Mechanical Properties of Metals Experiments with Steel, Copper, Tin, Zinc, and Soap Bubbles Margret J. Geselbracht and Arthur 6. EMS' Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706 Rona L. Penn and George C. Lisensky Department of Chemistry, Beloit College, Beloit, WI 53511 Donald S. Stone Department of Materials Science and Engineering, University of Wisconsin-Madison. Madison, WI 53706 The use of metals has been central to the advance of technology since before recorded history For most of this time, the underlying reason for their usefulness has been the mechanical properties of metals. For example, gold, which is most notable for its beautiful luster and resistance to corrosion, would nevertheless be of little value if it were not for the f a d that this metal can be worked easily into thin sheets or intricate shapes. The widespread use of electrical conductors would not be practical if metals could not be drawn into wires. The ductility of metals (their ability to be formed into new shapes) derives from the non-directionality of the metallic bond. Metals can also be processed by a number of means (e.g., alloying, cold working, heat treating) to give them high strength or other desirable properties by producing imperfections in the regular repeat pattern of the crystalline strnctnre within the metals. These imperfections range in dimensions from a few nanometers (10-'m) to tens or hundreds of micrometers m) across, and thus are referred to as microstrnctures. A common concern of metallurgy or materials science is the critical role of microstructure in determining the mechanical properties of a material. Many of the mechanical properties of metals are demonstrated easily in introductory chemistry courses and provide a natural link connecting the crystal structure and microstructure with macroscopic properties. In this paper, we will present a series of experiments that dramatically illustrate the mechanical properties of metals. These experiments can be used either as lecture demonstrations or in a laboratory setting. Copper, a metal known for its ductility, can be work hardened to yield a higher strength material and can be softened again by heating the metal. Bending a sample of tin or zinc produces an audible result from the collective motion of atoms in the crystal, moving fast enough to exceed the speed of sound in air. The use of solid-state phase transformations and temperature-dependent solubilities of impurities alter the strength and ductility of steel. Finally, these experiments can be explained on the atomic and microstructural levels using a bubble raft as a two-dimensional model of a metal ( I ) . Elemental metals comprise more than three-quarters of the elements in the periodic table, and together with metallic alloys, provide diverse mechanical properties that support a myriad of applications. Given their extreme importance to technology and human society, we believe that a more complete discussion of metals and the atomic basis of their mechanical properties fits naturally into introductory chemistry courses.
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Experiments with Metals Copper Work Hardening and Annealing at High Temperatures (2) Hammer a piece of thick copper wire (10 gauge or thicker, used for 120 V house wiring and available at a hardware store) to about half its original thickness. The thinner flattened (work hardened) piece will be much more difficult to bend than a piece of unaltered wire. Heat a piece of the flattened wire red hot by holding it with tongs in the flame of a Bunsen burner. Let it cool slowly on the iron plate of a ring stand. The heat treated (annealed)piece will be easy to bend at first, although continued bending will again work harden the copper. To conduct this experiment as a lecture demonstration, prepare a nine-inch long piece of 10-gauge copper wire by heating the wire red hot in the flame of a Bunsen burner (annealing) and letting it cool slowly on the iron plate of a ring stand. For the demonstration, either hold one end of the cool wire or support it in a hole drilled in an aluminum block, Figure 1. Hang about a 700 g weight on the other end of the wire (a large hammer with a hole in the handle can be used for this purpose). The annealed wire should bend under the applied weight. Work harden the wire by bending several times, or by hammering the wire as above. Show that the wire now supports the weight without bending. Anneal the wire by heating as above and show that the applied weight is again sufficient to bend the wire. Another way to demonstrate the same processes is to take a long piece of copper wire (we use about two feet of 18-gaugewire) and pull it two or three times over a nail or other small diameter object as if the wire was like the rope of a pulley. The wire will bend much more easily on the first pass than on the third pass. Heat a section of the strained wire red hot in a Bunsen burner, and let it cool slowly. The treated wire again will bend easily a t first. Bending of Tin or Zinc (3) Listen while quickly bending a piece of tin or zinc once. We use a piece about 6 mm in diameter (Aldrich,tin or zinc rod). Short pieces can be used by inserting the ends into two plastic tubes that serve as handles for bending. A series of quiet clicks will be heard. Amicrophone and amplifier is needed to hear the sounds in a large lecture hall. Eventually the rods will need to be heated to about 130 'C for 24 h in an oven (annealing) to restore the acoustical response to bending. Zinc is harder than tin and we have found it to be more robust in class use. 'Author to whom correspondence should be addressed.
