Interstitial diffusion
  • Other paths
  • Dislocations
  • Grain boundaries
  • Free surfaces

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f03_05_pg112.jpg

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Two conditions for diffusion

  • Empty site near the diffusing atom

  • Diffusing atom needs enough energy to break existing bonds.

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Fick’s First Law:

Steady-State Diffusion

Fick’s First Law—Steady-state diffusion

Factors affecting diffusion rates

  • Diffusing species & host material
  • Temperature
  • Concentration Gradient

Table 5.2

t02_05_pg119

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t02_05_pg119.jpg

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Diffusing Host Metal T(oC) D (m2/s)

species

Fe a-Fe (BCC) 500 3.0 x 10-21

Fe a-Fe (BCC) 900 1.8 x 10-15

Fe g-Fe (FCC) 500 1.1 x 10-17

Fe g-Fe (FCC) 900 7.8 x 10-16

Which diffuses faster?

  • Fe through a-Fe at 500oC
  • Fe through a-Fe at 900oC
  • Fe through g-Fe at 500oC
  • Fe through g-Fe at 900oC

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t02_05_pg119.jpg

The question that’s kept you up

late at night:

How does
diffusion
affect material properties?

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It strengthens

City Steel Heat Treating Co.

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Case Hardening – Hard Case w/ tough core

Low-C Steels

(> 0.30% C):

Carburizing,

Nitriding,

Carbonitriding

Carburized depth of 0.030” to 0.050”

in 4 hours @ 1700°F

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f16_07_pg190.jpg

Steel = C + Fe

  • Interstitial C imposes lattice strains in Fe
  • Strains near dislocations can bind atoms and reduce deformation

Carburizing—Solid Diffusion process

Begin at 1:35–2:40

YouTube: Vacuum Carburizing and Heat treating

Composition & processing affect material properties

Section 6.1-6.3
Mechanical Properties of Metals:

Elastic Deformation:
Stress-Strain behavior

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Material Properties:

Mechanical Properties

  • Stiffness
  • Ductility
  • Strength
  • Toughness
  • Hardness

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Tensile Test

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Tensile Test

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Tensile Test

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Strain gages

  • Metal-foil gage

  • Bond to testing material using epoxy

  • Variations in electrical resistance converted to strain measurements.

  • Grid shape maximizes resistance while keeping gage small

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t02_05_pg119.jpg

fig_06_11

Brittle Failure: Tensile test of Nodular Graphite Cast Iron

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Tensile Test

AlMgSi alloy ductile fracture

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Principles of
Stress and Strain

€

J = −D dC dx

 

J=-D

dC

dx

€

D = D0 exp −Qd RT

⎛

⎝ ⎜

⎞

⎠ ⎟

 

D=D

0

exp

-Q

d

RT

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