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NC State UniversityDepartment of Materials Science and Engineering 1
MSE 360: Phase Transformation and Diffusion
Instructor: Yuntian ZhuOffice: 308 RBIIPh: 513-0559
Lecture 17
NC State University
Transformation Kinetics: TTT Diagrams
Department of Materials Science and Engineering 2
Fig. 5.23 & 24p285-286
3 types of transformation:• Constant nucleation rate
• Site saturation
• Cellular transformationPrecipitationConstant nucleation rate
PrecipitationSite saturation
⍺ to btransformation
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Volume fraction vs time, f(t)
• Precipitation (f<<1, constant nucleation rate)
• Site saturation (precipitation or early stage transformation)
Department of Materials Science and Engineering 3
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
p.287
=Kt4
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Cellular transformation
Department of Materials Science and Engineering 4
f =1− exp −π3Vt
Nv3t 4"
#$
%
&'
f =1− exp −ktn( )
Johnson-Mehl-Avrami equationp.287
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Precipitation in Age-Hardening Al-Cu AlloysCrystal Structure Evolution
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GP zone θ" phase
θ� phase θ phase A fcc Alpp.288-293
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Transitional phases: Energy evolution
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GP zone: coherent, solute segragationθ� fully coherent
θ� semi-coherent
q incoherent
a0 è a1+GP zone è a2+θ�è a3+θ�èa4+θ1 2 3 4
p.291
NC State UniversityDepartment of Materials Science and Engineering 7
TTT curve
p.295
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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):p299
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Age Hardening: Mechanical properties
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p301
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Other alloy systems
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Home Work
• Reading assignment (to be quizzed at the next lecture): Ch. 5.5.5-5.8.3
• HW: none
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