Dispersion strengthening of metals

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 : Al 2 O 3 in Al or Cu, ThO 2 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) MAR 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 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, T m = 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 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% Al 2 O 3 Alloy 66% cold reduction of 8% Al 2 O 3 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 preserved , as indicating dashed lines.

16. As shown in Fig. Flat curve by Cu-Al 2 O 3 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% Al 2 O 3 100-200Å particles, Delays it by 1050˚C

18. Microstructures Electron Micrograph of Al-8 volume% Al 2 O 3 Alloy at 20000X, unetched showing oxide dispersion,

19. Microstructures Cu-0.77% Al Alloy internally oxidized at 950˚C, yielding Cu-3.5% Al 2 O 3 Alloy 10000X

20. Microstructures Ni-10% Al 2 O 3 prepared by powders and extruded. 1000X

21. Microstructures Longitudinal view extruded Cu-3.5% Al 2 O 3 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