Solution Treatment in Type III
If Alloy 4 (85A-15B) of Type III phase diagram [having microstructure shown at (a) in the figure given below] is reheated to point M, all the excess β will be dissolved and the structure will be a homogeneous α solid solution.
The alloy is now cooled rapidly (quenched) to room temperature. The quenching is generally carried out in a cold water bath or by a water spray. Drastic quenching may result in distortion in intricately designed parts. In such cases, boiling water is used as quenching media to minimize distortion. If α is a ductile phase, the alloy will be ductile immediately after quenching. This allows warped or distorted parts to be straightened easily. In view of this, straightening operation should be carried out as soon as possible after quenching. The microstructure is shown schematically at (b) in the above figure.
Congruent-melting Intermediate Phase (Type IV) When one phase changes into another phase isothermally (at constant temperature) and without any change in chemical composition, it is said to be congruent phase change or congruent transformation. All pure metals solidify congruently. Compounds are called intermediate phases because they are single phases that occur between the terminal phases on a phase diagram. Any intermediate phase may be treated as another component on a phase diagram. If the intermediate phase has a narrow range of composition, as in cases of intermetallic compounds and interstitial compounds, it is than represented on the diagram as a vertical line and labeled with the chemical formula of the compound. If the intermediate phase exists over a range of composition, it is usually an electron compound and is labeled with a Greek letter.
In the above figure, the intermediate alloy phase is shown as a vertical line. Since it is a compound, it is indicated as AmBn, where m and n are subscripts which indicate the number of atoms combined in the compound. It is apparent from the figure that the A-B system may be separated into two independent parts, one to show all the alloys between A and the compound AmBn and the other to show those between AmBn and B. The portion of the diagram between A and AmBn may be any of the types studied in the articles on phase diagrams, similarly for the portion between AmBn and B.
If the compound shows no solubility for either pure metal and the pure metals show some solubility for each other, the equilibrium diagram will look as shown in the figure given below.
Above diagram shows two different eutectic mixtures. The eutectic equations may be written as under.
The Peritectic Reaction (Type V)
In the peritectic reaction a liquid and a solid react isothermally to form a new solid on cooling. The reaction is expressed in general as under.
The new solid formed is usually an intermediate phase as shown in the figure given below, but in some cases it may be a terminal solid solution.
It can be seen in the above figure that the compound AmBn, 70A-30B, when heated to the peritectic temperature, point G, decomposes into two phases, liquid and solid A. Therefore, it can be said that it is an incongruent melting intermediate alloy. It can be seen that the peritectic reaction is just the reverse of the eutectic reaction, where a single phase formed two new phases on cooling. The liquidus line is TADETB and the solidus line is TATPGJTETB. The peritectic reaction line is TPD. Notice that only part of this line, the length TPG, coincides with the solidus line.
Two liquids partly soluble in the liquid state(Type VI)
Above is a diagram of a typical system with a monotectic reaction. The monotectic reaction is:
L1 -> alpha + L2
In this reaction, a liquid decomposes into a solid phase and a new liquid phase. Above the monotectic temperature Tm, we have two liquid phases. From the Tm, to Tc, the critical temperature, the two liquids are immiscible and will separate as two layers. Below the monotectic temperature, there is usually a single solid phase in equilibrium with a liquid phase. Examples of this are the Ga-Pb and Ga-Tl systems.The monotectic alloy goes through point M in the above diagram. At Tm, it undergoes the monotectic reaction. Once again, we see similarities to the eutectic reaction. As the solution passes point M, the alpha phase starts to nucleate. The L2 phase appears as well, dispersed among the alpha matrix. When the solution reaches the Te temperature, the L2 liquid solidifies into the beta phase.
Transformations in the Solid State
There are several equilibrium changes and reactions which take place entirely in the solid state. They are known as transformations in the solid state.
Allotropy Change
Several metals may exist in more than one type of crystal structure depending upon their temperature. This property of a metal is called allotropy. Iron, manganese, cobalt and tin have this property. On equilibrium diagram, allotropic change is indicated by a point on the vertical line for pure metals.
In the above figure, the gamma solid solution field is looped. The pure metal A and alloy rich in A undergo two transformations. Many of the equilibrium diagrams involving iron such as Fe-Mo and Fe-Cr show this type of looped solid solution field. Since the type of iron that exists in this temperature range is gamma iron, the field is usually called the gamma loop. However, it is not a must for all alloys to form loop. In some alloy systems involving iron for example, iron and nickel, the gamma loop is not closed.
