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 →

The crystalline structures.

In the solid metal the atoms are aligned in even lines, rows and layers, forming three-dimensional crystalline structures. By definition, the metals have the crystalline structure. It is obvious that any reasoning about «crystallization» as the cause of the destruction of metal has no basis for any reason. When solidified, the metal always acquires the crystalline structure. The formation of a fracture surface, which is mistakenly termed «crystal-like» because of its appearance, is typical for cases of fatigue or the brittle fracture.

The smallest number of atoms that form the structure that gives a complete Continue reading →

The features of AC arc burning. The diagrams of currents and voltages for the AC arc.

It should be noted that all the properties of the DC arc are inherent in the AC arc. When the arc burns at the alternating current of 50 Hz, there are conditions that reduce the stability of the arc combustion. First of all, it is worth noting that when the polarity of the arc is changed on the electrodes, when the arc voltage (transformer) is zero, the arc goes out. Thus, the welding arc goes out, and then ignites again 100 times in 1 second. This causes special requirements to the power source of the arc (transformer) and to the welding circuit. The main influence is the Continue reading →

The surface work of the critical dimension nucleus formation.

In the formation of the crystallization nucleus, the part of the energy is released when the bulk phase occurs, and part of the work is spent on creating the surface partition, the system resists, prevents the formation of the interface. We substitute rкр values into the energy balance formula:

If the amount of energy released by the passage of the certain volume of a liquid phase with a large level of free energy into the solid phase with a lower level of free energy would be equal to the amount of energy spent on the formation of the interface, then ΔG would be equal to zero. If the energy was released (allocated) more than was expended, then ΔG would be less than zero. In our case, ΔG is positive and equal to 1/3A of the nucleus formation. This means that the system Continue reading →

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