The heterogeneous nucleation.

If there are impurities in the liquid phase in the form of solid particles, then the crystallization often begins on them, as on finished substrates.

The role of such particles can play the walls of molds, non-metallic inclusions, oxides, etc. The foreign particles are active if the work of germs formation is less on them than the work of spontaneous generation of the germs. This is the case when the surface tension at the interface of the germ is an alien particle smaller than at the interface between the nucleus and the liquid phase. The interfacial energy is characterized by the wetting angle of the droplet of the mother phase on the Continue reading →

Dispersion hardening (aging) (Part 1).

The release of the excess phase from the supersaturated solution at the low temperatures leads to the increase in the strength and hardness of the alloy. This is called dispersion hardening or aging. During aging, there is usually a lot of precipitation, evenly distributed throughout the alloy. Especially, a lot of them during exposure at room temperature. In this case, the decomposition of the supersaturated solution occurs slowly and is termed natural aging.

With the heating of the alloy, the precipitation intensifies and is called artificial aging. At the initial stages of natural aging in the supersaturated solution (Fig. 1, a), Continue reading →

The amount of metal evaporating from the cathode spot (Part 2).

Since the total length of the cathode zone (the ionization zone and the free-motion zone of the ions adjacent to the cathode surface) is several mean free paths of atoms, we can assume that the concentration of iron atoms in the entire cathode zone is approximately equal to the concentration of atoms in the column of the arc and, taking that the flow of iron atoms emerging from the cathode zone displaces the atoms of the protective gas from the cathode spot, we can assume that the cathode zone is almost completely filled with only atoms of the cathode material, and the ion current in it is almost completely transferred by ions formed from the Continue reading →

The crystals shape (Part 2).

The accelerated growth of protruding dendrites (shoots) is due to several reasons. First, it can be associated with the features of packing atoms and placing defects on the surface of these areas. Secondly, the dendritic form of the crystal can be the result of uneven heat sink. The protruding sections of the crystal are in contact with the large volume of liquid per unit of their surface. This contributes to a better dissipation of the heat of crystallization released at the solidification front. With the growth of the crystal in the form of a tip, its apex always contacts the super cooled melt and advances more rapidly. Thirdly, impurities can contribute to the formation Continue reading →

The metallographical tests.

The various metallographic tests are another way to obtain information about the properties of the metal or weld. These tests are essentially in the excision and subsequent careful polishing of the sample from the metal or welded seam. After such processing of the sample, it is possible to evaluate the metal with the naked eye or with the help of magnifying optics.

The metallographic tests usually refer to the macroscopic or microscopic group. The difference is the magnification used. The macrostructure tests are carried out Continue reading →

Scroll Up