Fabrication and Characterization of Cold Rolled Ni/Al Multilayer Foils
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Fabrication and Characterization of Cold Rolled Ni/Al Multilayer Foils Xiaotun Qiu1, Jesse Harris Graeter1, Laszlo Kecskes2, and Jiaping Wang1 1 Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA, 70803 2 Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, MD, 21005 ABSTRACT Ni/Al reactive multilayer foils were fabricated by a cold rolling method and selfpropagating reactions in these foils were investigated. A two-stage phase formation process was observed in the ignition experiment. The first step was the lateral growth of Al3Ni phase from isolated nucleation sites, followed by coalescence into a continuous layer. The second step was the growth of such Al3Ni layers in the perpendicular direction to the Ni/Al interface until all Al was consumed. Because there was still Ni available, Al3Ni reacted with Ni to form the final reaction product AlNi. X-ray diffraction (XRD) experiments showed that the reaction product of the cold rolled foil was the same as the physical vapor deposition (PVD) foil. The reaction process was studied by differential scanning calorimetry (DSC). Three peaks could be identified from the constant heating rate DSC scan. Subsequently, additional cold rolled foils were heated to different peak temperatures obtained from the DSC curve with the same heating rate as DSC. XRD results of these foils showed that the first two peaks corresponded to the exothermic formation of Al3Ni, while the third one corresponded to the formation of AlNi. The enthalpy of the reaction for the cold rolled foil was calculated to be -57.5 KJ/mol, in good agreement with the formation enthalpy of AlNi (-59 KJ/mol). The reaction velocities of the first formation stage were measured to be 7mm/s for the cold rolled foils. These were much smaller than the reaction velocities of the PVD foils (which range between 1-30 m/s). INTRODUCTION Over the last decade, exothermic formation reactions have been reported to selfpropagate in a variety of reactive multilayer foils, such as Ni/Al and Nb/Si foils [1-2]. These foils consist of alternating layers of reactants with a large negative enthalpy of mixing. Self-propagating formation reactions in the multilayer foils are driven by a reduction in chemical bond energy. As atoms mix normal to the layers, heat is released and conducted parallel to the layers. If atomic mixing and energy release are sufficiently fast, then the reactions are self-sustaining. Such reactions can be ignited in these foils at room temperature with a small thermal pulse. Self-propagating reactions in multilayer foils can be used as local heat sources to melt solders or brazes and thus bond components in a variety of applications [3-7]. Multilayer foils are generally fabricated by physical vapor deposition (PVD) methods, such as magnetron sputtering or electron-beam evaporation. These methods involve creating a vapor of a material, known as the source material, and then depositing the vapor onto a substrate. The rate at which the vapor is deposited is controllable,
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