MOTIVATION

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

5. -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. Duplex Microstructure Lamellar Microstructure

6. 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) • 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. P2C 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

9. P2C CONSOLIDATION PARAMETERS FOR TiAL

10. 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. MICROSTRUCTURE • Scanning Electron Microscopy of TiAl samples annealed at 1400oC showed fully lamellar grains TiAl Sample Annealed at 1400oC

12. Element Element Atomic % Atomic % Al Al 32.92 43.49 Ti Ti 56.50 67.07 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) TiAl (gamma phase) Scanning Electron Micrograph of Consolidated TiAl Sample

13. 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 • 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. Density (gm/cm3) TiAl Sample

15. 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 Stress (MPa) % Strain Four-point Bend Test Results for Various TiAl Samples

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

18. 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 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 Strength (MPa) P2C consolidated Draper, et al 2003 Temperature (Celsius) High temperature mechanical properties of P2C consolidated TiAl were comparable to that of TiAl produced by extrusion process by Draper et al, 2003.