Order-disorder
Ordinarily in the formation of a substitutional type of solid solution the solute atoms do not occupy any specific position but are distributed at random in the lattice structure of the solvent. The alloy is said to be in a disordered condition. Some of these random solid solutions, if cooled slowly, undergo a rearrangement of the atoms where the solute atoms move into definite positions in the lattice. This structure as shown in figure below is now known as ordered solid solution or superlattice.
Order is most common in metals that are completely soluble in the solid state, and usually the maximum amount of ordering occurs at a simple atomic ratio of the two elements. For this reason, the ordered phase is sometimes given a chemical formula, such as AuCu and AuCu3 in the gold-copper alloy system. The effect of ordering on mechanical properties is negligible. However there is a significant reduction in electrical resistance.
The Eutectoid Reaction This is a common reaction in the solid state. It is very similar to eutectic reaction but does not involve the liquid. In this case, a solid phase transforms on cooling into two new solid phases. The general equation may be written as under.
The resultant eutectoid mixture is extremely fine, just like the eutectic mixture. Under the microscope both mixtures generally appear the same. An equilibrium diagram illustrating the eutectoid reaction is shown in the figure given below.
The liquidus line is TAETB and the solidus line is TAFGTB. The eutectic mixture is composed of the phases that occur at both ends of the eutectic temperature line, namely, γ solid solution (point F) and β solid solution (point G). Point M indicates an allotropic change for pure metal A. The significance of the solvus line MN is that, as the alloy composition is increased in B, the temperature at which the allotropic change takes place is decreased, reaching to a minimum at point N. The solvus line FN shows the decrease in solubility of B in γ as the temperature is decreased. Point N is known as eutectoid point. Its composition is the eutectoid composition, and the line OP is the eutectoid temperature line. Like the eutectic diagram, it is common practice to call all alloys to the left of eutectoid composition (point N) hypoeutectoid alloys and those to the right hypereutectoid alloys.
When the hypoeutectoid Alloy 1 is slowly cooled, γ solid solution is formed when the liquidus line is crossed at x1. More and more γ is formed until the solidus line is crossed at x2. It remains a uniform solid solution until the solvus line is crossed at x3. The pure metal A must now start to undergo an allotropic change, forming the α solid solution. It may be seen that the α solid solution dissolves much less of B than does the γ solid solution. Some of B atoms that are dissolved in the area that will undergo the allotropic change must now diffuse out of that area. When sufficient diffusion of B atoms has taken place, the remaining A atoms rearrange themselves into the new crystal structure, forming the α solid solution. The excess B atoms dissolve in the remaining γ solid solution, which becomes richer in B as the temperature falls. The composition of the remaining γ is gradually moving down and to the right along the solvus line MN. When the alloy reaches the eutectoid temperature x4, the remaining γ has now reached the eutectoid point N. The significance of the eutectoid line is that this temperature is the end of the crystal structure change that started at x3, and remaining γ must now transform by the eutectoid reaction, forming alternate layers of α and β in extremely fine mixture. The reaction may be written as under.
The microstructure at room temperature consists of primary α or proeutectoid α which was formed between x3 and x4, surrounded by the eutectoid mixture of α + β. In drawing the cooling curve for a given alloy from the phase diagram, it is important to remember that – whenever a line is crossed on the phase diagram, there must be a corresponding break in the cooling curve. Also, when a horizontal line is crossed, on the phase diagram, indicating a reaction, this will show on the cooling curve as a horizontal line. The cooling curve for Alloy 1 is shown in the following figure.
The Peritectoid Reaction
This is a fairly common reaction in the solid state and appears in many alloys. The peritectoid reaction may be written in general as under.
The new solid phase is usually an intermediate alloy, but it may also be a solid solution. The peritectoid reaction has the same relationship to the peritectic reaction as the eutectoid has to eutectic. Essentially, it is the replacement of a liquid by a solid.
Solid State Reactions
There are three major solid state reactions that arise:
• The eutectoid: alpha -> beta + gamma
• The monotectoid: alpha1 -> beta + alpha2
• The peritectoid: alpha + beta -> gamma
If Alloy 4 (85A-15B) of Type III phase diagram [having microstructure shown at (a) in the figure given below] is reheated to point M, all the excess β will be dissolved and the structure will be a homogeneous α solid solution.
