It's the mechanism of strengthening in metals by adding dispersed second phase material.

1. PRESENTATION
ON
DISPERSION STRENGHTHENING OF
COMPOSITES
By: Sanjeev Kumar Verma
M. Tech. 1st year (12307EN045)
Department of Metallurgical
Engg. IIT (BHU),Varanasi
4/12/2013

2. CONTENT
• Introduction
•Classification of composites materials
• Definition of dispersion strengthening
• Mechanism
• Influence of the strengthening
• Precipitation Vs dispersion strengthening
• Summary
• References
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3. Introduction:
 The strength and hardness in some metal alloys may be
enhanced by the presence of extremely small and uniformly
dispersed particles within the original phase matrix.
 Whether introduced as insoluble particles in powder
compaction (dispersion strengthening).
 Second phase particles are generally the most potent
strengthening agent in practical high strength engineering
materials.
 The size, shape, and amount of second phase particles
controls the mechanical properties of the alloy.
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4. Classification of composites
Composites
Particle-reinforced Fiber-reinforced Structural
Large Dispersion laminates Sandwich
particle strengthened
Continuous Discontinuous
(aligned) (short)
Randomly
Aligned
oriented
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5. Dispersion Strengthening
The process which produces dislocation pining sites due to
the presence of second phase particles in the matrix of the
first phase.
Normally Hard particles size rang .1nm up to 1μm
Hard Composite
Ductile Powder
particles materials
Matrix metallurgy
Examples:
14%Al2O3 in Al- “SAP” Composite
1-2%ThO2 in Ni-20%Cr-TD nickel
WC in CO- Cemented Carbide cutting tools
fig: 1 Powder metallurgy processes
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6. Mechanism
Orownan Model
Dislocation Bypass by the Orowan Bowing
Mechanism:
 The shear stress required to bow a two
particles separated by 
 ~ Gb / 
fig: 2 Stages in passage of a dislocation
 Straight dislocation line approaching
two particles.
 Initially line to bend.
 Reached to critical curvature.
 Leaving a dislocation loop around each
particle.
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7. Factors influencing second-phase particle
strengthening
 Interparticle spacing  
 Average particle diameterD  Strength
 Volume fraction V f  Ductility
 Distribution
(interspacing distance)
If the contributions of each phase are independent, the properties
of the multiple phase alloy is the summation of a weighted average
of individual phases.
For example:  avgV 1 1V 2 2       V n n
Where V= volume fraction
 avgV 1 1V 2 2     V n n V=V1+V2+----Vn
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8. Dispersion Hardening big and Small particles
Strengthening
Strengthening effect can be estimated as:
r
  6G ( ) 0.5 f 0.5 1.5
b
G – Shear Modulus
r – Particle radius
Particle Radius
b – Burgers vector
Curve for small particles
f – volume fraction of particles
 - strain field factor
4(1  f )r

Strengthening
3f
f – volume fraction of particles
r – particle radius
Particle Radius
Curve for big particles
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9. Difference between precipitation and
Dispersion Strengthening
Coherency occur in PS but No
coherency presence in DS.
DS stable at all temperatures but PS is
not stables.
fig:3 Coherency PS fig:4 No coherency DS
Time factor not important for DS but
PS time factor important.
 Any alloy can be made in case DS.
 In DS chemical stability is increase
than PS.
 PS-isotropic and DS- Anisotropic
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10. Advantage And Disadvantages
Advantages:
 Higher creep resistance
 Higher temperature sustain
 Automotives industries
 Aerospace areas
 Sporting goods industries
Disadvantages:
 Not use for only higher strength purpose.
 Uniform distribution of the particles
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11. Summary of dispersion strengthening
 Very favorable for high-temperature strengthening since
dispersoids can not dissolve.
 Due to incoherency particle cutting can not occur.
 Main problem: distribute fine particles homogeneously and at
high particle number density.
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12. References
 George E.Dieter,Mechanical Metallurgy SI Metric Edition, McGraw-
Hill Science in Materials and Engineering,
 William D.Callister , Jr.Materials Science and Engineering-In
Introduction six ediation,Joh Wiley& Sons’2007
 Luker Fischer, “Nano-Dispersion Particulate Strengthened Metal
Matrix Composites”,2004,P4-10
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