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Details Category: Metal

Metals and alloys

In industry, metals are used mainly in the form alloys: black (cast iron, steel) and colored (bronze, brass, duralumin, etc.)

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Steel And cast iron- This iron-carbon alloys . But steel has slightly less carbon content than cast iron.

IN cast iron contains from 2 to 4% carbon. Cast iron also contains silicon, manganese, phosphorus and sulfur. Cast iron- brittle hard alloy. Therefore, it is used in those products that will not be subject to shock. For example, heating radiators, machine beds and other products are cast from cast iron.

Steel, like cast iron, has impurities of silicon, phosphorus, sulfur and other elements, but in smaller quantities.
Steel not only durable, but also ductile metal. Thanks to this, it lends itself well machining ke. Steel It happens soft And hard .

Harder steel is used to make wire, nails, screws, rivets and other products.

Made from very hard steel metal structures (structural steel) And cutting tools (tool steel). Tool steel has greater hardness and strength than structural steel.

Adding elements to steel such as chromium, nickel, tungsten, vanadium , allows you to obtain alloys with special physical properties - acid-resistant, stainless, heat-resistant etc.

Cast iron smelted from iron ore blast furnaces. Ore together with coke (specially treated coal, which gives a high temperature when burning) is loaded into the blast furnace from above. Clean hot air is constantly blown into the blast furnace from below so that the coke burns better. A high temperature is generated inside the furnace, the ore melts, and the resulting pig iron flows to the bottom of the furnace. Molten metal flows from the blast furnace opening into the ladles. Steel is produced from a mixture of cast iron and steel scrap in open-hearth furnaces, converters and electric furnaces.

From non-ferrous alloys most widely used bronze, brass and duralumin.

Bronze- yellow-red alloy based on copper with the addition tin, aluminum tion and other elements. It is characterized by high strength and resistance to corrosion. Art objects are cast from bronze, plumbing fittings, pipelines, and parts that operate under conditions of friction and high humidity are made.

Brass - copper-zinc alloy , yellow. It has high hardness, ductility, and corrosion resistance. It is produced in the form of sheets, wire, hexagonal rolled products and is most often used for the manufacture of parts operating in conditions of high humidity.

Duralumin - aluminum alloy with copper, zinc, magnesium and other metals, silver in color. It has high anti-corrosion properties and is easy to process. Duralumin is widely used in aircraft manufacturing, mechanical engineering and construction, where lightweight and durable structures are required.

Basic properties of metals

You know that metals have different properties . Some of them soft, viscous , other hard, elastic ie or fragile . Knowing the properties of metals is necessary in order to correctly determine the most suitable material for a particular product.

Physical properties.

These properties include: color, specific gravity, thermal conductivity, electrical conductivity, melting point.

Color metal or alloy is one of the signs that allows us to judge its properties.
Metals vary in color. For example, steel - grayish color, zinc - bluish-white, copper - pinkish red.
When heated, the color of the metal surface can roughly determine to what temperature it is heated, which is especially important for welders. However, some metals (aluminum) do not change color when heated.

The surface of an oxidized metal has a different color than a non-oxidized one.

Specific gravity - the weight of one cubic centimeter of a substance, expressed in grams . For example, carbon steel has a specific gravity of 7.8 g/cm3. In the automotive and aircraft industries, the weight of parts is one of the most important characteristics, since structures must be not only strong, but also light. The greater the specific gravity of the metal, the heavier (with equal volume) the product turns out.

Thermal conductivity - the ability of a metal to conduct heat - measured by the amount of heat that passes through a metal rod with a cross-section of 1 cm2 in 1 minute. The greater the thermal conductivity, the more difficult it is to heat the edges of the part being welded to the desired temperature.

Melting point - the temperature at which a metal changes from solid to liquid . Steel, for example, has a much higher melting point than tin.

Pure metals melt at one constant temperature, and alloys melt at a temperature range.

Mechanical properties.

The mechanical properties of metals and alloys include strength, hardness, elasticity, plasticity, viscosity.
These properties are usually the decisive indicators by which the suitability of a metal for various operating conditions is judged.

Strength -the ability of a metal to resist fracture when a load is applied to it .

Hardness - the ability of a metal to resist the penetration of another harder body into its surface . If you hit a center punch placed on a steel plate with a hammer, a small hole will form. If you do the same with a copper plate, the hole will be larger. This indicates that steel is harder than copper.

Elasticity - the property of a metal to restore its shape and size after the load is removed . For example, springs and leaf springs must have high elasticity, which is why they are made from special alloys. Try simultaneously stretching and releasing springs made of steel and copper wire. You will see that the first one will shrink again, and the second one will remain in the same position. This means that steel is a more elastic material than copper.

Plastic - the ability of a metal to change shape and size under the influence of an external load and maintain a new shape and size after the force ceases . Plasticity is the opposite property of elasticity. The greater the ductility, the easier the metal is forged, stamped, and rolled.

Viscosity - the ability of a metal to resist rapidly increasing (shock) loads. For example, if you hit a cast iron stove, it will collapse. Cast iron is a brittle metal. Viscosity is the opposite property of brittleness. Viscous metals are used in cases where parts are subject to shock loads during operation (parts of carriages, cars, etc.).

The fact that the properties of metals change when they are fused became known back in ancient times. \(5\) thousand years ago our ancestors learned to make bronze - an alloy of tin and copper. Bronze is harder than both metals in its composition.

The properties of pure metals, as a rule, do not meet the necessary requirements, therefore, in almost all spheres of human activity, not pure metals, but their alloys are used.

An alloy is a material that is formed by the solidification of a melt of two or more separate substances.

In addition to metals, alloys may also contain non-metals, such as carbon or silicon.

By adding impurities of other metals and non-metals in a certain amount, you can obtain many thousands of materials with a wide variety of properties, including those that are not found in any of the elements that make up the alloy.

