Конструкциялық материалдар және термоөңдеу Конструкционные материалы и термообработка Constructional materials and heat treatment Учебное пособие для специальности: 5В071200– «Машиностроение»



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Crystallization process.
Transition metal from liquid to solid is called crystallization. The 
crystallization process starts with the formation of crystal nuclei and continues 
the growth in number and size. The crystallization process can take place only 
when the sub-cooling of metal below the equilibrium crystallization temperature 
at which the liquid and solid phases can co-exist and, moreover, forever. 
Difference between T and T, which can occur when the crystallization process is 
called the degree of sub-cooling . T = T-T When sub-cooling alloy below 
the Tm in many parts of the liquid alloy, a stable, capable of growing crystalline 
nuclei is called critical.
Polymorphic transformations in metals
Many metals, depending on the temperature can exist in different crystalline 
forms, or, as it is called, in different versions. As a result of the polymorphic 
transformation atoms crystalline solid with a lattice-type rearranged and form a 
crystalline lattice of another type. Polymorph, stable at lower temperatures, for 
most metals is denoted by the letter α, with higher β, γ, and so on. Pure metal 
transition from one polymorph to another in the condition of equilibrium occurs 
at a constant temperature (critical point) and is accompanied by heat, if the 
transformation is under cooling, and the absorption of heat in the case of heat.


177 
As a result of the polymorphic transformation creates new crystal grains with 
different size and shape. Therefore, such a transformation is called 
recrystallization. Polymorphic transformations occur in pure metals, alloys, 
chemical compounds. Polymorphous transformation is accompanied by an 
abrupt change in the properties of metals and alloys in the specific volume
specific heat, thermal conductivity, electrical conductivity, magnetic properties, 
mechanical and chemical properties, etc. 
Fig. 2.1. Types of crystal lattices: 
a) cubic body-centred lattice; b) cubic face-centered lattice
Fig. 2.2. Defects of a crystal structure: 
a, e - dot defects, b - linear defects 


178 
а б 
Fig. 2.3. Freezing of liquid metal and dendrite: 
a) dendrite; b) metal granules 


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