MSE 440/540: Processing of Metallic Materials

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MSE 440/540 Processing of Metallic Materials

• Instructors Yuntian Zhu
• Office 308 RBII
• Ph 513-0559
• ytzhu_at_ncsu.edu
• Lecture 17 Advanced Processing of Metastable
  Materials

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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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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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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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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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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

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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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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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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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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?v2--vIYNwgCY
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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

http//www.youtube.com/watch?vAXJ74gCe4ukfeature
rellistplaynext1listPLB03762EC69F8D650
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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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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

http//www.youtube.com/watch?v_JW0h9VyCQQ
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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 1010-1013 K/s (highest cooling rates for
  RSP)

http//www.youtube.com/watch?vBxxIVLnAbLw
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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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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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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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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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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 100-200 nm
  but in some cases, lt100 nm

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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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Surface Mechanical Attrition
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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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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