The alloy, compared to the parent metal, can be:

• mechanically stronger and harder,
• with a significantly higher or lower melting point,
• more resistant to corrosion,
• more resistant to high temperatures,
• practically do not change their size when heated or cooled, etc.

For example, pure iron is a relatively soft metal. When carbon is added to iron, its hardness increases significantly. Based on the amount of carbon, and therefore hardness, they distinguish steel(carbon content less than \(2\)% by weight), cast iron(\(C\) - more than \(2\)%). But it’s not just carbon that changes the properties of steel. Chromium added to steel makes it stainless, tungsten makes the steel much harder, the addition of manganese makes the alloy wear-resistant, and vanadium makes it durable.

Application of alloys as structural materials

The alloys used to make various structures must be strong and easy to process.

They are most widely used in construction and mechanical engineering. iron and aluminum alloys.

Iron alloys such as steel, are characterized by high strength and hardness. They can be forged, pressed, welded.

Cast iron used for the manufacture of massive and very durable parts. For example, central heating radiators and sewer pipes used to be cast from cast iron; boilers, railings and bridge supports are still made today. Cast iron products are made using casting.

Aluminum alloys, used in structures, along with strength, must be lightweight. Duralumin, silumin- aluminum alloys, they are indispensable in aircraft, carriage and shipbuilding.

Some aircraft components use magnesium alloys, very light and heat resistant.

Lightweight and heat-resistant materials are used in rocketry. titanium alloys.

To improve impact resistance, corrosion resistance, and wear resistance, alloys are alloyed - special additives are introduced. Additive manganese makes steel impact resistant. To obtain stainless steel, the alloy is composed of: chromium.

Tool alloys

Tool alloys are intended for the manufacture of cutting tools, dies and parts of precision mechanisms. Such alloys must be wear-resistant and durable, and when heated, their strength should not decrease significantly. Such requirements are met, for example, stainless steels, which have undergone special processing (hardening).

Adding substances to alloys that improve their properties is called alloying.

To impart the necessary properties, tool steels are usually alloyed with tungsten, vanadium or chromium.

Application of alloys in the electrical industry, electronics and instrument making

Alloys serve as an indispensable material in the manufacture of particularly sensitive and high-precision instruments, various types of sensors and energy converters.

For example, the production of transformer cores and relay parts is used Nickel alloy. Individual parts of electric motors are made from cobalt alloys.

Nickel-chrome alloy - nichrome, characterized by high resistance - used for the manufacture of heating elements for stoves and household electrical appliances.

From copper alloys In the electrical industry and instrument making, brass and bronze are most widely used.

Brass are indispensable in the manufacture of devices, parts of which are shut-off valves. Such devices are used in gas and water supply networks.

Bronze used for the manufacture of springs and spring contacts.

Application of low-melting alloys

The main demanded property of low-melting alloys is a given low melting point. This property is, in particular, used for soldering microcircuits. In addition, these alloys must have a certain density, tensile strength, chemical inertness, and thermal conductivity.

Low-melting alloys are made from bismuth, lead,cadmium,tin and other metals. Such alloys are used in temperature sensors, thermometers, fire alarms, for example, Wood's alloy. And also in foundry for the production of lost wax models, for fixation of bones and prosthetics in medicine.

Alloy sodium with potassium(melting point \(–\)\(12.5\) °C) is used as a coolant for cooling nuclear reactors.

Application of alloys in jewelry

The use of pure precious metals in jewelry is not always justified and advisable due to their high cost, physical and chemical characteristics.

To make gold jewelry more hard and wear-resistant, alloys with other metals are used.

The best additives are silver (lowers the melting point) and copper (increases hardness). Pure gold is used very rarely, as it is too soft and easily deformed and scratched.

Nozzles for laboratory instruments are made from alloys of gold with \(10–30\)% other noble metals (platinum or palladium), and jewelry and electrical contacts are made from an alloy with \(25–30\)% silver.

Jewelry made from gold alloys	Gold plated electrical contacts

Alloys in art

Tin bronze ( copper-tin alloy) is one of the first metal alloys mastered by man. Compared to pure copper, it has greater hardness, strength and is more fusible. Bronze is successfully used to produce castings with complex configurations, including art casting. The classic brand of bronze is bell bronze.

One of the new trends in art is the production of artistic cast iron products. Cast iron products are significantly superior in quality to forged products.

Cast iron is a much more brittle metal and not as malleable as steel. But even from such a seemingly rough material, real works of foundry art can be obtained by casting, for example, cast stairs or window bars. Such products are subject to only surface corrosion and do not require careful maintenance.

Metals have been used by humans for many millennia. The defining eras of human development are named after metals: the Bronze Age, the Iron Age, the Age of Cast Iron, etc. Not a single metal product around us consists of 100% iron, copper, gold or other metal. Each contains additives deliberately introduced by a person and harmful impurities introduced against the will of a person.

Absolutely pure metal can only be obtained in a space laboratory. All other metals in real life are alloys - solid compounds of two or more metals (and non-metals), purposefully obtained in the process of metallurgical production.

Classification

Metallurgists classify metal alloys according to several criteria:

Metals and alloys based on them have different physical and chemical characteristics.

The metal having the largest mass fraction is called the base.

Properties of alloys

The properties possessed by metal alloys are divided into:

To quantitatively express these properties, special physical quantities and constants are introduced, such as the elastic limit, Hooke's modulus, viscosity coefficient and others.

Main types of alloys

The most numerous types of metal alloys are made based on iron. These are steels, cast irons and ferrites.

