THE EARLY STAGES OF THE PRECIPITATION SEQUENCE IN NITRIDED DILUTE FE-TI ALLOYS

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 P zone with a maximum N/Ti ratio of 3.