Homogenous or self nucleatton of sand casting

- Formation of a critical solid from the liquid medium at the cluster together a large number of atoms at high undercooling (without external interface).

- Is one homogeneous nucleation occurs in perfectly homogeneous materials such as pure liquid metal.

- Grain nucleation the super cooled depending on two factors:

Factor A 1. The free energy available from the solidification process; depending on the volume of the particle formed. 2. The replacement passes old stage (ie molten metal) of the new phase (ie solid) reduced free energy per unit volume and this is contributing to the stability of the region (a new stage). 3. In the case of spherical particles, if the temperature suddenly dropped below the freezing point, the change of free energy per unit volume of metal transformation (ie solidified), and it is negative (due to energy cuts free); r radius of the particle.

Factor B 1. The second factor is the energy required to form a liquid-solid interface. 2. The particles are made, in some melting surface area. Steps are solid-liquid surface in between the two. A positive free energy per unit of surface it such. 3. The creation of a new interface (surface) of increasing the free energy is proportional to the surface area of ​​the particle and the free energy increase is equal to the free energy change so reported for particle ray. ? – As the radius increases particle, the free energy, also increases till grows the particle radius of critical and subsequent increase in radius of particles attached to reduce energy free and so much so that the energy Free also a negative charge. – Particles of radius smaller than the critical radius tend to redissolve and thus lower the free energy. Such particles are called embryos. – Particulate radius exceeds the critical radius tends to grow and also lower the free energy. Such particles are called nuclei. – Must critical particle size to create before the stable nucleus for certain super cooling temperature.