The eutec c reac on is an example of an invariant reac on Invariant reac ons refer to processes that occur where there are zero degrees of freedom F 0 For binary systems e g 2 components F 0 when three phases are in equilibrium Perhaps easier to no ce in binary systems the presence of horizontal lines are indica ve of an invariant reac on Note that in binary isomorphous systems e g Ni Cu there are no horizontal lines and no places where F 0 The iron iron carbide binary system is another example where invariant reac ons may be found 1 The Fe Fe3C system There are three invariant reac ons eutec c peritec c and eutectoid 2 Peritec c L Eutec c L Fe3C Eutectoid Fe3C 3 Things to no ce solubility of carbon in is much greater than in and Fe3C The eutectoid reac on at 727 C involves one solid transforming into two di erent solids The resul ng lamellar structure consis ng of alterna ng layers of and Fe3C and Fe3C is referred to as pearlite 4 Pearlite forma on occurs when the rela vely carbon rich phase decomposes into the carbon poor phase and carbon rich Fe3C 5 1040 Steel 0 40 wt C 6 7 Terms that come up oYen in the Fe Fe3C diagram is also ferrite is also BCC iron is also austenite is also FCC iron Fe3C is also cemen te is also iron carbide 10
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