Steel is an iron-based substance containing no more than 2.4% carbon, used for the manufacture of parts and housings for industrial installations and household appliances, water, land and air transport, tools and devices. Steels have a wide range of properties. The common ones are strength and elasticity. The individual characteristics of individual steel grades are determined by the composition of alloying additives introduced during smelting. Half of the periodic table is used as additives, both metals and non-metals. The most common of them are chromium, vanadium, nickel, boron, manganese, phosphorus.

If the carbon content is more than 2.4%, such a substance is called cast iron. Cast iron is more brittle than steel. They are used where it is necessary to withstand large static loads with small dynamic ones. Cast iron is used in the production of frames for large machine tools and technological equipment, bases for work tables, and in the casting of fences, gratings and decorative items. In the 19th and early 20th centuries, cast iron was widely used in building structures. Cast iron bridges have survived to this day in England.

Substances with a high carbon content and having pronounced magnetic properties are called ferrites. They are used in the production of transformers and inductors.

Copper-based metal alloys containing from 5 to 45% zinc are commonly called brasses. Brass is slightly susceptible to corrosion and is widely used as a structural material in mechanical engineering.

If you add tin to copper instead of zinc, you get bronze. This is perhaps the first alloy deliberately obtained by our ancestors several thousand years ago. Bronze is much stronger than both tin and copper and is second in strength only to well-forged steel.

Lead-based substances are widely used for soldering wires and pipes, as well as in electrochemical products, primarily batteries and accumulators.

Two-component aluminum-based materials, which contain silicon, magnesium or copper, are characterized by low specific gravity and high machinability. They are used in the engine, aerospace, and electrical component and appliance industries.

Zinc alloys

Zinc-based alloys are characterized by low melting points, corrosion resistance and excellent machinability. They are used in mechanical engineering, the production of computers and household appliances, and in publishing. Good anti-friction properties allow the use of zinc alloys for bearing shells.

Titanium alloys

Titanium is not the most affordable metal; it is difficult to produce and difficult to process. These shortcomings are compensated for by the unique properties of titanium alloys: high strength, low specific gravity, resistance to high temperatures and aggressive environments. These materials are difficult to machine, but their properties can be improved by heat treatment.

Alloying with aluminum and small amounts of other metals increases strength and heat resistance. To improve wear resistance, nitrogen is added to the material or cemented.

Titanium-based metal alloys are used in the following areas:

1. 1. • aerospace;
      • chemical;
      • atomic;
      • cryogenic;
      • shipbuilding;
      • prosthetics.

Aluminum alloys

If the first half of the 20th century was the century of steel, then the second was rightly called the century of aluminum.

It is difficult to name a branch of human life in which products or parts made of this light metal would not be found.

Aluminum alloys are divided into:

1. 1. • Foundry (with silicon). Used to produce conventional castings.
      • For injection molding (with manganese).
      • Increased strength, with the ability to self-harden (with copper).

Main advantages of aluminum compounds:

1. 1. • Availability.
      • Low specific gravity.
      • Durability.
      • Cold resistance.
      • Good machinability.
      • Electrical conductivity.

The main disadvantage of alloy materials is low heat resistance. When reaching 175°C, a sharp deterioration in mechanical properties occurs.

Another area of ​​application is the production of weapons. Aluminum-based substances do not spark under strong friction and collisions. They are used to produce lightweight armor for wheeled and flying military equipment.

Aluminum alloy materials are widely used in electrical engineering and electronics. High conductivity and very low magnetizability make them ideal for the production of housings for various radio and communications devices, computers and smartphones.

The presence of even a small proportion of iron significantly increases the strength of the material, but also reduces its corrosion resistance and ductility. A compromise on iron content is found depending on the requirements for the material. The negative effect of iron is compensated by adding metals such as cobalt, manganese or chromium to the alloy composition.

Magnesium-based materials compete with aluminum alloys, but due to their higher price they are used only in the most critical products.

Copper alloys

Typically, copper alloys refer to various grades of brass. With a zinc content of 5-45%, brass is considered red (tombac), and with a zinc content of 20-35%, it is considered yellow.

Thanks to its excellent machinability by cutting, casting and stamping, brass is an ideal material for the manufacture of small parts that require high precision. The gears of many famous Swiss chronometers are made of brass.

Brass is a mixture of copper and zinc

A little-known alloy of copper and silicon is called silicon bronze. It is highly durable. According to some sources, the legendary Spartans forged their swords from silicon bronze. If you add phosphorus instead of silicon, you get an excellent material for the production of membranes and leaf springs.

Hard alloys

These are wear-resistant and highly hard iron-based materials, which also retain their properties at high temperatures up to 1100 o C.

Chromium, titanium, and tungsten carbides are used as the main additive; nickel, cobalt, rubidium, ruthenium or molybdenum are auxiliary.

The main areas of application are:

1. 1. • Cutting tools (mills, drills, taps, dies, cutters, etc.).
      • Measuring tools and equipment (rulers, squares, calipers; working surfaces of special evenness and stability).
      • Stamps, dies and punches.
      • Rolls of rolling mills and paper machines.
      • Mining equipment (crushers, cutters, excavator buckets).
      • Parts and assemblies of nuclear and chemical reactors.
      • Highly loaded parts of vehicles, industrial equipment and unique building structures, such as the Burj Tower in Dubai.

There are other areas of application of carbide substances.

Metallurgy plays an extremely important role in our lives. No, not every one of us belongs to the glorious class of steelworkers, but we are faced with metal products every day. As a rule, they are made from a wide variety of alloys. By the way, what is this?

Basic definitions

In general, metal alloys are materials obtained by smelting, in the production of which two or more metal elements (in the chemical sense) were used, as well as (optionally) special additives. One of the first materials of this kind was bronze. It is composed of 85% copper and 15% tin (80:20 in the case of bell bronze). Currently, there are several varieties of this compound that do not contain tin at all. But they don't occur very often.

