Rev.Adv.Mater.Sci.Dislocation mechanics 19(2009) aspects 13-40 of energetic material composites 13 DISLOCATION MECHANICS ASPECTS OF ENERGETIC MATERIAL COMPOSITES R.W. Armstrong Center for Energetic Concepts Development, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, U.S.A. Received: November 19, 2008 Abstract. The dislocation mechanics based properties of solid energetic materials, particularly, of high explosives, are of particular interest in connection with issues of intrinsic chemical stability and with their fast chemical decomposition when employed as propellants or in explosive formula- tions. The ballistic impact and shock-associated plasticity responses of such materials present great experimental and model challenges for establishment of predictable performances. As dem- onstrated in the present report, much has been learned through direct investigation with a full range of scientific tools of the individual crystal and composite material properties and, also, through their comparison with relevant inert ionic and metallic material behaviors. Thus, in relation to other solid material structures, energetic crystals are elastically compliant, plastically strong, and prone to cleavage fracture. Somewhat surprisingly perhaps for such materials, individual dislocation self-energies are indicated to be relatively large while the intrinsic crystal-determined dislocation mobility is restricted because of the complicated and rather dense molecular packing of awk- wardly-shaped molecules that are self-organized within lower-symmetry crystal structures. Be- cause crack surface energies are low, cleavage is able to be initiated by relatively small dislocation pile-ups and, with the restricted dislocation mobility, there is little additional plastic work require- ment associated with cleavage crack propagation. Nevertheless, when compared with indentation fracture mechanics prediction, crack propagation appears to be controlled by the behavior of very limited dislocation activity at the crack tip. Adiabatic heating associated with dislocation pile-up avalanches provides an important mechanism for the thermal “hot spot” model of explaining the initiation of rapid chemical decompositions and relates directly to predicted influence of crystal (or particle) sizes. On such basis, desired characteristics of greater mechanical insensitivity to initia- tion but afterwards greater power dissipation are predicted to occur for energetic composite formu- lations made from smaller particle-sized ingredients. 1. INTRODUCTION rates of energy decompositions occurring within a shock wave associated environment. Beginning Solid energetic composite materials are included from the late 19th century until the present time, within the category of advanced materials, first, much chemical- and physics-based information has because complete information remains to be de- been gained on the component and formulated termined both for the individual component behav- energetic material properties and uses. More re- iors and for their interactions in relatively complex cently, additional information has been gained formulations; and, secondly, because there is the through a material science approach of establish- need for understanding of the composite perfor- ing microstructurally-based relations between the mances, for example, under mechanics-based high material compositions and their cumulative prop- rate loading conditions involving very substantial erties [1-4]. The multifunctional topic of reference Corresponding author: R.W. Armstrong, e-mail: [email protected] © 2009 Advanced Study Center Co. Ltd. 14 R.W. Armstrong Fig. 1. Schematic picture of structural elements associated with propellant combustion. [4] relates, in part, to the possible combination of with the high strain rate properties of metals has energetic and structural load-bearing properties of been recently reviewed [7]. An historical note in a composite material being built into a single de- that regard comes from B. Hopkinson, son of sign package. As will be seen in the present re- J. Hopkinson of split Hopkinson pressure bar port, the possibility is based in part on the achieve- (SHPB) fame, in contributing an early article on the ment of advantageous properties on both consid- topic of measuring the pressures generated in the erations at nanometric dimensions of the compos- detonation of high explosives or by the impact of ite ingredients [5]. In that regard, it’s interesting that bullets [8]. researches on the energetic materials themselves The occurrence of local thermal “hot spots” pro- are often intertwined with studies of their detona- vides a historically-established mechanism for ex- tion influences on the dynamic deformation prop- plaining the