), which has the same composition, structure, single state of aggregation and is separated from the rest of the system by an interface.

For example, a liquid metal is a single-phase system, and a mixture of two types of metals different in composition and structure, separated by an interface, or the simultaneous presence of an alloy in a liquid state and crystals forms a two-phase system.

The following phases can form in alloys:

A graphic representation of the lines of coexistence of phases depending on thermodynamic parameters is called "Phase Diagram".

1. Liquid solution

Liquid solutions are completely homogeneous mixtures of two (or more) substances, in which the molecules of one substance are evenly distributed among the molecules of another substance.

2. Solid solution

solid solutions called phases in which one of the components of the alloy retains its crystal lattice, and the atoms of the other component are placed in the crystal lattice of the first component (solvent), changing its size.

4. Intermediates

A significant number of compounds formed in metal alloys do not obey the laws of valency and do not have a stable ratio of components. The most important intermediate compounds formed in alloys are as follows:

• rooting phases;
• electronic connections;
• heterogeneous structures.

4.1. Rooting phases

The solid rooting solutions mentioned above are formed at much lower concentrations of the second component (C, N, H) and have a solvent metal lattice, while the rooting phases have a different lattice. The crystal structure of the rooting phases is determined by the ratio of the atomic radii of the non-metal (Rx) and metal (R m). If a R x / R m then the metal atoms in these phases are arranged according to the type of one of the simple crystal lattices (cubic or hexagonal), into which non-metal atoms are introduced, occupying certain places in it. If condition R x / R m is not fulfilled, as is observed for iron, manganese, chromium carbides, then complex lattices are formed and such compounds no longer belong to the rooting phases.

4.2. Electronic connections

Electronic connections formed between monovalent elements (Cu, Ag, Au, Li, Na) or transition metals (Fe, Mn, Co and etc.). And simple metals with a valency of 2 to 5 (Be, Mg, Zn, Cd, Al and etc..).

Electronic compounds have a crystal lattice that differs from the crystal lattices of their components and form alloys in a wide range of concentrations.

Such compounds have a certain electron concentration (a certain ratio of the number of valence electrons to the number of atoms):

• for compounds with an electron concentration of 3/2 (1.5), a volume-centered crystal lattice is characteristic and is called a β-compound (CuBe, Cu 3 Al, FeAl and etc..)
• compounds with a ratio of 21/13 (1.62) are characterized by a complex cubic lattice and are designated as γ-compounds (Cu 5 Zn 8, Fe 5 Zn 21 and etc.).
• compounds with an electron concentration of 7/4 (1.75) are characterized by a close-packed hexagonal lattice and are designated as the ε-phase (Cu 3 Si, Cu 3 Sn and etc.)..

4.3. heterogeneous structures

During the crystallization of many alloys (including and Fe-C) structures are formed, consisting of several phases, forming this heterogeneous structure, which is shown by microanalysis.

See also

Sources

• Lakhtin Yu. M. Fundamentals of Metallurgy Moscow: Metallurgy, 1988. 320 p. ISBN 5-229-00085-6
• Sych A. M., Nagorny P. G. Fundamentals of Materials Science: Textbook. - M. Publishing and printing center "Kyiv University", 2003.
• West A. Solid State Chemistry. - M.: Mir, 1988. - Ch. 1.2

A state diagram is a graphic representation of the state of any alloy of the system under study, depending on its concentration and temperature.

The study of any alloy begins with the construction and analysis of the state diagram of the corresponding system. The state diagram makes it possible to study the phases and structural components of the alloy. Using the state diagram, it is possible to establish the possibility of heat treatment and its modes, casting temperatures, hot plastic deformation.

In any system, the number of phases that are in equilibrium depends on internal and external conditions. The laws of all changes occurring in the system are subject to the general law of equilibrium, which is called the phase rule or the Gibbs law. The phase rule expresses the relationship between the number of degrees of freedom C (variance) of the system, the number of components K and the number of phases of the system Ф that are in equilibrium.

Degrees of freedom are called independent thermodynamic parameters, which can be given arbitrary (in a certain interval) values ​​so that the phase states do not change (old phases do not disappear and new ones do not appear).

Usually, all transformations in metals and alloys occur at constant atmospheric pressure. Then the phase rule is written as follows: C \u003d K - F + 1.

The phase rule equation allows you to correct the correctness of constructing state diagrams.

A phase is a homogeneous part of the system, which is separated from other parts of the system (phases) by the interface, when passing through which the chemical composition or structure of the substance changes abruptly.

A homogeneous liquid is a single-phase system, and a mechanical mixture of two crystals is a two-phase system, since each crystal differs from the other in composition or structure, and they are separated from one another by an interface.

