EXPERIMENTAL-DETERMINATION OF THE MISCIBILITY GAP IN HIGH-PURITY IRON-CHROMIUM ALLOYS

We have experimentally established the miscibility gap in the iron - c hromium system by studying ageing of alloys prepared from high purity materials. Samples were obtained by levitation melting. Alloys with le ss than 50% of chromium were cold worked. otherwise sheets were cut fr om the ingot. Evolution of samples with different compositions, from 1 7 to 90 %, initially quenched from 900 degrees C was followed during i sothermal annealing at different temperatures between 475 and 650 degr ees C. Electrical ratio R (He)= rho (4,2) (K) / rho (273) (K) and Vick ers hardness plots show that the kinetics are at a maximum for 35% chr omium alloys aged at 500 degrees C. For samples with a content lower t han 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 obtai ned when properties are unchanged by annealing. According to our resul ts, the limits of the unmixing zone are between 15 and 17,5% on the ir on side and 75 and 80% on the chromium side. The shape is asymmetric n ear 550 degrees C for 20% chromium and near 600 degrees C for 80% allo ys. Differential thermal analysis show that the evolution of the 80% P e -Cr alloy is complex.