Transcript Lecture17-Nov.12

MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu
Office: 308 RBII
Ph: 513-0559
[email protected]
Lecture 17: Advanced Processing of
Metastable Materials
Department of Materials Science and Engineering
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NC State University
Non-Equilibrium Processing
A metastable structure is considered to have local minima in
free energy, and not the lowest free energy of the system
1.Metastable state
2.Transitional “stable” state
3.Stable state
Department of Materials Science and Engineering
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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
Department of Materials Science and Engineering
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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
Department of Materials Science and Engineering
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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
Department of Materials Science and Engineering
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NC State University
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: Differential evaporation – different components have
different vapor pressures, so the composition of the film cannot be
easily controlled
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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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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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
Department of Materials Science and Engineering
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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
https://www.youtube.com/watch?v=cRE-PzI6uZA
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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Solid State Methods
Severe Plastic Deformation
- Mechanical Alloying or Mechanical Milling
The central event in mechanical milling or alloying is the ballpowder-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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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)
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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
Department of Materials Science and Engineering
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NC State University
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
Department of Materials Science and Engineering
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Surface Mechanical Attrition
Department of Materials Science and Engineering
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