The alloy is now cooled rapidly (quenched) to room temperature. The quenching is generally carried out in a cold water bath or by a water spray. Drastic quenching may result in distortion in intricately designed parts. In such cases, boiling water is used as quenching media to minimize distortion. If α is a ductile phase, the alloy will be ductile immediately after quenching. This allows warped or distorted parts to be straightened easily. In view of this, straightening operation should be carried out as soon as possible after quenching. The microstructure is shown schematically at (b) in the above figure.
Congruent-melting Intermediate Phase (Type IV) When one phase changes into another phase isothermally (at constant temperature) and without any change in chemical composition, it is said to be congruent phase change or congruent transformation. All pure metals solidify congruently. Compounds are called intermediate phases because they are single phases that occur between the terminal phases on a phase diagram. Any intermediate phase may be treated as another component on a phase diagram. If the intermediate phase has a narrow range of composition, as in cases of intermetallic compounds and interstitial compounds, it is than represented on the diagram as a vertical line and labeled with the chemical formula of the compound. If the intermediate phase exists over a range of composition, it is usually an electron compound and is labeled with a Greek letter.
In the above figure, the intermediate alloy phase is shown as a vertical line. Since it is a compound, it is indicated as AmBn, where m and n are subscripts which indicate the number of atoms combined in the compound. It is apparent from the figure that the A-B system may be separated into two independent parts, one to show all the alloys between A and the compound AmBn and the other to show those between AmBn and B. The portion of the diagram between A and AmBn may be any of the types studied in the articles on phase diagrams, similarly for the portion between AmBn and B.
If the compound shows no solubility for either pure metal and the pure metals show some solubility for each other, the equilibrium diagram will look as shown in the figure given below.
Above diagram shows two different eutectic mixtures. The eutectic equations may be written as under.
The Peritectic Reaction (Type V)
In the peritectic reaction a liquid and a solid react isothermally to form a new solid on cooling. The reaction is expressed in general as under.
The new solid formed is usually an intermediate phase as shown in the figure given below, but in some cases it may be a terminal solid solution.
It can be seen in the above figure that the compound AmBn, 70A-30B, when heated to the peritectic temperature, point G, decomposes into two phases, liquid and solid A. Therefore, it can be said that it is an incongruent melting intermediate alloy. It can be seen that the peritectic reaction is just the reverse of the eutectic reaction, where a single phase formed two new phases on cooling. The liquidus line is TADETB and the solidus line is TATPGJTETB. The peritectic reaction line is TPD. Notice that only part of this line, the length TPG, coincides with the solidus line.
Two liquids partly soluble in the liquid state(Type VI)
Above is a diagram of a typical system with a monotectic reaction. The monotectic reaction is:
L1 -> alpha + L2
In this reaction, a liquid decomposes into a solid phase and a new liquid phase. Above the monotectic temperature Tm, we have two liquid phases. From the Tm, to Tc, the critical temperature, the two liquids are immiscible and will separate as two layers. Below the monotectic temperature, there is usually a single solid phase in equilibrium with a liquid phase. Examples of this are the Ga-Pb and Ga-Tl systems.The monotectic alloy goes through point M in the above diagram. At Tm, it undergoes the monotectic reaction. Once again, we see similarities to the eutectic reaction. As the solution passes point M, the alpha phase starts to nucleate. The L2 phase appears as well, dispersed among the alpha matrix. When the solution reaches the Te temperature, the L2 liquid solidifies into the beta phase.
Transformations in the Solid State
There are several equilibrium changes and reactions which take place entirely in the solid state. They are known as transformations in the solid state.
Allotropy Change
Several metals may exist in more than one type of crystal structure depending upon their temperature. This property of a metal is called allotropy. Iron, manganese, cobalt and tin have this property. On equilibrium diagram, allotropic change is indicated by a point on the vertical line for pure metals.
In the above figure, the gamma solid solution field is looped. The pure metal A and alloy rich in A undergo two transformations. Many of the equilibrium diagrams involving iron such as Fe-Mo and Fe-Cr show this type of looped solid solution field. Since the type of iron that exists in this temperature range is gamma iron, the field is usually called the gamma loop. However, it is not a must for all alloys to form loop. In some alloy systems involving iron for example, iron and nickel, the gamma loop is not closed.