It is necessary to clearly understand that metal alloys in most cases are formed without any human involvement at all. The fact is that it is possible to obtain a material that is absolutely pure from a chemical point of view only in the laboratory. Any metal that is used in everyday life probably contains traces of another element. A classic example is gold jewelry. Each of them contains a certain proportion of copper. However, in the classical sense, this definition still means a compound of two or more metals, which was purposefully obtained by man.

The entire history of man is an excellent example of how metal alloys were able to have a huge impact on the development of our entire civilization. It is no coincidence that there is even a long historical period called the “Bronze Age”.

General characteristics of metal alloys

Now we will look at the general properties of metals and alloys by which they are characterized. They can very often be found in specialized literature.

Characteristic	Decoding
Strength	The ability of an alloy to withstand mechanical loads and resist destruction.
Hardness	A property that determines the resistance of a material to attempts to introduce a part from another alloy or metal into its thickness.
Elasticity	The ability to restore its original shape after the application of significant mechanical force or load.
Plastic	On the contrary, this is a property that characterizes the possibility of changing shape and size under the influence of applied force, mechanical load. In addition, this also characterizes the ability of a part to maintain its newly acquired shape for a long time.
Viscosity	- the ability of a metal to resist rapidly increasing (shock) loads

These are the qualities that characterize metal alloys. The table will help you understand them.

Production information

In principle, at present, an “alloy” may well be understood as a material based on only one chemical element, but “diluted” with a whole package of additives. The most common method of obtaining them, melting them to a liquid state, has changed little since ancient times.

For example, analysis of metals and alloys shows that the ancient Indians mastered a level of metal processing that was amazing for their time. They even began to create alloys using refractory zinc, which in our time is a rather labor-intensive and complex procedure.

Today, powder metallurgy is also widely used for these purposes. Ferrous metals and alloys based on them are especially often processed by this method, since in this case the maximum cheapness of both the process itself and the manufactured product is often required.

Distribution of alloys in modern industry

It should be noted that all metals that are intensively used by modern industry are alloys. Thus, more than 90% of all iron produced in the world is used for the production of cast iron and various steels. This approach to the matter is explained by the fact that metal alloys in most cases demonstrate better properties than their “progenitors”.

Thus, the yield strength of pure aluminum is only 35 MPa. But if you add 1.6% copper, magnesium and zinc to it in a ratio of 2.5% and 5.6%, respectively, then this figure can easily exceed even 500 MPa. Among other things, electrical conductivity, thermal conductivity, or other properties can be significantly improved. There is no mysticism in this: in alloys, the structure of the crystal lattice changes, which allows them to acquire other properties.

Simply put, the amount of this kind of material is large these days, but it is constantly growing.

Basic classification information

In general, there are no particular difficulties here: compounds that use non-ferrous metals and iron-based alloys. Below we will analyze both of these categories using the main types as an example, and also discuss the scope of their application in modern industry and production.

Steel

All iron compounds containing up to 2% carbon are called steels. If the composition contains chromium, vanadium or molybdenum, then they are called alloyed. We encounter these materials constantly, daily and hourly. The number of steels today is such that listing them alone could fill a not-too-thin book.

Oddly enough, lead has long been known to chefs and restaurateurs, as tableware and cutlery were often made from it. The alloy used for this is called pewter. It contains approximately 85-90% tin. The remaining 10-15% is occupied by lead (a standard alloy of two metals).

Technicians are also likely familiar with babbitts. These are also lead-based compounds that also contain tin, as well as arsenic and antimony. These alloys are very poisonous, but due to some special properties they are actively used in the bearing industry.

About light alloys

As we have already said, the properties of metals and alloys differ in that the latter in many cases have higher characteristics. This is especially noticeable in relation to modern industry. In recent years, it has required a huge amount of light alloys, which have increased mechanical strength, as well as resistance to adverse environmental factors and high temperatures.

Most often, aluminum, beryllium, and magnesium are used for their production. Compounds based on aluminum and magnesium are especially in demand, since the scope of their possible application is extremely wide.

Aluminum alloys

As we have already said, it is absolutely impossible to imagine modern industry without them. Judge for yourself: aluminum alloys are actively used in aviation, space, military, scientific and engineering and other industries. Without aluminum it is impossible to imagine manufacturers of modern household and mobile appliances, since cases made of this metal are increasingly used by modern flagships of these industries.

What are they?

Aluminum alloys are divided into three large groups:

• Foundry (Al - Si). They are especially widespread in the automotive and military industries.
• Alloys intended for injection molding (Al - Mg).
• High-strength, self-hardening joints (Al - Cu).

Advantages and disadvantages of this material

Many alloys made from this material are economical, relatively inexpensive and very durable, as they do not corrode. They are distinguished by high strength in conditions of extremely low temperatures (aerospace industries) and a very simple processing process. Their molding does not require particularly complex and expensive equipment, since they are relatively plastic and viscous (see table with characteristics).

Unfortunately, they also have their drawbacks. Thus, at temperatures above 175 °C, the mechanical properties of aluminum and alloys based on it begin to rapidly deteriorate. But thanks to the presence of amalgam on their surface (a protective film of aluminum hydroxide), they have outstanding resistance to aggressive chemical environments, including acids and alkalis.

They have excellent electrical and thermal conductivity and are non-magnetic. It is believed that they are absolutely harmless to human health, and therefore they can be used for the production of food utensils and cutlery. However, recent medical researchers still say that aluminum compounds in some cases can provoke the development of Alzheimer's disease.

The military fell in love with these materials because they do not produce sparks even under sudden mechanical impacts and impacts. In addition, they perfectly absorb shock loads. Simply put, some of these metal alloys (the composition of which is most often classified) are actively used for the production of light armor to equip a variety of armored personnel carriers, infantry fighting vehicles, BRDMs and other equipment.

