Solidification (Inflection points - phase changes)
When a molten alloy is allowed to cool at room temperature and under constant pressure (atmospheric pressure, for example), it solidifies.
This transition from the liquid to the solid phase is called a phase change. This phase change always occurs at a fixed temperature: the melting point. At the melting point, the liquid and solid phases coexist.
This point is determined by recording the cooling curve (temperature and time). Since crystallization is an exothermic phenomenon, the heat lost by the alloy's cooling is temporarily compensated as it passes through the melting point. This isothermal plateau is more pronounced the slower the cooling and the larger the mass of the alloy.
If, after the alloy is formed in a foundry, it is cooled, the phase change varies according to the relative changes in atomic size, the type of lattice structure, and the electronic properties of the different constituents. The more components an alloy contains, the more complex the solidification curves become.
The curve's behavior is then determined by inflection points (upper and lower red curves). Sometimes, these curves also include isothermal solidification plateaus.
The highest inflection point corresponds to the formation of the first crystal in the molten alloy (liquidus, upper red curve), while the lowest inflection point corresponds to the solidification of the last traces of molten alloy (solidus, lower red curve).