Dispersion Hardening: Hard particles: Mixed with matrix powder Consolidated Processed by powder metallurgy techniques Second phase – Very little solubility (Even at elevated temp.) No coherency So thermally Stable at very high temp. Resists : Grain growth Over aging Recrystallization Mobility of dislocation Different from particle Metallic Composites (Volume Fraction is 3 to 4% max.) (Does not affect stiffness) Examples : Al2O3 in Al or Cu, ThO2 in Ni

1. DISPERSION STRENGTHENING OF METALS
PREPARED BY
JAY NITESHBHAI PATEL AND DARSHAN SHAH,
FIRST YEAR M.E.-(MET. & MATS. ENG..)-WELDING TECHNOLOGY
GUIDED BY
MR. HEMANT N. PANCHAL

2. CONTENTS
 Introduction
 Difference between precipitation and dispersion strengthening
 History
 Method
 Advantage And limitation
 Strengthening mechanism
 Cremens’ approach
 References

3. INTRODUCTION
Dispersion Hardening:
 Hard particles:
 Mixed with matrix powder
 Consolidated
 Processed by powder metallurgy techniques
 Second phase – Very little solubility (Even at elevated temp.)
 No coherency
 So thermally Stable at very high temp.
 Resists :
 Grain growth
 Over aging
 Recrystallization
 Mobility of dislocation
 Different from particle Metallic Composites (Volume Fraction is 3 to 4% max.) (Does not affect stiffness)
 Examples : Al2O3 in Al or Cu, ThO2 in Ni

4. WHY DISPERSION STRENGTHENING?
 DESIGNERS of nuclear power plants, hypersonic aircraft, and space vehicles are seeking materials high
strength at elevated temperatures.
 The precipitation-strengthened “super alloys” suitable for applications around 1800°F.
 The refractory metals, tungsten, molybdenum, columbium, and tantalum, used when service temperature
exceeds the useful temperature of the super alloys. These are expensive, difficult to fabricate, and have
poor resistance to oxidation.
 Service Temperatures :
 Dispersion-strengthened alloys: up to 80 to 90% of the melting point of the base alloy
 Precipitation-strengthened alloys: 65 to 70%
 This boost means extending the use of
 nickel from about 1800° to about 2400°F.,
 aluminium from 500 ° to 900°F.

5. DIFFERENCE BETWEEN PRECIPITATION AND DISPERSION
STRENGTHENING
Dispersion Strengthening Precipitation Strengthening
No Coherency Coherency occurs
Stable at all Temp. Not stable
Time factor not important Time factor important
Any alloy can be made Specific Alloy can be made
Chemical Stability More Chemical Stability Less
Anisotropic Isotropic
No coherency Coherency

6. COMPARISON
Fig
Comparison of yield strength of
dispersion-hardened thoria-dispersed
(TD) nickel with two nickel-based super
alloys strengthened by precipitates (IN-
792) and directionally solidified (DS)
M 200.

7. HISTORY
 1922 - Franz Sauerwald - oxide film that forms on aluminum surfaces interfered with pressing and
sintering of the powders to such an extent that a coherent body could not be obtained. – Powder
metallurgy not possible for Al
 1940 - Max Stern - particulate aluminum and magnesium scrap—turnings, filings, grinding dust, etc. -
used hot pressing, hot forging, and hot extrusion at temperatures up to 900°F. to rupture the oxide film
on the particles and to obtain metal-to-metal contact. – PM possible for Al
 1949 - Alfred von Zeerleder & Roland Irmann – first time observed dispersion-strengthening
phenomenon in products made from sintered aluminum powder - realized that the high strength of these
specimens was due to the particles of oxide from the surface of the powder that became distributed
throughout the body by the compacting and extrusion processes
 Extensive research followed in the laboratories of the AIAG in Switzerland and the Aluminum Company
of America

8. METHODOLOGY
Ductile
Matrix
Hard
Particles
Composite
Material
• The process produces dislocation pining sites due to
the presence of hard particles in the matrix of the
ductile material.
• Normally Hard particles size range from 0.1nm up to
1μm

9. ADVANTAGE AND LIMITATION
 Advantages:
 Very favorable for high-temperature strengthening since dispersoids can not dissolve.
 Due to incoherency particle cutting can not occur.
 Allows the design of thin-walled structures for high-temperature application.
 Higher creep resistance
 limitations :
 distribute fine particles homogeneously and at high particle number density.
 Parts have lower ductility
 High cost of metal powders compared to the cost of raw material
 Not use for only higher strength purpose at room temp.

