Thermodynamic Interpretation of the Martensitic Transformation in Ultrafine γ-Fe(N) Particles

Abstract

Face centered cubic (fcc) γ-Fe (5.9–10.4 at.%N) ultrafine particles ranging in size from 30 to 100 nm were prepared at room temperature by laser induced pyrolysis of Fe(CO) 5 and NH 3 . It is found that these austenitic particles did not transform to martensite when subsequently quenched to 77 K or 4.2 K, which is greatly lower than the martensite-start temperature of the bulk alloy with the same composition. Many planar defects, which can serve as the favorable martensitic nucleation sites in bulk austenite alloy, have been observed in these particles. Taking into account the size effect of the ultrafine particles on the martensitic transformation, a modification for the conventional martensitic transformation has been made to interpret the stabilization behavior of the ultrafine γ-Fe(N) particles. It is indicated that the strengthening of the particles due to the reduction of the particle size and refinement of grain size may play important roles in both inhibiting the homogeneous nucleation and martensitic transformation occurred in the ultrafine particles.