Lecture_10_Phase_Transformations

# Lecture_10_Phase_Transformations - MECH 340 Engineering...

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MECH 340 Engineering Material Phase Diagrams Fe 3 C (cementite) α (ferrite) + (BCC)

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Issues To Address Transforming one phase into another takes time. How does the rate of transformation depend on time and T? How can we slow down the transformation so that we can engineering non-equilibrium structures? Are the mechanical properties of non-equilibrium structures better?
Non-Equilibrium Solidiﬁcation Insufﬁcient Time for Diffusion

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Coring C α changes as we solidify. Cu-Ni case: • Fast rate of cooling: Cored structure • Slow rate of cooling: Equilibrium structure First α to solidify has C α = 46wt%Ni. Last α to solidify has C α = 35wt%Ni. typical grain First α to solidfy: 46wt%Ni Uniform C α : 35wt%Ni Last α to solidfy: < 35wt%Ni
Eutectoid Reaction Calculate the amounts of ferrite and cementite present in pearlite. Since pearlite must contain 0.77% C, using the lever rule:

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Hypo & Hyper Eutectoid Steel
Phases in HypoEutectoid Plain Carbon Steel Calculate the amounts and compositions of phases and microconstituents in a Fe-0.60% C alloy at 726 o C. SOLUTION The phases are ferrite and cementite. Using a tie line and working the lever law at 726 o C, we ﬁnd: All of the austenite at 727 o C will have eutectoid composition (i.e., it will contain 0.77% C) and will transform to pearlite; all of the proeutectoid ferrite will remain as primary ferrite.

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Effect of Carbon on the Strength of Steel
Effect of Interlamellar Spacing ( λ )

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Eutectoid Transformation Rate ~ T γ α α α α α α pearlite growth direction Austenite ( γ ) grain boundary cementite (Fe 3 C) ferrite ( α ) γ Diffusive flow of C needed α α γ γ α Growth of pearlite from austenite: Reaction rate increases with Δ T. 675°C ( Δ T smaller) 1 10 10 2 10 3 time (s) 0 50 100 y (% pearlite) 0 50 100 600°C ( Δ T larger) 650°C % austenite
Nucleation & Growth Reaction rate is a result of nucleation and growth of crystals.

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## This note was uploaded on 02/07/2011 for the course MECH 340 taught by Professor Marwandarwiche during the Summer '07 term at American University of Beirut.

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Lecture_10_Phase_Transformations - MECH 340 Engineering...

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