Методическая разработка по работе со спецтекстами на английском языке для студентов физического факультета



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Structure and properties of materials

Solid exists in nature in two principal forms; crystalline and amorphous, which differ substantially in their properties.

Crystalline bodies remain solid, i.e. retain their shapes, up to a definite temperature (melting point) at which they change from the solid to liquid state. During cooling, the inverse process of solidification takes place, again at the definite solidifying temperature, or point. In both cases, the temperature remains constant until the material is completely melted or respectively solidified.

Amorphous bodies, when heated, are gradually softened in a wide temperature ranges and become viscous and only then change to the liquid state. In cooling, the process takes place in the opposite direction.

The crystalline state of a solid is stable than amorphous state.

Amorphous bodies differ from liquid in having a lower mobility of particles. An amorphous state can be fixed in many organic and inorganic substances by rapid cooling from the liquid state. On repeated heating, long holding at 20-25 C or, in some cases, deformation of an amorphous body, the instability of the amorphous state may result or complete change to the crystalline state.

Examples of such changes from amorphous to crystalline state are the turbidity effect appearing in inorganic glasses on heating or in optical glasses after a long use, partial crystallization of molten amber heating, or additional crystallization and strengthening of nylon fibres on tension.

Crystalline bodies are characterized by an ordered arrangement of their elementary particles (ions, atoms or molecules).

The properties of crystals depend on the electronic structure of atoms and the nature of their interactions in the crystal, on the spatial arrangement of elementary particles, and on the composition, size and shape of crystals.

The structure of crystals is described by using the concepts of fine structure and micro- and macrostructure depending on the size of structural components and the methods employed to reveal them.

Fine structure describes the arrangement of elementary particles in a crystal and of electrons in an atom. It is studied by diffraction methods (radiography, electron diffractometry, and neutron diffractometry). Analysis of a diffraction pattern on the interaction of atoms of a crystal with short waves of X –rays, electron or neutrons can offer vast information on the structure of crystals.

Most crystalline materials consist of fine crystals (grain) which constitute what is called the microstructure. Microstructure of crystalline materials can be observed with an optical or electron microscope.

Microscopic examinations make it possible to determine the size and shape of grains (crystals), the presence of crystals of different nature, their distribution and relative volume quantities, the shape of foreign inclusions and microvoids, orientations of crystals, and some special crystallographic characteristics (twin, slip lines, etc.).

Materials with high elastic properties

Steels and alloys with high elastic properties find various applications in machine building and instrument engineering.

Elastic elements of machines and instruments can be characterized by large diversity of shapes, dimensions, and operating conditions. A typical feature in their operation is that no residual (plastic) deformation is allowed in them even under heavy static, cyclic or impact loads. In that connection, all spring alloys must possess the combination of mechanical properties essential for all structural materials (strength, ductility, toughness, endurance and additionally have a high resistance to small plastic deformations. Under the conditions of short-term static loading, the resistance to small plastic deformations is characterized by the elastic limit and in long-term or cyclic loading, by relaxation stability.

The relaxation stability of a material is its resistance to stress relaxation. Stress relaxation may be dangerous because as an elastic deformation changes partially to plastic or residual deformation, the dimension of an elastic element after load release may change. For instance, if a spring has been compressed or a relay has been bent for a long time, they can not resume completely the original shape after load release and thus lose their elastic and operating properties.

Stress relaxation occurs by the mechanism of microplastic deformation in individual grains, which is accumulated in time. At a stress below the elastic limit, microplastic deformation can be caused; with low stresses, by dislocation bending or detachment of individual dislocations from locking points and with high stresses, by movement of pinned dislocations.

In that connection, a high elastic limit and high relaxation stability can be achieved in an alloy by forming a stable dislocation structure in which practically all dislocations rather than a major portion of them, are locked, or pinned. Besides, this structure must have a low level of microstresses which, when added to the working stresses, facilitate dislocation movement.



Corrosion – resistance materials


Structural materials intended for operation in aggressive media must posses a high corrosion resistance, as well as the desired mechanical properties. The most corrosion-susceptible materials are metals and alloys, which can be explained by their high chemical activity and high electrical conductivity.

Corrosion of metals is their spontaneous destruction due to chemical or electrochemical interaction with the surrounding medium. Corrosion – resistant metals and alloys are those which can withstand the corrosive action of a medium, i.e. corrosion processes proceed in them at a relatively low rate. It is usually distinguished between two principal kinds of corrosion: electrochemical and chemical.

Electrochemical corrosion. This kind of corrosion develops in liquid electrolytes: moist atmosphere and soil, salt and fresh water, aqueous solutions of salts, alkalies and acids. Electrochemical corrosion is characterized by the appearance of an electric current and dissolution of the metal due to its electrochemical interaction with the electrolyte.




Magnetic Materials




SINTERED NEODYMIUM-IRON-BORON

Neodymium-iron-boron permanent magnets are the second generation of the "Rare Earth" magnet family. Produced by a powder metallurgy process they still provide the highest energy per unit volume of any commercially available magnetic material.

Most stock items have a maximum working temperature of 120° C. But for specialist applications a range of grades are available with working temperatures between 80°C and 180°C. Being about 75% iron, the alloy is prone to corrosion so protective coatings are often applied to it.

