Dynamic Young's modulus and vibration damping for cryomilled NiAl- AlN composites: 5-20% AlN concentrations

High-temperature, high-strength nickel aluminidealuminum nitride (NiAl-AlN) composites have been created by utilizing a reaction milling process called cryomilling. These composites have similar compressive properties to those of single crystal Nibased superalloys [1]. Researching new NiAl-AlN composites may lead to high-strength, high-temperature alternatives to expensive superalloys [2]. High-intensity ball milling of NiAl powder in a liquid nitrogen-®lled attritor chamber was performed at NASA's Lewis Research Center to produce these high-strength, high-temperature NiAl-AlN composites. Through the cryomilling process, the NiAl particles became fragmented and the near-surface regions became supersaturated with nitrogen. Aluminium nitride (AlN) precipitated within the nitrogen-rich layers during powder consolidation operations at high temperatures [1, 3]. The powder was consolidated by hot isostatic pressing (HIP'ing) for 5%-AlN and 20%-AlN specimens. The 10%-AlN specimen was extruded. Hot isostatic pressing is a process of forming powder compacts at high temperatures. During HIP'ing, the powder was sealed in a metal container and was then simultaneously heated and compacted in a pressurized inert gas container. HIP'ing pressurized the powder isostatically. Although this process permits complex shapes to be produced, it is time-consuming and expensive [4]. The extrusion process is similar to the HIP'ing technique in that the powder was also sealed in a metal container. The container was then forced through a die to form the needed shape. The extrusion process, unlike the HIP'ing technique, does not allow complex shapes to be formed. Aluminum nitride (AlN) is an advanced ceramic that provides good electrical insulation and has a high thermal conductivity. Its high thermal conductivity allows it to dissipate and remove heat quickly. AlN has a density of 3260 kg my3 and a Young's modulus of 35.6 GPa [5]. The Piezoelectric Ultrasonic Composite Oscillator Technique (PUCOT) was utilized in this experiment to determine the dynamic Young's modulus and damping characteristics for the NiAl-AlN. Measurements of the dynamic Young's modulus and vibration damping were made for each specimen at 150 kHz, at temperatures ranging from 218C to 900 8C. Details about the PUCOT can be found elsewhere [6, 7]. This research complements that done recently by Wolfenden, Coan, and Hebsur on NiAl-AlN systems with concentrations of 1 and 30% AlN [8]. Table I contains the density and Young's modulus for the specimens of different compositions at room temperature (21 8C). Tables II±IV show the hightemperature results for the 5%, 10%, and 20% AlN