Engineering

Solidification of metals and alloys is an important industrial process in which metals and alloys are melted and then solidify or cast them in order to get desired finished or semi-finished products. For example once we get aluminium ingots through casting process will be further fabricated into many finished aluminium products.Solidification of pure metals takes place in the following two steps:(a) The formation of stable nuclei in the melt part or we can say the process nucleation.(b) The growth of nuclei into crystals which will further form grain structure.(a) The formation of stable nuclei in the melt part or nucleation process:- It is a physical reaction which occurs when components of a solution start to precipitate out and starts to form nuclei which attract more precipitate. Nucleation is the beginning process of a phase transformation. It involves The assembly of proper kinds of atoms by diffusion. Formation of critical sized particles of the new phase. Structural change into one or more unstable intermediate structures.There are two main mechanisms from which solid particles nucleation occur in a liquid metal are:-(i) Homogeneous nucleation.(ii) Heterogeneous nucleation.(i) Homogeneous nucleation:- This type of nucleation takes place when the solution is totally uniform or in pure metals and the metal itself provides the atoms spontaneously needed to form the nuclei. The formation of a nucleus means that at the new phase boundaries, an interface is formed. In case of solidification of pure metals cooling of a pure liquid metal takes place below freezing temperature at equilibrium to a required degree, it will result into many homogeneous nuclei which are created by slow moving atoms which bond together. For the homogeneous nucleation, it is necessary to process through a considerable amount of under cooling which include a wide range of temperatures. For a nucleus to be stable, it much reaches a critical size so that it can grow into the crystal. A cluster of atoms bonded together that is less in size than the critical size is called an embryo, and on the other hand those sizes are larger than the critical size is called a nucleus. For example all natural and artificial uniform solutions having different crystallization process starts with homogeneous nucleation.(ii) Heterogeneous nucleation:- This type of nucleation takes place when foreign particles are present as impurities in the melt part or in casting. It takes place in a liquid medium on the container surface or having impurities which are insoluble that decreases the critical free energy which is required for forming a stabilized nucleus.(b) The growth of nuclei into crystals:- Growth may be defined as the increase of the nucleus in size as it grows by addition of atoms. After the formation of stable nuclei due to nucleation in a solidifying metal, these stable nuclei will start to grow into crystals as shown in the figure (1.1) below.