These materials are brittle and should be handled with care.




BONDED NEODYMIUM-IRON-BORON

Bonded NdFeB is produced from melt spun powder bonded with a thermosetting epoxy resin to form net shape components. The product is magnetically isotropic enabling magnetisation in any direction although multi-pole magnetisation requires special fixtures.

For special shapes or prototypes the material is easily machined with conventional tooling. Operating up to 120°C the magnetic properties fall conveniently between Ferrites and the sintered rare earth materials.

(For specialist applications a bonded version of Samarium Cobalt is available to special order.)



ALNICO ALLOYS

The traditional cast Alnico alloys offer the highest operating temperature (550°C) and the lowest temperature coefficient of any magnetic material with good corrosion resistance.

A selection of rod diameters can be supplied cut to required lengths quickly. Limited ranges of blocks and rings are stocked together with horseshoe & button shapes.

SINTERED FERRITE

These offer excellent corrosion resistance and are available in both isotropic and anisotropic forms. Produced by sintering ceramic powder compacts, the preferred direction of the anisotropic grades are established at the pressing stage and offer good magnetic performance at an economical price.

Rings or blocks can be magnetised with multiple poles on the flat faces for improved holding or for magnetic drives or generators. An extensive range of rings are stocked for loudspeaker applications and arc segments for motors are available to order.

Isotropic discs can offer the cheapest holding devices whilst rings can be multi-poled around the circumference for encoder or sensing applications.

Ferrite powder can be compounded with rubber to produce a versatile flexible magnetic material for a variety of applications where sintered materials may not be suitable.


Pretreated aluminum and aluminum alloys




Field of the Invention

This invention relates to the protection and pretreatment of aluminum and aluminum alloys and to the surface treatment of aluminums with novel compositions and to the method of using these compositions for the pretreatment of aluminum and aluminum alloys at ambient temperatures. The pretreatment coatings provide improved corrosion resistance, adhesion of overlaying coatings e.g. paints etc., and maintains low electrical contact resistance in a corrosive environment.


Description of Prior Art

It is the current practice to improve the corrosion resistance and bonding of subsequent coatings to aluminum and its alloys by initially coating or pretreating the metal surface with protective films. The coating enhances the corrosion resistance of the unpainted metal surface and prepares the surface for a finish coating such as paint etc. These conversion coatings are most often applied by the use of hexavalent chromium-containing solutions. While these coatings provide good corrosion resistance, attempts have been made to provide a more acceptable non-chromate derived coating because of the growing concern regarding the occupational safety, health and environmental effects of hexavalent chromium. Hexavalent chromium is highly toxic and is a known carcinogen. Therefore, the solutions used to deposit these protective. Chromate films, however, provide outstanding paint adhesion and corrosion resistance and are easy to apply by various methods including immersion, spraying or by the wipe-on technique. However, the environmental laws and OSH regulations are forcing the military and commercial users to find other non-toxic, non-chromate pretreatments. Moreover, the use of chromate conversion coatings is becoming more expensive as the regulations are being enforced and costs become prohibitive with the restrictions being imposed by the EPA In addition, certain processes like spraying chromate conversion coatings are forbidden because of OSH, thereby forcing the use of less than optimum alternative methods.

More specifically, it is known that aqueous chromate solutions contain chemicals that partially dissolve the surface of the metal and form insoluble films known as chromate conversion coatings or pretreatments. These coatings are corrosion resistant and protect the metal from various elements which cause corrosion. Although the conversion coatings enhance corrosion resistance and improve the paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the solutions from which they are made and the presence of hexavalent chromium in the applied films. This is a serious problem from the standpoint of the operators handling the solution e.g. disposing the used chromate solution, the chromate-contaminated rinse water, and the coating systems contaminated with chromates. These problems, however, can be avoided by eliminating the hexavalent chromium from the process. However, this method is expensive and can be a major cost factor in the overall metal treating process. Therefore it is highly desirable to provide processes and protective coatings which are free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and bonding properties which are comparable to those imparted by conventional chromate-based conversion coatings. Of particular interest is the use of chromate conversion coatings on aluminum and its alloys e.g. the coating of large objects such as aircraft. It would be desirable to provide a protective coating for aluminum and its alloys utilizing relatively non-toxic chemicals that could serve as an alternative to the toxic hexavalent chromium.

Web and CD-ROM questions:




  1. Search the web for information about the latest Conferences, Forums and Symposiums on Materials Science.



  1. Search the web on new books and available CDs on “Materials Science.”



  1. Search the web on the new types of materials, and their purposes.



  1. Access the site www.kluweronline.com or other related sites and find interesting online news about properties and uses of materials. Answer the questions:


1) What new conducting, semiconducting, dielectric, magnetic and other types of materials do you know?
2) Describe the advantages of using new types of materials.

BIBLIOGRAPHY





  1. Arzamazov B. Materials Science. М.: Mir publisher, 1989.

  2. Tateev B. Electrical and radio engineering materials. М.: Mir publisher, 1989.

  3. Zolotorevsky V. Mechanical properties of metals. М.: Metallurgiya, 1983.

  4. Аlexandrov М. Materials handling equipment. М.: Mir publisher, 1981.

  5. Интернет ресурсы





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