Engineering Materials and Processes Lecture 4 – Crystal Structure of Metals Prescribed Text: Ref 1: Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. ISBN: 9781856177696 Readings: Callister: Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Edition, Wiley, New York. ISBN 9780470419977 Ashby 1: Ashby, M. & Jones, D., 2011, Engineering Materials 1: An Introduction to Properties, Applications and Design, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN: 9780080966656 Ashby 2: Ashby, M. & Jones, D., 2011, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN: 9780080966687   Lecture (2 hrs): Ref 1, Ch 1: Engineering materials; Ref 1 Ch 2: Properties of materials. Laboratory 1 (2 hrs): Hardness test Callister: Ch 1, 2, 18-21 Ashby 1: Ch 1, 2 Ashby 2: Ch 1 Grains after acid etching: Wikipedia 1

Crystal Structure of Metals Reference Text Section Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann Ch 4 Additional Readings Section Sheedy, P. A, 1994. Materials : Their properties, testing and selection Ch 1 Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Ed, Wiley, New York. Ch 3 Engineering Materials and Processes

Crystal Structure of Metals Crystals are the lattice structure that metal atoms form when they become solid. In engineering, crystals of metal are called grains. Grains in cast aluminium www.spaceflight.esa.int Engineering Materials and Processes

Solid, Liquid, Gas Everything can exist as either solid, liquid or gas. (state). This depends on temperature and pressure. Mercury freezes at -9°C and boils (to form a gas or vapour) at 357°C at 1 atm. Liquid Mercury: Wikipedia At the other end of the scale, tungsten melts at 3410°C and boils at 5930°C. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 1.2.1 Mercury vapour streetlights: theage.com.au Engineering Materials and Processes 4

Classification of Materials by State • Solid: Rigid structure of atoms (or molecules). A regular pattern of atoms is called a crystal (ceramics) or a grain (metals). Random is called amorphous (some plastics). Low temperature all materials freeze (become solid). • Liquid: A liquid is a substance that flows (fluid) but does not compress easily. Solids become liquid when heated to their melting point (fusion) • Gas: A gas is a compressible fluid, that expands to fill its container. At high temperature all materials vapourise to a gas. Engineering Materials and Processes

Melting / Boiling of Elements A classifiaction by atomic structure of all the elements (different types of atoms) Go to website for PT animation of temperature… www.ptable.com Engineering Materials and Processes

Kinetic Theory of Gases In any gas, the particles (whether atoms or molecules) are in constant motion. As these particles bounce off the walls it pushes them – which makes pressure exerted by the gas. As the temperature increases, the velocity and number of impacts increases, so the average pressure on the wall of the vessel increases. This is the kinetic theory of gases. Gas Animation Volume vs temperature Tim Lovett 2012 Engineering Materials and Processes 7

Gas to Liquid. (Metal Vapour Condensing) The temperature of a metal vapour (gas) falls until it reaches the boiling point where it starts to turn into liquid (condense). In a liquid the atoms are randomly mixed together and are free to slide around. The atoms are held together only by weak forces of attraction at this stage, the liquid lacks cohesion and will flow. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 1.2.1 Gas Animations: Tim Lovett 2012 Engineering Materials and Processes 8

Liquid to Solid. (Solidification) To turn the liquid metal into solid, each atom must lose more energy. For a pure metal this occurs at a fixed temperature (melting point). During solidification, the atoms arrange themselves according to some regular pattern, or 'lattice structure' which is called a crystal – or in the case of metals – a grain. Each atom is now connected to its neighbours like springs. This spring stiffness causes the Modulus of Elasticity E. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 http://www.ami.ac.uk/courses/topics/0131_mb/index.html Lava Solidification: http://www.geol.umd.edu Engineering Materials and Processes 9

Shrinkage Metal Contraction (%) Grey cast iron 0.7 to 1.05 White cast iron 2.1 Malleable iron 1.5 Steel 2.0 Brass 1.4 Alluminium 1.8 Aluminium alloys 1.3 to 1.6 Bronze 1.05 to 2.1 Magnesium Zinc 2.5 Manganese steel 2.6 Atoms in a solid fit closer together than in a liquid, so shrinkage occurs. A few substances expand when they solidify because they have a very loose packing arrangement. E.g. Water (ice), and Silicon. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 http://www.ami.ac.uk/courses/topics/0131_mb/index.html Based on; http://www.calculatoredge.com Titanic iceberg (maybe): Wikipedia Engineering Materials and Processes 10

