Challenges in Understanding the Dynamic Behavior of Heterogeneous Materials
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microstructures when isostatically compacted and are both heterogeneous and
topologically complex. Different load configurations, namely, uniaxial stress and
uniaxial strain loading configurations, were studied to discern their effects on the
reaction response and the local mechanical response leading to a reaction event. The
compacts showed an optimum stoichiometric configuration in the uniaxial stress
case whereby reaction thresholds were minimized. However, the uniaxial strain
loading condition complicated the reaction response due to the limiting effects from
the high crush strength of the boron constituents.
3.1 Impact-Induced Chemical Reactions
Impact- and strain-induced chemical reactions in Ti+B reactive powder mixtures
will be discussed in this subsection. Uniaxial stress configurations were chosen to
discern the effects of loading configuration on the local stress/strain states produced
in the reactive compacts, and high-speed photography using an IMACON framing
camera provided in situ imaging of the impact process. The procedure for discerning
reactivity from light emission is detailed, and the implications of exogenous sources
of light emission are discussed. The experiments show an optimal stoichiometry for
enhanced reactivity under these impact loading conditions. Meso-scale simulations
are employed to investigate the possible reason behind this optimal stoichiometry.
The simulations reveal that boron agglomeration likely inhibits reactivity of pure
Ti+2B but that adding aluminum (to a degree) lessens this effect by promoting a
uniform distribution of the boron particles. The aluminum likely participates in the
reaction as well, but the reaction pathway cannot be deciphered.
Uniaxial stress loading experiments were conducted using the setup described
by Gonzales [25]. Reactive pellets were pressed and mounted onto a copper rod and
shot at different velocities from a 7.62 mm helium-driven gas gun, and the reaction
event was captured by the framing and video cameras. Figure 9 shows a time series
of snapshots from a pellet-mounted rod-on-anvil Taylor test in the uniaxial stress
configuration. The emitted light is taken as evidence of a chemical reaction event.
Plotting the observed reaction events based on a go/no-go criterion as a function
of total kinetic energy of impact shows an interesting trend. Compacts were manufactured by controlling the volume fraction of Al but maintaining a stoichiometric
ratio of Ti+B in a 1:2 molar ratio. Figure 10 demonstrates that for a similar %TMD
(theoretical maximum density) of 75% for the given mixture stoichiometries, 3 there
exists an optimal stoichiometry that reduces the threshold input energy required for
impact-induced reactivity.
The enhanced reactivity may be due to a potential liquid-phase mechanism
resulting from the plastic heating of the softer Al phase enhancing the kinetics of any
3 This final density was selected because it was the maximum density achievable the cold isostatic
pressing setup used in our lab for the pure Ti+B mixtures.
385
microstructures when isostatically compacted and are both heterogeneous and
topologically complex. Different load configurations, namely, uniaxial stress and
uniaxial strain loading configurations, were studied to discern their effects on the
reaction response and the local mechanical response leading to a reaction event. The
compacts showed an optimum stoichiometric configuration in the uniaxial stress
case whereby reaction thresholds were minimized. However, the uniaxial strain
loading condition complicated the reaction response due to the limiting effects from
the high crush strength of the boron constituents.
3.1 Impact-Induced Chemical Reactions
Impact- and strain-induced chemical reactions in Ti+B reactive powder mixtures
will be discussed in this subsection. Uniaxial stress configurations were chosen to
discern the effects of loading configuration on the local stress/strain states produced
in the reactive compacts, and high-speed photography using an IMACON framing
camera provided in situ imaging of the impact process. The procedure for discerning
reactivity from light emission is detailed, and the implications of exogenous sources
of light emission are discussed. The experiments show an optimal stoichiometry for
enhanced reactivity under these impact loading conditions. Meso-scale simulations
are employed to investigate the possible reason behind this optimal stoichiometry.
The simulations reveal that boron agglomeration likely inhibits reactivity of pure
Ti+2B but that adding aluminum (to a degree) lessens this effect by promoting a
uniform distribution of the boron particles. The aluminum likely participates in the
reaction as well, but the reaction pathway cannot be deciphered.
Uniaxial stress loading experiments were conducted using the setup described
by Gonzales [25]. Reactive pellets were pressed and mounted onto a copper rod and
shot at different velocities from a 7.62 mm helium-driven gas gun, and the reaction
event was captured by the framing and video cameras. Figure 9 shows a time series
of snapshots from a pellet-mounted rod-on-anvil Taylor test in the uniaxial stress
configuration. The emitted light is taken as evidence of a chemical reaction event.
Plotting the observed reaction events based on a go/no-go criterion as a function
of total kinetic energy of impact shows an interesting trend. Compacts were manufactured by controlling the volume fraction of Al but maintaining a stoichiometric
ratio of Ti+B in a 1:2 molar ratio. Figure 10 demonstrates that for a similar %TMD
(theoretical maximum density) of 75% for the given mixture stoichiometries, 3 there
exists an optimal stoichiometry that reduces the threshold input energy required for
impact-induced reactivity.
The enhanced reactivity may be due to a potential liquid-phase mechanism
resulting from the plastic heating of the softer Al phase enhancing the kinetics of any
3 This final density was selected because it was the maximum density achievable the cold isostatic
pressing setup used in our lab for the pure Ti+B mixtures.