3.0 The Peritectic Reaction The peritectic reaction is a 3 phase reaction that takes place at a constant temperature tp, when a solid solution of constant composition reacts with a liquid of constant composition to form a second solid again of constant composition. A peritectic reaction usually occurs when the melting points of the 2 metals differ considerably. 3.1 Intermediate Phases The isomorphous, eutectic and peritectic diagrams so far showed solid phases called terminal solid solutions which existed over composition ranges near the concentration extremities of the phase diagram. We can also find intermediate solid solutions or intermediate phases at other than the two composition extremes For some systems, discrete intermediate compounds rather than solid solutions may be found on the phase diagram. These are called intermetallic compounds. This can be defined as a compound made up of 2 or more elements, producing a new phase with its own composition, crystal structure and properties. Intermetallic compounds are almost always very hard and brittle There are 2 types of intermetallic compounds which are often encountered Electron Compounds These compounds are of definite chemical crystal structure and arise if the two alloying metals are of different crystal structure, valency and if one of these metals is electropositive with the other being electro-negative Interstitial compounds These compounds form between metals, or metals and non-metals, with atoms very similar to those that form interstitial solid solution. The presence of an intermetallic splits the phase diagram so that each part can be treated separately as individual diagrams Stoichiometric intermetallic compounds Have a fixed composition and are represented by a vertical line in the phase diagrams Non-stoichiometric intermetallic compounds Have a range of compositions and are the intermediate solid solutions mentioned earlier3.2 Solid State Transformations The Eutectoid Reaction Three phase reaction in which one solid phase transforms to 2 different solid phases. (insert diagram + equations) The Peritectoid Reaction 3 phase reaction in which 2 solids combine to form a third solid on cooling ( insert diagram + equations)3.3 Summary on the types of phase diagrams All the possibilities which exist when an alloying element is added to the base metal Can be fully soluble, partly soluble or insoluble in the liquid state (we will only be dealing with full solubility in the liquid state) When fully soluble in the liquid state, it can remain fully soluble in the solid state, become partly soluble or completely insoluble on solidification. Similar changes can occur in the solid state, when a solid solution breaks down (eutectoid and peritectoid)

3.4 The Iron Carbon Phase Diagram 3.4.1. Polymorphism and Allotropy Polymorphism is a physical phenomenon where a material may have more than one crystal structure (BCC, FCC & HCP). A material that shows polymorphism exists in more than one type of space lattice in the solid state. If the change in structure is reversible with temperature or pressure then the polymorphic change is known as allotropy. The prevailing crystal structure depends on both the temperature and the external pressure. Allotropes of Iron Iron too can exist in more than one crystal structure and the change is reversible. The different allotropes of iron are designated with Greek letters. One way of demonstrating these changes is to heat an iron wire and to observe the changes in length. [graph] Low temperature iron is BCC and this form is called ALPHA iron (-Fe) At 910 degrees Celsius, -Fe changes to GAMMA iron (-Fe) which is FCC. This change causes contraction, since the FCC is close-packed, whilst the BCC form is not. This change from BCC to FCC therefor causes contraction. Further heating causes uniform expansion until at 1400 degrees Celsius, the FCC -Fe, reverts to the BCC form with a sudden expansion. This high temperature form is DELTA iron (-Fe) and it is stable up to the melting point of iron (1537 degrees Celsius.) The reverse occurs upon cooling Iron alloys are the most important and most widely applied metallic materials in industrial practice. Iron alloys contain Iron as the fundamental or base metal and always contain carbon which is considered as the basic alloying element of iron alloys. They can be called either steel or cast iron, depending on the amount of carbon present in the alloy. The iron alloys also contain several other elements in smaller quantities, including manganese, silicon, sulphur and phosphorous. These elements are found in a limited amount, and do not influence considerably the equilibrium diagram Fe-C. Therefor the metallographic concepts and processes connected to iron alloys can be studied in the two component Fe-C equilibrium diagrams 3.4.2 Solubility of Carbon in Iron Interstitial solid solution of iron and carbon Iron forms interstitial solid solutions with carbon. The amount of iron that iron can dissolve is different for the different allotropes of iron. The alpha iron is almost pure iron since it is only capable of dissolving 0.022% carbon in the equilibrium state. This solid solution is called FERRITE. Gamma iron can dissolve 2.14% carbon at a temperature of 1147 degrees. FCC can dissolve more carbon because of its empty space in the middle of the crystal structure. This solid solution is called AUSTENITE. Interstitial metallic compounds between iron and carbon. The interstitial compound of Fe & C is Fe3C. This is called iron carbide or cementite. This is very hard and rigid, it is practically non-deformable. By controlling the amount, size and shape of Fe3C we control the degree of dispersion strengthening and the properties of steel 3.4.3 Phases in the Fe-Fe3C There are 5 phases in the iron carbon phase diagram: liquid, ferrite, austenite, delta-ferrite and the fifth phase in which the insoluble carbon exists. This may be the metastable interstitial compound Fe3C or the stable form graphite. This diagram is in fact a dual phase diagram. Since complete stability is difficult to achieve, Fe3C is the more likely phase to form. However under certain favourable conditions, the higher carbon alloys can form graphite, giving the useful grey cast irons. In practice, iron carbon alloys are used with a limited carbon content, so the iron carbon phase diagram extends only to the value of C=6.67%. This carbon content corresponds to the Fe3C in the metastable system. Therefor the right hand side of the boundary line of the metastable phase diagram represents 100% Fe3C This equilibrium diagram is therefor often referred to as the Fe- Fe3C system. Cementite is a metastable phase, which on prolonged heating breaks up into iron + graphite. Thus the diagram cannot be called an equilibrium diagram but a phase diagram. 3.4.4 Steel and Cast Iron Classifications On the basis of the Fe- Fe3C phase diagram, iron-carbon alloys can be classified as: Up to 0.8%C Hypo-eutectoid steels 0.8%-2.0% - Hyper-eutectoid steels 2.0% - 4.3% - Hypo-eutectic cast irons 4.3% or more Hyper-eutectic Cast irons In practice plain carbon steels are classified as: Low Carbon or Mild Steels (up to 0.3%C) Medium carbon or midcarbon steels (0.3-0.7%C) High carbon or tool steels (0.7-1.4%C) 3.4.5 Reactions occurring in the Fe- Fe3C system Eutectic Occurs at T=1147 degrees between 2.06-6.67%C The two phase product of the eutectic reaction: [austenite() + cementite(Fe3C)] is called ledeburite Eutectoid Occurs at T=723 degrees between 0.025-6.67% [Ferrite() + cementite(Fe3C)] is called pearlite We can see that most of the pearlite is composed of ferrite (88.7%). The Fe3C lamellae are in fact surrounded by . The pearlite structure therefore provides dispersion strengthening the continuous ferrite phase is relatively soft and ductile and the hard brittle cementite phase is dispersed. 3.4.6 Mechanical Properties of Carbon Steels The mechanical properties of the crystalline3.4.1 Polymorphism & Allotropy3.4.2 Solubility of carbon in iron3.4.3 Phases in the Iron Carbon Phase Diagram3.4.4 Steel and Cast Iron Classifications3.4.5 Reactions occurring in the Fe-Fe3C system3.4.6 Development of microstructures in Fe-C alloys3.4.7 Mechanical properties of carbon steels