Order-disorder
Ordinarily in the formation of a substitutional type of solid solution the solute atoms do not occupy any specific position but are distributed at random in the lattice structure of the solvent. The alloy is said to be in a disordered condition. Some of these random solid solutions, if cooled slowly, undergo a rearrangement of the atoms where the solute atoms move into definite positions in the lattice. This structure as shown in figure below is now known as ordered solid solution or superlattice.
Order is most common in metals that are completely soluble in the solid state, and usually the maximum amount of ordering occurs at a simple atomic ratio of the two elements. For this reason, the ordered phase is sometimes given a chemical formula, such as AuCu and AuCu3 in the gold-copper alloy system. The effect of ordering on mechanical properties is negligible. However there is a significant reduction in electrical resistance.
The Eutectoid Reaction This is a common reaction in the solid state. It is very similar to eutectic reaction but does not involve the liquid. In this case, a solid phase transforms on cooling into two new solid phases. The general equation may be written as under.
The resultant eutectoid mixture is extremely fine, just like the eutectic mixture. Under the microscope both mixtures generally appear the same. An equilibrium diagram illustrating the eutectoid reaction is shown in the figure given below.
The liquidus line is TAETB and the solidus line is TAFGTB. The eutectic mixture is composed of the phases that occur at both ends of the eutectic temperature line, namely, γ solid solution (point F) and β solid solution (point G). Point M indicates an allotropic change for pure metal A. The significance of the solvus line MN is that, as the alloy composition is increased in B, the temperature at which the allotropic change takes place is decreased, reaching to a minimum at point N. The solvus line FN shows the decrease in solubility of B in γ as the temperature is decreased. Point N is known as eutectoid point. Its composition is the eutectoid composition, and the line OP is the eutectoid temperature line. Like the eutectic diagram, it is common practice to call all alloys to the left of eutectoid composition (point N) hypoeutectoid alloys and those to the right hypereutectoid alloys.
When the hypoeutectoid Alloy 1 is slowly cooled, γ solid solution is formed when the liquidus line is crossed at x1. More and more γ is formed until the solidus line is crossed at x2. It remains a uniform solid solution until the solvus line is crossed at x3. The pure metal A must now start to undergo an allotropic change, forming the α solid solution. It may be seen that the α solid solution dissolves much less of B than does the γ solid solution. Some of B atoms that are dissolved in the area that will undergo the allotropic change must now diffuse out of that area. When sufficient diffusion of B atoms has taken place, the remaining A atoms rearrange themselves into the new crystal structure, forming the α solid solution. The excess B atoms dissolve in the remaining γ solid solution, which becomes richer in B as the temperature falls. The composition of the remaining γ is gradually moving down and to the right along the solvus line MN. When the alloy reaches the eutectoid temperature x4, the remaining γ has now reached the eutectoid point N. The significance of the eutectoid line is that this temperature is the end of the crystal structure change that started at x3, and remaining γ must now transform by the eutectoid reaction, forming alternate layers of α and β in extremely fine mixture. The reaction may be written as under.
The microstructure at room temperature consists of primary α or proeutectoid α which was formed between x3 and x4, surrounded by the eutectoid mixture of α + β. In drawing the cooling curve for a given alloy from the phase diagram, it is important to remember that – whenever a line is crossed on the phase diagram, there must be a corresponding break in the cooling curve. Also, when a horizontal line is crossed, on the phase diagram, indicating a reaction, this will show on the cooling curve as a horizontal line. The cooling curve for Alloy 1 is shown in the following figure.
The Peritectoid Reaction
This is a fairly common reaction in the solid state and appears in many alloys. The peritectoid reaction may be written in general as under.
The new solid phase is usually an intermediate alloy, but it may also be a solid solution. The peritectoid reaction has the same relationship to the peritectic reaction as the eutectoid has to eutectic. Essentially, it is the replacement of a liquid by a solid.
Solid State Reactions
There are three major solid state reactions that arise:
• The eutectoid: alpha -> beta + gamma
• The monotectoid: alpha1 -> beta + alpha2
• The peritectoid: alpha + beta -> gamma
No comments:
Post a Comment