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MECH 210 081027 - Phase Transformations

MECH 210 081027 - Phase Transformations - MECH 210 081027...

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MECH 210 081027 Phases Transformations Classification of Phase Transformations 1. Diffusion-dependent transformation: accomplished by diffusion of atoms resulting in (a) no change in either the number or composition of the phases present – solidification of a pure metal, allotropic transformation, recrystallization and grain growth (b) change in the number or/and composition of the phases present – transformations at invariant points 2. Diffusionless transformation: results in metastable state (nonequilibrium state/structure that may persist indefinitely – martentistic transformation that results in the formation of martensite Kinetics of Phase Transformations Phase transformation proceeds in two stages of nucleation and growth 1. Nucleation: formation of stable nuclei (small particles of the new phase) by homogeneous nucleation or heterogeneous nucleation . (a) Homogeneous nucleation: nuclei of the new phase form uniformly throughout the parent phase Total free energy change = (volume free energy) + (surface free energy) γ π Δ π Δ 2 3 4 3 4 r G r G + = v where ( ) m m f T T T H G - = Δ Δ v The critical particle radius: r* - - = - = T T H T G m f m 1 2 2 Δ γ Δ γ v - a particle having a radius greater than r * is termed a nucleus that remains stable -
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