Latent Heat A pure metal solidifies at a fixed temperature (melting point). The liquid resists cooling below the melting point until the liquid has solidified. This requires removal of the Latent Heat. This energy is called the latent heat of fusion (solidification in this case). Alloys (metal mixtures) can have a range of melting temperatures. Higgins: Fig 4.1 Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 http://www.ami.ac.uk/courses/topics/0131_mb/index.html Engineering Materials and Processes 11

Crystals During solidification, the atoms fit into a regular pattern, or 'lattice structure' called a crystal. (or a metal grain). In a 2D plane, the tightest arrangement is like a honeycomb. (based on 120o). Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 http://www.chem.ufl.edu/~itl/2045/lectures/lec_h.html Engineering Materials and Processes 12

Crystals In 3D, the common crystal packing arrangements are; BCC, FCC and HCP. FCC and HCP pack the closest, while BCC packs slightly less dense (by a few %). Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 Higgins: Fig 4.2 Engineering Materials and Processes 13

Crystals The unit structures of the 3 crystal structures are a bit misleading. They are different SIZES! The packing difference is only a few %. A very 3 dimensional problem, so best explained in 3D video. Lattice structures: Part 1 thechemprofessor Local (mp4) Crystals Univ of Florida Local Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 Engineering Materials and Processes 14

Allotropy of Iron Iron is polymorphic - it forms more than one crystalline form (allotropes of Iron). The two main crystal structures are; BCC (a-iron) FCC (g-iron) above 910°C. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 BCC body-centred cubic (a-iron) FCC above 910°C (g-iron). Iron Engineering Materials and Processes 15

Iron BCC vs FCC Iron Since FCC is tighter than BCC, there is a sudden volume change during quenching. This can cause internal stresses, distortion or even cracking of the component. But, it is also the reason steel can be heat treated at all… making it by far the most important metal in engineering. Need to quench with care. Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 Cracking of steel due to quenching Engineering Materials and Processes 16

Dendritic solidification (Higgins 4.3.1) As the molten pure metal cools below its freezing point, crystallisation will begin. It starts out with a single unit – (e.g. BCC for Tungsten). New atoms will join the 'seed crystal' and grow onto the structure much like a snowflake (except the metal is forming in liquid, not a cloud of droplets). BCC Unit: Higgins Fig 4.3 The branched crystal is called a 'dendrite‘ (Greek for tree). Snowflake: Wikipedia Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 Higgins Fig 4.4 Engineering Materials and Processes 17

Dendrite of Silver: Wikipedia Engineering Materials and Processes

VIDEO: Excerpt from BBC Properties and Grain Structure 1973 Dendrites to Grains Each dendrite forms independently, so the outer arms of neighbouring dendrites make contact with each other at irregular angles and this leads to the irregular overall shape of crystals. VIDEO: Excerpt from BBC Properties and Grain Structure 1973 Grains form random orientations Wikipedia Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4 Grains after acid etching: Wikipedia Engineering Materials and Processes 19

Impurities Above: Dendritic formation of grains. Below: Etched metal showing dendritic structure with porosity where molten metal was not available to fill the voids. openlearn.open.ac.uk http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=2.7 The jumbled, chaotic area between grains. The irregular nature of the grain boundary is one source of creep in metals but it is a barrier to dislocation mobility. www.spaceflight.esa.int 20

Grains A section through a cast aluminium ingot, polished and etched in acid. The crystals are all the same (pure aluminium), but each crystal lattice is randomly tilted and so reflects the light differently. This is why they look bright or dark. Note a metal “grain” is not like the “grain” in wood. It means crystal. http://www.spaceflight.esa.int/impress/text/education/Solidification/Crystaline_Growth.html Grains in cast aluminium www.spaceflight.esa.int Engineering Materials and Processes 21

Grain Boundaries In pure metals, dentrites disappear once solidification is complete. If impurities were in the melt, they may get left out of the growing grain and end up at the grain boundaries. This is why a small amount of impurity can destroy the mechanical properties – causing brittleness and cracks along the crystal boundaries. http://www.spaceflight.esa.int/impress/text/education/Solidification/Crystaline_Growth.html The jumbled, chaotic area between grains. The irregular nature of the grain boundary is one source of creep in metals but it is a barrier to dislocation mobility. www.spaceflight.esa.int Engineering Materials and Processes 22

Grain Boundaries Bubble raft demonstrating grains in a metal under stress. Each bubble represents an atom. Under low stress, atoms stretch elastically. With higher deformation plastic deformation occurs. The bubble raft rearranges by dislocation motion. You Tube Offline (mp4) From TLP: Introduction to dislocations, http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_motion.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ Engineering Materials and Processes 23