n
Copper wire I
Remove the alligator clips and again wrap the piano wire around the wooden dowel. trvine to shaoe the wire into a coiled spring. This time, t h i -e r e t a i k the shape of a spring when the wooden dowel is removed. Test the springiness of the now-coiled wire by pulling gently on one end and observing the resulting behavior. The wire retains the shape of a spring, but the spring is not very elastic. To harden the svrina, connect one aUieator clio to each end, and again heat using a variable voltage power supply setting.of 20-30 Vuntil the wire iust turns red-hot. Try not to let adjacent coils touch each bther during this heating process. Turn t h e power supply off, and immediately quench the wire in a bucket of cold water. Test the springiness by pulling on one end of the coiled spring. The wire should break under a minimum of force. The final step in producing a useful spring is to temper it. Take a second oiece of wire and s h a ~ it e into a s ~ r i n as e described above. After the spring has been quenc0
-Interstitial Lo
Figure 3. Production of bubbles of uniform size and formation of a bubble raft. raft. (Blowing the bubbles across with one's breath also works.) Unwanted bubbles can be pushed aside with a slider (shown in Fig. 2) or scooped out. If the bubble raft is projected onto a screen with a n overhead projector, be sure to adjust the focus to the plane of the bubbles. Many parts of the raft will have a very regular hexagonal arrangement of hubbles. Ilnwevcr, there are always a numher nf imperfections. Individual bubbles can be popped to form vacancies by using a dry ol~ject.The slider is Lied to comnress (oush the slider eentlv toward one end of the tray) a i d stretch (pull the sliler gcntly in the opposite direction) the bubble raft without scratchine the bottom of the tray. Deformation of the raft will cause dislocations to zip through the raft (see below). One way to point out a dislocation is to begin to compress the raft with the slider.. s t . o a~s soon the demonstrator sees a dislocation move, point out the area where the movement occurred, and then stretch the raft to move the dislocation in the OPposite direction. A. long a s the dislocat~ondoes not reacha main boundary (see below1 it can be moved track and forth hy a ~ t e r n i l t e l ~ ~ s t r e t cand h i n cornpres4ng ~ the raR rplastic defi~rmatim~. When all the deformations have moved to the grain boundaries, larger energy input is required to move the bubbles (work hardening) and catastrophic failure of the raft may occur; the raft breaks up.
Discussion (6) Metallic bonding is, for the most part, non-directional and in manv metals the atoms arranee ., themselves to achieve the most efficient packing and the densest stmcture. Such metals will a d o ~ one t of the close-nacked strucr tures, either facecenteredcubic (e.g., Al, ~ u j ohexagonal close-packed (e.g., Zn, Ti). I n contrast, the central transition metals with many half-filled orbitals (e.g., W, Cr) often have the body-centered cubic crystal structure because a significant portion of the bonding is covalent. Covalent bonds, such a s those in diamond, lead to a relatively open structure dictated by the angles of the bonds. Thus, the types of bonding in materials are reflected in their shetures. A ~ ~ l i c a t i oofn a mechanical stress on a crvstal causes the-bonds between planes of atoms to stretch-and distort. If the displacement of each atom is small (less than halfway to the next equivalent location in the crystal), the strain is elastic and the planes relax back to their orizinal positions when the stress is removed. If the stress is iarge enough, the planes of atoms slide with respect to each other, giving ihe atoms new neighbors his-deformation that results from the ;il~d~ngof planes ofatoms 15known ns plastic deformation and generally does not reverse itself when the stress is r e r n o ~ e d . ~ The typr of bonding has an imponant impact on the mcchnnical properties of a material. In face-centered cubic (FCC, and hexagonal close-packed rHCP1 mcmls, it is easy to
