. What is the Difference Between Allotropes and Isotopes? If you cool it slowly, the carbon has time to get squeezed out and the iron can be BCC again. At a small content, manganese in bcc Fe alloys was experimentally shown to have either ferromagnetic [21, 22] or antiferromagnetic coupling with iron [23], and the uctuations between these However, tetrahedral sites are larger in BCC, and BCC has more tetrahedral and octahedral sites. When all the liquid iron has changed to solid -iron (BCC), the temperature starts falling again, and then the thermal arrest takes place when -Fe changes to -Fe (1394C). Delta-Fe is a high temperature manifestation of -Fe. Pure iron can have two different crystal structures as its temperature is increased from room temperature to its melting point. Iron atoms maintain their place on the lattice and carbon atoms become "interstitials." In the low-temperature ferrite, or BCC structure, however, there is no room for carbon atoms.2002-11-15 Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. High-temperature austenite, an FCC structure, allows enough space for carbon to squeeze in between the iron atoms. These transformations occur below the equilibrium temperature during cooling and above it during heating, and that is why the temperature of transformation during cooling is designated by the symbol, Ar (A letter is for arrest), where the letter V being taken from the French refroidissement meaning cooling. This strain does not allow large number of carbon atoms to be accommodated in BCC -iron. What is the name of new capital of Andhra Pradesh? We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Study with Quizlet and memorize flashcards containing terms like Calculate the radius of an iridium atom in cm, given that Ir has an FCC crystal structure, a density of 22.4 g/cm3, and an atomic weight of 192.2 g/mol, Iron (Fe) undergoes an allotropic transformation at 912C: upon heating from a BCC ( phase) to an FCC ( phase). Each of these two nearest (neighbour) iron atoms is displaced by 0.053 nm in one of the <100> directions and the unsymmetrical octahedral hole becomes symmetrical but causes tetragonal distortion of the lattice, i.e. FCC materials have more slip systems, or ways for dislocations to move, than BCC materials. for the bcc structure. (Simple Explanation), What are Space Groups? In this article we will discuss about:- 1. A.A. Bartlett, "Demonstration of a crystalline phase change in a solid," The Physics Teacher 13, 545-547 (1975).R.M. In body centred cubic (bcc) metals, the slip direction is also well established to be . However, elements like boron, carbon, nitrogen, oxygen, and hydrogen form interstitial solid solutions in -iron and -iron as their atomic sizes are sufficiently small relative to that of iron. How is ATP made and used in photosynthesis? The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". Solution . The non-metallic elements like carbon, nitrogen, oxygen, hydrogen, boron in iron sit in these sites to form their interstitial solid solutions. 3 Microstructure in iron-carbon alloys Eutectic--pearlite Hypoeutectoid alloys Hypoeutectoid steel: has a carbon . Finally, HCP lattices are closely packed, but not cubic. Why can FCC Fe accommodate more C atoms than BCC Fe? Yes the APF is important, the atomic packing factor, that is the reason FCC has more slip systems, because of the way the atoms are arranged in the crystal. Thus, BCC structure of a-iron is more loosely packed than that of FCC -iron, and that is why density of FCC -iron is 8.14 g/cm3 at 20C and 7.87 g/cm3 for -iron. Terms of Service 7. 2 The wire will glow red hot. 1.13 illustrates temperature dependence of mean volume per atom in iron crystal. The total energy of the crystal is made up of a balance of these two, with the energy due to entropy being linearly dependent on the absolute temperature. BCC metals are infact stronger than FCC metals. The presence of carbon in octahedral hole in BCC -iron causes tetragonal distortion of lattice, and this large strain permits ferrite to accommodate only a limited number of carbon atoms, and thus, the lattice remains essentially body centred cubic. Diamond Cubic3. Although BCC has more total room for interstitial atoms, FCC has the largest particular interstitial site (octahedral). we also compared FCC and HCP crystal structures here. The best known example of allotropy is exhibited by iron which (at normal one atmospheric pressure) is BCC at temperatures below 910C and above 1394C, but FCC between 910C and 1394C as is illustrated in Fig. The crystal structure is based on the Bravais lattice of the same name, with a single atom at each lattice point on the We're young materials engineers and we want to share our knowledge about materials science on this website! Even