Components are the substances that form the system.

The construction of state diagrams is carried out by various experimental methods. Thermal analysis is often used. Several alloys of this system are selected with different mass ratios of their components. The alloys are placed in refractory crucibles and heated in a furnace. After the melting of the alloys, the crucibles with the alloys are slowly cooled and the cooling rate is fixed. Based on the data obtained, thermal curves are built in time-temperature coordinates. As a result of the measurements, a series of cooling curves are obtained, in which at temperatures of phase transformations, inflection points 20b and temperature stops are observed. Temperatures corresponding to non-phase transformations are called critical points. The points corresponding to the beginning of crystallization are called liquidus points, and the points corresponding to the end of crystallization are called solidus points. Based on the obtained cooling curves for various alloys of the system under study, a phase diagram is constructed in coordinates; along the abscissa axis, concentration of components; along the ordinate axis, temperature.

In the process of crystallization, both the concentration of phases and the amount of each phase change. At any point in the diagram, when two phases simultaneously exist in the alloy, the amount of both phases and their concentration can be determined. For this, the rule of leverage or the rule of segments is used.

Segment rule. This diagram covers alloys whose components form mixtures of their practically pure grains with negligible mutual solubility. The abscissa shows the percentage of component B in the alloy.

The phase structure of alloys in the diagram depends on temperature. With the thermodynamic action of the components on each other, the temperature of their transition to the liquid state decreases, reaching a certain minimum at a composition determined for each pair of components. The composition of the alloy can be determined by projecting point C onto the x-axis (point B uh). An alloy of two components that melts at a minimum temperature is called a eutectic or eutectic.

The eutectic is a uniform mixture of simultaneously crystallized small grains of both components. The temperature at which both components melt or crystallize simultaneously is called the eutectic temperature.

Quantitative changes in the alloys of a given system of components during crystallization obey the rule of segments.

To determine the concentrations of components in the phases, a horizontal line is drawn through a given point characterizing the state of the alloy until it intersects with the lines limiting this area; the projections of the intersection points onto the concentration axis show the compositions of the phases.

By drawing a horizontal line through a given point, you can determine the quantitative ratio of the phases. The segments of this line between the given point and the points that determine the composition of the phases are inversely proportional to the quantities of these phases.

The segment rule in dual state diagrams is used only in two-phase areas. In a single-phase region, there is only one phase; any point inside the region characterizes its concentration.

| |

→ 20. Types of phases in metal alloys. Phase rule; lever rule

A state diagram is a graphic representation of the state of any alloy of the system under study, depending on its concentration and temperature.

The study of any alloy begins with the construction and analysis of the state diagram of the corresponding system. The state diagram makes it possible to study the phases and structural components of the alloy. Using the state diagram, it is possible to establish the possibility of heat treatment and its modes, casting temperatures, hot plastic deformation.

In any system, the number of phases that are in equilibrium depends on internal and external conditions. The laws of all changes occurring in the system are subject to the general law of equilibrium, which is called the phase rule or the Gibbs law. The phase rule expresses the relationship between the number of degrees of freedom C (variance) of the system, the number of components K and the number of phases of the system Ф that are in equilibrium.

Degrees of freedom are called independent thermodynamic parameters, which can be given arbitrary (in a certain interval) values ​​so that the phase states do not change (old phases do not disappear and new ones do not appear).

Usually, all transformations in metals and alloys occur at constant atmospheric pressure. Then the phase rule is written as follows: C \u003d K - F + 1.

The phase rule equation allows you to correct the correctness of constructing state diagrams.

A phase is a homogeneous part of the system, which is separated from other parts of the system (phases) by the interface, when passing through which the chemical composition or structure of the substance changes abruptly.

A homogeneous liquid is a single-phase system, and a mechanical mixture of two crystals is a two-phase system, since each crystal differs from the other in composition or structure, and they are separated from one another by an interface.

Components are the substances that form the system.

The construction of state diagrams is carried out by various experimental methods. Thermal analysis is often used. Several alloys of this system are selected with different mass ratios of their components. The alloys are placed in refractory crucibles and heated in a furnace. After the melting of the alloys, the crucibles with the alloys are slowly cooled and the cooling rate is fixed. Based on the data obtained, thermal curves are built in time-temperature coordinates. As a result of the measurements, a series of cooling curves are obtained, in which at temperatures of phase transformations, inflection points 20b and temperature stops are observed. Temperatures corresponding to non-phase transformations are called critical points. The points corresponding to the beginning of crystallization are called liquidus points, and the points corresponding to the end of crystallization are called solidus points. Based on the obtained cooling curves for various alloys of the system under study, a phase diagram is constructed in coordinates; along the abscissa axis, concentration of components; along the ordinate axis, temperature.

