1. MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu
Office: 308 RBII
Ph:
Lecture 17: Advanced Processing of Metastable Materials
Text book,
Office hour, by appointment
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2. Non-Equilibrium Processing
A metastable structure is considered to have local minima in free energy, and not the lowest free energy of the system
Metastable state
Transitional “stable” state
Stable state
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3. Non-Equilibrium Processing
We will be concerned with metastable structures, or more accurately “configurationally frozen” metastable structures.
Diamond is metastable at room temperature and atmospheric pressure
Microstructures that harden steel are metastable
Snow about to avalanche
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4. Non-Equilibrium Processing
Generalized procedure to obtain metastable materials:
“Energize” then “Quench” a material
The Energization consists of raising the energy of a phase at ambient temperature and pressure in various ways:
Raise the temperature or pressure to transform the material to a phase stable at higher temperatures (or pressure), e.g. a liquid or high temp allotrope
Evaporation
Dissolution
Irradiation
Severe Plastic Deformation
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5. Non-Equilibrium Processing
The Quenching back to ambient conditions can be characterized by cooling rate
In the Energize and Quench approach, the phase of the Energized matter can be gas, liquid, or solid; deposition from gas or liquid phase is most common
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6. Gas Phase Deposition Thermal Evaporation
Atoms are evaporated from one material, and deposited on a substrate
The cooling occurs on pico-second time scales, with cooling rates on the order of K/s
The atoms are frozen on the substrate/film; the low temperature prevents diffusion
Most times this is used to make amorphous metastable microstructures
- Problem: Differential evaporation – different components have different vapor pressures, so the composition of the film cannot be easily controlled
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7. Gas Phase Deposition Thermal Evaporation
Problem: Differential evaporation – different components have different vapor pressures, so the composition of the film is difficult to control
Ways to avoid Differential Evaporation
Flash evaporation – powdered material is dropped steadily onto a heated ribbon, thereby almost instantaneously vaporizing it
Use of separate power sources for each element and balance the evaporation rate to obtain desired composition
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8. Gas Phase Deposition Sputtering
Advantage over thermal deposition in that it is easier to control the composition since an average composition of the sputtering target is deposited on the substrate
Like thermal evaporation, this is also a thin film technique, with maximum film size ~10s of micron
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9. Liquid Phase Deposition
Rapid Solidification Processing (RSP)
Became popular in the 1960s and 1970s to create new materials with superior properties
The definition for “rapid solidification” is the cooling from the melt from its melting temperature to a low value (~room temperature) very rapidly – usually milliseconds or less
The range of quench rates can vary from 102 to 1010 C/s, but most techniques are in the 104 to 106 C/s range
Effects include
Decreased grain size
Increased chemical homogeneity
Extension of solid solubilities
Creation of metastable crystal structures
Creation of bulk metallic glasses
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10. Liquid Phase Deposition
Melt Spinning
Developed by Pond and Madden in 1969, but used more frequently in the 1970s and 80s
Free Flight Melt Spinning
This method consists of creating and subsequently solidifying a stable liquid jet on passage through a gaseous or liquid quenching medium
Problem: solidifying the metal into a wire prevents droplets from being formed
Advantage: Allows production of continuous filaments of circular cross section
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11. Liquid Phase Deposition
Chill Block Melt Spinning
Like free flight melt spinning, chill block melt spinning employs a jet of liquid metal extruded through an orifice.
-In chill block melt spinning, however, solidification is achieved when the molten jet impinges on the surface of a rotating solid substrate (rotating water cooled metal disks
- Filaments several mm wide and 25 micrometers thick are produced
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12. Spray and Droplet (Atomization) Methods
In these methods a continuous stream of liquid metal is atomized, that is, broken down into fine droplets by means of a gas or liquid
The resultant product after solidification is powder, which is desirable for large scale applications (with regard to consolidation)
Gas Atomization
involves breaking down of a continuous stream of liquid metal by one or more high velocity jets of gas; small particles solidify in flight by convection or radiation cooling. The solidification rate depends on needed particle size
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13. Spray and Droplet (Atomization) Methods
Gas Atomization
The smaller the particle, the higher the solidification rate
Higher solidification rates are achieved with smaller particle sizes and lighter gases (e.g. He is better than Ar
Water Atomization
Water is used instead of a gas
Used to make powders of tool and low alloy steels
Cooling rate of 102 to 104 K/s
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14. Laser or Electron Beam Methods (3D Printing)
Laser or Electron Beam Surface Melting
Involve local melting of the alloy; also called “self quenching”, “laser annealing:, “laser glazing”.
