What is an alloy?

A substance with metallic properties composed of one metal and another or several metals or non-metals is called an alloy. Alloys are generally prepared by fusing each component into a uniform liquid and then condensing it.

According to the number of elements that make up the alloy, there are binary alloys, ternary alloys and multi-element alloys; according to the different alloy structures, they can be divided into the following three basic types:

(1) Eutectic mixture. When the eutectic mixture solidifies, the components are crystallized separately into alloys, such as bismuth-cadmium alloys. The lowest melting temperature of bismuth-cadmium alloy is 413 K. At this temperature, the bismuth-cadmium eutectic mixture contains 40% cadmium and 60% bismuth.

(2) Solid solution. The components form an alloy in solid solution. A solid solution refers to a metal crystal in which the atoms of the solute dissolve into the lattice of the solvent, while still maintaining the lattice type of the solvent. Some solid solution alloys are formed by replacing part of the solvent atoms with solute atoms at the lattice nodes of the solvent metal, such as alloys of copper and gold; some solid solution alloys are formed by the solute atoms distributed in the interstices of the solvent lattice formed in.

(3) Intermetallic compounds. The components form an alloy of compounds with each other.

Generally speaking, the melting point of an alloy is lower than the melting point of any of the constituent metals. For example, the Wude alloy used as a power fuse has a melting point of only 67 °C, which is lower than the melting point of the four metals that make it up. The hardness of an alloy is generally greater than the hardness of its constituent metals. For example, the hardness of bronze is greater than that of copper and tin, and the hardness of pig iron is greater than that of pure iron. Alloys are both less conductive and thermally conductive than pure metals. Some alloys also vary considerably in their chemical properties. For example, iron is easy to rust. If about 15% of chromium and about 0.5% of nickel are added to ordinary steel, it becomes a stainless steel that is resistant to corrosion by acid and alkali.

"Qi" also means alloy. Alloys containing *** are often called *** qi. For example, sodium alloy is an alloy composed of sodium and zinc, and zinc alloy is an alloy composed of zinc and zinc.

What is an alloy and its structure?

Alloy is a substance with advance characteristics composed of a metal element and one or several other elements (metal or non-metal) after fusion. The most basic substances that make up alloys and can exist independently are called components, or elements for short. In most cases, the constituents are the elements that make up the alloy. However, there are also compounds that are used as constituents, provided that the compounds are neither decomposed nor undergo any chemical reaction within the studied range. According to the number of components, it can be divided into binary alloys, ternary alloys or multi-component alloys. For example, simple brass is a binary alloy composed of two elements, copper and zinc; duralumin is composed of three elements: aluminum, copper, and magnesium. composed of ternary alloys.

◆Classification of copper alloys

Copper alloys are divided into brass, bronze and cupronickel. Cupronickel is a copper-nickel alloy, which is mainly used to manufacture corrosion-resistant parts, resistors, thermocouples, etc. in precision machinery and precision instruments.

In mechanical production, brass and bronze are mainly used.

●Cast brass

The alloys known as copper and zinc are collectively referred to as brass. Among them, the copper-zinc binary alloy is called ordinary brass. In addition to zinc, the multi-component brass composed of other elements is called special brass.

Casting brass has higher mechanical properties, better casting performance, and lower price than bronze. Commonly used in general-purpose bearings, bushings, gears and other wear parts and valves and other corrosion-resistant parts.

●Cast bronze

It can be divided into two categories: ordinary bronze (tin bronze) and special bronze (aluminum bronze, lead bronze, silicon bronze, beryllium bronze, etc.).

◆Copper alloy casting process

The crystal characteristics of copper alloys of various compositions are different, the casting properties are different, and the casting process characteristics are also different.

1. Tin bronze: The crystallization feature is that the crystallization temperature range is large and the solidification area is wide. In terms of casting performance, it has poor fluidity, is prone to shrinkage, and is not easy to oxidize. The process is characterized by directional solidification (sequential solidification) for thick-walled parts, simultaneous solidification for complex thin-walled parts and general thick-walled parts.

