Synthesis of metastable phases via pulsed-laser-induced reactive quenching at liquid-solid interfaces

Abstract

High-power pulsed-laser-induced transformations at liquid-solid interfaces are examined with a view to synthesize new metastable phases of materials. Specifically, two types of problems are studied: (1) laser-induced synthesis of compound films at liquid-solid interfaces (called "reactive quenching" because the synthesized compound derives its atomic constituents from the participating liquid and solid systems), and (2) laser-induced alloying of layered structures under a liquid medium whose chemical participation in the process is minimal. The reactive-quenching process has been investigated for three different systems, viz., Fe:H₂O, Fe:NH₃ (liquid), and W:C₆H₆ (benzene), and the results clearly demonstrate that this process can lead to interesting possibilities of synthesis of metastable phases of materials. The identification of metastable compound phases and their microstructural transformations upon subsequent thermal annealing are investigated by using the techniques of conversion-electron Mossbauer spectroscopy, glancing-angle x-ray-diffraction measurements, Rutherford-backscattering spectrometry, x-ray-photoelectron spectroscopy, and transmission-electron microscopy. In the Fe:H₂O and Fe:NH₃ cases the as-irradiated state shows the presence of FeO and γ-Fe-N austenite, respectively, while in the W:C₆H₆ case a multiphase composite comprised of W₃C, β-W₂C, and WC_1-x is observed. Laser-induced alloying of layered structures in the liquid ambient has been studied in the case of the Fe-Al system, and it has been established that processing under liquid nitrogen leads to distinctly different results as compared to laser alloying in air or at liquid-nitrogen temperature in an inert-gas ambient. Time-resolved reflectivity measurements are carried out at the liquid-solid interface to obtain information about the possible mechanisms which could be responsible for the observed effects.