What is the lattice structure of metal?

According to the different arrangement of atoms in matter, matter can be divided into two categories: crystalline and amorphous. A substance in which the atoms are regularly arranged is called a crystal. All solid metals are crystals.

Substances with irregular arrangement of atoms inside are called amorphous. Such as glass, rosin, asphalt, etc.

The regular arrangement of atoms in the crystal is observed by electron microscope, which is called the crystal structure of metal. The arrangement of atoms in a crystal is called crystal structure.

Metal atoms are bound by the interaction between positive ions and free electrons, which is called metal bonds.

The common crystal structures of pure metals are body centered cubic lattice, face centered cubic lattice and closely spaced hexagonal lattice.

What is lattice?

Lattice: a three-dimensional space lattice formed by connecting atomic centers with imaginary straight lines. The intersection point (atomic center) of a straight line is called a node. Unit cell: the smallest geometric unit that can completely reflect the lattice characteristics.

Body centered cubic lattice (BCC)

The number of atoms in BCC cell is 1/8×8+1=2, and the density is 0.68.

Body centered cubic meter: Cr Cr, w w w, v v v, CB Nb, Ta TA, Mo Mo, steel( α- Fe、 δ- Fe)。

Face centered cubic lattice (FCC)

The number of atoms in FCC cell is 1/8×8+1/2×6=4, and the density is 0.74.

Face centered cubic: Al aluminum, Cu copper, Au gold, Pb lead, Ni nickel, Pt platinum, Ag silver, iron and steel( γ- Fe)。

Hexagonal close packed lattice (HCP)

The number of atoms in the close packed hexagonal cell is 1/6×12+1/2×2+3=6, and the density is 0.74.

Closely arranged hexagonal: Zn, Mg, Zr, CA, Co, Co, Mn, Ti.

Impact toughness refers to the energy consumption of a material when it breaks under an applied impact load.

The impact toughness of BCC lattice decreases sharply and has brittle ductile transition temperature.

The metal actually used is composed of many grains, also known as polycrystalline. Each grain is equivalent to a single crystal. The arrangement of atoms in the grain is the same, but the arrangement of atoms in different grains is different. The interface between grains is called grain boundary.

The process of high-temperature liquid metal cooling into solid metal is a crystallization process, that is, the transition of atoms from an irregular state (liquid) to a regular state (solid). The crystallization process always starts from the crystal nucleus, which is usually formed by adhering to the solid particle impurities in the liquid metal. The atoms in the liquid continue to gather towards the crystal nucleus, making the crystal nucleus grow up; At the same time, new grains are continuously produced in the liquid and continue to grow until all the grains grow to contact each other, and the crystallization is over.

The atomic arrangement of the actual crystal is not perfect. Due to various reasons, the atomic arrangement of many parts of the crystal is destroyed, resulting in a variety of defects.

Common defects are:

① Point defect —– vacancy, interstitial atom, substitutional atom;

② Dislocation.

Point defects (vacancies, interstitial atoms, replacement atoms) destroy the equilibrium state of atoms and cause curvature of the surrounding lattice, which is called lattice distortion. As a result, the yield point and tensile strength of the metal increase, while the plasticity and toughness decrease.

Dislocation refers to the local slip of one crystal relative to another in the lattice, and the boundary line between the slip region and the non slip region on the slip plane is called dislocation. The existence of dislocation makes the metal easy to plastic deformation and reduce its strength.

Iron carbon alloy

Steel and cast iron are generally referred to as iron carbon alloys. Iron carbon alloy is composed of more than 95% iron, 0.05% ~ 4% carbon and about 1% impurity elements. Iron carbon phase diagram, do you remember? (favorite Edition).

When the carbon content is less than 0.02%, it is called pure iron (industrial pure iron).

Generally, steel with carbon content of 0.02% ~ 2% is called steel.

Cast iron contains more than 2% carbon.

What is the alloy phase diagram?

Iron carbon alloy phase diagram, also known as iron carbon phase diagram or iron carbon alloy balance diagram, is established through experimental methods, which represents the structure and properties of iron carbon alloy at different compositions and temperatures, as well as the relationship between them.

What do the horizontal and vertical coordinates represent?

Abscissa: carbon content (0 ~ 6.69%), ordinate: temperature, multiple boundaries divide the phase diagram into multiple regions, and each region corresponds to a certain kind of structure.

What are the key points and lines in the iron carbon alloy phase diagram?

Point P: dividing point between pure iron and steel (0.0218% carbon content)

Point s: eutectoid line (0.77% carbon content)

Point E: dividing point between steel and pig iron (2.11% carbon content)

Point C: eutectic point (4.3% carbon content)

ACD: liquidus, aecf: solidus, gs:a3, es:acm, ECF: eutectic line (1148 ℃), PSK: eutectoid line, also known as A1 line (727 ℃).

Industrial pure iron (less than 0.0218% carbon content)

Microstructure: f+fe3cⅢ, ferrite (f) is the matrix structure of bright white equiaxed pure iron, mainly white. Ferrite with uniform grain distribution (Grade 6). The black thin strip in the figure is the grain boundary corrosion line.

The basic structure of steel includes austenite, ferrite and cementite (3 kinds)

① Ferrite (f-ferrite)

Carbon in α- The solid solution in Fe (below 910 ℃) is called ferrite, and f or α express.

Carbon in δ- Solid solution in Fe (between 1390 and 1535 ℃), weighing δ- Ferrite, with δ express.