though both FCC and BCC have equal number of slip systems present in the structure, FCC is more ductile. Why does iron have a BCC and FCC structure? This cookie is set by GDPR Cookie Consent plugin. What is the difference between FCC and BCC structures? This cookie is set by GDPR Cookie Consent plugin. When, -Fe is cooled slowly such as in furnace, small number of nuclei of -Fe form at the grain boundaries of -Fe [Fig. There are 6/2 + 12/4 = 6 octahedral site positions per unit cell in the BCC crystal structure. However, the comparison of atomic size of carbon with the available interstitial holes (these are much smaller) makes it clear that carbon forms interstitial solid solutions with -iron, called austenite and with -iron called ferrite. This phenomenon of materials to have more than one crystal structure is called allotropy or polymorphism, the former term is reserved for this behaviour in pure elements like metals, whereas polymorphism is a more general term, and the different phases are called allotropes, or polymorphs. The transition from BCC to FCC results in an 8 to 9% increase in density, causing the iron sample to shrink in size as it is heated above the transition temperature. Now you know all the differences between FCC and BCC! 1.12 (b) 2]. Double Hexagonal Close-Packed (La-type)5. (How it Works, Applications, and Limitations), What is Materials Science and Engineering? Body-Centered Cubic4. The relative magnitude of the free energy value governs the stability of a phase, that is, the phase having lowest free every in any temperature range is the stable phase. It will form a solid solution with , , and phases of iron. 3C (0.008-2.14wt%) Microstructures of iron -ferrite austenite. How does this influence the ductility of BCC, FCC, and HCP metals? What is the crystal structure of iron Mcq? What is the difference between platform and station? 4 What is the difference in FCC and hcp arrangements? The fcc lattice is both cubic and closely packed and forms more ductile materials. The lattice parameter of -iron increases with the increase of temperature due to increase of thermal vibrations, increase of vacancies etc., which also means increase in the size of the interstitial holes too. The interstitial solute atoms prefer to segregate to point, line and surface lattice defects which are responsible for strain ageing in steels. Iron is ferromagnetic (attracted to magnets), but only within a certain temperature range and other specific conditions. FCC unit cell has 4 atoms per unit cell as compared to BCC having 2 atoms per unit cell. At atmospheric pressure, three allotropic forms of iron exist: alpha iron (-Fe), gamma iron (-Fe), and delta iron (-Fe). Hexagonal Close-Packed4b. We call this the ductile-to-brittle transition temperature (DBTT). At very high pressure, a fourth form exists, called epsilon iron (-Fe). Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. For example, FCC metals, Cu, Au, Ag, are usually soft and ductile, which means they can be bent and shaped easily. Why? In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 C); other alloys of steel have different eutectoid temperatures. Explanation: Pure iron exists in three allotropic phases of iron, iron, and iron. Upon cooling, high carbon steels phase segregate into a mixture of bcc iron (light gray) and Fe3C (dark gray) microscopic grains. Why FCC is more ductile than BCC and HCP? When the temperature reaches 910 C , another phase . This state has low entropy as well as minimum internal energy, which makes BCC structure stable in iron at low temperatures. During the process to turn the metal (iron) into the alloy (steel), carbon is introduced. Why does iron change from bcc to fcc? What is the difference in FCC and hcp arrangements? From 912 to 1,394 C (1,674 to 2,541 F) alpha iron undergoes a phase transition from body-centred cubic (BCC) to the face-centred cubic (FCC) configuration of gamma iron, also called austenite. 9 Why is BCC the most stable crystal structure? The Body-Centered Cubic (BCC) crystal structure is one of the most common ways for atoms to arrange themselves in metals. There is, however, no cold shortness for fcc metals. Finally, at temperatures above 1135 degrees Celsius, the steel will change its crystal form again to delta with BCC structure. If a quantity increases as a second quantity decreases and decreases as the second quantity increases, the two quantities are said to be: A. directly proportional. On the other hand, more weakly bonded crystal structure, which has a higher H0 at low temperatures, is likely to be a stable phase at high temperatures, as the second term now becomes important. BCC metals are less ductile but stronger, eg iron, while HCP metals are usually brittle. High-temperature austenite, an FCC structure, allows enough space for carbon to squeeze in between the iron atoms. This is true to Le Chateliers principle. Iron is paramagnetic above this temperature and only weakly attracted to a magnetic field. Both steel and cast iron are a mix of iron and carbon. BCC metals are infact stronger than FCC metals. The cookie is used to store the user consent for the cookies in the category "Performance". This cookie is set by GDPR Cookie Consent plugin. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features Press Copyright Contact us Creators . For reference the distance for a Fe-C bond in FCC is about 1.8 angstroms while for BCC it's about 1.5 angstroms. Who was the first person to discover amethyst? %PDF-1.7 4 0 obj (Identity) endobj 5 0 obj (Adobe) endobj 8 0 obj /Filter /FlateDecode /Length 102730 /Type /Stream >> stream x |UE?)s'r"N \H ( D . Fig. The unsymmetrical octahedral holes in BCC structure have a size of radius 0.019 nm. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. 2 Interstitial sites of FCC Interstitial sites of BCC. 1.13 also illustrates that when -Fe transforms to -Fe (on cooling), expansion takes place by approximately 1%, which leads to the generation of internal stresses during transformation. Why does iron have a bcc and fcc structure? At this point, iron would lose most of its ductility. This, difference in temperatures during heating and cooling, called the thermal hysteresis, is because at the equilibrium temperature, the free energies of the two phases, one the parent phase and second the product phase, are equal. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.04% by mass at 1,146 C (2,095 F)). The first arrest takes place at the freezing temperature of pure (1539C) iron. (Definition, Types, Examples). 1.13, it ends in value for -Fe. The BCC phase is known as the alpha or ferrite phase of iron; the FCC phase is known as the gamma or austenite phase. They need thermal energy for atoms to overcome this extra distance and slip. These sites are named because of their nearest neighbors. Face-centered cubic (FCC) and body-centered cubic (BCC) are two of the most iconic crystal structures. (Metallurgy, How They Work, and Applications), What is the Difference Between Iron, Steel, and Cast Iron? A Variac supplies the adjustable AC heating current. It's pretty counterintuitive because you'd expect the tighter FCC packing structure would leave less room for carbon . Iron adopts two different packing types upon heating: BCC ( a -ferrite) at low temperature then FCC ( c -ferrite) upon heating, and back to a BCC system ( d -ferrite) at even higher. Also, both alloys contain a small amount of trace elements. Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features. However, you may visit "Cookie Settings" to provide a controlled consent. Can a general surgeon do a cardiothoracic surgery? 1.11 in the form of free energy versus temperature curve for pure iron, illustrating that -iron (FCC) has lower energy between 910C and 1394C, and is thus stable in that range. Depending on the element, the thermal energy to activate the slip systems will be different. Iron atoms are arranged in a body-centered cubic pattern (BCC) up to 1180 K. The transition from BCC to FCC results in an 8 to 9% increase in density, causing the iron sample to shrink in size as it is heated above the transition temperature. What is the atomic packing factor? When heated above 916 C, iron changes its BCC crystalline form to FCC without any change in the radius of atom. There are eight in number per unit cell. At this. This process is termed as polymorphism. Interstitial sites are the places in the unit cell between atoms. The bcc structure is often the high temperature form of metals that are close-packed at lower temperatures. The key difference between FCC and HCP is that FCC structure cycles among three layers whereas HCP structure cycles among two layers. Octahedral sites are much bigger in FCC than in BCC. How can I get that mathematically? Iron has two different crystal structures at atmospheric pressure: the body centered cubic (bcc) and the face centered cubic (fcc). The melting point of iron is 1539 C. Above this, it becomes liquid. You can check this article for details about that calculation. In this study, the structural transformation from bcc to fcc (bcc-fcc) or from fcc to bcc (fcc-bcc) during the heating process was studied by using the MD simulation with an embedded atom method. In BCC there are 2 atoms per unit . Its structure has been described as massive, cubic, lath-like, lenticular, subgrain-containing bundles at low C contents (i.e. Metals account for about two thirds of all the elements and about 24% of the mass of the planet. (Properties, Applications, and Metallurgy), Why Mercury is Used in Thermometers (and Modern Alternatives). The ability of iron to accommodate heavy interstitials, namely carbon and nitrogen, is mostly responsible for the strength and the hardening effects. Thus FCC metals deform easier than BCC metals and thus they are more ductile. