Glass Forming Ability in Transition Metal Films at 10 10 K/s
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GLASS FORMING ABILITY IN TRANSITION METAL FILMS AT 1010
K/s
M. VON ALLMEN, K. AFFOLTER Institute of Applied Physics, University of Bern, (C1-3012 Bern, Switzerland; * Present adress: California Institute of Technology, 116-81, Pasadena CA 91125, USA. ABSTRACT Glass formation by ns and ps laser quenching is investigated for the systems Au-Ti, Co-Ti, Cr-Ti and Zr-Ti. Glassy phases are found in all but the last system. These results (as well as earlier ones) show little correlation with published rules to predict glass forming ability, but are consistent with the kinetic analysis presented. Simple chysical parameters related to glass forming ability are discussed. Comparison with results for Cr-Ti allows a determination of the nucleation time-lag. INTRODUCTION Laser auenching with ns or ps pulses has been shown to be a powerfull method to produce metallic glasses and other metastable phases [1]. Thinfilm samples irradiated by ns pulses typically experience cooling rates of 1010 K/s (as inferred from numerical calculations), about four orders of maonitude more than achievable by mechanical quenching. This results in a greatly extended choice of material systems and compositions available in the classy state. New criteria to predict glass forming ability (GFA) of metallic materials at the cooling rates typical for laser quenching are therefore needed. In what follows we present recent experimental results with laser-quenched binary transition-metal films. We analyze these data from the point of view of predicting GFA from available material data. EXPERIMENTAL The samples were about 1000 A thick vaDor-deposited multilayers on sapphire substrates, prepared as described earlier [2]. Film compositions in the systems Au-Ti and Co-Ti were chosen to be in two-phase regions of the respective phase diagrams; the Cr-Ti and Ti-Zr systems are continous solid-solution formers (for details see below). Irradiation was mostly performed with 30 ns pulses from a Q-switched Nd-YAG laser, in some cases also with 90 ps pulses from a mode-locked Nd-YAG laser. In order to determine the correct level of irradiance a series of samples for each composition was irradiated at fluences ranging from the melt-threshold to the damage threshold. The structure of as-quenched films was investigated by x-ray diffraction using a Guinier-type thin film camera. Samples were then subjected to a series of 30 min. anneals in a vacuum furnace at orogressively increased temperature, each followed by x-ray analysis. We define an amorphous-crystalline transition temperature S Tac as the lowest annealing temperature leading to extra diffraction lines. The following is a short overview of our results as far as relevant for the present discussion. For more details see [3]. Au-Ti system Samples of composition Au3 5 Ti 6 5 proved to be ready glass formers by ns irradiation. However, as-irradiated films showed a few weak diffraction lines indicative of Ti precipitation, in addition to a broad halo. The Mat. Res. Soc. Symp. Proc. Vol. 28 (1984) CElsevier Science Publish
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