2. Aluminum bronze and aluminum brass: The crystallization feature is that the crystallization temperature range is small, which is a layer-by-layer solidification feature. In terms of casting performance, the fluidity is good, it is easy to form concentrated shrinkage cavities, and it is easy to oxidize. The process feature is that the aluminum bronze pouring system is bottom pouring, and the aluminum brass pouring system is open.

3. Silicon brass: The crystalline characteristics are between tin bronze and aluminum bronze. Best casting properties (in special brass). The process features are sequential solidification process, medium-casting gating system, and small size of dark riser.

◆Classification of aluminum alloy castings

Cast aluminum alloys can be divided into aluminum-silicon alloys, aluminum-copper alloys, aluminum-magnesium alloys and aluminum-zinc alloys according to their chemical composition.

The casting process of aluminum alloy

The casting properties of aluminum alloys are closely related to the chemical composition. The Al-Si alloy is near the eutectic composition and has the best casting properties, similar to gray cast iron. Al-Cu alloy is far away from eutectic composition, has a large solidification temperature range, and has the worst casting performance. In actual production, aluminum castings all have riser feeding. The solidification temperature range of Al-Si alloys is small, the riser feeding efficiency is high, and it is easy to obtain castings with dense structures. Other types of cast aluminum alloys have a large solidification temperature range, low riser feeding efficiency, and poor casting compactness.

Aluminum alloy is easy to inhale and oxidize, so the pouring system must ensure that the molten aluminum flows in quickly and smoothly to avoid stirring.

Various casting methods are suitable for aluminum alloy castings. When the production volume is small, sand casting can be used, and fine sand should be used for modeling; for important castings produced in large quantities, special casting is used. Metal mold casting has high efficiency and good casting quality. Low pressure casting is suitable for water-resistant die castings that require high compactness. Die casting can be used for thin-walled and complex small parts.

●Smelting characteristics of cast aluminum alloy

Aluminum alloy is easily oxidized in liquid state, its product is Al2O3, the melting point is as high as 2050 ℃, the density is slightly higher than that of aluminum, and the solid inclusions are suspended in the aluminum liquid, which is difficult to remove, which not only deteriorates the casting properties, but also reduces the mechanical properties, making the castings difficult to remove. Density is reduced. The molten aluminum is also very easy to absorb hydrogen, and precipitate out during solidification, forming defects such as pores or pinholes.

1. Refining method In order to slow down the oxidation and inhalation of the molten aluminum, the aluminum alloy should be smelted under the coverage of the flux layer. KCl, NaCl, etc. can be added to the crucible as a flux to isolate the molten aluminum from the furnace gas. In order to drive off the inhaled hydrogen in the molten aluminum and prevent the generation of pinholes, the hydrogen-flooding refining should be carried out before the molten aluminum is released from the furnace. There are many methods, the simpler one is to use a bell jar to press chloride salts or chlorides such as zinc chloride (ZnCl2) or hexachloroethane (C2Cl6) into the aluminum liquid, and the following reaction occurs:

3ZnCl2+2Al=3Zn+2AlCl3

3C2Cl6+2Al=3C2Cl4+2AlCl3

The boiling point of AlCl3 produced by the reaction is 183 °C, and the boiling point of C2Cl4 is 121 °C, so bubbles are formed, and the gas H2 and Al2O3 in the aluminum liquid are mixed and brought out of the liquid surface during the floating process.

2. There are many types of aluminum alloy furnaces for melting equipment, and coke crucible boilers are generally used. Resistance crucible boilers are also available. In addition, induction furnaces (power frequency, intermediate frequency) are also used.

The structure of alloys is much more complex than that of pure metals. Because alloys are composed of two or more elements, the interaction between the elements can form various phases. In metals and alloys, we call the homogeneous components that have the same chemical composition, the same structure and are separated from other parts by the interface.

