The influence of temperature on σ-phase formation and the resulting hardening of Fe-Cr-Mo-alloys

Hardening in Fe-Cr-Mo-alloys due to the formation of sigma-phase, has been the subject of theoretical and experimental interest. In the present investigation Fe-Cr-alloys containing 0, 2, 4 and 6% Mo were prepared and were fully transformed to the sigma-phase by isothermally annealing the samples for various periods at temperatures of 600-800 degrees C. After each annealing cycle room temperature CEMS-spectra were recorded and micro-hardness tests were performed. The micro-hardness increases with annealing time and temperature, in accordance with the fraction of sigma-phase present, and ranged from about 140 HV to 200 HV. From the measurements, activation energies were also deduced.