10. STRENGTHENING
MECHANISM
 Deformation- Due to movement of Dislocation
 Increased strength is a result of interference of the dispersed particles with the
movement of dislocations through the crystal lattice.
 Many theories – to explain strengthening mechanism
 Theory by Lenel and Ansell is generally accepted
 first dislocation passes between the particles, leaving a dislocation loop around each.
 Successive dislocations pile up around the particles until the accumulated stress causes them
to yield or fracture.

11. IN ORDER TO GET HIGH STRENGTH AT HIGH
TEMPERATURE :
 Matrix metal Should have
 High Melting point
 High Strength
 Dispersed phase should have
 high thermal stability in contact with the matrix
 a low diffusivity in the matrix
 low solubility in the matrix
 high strength
 uniform distribution of particles less than one micron in size
 There must be wetting or bonding between the matrix and the dispersoid.

12. 0.25% OFFSET YIELD STRENGTH—HOMOLOGOUS
TEMPERATURE
T = TEST TEMPERATURE °K, TM = MELTING POINT °K;

13.  From ∞ to 2 microns stress for
 10 hour life improves by factor 4.5
 100 hour life improves by factor 5.5
 1000 hour life improves by factor 17
 Here it is evident that
 Strength α
1
𝐢𝐧𝐭𝐞𝐫 𝐩𝐚𝐫𝐭𝐢𝐜𝐥𝐞 𝐬𝐩𝐚𝐜𝐢𝐧𝐠
 Strength increase α Rupture Time
Ni-Al2O3 alloy at 815˚ C

14. CREMENS’ APPROACH
 To study nature and stability of the structure.
 They noted that
 30% cold reduction of SAP 14% Al2O3 Alloy
 66% cold reduction of 8% Al2O3 Alloy
 Few Rockwell F Hardness points increases
 Few thousand psi tensile & yield stress Increases
 Small reduction in Ductility
 150 hours at 1180˚ F gives the as extruded property back.
 Shows high level of stored energy of cold work in as extruded alloys
 But it is not sufficient to explain time-temp-stress stability of Alloys.

15.  By 30% CW Short time
rupture stress increases by 2
times
 Fails in ductile – Trans
granular manner
 Above 1300˚F grain
boundary sliding and
recrystallization occurs in
longer time tests and
weakening of alloy results
 Increasing cold work speeds
up weakening process
 If the flat slope obtained in
short time test could be

16. As shown in Fig. Flat curve by Cu-
Al2O3 Alloys is due to
combination of
Cold work
Pinning of Dislocation & Grain
Boundary
Pinning prevents intercrystalline
cracking.
Some dislocation still climb at high
temp.
Resulting Some Ductility

17. Figure shows remarkable
restriction of recrystallization
Pure Cu recrystallizes at 250 to
300˚ C
As little as 0.4% Al2O3 100-
200Å particles,
Delays it by 1050˚C

18. MICROSTRUCTURES
Electron Micrograph of Al-8 volume% Al 2O3 Alloy at 20000X, unetched showing oxide
dispersion,

19. MICROSTRUCTURES
Cu-0.77% Al Alloy internally oxidized at 950˚C, yielding Cu-3.5% Al2O3 Alloy 10000X

20. MICROSTRUCTURES
Ni-10% Al2O3 prepared by powders and extruded. 1000X

21. MICROSTRUCTURES
Longitudinal view extruded Cu-3.5% Al2O3 Alloy prepared by internal oxidation 500X

22. REFERENCES
 George E. Dieter, Mechanical Metallurgy
 Marc Meyers & Krishan Kumar Chawla, Mechanical Behavior of Materials
 RICHARD B. ELLIS , Dispersion Strengthening of Metals
 Donald Peckner , The Strengthening of Metals

23. Thank You