Iron and Its Interstitial Solid Solutions

Ch01-H8084.tex 5/5/2006 12: 15 Page 1 1 IRON AND ITS INTERSTITIAL SOLID SOLUTIONS 1.1 INTRODUCTION Steel is frequently the ‘gold-standard’ against which emerging structural materials are compared. What is often not realized is that this is a moving standard, with notoriously regular and exciting discoveries being made in the context of iron and its alloys.This is why steel remains the most successful and cost-effective of all materials, with more than a billion tonnes being consumed annually in improving the quality of life. This book attempts to explain why steels continue to take this pre-eminent position, and examines in detail the phenomena whose exploitation enables the desired properties to be achieved. One reason for the overwhelming dominance of steels is the endless variety of microstructures and properties that can be generated by solid-state transformation and processing. Therefore, in studying steels, it is useful to consider the behaviour of pure iron first, then the iron–carbon alloys, and finally the many complexities that arise when further solutes are added. Pure iron is not an easy material to produce. It has nevertheless been made with a total impurity content less than 60 parts per million (ppm), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur and phosphorus, with the remainder representing metallic impurities. Iron of this purity can be extremely weak when reasonably sized samples are tested: the resolved shear stress of a single crystal at room temperature can be as low as 10 MN m−2, while the yield stress of a polycrystalline sample at the same temperature can be well below 50 MN m−2. However, the shear strength of small single crystals has been observed to exceed 19,000 MN m−2 when the size of the sample is reduced to about 2 µm. This is because the chances of finding crystal defects such as dislocations become small as the size of the crystal is reduced. The theoretical shear strength of a perfect crystal of iron is estimated to be about 21,000 MN m−2, equivalent to a tensile strength of about 11,000 MN m−2. 1 Ch01-H8084.tex 5/5/2006 12: 15 Page 2 2 CHAPTER 1 IRON AND ITS INTERSTITIAL SOLID SOLUTIONS For comparison purposes the breaking strength of a very small carbon nanotube has been measured to be about 130,000 MN m−2; this number is so astonishing that it has led to exaggerated statements about their potential in structural applications. For example, the tubes are said to be a hundred times stronger than steel; in fact, there is no carbon tube which can match the strength of iron beyond a scale of 2 mm, because of the inevitable defects which arise as the tubes are grown. The lesson from this is that systems which rely on perfection in order to achieve strength necessarily fail on scaling to engineering dimensions. Since perfection is thermodynamically impossible to achieve in large samples, steels must in practice be made stronger by other means which are insensitive to size. The mechanisms by which the strength can be increased will be discussed – suffice it to state here that it is possible to commercially buy steel with a strength of 5500 MN m−2, with sufficient ductility to ensure safe application. Some of the methods by which such impressive combinations of properties are achieved without compromising safety will be discussed,before the wide range of complex structures which determine the properties is dealt with. 1.2 THE ALLOTROPES OF PURE IRON At least three allotropes of iron occur naturally in bulk form, body-centred cubic (bcc, α, ferrite), face-centred cubic (fcc, γ, austenite) and hexagonal close- packed (hcp, ). The phase β in the alphabetical sequence α, β, γ, δ...is missing because the magnetic transition in ferrite was at one time incorrectly thought to be the β allotrope of iron. In fact, there are magnetic transitions in all of the allotropes of iron. The phase diagram for pure iron is illustrated in Fig. 1.1. Each point on any boundary between the phase fields represents an equilibrium state in which two phases can coexist. The triple point where the three boundaries intersect represents an equilibrium between all three phases which coexist. It is seen that in pure iron, the hcp form is stable only at very large pressures, consistent with its high density. The best comparison of the relative densities of the phases is made at the triple point where the allotropes are in equilibrium and where the sum of all the volume changes is zero: V(bcc → hcp) =−0.34 → =+ 3 −1 V(hcp ccp) 0.13 cm mol V(ccp → bcc) =+0.21 There may exist a fourth natural allotrope