Thanks to all these properties, base alloys are widely used for the production of pistons for internal combustion engines, as well as in the production of building structures (corrosion resistance). Aluminum and materials based on it are widely used in the production of reflectors for lighting displays, electrical wiring, as well as for the manufacture of housings for various equipment (not magnetized).

It is important to note that even theoretically pure aluminum sometimes contains a significant admixture of iron. It can contribute to higher mechanical strength of the material, but its presence makes the aluminum-based alloy highly susceptible to corrosion processes. In addition, the alloy largely loses its ductility, which is also not very good in most cases.

Cobalt, chromium or manganese helps to weaken the negative effect of iron impurities. If the alloy contains lithium, the result is a very strong and elastic material. It is not surprising that this connection is very popular in the aerospace industry. Alas, lithium alloys with aluminum have an unpleasant property, which again manifests itself in poor ductility.

Let's summarize some results. It turns out that the main metal alloys in astronautics, aviation and other high-tech industries contain aluminum. In general, this is exactly how things stand today, but magnesium and its alloys are often used in modern industry.

Magnesium alloys

They have extremely low weight and are also characterized by very impressive strength. In addition, these materials are excellent for the foundry industry, and the workpieces are perfectly amenable to turning and milling. Therefore, they are actively used in the production of missiles and aircraft turbines, instrument housings, car wheel rims, as well as some types of armor steel.

Some varieties of these alloys are distinguished by excellent viscous damping properties, and therefore they are used in the production of parts and structures that have to work under conditions of extremely high levels of vibration.

Advantages and disadvantages of magnesium alloys

They are quite soft, resist wear relatively well, but are not very impressive in their ductility. But they are distinguished by their excellent adaptability to forming at high temperatures, are perfectly suited for joining using all existing types of welding, and can also be connected by bolting, riveting and even gluing.

Alas, all these alloys are not particularly resistant to acids and alkalis. They are extremely negatively affected by prolonged exposure to sea water. However, magnesium alloys are surprisingly stable in air conditions, so many of their disadvantages can be neglected. If it is necessary to reliably protect such parts from corrosion, then chrome plating, anodizing or similar methods are used.

They can be clad with nickel, copper or chromium, after first immersing them in a melt of chemically pure zinc. With this treatment, their strength and abrasion resistance increase sharply. It must be recalled that magnesium is a fairly active metal from a chemical point of view, and therefore when working with it it is necessary to observe at least basic safety measures.

Thus, the production of metals and alloys is a key feature of modern industry. Every year people are inventing more and more ways to obtain new materials, so soon we will probably get absolutely incredible compounds that will combine the beneficial properties of several groups of materials and chemical elements at once.

Metals and their alloys

Metals are substances characterized under ordinary conditions by high electrical and thermal conductivity, malleability, “metallic” luster, opacity and other properties due to the presence in their crystal lattice of a large number of mobile conduction electrons not associated with atomic nuclei.

In technology, metals are usually divided into ferrous (iron and alloys based on it) and non-ferrous (all others).

The properties of metals are explained by the features of their structure:

The location and nature of the movement of electrons in atoms;

The location of atoms, ions and molecules in space;

Size, shape and nature of crystalline formations.

Features of the atomic structure determine the nature of the interaction of metals, their ability to produce various types of compounds, which include several metals, metals with non-metals, etc.

At different temperatures, some chemical elements have 2 or more stable types of crystal lattices. The existence of one metal in different crystalline forms (modifications) at different temperatures is called polymorphism, or allotropy, and the transition from one structure to another is called a polymorphic (allotropic) transformation. Allotropic forms resulting from polymorphic transformation are usually designated by the initial letters of the Greek alphabet.

Such polymorphic metals include, for example, cobalt (Co), tin (Sn), manganese (Mn), iron (Fe). In turn, a change in the structure of the crystal lattice causes a change in properties - mechanical, chemical and magnetic properties, electrical conductivity, thermal conductivity, heat capacity, etc.

Metals that have only one type of crystal lattice and are called isomorphic include aluminum (Al), copper (Cu), nickel (Ni), chromium (Cr), vanadium (W), etc. The most complete information on the structure and properties of metals obtained by using a set of research methods:

Structural (based on direct observation of the structure of a metal or alloy: macroscopic analysis, microscopic analysis, etc.);

Physical (based on the measurement of various physical properties: thermal, magnetic, etc.).

For example, the method of elemental microanalysis of changes in the surface of dental alloys in the oral cavity is used by many researchers [Gani G. et al., 1989].

Metal alloys are macroscopically homogeneous systems consisting of two or more metals with characteristic metallic properties. In a broad sense, alloys are any homogeneous systems obtained by fusing metals, non-metals, oxides, and organic substances.

The structure and properties of pure metals differ significantly from the structure and properties of alloys consisting of two or more metals. Based on the number of elements (alloy components), two-, three-, or multi-component alloys are distinguished.

The formation of new homogeneous substances during the mutual penetration of atoms is called alloy phases.

In the molten state, all components are usually in an atomic state, forming an unlimited liquid homogeneous solution, at any point of which the chemical composition is statistically the same. When the melt solidifies, the atoms of the components are arranged in the order of the crystal lattice, forming a solid crystalline substance - an alloy.

There are three types of relationships between alloy components:

1) the formation of a mechanical mixture, when each element crystallizes independently, and the properties of the alloy will be the average properties of the elements that form it;

2) the formation of a solid solution, when the atoms of the components form a crystal lattice of one of the elements, which is a solvent, while the type of lattice of the base metal is preserved;

3) the formation of chemical compounds, when, during crystallization, dissimilar atoms can combine in a certain proportion to form a new type of lattice, different from the lattices of alloy metals. The formation of a chemical compound is a complex process in which a new substance with new qualities is created, and the lattice has a more complex structure. The connection loses the main property of metal - the ability to undergo plastic deformation, and becomes brittle.

