Text 6. Plastics vs. Metals

Properties of plastics which may be...

Favorable: 1. Lighter weight. 2. Better chemical and moisture resistance. 3. Better resistance to shock and vibration. 4. Transparent or translucent. 5. Tend to absorb vibration and sound. 6. Higher abrasion and wear resist­ance. 7. Self-lubricating. 8, Often easier to fabricate. 9. Can have integral color. 10. Cost trend-is downward.

Unfavorable: 1. Lower strength. 2. MucK higher ther­mal expansion. 3. More susceptible to creep, cold flow, and deformation under load. 4. Lower heat resistance — both to thermal degradation and heat distortion. 5. More subject to embrittlement at low temperature. 6. Softer. 7. Less ductile. 8. Change dimensions through absorption of moisture or solvents. 9. Flammable. 10. Some varieties are degraded by ultraviolet radiation. 11. Most cost more (per cu in.) than competing metals. Nearly all cost more per pound.

Either favorable or unfavorable: 1. They are flexible. Even rigid varieties are more resilient than metals. 2. They are electrical non-conductors. 3. They are thermal insula­tors. 4. They are formed through the application of heat and pressure.

Exceptions: 1. Some reinforced plastics (glass-rein­forced epoxies, polyesters, and phenolics) are nearly as rigid and strong (particularly in relation to weight) as most steels. They may be even more dimensionally stable.

2. Some oriented films and sheets (oriented poly­ esters) may have greater strength-to-weight ratios than cold-rolled steels.

3. Some plastics are now cheaper than competing met­als (nylons vs. brass, acetal vs. zinc, acrylic vs. stainless steel).

4. Some plastics are tougher at low than at normal temperatures (acrylic has no known brittle point).

5. Many plastic-metal combinations extend the range of useful applications of both (metal-vinyl laminates, leaded vinyls, metallized polyesters).

6. Plastic and metal components may be combined to produce a desired balance of properties (plastic parts with molded-in, threaded metal inserts; gears with cast- iron hubs and nylon teeth; gear trains with alternate steel and phenolic gears; and rotating bearings with metal shaft and housing and nylon bearing liner).

7. Metallic fillers in plastics can make them electrical­ ly or thermally conductive or give them magnetic prop­erties.

AFTER-TEXT DISCUSSION

Practice 1. Расскажите об основных свойствах пластмасс.

Practice 2. Расскажите о свойствах, которые можно рассматривать как положительные или отрицательные.

Practice 3. Сравните осноные свойства пластмасс и металлов.

Text 7. Aluminium Alloys.

Pure aluminum has good corrosion resistance and working and forming properties but poor machining char­acteristics and low mechanical strength. By adding other elements to aluminum, its strength and machining char­acteristics can be improved. Such a combination of two or more elements, at least one of which is metallic, is cal­led an alloy and the predominant metal in the system is referred to as the base metal.

Silicon, copper, zinc and magnesium are common al­loying elements and are often added to aluminum in sub­stantial proportions. Iron, manganese, nickel, chromium, titanium, antimony, cadmium, cerium, lithium, beryllium and molybdenum are also added in smaller proportions with various beneficial effects.

Titanium, tungsten, cerium and molybdenum all con­tribute to grain refinement of cast aluminum. Manganese and antimony are often added to improve corrosion resist­ance. Cobalt and nickel affect strength and workability, while cadmium and tin increase hardness in heat treatable alloys.

The market penetration of ZA alloys has been aided by the fact that traditional high volume foundry metals have significant shortcomings that detract from their inherent advantages:

cast iron has high energy and machining costs, protec­tive finishes are nearly always required and there are in­dustry environmental problems;

bronze has high material and energy costs and the environmental problem of lead for many important alloys;

aluminum has limitations in strength, bearing proper­ties and finishing along with moderately high energy costs. Of course, each of these classic materials does have distinct advantages in given applications.

In contrast, the zinc casting alloys have advantages that are highly attractive to foundries:

excellent casting properties;

low energy consumption;

pollution free melting and casting;

excellent machinability;

lower material cost and density than bronze.

Answer the following questions:

1. What properties does pure aluminium have?

2. What properties of aluminium can be improved by adding other elements to it?

3. What elements are often added to aluminium to improve its properties?

4. What is an alloy?

5. What metal in an alloy is called the base metal?

6. What are the disadvantages of traditional high volume foundry metals?

7. What are the advantages of zinc casting alloys?

Text 8. Titanium, a Wonder Metal

The story of titanium is extraordinary. To begin with, it was discovered twice. A British scientist, William Gregor, found it first and called it menachanite, and six years later, in 1797, M. H. Klaproth, a German chemist, also found it and gave it its present name.

For many years, titanium was of inretest only to re­search chemists — ii was considered too brittle to be of any practical value. Yet it was the impurities with which it was usually associated (it forms compounds easily with nearly every known element) that made it brittle.

It cost the chemists in many countries endless efforts to isolate pure titanium and even more to start producing it commercially. In 1948 the world stock of pure tita­nium was only ten tons. Today the output is much larger.

Titanium has one surprising property — it is complete­ly inert in biological media, something the medical com­munity was quick to notice. It is being used to make artifi­cial joints and many other things necessary in surgery at the Priorov Central Institute of Traumatology and Ortho­pedics. Titanium instruments do not corrode, and are thir­ty per cent lighter than instruments made of stainless steel.

