NC State UniversityDepartment of Materials Science and Engineering 1

MSE 360: Phase Transformation and Diffusion

Instructor: Yuntian ZhuOffice: 308 RBIIPh: 513-0559

ytzhu@ncsu.edu

Lecture 19

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Martensitic formation

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http://.blogspot.com//martensite-formation_08.hmetallu2010/04rgyfordummiestml

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Diffusionless Transformation (Martensitic transformation)

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High c% Medium c% Low c%Plate Martensite Lathe martensiteTwin Dislocation

Morphology of Martensites

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Plate (Lenticular) Martensite

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Lathe Martensite

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Characteristics of martensite

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• Surface tilting• Surface continuous• Coherent with the matrix• Grow very fast (near the

speed of sound)• Military transformation• Accompanied by a volume

expansion• Supersaturated solution• Metastable

Habit plane is the interface plane of the α’/γHabit plane is not distorted

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Thermodynamics and kinetics of martensitic transforamtion

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ΔGγ→ #α = ΔH γ→ #α T0 −MS

T0

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Effect of alloy elements

• Increased hardenability• Mechanisms

• Austenite stabilizer– Ni, Mn, Cu

• Ferrite stabilizer– Cr, Mo, Si

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Home Work

• Reading assignment (to be quizzed at the next lecture): Ch. 6.2, 6.3

• HW: none

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