Evolution of the σ phase morphology during its isothermal precipitation in the 2205 duplex stainless steel

Abstract

The present work aims at addressing the change in the σ phase morphology during its isothermal precipitation at 850 °C in a 2205 Duplex Stainless Steel (DSS) and correlate this morphology change with the evolution of the Avrami exponent n obtained using a modified Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation, which account for the time-dependence of n. The microstructural analysis depicted that the σ phase morphology evolves from a blocky-like shape at the beginning of the transformation (t < 1h50) to a choral-like shape at the end of the transformation. The modified KJMA equation, combining the hard impingement introduced using an impingement exponent c and the time-dependent Avrami exponent n, has been used to assess the observed change in the σ phase morphology. N increases to reach a first plateau at n = 1.5 approximately. Then, it increases again and reaches 2–2.3 at the end of the transformation. Very good agreement has been obtained between the experimental and the calculated transformed fraction with a coefficient of determination R2 = 0.999. The evolution of the Avrami exponent n and values it reaches correlate well with the morphological changes observed in the σ phase.