Accordingly, the properties of alloys will depend on what phases are formed in them: solid solutions, chemical compounds or mixtures of pure metals. If the atomic volumes of two metals and their melting temperatures differ sharply, then in the liquid state such elements, as a rule, have limited solubility.

At the same time, unlimited solubility, i.e. Only metals with a single type of crystal lattice have the ability to form solid solutions in any proportions. Metals located close to each other in the periodic table (Cu and Ni; Fe and Ni; Fe and Cr; Fe and Co; Co and Ni), or located in the same group (As and Sb; Au and Ag; Au and Cu; Big and Sb), have unlimited solubility.

Thus, the interaction of elements in alloys and the nature of the resulting structure are determined by the position of the elements in the periodic table, the type of crystal lattice, the size of the atoms, that is, the physical nature of the elements.

Dependence of properties on alloy composition:

1) in alloys having the structure of mechanical mixtures, the properties change mainly linearly. Some properties of mechanical mixtures, primarily hardness and strength, depend on the particle size (that is, on the degree of dispersion) - they increase significantly during grinding;

2) in alloys - solid solutions, the properties change according to a curvilinear dependence;

3) when chemical compounds are formed, the properties change abruptly.

Many physical and mechanical properties of alloys clearly depend on the structure, but some technological properties, such as castability (i.e., the ability to provide good quality casting) or weldability, depend not so much on the structure, but on the temperature conditions under which solidification took place alloys

For example, dental gold alloys, cast into a mold and quickly cooled in water, will have the form of a solid solution, characterized by characteristic softness, malleability and lower strength than alloys with an ordered arrangement of atoms [Kopeikin V.N., 1995]. However, if the same casting is cooled slowly to room temperature, then the solid solution, which prevails at temperatures above 424° C, completely transforms into the AuCu phase by redistributing atoms in the spatial crystal lattice into a more ordered structure. This leads to an increase in strength and hardness while losing the ductility of the alloy. Alloys with a high gold content (above 88%) do not form an ordered phase.

Therefore, the dependence of the mechanical and physical properties of single-phase alloys (a and p) is indicated by the following provisions, known from the course of metallurgy:

The hardness, strength and electrical resistance of solid solutions are higher than that of pure metals;

The electrical conductivity and temperature coefficient of electrical resistance of solid solutions is lower than that of pure metals;

In this case, the electrochemical potential changes according to smooth crooked.

In addition to the properties of the metal matrix, which has a certain crystal lattice and thereby determines the main parameters of mechanical properties, the latter can be influenced by additional alloying with elements such as molybdenum, tungsten, niobium, carbon, nitrogen, etc. Their presence in alloys, even in small quantities, is significant increases strength, wear resistance, heat resistance and other properties necessary for the operation of structures.

The addition of small amounts (0.005%) of iridium and ruthenium transforms the coarse grain structure of gold alloys into a fine grain structure, which makes it possible to improve the tensile strength and elongation strength by 30%, without affecting the hardness and yield strength. Strength is especially effectively increased when alloying cobalt-chromium alloys with 4-6% molybdenum and an additional 1-2% niobium in the presence of 0.3% carbon. In metal alloys, various chemical compounds are formed both between two or more metals (they are called intermetallic compounds) and between a metal and a non-metal (carbides, oxides, etc.).

The presence of non-metallic inclusions in the alloy structure leads to the formation of fatigue, cracks, internal pores and cavities, corrosion cracking of castings, which ultimately leads to destruction. Non-metallic inclusions play a significant role in the process of ductile and fatigue fracture. The basis of non-metallic inclusions in the alloy Vitallium consists of manganese and silicon. Cobalt-chrome alloy (CCH) contains inclusions of titanium nitrides and silicates.

Due to metal fatigue, microcracks appear at the boundaries of non-metallic inclusions and metal grains, which during cyclic loading increase their size, forming a main crack leading to metal destruction.

The main characteristic determined when testing a material for fatigue is the endurance limit - the greatest stress that the material can withstand without destruction under an arbitrarily large number of load changes (cycles). The maximum stress that does not cause destruction corresponds to the endurance limit.

In addition to mechanical tests, metal materials are subjected to technological tests(bending, bending, etc.) in order to determine their suitability for various technological operations during use.

Applying a load to a sample during a mechanical test results in deformation.

Physico-mechanical properties of metals and metal alloys. Metals have different color shades across almost the entire spectrum, however, as a rule, for base metals it is gray, bluish, bluish in varying degrees of severity and in different combinations. Precious metals are characterized by a yellow-orange hue and a whitish-silver tint; these substances have a fairly high density. Thus, the density of gold-containing alloys is 14-18 g/cm 3 , the density of cobalt-chrome alloys is 8.4 g/cm 3 , and the density of nickel-chrome alloys is 8.2 g/cm 3 . As already indicated, they are thermally conductive and electrically conductive, and also expand and contract when heated and cooled, respectively.

The melting point of metals varies widely. In this regard, low-melting metals are distinguished with a melting point lower than that of pure tin (232 ° C), as well as refractory metals, the melting point of which is higher than that of iron (1535 ° C). Between these poles are the average melting temperatures characteristic of most metals and alloys. The melting point and solidification temperature of pure metals are always constant, and until one phase disappears - the melting of the solid part when heated or the solidification of the liquid part when cooled - the temperature remains unchanged.

Plastic deformation leads to a change in the physical properties of the metal, namely:

Increased electrical resistance;

Reducing density;

Changes in magnetic properties.

All internal changes that occur during plastic deformation cause strengthening of the metal. Strength characteristics (tensile strength, yield strength, hardness) increase, and plastic characteristics decrease.

