Investigation on the martensitic transformation in γ-Fe(N) nanoparticles

γ-Fe(5.90-10.9 at.% N) nanoparticles ranging in diameters from 30 to 100 nm were prepared by laser-induced pyrolysis of mixtures of Fe(CO)₅ and NH₃. During the quenching from high temperature to room temperature and even down to 4.2 K, no martensitic transformation occurred in these particles. Based on the effects of surface tension and refinement of grain size on the yield strength of the nanoparticles, the dependencies of nucleation driving force and the martensite-start temperature on the particle size was revealed,from a viewpoint of thermodynamics of martensitic transformation in the γ-Fe(N) nanoparticles. It is suggested that the yield strength increment due to the surface tension and to the very fine crystallite grains is probably the predominant aspect responsible for inhibiting the nucleation of martensite and drastic decrease of the martensite-start temperature of the nanoparticles. High pressure of 0.5 - 4.0 GPa were adopted to induce the martensitic transition of the γ-Fe nanoparticles. It is found that the extent of martensitic transformation increased with the increasing pressures, most individual γ-Fe particles were partially transformed to martensite after pressurization.