Phase Selection During Pulsed Laser Annealing of Fe-V Alloys

Abstract

Pulsed laser melting of the low-temperature σ (tetragonal, D8_b) phase has been used to generate a liquid undercooled with respect to the melting point of the higher-temperature, equilibrium α (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 σ phase is about 1720 K for an Fe-50 at.% V alloy, which is 54 K below the melting temperature of the α phase. During rapid heating of well-annealed σ-phase material with a 30 ns laser pulse to above melt threshold, the σ → α reaction is suppressed, so that the melt zone is undercooled by ∼ 54 K with respect to the equilibrium α phase. The α phase nucleates from the undercooled molten surface layer and is retained during the subsequent rapid cooling (∼ 10¹⁰ K/s) because of the relatively sluggish α → σ transformation. X-ray diffraction (Read camera) and TEM identified the α phase in the near-surface after melting σ with incident laser energies (1.0–1.41 J/cm²) which are well above the melt threshold as determined by changes in reflectivity (∼ 0.7 J/cm²). The α phase nucleated from the undercooled liquid within ∼ 20 ns.