The main mechanical properties of metals. Technological properties of metals
In our time, for the manufacture of machines and devices are mainly used materials, which are treated with other metals and non-metals. Therefore, it is very important to determine the mechanical properties of metals. No less important is the knowledge of such general laws as the frequency of changes in the capabilities of their elements and their compounds, the dependence of properties on the types and characteristics of chemical bonds in alloys based on them.
Basic mechanical properties of metals
Metals are substances that are characterized by heat conduction, electrical conductivity, plasticity. All of them, with the exception of mercury, are solids at room temperature. Melting point ranges from -38.78 to +3380aboutC. The mechanical and technological properties of metals have a high ability to absorb light, and therefore they are opaque even in very thin layers.However, the smooth and clean surface layer reflects the light well and gives it a characteristic shine. Most surfaces are white and gray. Only copper and gold have a yellow hue. Some metals have a gray color with a weak bluish, yellowish or reddish tint. In the solid state, they all have a crystalline form. In the vapor state, monoatomic metals. According to their specific weight they are divided into light and heavy ones. There is another division - for ferrous and non-ferrous metals.
Metals in nature and methods of their extraction
In nature, metals are found both in the free state (Cu, Au, Ag, Hg, Pt) and in the form of various compounds - oxides, sulfides, carbonates, sulfates, phosphates, chlorides, nitrates and other compounds. When extracting them from ores and minerals, various ways of reduction are used. In practice, those compounds and minerals have a value from which the industry can obtain pure metal easily and without great expense. Carbon is used to produce iron from iron ore. Reducing agents can be hydrogen, aluminum, calcium, sodium, which have a greater ability to add oxygen.The production of iron from sulphides takes place in two stages: first, sulphate is obtained, and then burned out and converted to oxides, then the resulting oxide is reduced according to the production technology from oxides. Of carbonates, carbonate is first laid out when heated. Similar actions can be obtained from various types of iron from various natural compounds. The method of electrolysis produces active metals, alkali, alkaline earth, aluminum, magnesium, etc. The latter are produced during the electrolysis of melts (molten salts). By passing a direct electric current, ions are released at the cathode. The difficult-to-melt technological properties of metals are used to obtain them in the form of a powder or a spongy state, followed by pressing at high temperature.
The structure of metals and their physical properties
The mechanical properties of metals are influenced by the characteristics of their internal structure in the solid state. The metal lattice has such a feature that there are molecular particles in its nodes, that is, there is an equilibrium. Valence electrons are in a relatively free state and are not fixed strictly to each atom, forming a so-called electron gas.That is, the crystal lattice consists of positive ions, and the gaps between the ions are filled with electrons. If there is a temperature difference or under the influence of an external potential difference, these electrons move easily and conduct heat and electric current without displacing the material particles. In the vapor state, the mechanical properties of metals contribute to the conduction of electric current only in ionized form. It is characteristic that with increasing temperature the electrical conductivity decreases due to the fact that their volume resistance increases. When heated or (even when exposed to photons) the energy of electrons increases, as a result, they can even be easily emitted (the appearance of cathode rays and photoelectron emission, is used in radio engineering, in electronic tubes and in measuring light intensity using photocells). Thus, the metal lattice is actually an ionic lattice, at the vertices of which there are positive ions of the same name, the mutual repulsion of which is compensated not by the opposite charged anions, but by the joint efforts of free electrons.
Testing of the mechanical properties of metals
Dissolution can be carried out only when they are converted to water-soluble compounds, that is, by chemical means. Some may liquefy in liquid mercury (silver, gold), forming the so-called amalgam. Iron can form both mixtures and intermetallic compounds (intermetallic phases), which have a certain composition. To obtain a picture of the change in properties with temperature, the cooling curves obtained by studying the cooling rate are used. The preheated substance is allowed to cool and the temperature is measured every hour. The results are plotted on a diagram, where time is plotted on the abscissa, and temperature is plotted on the ordinate. If the technological properties of metals, accompanied by the release of heat, do not change during cooling, the temperature decreases gradually. If there are any changes in the system, then there is a time delay in cooling the system, caused by phase transitions. Using thermal analysis on the cooling curves it is possible to investigate the composition of compounds that can be formed between the constituent parts of the alloys.
