Transcript MOTIVATION - Super Materials

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
gammatitanium aluminides (-TiAl) have been
identified as material of choice for
aerospace
applications
in
the
o
o
temperature range of 600 C to 900 C.
 -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.
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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
Ti-45Al-X(Nb, B, C)
Draper et al, 2003
 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
Example of advanced concept for TiAl
Bartolotta, et al 1999
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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
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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
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.
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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.
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
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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 γ-TiAl and α2-Ti3Al.
O, Ti and Al
Ti3Al (alpha phase)
Element
Atomic %
O
61.77
Element
Atomic %
Al
32.39
Al
32.92
Ti
5.83
Ti
67.07
TiAl (gamma phase)
Element
Atomic %
Al
43.49
Ti
56.50
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
Ti49.5(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)
3.98
1000
1000
1200
1200
3.96
3.94
3.92
deg
deg
deg
deg
C,
C,
C,
C,
No pulsing
Pulsing at 600 A
No pulsing
Pulsing at 600 A
3.9
3.88
3.86
3.84
3.82
3.8
0
1
2
3
4
5
 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 Young’s modulus and fracture
toughness.
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MECHANICAL PROPERTIES OF TiAl
Four-point Bend Test Results for Various TiAl Samples
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Flexure Strength (MPa)
HIGH TEMPERATURES TEST RESULTS
1200
Air
Vacuum
1000
800
600
400
200
0
15
400
600
800
950
High Temperature Tests for Ti-50Al (at%)
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HIGH TEMPERATURES TEST RESULTS
Air
Vacuum
Flexure Strength (MPa)
1800
1600
Maximum sustained
stress
1400
1200
1000
800
600
400
200
0
200
400
750
950
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
1800
TiAl-Nb-Cr
TiAl
Strength (MPa)
1600
1400
1200
1000
800
600
400
P2C consolidated
200
Draper, et al 2003
0
0
200
400
600
800
1000
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.
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