Phase Diagrams
Iron-Chromium (Fe-Cr) Phase Diagram
In pure iron, the A4 (1394 °C) and A3 (912 °C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the Phase Rule to occur over a range of temperature.
Some elements, such as chromium, lower the A4 and raise the A3 transformation temperatures, restricting the gamma loop (γ loop) in the iron-carbon phase diagram. As the binary iron-chromium phase diagram shows, the presence of chromium restricts the gamma loop (Figure 1).
Notice that above approximately 13 wt. % Cr, the binary Fe-Cr alloys are ferritic over the whole temperature range. A narrow (α + γ) range that exists between approximately 12 wt. % Cr and 13 wt. % Cr is also worth noting.
The addition of carbon to the Fe-Cr binary system widens the (α + γ) field and extends the gamma-loop to higher chromium contents (see, for example, the Fe-Cr-C ternary system at 1000 °C and 1100 °C).
Figure 1. Fe-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature. The Fe-Cr phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database. The melting point of iron and chromium at the pressure of 101325 Pa is 1538 °C and 1907 °C, respectively.
The sigma (σ) phase, which is an intermetallic FeCr compound, can sometimes form in Fe-Cr alloys, such as AISI 316 or AISI 310 stainless steels. The harmful effects of the sigma phase on mechanical properties (e.g., ductility) and corrosion resistance are well documented.
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• The phase diagram of Iron-Chromium binary system
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