Development of next generation space exploration vehicles and space structures require high temperature materials with

1
MOTIVATION

• Development of next generation space exploration
  vehicles and space structures require high
  temperature materials with
• Low density
• High strength and ductility
• Oxidation resistance
• Good creep properties
• Metal Matrix Composites based on intermetallics
  such as gamma-titanium aluminides (?-TiAl) have
  been identified as material of choice for
  aerospace applications in the temperature range
  of 600oC to 900oC.
• ?-TiAl have been identified as possible
  replacement for superalloys in engine components
  and nozzles due to their high specific strength
  and oxidation resistance at high temperatures.

2
STATE OF THE ART IN TITANIUM ALUMINIDES

• Current state of the art manufacturing techniques
  have produced ?-TiAl based alloys with
• Strength of 1 GPa
• Density of 3.8 gm/cm3
• Thus, they have posed a stiff competition for
  superalloys which have
• Strength of 1.2 GPa
• Density of 8 gm/cm3
• In order to capitalize on these advancements on
  ?-TiAl, further work is needed in the areas of
• Near-net shape manufacturing
• Low cost material production
• Materials property database

Ti-45Al-X(Nb, B, C) Draper et al, 2003
Example of advanced concept for TiAl Bartolotta,
et al 1999
3
INTERMETALLICS

• High strength compounds of metals whose crystal
  structures are different from the constituent
  metals.
• They form because the strength of bonding between
  unlike atoms is greater than that between like
  atoms.
• Examples are TiAl, Ti3Al, TiAl3, Ni3Al, Co3Ti.

Al
Ti
TiAl Face Centered Cubic Structure
Ti3Al Hexagonal Closed Packed Structure
4
PHASES OF TITANIUM ALUMINIDES

• ?-TiAl can exist in two different phases
• Pure ?-TiAl phase
• Mixture of ?-TiAl and ?2-TiAl
• Pure ?-TiAl has high strength and oxidation
  resistance, but it shows almost no ductility.
  Thus, not much research has been done on pure
  ?-TiAl.
• Mixture of ?-TiAl and ?2-TiAl has high strength
  and good ductility, but does not show good
  oxidation resistance.
• But the properties of this phase can be improved
  by
• Control of its microstructure
• Small additions of TiB2, Nb, and Cr.
• A lot of research has been concentrated on this
  phase of TiAl.

Phase diagram - Titanium Aluminide
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?-TiAl MICROSTRUCTURES
Duplex Microstructure
Lamellar Microstructure

• Two main characteristic microstructures possible
  in ?-TiAl.
• Duplex Microstructure Exhibits good strength
  and ductility.
• Lamellar Microstructure Has good creep
  properties.
• These microstructures can be produced with
  appropriate heat-treatments.
• Refinement and control of grain size of these
  microstructures have shown improved mechanical
  properties.

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MANUFACTURING TECHNIQUES

• The state of the art manufacturing techniques of
  ?-TiAl involve ingot metallurgy and extrusion
  processes, which are often time consuming and
  expensive.
• Other methods follow the powder metallurgy route
  such as
• Sintering
• Hot Pressing
• Hot Isostatic pressing
• Powder consolidation methods usually have the
  advantage of yielding near-net shape parts.
• But the methods mentioned above require exposure
  to high temperatures for long time to achieve
  full densification.
• Such extended exposure at high temperatures leads
  to grain growth and deterioration in mechanical
  properties. Controlling or minimizing grain
  growth has long been known to increase strength
  and ductility of materials.
• Rapid consolidation can be a potential solution
  since it generally reduces segregation, refines
  microstructure and thus produces a more
  homogeneous material.

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PLASMA PRESSURE COMPACTION (P2C)

• Developed by Materials Modification, Inc., P2C
  is designed for rapid consolidation of
  nanocrystalline and micron-sized powders.
• The powder is loaded into a graphite die.
• An electrical discharge between the particle
  surfaces provides electrical resistance and
  surface heating.
• Before applying high temperatures and pressures,
  a plasma activation stage removes all adsorbed
  surface oxides and contaminants.
• The P2C process has the following advantages
• Rapid consolidation of powders (minutes vs
  hours).
• No sintering additive required.
• Near-net shape processing.
• Fewer impurities.
• Lower oxygen content in consolidated part
  compared to starting powders.

