How is the grain structure of metals formed? Solidification/crystallization of melts! скачать в хорошем качестве

How is the grain structure of metals formed? Solidification/crystallization of melts!

The solidification of metals is a central process in metallurgy and materials science. The phenomena of supercooling and nucleation play a decisive role in this process. Solidification begins with a melt in which metal atoms move randomly with high kinetic energy. When the temperature drops below a critical point, the attractive forces between the atoms dominate, initiating the solidification process. This leads to the formation of crystal grains, which together make up the grain structure of a metal. Crystallization does not occur spontaneously when the solidification temperature is reached but requires a certain degree of supercooling. Supercooling refers to the difference between the current temperature of the melt and its solidification temperature. It ensures that atoms attach to existing crystal lattices until the entire material solidifies. In many cases, a melt can be cooled below the solidification temperature without solidifying. A well-known example is the hand warmer, which contains a supercooled solution of sodium acetate. Despite significant supercooling, this liquid remains in a metastable state until a trigger, such as a metal plate, initiates crystallization. Besides supercooling, nucleation is a second essential condition for solidification. Nuclei are tiny sites in the melt where crystallization begins. These can be classified as heterogeneous (foreign nuclei) or homogeneous (self-nuclei). Foreign nuclei arise from impurities or from the container walls in which the melt is held. They facilitate solidification by providing a crystal structure onto which further atoms can attach, a process known as heterogeneous nucleation. In contrast, homogeneous nucleation occurs when atoms in the melt randomly arrange themselves into a crystalline structure. Since homogeneous nucleation relies on random atomic arrangements, it is less likely than heterogeneous nucleation. However, a high degree of supercooling can increase the probability of homogeneous nucleation. A common example of supercooled liquids is freezing rain. Supercooled water remains liquid as long as it does not come into contact with impurities or surfaces. However, when it touches a solid surface, the existing impurities act as nuclei, triggering rapid crystallization. In metallurgy, various methods are used to control nucleation. Strong supercooling leads to finer grain structures since many small nuclei form simultaneously. A fine-grained microstructure improves the mechanical properties of a metal, increasing its toughness and strength. Another technique to influence solidification is seeding, where foreign particles are intentionally introduced into the melt to serve as nuclei. For example, silicon compounds are added to ductile cast iron to promote a fine-grained structure. Another important aspect of crystallization is the release of heat during solidification. This process is known as the heat of crystallization or heat of solidification. The energy released during crystallization must be dissipated; otherwise, the heat would remelt already solidified regions. The temperature behavior during solidification can be analyzed using cooling curves. Pure substances exhibit a thermal arrest, where the temperature remains constant throughout the solidification process. In alloys, however, a thermal gradient often occurs because the different components interact chemically and influence heat dissipation differently. As a result, the temperature decreases more gradually instead of remaining constant. 00:00 Solidification of metals 00:23 Liquid state (melt) 00:46 Supercooling (undercooling) 02:45 Hand warmer 03:45 Nuclei 04:17 Supercooled water (freezing rain) 05:17 Heterogeneous nucleation 06:19 Homogeneous nucleation 07:05 Influencing nucleation by supercooling 07:45 Influencing nucleation by seeding 08:30 Heat of solidification 09:55 Thermal arrest