Experimental Determination of the Miscibility Gap in High Purity Iron-Chromium Alloys

Abstract
We have experimentally established the miscibility gap in the iron - chromium system by studying ageing of alloys prepared from high purity materials. Samples were obtained by levitation melting. Alloys with less than 50% of chromium were cold worked, otherwise sheets were cut from the ingot. Evolution of samples with different compositions, from 17 to 90 %, initially quenched from 900° C was followed during isothermal annealing at different temperatures between 475 and 650°C. Electrical ratio R He =p_4.2K/p_273K and Vickers hardness plots show that the kinetics are at a maximum for 35% chromium alloys aged at 500° C. For samples with a content lower than 66 % ageing produced an increase of the electrical ratio R_He and hardness. At 66% there is an instability in R_He, whereas for 80%, it decreases. The limit of the miscibility gap is considered to be obtained when properties are unchanged by annealing. According to our results, the limits of the unmixing zone are between 15 and 17,5% on the iron side and 75 and 80% on the chromium side. The shape is asymmetric near 550°C for 20% chromium and near 600°C for 80% alloys. Differential thermal analysis show that the evolution of the 80% Fe -Cr alloy is complex.