Phase Selection During Pulsed Laser Annealing of Fe-V Alloys
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PHASE SELECTION DURING PULSED LASER ANNEALING OF Fe-V ALLOYS J. H. PEREPEZKO*, D. M. FOLLSTAEDT** and P. S. PEERCY** *University of Wisconsin, 1509 University Avenue, Madison, WI 53706 "**Sandia National Laboratories, Albuquerque, NM 87185-5800 ABSTRACT Pulsed laser melting of the low-temperature a (tetragonal, D8b) phase has been used to generate a liquid undercooled with respect to the melting point of the higher-temperature, equilibrium a (bcc) solid solution in equiatomic Fe-V alloys. From calculations based on reported thermodynamic data and equilibrium transformation temperatures, the metastable melting point of the a phase is about 1720 K for an Fe-50 at.% V alloy, which is 54 K below the melting temperature of the a phase. During rapid heating of well-annealed a-phase material with a 30 ns laser pulse to above melt threshold, the a + a reaction is suppressed, so that the melt zone is undercooled by - 54 K with respect to the equilibrium a phase. The a phase nucleates from the undercooled molten surface layer and is retained during the subsequent rapid cooling (- 10 K/s) because of the relatively sluggish a + a transformation. X-ray diffraction (Read camera) and TEM identified the a phase in the n$ar-surface after melting a with incident laser energies (1.0-1.41 J/cm ) which are well abovF the melt threshold as determined by changes in reflectivity (- 0.7 J/cm ). The a phase nucleated from the undercooled liquid within - 20 ns. INTRODUCTION A novel approach to the study of nucleation of phases during liquid quenching has been developed using the rapid resolidification conditions following pulsed-laser melting [1,2]. The strategy of the method is based upon the use of metals with more than one structure in the solid-state and has been demonstrated in previous experiments on pure manganese. The metal is equilibrated initially in a low-temperature phase which has a melting temperature that can be determined by thermodynamij calculations [3]. The melting temperature of the H'ow-temperature phase, TM, is below that for the high temperature phase, TM. During ultrafast pulsed laser heating, solidstate transformation of the low-temperature phase into the high temperature phase can be suppressed to melt the laser-heated surface when the temperature reaches T . After the laser pulse, heit flows from the liquid into the solid substrate and the liquid cools to TM. The resulting liquid layer is undercooledHwith respect to the equilibrium (i.e. high-temperature) solid phase by AT=T -T . If the melt duration and undercooling level are sufficient, the high-temperature phase may nucleate from the undercooled liquid and be retained during subsequent rapid quenching to low temperatures. In general, favorable kinetic conditions would involve a rapid nucleation rate for the high-temperature phase so that it can dominate the resolidification microstructure before regrowth of the low-temperature substrate phase consumes the melt zone. In previous studies with manganese, which has four allotropes, substrates of both a-Mn and 8-Mn w
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