In the process of crystallization, both the concentration of phases and the amount of each phase change. At any point in the diagram, when two phases simultaneously exist in the alloy, the amount of both phases and their concentration can be determined. For this, the rule of leverage or the rule of segments is used.

Segment rule. This diagram covers alloys whose components form mixtures of their practically pure grains with negligible mutual solubility. The abscissa shows the percentage of component B in the alloy.

The phase structure of alloys in the diagram depends on temperature. With the thermodynamic action of the components on each other, the temperature of their transition to the liquid state decreases, reaching a certain minimum at a composition determined for each pair of components. The composition of the alloy can be determined by projecting point C onto the x-axis (point B e). An alloy of two components that melts at a minimum temperature is called a eutectic or eutectic.

The eutectic is a uniform mixture of simultaneously crystallized small grains of both components. The temperature at which both components melt or crystallize simultaneously is called the eutectic temperature.

Section pages: 1

At any point in the equilibrium diagram, when two phases simultaneously exist in the alloy, the concentration and amount of both phases can be determined. This is the rule of segments or the rule of the lever.

The first position of the rule of segments: in order to determine the concentration of components in the phases, a horizontal line is drawn through a given point characterizing the state of the alloy until it intersects with the lines limiting this area; the projections of the intersection points onto the concentration axis show the composition of the phases.

For example, consider an alloy X at a temperature t 1 in a diagram of the 1st kind (Fig. 2.6).

Figure 2.6 Type I state diagram

(with the rule of segments applied to it)

Therefore, for alloy X at temperature t 1, the phase compositions are determined by the projections of the corresponding points. The composition of the liquid phase will correspond to point B, and the solid phase - to point C 1.

The second position of the rule of segments: in order to determine the quantitative ratio of the phases at a given temperature, a horizontal line is drawn through a given point. The segments of this line between the given point and the points that determine the composition of the phases are inversely proportional to the quantities of these phases.

For alloy X at temperature t 1 this ratio will be

or
,

where Q L is the amount of the liquid phase; Q B is the number of crystals of component B; Q is the total amount of the alloy.

From here, the percentage of the liquid phase will be

those. at a temperature t 1 alloy X will consist of 66.7% crystals of component B and 33.3% liquid solution of components A and B.

Using the rule of segments, in a similar way, you can determine the volume of the eutectic and the volume of crystals B after solidification.

For Alloy X

The cut rule applies to all two-phase regions of any state diagrams.

STUDY OF COMPLEX DIAGRAM OF STATES OF BINARY SYSTEMS

Most binary alloys have more complex (combined) state diagrams. Knowing the main types of state diagrams, each complex diagram can be mentally divided into constituent parts corresponding to the main types, and, depending on the composition of the alloy, consider the corresponding part of the diagram.

As an example, let's analyze the state diagram of aluminum-calcium alloys. On fig. The phase diagram of the state and the cooling curve of the alloy with 25% calcium are presented, in fig. - structural diagram of the state of aluminum-calcium alloys.

Figure 2.7 Phase diagram of the state of the Al-Ca system and

cooling curve

In a general examination of the diagram, it is necessary to highlight its parts corresponding to typical state diagrams; the area of ​​existence of the liquid phase, solid and liquid phases, the area of ​​solid solutions; find eutectic, eutectoid and peritectic points and lines; liquidus and solidus lines, find out what phases exist in a given system. Phases can be: solid solutions, chemical compounds, pure components and liquid. For our example, the region of the liquid phase lies above the ABCDEF line, and the region of the simultaneous existence of the liquid and solid phases lies between the liquidus ABCDEF and solidus AKBLGHMEN lines.

Fig 2.8 Structural diagram of the state of the Al-Ca system

In the system under consideration, there is one solid solution α corresponding to the AKS region. It is a solid solution of calcium in aluminum. Point K is the point of maximum solubility, KS is the line of the limiting solubility of calcium in aluminum. Aluminum does not dissolve in calcium.

Thus, the phases in this system are: liquid, -solid solution, chemical compounds Al 3 Ca, Al 2 Ca, Ca crystals.

The aluminum-calcium diagram is characterized by the following:

1. Line KBL - line of eutectic transformation, then B - eutectic point. A eutectic is a mechanical mixture of crystals of an α-solid solution and a chemical compound Al 3 Ca. The eutectic transformation proceeds according to the equation

W in α to + Al 3 Ca

In accordance with the phase rule, the eutectic transformation proceeds at a constant temperature, since the alloy is in a three-phase equilibrium state. Under these conditions, the number of degrees of freedom will be zero: C = K - + 1 = 2 – 3 + 1 = 0, where K is the number of components (Al and Ca), and - number of phases (l, α, Al 3 Ca).