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CONSOLIDATION OF TITANIUM ALUMINIDE

• Two different compositions of Titanium Aluminides
  powders were consolidated
• Commercially available micron sized powders of
  composition Ti-50Al (at) were procured from
  CERAC, Milwauke, WI, and ESPI, Inc., OR.
• Specialized micron sized powders of composition
  Ti-46Al-2Cr-2Nb (at) were procured from Oak
  Ridge National Laboratories, Infrared Processing
  Center, Department of Energy, Oak Ridge, TN.

SEM of micron-sized titanium aluminide powder,
average particle size 10 µm
3 inch x 2.25 inch x 0.25 inch TiAl tile
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P2C CONSOLIDATION PARAMETERS FOR TiAL
P2C Sample Consolidation Time Temperature (Celsius) Pressure (MPa)
CERAC Disc 20 mins 1000 100
ESPI Disc 1 10 mins 1000 54
ESPI Disc 2 10 mins 1200 54
DOE Tile 1 20 mins 1000 30
DOE Tile 2 20 mins 1200 30
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MICROSTRUCTURE

• Optical and Scanning Electron Microscopy showed
  duplex microstructure

10 ?m
TiAl
TiAl-Nb-Cr

• Average measured grain size 5 to 10 µm
• Average powder particle size 5 to 10 µm
• Micrographs showed no grain growth.

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MICROSTRUCTURE

• Scanning Electron Microscopy of TiAl samples
  annealed at 1400oC showed fully lamellar grains

TiAl Sample Annealed at 1400oC
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MICROSTRUCTURAL CHARACTERIZATION

• Energy Dispersive Spectroscopy (EDS) of the
  scanning electron micrographs (SEM) showed
  presence of both ?-TiAl and a2-Ti3Al.

O, Ti and Al
Ti3Al (alpha phase)
Element Atomic
O 61.77
Al 32.39
Ti 5.83
TiAl (gamma phase)
Scanning Electron Micrograph of Consolidated
TiAl Sample
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CHEMICAL COMPOSITION

• Chemical composition analyses of the CERAC/ESPI
  powders and consolidated samples revealed the
  chemical composition as Ti-49.5(at)Al.
• Presence of alpha2 phase is very less in this
  composition.
• In order increase alpha2 composition, the
  aluminum must be decreased up to 46 to 48
• New powders were procured from Oakridge National
  Laboratories with 46 Al and additions of Nb and
  Cr.

Phase Diagram
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DENSITY
Density (gm/cm3)
TiAl Sample

• The average density of the consolidated samples
  was found to be 3.9 gm/cm3.
• The density of the gamma phase is 3.76 gm/cm3,
  while that of the alpha2 phase is 4.2 gm/cm3.
• The theoretical density of the samples will be
  determined by calculating the amount of alpha2
  phase present in the sample.
• From the micrographs and the density data, the
  consolidated samples seem fully dense.

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MECHANICAL TESTING

• Mechanical testing was conducted via four-point
  bending tests in a self-aligning silicon carbide
  fixture
• The test was conducted as per ASTM 1161 and 1421
  specifications.
• The test specimen was mounted with a strain gage
  for tests conducted at room temperature
• The four-point bending tests revealed flexure
  strength and Youngs modulus and fracture
  toughness.

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MECHANICAL PROPERTIES OF TiAl
Stress (MPa)
Strain
Four-point Bend Test Results for Various TiAl
Samples
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HIGH TEMPERATURES TEST RESULTS
Flexure Strength (MPa)
Temperature (Celsius)
High Temperature Tests for Ti-50Al (at)
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HIGH TEMPERATURES TEST RESULTS
Maximum sustained stress
Flexure Strength (MPa)
Temperature (Celsius)
High Temperature Tests for Ti-46Al-2Al-2Cr (at)
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HIGH TEMPERATURES TEST RESULTS
Stress v. Displacement Plot for TiAl-Nb-Cr at
950oC
TiAl and TiAl-Nb-Cr Samples Tested at 950oC in Air
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COMPARISON WITH STATE OF THE ART
Strength (MPa)
P2C consolidated
Draper, et al 2003
Temperature (Celsius)
Temperature P2C consolidated (flexure Strength) As-extruded (tensile strength)
400oC 1600 MPa 1100 MPa
800oC 1000 MPa 700 MPa
950oC 800 MPa 500 MPa
High temperature mechanical properties of P2C
consolidated TiAl were comparable to that of TiAl
produced by extrusion process by Draper et al,
2003.