1. ASTP MMR
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
ASTP MMR
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. Hexagonal Closed Packed
ASTP MMR
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
ASTP MMR
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

5. -TiAl MICROSTRUCTURES
ASTP MMR
-TiAl MICROSTRUCTURES
Lamellar Microstructure
Duplex 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.

6. MANUFACTURING TECHNIQUES
ASTP MMR
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.

7. PLASMA PRESSURE COMPACTION (P2C)
ASTP MMR
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.

8. CONSOLIDATION OF TITANIUM ALUMINIDE
ASTP MMR
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.
P2C
SEM of micron-sized titanium aluminide powder,
average particle size ~ 10 µm
3 inch x 2.25 inch x 0.25 inch TiAl tile

9. P2C CONSOLIDATION PARAMETERS FOR TiAL
ASTP MMR
P2C CONSOLIDATION PARAMETERS FOR TiAL
P2C
Sample
Consolidation
Time
Temperature
(Celsius)
Pressure
(MPa)
CERAC Disc
20 mins
1000
100
ESPI Disc 1
10 mins 54
ESPI Disc 2
1200
DOE Tile 1 30
DOE Tile 2

10. MICROSTRUCTURE TiAl TiAl-Nb-Cr
ASTP MMR
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.

11. TiAl Sample Annealed at 1400oC
ASTP MMR
MICROSTRUCTURE
Scanning Electron Microscopy of TiAl samples annealed at 1400oC showed fully lamellar grains
TiAl Sample Annealed at 1400oC

12. MICROSTRUCTURAL CHARACTERIZATION
ASTP MMR
MICROSTRUCTURAL CHARACTERIZATION
Energy Dispersive Spectroscopy (EDS) of the scanning electron micrographs (SEM) showed presence of both γ-TiAl and α2-Ti3Al.
O, Ti and Al
Ti3Al (alpha phase)
Element
Atomic % O
61.77 Al
32.39 Ti
5.83
Element
Atomic % Al
32.92 Ti
67.07
TiAl (gamma phase)
Element
Atomic % Al
43.49 Ti
56.50
Scanning Electron Micrograph of Consolidated TiAl Sample

13. CHEMICAL COMPOSITION Phase Diagram
ASTP MMR
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

14. Density (gm/cm3) TiAl Sample DENSITY
ASTP MMR
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.

15. ASTP MMR
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 Young’s modulus and fracture toughness.

16. MECHANICAL PROPERTIES OF TiAl
ASTP MMR
MECHANICAL PROPERTIES OF TiAl
Stress (MPa)
% Strain
Four-point Bend Test Results for Various TiAl Samples

17. HIGH TEMPERATURES TEST RESULTS
ASTP MMR
HIGH TEMPERATURES TEST RESULTS
Flexure Strength (MPa)
Temperature (Celsius)
High Temperature Tests for Ti-50Al (at%)

18. HIGH TEMPERATURES TEST RESULTS
ASTP MMR
HIGH TEMPERATURES TEST RESULTS
Maximum sustained
stress
Flexure Strength (MPa)
Temperature (Celsius)
High Temperature Tests for Ti-46Al-2Al-2Cr (at%)

19. HIGH TEMPERATURES TEST RESULTS
ASTP MMR
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

20. COMPARISON WITH STATE OF THE ART
ASTP MMR
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.