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MSE 360: Phase Transformation and Diffusion

Instructor: Yuntian Zhu Office: 308 RBII Ph: 513-0559

ytzhu@ncsu.edu

Lecture 16

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Transformation Kinetics: TTT Diagrams

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Fig. 5.23 & 24

3 types of transformation: •  Constant nucleation rate

•  Site saturation

•  Cellular transformation

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Volume fraction vs time, f(t)

•  Precipitation (f<<1, constant nucleation rate)

•  Site saturation

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V = π3Nv3t 4 ,

f = 1Vt

π3Nv3t 4

v = linear growth rate, r= vt N = nucleation rate (nuclei/unit time

€

V = N 4π3r3 = N 4π

3(νt)3

f =V /Vt = Kt 3

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Cellular transformation

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f =1− exp −π3Vt

Nv3t 4"

#$

%

&'

f =1− exp −ktn( )

Johnson-Mehl-Avrami equation

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Precipitation in Age-Hardening Al-Cu Alloys Crystal Structure Evolution

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GP zone q” phase

q’ phase q phase A fcc Al

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Transitional phases: Energy evolution

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GP zone: coherent, solute segragation

q” fully coherent

q’ semi-coherent

q incoherent

a0 è a1+GP zone è a2+q” è a3+q’ èa4+q 1 2 3 4

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a0 è a1+GP zone è a2+q” è a3+q’ èa4+q

a0 èa1+GP zone: solute atom segregation GP zone èq” : In-situ conversion of GP zone to q” q” è q’ : q” dissolve, q’ nucleate on dislocations q’ èq: q’ dissolve, q phase nucleate on GB and q’/matrix interface

TTT curve

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Quenched in vacancies

•  Form vacancy clusters •  Increase diffusion rate •  Critical vacancy supersaturation is needed for

GP zone formation

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Precipitate free zone (PFZ):

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Age Hardening: Mechanical properties

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Other alloy systems

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Home Work

•  Reading assignment : Ch. 5.5.5-5.8 •  HW: none

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