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Allotropic Transformation

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.

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Allotropic Transformation

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

  2. 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

  3. G = - (4/3)  r3 [GV - GS] +4 r2 / - 4  (r*)2 [GV - GS] + 8  r* /= 0

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

  5. 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 ()

  6. 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”.

  7.  - Grain boundary width, D- Grain size Grain corner Grain edge

  8. (Chet.,i / C0 ) = exp [- (G*het.,i - G*hom) / kT ]

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