Titanium's high standard of corrosion resistance, lightness, tensile strength, and the ease of forging, rolling and stamping are finding it more and more uses. Tita­nium alloys are very useful in mechanical engineering, and for chemical and refractory apparatus. Titanium helped Soviet design engineers to surmount the sound and heat barriers in supersonic and high-altitude aircraft designing. On earth, it shows good work at chemical plants, in the_pulp-and-paper and food industries. More­over, it is іШПа source of surprise for the investigator.

A group of researchers at our Institute, under the leadership of Professor. I. Kornilov, D. Sc. (Chemistry), produced a material that has a kind of "memory", as the following experiment shows: a thin bent strip of the new alloy was clamped to a stand, a 500-gram weight hung on the free end. A current was passed through for several seconds, which heated the strip to more than 100 °С. As if commanded by an enigmatic force, it straightened out

like a tight spring and lifted the load. When the current was switched off, the strip gradually went back to its orig­inal shape. The cycle was repeated a number of times, and the strip always "remembered" its original shape. The surprising phenomenon of direct conversion of ther­mal energy into mechanical is seen with the naked eye.

The explanation is in the crystalline modifications of titanium-nickel alloy which, changing with the tempera­ture, also changes back again.

This is why the material has a "memory" and special acoustic properties. At room temperature, the alloy called titanium nickeloid becomes soft, ductile and does not pro­duce the characteristic metallic sound when struck. How­ever, when it is heated to a certain temperature, it becomes hard, resilient and ringing.

There will undoubtedly be some unusual applications for this phenomenon in the future —even at this early stage it is clear that titanium-nickeloid-based alloys will be useful in many areas, For instance, in sensitive pickups which are activated by a change in temperature, in acoustics for sound absorption, etc., etc.

Titanium and its alloys are coming out in the com­mercial field — they have already made quite a name for themselves as structural materials.

brittle хрупкий л ю лоз но -бумажна я промыш-

Impurity примесь ленность

output выпуск strip полоска

media среда to clamp закреплять

artificial Joints искусственные су- current ток

ставы to straighten out выпрямляться

surgery хирургия tight spring тугая пружина

tensile strength прочность на раз- load = weight ад. груз

рыв conversion преобразование, прев-

refractory огнеупорный ращение

to surmount the barrier преодо- when struck при ударе

леть барьер sensitive pickup чувствительный

pulp-and-paper industry цел- адаптер, звукосниматель

AFTER-TEXT SECTION

Practice 1. Вы ознакомились с содержанием текста. Отметьте, какие из нижеприведенных утверждений соответствуют содержанию текста.

I. Titanium was discovered twice. 2. Pure titanium is found in nature. 3. Titanium forms compounds with many elements. 4. To isolate pure titanium isn't difficult. 5. Titanium is light, strong and corrosion resistant. 6. It is active in biological media. 7. Titanium can be used in sur­gery. 8. Titanium alloys can't be used as structural ma­terials,

Practice 2. Перечислите области применения титана и его сплавов.

Practice 3. Скажите, о чем свидетельствует результат опыта, описан­ного в тексте. Подтвердите свой ответ соответствующими положениями текста.

Text 9. Working with New Materials

A successful design is almost always a compromise among highest performance, attractive appearance, effi­cient production, and lowest cost. Achieving the best com­promise requires satisfying the mechanical requirements of the part, utilizing the most economical material that will perform satisfactorily, and choosing a manufactur­ing process compatible with the part design and material choice. Stating realistic requirements for each of these areas is of the utmost importance.

The rapidity of change in materials technology is typified by the factthat plastics» a curiosity at the turn of the century, are now being used in volumes which have for many years exceeded those of all the non-ferrous metals put together, and which are beginning to rival

steel.

The changes which are taking place are, of course, not only quantitative. They are associated with radical changes in technology —in the range and nature of the materials and processes available to the engineer.

The highest specific strength (i. e. the strength avail­able from unit weight of material) now available comes from non-metals, such as fibreglass, and from metals, such as berillium and titanium, and new ultra-high strength steels.

Fibre technology, in its modern form, is of more recent - origin than plastics, but composites based on glass and/or on carbon fibres are already being applied to pressure vessels, to lorry cabs and to aircraft engines, and may well replace aluminium for the skin and structure of air­craft. An all plastic car has been exhibited: nearly the whole car, except the engine and transmission is of plas­tics or reinforced plastics.

It is not only plastics and their reinforcement which are changing the materials scene. Ceramics too are gam­ing an increasing foothold. Their impact as tooling mate­rials in the form of carbides, nitrides and oxides is also well known — cutting tools made of these materials are allowing machining rates which had previously been

considered quite impossible. Silicon nitride seems to offer particular promise for a wide variety of applications. Amongst these is liquid metal handling. Pumps for conveying liquid aluminium are now on trial which could revolutionize the foundry industry. Silicon nitride is also being tested for the bearing surfaces of the Wankel rotary engines which are being developed as potential replace­ments for the conventional piston engines of our motor cars. And ceramic magnets have replaced the traditional steel pole-piece plus copper field coil for providing the engineering field for many electric motors.

It is clear that the number of combinations of all kinds of original trends in the production of new materials is practically unlimited. This, in turn, opens new realms for the designing of still cheaper, effective and unthinkably perfected, compared to that we have today, machines and mechanisms.