Critical Examination of the Representativeness of Austenite Grain Growth Studies Performed In Situ Using HT-LSCM and Application to Determine Growth-inhibiting Mechanisms

Abstract

This contribution critically addresses the potential of HT-LSCM experiments for in situ observations of austenite grain growth (AGG). By quantifying AGG for various alloys, the impact of impurity induced solute drag effects (SDE) and second phase precipitation Zener pinning forces (PZ) on AGG can be estimated. Also the grain boundary mobility (GBM) can be determined. The measured arithmetic mean of the time-resolved grain size distributions as a function of temperature and chemical composition is the most important value for quantification. The obtained data is then used to contribute to mathematical models of classical grain growth theory and to allow conclusions on parametrization of SDE and PZ. In this contribution, grain size measurements at the sample surface (in situ and ex situ) are compared with ex situ bulk measurements and experiments on grain growth in the single-phase austenite region (γ-Fe) under isothermal annealing conditions at different temperatures are presented. Grain growth results include high-purity Fe (Fe > 99.98%), binary Fe‑P, Fe‑C, and quaternary Fe-C-Nb‑N systems. For the alloys investigated, it is assumed that grain growth in high-purity Fe occurs without the influence of solute drag or precipitation mechanisms. In Fe‑P, it is shown that grain growth is inhibited by the segregation of impurity atoms at the grain boundaries (GB), which allows conclusions to be drawn about the influence of SDE. In the case of Fe-C-Nb‑N systems, the influence of Nb(C,N) precipitation on grain growth due to Zener pinning forces is presented.