Transformations

• Diffusional (require chemical potential gradients)

  • Spinodal (small degree large extend)

  • Nucleation and growth (small extend large degree)

• Non diffusional (eg. cooperative shearing action for martensite)

A spinodal transformation is a solubility gap in a other wise fully soluble part. It describes a unstable solution, which can decompose spontaniously.

The diffusion works “uphill”, against a concentration equalisation, which leads to a negative diffusion constant.

In case G3 the dissolution leads to a lower enthalphy (=more stable) in comparison to the normal case G1, where a dissolution increases the energy.

Nucleation: a nucleus of the right composition emerges, which grows via diffusion. The composition of the nucleus stays roughly the same.

Spinodal transformation: A transformation occurs until the equilibrium composition is reached. The composition changes until equilibrium.

Homogenous: The nucleation occurs via fluctuation in the bulk.

Heterogenous: The nucleous emerges on a foreign particle or the sidewalls etc. (vgl. adding nucleation powder in steelmaking)

decrease in free energy

r* is the critical radious that needs to be passed for the nucleous to be stable

The green part stands for the increase in enthalphy due to the increase in surface

The blue part stands for the enthalphy decrease due to solidification

with gamma sl = surface energy nucleous-liquid

gamma cs = surface energy catalyst-nucleous

gamma cl = surface energy catalyst-liquid

It does not:

The wettability of the catalysts

Undercooling

∆G*:free energy change for formation of critical embryo

∆Gm:free energy barrier between liquid and solid