The structure of pure metals and alloys is analyzed according to this concept below. Pure metal is a single phase when it is in liquid state; solid state is composed of the same element and the same crystal lattice, so it is a single phase; during the crystallization process, there are both liquid and solid phases, that is, two phases. When the alloy is in a liquid state, it is an alloy liquid with a certain uniform chemical composition, which is a single phase. After the alloy changes from a liquid state to a solid state, the elements dissolve with each other to form a solid solution; the elements may also react with each other to form metal compounds. Solid solutions and metal compounds are the two basic phases of solid state alloys. Therefore, when the alloy is in the solid state, it may be a single-phase structure or a multi-phase structure. When analyzing the alloy structure, it is to analyze its phase structure to see that it consists of several solid solutions or metal compounds, that is, several phases.

◆Solid solution

The so-called solid solution refers to the alloy phase composed of solute atoms dissolved in the crystal lattice of the metal solvent. The two components dissolve in each other in the liquid state, and the solid state also dissolves in each other, and form a uniform and consistent substance. When a solid solution is formed, the one with the largest content is the solvent, and the one with the less content is the solute; the crystal lattice of the solvent is the crystal lattice of the solid solution.

●Classification of solid solutions

According to the different positions of solute atoms in the crystal lattice, they can be divided into substitutional solid solutions and interstitial solid solutions.

1. The solid solution formed by replacing the solute atoms of the solid solution to occupy the node positions in the solvent lattice is called a replacement solid solution. When the atomic diameter of the solvent and the solute is not much different, generally within 15%, it is easy to form a substitutional solid solution. The copper-nickel binary alloy forms a substitutional solid solution, and nickel atoms can replace copper atoms in any position of the copper lattice.

2. Interstitial solid solution A solid solution formed by distributing the solute atoms in the interstitial space of the solvent lattice is called an interstitial solid solution. The solvent of the interstitial solid solution is a transition group metal with a larger diameter, while the solute is a non-metallic element such as carbon and hydrogen with a small diameter. Its formation condition is that the ratio of solute atom to solvent atom diameter must be less than 0.59. For example, in iron-carbon alloys, the solid solutions formed by iron and carbon - ferrite and austenite, are interstitial solid solutions.

In addition, according to the solubility of solute elements in solid solution, it can be divided into finite solid solution and infinite solid solution. But only the substitutional solid solution has the potential to become an infinite solid solution.

The properties of solid solution

When the content of solute elements is small, the properties of solid solutions are basically the same as those of solvent metals. However, with the increase of the content of solute elements, the strength and hardness of the metal will increase, which is called solid solution strengthening. Both substitutional solid solutions and interstitial solid solutions produce solid solution strengthening.

Appropriate control of the solute content can significantly improve the strength and hardness, while still ensuring high enough plasticity and toughness, so the solid solution generally has better comprehensive mechanical properties. Therefore, structural materials with comprehensive mechanical properties are required, and almost all use solid solution as the basic phase. This is why solid solution strengthening has become an important strengthening method and is widely used in industrial production.

◆Metal compounds

The metal compound is a new phase formed by the interaction of the two components, its crystal structure is different from the two components, and has metallic properties. There are many kinds of metal compounds, their common features are high melting point and high hardness, and they are generally used as hardening phases in alloys. Such as Fe3C in carbon steel, TiC, VC, W2C in alloy steel, etc.

In the alloy, there are fewer cases in the form of single-phase solid solution or metal compound, and most of them exist in the form of two-phase mechanical mixture. For example, pearlite in carbon steel is a mechanical mixture composed of solid solution (ferrite) and metal compound (cementite Fe3C).

◆Determination of alloy phase diagram

As mentioned earlier, the crystallization of pure metals is carried out at a constant temperature, which can be described by a cooling curve. The transformation of an alloy from a liquid state to a solid state occurs within a certain temperature range, which can also be represented by a cooling curve. However, the crystallization process of an alloy system needs to be represented by a phase diagram in order to be clearly expressed.

The alloy phase diagram is a graph showing the relationship between the composition, temperature and structure of the alloy, and it is an important tool for studying the alloy. Alloy phase diagrams are generally obtained by experimental methods. Now take the Pb-Sb binary alloy as an example to illustrate the determination process of the binary alloy phase diagram.

