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The shock-compression response of as-blended and ball-milled elemental Ni and Ti powder mixtures is investigated in this study. An 80-mm diameter single-stage gas-gun was employed to perform time-resolved measurements using piezoelectric stress gauges to monitor the stress wave profiles at the front (input) and back (propagated) surfaces of the ∼50% dense Ni+Ti powder mixture compacts. Shock-compression experiments performed on as-blended Ni+Ti powder mixtures in the range of 522 m/s to 1046 m/s impact velocities, showed characteristics of powder densification at measured input stress of 1.12 GPa, and shock-induced chemical reaction indicated by volume expansion and wave speed increase at measured input stress of 3.2 GPa. Ball-milled powder mixtures also showed similar evidence of shock-induced chemical reaction at stresses exceeding 3 GPa, with the degree of expansion depending on the energy release associated with the reaction in the powder mixtures ball-milled for various times. The measured high-pressure expanded state was observed to correspond to that calculated using a thermodynamic “ballotechnic” model for shock-induced formation of reaction products in as-blended and ball-milled powder mixtures. The results of instrumented experiments provide clear evidence of shock-induced chemical reactions occurring in Ni+Ti powder mixtures (following densification at stresses exceeding the crush strength) as evidenced by increased shock velocity and generation of an expanded state of products resulting from the associated heat of reaction. [ABSTRACT FROM AUTHOR]

Two types of microstructurally distinct ball-milled Ni+Al powder compacts are characterized for the investigation of reaction initiation threshold under high-rate mechanical loading using a modified rod-on-anvil Taylor impact-test setup. It is observed that the kinetic energy threshold for reaction decreases to a minimum then increases with milling time. It is also observed that the kinetic energy required for reaction initiation is lower for the 95% theoretical maximum density (TMD) ball-milled powder compacts than for the 65% theoretical maximum density (TMD) compacts. The results are discussed on the basis of competing effects of reactivity enhancement and deformability reduction caused by prior ball-milling of the powder mixtures. [ABSTRACT FROM AUTHOR]

A kinetics model based on mass and heat transport has been developed for Ti3SiC2 formation via shock-activated reaction synthesis of powder precursors. The model allows prediction of heat treatment conditions under which an otherwise steady-state reaction is taken over by a "run-away" combustion-type reaction during post-shock reaction synthesis of Ti3SiC2. Shock compression of Ti, SiC, and graphite precursors generates a densely packed highly activated state of reactants, which lowers the activation energy and results in an increased rate of formation of Ti3SiC2 at a lower temperature and in shorter times. The predictive model correlated with experimental results of fraction reacted as a function of time at heat-treatment temperatures of 1400 and 1600 °C illustrates an increased rate of reaction due to lowering activation energy, which also results in the reaction at 1600 °C being taken over by a "run-away" combustion-type reaction, as the rate of heat release due to reaction exceeds the rate of heat dissipation through the compact. Correlation of the model with experimental results illustrates that the predictive model can be used to optimize reaction synthesis conditions in shock-densified compacts of Ti3SiC2-forming powder precursors, to better understand the processes leading to a steady-state reaction being taken over by the combustion mode. [ABSTRACT FROM AUTHOR]