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TAFE NSW -Technical and Further Education Commission www.highered.tafensw.edu.au ENMAT101A Engineering Materials and Processes Associate Degree of Applied.

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Presentation on theme: "TAFE NSW -Technical and Further Education Commission www.highered.tafensw.edu.au ENMAT101A Engineering Materials and Processes Associate Degree of Applied."— Presentation transcript:

1 TAFE NSW -Technical and Further Education Commission ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies) Lecture 4 – Crystal Structure of Metals Grains after acid etching: Wikipedia

2 TAFE NSW -Technical and Further Education Commission Crystal Structure of Metals EMMAT101A Engineering Materials and Processes Reference TextSection Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann Ch 4 Additional ReadingsSection Sheedy, P. A, Materials : Their properties, testing and selectionCh 1 Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Ed, Wiley, New York. Ch 3

3 TAFE NSW -Technical and Further Education Commission Crystal Structure of Metals EMMAT101A Engineering Materials and Processes 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

4 TAFE NSW -Technical and Further Education Commission 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. EMMAT101A Engineering Materials and Processes Liquid Mercury: Wikipedia At the other end of the scale, tungsten melts at 3410°C and boils at 5930°C. Mercury vapour streetlights: theage.com.au

5 TAFE NSW -Technical and Further Education Commission 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. Classification of Materials by State EMMAT101A Engineering Materials and Processes

6 TAFE NSW -Technical and Further Education Commission Melting / Boiling of Elements EMMAT101A Engineering Materials and Processes

7 TAFE NSW -Technical and Further Education Commission Kinetic Theory of Gases EMMAT101A Engineering Materials and Processes 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

8 TAFE NSW -Technical and Further Education Commission 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. Gas Animations: Tim Lovett 2012 EMMAT101A Engineering Materials and Processes

9 TAFE NSW -Technical and Further Education Commission 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. Lava Solidification: Each atom is now connected to its neighbours like springs. This spring stiffness causes the Modulus of Elasticity E. EMMAT101A Engineering Materials and Processes

10 TAFE NSW -Technical and Further Education Commission Shrinkage 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. Based on; Metal Contraction (%) Grey cast iron0.7 to 1.05 White cast iron2.1 Malleable iron1.5 Steel2.0 Brass1.4 Alluminium1.8 Aluminium alloys1.3 to 1.6 Bronze1.05 to 2.1 Magnesium1.8 Zinc2.5 Manganese steel2.6 Titanic iceberg (maybe): Wikipedia EMMAT101A Engineering Materials and Processes

11 TAFE NSW -Technical and Further Education Commission 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 EMMAT101A Engineering Materials and Processes

12 TAFE NSW -Technical and Further Education Commission 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 120 o ). EMMAT101A Engineering Materials and Processes

13 TAFE NSW -Technical and Further Education Commission 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: Fig 4.2 EMMAT101A Engineering Materials and Processes

14 TAFE NSW -Technical and Further Education Commission 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 Crystals Univ of Florida Local Local (mp4) EMMAT101A Engineering Materials and Processes

15 TAFE NSW -Technical and Further Education Commission Allotropy of Iron Iron is polymorphic - it forms more than one crystalline form (allotropes of Iron). The two main crystal structures are; BCC (  -iron) FCC (  -iron) above 910°C. Iron BCC body-centred cubic (  -iron) FCC above 910°C (  -iron). EMMAT101A Engineering Materials and Processes

16 TAFE NSW -Technical and Further Education Commission Iron BCC vs FCC 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. Iron 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. Cracking of steel due to quenching EMMAT101A Engineering Materials and Processes

17 TAFE NSW -Technical and Further Education Commission 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 Snowflake: Wikipedia The branched crystal is called a 'dendrite‘ (Greek for tree). Higgins Fig 4.4 EMMAT101A Engineering Materials and Processes

18 TAFE NSW -Technical and Further Education Commission Dendrite of Silver: Wikipedia EMMAT101A Engineering Materials and Processes

19 TAFE NSW -Technical and Further Education Commission 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. Grains after acid etching: Wikipedia VIDEO: Excerpt from BBC Properties and Grain Structure 1973 Grains form random orientations Wikipedia EMMAT101A Engineering Materials and Processes

20 TAFE NSW -Technical and Further Education Commission 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 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.

21 TAFE NSW -Technical and Further Education Commission 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. Grains in cast aluminium EMMAT101A Engineering Materials and Processes

22 TAFE NSW -Technical and Further Education Commission 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. 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. EMMAT101A Engineering Materials and Processes

23 TAFE NSW -Technical and Further Education Commission 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. EMMAT101A Engineering Materials and Processes You TubeOffline (mp4) From TLP: Introduction to dislocations, _motion.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non- Commercial-Share Alike licence _motion.php

24 TAFE NSW -Technical and Further Education Commission 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. EMMAT101A Engineering Materials and Processes

25 TAFE NSW -Technical and Further Education Commission 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, p Van Nostrand Reinhold (UK) Co. Ltd. ISBN: EMMAT101A Engineering Materials and Processes

26 TAFE NSW -Technical and Further Education Commission 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 owth/2dcomputersimulation.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial- Share Alike licence owth/2dcomputersimulation.php You Tube Offline (mp4) EMMAT101A Engineering Materials and Processes

27 TAFE NSW -Technical and Further Education Commission 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. Zones of different crystal forms in an ingot. Higgins Figure 4.12 EMMAT101A Engineering Materials and Processes

28 TAFE NSW -Technical and Further Education Commission Make sense now? Grains in cast aluminium EMMAT101A Engineering Materials and Processes

29 TAFE NSW -Technical and Further Education Commission Forging Forging improves steel – because it mixes up the bad grain structure of a casting. Forging of Samurai sword. You Tube Offline (mp4) EMMAT101A Engineering Materials and Processes

30 TAFE NSW -Technical and Further Education Commission 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 o C per sec) means only very thin bits have been made. Amorphous (Glassy) Metals EMMAT101A Engineering Materials and Processes

31 TAFE NSW -Technical and Further Education Commission 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. From TLP: Casting, dev2008/casting/casting_other.phphttp://www.doitpoms.ac.uk/tlplib- dev2008/casting/casting_other.php DoITPoMS, The University of Cambridge. EMMAT101A Engineering Materials and Processes Offline You Tube

32 TAFE NSW -Technical and Further Education Commission VIDEO: Crystals and Grain Structure Properties and Grain Structure BBC (1973) EMMAT101A Engineering Materials and Processes What is a grain? Recrystalisation

33 TAFE NSW -Technical and Further Education Commission EMMAT101A Engineering Materials and Processes Graphical comparison of materials properties. Wikipedia: Materials properties Online Properties Resources. Lattice Structure BCC, FCC,HCP Metal Grains and processing DoITPoMS: Dissemination of IT for the Promotion of Materials Science

34 TAFE NSW -Technical and Further Education Commission 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 EMMAT101A Engineering Materials and Processes

35 TAFE NSW -Technical and Further Education Commission QUESTIONS Callister: Ch3 (Mostly about calculating atomic packing factors - too esoteric) Moodle XML: Some questions in Classification and Steel 1.Define all the glossary terms. 2.What is the difference between atomic structure and crystal structure? 3.What is a dendrite? 4.Explain why ice floats. 5.As a pure metal constantly cools, what happens to the temperature at the melting point? Explain. 6.Determine the coordination number (number of direct neighbours) for BCC, FCC and HCP lattice packing. 7.As pure iron cools, at 910 o C it switches from ….. lattice to ….. 8.Explain why grain size is important in engineering metals, and which is best. EMMAT101A Engineering Materials and Processes


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