The nanostructured materials program at the University of Idaho is designed to produce nanostructured materials with enhanced mechanical and physical properties in a cost-effective fashion. The program can be conveniently split up into four segments: (1) nanostructured materials production, (2) compaction, (3) characterization, and (4) testing.
A. Plasma Processing (Dr. P.R. Taylor). A non-transferred arc thermal plasma reactor has been designed, built, and operated to vaporize coarse metal powders. Quenching is performed using a supersonic nozzle, and the powders are collected in a filter system that can be sealed and dismantled in a glove box. Experiments have been performed to generate nanometer-sized copper and iron. Handling the fine powders without oxidation is very difficult, and recent experiments have been performed to evaluate improvements in materials handling.
B. Combustion Synthesis (Dr. S. Bhaduri). The "combustion synthesis" process utilizes oxidizers (e.g., metal salts) and fuels (typically organic compounds). When properly controlled, high temperatures can be generated by the exothermicity of reduction-oxidation (redox) reactions between decomposition products of the oxidizer and the fuel. Because of the fast heating and cooling, there is nucleation of crystallites but little growth, resulting in nanocrystalline ceramics. The following are the important features of this process:
- It is a versatile process leading to the synthesis of single-phase, solid solutions, and composites as well as complex compound oxide phases.
- It uses cheap raw materials.
- It is a scaleable and a high production rate process.
- The products are of high purity and loosely agglomerated, resulting in high sinterability. Examples of materials include n-(-Al2O3, n-spinel, n-ZrO2, etc.
C. Mechanical Alloying (Dr. O.N. Senkov). Mechanical alloying (MA'ing), a high-energy ball milling process, has been used to produce nanostructured powders.
- Low-cost nanostructured titanium powders have been produced by reduction reactions from titanium tetrachloride (replacement reaction with Mg) and titanium dioxide (replacement reaction with hydrides of light metals, such as CaH2).
- Amorphous powders have been synthesized by MA'ing pre-alloyed gas atomized powders of TiAl-based alloys in order to produce nanocrystalline compacts after hot isostatic pressing (HIP'ing).
- Nanocrystalline composite powders have been produced by MA'ing of blended elemental powders of TiH2, Ti, Al, Si, and B. Titanium aluminide - titanium silicide and titanium - titanium diboride nanocrystalline composite powders have been produced and characterized.
- The effect of replacing titanium with titanium hydride in the MA process has been investigated, including effects on contamination and the scale of the microstructure.
D. Supercritical Fluid (Dr. C. Wai and Dr. E.G. Baburaj). The production of nanostructured particles or their films depends upon the substantial change in solubility of a solute in a solvent, at the supercritical point (temperature and pressure) of the solvent. For example, carbon dioxide, which is a good solvent for dissolving organometallic compounds, becomes a supercritical fluid at relatively low temperature and pressure (31°C and 73 atmosphere). Thus, metal or oxide particles can be produced from organometallic compounds dissolved in supercritical carbon dioxide. We have produced Cu films by hydrogen reduction of an organo-copper compound dissolved in supercritical carbon dioxide, and have made SnO2 particles by heating an organo-tin compound dissolved in supercritical carbon dioxide.
A. Press and Sinter (Dr. S. Bhaduri). The powders produced by the combustion synthesis process were heat-treated at various temperatures to examine the extent of grain growth. It was determined that the grains remained less than 100 nm at temperatures less than 1200oC because of microstructural tailoring. The powders were cold isostatically pressed at about 300 MPa and were glass encapsulated and hot isostatically pressed at temperatures between 1000°C and 1200°C with pressure levels of about 175 MPa, which led to very dense samples (>95%).
B. Hot Isostatic Pressing (Dr. F.H. [Sam] Froes). Although the fine temperature exposure experienced during hot isostatic pressing is extreme, it has been shown that nanograins can be retained in alloys such as TiAl by low temperature HIP'ing while achieving full density. With nanostructured material, full density can be achieved at 700°C compared to 1100°C for material with micron-sized grains.
III. Characterization (Dr. O.N. Senkov)
A. DTA, SEM, TEM, and EDS
C. Thermal stability of the nanocrystalline HIP'd Ti Al-based compacts
D. Thermal stability of the nanocrystalline intermetallic-ceramic powders
IV. Testing (Dr. R. Stephens)
Testing of nanostructured materials will commence on small-scale test specimens in the near future.
In other activity, the superplastic forming behavior of nanostructured TiAl has been studied in a cooperative program with Professor A. Mukherjee's group at University of California, Davis.
This program is predominantly funded by the Idaho State Board of Education.