• PDF / 316,725 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 0 Downloads / 253 Views

DOWNLOAD

REPORT

---

0896-H01-08.1

Reactive Al-Li Powders Prepared by Mechanical Alloying Xiaoying Zhu, Mirko Schoenitz, Vern K. Hoffmann, and Edward L. Dreizin New Jersey Institute of Technology Newark, NJ 07102 ABSTRACT Mechanically alloyed powders with the composition Al0.7Li0.3 are synthesized. Materials milled for different times are studied using electron microscopy, x-ray diffraction, and thermal analysis. A solid solution of Li in Al (α-phase) is formed with as much as 10 at % of dissolved Li. The LiAl intermetallic δ-phase is readily produced by mechanical alloying but disappears after extended milling times. The final product of milling for 102 hours consists of an x-ray amorphous phase. Mechanically alloyed powders heated in inert environment exhibit several weak exothermic reactions between 420 and 700 K, and two endothermic reactions, around 810 and 870 K. All the observed relaxation processes become less pronounced and eventually become undetectable as the milling time increases and an amorphous material is produced. Ignition experiments performed for the powders coated on an electrically heated filament showed that the powders ignited in the vicinity of 1250 K. An experimental setup for studying combustion of reactive mechanically alloyed powders is developed and initial experimental results are described. INTRODUCTION Al-Li cast alloys have been extensively used in structural applications due to their good mechanical properties and low density [1, 2]. Mechanical alloying (MA) has been used to improve the ductility of ternary Al-Li alloys [3, 4]. Typically, low concentrations of lithium in the structural alloys are maintained to ensure stability during exposure to air. However, powders of Al-based alloys with increased concentrations of Li are attractive as high energy density fuels or additives to metallized propellants [5, 6]. Such materials are expected to be more reactive than pure Al, which is most commonly added to energetic materials. Several related Al-based alloys were recently prepared by MA [7, 8], a low temperature process that is well suited for the production of reactive powders on a practical scale. In this work, mechanically alloyed Al-Li powders were produced with 30 at % (10 wt %) of lithium. The progress of mechanical alloying was assessed by collection and characterization of powders at different milling times. The ignition and combustion characteristics of the alloys were evaluated experimentally. EXPERIMENTAL Synthesis Mechanically alloyed powders with the composition Al0.7Li0.3 were prepared in a Retsch PM 400 MA planetary mill. Starting components were a 99.8 % pure, -40+325 mesh aluminum powder by Alfa Aesar, and a 99.9% pure, 0.75 mm thick, 45 mm wide lithium ribbon by SigmaAldrich. Batches of 30 g were prepared in steel milling jars using steel milling media (10 mm diameter balls). The ball-to-powder weight ratio was 10, and 4 mass % of stearic acid was added as a process control agent. An air conditioner was used to maintain the temperature in the milling chamber at about 15 ºC and custom desi