We showed how intermetallic formation reactions can be studied under rapid heating ͑10 6 -10 7 K s −1 ͒ using x-ray microdiffraction with temporal resolution on microsecond time scales. Rapid heating was achieved by initiating an exothermic reaction in multilayer foils comprising alternating nanoscale layers of elemental metals. The reaction occurred in a front ϳ100 m wide which propagated across the foil at ϳ1-10 m s −1 . By using synchrotron x-rays focused to a small spot ͑60 m diameter͒ and a fast pixel-array detector, we were able to track the evolution of phases in the reaction front during the initial heating transient, which occurred in approximately 1 ms, through cooling over a period of hundreds of milliseconds. In Al/Ni multilayer foils, the first phases to form were an Al-rich liquid and the cubic intermetallic AlNi ͑which likely formed by nucleation from the liquid͒. In foils of overall composition AlNi, this is the stable intermetallic and the only phase to form. In foils of composition Al 3 Ni 2 , during cooling we observed a peritectic reaction between AlNi and the remaining liquid to form Al 3 Ni 2 , which is the stable phase at room temperature and the final product of the reaction. This is in contrast to the sequence of phases under slow heating, where we observed formation of nonequilibrium Al 9 N 2 first and do not observe formation of a liquid phase or the AlNi intermetallic. We also observed formation of an amorphous phase ͑along with crystalline ZrNi͒ during rapid heating of Zr/Ni multilayers, but in this system the temperature of the reaction front never reached the lowest liquidus temperature on the Zr-Ni phase diagram. This implies that the amorphous phase we observed was not a liquid arising from melting of a crystalline phase. We suggest instead that a Zr-rich amorphous solid formed due to solid-state interdiffusion, which then transformed to a supercooled liquid when the temperature exceeded the glass transition temperature. Formation of the supercooled liquid presumably facilitated continued rapid intermixing, which may be necessary to sustain a self-propagating reaction front in this system.

We report on intermetallic formation reactions in vapor-deposited multilayered foils of Ni/Zr with 70 nm bilayers and overall atomic ratios of Ni:Zr, 2 Ni:Zr, and 7 Ni:2 Zr. The sequence of alloy phase formation and the stored energy is evaluated at slow heating rates (∼1 K/s) using differential scanning calorimetry traces to 725 °C. All three chemistries initially form a Ni–Zr amorphous phase which crystallizes first to the intermetallic NiZr. The heat of reaction to the final phase is 34–36 kJ/mol atom for all chemistries. Intermetallic formation reactions are also studied at rapid heating rates (greater than 105 K/s) in high temperature, self-propagating reactions which can be ignited in these foils by an electric spark. We find that reaction velocities and maximum reaction temperatures (Tmax) are largely independent of foil chemistry at 0.6±0.1 m/s and 1220±50 K, respectively, and that the measured Tmax is more than 200 K lower than predicted adiabatic temperatures (Tad). The difference between Tmax and Tad is explained by the prediction that transformation to the final intermetallic phases occurs after Tmax and results in the release of 20%–30% of the total heat of reaction and a delay in rapid cooling.

A pair of techniques have been developed for performing time-resolved X-ray microdiffraction on irreversible phase transformations. In one technique capillary optics are used to focus a high-flux broad-spectrum X-ray beam to a 60 mm spot size and a fast pixel array detector is used to achieve temporal resolution of 55 ms. In the second technique the X-rays are focused with Kirkpatrick-Baez mirrors to achieve a spatial resolution better than 10 mm and a fast shutter is used to provide temporal resolution better than 20 ms while recording the diffraction pattern on a (relatively slow) X-ray CCD camera. Example data from experiments are presented where these techniques are used to study self-propagating high-temperature synthesis reactions in metal laminate foils.

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