in the core of the earth, where the pressure reaches some three million times that at the surface and where the temperature is estimated to be about 6000◦C.The core of the earth is predominantly iron, and consists of a solid inner core surrounded by a liquid outer core. Know- ledge of the core is uncertain, but it has been suggested that the crystal structure of the solid core may be double hcp, although calculations which assume pure iron,indicate that the -iron remains the most stable under inner-core conditions. Ch01-H8084.tex 5/5/2006 12: 15 Page 3 1.2 THE ALLOTROPES OF PURE IRON 3 Fig. 1.1 The phase diagram for pure iron (data from Bundy,1965).The triple point temperature and pressure are 490◦C and 110 kbars, respectively. α, γ and refer to ferrite, austenite and -iron, respectively. δ is simply the higher temperature designation of α. 1.2.1 Thin films and isolated particles There are two further allotropes which can be created in the form of thin films. Face-centred tetragonal iron has been prepared by coherently depositing iron as a thin film on a {100} plane of a substrate such as copper with which the iron has a mismatch. The position of atoms in the first deposited layer in this case replicates that of the substrate. A few monolayers can be forced into coherency in the plane of the substrate with a corresponding distortion normal to the substrate. This gives the deposit a face-centred tetragonal structure. Growing iron on a misfitting {111} surface of a fcc substrate leads to trigonal iron. Very thin films of iron retain their ferromagnetic character, but there are special effects due to the small dimensions. The magnetic moment per atom becomes very large: 3.1 Bohr magnetons compared with 2.2 for bulk α-iron. This is due to the smaller coordination number for atoms in a thin film. The second effect is that magnetic anisotropy greatly increases for thin films because the spins tend to align normal to the surface. The Curie temperature is greatly reduced, again because of the change in coordination. For a monolayer of iron the temperature is just 280◦C. Many classical studies of nucleation theory have been conducted on minute (5–1000 nm) particles of iron where defects responsible for heterogeneous nucleation can be avoided. Such particles have acquired new significance in that they are exploited in the manufacture of carbon nanotubes. The particles are deposited due to the decomposition of ferrocene in chemical mixtures which also contain the ingredients necessary to grow the tubes. Ch01-H8084.tex 5/5/2006 12: 15 Page 4 4 CHAPTER 1 IRON AND ITS INTERSTITIAL SOLID SOLUTIONS Fig. 1.2 A multi-walled carbon nanotube containing a particle of iron (unpublished micrograph courtesy of I. Kinloch). It is expected that the coarser particles will have the bcc crystal structure of ferrite, but it has to be appreciated that a 5 nm particle has about half its atoms at the surface. Metal surfaces are prone to reconstruction into a variety of two-dimensional structures which will complicate the interpretation of the structure of the particle as a whole. The surface also plays another role, in that it alters the total free energy of the particle leading to a depression of its melting temperature. It has been estimated that a 5 nm diameter iron particle could melt at a temperature as low as 500◦C. Figure 1.2 illustrates an iron particle inside a carbon nanotube – its blobby character has been speculated to be due to melting. Small metal particles in the size range 1–5 nm are close to a metal/insulator transition. When observed at the tips of carbon nanotubes using scanning elec- tron microscopy, the iron particles have shown a tendency to charge, possibly indicating a loss of metallic behaviour. 1.3 THE PHASETRANSFORMATION: α- AND γ-IRON The vast majority of steels rely on just two allotropes, α and γ. Iron is a peculiar element in that at ambient pressure, bcc ferrite is stable from all temperatures up ◦ to 910 C (the A3 point), when it transforms into the fcc austenite, only to revert ◦ to ferrite at 1390 C (the A4 point). This high-temperature ferrite is traditionally labelled δ, although it is no different in crystal structure from α.Theδ-ferrite remains the stable phase until melting occurs at 1536◦C. Figure 1.3 shows the phase changes in a plot of the mean volume per atom of iron as a function of temperature. It should be noted that the γ-to α-transformation is accompanied by an atomic volume change of approximately 1%, which can lead to the generation of internal stresses during transformation.

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