TECHNICAL: Nickel-based super alloys are outstanding high temperature materials. However, the desire to reach even higher operating temperatures as a way to improve performance and thermodynamic efficiency (thereby reduce fuel consumption and pollution) requires the development of a new class of high-temperature materials. Research will improve high temperature creep strength of NiAl and IrAl intermetallics by incorporating a high number of thermally stable Y-Ti(Zr,Hf)-O nanoclusters. Research would involve computational modeling, including electronic structure calculations, atomistic lattice and off-lattice Monte Carlo simulations, and molecular dynamics simulations, to investigate the structure, composition and thermal stability of the nano-clusters. Materials will be prepared by mechanical alloying and consolidation through hot isostatic pressing. Advanced experimental characterization techniques, combined with micro-hardness measurements will be used to investigate the material microstructure and to evaluate thermal stability during long-term annealing. In particular, the size, density, and composition of the nanoclusters will be assessed and compared to the computational predictions. Computational molecular dynamics simulations will be used in the later stage of the program to investigate the atomistic sequence of events controlling dislocation interaction and detachment from the nanoclusters. NON-TECHNICAL: Results would lead to an optimized alloy system for high temperature applications for jet engines, power plant applications, etc. It would also provide an inter-disciplinary environment to attract and educate graduate students in materials science research. This will be achieved by incorporating computational and experimental tools as well as the specific knowledge gained from this program into classroom teaching, research seminars, and educational outreach activities. Java-based modules that demonstrate materials behavior will be developed during this program for undergraduate and graduate courses, and translated into a format applicable to educational outreach activities for high school students. Outreach activities will be aimed at high school students will specific focus on students from minority, underrepresented, or low-income backgrounds in the Berkeley and Oakland school districts.
The scientific objective of the research program was to investigate the potential for improved high temperature creep strength of B2 NiAl intermetallics by incorporating a high number density of very thermally stable Y-Ti-O nanoclusters, akin to the nanostructured ferritic alloy (NFA) development strategy. The research involved a combination of computational modeling to predict the structure and composition of such precipitates combined with an experimental program to fabricate such alloys using mechanical alloying with an attritor mill, and to characterize the resulting precipitate structures following processing and thermal aging. The key conclusions of the modeling studies obtained in this program are that i) the compositions of the Y-Ti-O nanoclusters formed in ferritic alloys are in the range of those observed experimentally by atom probe tomography, but only for the case that substantial additional atomic volume is included to relieve the strain energy density associated with the precipitation of larger atoms; and ii) that Y-Ti-O rich nanoclusters can be formed in NiAl intermetallic alloys with the B2 crystal structure, but that those precipitate clusters will likely contain Al. At this time, the possible detrimental effect of Al on the thermal stability of the precipitates has not been entirely evaluated. The key conclusions of the experimental studies indicate that stronger NiAl alloys do result from the addition of Y2O3 and Ti. Furthermore, the results of the detailed characterization and thermal annealing studies of the mechanically alloyed and consolidated NiAl + Y2O3 + Ti alloys indicate some promise for improved high temperature strength, including a higher precipitate number density of features with sizes in the range of 10 nanometers and initial indications of retained strengthening properties following high temperature exposures. However, there is also an indication of the possibly detrimental incorporation of aluminum into the dispersed oxide precipitates in the 20 – 40 nm size range and thus it is still unclear whether the overall objective of introducing an ultrahigh density of nanometer-scale and thermally stable Y-Ti-O precipitate clusters in NiAl has been successful.