Alloys, in the structure of which there is a eutectic, are divided into hypoeutectic, eutectic and hypereutectic.

For alloys located below the KB line, the structure will consist of an α-solid solution and eutectic, for alloys below the BL line, from the Al 3 Ca chemical compound and eutectic; the eutectic alloy in t. B consists of one eutectic.

2. CGH line - the line of formation of an unstable chemical compound Al 3 Ca. Point G is a peritectic point. Peritectic transformation reaction:

W s +Al 2 CaAl 3 Ca.

Peritectic transformation consists in the formation of Al 3 Ca crystals during the interaction of liquid and solid phases of certain chemical compositions. For an alloy at point G, as a result of the completion of the peritectic transformation, the entire alloy will consist of the chemical compound Al 3 Ca. For alloys located to the left of point G (point G to point C), the liquid phase will remain in excess; for alloys located to the right of point G (from point G to point H), the Al 2 Ca compound will remain in excess. In accordance with the phase rule, the peritectic transformation also proceeds at a constant temperature.

3. Line MEN - line of the second eutectic transformation:

F E Al 2 Ca+Ca

The eutectic will consist of fine crystals of Ca and chemical. Al 2 Ca compounds. Alloys below the ME line are hypoeutectic, their structure consists of eutectic and Al 2 Ca; alloys below the EN line are hypereutectic, the structure consists of Ca and eutectic.

In the process of crystallization, both the concentration of phases 1 (therefore, the composition of the liquid changes) and the amount of each phase (during crystallization, the amount of the solid phase increases, and the liquid phase decreases). At any point in the diagram, when two phases simultaneously exist in the alloy, the amount of both phases and their concentration can be determined. For this, the so-called lever rule, or the rule of segments, is used.

At point a, showing the state of alloy K at temperature (Fig. 95), the alloy consists of crystals B and liquid. Above the point, the alloy is in a single-phase state, and the concentration of components in this phase (i.e., in the liquid) was determined by the projection of the point. When cooled, crystals B precipitate from the alloy and the composition of the liquid changes in the direction of increasing component A in it. At temperature, the concentration of component B in liquid is determined by the projection of the point this is the maximum amount of component B that the liquid can contain when reaching the eutectic temperature, the liquid takes on the eutectic concentration. Therefore, when alloy K is cooled, the liquid concentration changes along the curve. The precipitated crystals B have a constant composition - this is a pure component B, the concentration of which lies on the vertical axis

The first provision of the rule of segments is formulated as follows. To determine the concentrations of components in the phases, a horizontal line is drawn through a given point characterizing the state of the alloy until it intersects with the lines limiting this area; the projections of the intersection points onto the concentration axis show the compositions of the phases.

Consequently, for alloy K at temperature, the compositions of both phases are determined by the projections of points and c, since these points are at the intersection of the horizontal line passing through point a with the lines of the diagram.

The number of these phases can also be determined. To determine the amount of each phase (the second position of the rule of segments), we assume that alloy K is at a temperature

Rice. 95. State diagram (to apply the rule of segments on it)

Alloy K contains Therefore, if the segment determines the entire amount of the alloy, then the segment A is the amount of B in the alloy, and the segment is the amount of component A in the alloy.

At point a, the alloy consists of crystals B and a liquid of concentration Liquid contains , or in a liquid, the amount of component B is determined by the segment

With the total weight of the alloy equal to one, the desired number of precipitated crystals is x, and the amount of liquid is 1 - x. In this case, the amount of the component that is only in the liquid is

i.e. if the mass of the alloy is equal to unity and is represented by a segment, then the mass of crystals at point a in alloy K is equal to the ratio

Amount of liquid

i.e. the amount of liquid is determined by the ratio

The ratio of the amount of solid and liquid phases is determined by the ratio

If point a determines the state of the alloy, point determines the composition of the liquid phase, and point c determines the composition of the solid phase, then the segment determines the entire amount of the alloy, the segment determines the amount of liquid and the segment determines the number of crystals.

The second provision of the rule of segments is formulated as follows. In order to determine the quantitative ratio of the phases, a horizontal line is drawn through a given point. The segments of this line between the given point and the points that determine the composition of the phases are inversely proportional to the amounts of these phases.

The segment rule in dual statecharts can only be applied in two-phase areas. In a single-phase region, there is only one phase; any point inside the region characterizes its concentration.