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Figure 4. (a) Representation of a crystal by a two-dimensional array of circles similar to a bubble raft. Crystalline defecls include point defects such as the vacancy, interstitial, and substitutional, and extended imperfectionssuch as dislocations. To more easily see the dislocation, sight along the arrows while viewing from an oblique angle to the page. (b) Representation of a three dimensional crystal containing a dislocation, emphasizing the dislocation as a line defect. The symbol Idesignates the core of the dislocation. Although the dislocation is recoonizable from what amears to be an extra half olane of atoms inshed Into the structure'lt ~. is the /me that termmates ~ ihe enra na f-planethat s mponanr and character zes tne n gnly o stone0 reglon. n fan, me exlra nalf plane is ralner un merest ng. For Inslance, f tne o slocat on were to move to the r ght oy breanlng me atomic bonds on one side of it and attaching new ones on the other side, it would be associated with a new half-plane.It is clearly the line that moves, and not the extra half-plane ~
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slide planes of atoms past each other, because the bonds are not directional. FCC metals are realatively easy to deform plastically because they have multiple sets of closepacked planes. In covalent solids, i t is more difficult to slide planes of atoms past one another. Covalent solids, like Si and diamond, can be quite brittle (i.e., they tend to fracture before they plastically deform).
Defects To understand the processes underlying the mechanical properties of metals, we need to appreciate some of the defect structures that can be found in metal^.^ Defects in crystals are imperfections in the regular repeat pattern of the crystal, can drastically affect the strength 01a metal, and may be classified i n terms of their dimensionality. Some imperfections such a s vacancies (a missing atom) and substitutional or interstitial impurity atoms are located a t individual sites, and are, therefore, regarded a s zero-dimensional point defects. Adislocation, on the other hand, is a one-dimensional line defect. I t runs somewhat 2Polymersalso ~ndergop astlcoeformaton, but tne deformaton of polymer structures &La y re% 1s n a change n o rect~onall y or preferred orientation of the polymer chain. 3These defectsalso are also important to other types of properties: electrical, magnetic, thermal, and optical properties.
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l i k e a s t r i n g t h r o u g h t h e crystal? Examples o f defects are shown in F i g u r e 4. Metals, l i k e other crystalline solids, are u s u a l l y polycrystalline. T h e y a r e composed of m a n y s m a l l crystals called grains (Fig. 5). The grains within a m e t a l u s u a l l y have r a n d o m orientations w i t h respect t o one another a n d are separated by g r a i n boundaries. G r a i n boundaries a n d even t h e surface o f a single crystal are two-dimensional planar defects. A l o n g a g r a i n boundary, t h e atomic packing i s imperfect a n d t h e energies o f atoms a t a g r a i n boundary are higher t h a n those o f atoms w i t h i n t h e grains, m a k i n g atoms a t m a i n boundaries more reactive. G r a i n boundaries o f t e n can b e seen by u s i n g a microscope to view a polished, t h e n chemically-etched, f l a t m e t a l surface. Etchingw i t h a n acid removes m a t e r i a l f r o m g r a i n boundaries more r a p i d l y due t o t h e h i g h e r energy o f these sites.
Figure 5. Grains in a polycrystalline material; different shading is used for atoms that belong to different grains. Grain boundaries are higher in energy than the surrounding grains due to less efficient packing.
4An important difference between a vacancy and a dislocation is that if you sketch a regular polygon around the vacancy in Figure 4 by traveling along atoms, the polygon will close. The same polygon will not close around a dislocation. Try it. 5 ~ ~ t a tmodel i c illustrating many of the same concepts of structure and microstructure can be made with a sealed plastic tray of metal BB pellets. A description of the construction of this BB board is currently in press and will be available from ICE Publications, Department of Chemistry, University of Wisconsin-Madison, Madison. WI 53706.
