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Mechatronics Engineering · 300 Level
This is the complete lecture note for MME 312 Physical Metallurgy 1, prepared for Mechatronics Engineering students at 300 Level. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.
Course code: MME 312 Course title: Physical Metallurgy I Course credit load: 2 Units Series 1: Introduction to Metals, the Metallic Bond and Solidification of Materials ● Part 1.1: Introduction to Metals and Metal Alloy Systems o Sub Part 1.1.1: The Nature and General Properties of Metals o Sub Part 1.1.2: Pure Metals and Alloys o Sub Part 1.1.3: Classification of Metal Alloy Systems ● Part 1.2: The Metallic Bond and Structure of Metals o Sub Part 1.2.1: The Metallic Bond and the Electron Sea o Sub Part 1.2.2: Unit Cells and Common Crystal Structures o Sub Part 1.2.3: Atomic Packing, Coordination and Properties ● Part 1.3: Solidification of Pure Metals o Sub Part 1.3.1: Freezing, Latent Heat and Undercooling o Sub Part 1.3.2: Nucleation: Homogeneous and Heterogeneous o Sub Part 1.3.3: Crystal Growth and Grain Formation ● Part 1.4: Solidification Variables and Non-Crystalline Materials o Sub Part 1.4.1: Effect of Cooling Rate on Structure o Sub Part 1.4.2: Solidification of Non-Crystalline Materials o Sub Part 1.4.3: Crystalline and Non-Crystalline Solidification Compared Introduction Every metal object around you, from the cutlery on a dining table to the girders of a bridge, began life as a liquid that cooled and set. The way that change happened quietly decided how strong, how tough and how reliable the finished object would be. Physical metallurgy is the study of the link between the internal structure of metals and their behaviour, and it begins with understanding what metals are, how their atoms are held together and how they freeze.
The aim of this Series is to equip you to describe the nature of metals and alloys, explain the metallic bond and the crystal structures of metals, and analyse how pure metals and non-crystalline materials solidify. We shall commence with metals and metal alloy systems, where you will learn how metals differ from alloys and how alloy systems are described. Next, we will explore the metallic bond and the structure of metals, including unit cells and atomic packing. Afterwards, we will follow the solidification of pure metals from nucleation to grain formation. We will wrap up with the variables that shape solidified structures and the solidification of non-crystalline materials. Series Learning Outcomes Upon completing this Series, you should be able to: ● SLO 1.1: define a metal and an alloy, and state the general properties of metals, ● SLO 1.2: classify metal alloy systems according to their components and phases, ● SLO 1.3: explain how the metallic bond arises and how it accounts for the properties of metals, ● SLO 1.4: describe the BCC, FCC and HCP crystal structures in terms of atomic packing, ● SLO 1.5: explain the solidification of a pure metal using its cooling curve, ● SLO 1.6: distinguish homogeneous nucleation from heterogeneous nucleation, ● SLO 1.7: analyse the effect of cooling rate and temperature gradient on the structure of a solidified metal, ● SLO 1.8: explain how the solidification of non-crystalline materials differs from that of crystalline metals. 1.1 Introduction to Metals and Metal Alloy Systems 1.1.1 The Nature and General Properties of Metals A metal is an element that readily gives up its outer electrons and, in the solid state, shows a characteristic lustre, high density and good ability to conduct heat and electricity. Most of the elements in the periodic table behave in this way, including iron, copper, aluminium and zinc. Metals are also ductile, meaning they can be drawn into wires without fracture, and malleable, meaning they can be hammered into thin sheets. These properties explain why metals dominate structural, electrical and manufacturing applications. The mechanical behaviour of metals is described by properties such as strength, the ability to resist an applied load, and hardness, the resistance to indentation or scratching of the surface.