Cold-worked (work-hardened) metals are more prone to corrosion damage during operation. To completely remove hardening, metals undergo recrystallization annealing, that is, the process of the emergence and growth of new undeformed crystalline grains of a polycrystal at the expense of other grains.

Recrystallization is used in practice to give the material the greatest plasticity. Moreover, it occurs especially intensely in plastically deformed materials at higher temperatures. The recrystallization temperature has important practical significance. To restore the structure and properties of cold-worked metal (for example, when continuing to stamp a crown under a press after hammering a sleeve on a chalk model), it must be heated above the recrystallization temperature.

The set of properties that characterize the resistance of a metal and alloy to the action of external mechanical forces (loads) applied to it is commonly called mechanical properties.

Forces can be applied in the form of a load:

Static (smoothly increasing);

Dynamic (increasing sharply and at high speed);

Repeatedly variable (repeatedly applied, changing in magnitude and direction).

Accordingly, mechanical tests are divided into:

Static (tension, compression, bending, torsion, hardness);

Dynamic (impact bending);

Fatigue (with repeated-variable application of load);

High temperature (for example, long-term strength). As a rule, all tests are carried out under certain conditions on samples of a given shape and size, i.e., according to international and national standards, which ensures the comparability of the results obtained and their correct interpretation.

When stretched or compressed, the sample has the ability to resist elastic deformations, which determines rigidity material - elastic modulus E. The dimension of the elastic modulus E in the SI system is Pascal (Pa, N/m 2) or Mega-pascal (MPa, N/mm 2). The elastic limit is indicated as follows - d 0.05.

Metals are characterized by high strength. Moreover, some of them can be plastic or elastic (springy), while others, on the contrary, are fragile. The ultimate strength of gold alloys is lower than that of cast cobalt-chrome alloys. High strength makes it difficult to finish the structure of the prosthesis, but resists damage during its operation (primarily abrasion).

From all mechanical tests hardness is determined most often, since the method is easy to use.

The main methods for determining hardness are methods of introducing standard tips made of hard, non-deformable materials into the surface of the tested metal under the influence of static loads:

Brinell method (pressing a steel ball of a certain diameter);

Rockwell method (pressing a diamond cone or a hardened steel ball with a diameter of 1.58 mm);

Vickers method (indentation of a tetrahedral diamond pyramid with a square base).

The Brinell hardness indicator is the hardness number, designated HB (H - Hardness, English- hardness, B - the initial of the surname of the author of the method - Brinell). The Brinell method can be used to test materials with a hardness of no more than HB 450. Brinell hardness is expressed in kgf/mm 2. If the load is expressed in newtons (N), then the Brinell hardness number is expressed in MPa. In this case, the dimension is written as follows: HB 320 MPa.

Rockwell hardness is designated HRA, HRB, HRC (depending on the A, B or C scale used). Vickers hardness (HV) has the same dimension as Brinell hardness numbers, i.e. MPa or kgf/mm. Vickers and Brinell hardness numbers for materials with hardness up to HV 400-450 are actually the same.

Hardness as a characteristic of an alloy is closely related to its other parameters. For example, as the hardness of gold alloys increases, the yield strength and tensile strength also increase, and as the hardness and strength increase, the elongation decreases.

The microhardness of a metal alloy can be changed during the casting process by exposing it to an electromagnetic field of different frequencies, which makes it possible to obtain an alloy with specified properties [Bobrov A.P., 2001].

As a result of cyclic stresses, the metal “gets tired,” its strength decreases, and the sample (prosthesis) fails. This phenomenon is called fatigue, and fatigue resistance - endurance. Failure from fatigue always occurs suddenly due to the accumulation of irreversible changes in the metal, which lead to the appearance of microscopic cracks - fatigue cracks that occur in the surface zones of the sample. Moreover, the more scratches, gouges and other defects on the surface that cause stress concentration, the faster fatigue cracks form.

Chemical properties of metals and metal alloys. These include solubility, oxidation, corrosion resistance.

The ability of metals to dissolve various elements allows, at elevated temperatures, the atoms of the substance surrounding the surface of the metal to diffuse into it, creating a surface layer of altered composition.

During this treatment, not only the composition but also the structure of the surface layers, and often the core, changes. This treatment is called chemical-thermal.

Corrosion(from Latin corrosio - corrosion) - destruction of solids caused by chemical and electrochemical processes developing on the surface of the body during its interaction with the external environment.

Corrosion resistance- the ability of materials to resist corrosion. For metals and alloys, corrosion resistance is determined by the corrosion rate, that is, the mass of material converted into corrosion products per unit surface per unit of time, or the thickness of the destroyed layer in millimeters per year.

Corrosion fatigue - lowering the endurance limit of a metal or alloy under simultaneous exposure to cyclic stresses and a corrosive environment. There are at least 3 forms of corrosion destruction: uniform, local, intercrystalline corrosion.

Uniform corrosion destroys the metal, having little effect on its mechanical strength. It is found in silver solder.

Local corrosion leads to the destruction of only individual sections of the metal and manifests itself in the form of spots and pinpoint lesions of varying depths. It occurs in the case of a non-uniform surface, in the presence of inclusions or internal stresses with a rough metal structure. This type of corrosion reduces the mechanical properties of parts.

Intercrystalline corrosion is characterized by the destruction of metal along the grain (crystal) boundaries. In this case, the connection between the crystals is disrupted, and the aggressive environment, penetrating deep, destroys the metal. Stainless steels are especially susceptible to this.

Crystals have symmetry of the atomic structure, a corresponding symmetry of the external shape, as well as anisotropy of physical properties (i.e., the dependence of properties on the shape and type of crystal). Crystals are an equilibrium state of solids: each substance located at a given temperature and pressure in the crystalline state corresponds to a certain atomic structure. When external conditions change, the crystal structure may change.