Figure 6. A photograph of a typical bubble raft produced with the apparatus described In the text. The positions of vacancies, grain boundaries, and dislocations are indicated wlth arrows. Volume 71
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arranged in a hexagon), because the distortion required for the atoms to move from one stable position to another is smaller. These close-packed planes act a s sllp planes. Facecentered cubic or cubic close-packed metals such as copper, aluminum, lead, and gold have four sets of close-packed planes in a unit cell (Fig. 8).On the other hand. hexa~onal close-packed (HCP) metals such a s titanium, magne&m, and zinc have close-~ackedlanes alona onlv " one direction in the unit cell. When scientists first began to understand that plastic deformation involves the sliding of atomic planes oast one another, they assumed that t h e sliding oc&s all-at once. Onlv later did i t become amarent that this is not the case. ~ e l i a b l etheoretical calciiations showed that if all the bonds between two planes were distorted a t the same time, the stress required to do this would be high, roughly lo6 psi (-lo7 kPa; -lo5 atm) in the case of high purity iron (7). These calculations presented a direct conflict with exoerimental evidence, which showed that the stress required to deform Dure metals is actuallv much lower. on the order of a few tens or hundreds of psi:~he explanation for the discrepancy lies in a type of crystal defect, the dislocation, first postulated in the 1930's (8). he presence of a line dislocation relaxes the requirement that entire olanes of atomic bonds must distort and break simultane&sly for plastic deformation to occur. Dislocations are illustrated schematically in Figure 4. In Figure 4a, the crystal consists of a two-dimensional array of circles similar to the bubble raft. The core of the dislocation resides in the highly distorted region located by the two arrows a t the bottom of the figure. Viewing the array from a n oblique angle along the directions indicated by the arrows, the reader will notice that the lines of atoms terminate in this core region. The line-like character of a dislocation in a three-dimensional crystal is apparent in Figure
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Distance Figure 7. Potential energy diagram for sliding a row of spheres across another row. The Bubble Raft ~ o d e l '
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Point defects. dislocations. and main boundaries all can be easily observed i n a two-dimensional bubble raft modeL6 A Dhotoma~h - . of a tvoical bubble raft is shown in Figure 6 where the positions of vacancies, grain boundaries. and dislocations are indicated. The bubble raft is a good two-dimensional model of a metal on the atomic level, because the attraction of small bubbles for one another mimics the attractions between metal atoms in a crystal. Bubbles of uniform size correspond to the metal atoms, and the non-directional forces that make the bubbles stick together correspond to the metallic bonding. The hexagonal arrangement of bubbles, the closest possible packing of bubbles, is caused by surface tension. The same hexagonal arrangements of atoms a r e found in hexagonal closepacked and face-centered cubic structures of metals. The growth of the bubble raft is a dynamic process. As the bubble raft is forming, it undergoes reconstruction. Grains form, grow, and disappear. Dislocations move rapidly across grains a s they are emitted and absorbed by main boundaries. Vacancies resent in the bubble raft do not move as the raft is deformed. Vacancies are frequently annihilated, however, when they interact with dislocations that pass nearby. Mechanical stress can cause dislocations to move through the grain until they are trapped or annihilated. When the bubble raft is stressed using the slider, dislocations zip through the grains, moving to the grain boundaries and causing plastic deformation.
".
Plastic Deformation and the Dislocation Two rows of atoms sliding past one another pass through a series of energetically stable positions a s shown in Figure 7. Sliding is preferred between close-packed planes (planes in which each atom is surrounded by six neighbors
Figure 8. Orientation of the four close-packed planes in a face-centered cubic unit cell.
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Journal of Chemical Education
4h
Plastic or irreversible deformation of crystalline substances occurs primarily by the movement of dislocations within grains. 'the motion &a' disloration from left to right through - a rrvsral IS illustrated in Fimre 9. As the disloration moves and becomes associated with new half planes, bonds on one side of the dislocation are broken and bonds on the other side are formed. As a consequence of the motion ofthe dislocation, the top half of the crystal slips to the right by one olane of atoms. Dislocations move through the &n ( ~ i 9~i s. reversible and the motion of the deformation could be started or stopped a t any stage) until they are trapped or annihilated. The snapping sounds heard when tin is bent are u result oSolanei ol'atorn; slidine -.oust each other. in the rase of tm or zinc a large number of dislocations move cooperatively and a t the speed of sound in the metal, which exceeds the speed of sound in air ("Mach I " ) . The ~ sudden reorientation of the crystal lattice results in an audible "click". Once again the bubble raft model provides an excellent "live" demonstration of plastic deformation. When the raft is stressed (compressed toward one end of the tray using the slider), dislocations zip through the mains. moving - to the grain boundaries andiausingplastii deformation. As the raft is reoeatedlv. e x.~ a n d e dand com~ressed.dislocations arc created and annih~lated.
