Some of the Diffusional transformations Allotropic Transformation Precipitation Reaction Eutectoid Transformation Order-disorder Transformation Massive Transformation

HOMOGENEOUS NUCLEATION IN SOLID STATE ’     Supersaturated B-rich precipitate solid solution G = - V GV + V Gs + A  = - (4/3)  r3 GV + (4/3)  r3 GS +4 r2 a/b = - (4/3)  r3 [GV - GS] +4 r2 a/b

G = - (4/3)  r3 [GV - GS] +4 r2 /

Gn = G2 - G1 Gv = Gn / Vm  X where X = X0 - Xe

Gheterogeneous = - V [ GV - Gs] + A   A  Heterogeneous Nucleation 2  Cos  =  Gheterogeneous = - V [ GV - Gs] + A   A  A =  (r Sin )2 A = 4  r2 (1-Cos) V = (2/3) r3 (2 + Cos ) (1-Cos)2 G*heterogeneous = [- (4/3)  r*3 (GV - Gs) + 4 (r*)2 a/b ] . [(2 + Cos ) (1-Cos)2 /2] = G*homogeneous . S ()

S () = (1/2) (2 + Cos ) (1-Cos )2 = (G*heterogeneous /G*homogeneous) = V*heterogeneous / V*homogeneous C*hom = C0 exp [ - G*hom / kT ] C*I,het. = Ci exp [ - G*i, het. / kT ] i refers to heterogeneous nucleation site type “i”, Ci refers to number of heterogeneous nucleation sites of type “i” per unit volume, C*i refers to rate of formation of stable nucei per unit volume at heterogeneous nucleation sites of type “i”.

 - Grain boundary width, D- Grain size Defect site type Ci/C0 Grain boundary (/D) Grain corner (/D)3 Grain edge (/D)2  - Grain boundary width, D- Grain size Grain corner Grain edge

Ci G* Type of site Homogeneous sites Vacancies Dislocations Stackin faults Grain boundaries and interface boundaries Surfaces (Chet., i / C0 ) = exp [- (G*het.,i - G*hom) / kT ]