Advanced Processing of Metastable Materials

1. MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 17: Advanced Processing of Metastable Materials 1

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 2

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 3

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 4

5. Non-Equilibrium Processing • The Quenching back to ambient conditions can be characterized by high 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 5

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 1013 -1014 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 for alloy: Differential evaporation – different components have different vapor pressures, so the composition of the film cannot be easily controlled 6

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 7

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 8

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 9

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 http://www.youtube.com/watch?v=2--vIYNwgCY 10

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 https://www.youtube.com/watch?v=L00HbH8Vla8 11

12. Metastable Materials • 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 12

13. 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 13

14. Laser or Electron Beam Methods (3D Printing)Additive Manufacturing • 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 1010-1013 K/s (highest cooling rates for RSP) http://www.youtube.com/watch?v=BxxIVLnAbLw 14

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 15

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 16

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 17

18. Severe Plastic Deformation • 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) 18

19. Severe Plastic Deformation • 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 100-200 nm but in some cases, <100 nm 19

20. Severe Plastic Deformation • 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 20

21. Surface Mechanical Attrition 21