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bBeca~seme b-oble rah 1s a two-d rnenslona model, the 08s ocation line defen becomes a zero-dimensional point defect, and the grain boundary. planar defect becomes a one-dimensional line de. iect. 'The sound is due to a twinning reorientation involving a large number of dislocations moving cooperatively. Twinning is the existence of two differentsyrnmetry-related orientations of a lanice in what appears to be one crystal. :
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the many solid phases, both equilibrium and metastable, present in binary and multicomponent alloys. The fact that the metallic bond is not particularly selective allows metal atoms of different kinds to intermingle and form alloys over wide ranges of composition. Changing the composition on a n atomic scale also can increase the strength of a metal. The presence of dissolved i m ~ u r i t i e sin a metal. called solid solution hardening,
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makes it more locations to move, difficult especially for disif the sizes of the atoms of the impurity and host metal differ significantly? For example, tin in copper produces bronze which is harder than either pure copper or pure tin. The a d d i t i o n of even small 0 0 a, O 0 0 0 O O 0 amounts of alloying elements 0 00) CXDma f G ~ C D c O ~ ~g)O puts the structure under C D CD o?CaCYX)CD h ) O 0 0 0 0 0 0 0 strain, erecting energy bar0 a 0) ~ C X X ) ~ 0 0 0 0 0 0 0 oaacnccomo riers to the movement of dislooa,aaaya 0 0 0 0 0 0 0 0 i:CDCZ,Cn~EOcO cations and strengthening the a Q 0 0 B O O 0CB00000 0 0 0 0 0 0 0 metal (9)a s seen in Figure 10. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Loosely speaking, the difference in the size of the atoms 0 000 0 00 00 000 00 0 0 0 0 0 0 0 roughens the slip plane, making it harder for dislocations to move. Figure 9. When a crystal is subjected to a shearing force (represented by the bold arrows), dislocations Precipitation hardening ocmove along crystallographic slip planes. Atoms above the slip plane shear away from their Original neigh- curs when the room temperabors below the slip plane, then bond with a new set of neighbors to restore the internal structure of the tu, solubility of an impurity crystal. l designates the core of the dislocation. Each circle in this figure represents a line of atoms going is exceeded, and a fine grained into the page. For reference,the original atom positions are shown shaded. precipitate forms throughout the metal, again blocking the Work Hardening and Annealing at High Temperatures motion of line dislocations (see Fig. 10). For example, Cu dissolves in Al a t high temperatures, but its lower solubilB~~~~~~the movement of the line dislocation is the key ity a s the solid solution cools results in precipitation of to plastic deformation, in order to strengthen a metal by C a z . Second phases also can precipitate a t the grain increasing its resistance to deformation we must either get boundaries, where they a r e accommodated by the imrid of dislocations or prevent them from moving. In pracperfect nature of the atomic packing. tice, removing all of the dislocations from a crystal or growing a crystal without dislocations is practically impossible The piano wire demonstration provides a dramatic except for very small crystals. Therefore, metallurgists go illustration of how phase transformations and the solubility of carbon are used for strengthening in carbon steels.'0 to great lengths to find clever ways of entangling or pinning the dislocations by other defects in the crystal. New Pure iron melts a t 1538 T. Between room temperature dislocations are created during deformation and become and the melting point, however, iron undergoes several alpinned by the initial dislocations. The effect cascades a s lotropic phase transformations, from a body-centered cubic the material is repeatedly deformed, and dislocations col(BCC) phase a t room temperature, called a-iron or ferrite, lect in tangles in the metal! This build-up of pinned didoto a face-centered cubic (FCC) phase a t temperatures becations leads to the hardening of the metal in a process tween about 900 -C and 1400 'C, called y-iron or austenite. known a s work hardening. At temperatures above 1400 'C, iron transforms to another BCC structure, but this is generally not a n important feaA work hardened metal can be softened again by annealture in the heat treatment of steels. ing (heating) a t high temperatures, generally a t temperatures above one half of the melting point on the absolute Austenite, the FCC structure of iron, has the ability to temperature scale. As a piece of metal is heated, the indissolve much more carbon than ferrite, the BCC structure creased thermal vibrations allow atoms to rearrange to a ,firon, ( ~ maximum h ~ solubility of carbon in austenite is more crystalline ordering and thereby reduce the number of dislocations present. 'The density of dislocations stored in a work hardened metal can become huge, on the order of lo6 km of dislocation line per cm3 of Hardening of Alloys material. '11 the atomic radii of lwo elements differby more than -15% the To work harden a metal by deforming it is one way to solubility of an impurity element in a host metal is very low. "For a thorough discussion of the ironiron carbide phase diastrengthen it. However, metallurgists often rely on more gram. see reference 10. : sophisticated means of hardening and take advantage of
