Rapid e-beam heating for studying metastable transitions in Mn
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INTRODUCTION
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An understanding of thermodynamic transitions involving metastable phases is increasingly important as rapid heating and quenching are used to search for materials with new properties. Such basic thermodynam!c information as metastable transition temperatures between phases can be difficult to obtain by conventional experimental means, although in some cases it can be accurately calculated.1 We have developed a novel experimental approach to study the thermodynamics of metastable phases.2 Thin, deposited layers of the material of interest on a sapphire substrate are heated in the time scale of 100-200 pts with a line-source e-beam system.3 Heating and subsequent cooling rates greater than 106 K/s can be achieved. Examining changes in microstructures after pulsed heating to a range of known (calculable) peak temperatures allows transformation temperatures to be identified, and in certain cases, relative transformation kinetics can be deduced. Here, we examine transitions in pure Mn. Manganese was chosen for study because it has four equilibrium allotropes (with increasing temperature: a, /3, y, and 8) and the thermodynamics of its equilibrium transitions are well known, permitting metastable transition temperatures to be calculated accurately. Equilibrium stability ranges for these phases are shown on the left of Fig. 1, and metastable transitions calculated by Perepezko1 in the middle. Unlike our previous work with the metastable icosahedral Al-Mn phase,2 where all solid-state transitions were suppressed during our rapid heating to the melting point, such transitions did occur with pure Mn and thus direct melting of the initial a phase was not detected. We will show that the solid-state a —» /3 transition was suppressed, and the metastable a —> y transition occurred instead at a temperature which agrees with the calculated value. II. EXPERIMENTAL Rapid heating was done with a line-source electronbeam annealing system3 which focuses a sheet beam of electrons to a line 1 X 20 mm at the sample table. Samples are moved under the beam on this rotating table at speeds up to 5000 cm/s. Pulsing the beam on while the
Equilibrium Transitions
Metastable Transitions
Observed Boundaries
Inferred Transitions liquid
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I 1270 I ±30°C
1 T 6
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I 1150 I ±30°C
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i i
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• a "^8 a~y
a—p 600 -
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FIG. 1. Schematic showing the equilibrium phases of Mn, the calculated metastable transition temperatures, experimentally observed transition temperatures, and the inferred phase transitions.
sample is moving under the beam treats an area whose width is equal to (sample speed) x (pulse time). The sweep speeds used above of —100 cm/s are much faster than thermal diffusion along the sweep direction, which simplifies the calculation of the sample's temperature history: the heat flow is essentially one-dimensional (into the substrate) and the gaussian profile of the beam in its narrow dimension gives an effective power input which is gaussian in tim
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