The crystallization process mechanisms.

The crystallization takes place in two stages: in the beginning, nucleation center arise, and then nucleation center grow. There are two mechanisms for the nucleation of the nuclei: crystallization centers:

  1. Homogeneous (spontaneous, spontaneous without extraneous particles);
  2. Heterogeneous (not spontaneous, with the participation of foreign impurities at the ready interfaces.)

The growth of crystallization centers does not depend on the method of nucleation of crystallization centers and obeys general laws.

  The homogeneous crystallization theory.Continue reading →

The isolation of the excess phase from the supersaturated solid solution (Part 1).

The metals in the solid state can only dissolve other metals and nonmetals. As the pressure and temperature change, the solubility changes, this leads to the separation or dissolution of excess phases. If the alloy is in an unstable state, these processes can occur without changing the external conditions. Structural changes due to the limited solubility of the components in the solid state are widely used in the technique of heat treatment.

Let us consider the system whose components are bounded soluble in one another in the solid state. In this case, the field of the diagram below the solidus line is Continue reading →

The amount of metal evaporating from the cathode spot (Part 1).

When welding with the direct polarity, the dimensions of the anode spot determine the width of the weld. At the same time, at the given current and voltage, on the arc from the width of the seam depends on the depth of penetration of the product, since it is determined by the arc pressure on the weld pool. Thus, the size of the anode spot affects not only the technological parameters of the welding process, but also the strength properties of the welded joint. The size of the anode spot is directly influenced by the diameter of the column of the arc, which in turn depends on the composition of the arc gas, which is significantly influenced by metal vapor from the cathode spot on the electrode. Similarly, when welding on the reverse Continue reading →

The crystals shape (Part 1).

The shape formed in the melt of the crystals depends on the growth conditions (overheating and supercooling liquid heat removal direction, and other impurities) and the metal nature. In conditions close to equilibrium, properly cut crystals are formed. They grow slowly; their surface is smooth, although it contains steps necessary for the construction of faces. If the impurities are small and the temperature of the liquid increases with the distance from the surface of the crystal, the zone advanced for some reason or another enters the zone with the Continue reading →

The polymorphic transformations in alloys (Part 4).

The polymorphic transformations in alloys lying outside the eutectoid (KML) and peritectoid (MKL) lines do not differ from the one considered earlier (see Fig. 1).

The transformations begin with the appearance of embryos of the new modification and end with a complete transition of the high-temperature modification β into one of the low-temperature α1 or α2. The polymorphic Continue reading →

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