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PHASE TRANSFORMATIONS IN Cu-Zr MULTILAYERS T.P.Weihs, T.W. Barbee, Jr., and M.A. Wall, Lawrence Livermore National Laboratory, Livermore, CA 94550

ABSTRACT A study of phase transformations is reported for Cu-rich, Cu-Zr multilayer foils that were synthesized using magnetron sputter deposition and annealed using a differential scanning calorimeter. The foils range in composition from 1.6 at% to 9.0 at% Zr and consist of alternate layers of polycrystalline Cu and Zr. Differential scanning calorimetry, X-ray analysis and electron microscopy were used to examine three distinct reactions in the foils: a mixing and an amorphization of the Cu and the Zr, a crystallization to the metastable intermetallic, Cu5lZrl4, and a transformation of the Cu51Zr14 phase into the equilibrium phase, Cu9Zr2. The asdeposited layering remained stable during the first two reactions and then broke down in the third reaction as large grains of Cu9Zr2 encompassed the smaller Cu grains. The heats of the reactions and the activation energies of these reactions are measured and are compared to values reported for bulk samples. The measured heats support the observation that amorphous Cu-Zr alloys phase separate and provide evidence that mixing and short range ordering produce 3.5 times more heat than long range ordering when Cu and Zr react and form Cu51Zrl4. INTRODUCTION As applications of multilayered films extend into high temperature environments, the need to understand their structural stability increases. The transformation of phases and the break-down of layering are particularly important. This paper examines both issues for magnetron sputter deposited Cu-Zr multilayers. While the Cu-Zr system offers few applications at elevated temperatures due to the rapid oxidation of Zr, it provides useful insights as a model system. It contains a large number of stable intermetallic compounds [1]; it amorphizes in the solid state [2]; it phase separates while amorphous [3]; and it deposits readily into a layered structure [2]. Here, we investigate both the evolution of phases and the stability of layers in Curich, Cu-Zr multilayers by varying the average compositions, the layer thicknesses and the thermal histories of free-standing foils. Heats of reactions and activation energies of reactions are measured for the Cu-Zr multilayers and the results are compared with data reported in the literature for bulk alloys. EXPERIMENTAL PROCEDURES Cu and Zr were alternatively magnetron sputter deposited onto 3in and 6in (100) Si wafers at applied powers between 41W and 645W. The multilayers were approximately 26gm thick and were easily handled as free standing foils. This capability greatly facilitated thermal, structural and mechanical characterizations. The compositions of the samples, the layer thicknesses and the foil thicknesses were based on calibrated deposition parameters and are listed in Table 1. The particular thicknesses were chosen so that once the Zr reacted with the excess Cu, the remaining Cu layers would be approximately 350A thick in all sampl