Ferritic Iron-Chromium-Aluminium (Fe-Cr-Al) alloys are known for their excellent oxidation resistance and high-temperature corrosion resistance. These alloys are widely used as radiant tubes in electrical and gas-fired furnaces, thermocouple protection tubes and furnace muffles for sintering applications. Dissimilar welds of Fe-Cr-Al are encountered in several instances such as the construction of high-temperature application equipment and as coatings for high-temperature corrosion protection. Another application is weld cladding, where Fe-Cr-Al alloys are in contact with austenitic high-temperature alloys. At high temperatures, the interdiffusion in the dissimilar weld may result in precipitation of secondary phases. Thermokinetic modelling tools such as diffusion module (DICTRA) of Thermo-Calc suit of programs can be used to investigate the propensity of phase formation in dissimilar welds as a function of time to estimate the lifetime of such components. This, however, requires reliable thermodynamic and mobility databases.

Fe-Cr-Ni-Al is an important sub-system for many important engineering alloys. In the present work, phase equilibria and element diffusion profiles in this system are reported. Phase equilibria studies in the solid-state were performed on three alloys, namely, Fe-18Cr-9Ni-3Al, Fe-16Cr-18Ni-5Al and Fe-20Cr-20Ni-5Al (wt%) at 600-1100°C. Differential scanning calorimetry was used to determine the solidus and liquidus temperatures.

28 diffusion couple experiments were carried out at 800°C and 1000°C for different compositions of terminal Fe-Cr-Al alloys and Fe-Ni alloys, to validate the reliability of databases for ferrous alloys (TCFE9+MOBFE4) and Ni-base superalloys (TCNI9+MOBNI5) from Thermo-Calc Software AB. The Cr and Al contents of terminal Fe-Cr-Al alloys were varied between 16-30 wt% and 2-5 wt%, respectively. The Ni content of Fe-Ni terminal alloys was varied between 15-35 wt%. The phase formation and element diffusion profiles across the interface were characterized using EPMA. The results were compared with calculations performed using homogenization model in DICTRA using TCFE9+MOBFE4 and TCNI9+MOBNI5 databases. Several tie-lines and phase boundary information for B2-NiAl were generated and compared with the existing thermodynamic databases. This information can be used to update the thermodynamic descriptions in the Fe-Cr-Al-Ni system.