High power densities are concentrated on a small spot (0.1 to 1.0 mm) for short times (~10-5 s)
Cooling rate of 106 – 108 K/s are reported, but for very thin layers (0.01 to 0.1 micrometers), can be K/s (highest cooling rates for RSP)
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15. Solid State Methods Severe Plastic Deformation
Mechanical Alloying or Mechanical Milling
Ball milling of either dissimilar powders (MA) or single composition powders (elements or compounds (MM) ) has been found to induce metastable structures in many materials. These metastable phases include:
Amorphous
Metastable crystalline compounds
Supersaturated solid solutions
Quasicrystalline phases (2011 Nobel Prize in Chemistry – Schectman)
Nanocrystalline microstructures
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16. Solid State Methods Severe Plastic Deformation
Mechanical Alloying or Mechanical Milling
MM/MA has been carried out in a variety of high energy shaker, vibratory, or planetary mills, as well as larger attritor and ball mills.
Conventional low energy mills are used, but long times (weeks to months) are required to obtain the same microstructures a high energy mill can get in a day or less
Usually carried out in inert atmosphere to prevent oxidation
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17. Solid State Methods Severe Plastic Deformation
Mechanical Alloying or Mechanical Milling
The central event in mechanical milling or alloying is the ball-powder-ball collision, where powder particles are trapped between the colliding balls during milling and undergo deformation and/or fracture processes which define the ultimate structure of the powder
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18. Solid State Methods Severe Plastic Deformation of Bulk Samples
Plastic deformation of bulk samples gets around the problem of consolidation of powder
In recent years, there has been high interest in methods which give submicron and nanocrystalline materials
The two most common methods are:
High Pressure Torsion (HPT)
Equal Channel Angular Extrusion (ECAE)
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19. Solid State Methods Severe Plastic Deformation of Bulk Samples
High Pressure Torsion (HPT) involves superimposing high hydrostatic pressure on a sample being sheared in torsion.
Very high strains can be achieved by this technique
Grain sizes obtained are typically nm but in some cases, <100 nm
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20. Solid State Methods Severe Plastic Deformation of Bulk Samples
Equal Channel Angular Extrusion (ECAE), also called Equal Channel Angular Pressing (ECAP) involves bending and rebending a rod through a special die, which is typically 90 degrees
The advantage is that larger samples can be processed than with HPT – Army testing 15” x 15” x 3.4” billets of Al-alloys
The disadvantage is that only materials with ductility can be processed, and grain sizes are typically 200 – 500 nm
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21. Surface Mechanical Attrition
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22. Metastable Materials
Some examples of metastable materials made by nonequilibrium processing methods are presented
Metastable crystalline alloys made by rapid solidification
Among the differences which may result from rapid solidification are:
- Microstructural refinement
- Solid solubility extension
- Formation of unique metastable phases
- Greater chemical homogeneity
- Changes in crystal morphology
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23. Metastable Materials
Most alloy development via Rapid Solidification has been with Al alloys, tool steel and superalloys involving precipitation hardening or solution hardening mechanisms
The greater homogeneity and extended solid solubility which may result from rapid solidification alloy greater freedom in alloy design
Alumimum alloys: Al alloys for higher strength, higher elastic modulus for aerospace applications include
- Al-Li-X alloys which involve precipitation of Al3Li from a supersaturated solid solution.
- Al-Mn-X and Al-Fe-N-Co alloys have also been developed
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24. Metastable Materials
Irons and Steels: High speed tool steels have rapidly solidified gas-atomized powders which are consolidated by HIPing. These have finer, more uniform microstructures (distribution of carbides) than the same alloys made by ingot metallurgy
Superalloys and Titanium: Rapidly solidified Ni-based superalloys (Ni-Al-Mo) powder has been consolidated by HIPing, hot pressing or extrusion for use in gas turbine blades to increase their operating temperatures.
Ti alloys (Ti-6Al-4V) have been rapidly solidified by the melt extraction method and by powder atomization methods. Finer, more uniform microstructures and chemical homogeneity give better mechanical properties
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