The change in the characteristics of the alloys depending on the composition
In general, when a substance passes from a liquid to a solid state, a substance is released in the form of more or less large particles - crystals, or a shapeless amorphous mass (adhesives, rubber, etc.). The smallest possible volume of the crystal lattice, which reproduces the features of its structure, is characterized by the unit cell. The form of a solid depends on the nature of the substance and on the conditions in which the transition to the solid state takes place. If at the vertices there are identical atoms, then the distance between them in the crystal is equal to the sum of their radii, that is, the radius of the atom is equal to half of this distance. The filling of the crystal lattices with molecules and ions occurs with the most dense packing, that is, ions and molecules fill the space with the minimum volume. The symmetry elements of a solid crystal are its center, planes and axes. Their most characteristic feature is anisotropy, that is, the dissimilarity of their characteristics (strength, thermal conductivity, dissolution rate, etc.) in different directions. The absence of strictly directed bonds between atomsThe mechanical properties of metals make it possible to place two or more elements in a metal lattice, which are arranged in a specific order, forming intermetallic structures.
When mixing different metals in the molten state, the particles of the main component can be replaced by particles of another or several elements without changing the crystal lattice, forming solid solutions. Materials containing two or more types of atoms and have characteristic properties (gloss, thermal conductivity, electrical conductivity) are called alloys. In the molten state, the metals dissolve well in each other and, as a rule, without restrictions. Often in these solutions can form a number of heterogeneous zones, indicating their limited solubility. The mechanical properties of metals, on the basis of which an alloy is formed, differ from the physical and mechanical properties of alloys. When dissolved in mercury, so-called amalgams are formed. In practice, there are three types of alloys: solid solutions, those that have the character of chemical compounds of metals, and a mixture of crystals.
Formation of an elementary crystal lattice of alloys
A variety of methods for producing alloys makes it possible to produce them with desired properties. In practice, compounds based on iron, copper, nickel, etc. are widely used. The physical and mechanical properties of metals, on the basis of which an alloy is produced, differ significantly from the properties of alloys. The added atoms can form more “rigid” localized bonds, and the slip of the layers of atoms decreases. This leads to a decrease in ductility and an increase in the rigidity of the alloys. Thus, the strength of iron is increased 10 times with the addition of 1% carbon, nickel or manganese. In brass, which contains 65-70% chromium and 30-5% zinc, the strength is 2 times more than in pure copper, and 4 times more than in pure zinc. The industry produces very many varieties of alloys of various metals with desired properties.
Studying the structure of atoms, one can observe that all of them have a small number of electrons at the external energy level, and they are characterized by the ability to only give away electrons when compounds are formed.In compounds, metals always have a positive oxidation state. In the formation of compounds, the particles donate electrons, showing the properties of the reducing agent. The ability to donate electrons is different and depends on the structure of the atom. The easier it gives electrons, the more active it is. The quantitative characteristic of the mechanical properties of metals to give away an electron is an ionization potential. By this is meant the minimum voltage of the electric field (in volts) at which the electron receives such acceleration that it can cause ionization of the atom. Activity in aqueous solutions is characterized by a standard electrode potential and can be quantified using a standard hydrogen electrode, the potential of which is taken as ± 0. Noble metals have a positive standard potential. By chemical properties, they are able to interact with water, acids, alkalis, salts, oxides, organic substances.
Interaction with non-metals
In all cases of formation of compounds with non-metals, the transition of electrons from metal atoms to non-metal atoms occurs. Hydrides are compounds with hydrogen.Alkaline and alkaline-earth are formed by direct interaction with hydrogen. Halides are salts of hydrohalic acids, polar molecules, which for metals 1, 2 groups are well soluble in water. They are formed by the direct interaction of iron with halogens, hydrohalic acids with iron. In their environment, metals interact with him very actively. Oxides are predominantly fundamental in nature, these include oxides of aluminum, zinc, lead (II), chromium (III). They can be obtained from the elements during the decomposition of salts with hydroxide, roasting sulphides. The main mechanical properties of metals in air contribute to their coating with an oxide film. If it does not cover the surface loosely, it does not protect against destruction; chemical corrosion is underway. Some metals form a very dense oxide film that does not allow oxygen from the air and other oxidants to penetrate through it and protects the metal from corrosion.