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We investigate the effects of bulk composition and reactant configuration on the strain-induced reaction behavior of Ti/Al/B powder mixture compacts under uniaxial stress loading conditions. Impact experiments reveal a varying degree of reactivity as a function of Al content in the ternary mixture. Impact simulations are conducted on both real and synthetic microstructures of the same composition and with a random distribution of particles. An increase in hot-spots and strain localization is observed to occur at higher impact velocities, contributing to greater reactivity. The synthetic structures behave similarly to the real structures providing a preliminary validation of the synthetic scheme. A number of stereological metrics were used to describe the microstructural evolution during simulation as a function of time. [ABSTRACT FROM AUTHOR]

Instrumented reverse Taylor impact experiments were conducted on pure epoxy and epoxy-cast Al+Fe2O3 composites to determine the incremental stress-strain response under dynamic loading. High-speed camera images were used to measure transient (axial and areal) deformations, and velocity interferometry was used to record complex elastic and plastic wave interactions. For polymeric materials, elastic strains are generally not negligible compared to plastic strains and the rigid-plastic material behavior assumed in typical Taylor tests for metallic materials cannot be applied. Hence, in this work, a one-dimensional elastic-plastic wave propagation analysis, developed by Hutchings [J. Mech. Phys. Solids 26, 1979] to account for the appreciable elastic strains that develop before the material yields, was used to obtain stress-strain behavior for each polymer composition. The relative strengths between each composition are compared to ascertain the influence that particle reinforcement has on material properties. Post-mortem analysis for select impact velocities gave evidence of localized chemical reaction initiation, possibly triggered by bulk temperature rise above the Tg leading to softening and a series of events responsible for strain-induced reaction. [ABSTRACT FROM AUTHOR]

Shock-induced chemical reactions in ∼58% dense Mo+2Si powder mixtures were investigated using time-resolved instrumented experiments employing PVDF-piezoelectric stress gauges placed at the front and rear surfaces of the powders to measure the input and propagated stresses and wave speed through the powder mixture. Experiments performed on the powders at input stresses less than 4 GPa showed characteristics of powder densification and dispersed propagated wave stress profiles with rise time >∼40 nanoseconds. At input stresses between 4-6 GPa, the powder mixtures showed a sharp rise time (<∼10 ns) of propagated wave profiles and an expanded state of products revealing evidence of shock-induced chemical reaction. At input stresses greater than 6 GPa, the powder mixtures showed a slower propagated-stress-wave rise time and transition to a lowcompressibility (melt) state indicating lack of shock-induced reaction. The results illustrate that premature melting of Si, at input stresses less than the crush-strength of the powder mixtures, restricts mixing between reactants and inhibits "shock-induced" reaction initiation. [ABSTRACT FROM AUTHOR]

Instrumented experiments using PVDF stress gauges were employed to investigate the occurrence of shock-induced chemical reactions in ∼50% dense Ni+Ti powder mixtures. At low input stresses (∼1 GPa), the as-blended powder mixture showed characteristics of powder densification and dispersed propagated-wave stress profiles with rise-time of 36 nanoseconds. At input pressure as high as 3.22 GPa, the as-blended mixture showed a sharp rise-time (< 15 ns) of the propagated-wave profile and an expanded state of products revealing evidence of shock-induced chemical reaction. Experiments performed on the ball-milled Ni+Ti powder mixtures showed that while the powder ball-milled to the state of becoming fully alloyed, mechanically amorphized, remained inert and showed no expansion, those powder mixtures ball-milled for intermittent times underwent shock-induced reaction. The expansion due to the resulting shock-induced reaction increased with decrease in ball-milling time. These results support previous studies on other intermetallic forming systems that show similar volume expansion. [ABSTRACT FROM AUTHOR]