The early stages of the precipitation sequence in nitrided dilute fe-ti alloys

Abstract

The early stages of the precipitation sequence in nitrided dilute Fe-Ti alloys were investigated by internal friction measurement and electron microscopy. A maximum nitrogen-to-titanium (N/Ti) atom ratio of approaching 3 was achieved prior to the formation of stable titanium nitride. A Ti-N clustering process precedes the formation of the substitutional-interstitial Ti-N Guinier-Preston (GP) zone, characterized by the evolution of two abnormal internal-friction peaks, P₁ (about 504 K) and P₂ (ranging from about 373 to about 433 K), and changes in the electron diffraction patterns during the pre-precipitation process. The stages are indicated by the reduction and disappearance of P₁ concomitant with the enhancement of P₂, and then the reduction and disappearance of P₂ with corresponding occurrence and disappearance of the <100> side bands, and then the occurrence of <100> streaks. P₁ is due to isolated N-Ti-N triplets and P₂ is due to segregating Ti-N clusters. The mechanism of this pre-precipitation is probably spinodal clustering. The early stages of the precipitation sequence can be depicted as follows: titanium atom absorbing one nitrogen atom → isolated Ti-N pair absorbing another nitrogen atom → isolated N-Ti-N cluster absorbing nitrogen and decomposing spinodally → monolayer Ti-N GP zone with a maximum N/Ti ratio of 3.