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. The fcc lattice is both cubic and closely packed and forms more ductile materials. Why does iron have a bcc and FCC structure? This cookie is set by GDPR Cookie Consent plugin. This is not the case! Gamma-iron, silver, gold, and lead have fcc structures. But opting out of some of these cookies may affect your browsing experience. You might think thatbecause FCC and BCC are cubicthey are much more similar than HCP. FCC and BCC crystals have different packing, slip systems, ductility, and more! This demonstration has been around for some time (see Sutton, for example). Face-centered cubic (FCC) and body-centered cubic (BCC) are two of the most iconic crystal structures. In the example of an FCC structure, it is able to absorb higher quantities of Carbon than in a BCC structure. The cross slips in hexagonal close-packed (HCP) metals are not easy because the slip planes do not intersect. engineeringenotes.com All rights reserved. The ratio of density of the crystal before heating and after heating is [atomic weight of F e =56] Q. Thus, BCC structure of a-iron is more loosely packed than that of FCC -iron, and that is why density of FCC -iron is 8.14 g/cm3 at 20C and 7.87 g/cm3 for -iron. In general, alloys with these metals will have the same crystal structure as the most common element. Titanium, too, possesses such a property of changing the lattice structure. 7 Why can FCC Fe accommodate more C atoms than BCC Fe? The bcc!fcc phase transformation in pure Fe is studied with the help of the free-energy difference DFFe bcc!fcc = F Fe bcc F Fe fcc. Fig. When pure iron is cooled from the molten state from temperature above its freezing temperature to room temperature, heat is evolved as a phase change takes place, leading to arrest of fall of temperature and the arrest lasts as long as the phase transformation is taking place, that is, the temperature remains constant during the phase change as illustrated by a horizontal line in the thermal analysis curve. Relationship Between Cube Edge Length a and the Atomic Radius R. This cookie is set by GDPR Cookie Consent plugin. Face-Centered Cubic2a. The form occurs below a special temperature called the Curie point, which is 770 C. You also have the option to opt-out of these cookies. A sample of bcc metal with the lattice parameter a = 0.33nm was placed in a X-ray diffractometer using incoming x-rays with = 0.1541nm. What is the shape of BCC and FCC in iron? 1. Set the Variac voltage to 0, switch on the Variac, and then increase the voltage slowly to about 95 volts. Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features. When heated above 916C, iron changes its BCC crystalline form to FCC without the change in the radius of atom. In the ground state the bcc -phase is stable, and at the temperature T=1184 K (A3 point), -Fe transforms into fcc -Fe, which is stable up to 1665 K(A4 point). We talk about these structures under the field of crystal lattices. Also, at any temperature, the substitutional atoms in iron move (diffuse) several orders of magnitude more slowly than interstitial atoms. Necessary cookies are absolutely essential for the website to function properly. The important thing to note is the effect of the increased atoms in the lattice. If youre reading this article as an introductory student in materials science, welcome! This is because the stress required to move dislocations is not strongly temperature-dependent in FCC metals, and thus failure occurs by plastic flow instead of crack propagation. As you heat up iron, it will change from BCC to FCC at 912C. 1.12 (b) 1]. Which is more ductile, HCP or fcc lattice? Answer: d Explanation: Similar to metallic crystals, a few non-metallic crystals also change form due to temperature and pressure differences. How it works: If you read a very old textbook, you may even see something called a cubic close-packed (CCP), which is another name for FCC. Accompanying this transformation is a change in the atomic . Use this to determine whether iron expands or contracts when it undergoes transformation from the BCC to the FCC structure. Here is this list, in recommended reading order: Introduction to Bravais LatticesWhat is the Difference Between Crystal Structure and Bravais LatticeAtomic Packing FactorHow to Read Miller IndicesHow to Read Hexagonal Miller-Bravais IndicesClose-Packed Crystals and Stacking OrderInterstitial SitesPrimitive CellsHow to Read Crystallography Notation What are Point GroupsList of Point Groups. Note the amount