Phase Transformations Nucleation nuclei seeds act as templates on which crystals grow for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss once nucleated growth proceeds until equilibrium is attained Driving force to nucleate increases as we increase T supercooling superheating Small supercooling Large supercooling 1 Solidification Nucleation Types Homogeneous nucleation Heterogeneous nucleation 2 Homogeneous Nucleation Energy Effects 3 Solidification 2 Tm r Hf T r critical radius surface free energy Tm melting temperature Hf latent heat of solidification T Tm T supercooling Note Hf and are weakly dependent on T r decreases as T increases For typical T r 10 nm 4 Rate of Phase Transformations Kinetics study of reaction rates of phase transformations To determine reaction rate measure degree of transformation as function of time while holding temp constant How is degree of transformation measured X ray diffraction many specimens required electrical conductivity measurements on single specimen measure propagation of sound waves on single specimen 5 Fraction transformed y Rate of Phase Transformation Fixed T 0 5 t0 5 log t Avrami equation y 1 exp kt n fraction transformed time k n are transformation specific parameters By convention rate 1 t0 5 6 Temperature Dependence of Transformation Rate 135 C 119 C 1 10 113 C 102 C 88 C 102 43 C 104 For the recrystallization of Cu since rate 1 t0 5 rate increases with increasing temperature Rate often so slow that attainment of equilibrium state not possible 7 Transformations Undercooling a Fe3C Eutectoid transf Fe Fe3C system 0 76 wt C 6 7 wt C For transf to occur must 0 022 wt C cool to below 727 C i e must undercool T C 1600 d 1200 L Fe3C 1148 C 1000 Fe3C Eutectoid Equil Cooling Ttransf 727 C 800 727 C 400 0 Fe T a Fe3C Undercooling by Ttransf 727 C 0 76 600 0 022 a ferrite L austenite 1 2 3 4 5 6 Fe3C cementite L 1400 6 7 C wt C 8 The Fe Fe3C Eutectoid Transformation Transformation of austenite to pearlite a a a a a a For this transformation rate increases with Teutectoid T i e T cementite Fe3C Ferrite a a pearlite growth direction a a 100 y pearlite Austenite grain boundary Diffusion of C during transformation 600 C T larger 50 Carbon diffusion 650 C 675 C T smaller 0 Coarse pearlite formed at higher temperatures relatively soft Fine pearlite formed at lower temperatures relatively hard 9 Generation of Isothermal Transformation Diagrams Consider y transformed The Fe Fe3C system for C0 0 76 wt C A transformation temperature of 675 C 100 T 675 C 50 0 10 2 1 T C Austenite stable 10 4 time s TE 727 C 700 Austenite unstable 600 Pearlite isothermal transformation at 675 C 500 400 1 10 10 2 10 3 10 4 10 5 time s 10 Austenite to Pearlite Isothermal Transformation Eutectoid composition C0 0 76 wt C Begin at T 727 C Rapidly cool to 625 C Hold T 625 C constant isothermal treatment T C Austenite stable 700 Austenite unstable 600 500 TE 727 C Pearlite 400 1 10 10 2 10 3 10 4 10 5 time s 11 Transformations Involving Noneutectoid Compositions Consider C0 1 13 wt C T C T C 900 d A 1200 C A P a P L Fe3C austenite 1000 Fe3C 800 600 500 1 L 10 102 103 time s 104 T 400 0 Fe 0 76 600 A TE 727 C A 1 13 700 L 1400 0 022 800 1 727 C a Fe3C 2 3 4 5 6 Fe3C cementite 1600 6 7 C wt C 12 Bainite Another Fe Fe3C Transformation Product Bainite elongated Fe3C particles in a ferrite matrix diffusion controlled Isothermal Transf Diagram C0 0 76 wt C 800 Austenite stable T C A TE P 600 5 mm 100 pearlite 100 bainite 400 B A 200 10 1 10 103 105 time s 13 Spheroidite Another Microstructure for the FeFe3C System Spheroidite Fe3C particles within an a ferrite matrix formation requires diffusion heat bainite or pearlite at temperature just below eutectoid for long times driving force reduction of a ferrite Fe3C interfacial area 60 mm 14 Martensite A Nonequilibrium Transformation Product Martensite Fe atom sites x x x x x 60 mm FCC to Martensite BCT potential C atom sites x Isothermal Transf Diagram 800 Austenite stable T C A 10 1 to martensite M transformation B A 200 TE P 600 400 Martensite needles Austenite 0 50 90 M A M A M A 10 103 105 is rapid diffusionless transf depends only on T to which rapidly cooled time s 15 Martensite Formation FCC slow cooling a BCC Fe3C quench tempering M BCT Martensite M single phase has body centered tetragonal BCT crystal structure Diffusionless transformation BCT BCT if C0 0 15 wt C 16 Phase Transformations of Alloys Effect of adding other elements Change transition temp Cr Ni Mo Si Mn retard a Fe3C reaction and formation of pearlite bainite 17 Continuous Cooling Transformation Diagrams Conversion of isothermal transformation diagram to continuous cooling transformation diagram Cooling curve 18 Isothermal Heat Treatment Example Problems On the isothermal transformation diagram for a 0 45 wt C Fe C alloy sketch and label the time temperature paths to produce the following microstructures a b c d 42 proeutectoid ferrite and 58 coarse pearlite 50 fine pearlite and 50 bainite 100 martensite 50 martensite and 50 austenite 19 Solution to Part a of Example Problem a 42 proeutectoid ferrite and 58 coarse pearlite Fe Fe3C phase diagram for C0 0 45 wt C Isothermally treat at 680 C 800 all austenite transforms to proeutectoid a and coarse pearlite Wpearlite C0 0 022 0 76 0 022 A T C 0 45 0 022 0 58 0 76 0 022 A a P B 600 A P A B A 400 50 M start M 50 M 90 200 Wa 1 0 58 0 42 0 0 1 10 103 time s 105 20 Solution to Part b of Example Problem b 50 fine pearlite and 50 bainite Fe Fe3C phase diagram for C0 0 45 wt C 800 Isothermally treat at 590 C T C 50 of austenite transforms to fine pearlite A P B 600 Then isothermally treat at 470 C all remaining austenite transforms to bainite A a A P A B A 400 50 M start M 50 M 90 200 0 0 1 10 103 time s 105 21 Solutions to Parts c d of Example Problem c 100 martensite rapidly quench to room temperature Fe Fe3C phase diagram for C0 0 45 wt C d 50 martensite 800 50 austenite T C rapidly quench to 600 290 C hold at this temperature 400 A A a P B A P A B A 50 M start M 50 M 90 d 200 c 0 0 1 10 103 time s 105 22 Mechanical Props Influence of C Content Pearlite med Cementite hard Pearlite med ferrite soft C0 0 76 wt C Hypoeutectoid Hypo Hyper …
View Full Document
Unlocking...