Rapid E-Beam Heating for Measuring Thermodynamics of Metastable Materials
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RAPID E-BEAM HEATING FOR MEASURING THERMODYNAMICS OF METASTABLE MATERIALS J. A. KNAPP AND D. M. FOLLSTAEDT Sandia National Laboratories, Albuquerque,
New Mexico 87185
ABSTRACT A line-source electron-beam system has been used to heat thin surface of metastable phases at a rate which precludes solid-state layers transformations to stable phases, thus permitting the observation of melting transitions normally missed with slow heating. A detailed example of a new shown for metastable icosahedral Al-Re and. approach to this method is crystalline Al 6Re. INTRODUCTION A major challenge in the study of metastable materials is the determination of high-temperature thermodynamic properties, since heating normally results in transformation of the metastable phase of interest to more stable phases. One method of deducing high temperature properties such as melting temperatures is to heat the material at a rate which precludes solid-state transformations. Such rapid heating is easily accomplished with a pulsed laser, and is the basis for studies such as the thermodynamic evaluation of amorphous Si.[l] However, for typical laser pulses of 100 ns or less, the thermal and optical properties of the treated material are important parameters in the calculation of its temperature history. Consequently, real-time property changes, such as reflectivity and conductivity, must be measured during the shot for accurate results.[l] An alternative method is to deposit a thin surface layer of the material of interest on a substrate with known properties and then do a heat treatment which is sufficiently slow and extends deep enough that the thermal properties of the surface layer may be neglected, but still fast enough to bypass solid-phase transformations. The latter approach has been the basis for our studies of the metastable quasicrystalline phases.[2]
EXPERIMENTAL METHOD The line-source electron-beam annealing system [3] we have been using for these measurements is shown in Fig. 1; it provides a sheet beam of electrons focussed to a line 1 x 20 mm at the sample, with a gaussi~n profile in the narrow dimension and a peak power density up to -100 kW/cm-. Samples are mounted on a rotating table and swept under the beam at speeds up to 5000 cm/s. The beam is pulsed on for up to 2 ms duration when the sample is in position. Beam power parameters and sweep speed are measured under computer control; beam profiles are reproducible from shot to shot within a few percent. Peak 6 temperatures of 1500-2000°C and heating and cooling rates in excess of 2x10 K/s are easily achieved. Thin surface layers of the phase of interest are formed on a convenient, inert substrate whose thermal properties are well known. For the icosahedral phases of Al-Mn, Al-Ru and Al-Re a sapphire substrate was used. The icosahedral phases were formed in the solid state by multi-layer deposition followed by ion beam mixing.[4,5] The surface layer thickness of 50-100 nm is much less than the characteristic thermal diffusion length (>10 pm) for the e-beam dwell times used in t
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