The role of microstructure refinement on the impact ignition and combustion behavior of mechanically activated Ni/Al reactive composites

Abstract

Metal-based reactive composites have great potential as energetic materials due to their high energy densities and potential uses as structural energetic materials and enhanced blast materials however these materials can be difficult to ignite with typical particle size ranges. Recent work has shown that mechanical activation of reactive powders increases their ignition sensitivity, yet it is not fully understood how the role of microstructure refinement due to the duration of mechanical activation will influence the impact ignition and combustion behavior of these materials. In this work, impact ignition and combustion behavior of compacted mechanically activated Ni/Al reactive powder were studied using a modified Asay shear impact experiment where properties such as the impact ignition threshold, ignition delay time, and combustion velocity were identified as a function of milling time. It was found that the mechanical impact ignition threshold decreases from an impact energy of greater than 500 J to an impact energy of $50 J as the dry milling time increases. The largest jump in sensitivity was between the dry milling times of 25% of critical reaction milling time (t cr ) (4.25 min) and 50% t cr (8.5 min) corresponding to the time at which nanolaminate structures begin to form during the mechanical activation process. Differential scanning calorimetry analysis indicates that this jump in the sensitivity to thermal and mechanical impact is dictated by the formation of nanolaminate structures, which reduce the temperature needed to begin the dissolution of nickel into aluminum. It was shown that a milling time of 50%-75% t cr may be near optimal when taking into account both the increased ignition sensitivity of mechanical activated Ni/Al and potential loss in reaction energy for longer milling times. Ignition delays due to the formation of hotspots ranged from 1.2 to 6.5 ms and were observed to be in the same range for all milling times considered less than t cr . Combustion velocities ranged from 20-23 cm/s for thermally ignited samples and from 25-31 cm/s for impacted samples at an impact energy of 200-250 J.

Properties of reactive Al:Ni compacts fabricated by radial forging of elemental and alloy powders

“…However, such processes are more expensive than mechanical methods due to high capital costs and lower throughput on a per mass basis. Mechanical methods for fabricating RMs include ball-milling [9,[11][12][13][14][15][16][17][18][19][21][22][23][24][25][26][27][28], rolling [10,[41][42][43][44][45], and cold isostatic pressing [3]. These methods typically have lower capital costs and higher throughputs but cannot achieve the same microstructural uniformity as PVD methods.…”

“…Exothermic formation reactions that generate intermetallic compounds and heat have been studied extensively [1][2][3][4][5][6][7][8] and can occur in compacts of homogeneous [1][2][3][4][5][6][7][8], composite [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] or core shell particles [29][30][31], or in layered films [32][33][34][35][36][37][38][39][40] or foils [41][42][43]…”

Exothermic Reaction Kinetics in High Energy Density Al-Ni with Nanoscale Multilayers Synthesized by Cryomilling

“…The thermodynamic modification is therefore necessary to improve its usability and reliability. Several reactive Al-Ni systems have been produced by methods like combustion synthesis [8], mixing and pressing of powders [9,10], welding [11,12], forging [13], rolling [14,15], vacuum deposition [11,15], cladding [16], and high energy ball milling [17][18][19]. It is well-known fact that the reactivity of these composites depends strongly on their corresponding microstructures.…”

Impact-initiated behavior and reaction mechanism of W/Zr composites with SHPB setup

“…The most studied of these reactive materials are Al-based composite such as Al/Ni and Al/W owing to their high strength, high energy density, easy processing characteristics and rapid energy release properties. [5][6][7][8][9][10][11][12] To improve their intercept efficiency, some high density reactive fragments have been proposed as alternatives to traditional steel fragments of anti-air warheads. For instance, tungsten/zirconium (W/Zr) composites have enough mechanical strength to maintain stability under overload during warhead detonation, and can penetrate targets and rapidly react.…”