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If the steel is quenched from hiah tem~erature by rapid cooling n: a water bath, the driving force for the austenitefemite phase transformation becomes very large. However, at the same time, the steel is cooled so quickly that the carbon has insufficient time to concentrate by diffusion into the Fe& precipitates. The result is a new phase, called martensite, which is essentially a distorted form of the BCC ferrite structure, distorted because it wntains large amounts of carbon trapped during the transformation (see Fig. 10). Martensite, when it contains large amounts of carbon. is a verv hard material, bui the large distortiods of the crystal lattice that occur durine the ohase transformation produce high internal &es& that promote fracture. Therefore, martensite can be verv brittle and can shatter like glass when dropped if it has not been treated by a process called temoerina. In tempering, the sample is gently heated aft& it has been quenched by sudden cooling. Tempering the steel allows the carbon to diffuse short distances and allows some of the internal stresses to relax, yielding a greater resistance to fracture. Conclusions It is valuable to summarize the processes we 0000000 have discussed in this paper and to emphasize that 0000000 0 0 0 0 0 0 0 while the details may vary for a given metal or allox the microsco~ic . ohenomena are universal. Figure 10. Solution hardening due to crystal structure distortion by an interstitial ~peiifically,the movement of dislocations in a atom such as carbon in iron (top left)or a substitutional atom such as phosphorus metal causes plastic deformation. Bending a piece in iron (top right) and precipitation hardening due to crystal structure distortion by of copper wire leads to the creation of new dislocaa second crystalline phase that either almost matches the lanice (bonom left)or tions, which become pinned and then other disdoes not match the latttice (bottom right). In all four cases, slippage of crystal locations in a process known as work hardening, planes and movement of dislocations is impeded. Precipitates, as are found in steel and almost aU othefalloys, are used as hardening agents because they pin dislocations.To a lesser degree, solid solu2.11 wt % at about 1148 'C compared to the maximum soltion impurities also pin dislocations and strengthen metubility of carbon in ferrite of only 0.02 w t % at 727 'C (10). als. Heat treatments are used to tailor the mechanical Although the structure of ferrite is more open in terms of properties of a metal, but how a heat treatment affects the the total volume of interstices or "voids" between iron properties depends on the particular alloy system as well atoms, the structure of austenite wntains fewer but larger as the details of how the heat treatment is performed. For interstitial sites and is able to dissolve larwr amounts of example, in the case of heating the work hardened copper, carbon as an interstitial defect in the lattice. The carbon dislocations were removed which resulted in a net soRenatoms do not dissolve well in ferrite, because the carbon is ing effect. In the case of the steels described above, the vartoo big to fit in the interstices. ious heat treatments take advantage of the phase chemisMany of the important heat treatments of steels involve try and the transformation kinetics of the steel, leading to phase transformations across the austenitelfemte phase a variety of phases present and a range of mechanical boundary. When steel, including the specially processed properties. type used to make piano wire (containing about 1 wt % In this paper, we have discussed some of the processes carbon), is slowly woled below the austenitelferrite phase underlying plastic deformation in metals. We have transition temperature, the excess carbon, which is no illustrated how metallurgists apply solid-state chemistry longer soluble in ferrite, precipitates as iron carbide to tailor the properties of alloys. Through time, metallur(Fe7C).11This Drocess reauires the carbon to diffuse modergists have combined mechanical processes such as work atebistances (-0.1-1 &through the austenite. The preshardening with techniques such as precipitation hardenence of iron carbide precipitates serves to strengthen the ing and solid solution strengthening to optimize the steel bv in nine dislocations. The more carbon there is in strength of materials. The bottom line is that much of metthe steel, the more iron carbide there will be, and generallurgy is really chemistry. We hope it is clear to the reader ally, the stronger and more brittle the steel will be. The that a more complete discussion of metals and the atomic kinetics of the phase transformation depend strongly on basis for their mechanical properties has a place in a general chemistry course and that many valuable concepts tem~erature.a fact that allows careful tailorine of the size can be demonstrated to students using common materials distributions and types of second-phase precipitates found around us. within steels.