initiation of fast chemical decomposi- erties of structural materials [6]. Such connection tion within the various energetic materials; see, for Dislocation mechanics aspects of energetic material composites 15 2. CRYSTAL INGREDIENTS/ STRUCTURES There is an important role of structural chemistry in understanding the nature of energetic crystal properties. Fig. 3 shows the crystal unit cell struc- ture of cyclotrimethylenetrinitramine that is known by the substitute designation RDX and has an indi- vidual molecular formula [CH2·N·NO2]3. The brack- eted manner of specifying the formula relates to the configuration of the molecule. Eight such RDX molecules, with individual covalent bonding be- tween the carbon, C, hydrogen, H, nitrogen, N, and oxygen, O, elements, are relatively closely packed Fig. 2. Scanning electron microscope image of then within a molecularly-bonded orthorhombic unit cyclotrimethylenetrinitramine (RDX) crystals. cell with a Pbca crystallographic space group des- ignation. The crystal lattice parameters are a = 1. 3182 nm, b = 1.1574 nm, and c = 1.0709 nm so having a unit cell volume of ~ 1.63 nm3 containing 168 atoms [20]. Identification in the reported struc- tural analysis of any smaller length for a typical covalent intramolecular bond distance, for example, for a labeled C3 – H5 bond length of 0.11 nm may example, reference [9]. The task of tracking such be compared with a larger intermolecular bond dis- proposed hot spots is made more difficult by hav- tance, say, O1 – H2 of 0.25 nm length. ing to ferret through the complicated environment Two other energetic crystal materials to be es- of energetic crystal responses occurring within a pecially referenced in the present article are: chemi- typical propellant formulation, as schematically cally-related to RDX, tetramethylenetetranitramine depicted in Fig. 1, or within the more heavily filled (HMX), [CH2·N·NO2] 4; and, pentaerythritol constitution of an explosive formulation; see for tetranitrate (PETN), [C·(CH2·O·NO2)4]. The example references [10-12], including in the last Arrhenius-based hot spot model assessment men- reference the formulation of a reactive metal sys- tioned above [9] lists RDX as being relatively stable tem. And, for whatever energetic composite de- thermally when compared to HMX and PETN that, scription is of interest, including that of a more in turn, are individually about equally less so but heavily-filled plastically-bonded explosive (PBX), with PETN showing a stronger dependence of its the decomposition behaviors, whether from direct critical hot spot size on temperature. The energetic thermal stimulus or under mechanical loading at crystal triaminotrinitrobenzine, (TATB), the highest applied rates, including shock wave [(NH2)·C·C·(NO2)]3, that is relatively more stable loading, are connected generally with variously mechanically in comparison to RDX, is an impor- modified Arrhenius-type thermal activation equa- tant ingredient of certain PBX formulations. Addi- tion descriptions for either the measured or pre- tional newer energetic crystals whose properties dicted temperature dependencies of the hot spot are being researched are CL-20, 2[C·N·NO2]3, see ignitions and their growth [13-20]. A principal con- [23], and FOX-7, [(NH2) ·C·C·(NO2)2], see [24,25]. cern of the present report is to relate to such con- Research continues on the design and synthesis siderations on two counts: (1) a dislocation defect of new energetic crystal structures and their role in providing localized sources for potential hot thermo-mechanical response on the molecular spot development; and, (2) the action of disloca- scale [26-29]. tion pile-up avalanches under mechanical loading Included among a number of other energetic in providing an important mechanism for initiation composite ingredients whose properties are of in- of fast chemical decomposition [21]. On this basis, terest, as to be described here, are so-called oxi- special importance is attributed both to the role of dizer crystals such as ammonium perchlorate (AP), dislocations in the original production of an ener- [NH4·Cl4], and the newer ingredient, ammonium getic crystal as well as to the subsequent defor- dinitramide (ADN), [NH4·N·(NO2)2]. Aluminum, in mation-induced crystal responses. polycrystalline form, is also an important additional 16 R.W. Armstrong composite ingredient for energy release by oxida- tion. Recent research effort has been directed to the increased burning rate achieved for aluminum at nanoparticle sizes [30,31]. For comparison with the larger RDX unit cell volume, the well-known aluminum face-centered cubic (fcc) unit cell vol- ume is ~ 0.064 nm3 and contains four atoms. On balance, the mixture of covalent and molecular bonding among the