Exothermic reactions in cold-rolled Ni/Al reactive multilayer foils
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L. Kecskes United States Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005
J. Wanga) Department of Physics and Tsinghua–Foxcoon Nanotechnology Research Center, Tsinghua University, Beijing 100084, China; and Department of Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 3 April 2007; accepted 25 September 2007)
Exothermic reactions in cold-rolled Ni/Al reactive multilayer foils were investigated in this study. A two-stage reaction process was observed in the self-propagating reactions in the cold-rolled foils that were ignited by a point-source flame. Foils taken out of the flame after completing the first stage of the reaction process were compared to those allowed to complete both stages. Differences in the phase-evolution sequence from the two types of foils were studied by differential scanning calorimetry (DSC), using slow and controlled heating of the samples. Several exothermic peaks could be identified from the DSC thermograms for both types of foils. Using the DSC, both the as-cold-rolled and partially reacted foils were heated to each peak temperature to identify the reaction product associated with each peak. X-ray diffraction and scanning electron microscopy analyses showed that the first two peaks corresponded to the formation of Al3Ni, while the third peak corresponded to the formation of AlNi. I. INTRODUCTION
Over the last decade, exothermic formation reactions have been reported to self-propagate in a variety of reactive systems, such as Ni/Al, Ti/Al, or Nb/Si multilayered foils.1–3 These materials consist of alternating layers of reactants with a large negative enthalpy of mixing. The self-propagating high-temperature synthesis (SHS) reaction in these multilayers is driven by a reduction in chemical-bond energy. The local reduction in chemicalbond energy produces a large quantity of heat that is conducted down the multilayers and facilitates further atomic mixing and compound formation. If atomic mixing and energy release occur sufficiently fast, compared to heat losses, then the reactions are self-sustaining. SHS reactions can be initiated in these multilayers at room temperature with a small thermal pulse. Such exothermic reactions in multilayers can be used as local heat sources to melt solders or brazes and thus join components in a variety of applications, such as making stainless steel– stainless steel, Al–Al, Ti–Ti, SiC–Ti, Si–Si, and bulk metallic glass (BMG)–BMG joints.4–10 Reactions in the Ni/Al multilayer system have been
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0043 J. Mater. Res., Vol. 23, No. 2, Feb 2008
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extensively investigated both theoretically and experimentally. Reactive multilayers are generally fabricated using physical vapor deposition (PVD) methods, such as magnetron sputtering or electron-beam evaporation. In PVD multilayers, Ma et al.11 performed calorimetric measurements on Ni/Al multilayer films p
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