α Iron and δ Iron is a body centered cubic lattice (with cold brittleness). The amount of carbon dissolved in ferrite is very poor, which is 0.02% at 727 ℃; 0.0008% at room temperature, almost zero. The metallographic structure is bright polygonal grains. It has high strength and hardness, good plasticity and toughness. It has ferromagnetism below 770 ℃ and loses ferromagnetism above 770 ℃. Grain size: common grade 1 ~ 8. Grade 8 is fine and uniform with good comprehensive mechanical properties.

② Austenite (a-austenite)

Carbon melts in γ- Solid solution formed in Fe (910~1390 ℃). γ- Iron is a face centered cubic lattice. Indicated by A.

The carbon dissolving capacity of austenite is higher than that of ferrite, reaching 2.11% at 1148 ℃ and 0.77% at 727 ℃. Austenite has higher plasticity and lower hardness and yield point than ferrite. In the iron carbon alloy system, it only exists in the high temperature range above 727 ℃ and does not have ferromagnetism. Therefore, it is often heated to austenite in rolling and forging to improve its plasticity. The microstructure of austenite is irregular polyhedral grain, and the grain boundary is straighter than that of ferrite.

③ Cementite (Fe3C cementite)

A metallic compound of iron and carbon with a complex lattice structure. The melting temperature of cementite is 1600 ℃, the carbon content is 6.67%, the hardness of cementite is very high, the brittleness is very great, and the plasticity and toughness are almost zero. Cementite is weakly magnetic at low temperature, and its magnetism disappears above 217 ℃.

When the carbon content of iron carbon alloy is less than 2%, its structure is cementite scattered in ferrite, which is carbon steel. When the carbon content is 2%, part of the carbon exists in the form of graphite, which is called cast iron. The tensile strength and plasticity are lower than those of carbon steel. However, cast iron has certain shock absorption capacity. Because carbon is α- The solubility in Fe is very small, so most carbon exists in the form of cementite Fe3C at room temperature.

In addition to the single-phase structure composed of austenite, ferrite and cementite, the basic structure of steel also has a multiphase structure composed of two basic phases, namely pearlite and ledeburite.

④ Pearlite (p-pearlite)

Pearlite is a mechanical mixture of ferrite and cementite arranged in interlaced layers. The lamellar spacing and thickness mainly depend on the undercooling during austenite decomposition. According to the lamellar thickness, it is divided into coarse pearlite P, sorbite s and troostite t.

Under the condition of slow cooling, the Fe-C alloy with carbon content of 0.77% only undergoes eutectoid reaction, and its structure is 100% pearlite, which is called eutectoid steel.

The properties of pearlite are between ferrite and cementite, with higher strength, hardness and plasticity.

The iron carbon alloy with carbon content greater than 0.77% is hypereutectoid steel, and its structure is p+fe3c.

The iron carbon alloy with carbon content less than 0.77% is hypoeutectoid steel, and its structure is f+p.

⑤ LD ledeburite

Ledeburite is a mixture of austenite and cementite. Ledeburite is a kind of high-temperature structure, which exists above 1148 ℃, 4.3%c.

Ledeburite has high hardness, great brittleness and poor plasticity.

Low carbon steel is hypoeutectoid steel, and its normal structure is ferrite F + pearlite P. The lower the carbon content, the more ferrite f in the structure, the better the plasticity and toughness of the material, but the strength and hardness will decrease.

Strength: when c<0.9%, the strength increases with the increase of C; When c>0.9%, the strength of the steel decreases due to the network distribution of cementite at the grain boundary.

Hardness: increases with the increase of C.

Plasticity: decreases rapidly with the increase of C.

Impact toughness: decreases rapidly with the increase of C.

Heat treatment knowledge

General process of heat treatment

Heat treatment process: the heat treatment process is mainly composed of three stages: heating, holding (time) and cooling. Temperature and time are the main factors affecting the heat treatment, so the heat treatment process can be expressed by temperature time curve.

The cooling of steel is the key process of heat treatment. After the steel with the same composition is heated to obtain austenite structure, it will obtain different mechanical properties when cooled at different speeds.

When the alloy is heated, the phase transformation takes place when the temperature is higher than the critical temperature of the alloy phase diagram. As shown in the figure, AC3, AC1 and ACM are the critical temperatures of steel during heating.

In actual production, the cooling of steel heat treatment is always carried out at a certain speed, that is, there is a phenomenon of supercooling. The difference between the theoretical critical point and the actual critical point is the degree of supercooling. For the same metal, the faster the cooling rate, the greater the component undercooling.

Solution treatment and stabilization of austenitic stainless steel

Heat austenitic stainless steel to 1050 ~ 1100 ℃ (at this temperature, carbon can be dissolved in austenite), keep it for a certain time (about no less than 1 hour per 25mm thickness), and then quickly cool it to below 427 ℃ (cooling time from 925 ℃ to 538 ℃ is required to be less than 3 minutes) to obtain uniform austenite structure. This method is called solution treated chromium nickel austenitic stainless steel, which has low strength and hardness and good toughness, It has high corrosion resistance and good high temperature performance.

For chromium nickel austenite stainless steel containing titanium or niobium, in order to prevent intergranular corrosion, all carbon in the steel must be fixed in titanium carbide or niobium carbide. The heat treatment for this purpose is called stabilization treatment. The stabilization process is: heat the workpiece to 850 ~ 900 ℃, keep it warm for 6 hours, and cool it in air or slow cooling.

Stabilization treatment is only suitable for chromium nickel austenitic stainless steel containing titanium or niobium.

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