lecture8 - Chapter Chapter 10 10 Phase Transformations...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter Chapter 10 10 Phase Transformations Phase Transformations se s o o s se s o o s 1 Eqm Eqm. Vs Non . Vs Non-Eqm Eqm Cooling Cooling qm Eqm Cooling hase Transformations hase Transformations SUES TO ADDRESS Phase Transformations Phase Transformations ISSUES TO ADDRESS... Transforming one phase into another takes time. Fe (Austenite) Eutectoid transformation C CC Fe 3 C (cementite) errite) + CC) How does the rate of transformation depend on me nd FCC (ferrite) (BCC) time and T ? How can we slow down the transformation so that we can engineering non-equilibrium structures? Are the mechanical properties of non uilibrium Are the mechanical properties of non-equilibrium structures better? Phase Transformations Phase Transformations Nucleation Nucleation uclei (seeds) act as template to grow crystals 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, all crystals small crystals Solidification Solidification : Nucleation Processes : Nucleation Processes omogeneous nucleation Homogeneous nucleation nuclei form in the bulk of liquid metal quires percooling ypically 80- 00 max) 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 hase phase allows solidification with only 0.1-10C supercooling Homogeneous Nucleation & Energy Effects Homogeneous Nucleation & Energy Effects Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface) = 2 4 r G S g = surface tension G T = Total Free Energy = G S + G V Volume (Bulk) Free Energy stabilizes the nuclei (releases energy) 3 4 = G r G V 3 olume nit energy free volume = G r * = critical nucleus : nuclei < r * shrink; nuclei> r * grow (to reduce energy) volume unit Solidification: Critical Nucleus Size Solidification: Critical Nucleus Size T m 2 * = surface free energy r* = critical radius T H r S = H S = latent heat of solidification T m = melting temperature surface free energy Note: H = strong function of T T = T m- T = supercooling S = weak function of T d r * decreases as T increases For typical T r * ca. 100 Rate of Phase Transformations Rate of Phase Transformations inetics easure approach to equilibrium vs Kinetics- measure approach to equilibrium vs....
View Full Document

Page1 / 35

lecture8 - Chapter Chapter 10 10 Phase Transformations...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online