Classification 上游充通大学 SHANGHAI JIAO TONG UNIVERSITY Based on whether there are "thermally activated atomic movements": Diffusional transformations>Majority of transformations! Diffusionless transformations>Martensitic transformation Diffusional transformations: induced by a change of temperature ■ in a solid-state alloy of a fixed bulk composition AO TONG K.Chen 12
K. Chen Based on whether there are “thermally activated atomic movements”: Diffusional transformations Majority of transformations! Diffusionless transformations Martensitic transformation Diffusional transformations: induced by a change of temperature in a solid-state alloy of a fixed bulk composition Classification 12
Classification 上游充通大兽 SHANGHAI JIAO TONG UNIVERSITY Diffusional transformations can be divided into: ■Precipitation reactions(脱溶沉淀) ■ Eutectoid transformations(共析转变) Ordering reactions(有序化转变) ■ Massive transformations(块状相变) ■ Polymorphic changes(同素异构转变（多形性转变）》 AJ JIAO TONG UNIV K.Chen 13
K. Chen Diffusional transformations can be divided into: Precipitation reactions (脱溶沉淀) Eutectoid transformations (共析转变) Ordering reactions (有序化转变) Massive transformations (块状相变) Polymorphic changes (同素异构转变(多形性转变)) Classification 13
Precipitation reactions 上游充通大粤 SHANGHAI JIAO TONG UNIVERSITY Precipitation reactions a+B a+B A (i) B A Gii) B A (ii) B a'is metastable supersaturated solid solution ■ B is a stable (or metastable)precipitate ■ a is a more stable solid solution with the same crystal structure as a',but with a composition closer to equilibrium K.Chen 14
K. Chen Precipitation reactions α’ is metastable supersaturated solid solution β is a stable (or metastable) precipitate α is a more stable solid solution with the same crystal structure as α’, but with a composition closer to equilibrium Precipitation reactions 14
Eutectoid transformations 上浒充通大粤 SHANGHAI JIAO TONG UNIVERSITY Y→a+β A Replacement of a metastable phase (y)by a more stable AC mixture of two other phases (a+β) ⊕ Eg.in steel::Y→a+Fe,C >Pearlite reaction in Fe-C Alloys K.Chen 15
K. Chen Replacement of a metastable phase (γ) by a more stable mixture of two other phases (α + β) Eg. in steel: γ→α+Fe3C Pearlite reaction in Fe-C Alloys Eutectoid transformations 15
Ordering Massive 上海充夏大￥ SHANGHAI JIAO TONG UNIVERSITY Ordering reactions a(disordered)a'(ordered) (i) B (i) Disordered dered ●Cu OAu Massive transformations B→a Original phase decomposes into one or more new phases which have the same composition as the parent (i) phase,but different crystal B (ii) B structures Stable a Metastable a K.Chen 16
K. Chen Ordering reactions Massive transformations Ordering & Massive 16 Original phase decomposes into one or more new phases which have the same composition as the parent phase, but different crystal structures Stable α Metastable α Disordered Ordered