先进材料疑固实验室 Nucleation Laboratory of Advanced Materials Solidification Only when T>,we can conclude a phase will nucleate before βphase. ⊕ In general 4T+4,and thus the a-B transition composition C is shifted with respect to Cper,it is given by C-Cpor ATS -ATE T T B mg -m △TR ⊕ m mf are the liquidus slope of a and B D1 Effective liquidus line △Tn 上浒充通大学 SHANGHAI JIAO TONG UNIVERSITY
Nucleation Only when > , we can conclude α phase will nucleate before β phase. In general , and thus the α→β transition composition Cn α→β is shifted with respect to Cl per, it is given by are the liquidus slope of α and β
先进材料疑固实验室 nucleation Laboratory of Advanced Materials Solidification Consider now a hyperperitectic alloy oc composition close to the peritectic composition Cper,we assume the a/l interface to be at equilibrium,i.e.,that the curvature and kinetic contributions are negligible.Therefore,the undercooling for the nucleation of B for T<Tper is given by 4=(Ga(G)-Ta(G)=(m-m）(G-C) T(C) Since the a phase is already present, the a/liquid interface is the most △ probable site for nucleation of B.There T) is usually a coherency relationship between the phases. 上浒充通大学 SHANGHAI JIAO TONG UNIVERSITY
nucleation Consider now a hyperperitectic alloy oc composition close to the peritectic composition Cper, we assume the α/l interface to be at equilibrium, i.e., that the curvature and kinetic contributions are negligible. Therefore, the undercooling for the nucleation of β for T<Tper is given by Since the α phase is already present, the α/liquid interface is the most probable site for nucleation of β. There is usually a coherency relationship between the phases. Cper β
先进材料疑固实验室 Solidification of peritectics Laboratory of Advanced Materials Solidification Normal growth speed: A quenched peritectic microstructure observed in Sn- 1000 900 bronze,with the B-phase 800 completely surrounding the 700 primary a-phase. 600 10 20 30 Cs [wt6] (a)Cu-Sn phase dingram (b)Cu-Sn microstructure A micrograph of Fe-50wt%Au 1400 alloy with the peritectic phase 1200 has the same crystal structure as the primary phase.The 1000 primary y-phase solidifies first 800 and thin layer y'-phase forms 204060C. 10m around y. (e)Fe-Au phase diagram (d)Fe-Au mierostructure Fig.9.27 Examples of peritectie microstructures and their associated phase dia. grams:(a)The Cu-Sn phase diagram and (b)mierostructure observed in a quenched Cu-20wt%Sn specimen [19).(c)The Fe-Au phase diagram and (d) mierostructure in a quenched Fe-50wt Au specimen.(Courtesy,D.Favez.) 上游文通大学 SHANGHAI JIAO TONG UNIVERSITY
Solidification of peritectics Normal growth speed: A quenched peritectic microstructure observed in Snbronze, with the β-phase completely surrounding the primary α-phase. A micrograph of Fe-50wt%Au alloy with the peritectic phase has the same crystal structure as the primary phase. The primary γ-phase solidifies first and thin layer γ’-phase forms around γ
先进材料疑固实验室 Solidification of peritectics Laboratory of Advanced Materials Solidification Slow growth speed: (a) (b) (c) (d) (e) (D Fig.9.30 A summary of peritectic microstructures observed at very low speed dur ing solidification in a vertical thermal gradient:(a)Bands;(b)Islands;(c)Single transition from a to B;(d)Coupled growth of o-and B-lamellae;(e)Nucleation of in the liquid ahead of a 3-e planar front;(f)Helicoidal structure.(a-d)Hypoperi tectic compositions;(e-f)Hyperitectic compositions.(After Boettinger et al.[61.) 上浒充通大 SHANGHAI JIAO TONG UNIVERSITY
Solidification of peritectics Slow growth speed:
先进材料疑固实验室 Solidification of peritectics Laboratory of Advanced Materials Solidification Cper 1.4 bands plane coupled front 1.0 mixed t118601202 bands yplane front 0.6 6cells dendrites 0.2 Ycells dendrites 3.0 3.5 4.0 4.5 15.0 5.5 CNi [at%] Cper (a)Coupled growth (b)Morphology map Fig.9.31 (a)Coupled (or cooperative)growth of y and 6 phases in Fe-Ni.(b)The G/v-Co diagram showing the various morphologies that can be observed in Fe-Ni. (After Hunziker et al.[15].) 熟 上游文通大学 SHANGHAI JIAO TONG UNIVERSITY
Solidification of peritectics