Shock-induced reaction synthesis (SRS) of nickel aluminides
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R. A. Graham and W. F. Hammetter Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 29 July 1991; accepted 8 January 1992)
Shock-induced chemical reactions between nickel and aluminum powders (mixed in Ni3Al stoichiometry) are used for the synthesis of nickel aluminides. It is shown that the extent of shock-induced chemical reactions and the nature of the shock-synthesized products are influenced by the morphology of the starting powders. Irregular (flaky type) and fine morphologies of the powders undergo complete reactions in contrast to partial reactions occurring in coarse and uniform morphology powders under identical shock loading conditions. Furthermore, irregular morphology powders result in the formation of the equiatomic (B2 phase) NiAl compound while the Ni3Al (Ll 2 phase) compound is the reaction product with coarse and regular morphology powders. Shock-induced reaction synthesis can be characterized as a bulk reaction process involving an intense "mechanochemical" mechanism. It is a process in which shock compression induces fluid-like plastic flow and mixing, and enhances the reactivity due to the introduction of defects and cleansing of particle surfaces, which strongly influence the synthesis process.
I. INTRODUCTION
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Nickel aluminides are considered to be important structural materials because of their many attractive features such as lower densities, higher melting temperatures, and excellent isothermal creep resistance.1^6 The Ni-Al system contains four major intermetallic compounds, NiAl 3 , Ni 2 Al 3 , NiAl, and Ni 3 Al. The phase diagram and heats of formation of different phases are shown in Fig. I. 7 8 The most aluminum-rich of these, NiAl 3 , melts peritectically at 854 °C, while the equiatomic NiAl compound has the highest melting temperature (1638 °C). The nickel-rich Ni3Al compound, on the other hand, melts at 1395 °C. Additionally, certain nickel-rich alloys also undergo a martensitic transformation when quenched to room temperature from the high temperature beta phase which gives rise to a shape memory effect.9'10 In the present work shock-induced chemical reactions between elemental powder mixtures were used for the synthesis of nickel aluminides. Shock-compression loading of a porous material produces high pressures and significant increases in temperatures over a duration of a few microseconds.11^13 These high pressures and accompanying mass velocities can cause severe plastic deformation with relative motion between particles, or even fracture of individual particles. Extensive mechanical mixing, surface cleansing, and opening of fresh surfaces occur under these conditions.11 Thus, it is expected and is also observed that shock compressed materials undergo not only interparticle bonding between powder particles, but chemical reactions and molecular J. Mater. Res., Vol. 7, No. 5, May 1992 http://journals.cambridge.org
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At. % Nickel
(Nj
298 OK
Al 3 Ni o
10 Al 3 Ni 2 14
20
40
60
80
100
At.% Ni FIG. 1. Equilibrium phase diagr
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