Superalloys created by the self-propagating high-temperature synthesis of nano-composite nickel aluminide

Advancements in nanotechnology for material processing via combustion synthesis have spurred the development of superalloys that provide improved protective properties. Nano-scale reactant particles offer unique thermal properties and increased homogeneity that improve the micro-structural features and macroscopic properties of the synthesized product. The ignition and combustion behaviors of Nickel (Ni) and Aluminum (Al) thermites were studied as a function of Al particle size. In particular, nano-scale Ni/Al composites were compared to micron-scale Ni/Al composites. Laser ignition experiments were performed on pressed Ni/Al pellets to determine ignition time and temperature as a function of Al particle size. Flame propagation behavior and burn rates were also examined using high-speed diagnostics. Results show that nano-scale composites have significantly reduced ignition times over micron-scale composites owing primarily to the unique thermal properties associated with nano-particles. Flame propagation and overall burn rate was also influenced by Al particle size and physical properties. Electron micrographs of the products reveal the formation of whiskers in nano-scale composites but not in micron-scale composites. In order to examine the effects of a nano-scale additive on the Ni/Al alloy, nano-scale molybdenum tri-oxide (MoO3) particles were added to micron scale Ni and Al. The goal was to incorporate a nano-scale additive within the reactant matrix that would produce a superalloy by generating excessively high heating rates and creating controlled quantities of Al2O3 (a strengthening agent) within the microstructure of the alloy. Ignition and flame propagation were examined using a CO2 laser and imaging diagnostics that include a copper-vapor laser coupled with a high-speed camera. Product microstructure was examined using micro-XRD analysis and scanning electron microscopy. Abrasion testing was performed to evaluate the wear resistance properties of the superalloy. Results show that adding MoO3 increases the flame temperature, results in greater ignition sensitivity, produces a more homogeneous microstructure and increases the overall wear resistance of the product. Ignition behaviors associated with nano and micron scale particulate composite thermites were studied experimentally and modeled theoretically. The experimental analysis utilized a CO2 laser ignition apparatus to ignite the front surface of compacted Ni and Al pellets at varying heating rates. Ignition delay time and ignition temperature as a function of both Ni and Al particle size were measured using high speed imaging and micro-thermocouples. The activation energy was determined from this data using a Kissinger isoconversion method. This is the first study to show that the activation energy is significantly lower for nano- compared with micron-scale particulate media (i.e., as low as 17.4 compared with 162.5 kJ/mol, respectively). Two separate Arrhenius-type mathematical models were developed that describe ignition in the nano- and the micron- composite thermites. The micron-composite model is based on a heat balance while the nano-composite model incorporates the energy of phase transformation in the alumina shell theorized to be an initiating step in the solid-solid diffusion reaction and uniquely appreciable in nano-particle media. These models were found to describe the ignition of the Ni/Al alloy for a wide range of heating rates. A highly porous intermetallic alloy was created through self-propagating high-temperature synthesis. The reactants are composed of nano-scale particles of Ni, micron-scale particles of Al, and nano-scale Al particles passivated with a gasifying agent, C13F27COOH. The concentration of nm Al particles present in the reactant matrix was controlled according to the wt % of gasifying agent. The reactant mixture was cold-pressed into cylindrical pellets with a constant density equal to 70% of the theoretical maximum density. Once ignited, flame propagation was observed to transition from normal to convectively dominant burning as more gasifying agent became present in the reactants. A critical Andreev number of 6 was determined to represent this transition. Ignition delay times were reduced by two orders of magnitude when only 2.24 wt % nm Al particles were present. The product alloy expanded by a factor of 14 in the axial direction with 1.6 wt % nm Al (corresponding to 10 wt % gasifying agent). Total porosity of the pellets was also measured and found to increase with increasing wt % of the nm Al and gasifying agent.