Formation and stability of metastable structures and amorphous phases in PU-V, PU-TA, and PU-YB systems with positive he
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INTRODUCTION
V A P O R quenching techniques such as sputterdeposition or thermal evaporation are being used increasingly for exploration of alloying behavior and phase formation, particularly at low temperatures. Under such conditions, nonequilibrium crystalline alloy phases or amorphous phases are frequently observed. Sputtering from a "split target" makes it possible to produce alloy film deposits that change continuously in composition along a single sample. [2j This in turn allows the exploration of wide composition ranges in a single experiment. This approach is especially helpful with highly reactive systems such as those involving Pu because it eases the problems of handling and characterization. Work with vapor deposition has demonstrated that single-phase regions of primary solid solutions can be greatly extended beyond the regions indicated by equilibrium phase diagrams. These metastable extensions are often followed by wide ranges of amorphous phases. It has been shown that the compositional extent of the amorphous phases can be correlated with the atomic size differences of the elements involved 13] and with the H.F. RIZZO, Research Associate, and A. ECHEVERRIA, Senior Technical Associate, are with Lawrence Livermore National Laboratory, Livermore, CA 94550. T. ZOCCO and M. NASTASI, Technical Staff Members, are with Los Alamos National Laboratory, Los Alamos, NM 87545. T.B. MASSALSKI, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA. Manuscript submitted January 5, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
thermodynamic considerations t4-Tj related to the relative stability of the competing phases, as manifested by the heats of formation (A/-//). Superimposed on these factors are of course the usual kinetic considerations that determine the rate of approach to equilibrium. I41 Previous investigations of Pu systems, using the split-target sputtering technique, have shown wide regions for the formation of amorphous and metastable phases. [8-1~ Of particular interest in this study is the behavior of Pu-based alloy systems that show little or no equilibrium miscibility. Such systems are characterized by large and positive heats of mixing and atomic size differences between the component atoms of more than 10 pct. Experiments involving solid-state amorphization occurring between codeposited crystalline films [61 and similar explorations [1~1 involving ion-beam mixing suggested that amorphous phases are to be expected mainly when the z3d-//is negative. However, amorphous phase formation has also now been reported in several systems with positive M-//.l~2J In this work, the triode sputtering technique has been used to produce metastable crystalline and amorphous phases in the systems Pu-V, Pu-Ta, and Pu-Yb. The calculated phase diagrams for these systems, based on heats of formation derived from the semiempirical Miedema model tq all indicate liquid and solid immiscibility, as illustrated in Figure 1 for the Pu-Yb system, t~3~The fo
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