Chemical corrosion- interaction of metal with aggressive media that do not conduct electric current. Thus, strong heating of iron in the presence of atmospheric oxygen is accompanied by the formation of oxides (scale). The resulting oxide film can protect the metal from the diffusion of an aggressive agent into it.

In the oral cavity, metals are found in the moist environment of the oral fluid. The latter, being an electrolyte, creates conditions for electrochemical corrosion of metal fillings, inlays and other metal prostheses.

Characteristics of alloys used in orthopedic dentistry. Currently, over 500 alloys are used in dentistry. International standards (ISO, 1989) divide all metal alloys into the following groups:

1. Alloys of precious metals based on gold.

2. Alloys of noble metals containing 25-50% gold or platinum, or other precious metals In the specialized literature, until recently, there has been a lexical substitution of two terms - noble metal and precious metal, which are not synonymous: precious indicates the cost of the metal, and noble - refers to its chemical properties. Therefore, the elements gold and platinum are both noble and precious, palladium is noble, but much cheaper. Silver has earned a place in the precious metal classification, but is not a noble metal. Note editors).

3. Alloys of base metals.

4. Alloys for metal-ceramic structures:

a) with a high gold content (>75%);

b) with a high content of noble metals (gold and platinum or gold and palladium - > 75%);

c) based on palladium (more than 50%);

d) based on base metals:

Cobalt (+ chromium > 25%, molybdenum > 2%);

Nickel (+ chromium > 11%, molybdenum > 2%).

The classic division into noble And base alloys.

In addition, alloys used in orthopedic dentistry can be classified according to other criteria:

By purpose (for removable, metal-ceramic, metal-polymer prostheses);

By the number of alloy components;

According to the physical nature of the alloy components;

By melting point;

According to processing technology, etc.

Summarizing the above about metals and metal alloys, it is necessary to once again emphasize the main general requirements for metal alloys used in an orthopedic dentistry clinic:

1) biological indifference and anti-corrosion resistance to acids and alkalis in small concentrations;

2) high mechanical properties (ductility, elasticity, hardness, high wear resistance, etc.);

3) the presence of a set of certain physical (low melting point, minimal shrinkage, low density, etc.) and technological properties (malleability, fluidity during casting, etc.), determined by a specific purpose.

If a metal alloy is intended for ceramic veneering, it must meet the following specific requirements:

1) be capable of adhesion to porcelain;

2) the melting temperature of the alloy must be higher than the firing temperature of porcelain;

3) the coefficients of thermal expansion (CTE) of the alloy and porcelain should be similar.

It is especially important to match the coefficients of thermal expansion of the two materials, which prevents the occurrence of force stresses in the porcelain, which can lead to chipping or cracking of the coating. On average, the coefficient of thermal expansion for all types of alloys that are used for ceramic veneering ranges from 13.8 × 10 -6 °C -1 to 14.8 × 10 -6 °C -1 .

The coefficient of thermal expansion of the ceramic mass can be changed by introducing certain additives. Thus, the introduction of leucite into the ceramic mass allows you to change the coefficient of thermal expansion from 12.5 × 10 -6 °C -1 to 16 ×10 -6 °C -1 .

The combination of the high strength properties of the cast metal frame of a denture and the appearance of the veneer, which quite accurately imitates the appearance of natural teeth, makes it possible to create effective and aesthetic dentures.

As mentioned above, alloys used in orthopedic dentistry are divided into 2 main groups - noble and base.

Alloys based on noble metals are divided into:

Gold;

Gold-palladium;

Silver-palladium.

Alloys of noble group metals have better casting properties and corrosion resistance, but are inferior in strength to alloys of base metals.

Alloys based on base metals include:

Chrome-nickel (stainless) steel;

Cobalt-chrome alloy;

Nickel-chrome alloy;

Cobalt-chromium-molybdenum alloy;

Titanium alloys;

Auxiliary alloys of aluminum and bronze for temporary use. In addition, an alloy based on lead and tin is used, which is characterized by low fusibility.

Alloys of gold, platinum and palladium. These alloys have good technological properties, are resistant to corrosion, durable, and toxicologically inert. Idiosyncrasy is less common with them than with other metals.

Pure gold is a soft metal. To increase elasticity and hardness, so-called alloy metals are added to its composition - copper, silver, platinum.

Gold alloys vary in the percentage of gold content. Pure gold in the metric hallmark system is designated as 1000 fineness. In Russia, until 1927, there was a spool-type assay system. The highest standard in it corresponded to 96 spools. The English carat system is also known, in which the highest standard is 24 carats.

900 gold alloy used in prosthetics with crowns and bridges. Contains 90% gold, 6% copper and 4% silver. The melting point is 1063°C. It has plasticity and toughness, and can be easily stamped, rolled, forged, and casted.

750 gold alloy used for frames of arched (clasp) dentures, clasps, inlays. Contains 75% gold, 8% copper and silver each, 9% platinum. It has high elasticity and low shrinkage during casting. These qualities are acquired by adding platinum and increasing the amount of copper. 750 gold alloy serves as solder when 5-12% cadmium is added to it. The latter reduces the melting temperature of solder to 800°C. This makes it possible to melt it without melting the main parts of the prosthesis. The bleach for gold is hydrochloric acid (10-15%).

Super-TZ is “hard gold,” a heat-hardening, wear-resistant alloy that contains 75% gold and is yellow in color. It is universal and technologically advanced - it can be used for stamped and cast dental structures: crowns and bridges.

For the first time in Russia production began gold-palladium alloy for metal-ceramic dentures Superpal(I.Yu. Ebedenko et al.), which contains 60% palladium and 10% gold.

Abroad, for the needs of orthopedic dentistry, alloys of precious metals are produced with different contents of gold and precious metals, which therefore have different mechanical properties - M-Palador,V-Classic, Stabilor-G, Stabilor-G.L. etc.