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.

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We have used self-propagating exothermic reactions in Al/Ni multilayers as a means to explore the effect of rapid heating on phase transformations. Using time-resolved synchrotron x-ray microdiffraction with an extremely fast detector, we were able to examine the reaction sequence in detail at heating rates of ϳ10 6 K s −1. We observed that the intermediate phases formed during the self-propagating reactions are different from those formed at lower heating rates, even though the final phases are the same. In situ characterization is essential, as other means of studying self-propagating reactions ͑such as quenching the reaction followed by ex situ analysis͒ provide different-and potentially misleading-results.

An instrument capable of performing nanoindentation at temperatures up to 500 o C in inert atmospheres, including partial vacuum and gas near atmospheric pressures, is described.Technical issues associated with the technique (such as drift and noise) and the instrument (such as tip erosion and radiative heating of the transducer) are identified and addressed. Based on these considerations, preferred operation conditions are identified for testing on various materials. As a proof-of-concept demonstration, the hardness and elastic modulus of three materials are measured: fused silica (non-oxidizing), aluminum and copper (both oxidizing). In all cases, the properties match reasonably well with published data acquired by more conventional test methods.2

The influence of temperature on the indentation size effect is explored experimentally. Copper is indented on a custom-built high-temperature nanoindenter at temperatures between ambient and 200 °C, in an inert atmosphere that precludes oxidation. Over this range of temperatures, the size effect is reduced considerably, suggesting that thermal activation plays a major role in determining the length scale for plasticity.