Lecture 8 - 8th Ed. Ch. 10

Lecture 8 - 8th Ed. Ch. 10 - Chapter 10 Phase...

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1 Chapter 10 Phase Transformations
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Eqm. Vs Non-Eqm Cooling Eqm Cooling
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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 manipulate nonequilibrium structures? Are the mechanical properties of nonequilibrium structures better? Fe (Austenite) Eutectoid transformation C FCC Fe 3 C (cementite) (ferrite) + (BCC) Phase Transformations
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Phase Transformations Nucleation nuclei (seeds) act as template to grow crystals for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss once nucleated, grow until reach equilibrium Driving force to nucleate increases as we increase T supercooling (eutectic, eutectoid) superheating (peritectic) Small supercooling few nuclei - large crystals Large supercooling rapid nucleation - many nuclei, small crystals
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Phase Transformations Every Phase Transformation has to main steps: Nucleation Growth
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Phase Transformations : ......... total nucleation growth nucleation growth total nucleation R rate of transformation R R R Kinetically 2 processes in series if R R R R then rate limiting step is growth Virtually all diffusional trans  . ....... nucleation growth total growth formations if R R R R then rate limiting step is nucleation Virtually all martensitic transformations  Please refer to the supplemental lecture notes provided on series and parallel processes in kinetic. Study them thoroughly. The concepts contained therein are very fundamental which you’d make use in all aspects in your engineering careers
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Ex: Solidification: Nucleation Processes Homogeneous nucleation nuclei form in the bulk of liquid metal requires supercooling (typically 80-300°C max) Heterogeneous nucleation much easier since stable “nucleus” is already present Could be wall of mold or impurities in the liquid phase allows solidification with only 0.1-10ºC supercooling
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Homogeneous Nucleation & Energy Effects 2 4 r G S G r G V 3 3 4 * 32 4 3 4 0 v r G r G r G critical stable nucleus size r     
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r * = critical nucleus : nuclei < r * shrink; nuclei> r * grow (to reduce energy) Homogeneous Nucleation & Energy Effects G T = Total Free Energy = G S + G V Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface) 2 4 r G S g = surface tension Volume (Bulk) Free Energy stabilizes the nuclei (releases energy) G r G V 3 3 4 volume unit energy free volume G
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Homogeneous Nucleation & Energy Effects * 32 4 3 * * 22 4 0 () 2 .
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This note was uploaded on 02/20/2012 for the course MSE 14:635:407 taught by Professor Dr.e.korayakdoğan during the Spring '12 term at Rutgers.

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Lecture 8 - 8th Ed. Ch. 10 - Chapter 10 Phase...

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