Characterization of the γ-loop in the Fe-P system by coupling DSC and HT-LSCM with complementary in-situ experimental techniques

Solid-state phase transformations in the ?-loop of the binary Fe-P system were studied using differential scanning calorimetry (DSC) and high-temperature laser scanning confocal microscopy (HT-LSCM). In total, eight alloys with varying P content from 0.026 to 0.48 mass pct. P were investigated in the temperature range of 800 ?C to 1450 ?C. The first part of the present work deals with the critical evaluation of the approach to couple DSC experiments and HT-LSCM observations in order to characterize bcc/fcc phase equilibria in Fe-based ?-loops. The phase transformation temperatures of a selected alloy with 0.394%P were analyzed by DSC and HT-LSCM and compared with results of the well-established techniques of dilatometry and high-temperature X-ray diffraction (HT-XRD). Then, the overall phase boundaries of the ?-loop were reconstructed by HT-LSCM and DSC data and the phase diagram was compared with thermodynamic assessments from literature. Finally, the quantitative phase fractions of fcc and bcc at 0.394%P were analyzed by Rietveld refinement at temperatures of 1050 ?C, 1100 ?C and 1150 ?C using in-situ HT-XRD. Although the phase boundaries of the ?-loop and phase transformation temperatures have been reproduced accurately by recently published thermodynamic optimizations, larger deviations between HT-XRD measurements and the calculations were identified for the phase fraction prediction. The present work clearly demonstrates that coupling DSC and HT-LSCM is a powerful tool to characterize ?-loops in steel for future research work.