Challenges in Understanding the Dynamic Behavior of Heterogeneous Materials
383
2.3 Reactive Powder Mixtures and Explosives
Microstructurally sensitive and physics-based constitutive models [5–8, 12, 15, 76]
provide greater resolution of the physical phenomena by incorporating fine-scale
detail through direct numerical simulation on the microstructure or by suitable
homogenization procedures to account for the microstructural effect on the material
behavior. They depend on meso-scale information, equations of state, and material
characteristics and properties that can be obtained from structural characterization.
Many of the original simulations have involved idealized microstructures using
simple shapes and packing routines, which do not wholly capture the nuances
of real microstructures. Recent work by Gonzales et al. [25, 27–29, 37] has
attempted to use real microstructures and synthetic representations which were
validated via two-point correlation functions [27, 29, 37] to help generate valid
synthetic microstructures. Prior work investigating energetic and reactive materials
has involved flyer-plate gas gun experiments and explosively driven flyers.
Elemental powder precursors can undergo chemical reactions under shock
compression through mechanisms that are fundamentally different from firstorder physical and chemical changes [16]. The turbulent state within the shock
wave, facilitated by the local heterogeneity within the powder, leads to extreme
deformation of particles and enhanced material mixing, which can cleanse oxidized
surfaces and lead to conditions favorable to chemical reactions [74, 75]. The natural
porosity in powder compacts (of green strength or higher) provides sites for mixing
and thermal buildup due to pore collapse during the densification process. Horie
et al. [40, 41] were the first to study intermetallic-forming powder mixtures of Ni/Al
and Ti/Al under shock compression. They observed an ordered phase of Ni 3 Al in
the high-temperature zones of the post-impact recovered material and various other
stoichiometries interdispersed within the low-temperature zones [40], as shown in
Fig. 8. The high-temperature regions were found near the periphery of the samples
due to wave interactions at the edges of these regions. They also observed a region
of a nearly homogeneous distribution of the Ni 3 Al product dispersed alongside
inhomogeneous, irregularly shaped Ni, which was partly attributed to localized
inhomogeneous distributions of starting particles. Finally, regions of NiAl and
NiAl 3 grains were also observed. The NiAl 3 phase was postulated to have formed by
precipitation from the liquid state, as the structure had the classic eutectic form [40].
Horie’s original works stimulated further research into intermetallic-forming
systems. Thadhani et al. [72–74, 77] explored a number of materials systems under
shock compression to investigate their reactivities and equations of state. Novel
manufacturing strategies to develop ordered topologies have also been explored by
Weihs and his group with Ni+Al laminates [70, 81, 82] to promote more intimate
mixing and control the seemingly random nature of conventional powder mixtures.
Meso-scale simulations provide a way to probe the structure property relations
between these topologies and the bulk shock response of intermetallic-forming
powder mixtures. The next section details a case study from the authors’ own work
on Ti+B mixtures.
383
2.3 Reactive Powder Mixtures and Explosives
Microstructurally sensitive and physics-based constitutive models [5–8, 12, 15, 76]
provide greater resolution of the physical phenomena by incorporating fine-scale
detail through direct numerical simulation on the microstructure or by suitable
homogenization procedures to account for the microstructural effect on the material
behavior. They depend on meso-scale information, equations of state, and material
characteristics and properties that can be obtained from structural characterization.
Many of the original simulations have involved idealized microstructures using
simple shapes and packing routines, which do not wholly capture the nuances
of real microstructures. Recent work by Gonzales et al. [25, 27–29, 37] has
attempted to use real microstructures and synthetic representations which were
validated via two-point correlation functions [27, 29, 37] to help generate valid
synthetic microstructures. Prior work investigating energetic and reactive materials
has involved flyer-plate gas gun experiments and explosively driven flyers.
Elemental powder precursors can undergo chemical reactions under shock
compression through mechanisms that are fundamentally different from firstorder physical and chemical changes [16]. The turbulent state within the shock
wave, facilitated by the local heterogeneity within the powder, leads to extreme
deformation of particles and enhanced material mixing, which can cleanse oxidized
surfaces and lead to conditions favorable to chemical reactions [74, 75]. The natural
porosity in powder compacts (of green strength or higher) provides sites for mixing
and thermal buildup due to pore collapse during the densification process. Horie
et al. [40, 41] were the first to study intermetallic-forming powder mixtures of Ni/Al
and Ti/Al under shock compression. They observed an ordered phase of Ni 3 Al in
the high-temperature zones of the post-impact recovered material and various other
stoichiometries interdispersed within the low-temperature zones [40], as shown in
Fig. 8. The high-temperature regions were found near the periphery of the samples
due to wave interactions at the edges of these regions. They also observed a region
of a nearly homogeneous distribution of the Ni 3 Al product dispersed alongside
inhomogeneous, irregularly shaped Ni, which was partly attributed to localized
inhomogeneous distributions of starting particles. Finally, regions of NiAl and
NiAl 3 grains were also observed. The NiAl 3 phase was postulated to have formed by
precipitation from the liquid state, as the structure had the classic eutectic form [40].
Horie’s original works stimulated further research into intermetallic-forming
systems. Thadhani et al. [72–74, 77] explored a number of materials systems under
shock compression to investigate their reactivities and equations of state. Novel
manufacturing strategies to develop ordered topologies have also been explored by
Weihs and his group with Ni+Al laminates [70, 81, 82] to promote more intimate
mixing and control the seemingly random nature of conventional powder mixtures.
Meso-scale simulations provide a way to probe the structure property relations
between these topologies and the bulk shock response of intermetallic-forming
powder mixtures. The next section details a case study from the authors’ own work
on Ti+B mixtures.