Materials science

1 - laboratory work
Topic: Study of the crystallization process.


The purpose of the work: to create a state diagram of lead-antimony by the thermal method. To study the crystallization process, to independently determine the crystallization temperature of metals and alloys, to draw up a cooling curve and to create a state diagram of alloys based on these curves.
Theoretical information:
Crystallization process
Any substance can be solid, liquid and gaseous. The transition of a substance from one state of matter to another state of matter is called a phase transition .
The change of aggregate states of pure metals occurs at a certain temperature, at the melting temperature it goes from a solid state to a liquid state, and at a boiling temperature it changes from a liquid state to a gaseous state. Pure metals change from liquid state to solid state at crystallization temperature T _{kr .} This temperature is constant I atm. determined at pressure, such as: iron (Fe) T _kr =1535 ⁰ C; tungsten (W) T _kr =3390 ⁰ C; molybdenum (Mo) T _kr =2600 ⁰ C.
The process of transition from a liquid state to a solid state (formation of crystal lattices) is called crystallization.
The shape, size and orientation of crystals of metals and alloys affect all its properties. For this reason, it is necessary to study the crystallization process in order to control the crystallization, therefore, to create the necessary properties of the crystal.
In nature, only the process of decreasing the free energy of the system can occur by itself. Free energy (E) refers to the amount of internal energy of the system that can be converted into work. The change of the free energy of liquid and solid states depending on the temperature is shown in Fig. I. Liquid and solid states The temperature T ₃ when the free energies are equal (F _suy = F _{kr )} is called the equilibrium temperature or theoretical crystallization. It can not crystallize and proceed freely at T _{3 temperature.} To accelerate the crystallization process, F _suy >F _kr should be.
Figure 1.
This is possible only when the alloy is cooled to a certain degree, as can be seen from the graph . is called the degree of supercooling and is equal to the difference between the theoretical and practical crystallization temperatures: (here: T _kr - practical crystallization temperature).

2 - picture. The free energy of liquid and crystalline solids changes depending on the temperature. Figure 2 shows the cooling curve representing the process of transition from the liquid state to the solid state in temperature-time coordinates. Cooling of liquid metal occurs with a uniform decrease in temperature. (Fig. 2 part "a" 0-1). When the crystallization temperature is reached, a horizontal section is formed in the cooling curve (section 1-1 ) . This
Part 1-1h represents the latent heat release of crystallization, this released heat is compensated by the surrounding environment, and the temperature of crystallization does not change until the transition from the liquid phase to the solid phase. Curve "a" in Figure 2 describes the theoretical crystallization process, "b ” curve shows the real transition of the crystallization process, i.e. crystallization occurs with some kind of supercooling.
In some metals (for example, antimony C), due to excessive supercooling, the latent heat of crystallization is dissipated so rapidly that the temperature jumps and approaches the theoretical crystallization temperature ("b" curve). As soon as the crystallization process stops, the solid metal starts to cool down and the temperature drops again.
DK Chernov showed that the crystallization process consists of two elementary processes: formation of crystallization centers and their growth processes.
The rate of formation of crystallization centers is measured by the number of them formed per unit time, the unit of measure is 1 m ^-3 c ^-1 ;
The growth rate of the crystal is measured by increasing the linear dimensions of the growing edges of the crystal per unit of time (m ^-1 c ^-1 ). The change of the crystal growth rate and the number of crystallization centers depending on the degree of supercooling is depicted in Figure 3.
It can be seen from Fig. 3 that at small values of the supercooling level, the growth rate of crystallization is large compared to the number of crystallization centers. Therefore, the alloy has a large grain structure
will have and vice versa. Therefore, such structural alloys differ in terms of mechanical properties. It should also be noted that metals have a low tendency to transfer, and in their liquid state, they are not very pure, but have insoluble particles. These particles are ready crystallization centers and affect the crystallization process.
Concept of state diagram.
A diagram showing the change of states of a compound as a function of temperature and concentration is called a state diagram.
All the processes that take place in the alloy depending on the temperature and concentration are reflected in the state diagram.
For specific alloys, the hollar diagram allows you to observe the processes that occur when they are heated and cooled. In particular, it is necessary to determine that the alloy has good fluidity, at the same time, the change in its mechanical properties due to the thermal or pressure operation of the alloy, the correct determination of the thermal, chemical-thermal and thermal processing mode, how the alloy in the equilibrium state it can be shown to have a structure, to correctly select the composition of the alloy with the desired properties, and so on.
N ote: case diagrams are studied in detail from recommended literature in preparation for laboratory work.
General instructions for creating a state diagram.
If the system is one-component, its state diagram is a one-dimensional axis (temperature scale), which shows the temperature of change of the aggregate state of the system of corresponding points in it (Fig. 4). If the system is two-component, then the second dimension is the concentration of the alloy, and the diagram consists of two dimensions (temperature-concentration) (Fig. 5).
Os T

10 20 30 40 50 60 70 80 90 100
Temperature is placed on the ordinate axis, concentration on the abscissa axis. The total amount of both components in the mixture is 100%. Points A and B correspond to pure components, and between them are two-component alloys. Each point on the state diagram represents the state of the alloy at a given concentration and temperature.
Each vertical axis corresponds to the change in temperature of a given alloy. Lines connecting points of similar changes demarcate areas of similar phase states on the diagram. The type of state diagram depends on how the two components interact with each other in their solid and liquid states, whether they dissolve in their solid and liquid states, whether they form chemical compounds, etc.
Assignment

1. 
   lead, antimony and P _b -S _b alloys.
   
2. 
   Drawing the cooling curve of lead, antimony and P _b -S _{b alloys.}
   
3. 
   Draw the state diagram of P _b -S _{b alloy.}
   
4. 
   Writing a report on the work.
   

Tools and materials
200 grams each ), pistachios mir wave, thermocouple graduation curve and stopwatch.
Order of work
Students are divided into teams of 3-4 people. Two brigades are given pure metals (lead and antimony), and the rest are given different alloys of lead and antimony (5, 10, 13, 25%).
Method for determining the crystallization temperature.

The crystallization temperature is determined in the device shown in Figure 6.

1. 
   Prepared pure metal (alloy) is placed in crucible 1 and placed in an electric furnace.
   
2. 
   The electric furnace is connected to the electrical network and the metal (alloy) in the crucible is heated and melted for some time.
   
3. 
   To protect the metal (alloy) from oxidation, a layer of coal is formed by sprinkling pistachio flakes on the surface of the molten metal (alloy).
   
4. 
   3 of the Crucible the lid closes.
   
5. 
   4, the heated weld of the thermocouple is lowered into the molten metal (alloy) in the crucible through the cover hole. The cold junction of the thermocouple is inside the thermostat 5, and the galvanometer 6 is connected to the thermocouple with copper wires.
   
6. 
   When the metal (alloy) melts, the furnace is disconnected from the electrical network.
   
7. 
   After every 30 seconds, the reading of the galvanometer needle is recorded. Recording is stopped after 2-3 minutes after the metal (alloy) solidifies. If the scale of the galvanometer (MB) is graduated in millivolts, then the indication of the arrow is written in the 3rd column of the protocol, and using the calibration curve of the galvanometer, the temperature is determined and it is written in the 4th column.
   

Galvanometer display protocol

No	Cooling time, sec	Thermoelectric driving force mV	temperature,^0C
1	2	3	4

8. 
   "Temperature-time" cooling according to the critical points obtained in the protocol
   

Temperature С