The Effects of Nonuniform Microstructures Ni/Al Reactive Laminate Sheets and Powders
Embargo until
2015-08-01
Date
2014-07-18
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Publisher
Johns Hopkins University
Abstract
This work aims to make progress in understanding the relationship between nonuniform microstructures in Ni/Al reactive systems and reactive properties. To this end, alternating Ni and Al sheets were cold-rolled into fully dense structures and had their reactivity tuned by further cutting, stacking and rerolling. In addition, some of the resulting foils were mechanically ground into mechanically processed composite powders.
The cold-rolled microstructure was extensively characterized via scanning-electron microscopy (SEM) and a ‘home-built’ MATLAB code that determined the bilayer spacing and the local Ni atomic fraction associated with each bilayer. Differential scanning calorimetry, gas pycnometry density analysis and self-propagating reaction velocity/maximum temperature determination were performed on all of the foil types. The reaction velocity and maximum temperature were found to vary with processing conditions rather than the volume average bilayer thickness, with foils that had more gradual rolling passes having a hotter and faster reaction compared to foils with more severe plastic deformation during rolling. The detailed microstructural information gathered from SEM images was used to simulate diffusion during self-propagating reaction to predict the maximum temperature. While there is a lot of variation between SEM images, we found that the distribution of bilayer thicknesses and local chemistries in the foils with gradual rolling passes have a more uniform bilayer thickness distribution, which promotes more complete reaction on the time scale of the self-propagating front and therefore a higher maximum temperature. The source for the variation in propagation velocity is currently unknown.
An analogous set of rolled foils were fabricated to investigate the propagation mechanism using high-speed cameras. However, their propagation velocities vary with bilayer, but the mechanism of propagation is similar to a previously reported one in the Al/Co system. Furthermore, the measured maximum temperatures are constant. The net velocities of the foils are shown to vary inversely with the volume average bilayer thickness, as well the local propagation rate perpendicular to that direction, which agrees with previously reported results.
The mechanically fabricated powders show variation in both physical density and DSC behavior as a function of both original foil microstructure and particle size. As the particle size is decreased, the DSC heat release decreases as Al melting and mixing becomes more prevalent, with the strongest decreases seen in particles with the coarsest microstructure. The evolved heats are also plotted as a function of the average number of bilayers in each particle and the result is that they all fall on one curve. This is an important parameter to bear in mind when using these powders for commercial applications. Furthermore, of the three particle types fabricated, one becomes Ni-rich at the smallest particle sizes; one becomes Al-rich while one shows little variation.
We attribute this variation to non-random breakup during the grinding process, resulting in the smallest particle sizes having the highest concentration of Ni-rich and Al-rich bilayers, which leads to Al melting and mixing as well as a reduction in total heat release during DSC scans. We are able to model the reduction in heat by using a simulated bimodal distribution in bilayer Ni atomic fraction taken as a subset of the total bilayer chemistry distribution, leading us to believe that our hypothesis of non-random breakup is a reasonable one.
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Keywords
Metallurgy, Thin Films