1. Structure and crystallization of pure metal
1. Crystal structure of the metal
Metals are crystals in the solid state.
The crystalline structure is related to the properties, plastic deformation and heat treatment phase transformations of metals.
The three most common lattices in metals are the body-centered cubic lattice, the face-centered cubic lattice, and the close-packed hexagonal lattice.
Crystalline defects can be classified into three categories based on their geometric shapes: point defects, linear defects and plane defects.
2. Crystallization of metals
The process by which a metal passes from a liquid state to a solid (crystalline) state is known as crystallization of the metal.
(1) Cooling curve and supercooling phenomenon
The cooling curve is a graph that shows the relationship between temperature and time during the cooling process of a material. The cooling curve of a metallic crystal can be determined using thermal analysis methods. The process involves melting the metal to achieve as uniform a temperature as possible, cooling it at a set rate, recording the temperature changes over time, and plotting the data on a temperature-time graph to obtain the cooling curve, as shown. shown in Figure 1.
The latent heat of crystallization released during crystallization counteracts the metal's heat loss to the outside, causing a horizontal line to appear on the cooling curve. The temperature corresponding to this line is the actual crystallization temperature of the metal.
Experiments show that the actual crystallization temperature (T1) of the metal is always lower than the theoretical crystallization temperature (T0). This phenomenon is known as supercooling. Supercooling is a requirement for crystallization to occur. The difference between T0 and T1, △T = T0 – T1, is known as the degree of supercooling.
Fig. 1 Cooling curve of pure iron crystal
(2) Crystallization Process
The crystallization process involves the creation and expansion of nuclei. This process is known as nucleation and growth.
3. Isomeric transformation of metals
The phenomenon in which a metal transforms from one lattice structure to another as the temperature changes in the solid state is known as isomorphic transformation.
Some metals that exhibit this transformation include iron, cobalt, titanium, tin, and manganese.
Crystals of the same metallic element that exist in different lattice forms are called allotropic crystals of the metal.
2. Structure and crystallization of the alloy
Phase: Refers to the uniform components within an alloy (or pure metal) that have the same composition, structure and properties and are separated from each other by an interface.
1. League phase structure
The phase structure in alloys can be divided into two types based on the interaction between the constituent elements: solid solution and metallic compound.
(1) Solid Solution: When the liquid alloy solidifies, the elements can still dissolve into each other to form a phase in which the atoms of one element are dispersed throughout the lattice of another element. This phase is known as solid solution.
(2) Metallic Compound.
2. Binary alloy state diagram
The alloy phase diagram, also known as the alloy equilibrium diagram or alloy state diagram, is a diagram that illustrates the relationship between the temperature, composition, and state of an alloy under equilibrium conditions. It describes the laws of structural change of alloys with varying compositions as they are slowly cooled or heated to infinity.
The alloy phase diagram is an important tool for selecting the correct alloy composition, analyzing the alloy's microstructure, studying its properties, and determining the casting, forging, and heat treatment processes.
(1) Homogeneous phase diagram: This type of diagram represents an alloy system where two components can be infinitely miscible in liquid and solid states. During solidification, the alloy forms a solid solution from the liquid phase, a process known as homogeneous transformation.
(2) Eutectic phase diagram: In this diagram, two components are completely miscible in the liquid state and exhibit a eutectic transformation. Eutectic Transformation refers to the simultaneous crystallization of two solid phases with a specific composition from a uniform liquid phase with a specific composition at a specific temperature.
(3) Peritectic phase diagram: In this diagram, two components are infinitely miscible in the liquid state and form a finite solid solution in the solid state. There is also a state of peritectic transformation. Peritectic Transformation refers to the reaction between the liquid phase of a given component and the solid phase of another component, which results in the formation of a new solid phase at constant temperature.
3. Iron-carbon phase diagram
1. Iron-carbon phase diagram
Steel is an iron-carbon alloy with a specific composition range.
The iron-carbon alloy phase diagram illustrates the various equilibrium structures of iron-carbon alloys with varying compositions at different temperatures, as represented in the Fe-Fe3C phase diagram.
From the Fe-Fe3C phase diagram, we can determine the temperature at which phase transformation occurs in an iron-carbon alloy of a given composition, also known as the critical point.
By analyzing the Fe-Fe3C phase diagram, it is possible to predict the phase transformation process in different temperature regions and the equilibrium potential structure after cooling to room temperature.
See Characteristic Points on the Fe-Fe3C Phase Diagram for a description of each point on the Iron-Carbon Alloy Phase Diagram and Characteristic Lines for a description of each line.
According to the phase diagram of the iron-carbon alloy, carbon steel with a carbon content of less than 2.11% and cast iron with a carbon content of more than 2.11% are distinguished.
Based on structural characteristics, iron-carbon alloy is divided into seven categories based on the carbon content in the iron-carbon alloy phase diagram:
(1) Industrial pure iron, with carbon content < 0.0218%;
(2) Eutectoid steel, with a carbon content of 0.77%;
(3) Hypoeutectoid steel, with carbon content ranging from 0.0218% to 0.77%;
(4) Hypereutectoid steel, with carbon content ranging from 0.77% to 2.11%;
(5) Eutectic white cast iron, with a carbon content of 4.30%;
(6) Subcrystalline white cast iron, with carbon content ranging from 2.11% to 4.30%;
(7) Supercrystalline white cast iron, with carbon content ranging from 4.30% to 6.69%.
2. Metal structure
Metal: Material with good thermal and electrical conductivity, characterized by its opaque appearance and metallic luster. The conductivity of metals decreases as temperature increases, and they are known for their ductility and ability to expand.
A metallic crystal is a solid in which the atoms are arranged in a regular pattern.
Alloy: Substance with metallic properties composed of two or more elements, including metals and non-metals.
Solid solution strengthening: This occurs when solute atoms occupy the spaces or interstices of the solvent network, causing the network to become distorted and increasing the hardness and strength of the solid solution.
Compound: A new crystalline solid structure with metallic properties is formed by combining alloy components.
Mechanical Mixture: Alloy composition composed of two distinct crystalline structures, although it appears as a single entity with independent mechanical properties.
Ferrite: An interstitial solid solution of carbon in alpha-Fe (body-centered cubic iron).
Austenite: An interstitial solid solution of carbon in gamma-Fe (face-centered cubic iron).
Cementite: Stable compound (Fe3C) formed by the combination of carbon and iron.
Perlite: Mechanical mixture composed of ferrite and cementite (F + Fe3C, containing 0.8% carbon).
Ledeburite: Mechanical mixture composed of cementite and austenite (containing 4.3% carbon).
The heat treatment of metals is a crucial process in mechanical manufacturing. Unlike other processing methods, heat treatment does not change the shape or overall chemical composition of the part, but instead improves its performance by modifying its microstructure or surface chemical composition.
The objective of heat treatment is to improve the internal quality of the part, which is often not visible to the naked eye. To achieve the desired mechanical, physical and chemical properties of a metal part, heat treatment is often required, in addition to appropriate material selection and various forming processes.
Steel is the most used material in the mechanical industry and its microstructure can be controlled through heat treatment. As a result, steel heat treatment is an important aspect of metal heat treatment.
In addition to steel, heat treatment can also be used to modify the mechanical, physical and chemical properties of aluminum, copper, magnesium, titanium and their alloys, allowing the obtaining of various service properties.