(1) Prepare several alloys of different compositions and make their cooling curves respectively.

(2) Find out the crystallization transition temperature of each alloy, that is, the crystallization start temperature and the crystallization end temperature.

(3) The crystallization start temperature and crystallization end temperature of each alloy are respectively marked in the temperature-composition coordinate system.

(4) Connecting the crystallization start temperature points of each alloy is the liquidus line; connecting the crystallization end temperature points is the solidus line. This constitutes the Pb-Sb binary alloy phase diagram. Other alloy phase diagrams can also be determined in this way.

The Pb-Sb alloy is composed of a binary eutectic phase diagram. Other alloys can also form binary eutectoid phase diagrams, binary phase diagrams that form stable compounds, binary homogeneous phase diagrams, and the like.

◆Iron carbon alloy

Iron-carbon alloy is a general term for steel and iron, and it is the most widely used alloy in industry. Iron-carbon alloy is an alloy composed of iron as the basic element and carbon as the main element. In the liquid state, iron and carbon are infinitely soluble in each other. In the solid state, carbon dissolves in iron to form a solid solution. Metal compounds occur when the carbon content exceeds the solid-state solubility of carbon in iron. In addition, mechanical mixtures consisting of solid solutions and metal compounds can also be formed.

The following describes several basic structures of iron-carbon alloys in the solid state.

●Ferrite

Ferrite is an interstitial solid solution in which carbon is dissolved in a-Fe, and is commonly represented by the symbol F. It still maintains a body-centered cubic lattice, and its carbon-dissolving ability is very small. The maximum carbon-dissolving ability is 0.0008% at room temperature, and the maximum carbon-dissolving ability is 0.02% at 727 °C.

Due to the low carbon content of ferrite, its properties are similar to those of pure iron, with good plasticity and toughness, and elongation δ=45% to 50%. The strength and hardness are low, σb≈250MPa, and HBS=80.

●Austenite

Austenite is an interstitial solid solution in which carbon is dissolved in γ-Fe, and is commonly represented by the symbol A. It still maintains the face-centered cubic lattice of γ-Fe. Its carbon-dissolving ability is relatively large, the dissolved carbon is ωc=0.77% at 727°C, and the soluble carbon is 2.11% at 1148°C. Austenite is a structure that can only exist stably at high temperatures greater than 727 °C. Austenitic plasticity is good, and it is the structure required by most steel grades for pressure working at high temperature. Austenite is not magnetic.

●Cementite

Cementite is a metal compound formed by iron and carbon, and its chemical formula is Fe3C. The carbon content of cementite is ωc=6.69%, and the melting point is 1227°C. Its crystal lattice is a complex orthorhombic lattice, with high hardness HBW=800, almost zero plasticity and toughness, and great brittleness.

There are different forms of cementite in iron-carbon alloys, and their quantity, shape and distribution have a direct impact on the properties of iron-carbon alloys.

●Pearlite

Pearlite is the eutectoid of ferrite and cementite formed by the eutectoid transformation of austenite. Its form is a layered complex in which thin layers of ferrite and thin layers of cementite are alternately overlapped, also known as flake pearlite. Indicated by the symbol P, the carbon content is ωc=0.77%. Its mechanical properties are between ferrite and cementite, and are determined by the pearlite lamellar spacing, that is, the average value of the thickness of a layer of ferrite and a layer of cementite.

●Ledburite

Ledburite is a eutectic composed of austenite and cementite formed by eutectic transformation of liquid iron-carbon alloy, and its carbon content is ωc=4.3%. When the temperature is higher than 727 ℃, ledeburite is composed of austenite and cementite, which is represented by the symbol Ld. When the temperature is lower than 727 °C, ledeburite is composed of pearlite and cementite, which is represented by the symbol Ld', which is called transformed ledeburite. Because the matrix of ledeburite is hard and brittle cementite, it has high hardness and poor plasticity.

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