Coarsening of needle-shaped precipitates in CoNiCrAl alloys during isothermal heat treatment: Kinetics in length and thickness

This paper investigated the coarsening kinetics of needle-shaped γ precipitates in CoNiCrAl alloys after isothermal heat treatments at 900 °C and 1100 °C. The CoNiCrAl alloys were prepared by vacuum induction melting and casting process. They were characterised to exhibit a two-phase γ + β structure and some needle-shaped secondary γ precipitates were formed within the primary β phase during isothermal heat treatments. The coarsening of the needle-shaped secondary γ precipitates occurred with heat treatment time and became more rapid at elevated temperatures. In particular, the coarsening kinetics of needle-shaped γ precipitates in length and thickness were specifically studied. It was found that the coarsening kinetics in length and thickness was a volume diffusion governed process and generally followed the classic Lifshitz-Slyozov-Wagner (LSW) theory. An analytical model was proposed to estimate the lengthening and thickening of the precipitates, which considered the aspect ratio and volume fraction of the precipitates. The calculated coarsening rate in thickness agreed well with the experimentally determined value. But some discrepancies were found between the predicted and measured coarsening rate in length. This might be due to the termination migration effects of the needle-shaped precipitates which restricted the coarsening rate in length.