Porosity Porosity occurs because the casting shrinks on solidification. High pressure can fix this, but it makes the mould very expensive. Another way is to use a reservoir of molten metal that feeds more liquid as it solidifies. In this low pressure casting, aluminium alloy was poured down the runner into the mould. The hollow in the top of the runner caused by liquid flowing from the runner into the mould as the casting solidified. As well as the hollow at the top, you can see some holes in the runner and one hole within the casting itself. The runners and risers will later be cut off and discarded. http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=2.7 http://openlearn.open.ac.uk Engineering Materials and Processes 24

Cooling rates and grain size Slow cooling = more time to form = larger grains. Rapid cooling = fine grains. For the same metal grain, finer grains are stronger and tougher. Grains are typically 0.1 to 100 microns. Note: This is NOT referring to quenching of Carbon steel. Quenching produces a different type of grain - Martensite. Grain size vsd yield strength. Low C steel.   W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers, p63-71. 1985. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5 http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=2.7 Engineering Materials and Processes 25

Grain Growth If solid metal is above a certain temperature (recrystalisation), certain grains will grow at the expense of their neighbours. Growth of a grain structure http://www.doitpoms.ac.uk/tlplib/grainGrowth/2dcomputersimulation.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence You Tube http://www.youtube.com/watch?v=RX6xAeCForM Offline (mp4) Engineering Materials and Processes 26

Cooling of a large casting When a large ingot solidifies, the outer skin cools rapidly (small grains). Cooling then travels inwards, creating inward grain growth. (Columnar) The left over molten metal at the centre, cools slowly (large grains). Cooling in all directions, so equi-axed. So the grain structure makes castings weak. http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=2.7 Zones of different crystal forms in an ingot. Higgins Figure 4.12 Engineering Materials and Processes 27

Make sense now? Engineering Materials and Processes http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=2.7 Grains in cast aluminium www.spaceflight.esa.int Engineering Materials and Processes 28

Forging Forging improves steel – because it mixes up the bad grain structure of a casting. Forging of Samurai sword. http://www.roninkatana.com You Tube http://www.youtube.com/watch?v=RX6xAeCForM Offline (mp4) Engineering Materials and Processes 29

Rapid solidification (Higgins 4.5.1) By cooling fast enough, tiny crystals are formed – or even none at all. (glassy metal) Any impurities dispersed in the melt will remain so in the amorphous solid. The metal is more uniform in composition, stronger and tougher, but still malleable and ductile. But cooling so fast (a million oC per sec) means only very thin bits have been made. http://metallurgyfordummies.com/amorphous-metal/ Amorphous (Glassy) Metals http://metallurgyfordummies.com/amorphous-metal/ Engineering Materials and Processes 30

Glassy Metal 'Melt Spinning' or 'Splat quenching' to produce metallic glass ribbon. Molten tin alloy is fed through several nozzles onto a rotating metal drum, resulting in extremely high cooling rates (approx. 1 million Kelvin per second). High speed photography at 4000 fps. You Tube Offline http://metallurgyfordummies.com/amorphous-metal/ From TLP: Casting, http://www.doitpoms.ac.uk/tlplib-dev2008/casting/casting_other.php DoITPoMS, The University of Cambridge. Engineering Materials and Processes 31

VIDEO: Crystals and Grain Structure Properties and Grain Structure BBC (1973) What is a grain? Recrystalisation Engineering Materials and Processes 32

Online Properties Resources. Graphical comparison of materials properties. DoITPoMS: Dissemination of IT for the Promotion of Materials Science Wikipedia: Materials properties Metal Grains and processing h ttp://www.matweb.com S how this website on screen. Will be using this later. Lattice Structure BCC, FCC,HCP Engineering Materials and Processes 33

Body-centred cubic (BCC) Coordination number Crystalline GLOSSARY Allotropy Amorphous Anisotropy Body-centred cubic (BCC) Coordination number Crystalline Face-centred cubic (FCC) Grain Grain boundary Hexagonal close-packed (HCP) Isotropic Lattice Noncrystalline Polycrystalline Polymorphism Single crystal Unit cell http://www.youtube.com/watch?v=1GFst2IQBEM Engineering Materials and Processes 34

Define all the glossary terms. QUESTIONS Callister: Ch3 (Mostly about calculating atomic packing factors - too esoteric) Moodle XML: Some questions in 10102 Classification and 10105 Steel Define all the glossary terms. What is the difference between atomic structure and crystal structure? What is a dendrite? Explain why ice floats. As a pure metal constantly cools, what happens to the temperature at the melting point? Explain. Determine the coordination number (number of direct neighbours) for BCC, FCC and HCP lattice packing. As pure iron cools, at 910oC it switches from ….. lattice to ….. Explain why grain size is important in engineering metals, and which is best. http://www.youtube.com/watch?v=1GFst2IQBEM Engineering Materials and Processes 35