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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.

MOTIVATION

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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 TiAlBartolotta, et al 1999

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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.

INTERMETALLICS

TiAlFace Centered Cubic

Structure

Ti3AlHexagonal Closed Packed

Structure

Al

Ti

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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

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.

Lamellar Microstructure Duplex Microstructure

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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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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.

PLASMA PRESSURE COMPACTION (P2C)

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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

CONSOLIDATION OF TITANIUM ALUMINIDE

3 inch x 2.25 inch x 0.25 inch TiAl tile

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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

P2C CONSOLIDATION PARAMETERS FOR TiAL

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Optical and Scanning Electron Microscopy showed duplex microstructure

Average measured grain size ~ 5 to 10 µm

Average powder particle size ~ 5 to 10 µm

Micrographs showed no grain growth.

MICROSTRUCTURE

10 m

TiAl TiAl-Nb-Cr

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

MICROSTRUCTURE

TiAl Sample Annealed at 1400oC

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Energy Dispersive Spectroscopy (EDS) of the scanning electron micrographs (SEM) showed presence of both γ-TiAl and α2-Ti3Al.

Scanning Electron Micrograph of Consolidated TiAl Sample

Element Atomic %

Al 32.92

Ti 67.07

Element Atomic %

Al 43.49

Ti 56.50

TiAl (gamma phase)

Ti3Al (alpha phase)Element Atomic %

O 61.77

Al 32.39

Ti 5.83

O, Ti and Al

MICROSTRUCTURAL CHARACTERIZATION

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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

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.

3.8

3.82

3.84

3.86

3.88

3.9

3.92

3.94

3.96

3.98

0 1 2 3 4 5

1000 deg C, No pulsing1000 deg C, Pulsing at 600 A1200 deg C, No pulsing1200 deg C, Pulsing at 600 A

Den

sity

(g

m/c

m3 )

TiAl Sample

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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 Young’s modulus and fracture toughness.

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MECHANICAL PROPERTIES OF TiAl

Str

ess

(MP

a)

% Strain

Four-point Bend Test Results for Various TiAl Samples

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HIGH TEMPERATURES TEST RESULTS

High Temperature Tests for Ti-50Al (at%)

0

200

400

600

800

1000

1200

15 400 600 800 950

AirVacuum

Fle

xure

Str

eng

th (

MP

a)

Temperature (Celsius)

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HIGH TEMPERATURES TEST RESULTS

High Temperature Tests for Ti-46Al-2Al-2Cr (at%)

Fle

xu

re S

tre

ng

th (

MP

a)

0

200

400

600

800

1000

1200

1400

1600

1800

200 400 750 950

AirVacuum

Maximum sustainedstress

Temperature (Celsius)

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HIGH TEMPERATURES TEST RESULTS

TiAl and TiAl-Nb-Cr Samples Testedat 950oC in Air

Stress v. Displacement Plot for TiAl-Nb-Cr at 950oC

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COMPARISON WITH STATE OF THE ART

0

200

400

600

800

1000

1200

1400

1600

1800

0 200 400 600 800 1000

TiAl-Nb-CrTiAl

Str

en

gth

(M

Pa

)

Temperature (Celsius)

P2C consolidated Draper, et al 2003

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.