Self-propagating high-temperature synthesis (SHS) of rotator mixed and mechanically alloyed Ni/Al powder compacts

A significant amount of work has been published on the combustion synthesis of nickel aluminides including thermal explosion and SHS type of ignition, [1–4]. In the SHS of nickel aluminides, nickel and aluminum powders are mixed in the desired composition and pressed into a compact, which is then ignited at one end, usually with the aid of a tungsten-heating coil (Fig. 1). An exothermic reaction proceeds converting the nickel and aluminum powders into the desired intermetallic compound, and raises the compact temperature to the maximum combustion temperature. The elemental powders are normally turbula or rotator mixed prior to compaction. On the other hand, recently, mechanical activation of elemental powders prior to SHS (mechanically activated SHS (MASHS)) has also been conducted [5–9]. In MASHS the powders are mechanically alloyed by ball milling under certain conditions that do not give rise to reactions between the elemental powders. In this process nanostructured elemental powders are generated prior to SHS. The process has a number of advantages including the production of a nanostructured product phase; however the resulting product is reported to contain high levels of porosity attributed to entrapped gas during the mechanical alloying process [10]. In the present paper, the concepts of the theory of plasticity of porous bodies are employed to assess the influence of mechanical alloying (MA) on the theoretical yield stress of Ni–Al powder compacts (for nickel particle sizes of 8–15 and 3–7 lm). The effect of MA (low-energy mechanical mixing) on SHS characteristics are reported, together with the influence of degassing-MA-SHS sequential order on resulting product porosity, which may possibly have implications for MASHS in general. Powders of nickel (Novamet Inc., USA) and aluminum ()325 mesh, average particle size ~11 lm, Atlantic Engineers, USA), were mixed in the composition of Ni3Al. Two sample compositions were produced, each with a different nickel particle size (3–7 and 8–15 lm, respectively). For each composition, mixing was conducted over two stages. First, the powders were gently mixed for 1.5 h using a powder rotator mixer. Second, approximately 5–6 g of Ni/Al powders were separately mechanically alloyed (low-energy mechanically mixed) in hardened steel vials, using twenty tungsten carbide balls (diameter 3 mm) at 240 rpm for 27 h. Specimens that have just been mixed using the powder rotator without MA are referred to in this paper as have undergone rotator mixing. Powders that were rotator mixed followed by MA for 27 h are referred to as MA 27 h. Each powder was then uniaxially pressed into rectangular specimens of dimensions shown in (Fig. 1). Zinc stearate lubricant was mixed in acetone and applied to the inner walls of the hardened steel compaction die used. All powder compacts were vacuum degassed at 150 C for 10.5 h. A K-type thermocouple was inserted at half height into the specimen to monitor the temperature– time profile during the reaction. Specimens were then reacted in an SHS reactor under an argon atmosphere (three specimens were used for each particle size and mixing sequence, and average combustion temperatures were then reported). The top of the specimens were placed at a distance approximately 2–3 mm beneath the tungsten coil, which was subsequently subjected to 750 W. Each powder mixture (mechanically alloyed and K. Morsi (&) AE S. Shinde AE E. A. Olevsky Department of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA e-mail: kmorsi@mail.sdsu.edu J Mater Sci (2006) 41:5699–5703 DOI 10.1007/s10853-006-0068-x