Metals generally combine reasonable strength with toughness, so they can absorb energy before breaking. Their thermal conductivity makes them useful for heat exchangers, while their electrical conductivity underpins cables and contacts. Because these properties depend on internal structure as well as composition, a physical metallurgist studies how structure can be controlled to obtain the properties required. Worked example: A copper wire drawn through a die to a smaller diameter without breaking demonstrates ductility, while a copper sheet hammered flat without cracking demonstrates malleability. Application: Recognising ductility and malleability allows an engineer to match a metal to a forming operation such as wire drawing or sheet rolling. Fill in the blank: A metal that can be drawn into wires without fracture is described as __________. 1.1.2 Pure Metals and Alloys A pure metal consists of atoms of a single element, although even the purest commercial metal contains small traces of impurities. Pure metals melt and freeze at one fixed temperature and are usually soft and weak, even though they conduct electricity very well. An alloy is a metallic material made by combining a metal with one or more other elements, which may be metals or non-metals such as carbon. Brass, formed from copper and zinc, and steel, formed from iron and carbon, are familiar examples. Alloying is carried out deliberately because it improves properties that pure metals lack. The added atoms disturb the regular arrangement of the host atoms, making it harder for layers of atoms to slide over one another, so strength and hardness rise. Alloying can also improve corrosion resistance, castability and response to heat treatment. However, the gain in strength often comes with some loss of electrical conductivity, which means that the alloy chosen must always balance competing requirements for a given application. Worked example: Adding about 30 per cent zinc to copper produces brass, which is stronger and harder than pure copper but conducts electricity less well. Application: Choosing between a pure metal and an alloy depends on whether conductivity or strength matters more for the intended use. Fill in the blank: A metallic material made by combining a metal with one or more other elements is called an __________. 1.1.3 Classification of Metal Alloy Systems Metallic materials are commonly grouped into ferrous metals, which are based on iron, and non-ferrous metals, which contain little or no iron. Steels and cast irons belong to the ferrous group, while copper, aluminium and their alloys belong to the non-ferrous group. This grouping is useful because ferrous materials share a common set of transformations and heat treatments, whereas non-ferrous materials usually rely on different strengthening routes. You will meet both groups repeatedly throughout this course.
Alloys can be described further using the language of systems. A component is one of the chemically distinct constituents from which an alloy is made, and a phase is a physically distinct, homogeneous portion of the material with its own composition and structure. An alloy system is the complete range of alloys that can be formed from the same set of components, for example all the copper and nickel alloys. A system with two components is called a binary system, and it forms the basis of the phase diagrams studied later. Worked example: In a copper-nickel binary system, copper and nickel are the two components, and an alloy that is uniform throughout consists of a single phase. Application: Correctly identifying components and phases is the essential first step in reading any phase diagram. Fill in the blank: A homogeneous, physically distinct portion of an alloy with its own composition and structure is called a __________. Figure 1.1: Classification of metallic materials and alloy systems. Pilot questions 1.1 1. Name the property that describes the ability of a metal to be hammered into thin sheets. (Linked to SLO 1.1) 2. True or false: A pure metal usually melts over a wide range of temperatures. (Linked to SLO 1.1) 3. State one reason why metals are alloyed. (Linked to SLO 1.1) 4. Which group of metals is based on iron? (Linked to SLO 1.2)
5. How many components are present in a binary alloy system? (Linked to SLO 1.2) 1.2 The Metallic Bond and Structure of Metals 1.2.1 The Metallic Bond and the Electron Sea The metallic bond is the attraction between positively charged metal ions and the cloud of freely moving valence electrons that surrounds them. Metal atoms have few outer electrons, and these are loosely held, so in the solid they detach and become delocalised, meaning they are not attached to any single atom. The resulting arrangement is often called the electron sea model, in which a lattice of positive ions is held together by mobile electrons. The bond is non-directional, so it acts equally in all directions around each ion. This model explains many familiar properties of metals. Because the electrons move freely, metals conduct electricity and heat well. Because the bond is non-directional, layers of ions can slide past one another without breaking the bonding, which accounts for ductility and malleability. Free electrons also absorb and re-emit light, giving metals their lustre. The strength of the bond varies between metals, so tungsten, which contributes many bonding electrons, melts at a far higher temperature than sodium, which contributes only
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