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 manganese, raise the A4 and lower the A3 transformation temperatures below increasing, in effect, the extent of the gamma field in the iron-carbon phase diagram.

Fe-Mn phase diagram shows which phases are to be expected at equilibrium for different combinations of manganese content and temperature.  The melting point of iron and manganese at the pressure of 101325 Pa is 1538 °C and 1246 °C, respectively.

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