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"Iron carbide (Fe3C)is a metastable phase, meaning that it is unstable with respect to decomposition to pure iron and graphite. However. in most allovs. this decomoosition is so sluooish that it is L n mportanl In certaln cast Irons tne carbon is present as graphre, ana in these alloys,tne formatoon ofgrapnlle nas been cata yzw w th the add rlon of an element I ke s~l~con
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Journal of Chemical Education
Acknowledgment The authors would like to thank Frank J. Wonala for making us aware of the piano wire demonstration; Art Dodd, Kathleen Gisser, Brian Johnson, and William Robinson for helpfnl discussions; and the Dreyfus Foundation
and the National Science Foundation (grant USE9150484) for financial support.
Dohertx P and hihen. D. Eds.Explomforium scioncosnoehbmk: ~ x p ~ o r ~ t o " u m Teacher Institute: Ssn Raneism, 1991; p 161. 6. General refereneesanthistopiiiidudd: Cottrel1,A. H. S c i . A m r 1881, ZIT@), 9& A. and J U I I I ~R. ~ ,~ h s .o l a stofe. hrn svprmnductors to suporO I ~ W S ,&ford univereity press: ~ l f o r d1989, , PP 171-241. &hby. M F,; JO"~S,D. R. H. Englmering Malonols:An intmduetran to &.ir Pmperirea andApplimf,ons; P e ~ m o nW : ord. 1980. Hayden, H.W.; Moffatt, W 0 ;WulK J. The Strudilm and Ploprlles of Mobrials, Vol. 3 Mechanical Behavior, Wiley: New York, 1 9 6 1 Moffatt, G. W.: Pearsal1.C. W.; W u l f f , ~ .he S t r u d m o n d ~ m p n i p s a f ~ o ~ & i s . Val. 1Shwcture: Wiley: New York. 19M.
~w.
Literature Cited R q Soe. London 1947, 190, 4 7 4 4 8 1 and reprinted in refereneed. Bmm. B, 2 0 0 E x p r i m t s f o r B o y s and Girls; Collms: Cleueland, 1973: p 24. Fqaman,R. P : Leighton, R B.; Sssds,M. T h d s F q n m k t t m t on Phy~icre,Vd. 2, Addison-Wesley: Reading. MA. 19% pp 30-830-10. Institute for Chemical Education. ChemiofryCan Be Fun; UniveraityofWiwonpmMadison: Madison. WI.1S84. The soap solution ie based on recipes found in: Ontario Science Centre. Seiene~uorks,Addison-Wesley: Resding.MA.1984;p M. Katz, D. A. Chemistry in the 7b.v Store, 5th ed.; Cornmu& College of Philadelphia: Philadelphia, 1990.
1. Bragg, L.; Nye, J F Pme
2. 3. 4. 5.
I. Dieter, G . F. Mechnnicol Mdallurg~,3rd ed., McGraw-Hill: New York,1986, p 119. 8. Taylor, 0.I . P l a . Rgv Soe 19.34, W145,882. 9. Ohashi,N.Amr Sci lsBa,80,540-555. 10. hng, 0.J.: Leighly, H. P. Jr. J. Chem. Edue 1982,59,94&963.
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