of sag and then switch off the Variac. Phase transitions are one of the most fundamental phenomena of matter. The cookies is used to store the user consent for the cookies in the category "Necessary". The sagging wire will rise a few centimeters, halt momentarily and drop two to three centimeters, and then continue rising back to its original height. energy. The occurrence of BCC iron structure above 1394C is due to large electronic specific heat of iron in general (on heating). The cookie is used to store the user consent for the cookies in the category "Analytics". It does not store any personal data. We also use third-party cookies that help us analyze and understand how you use this website. Gamma-iron, silver, gold, and lead have fcc structures. This is not true for BCC. Iron atoms are arranged in a body-centered cubic pattern (BCC) up to 1180 K. The transition from BCC to FCC results in an 8 to 9% increase in density, causing the iron sample to shrink in size as it is heated above the transition temperature. The weight percentage scale on the X-axis of the iron-carbon phase diagram goes from 0% up to 6.67% Carbon. Iron has two different crystal structures at atmospheric pressure: the body centered cubic (bcc) and the face centered cubic (fcc). Because FCC has truly close-packed planes, FCC metals do not have a DBTT. These cookies will be stored in your browser only with your consent. Metals are widely used because of their properties: strength , ductility, high melting point, thermal and electrical conductivity, and toughness . (We call this APF, and you can read more about APF here). Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. Additionally, FCC metals tend to be more dense, more stable at high temperatures, and more ductile than BCC metals. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. 3. The wire is secured to lab clamps/supports on top of the lecture bench. (With Examples Beyond Carbon). Cast iron objects (frying pans, radiators, etc) have a higher carbon content and are therefore very strong, but tend to fracture rather than bend because of the larger fraction of the brittle Fe 3 C phase in the alloy. In the book Condensed Matter Physics by Marder I have read that an FCC lattice can be obtained by expanding a bcc lattice along one axis by a factor of $\sqrt{2}$. These holes or voids are called interstitial holes, or sites, or voids. Some common BCC metals are chromium, iron, and tungsten. FCC unit cell has 4 atoms per unit cell as compared to BCC having 2 atoms per unit cell. Show that the FCC structure is denser than the BCC structure. How to determine interplanar spacing for BCC/FCC crystals? B. inversely proportional. [] or not rated [], Copyright 2023 The President and Fellows of Harvard College, Harvard Natural Sciences Lecture Demonstrations, Temperature and Thermal Properties of Matter. Triclinic. Octahedral sites have 6 nearest neighbors, and tetrahedral sites have 4 nearest neighbors. Slip in face centred cubic (fcc) metals is well documented to occur on {111} planes in n110m directions. Thus, it can be generalised that whenever a phase change occurs, the more close-packed structure usually exists at low temperatures, whereas the more open structure is stable at the higher temperatures. Geometry of BCC and FCC Crystal Structures of Pure Iron: Pure iron has essentially two crystal structures, one BCC and the other FCC. The alloy may form a face-centred cubic (FCC) lattice or a body-centred cubic (BCC) lattice. (Crystal Structure, Properties, Interstitial Sites, and Examples), What is the Difference Between FCC and HCP? The body centered cubic crystal structure and icrystal structure of face centered cube. Note that the increase in sag (at the transition temperature) happens very quickly and it is helpful to repeat the demonstration for the class. Weve got your back. This cookie is set by GDPR Cookie Consent plugin. It does not store any personal data. Gamma-iron unit cell has greater lattice parameter than -iron unit cell, but atomic packing factor of FCC is 0.74, that is, 26% of the volume of unit cell is unoccupied by atoms; and is 0.68 in BCC, that is, 32% of the volume of unit cell is unoccupied by atoms. Almost all the properties are changed when one modification changes to another. What are Alloys? Image Guidelines 4. Combining the two gives: sin = n h 2 + k 2 + l 2 2 a. Thus, tetrahedral holes are not preferred sites in -iron (also ferrite) for carbon but prefer smaller octahedral holes. Because the slip planes in the FCC structure are of the closest packing. If you are interested in more details about any specific crystal structure, I have written individual articles about simple crystal structures which correspond to each of the 14 Bravais lattices: 1. Note that centre of this hole is at a distance of a/2 from two atoms and at a distance of a/2 from four atoms. How HCP BCC and FCC metals have different ductility? The different cells leads to different physical properties of bulk metals. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc. When you cool down the iron, the atoms want to shift back to BCC, but now there's too much carbon! This cookie is set by GDPR Cookie Consent plugin. Metals of bcc structure are hardened much more by interstitial than by substitutional solute atoms. This website uses cookies and third party services. Therfore interstitial gap in BCC is much smaller than that of FCC. Specific heat is primarily contributed by the lattice vibration of atoms and partly by the vibrations of electrons. These cookies track visitors across websites and collect information to provide customized ads. iron prefers to be FCC. Thus, I was surprized that alpha-Fe as body-centered cell has practically a higher density than the respective fcc if one uses as lattice parameters 3.656 A for fcc and 2.8664 A for bcc. These nuclei grow to impinge on the neighbouring grains to complete the transformation. At room temperature, some common FCC metals are aluminum, nickel, and copper. A vertical scale (meter stick) placed behind the wire may help to make its motion more evident. FCC actually has the most efficient atomic arrangement possible (tied with HCP). Effect of Pressure on Allotropy of Iron: As the pressure is increased, the -Fe -Fe transition temperature is lowered, whereas -Fe Fe transition temperature is raised. In FCC structure, there are 4 atoms per unit cell. A three meter length of iron wire is horizontally stretched above the lecture bench. FCC and BCC crystal structure also have different arrangements of interstitial sites. A metal needs five independent slip systems to plastically deform. C. comple. What is the Difference Between Crystal Structure and Bravais Lattice, How to Read Hexagonal Miller-Bravais Indices. Alpha - iron refers only to the bcc form of pure iron that is stable below 912 deg C. Ferrite is a solid solution of one or more elements in bcc iron. Thus, the critical temperatures in pure iron during cooling are designated as Ar4, Ar3 and Ar2. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. But they do not change at the same ratethe BCC phase increases at a faster rate than the FCC phase. Run hefty leads from the Variac to the wire as quite a bit of current will be required ( 9 amps). High-temperature austenite, an FCC structure, allows enough space for carbon to squeeze in between the iron atoms. For some space, in FCC materials, about 74% of the space is occupied by atoms, leaving 26% of the space empty. 1 Oxford St Cambridge MA 02138 Science Center B-08A (617) 495-5824. (List of Ferromagnetic and Ferrimagnetic Materials). Who wrote the music and lyrics for Kinky Boots? Unlike FCC slip systems, where planes are truly close-packed, BCC slip planes dont have atoms touching. Ferrite. 1.15 (b) illustrates location of other octahedral holes in FCC structure by solid spheres which are at the centres of cube edges [also Fig. On cooling further, the phase change occurs at 1401 C and the atoms rearrange themselves into the form which is F.C.C and non magnetic. But in this form, the steel is no longer magnetic. c-axis tries to become larger than -axis. When iron first solidifies 1539 c. It is in the form of B..C.C structure. On heating from room temperature, iron changes from bcc to fcc metal structure at 912 oC. Because, during the transition from the bcc to the fcc structure, the interatomic bond energy of a crystal is redistributed from kn = 8 bonds with nearest neighbors to kn = 12 bonds, the depth of the pair interatomic bond for an fcc crystal can be estimated by formula: D (12) = (8/12) D (8). Account Disable 12. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843 . Content Filtration 6. Basically, it is because the potential energy function a bulk iron system has local minima corresponding to both crystal forms, separated by an energy barrier which can be overcome through the inclusion of other atoms (carbon) in the iron lattice combined with heating (to overcome the energy barrier between the two minima) and very rapid cooling For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. For an elemental crystal, there exist a number of methods to calculate this quantity, such as the Overall, we call the slip system {111}<110>. Sodium has bcc packing. All BCC metals have a ductile-to-brittle transition temperature when there is not enough thermal energy to activate 5 independent slip systems. Iron atoms maintain their place on the lattice and carbon atoms become "interstitials." In the low-temperature ferrite, or BCC structure, however, there is no room for carbon atoms.