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Engineering Materials and Processes Lecture 4 – Crystal Structure of Metals Grains after acid etching: Wikipedia Crystal Structure of Metals Reference Text Section Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, Ch 4 5th ed, Butterworth Heinemann 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 Ch 3 Engineering: An Introduction, 8th Ed, Wiley, New York. 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 ° Mercury vapour streetlights: theage.com.au boils at 5930 C. Engineering Materials and Processes 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 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 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 Engineering Materials and Processes 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. Lava Solidification: http://www.geol.umd.edu Engineering Materials and Processes Shrinkage Contraction Metal (%) Atoms in a solid fit closer together than in Grey cast iron 0.7 to 1.05 a liquid, so shrinkage occurs. White cast iron 2.1 A few substances expand when they Malleable iron 1.5 solidify because they have a very loose Steel 2.0 packing arrangement. E.g. Water (ice), Brass 1.4 and Silicon. Alluminium 1.8 Aluminium alloys 1.3 to 1.6 Bronze 1.05 to 2.1 Magnesium 1.8 Zinc 2.5 Manganese steel 2.6 Based on; http://www.calculatoredge.com Titanic iceberg (maybe): Wikipedia Engineering Materials and Processes 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 Engineering Materials and Processes 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). http://www.chem.ufl.edu/~itl/2045/lectures/lec_h.html Engineering Materials and Processes 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 Engineering Materials and Processes Crystals The unit structures of the 3 Lattice structures: Part 1 crystal structures are a bit thechemprofessor misleading. Local (mp4) They are different SIZES! The packing difference is only a few %. Crystals A very 3 dimensional Univ of Florida problem, so best explained Local in 3D video. Engineering Materials and Processes Allotropy of Iron Iron is polymorphic - it forms more than one crystalline form (allotropes of Iron). The two main crystal structures are; BCC (α-iron) FCC (γ-iron) above 910°C. BCC body-centred cubic FCC above 910°C Iron (α-iron) (γ-iron). Engineering Materials and Processes 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. Cracking of steel due to quenching Engineering Materials and Processes 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 BCC Unit: Higgins Fig 4.3 structure much like a snowflake (except the metal is forming in liquid, not a cloud of droplets). The branched crystal is called a Snowflake: Wikipedia 'dendrite‘ (Greek for tree). Higgins Fig 4.4 Engineering Materials and Processes Dendrite of Silver: Wikipedia Engineering Materials and Processes 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 Grains after orientations acid etching: Wikipedia Wikipedia Engineering Materials and Processes 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. www.spaceflight.esa.int 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 www.spaceflight.esa.int Engineering Materials and Processes 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. www.spaceflight.esa.int Engineering Materials and Processes 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 You Tube Offline (mp4) raft rearranges by dislocation motion. 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 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 Engineering Materials and Processes 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 Grain size vsd yield strength. Low C steel. quenching of Carbon steel. Quenching produces a different type W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers, p63-71. 1985. of grain - Martensite. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5 Engineering Materials and Processes Grain Growth If solid metal is above a certain temperature (recrystalisation), certain grains will grow at the expense of their neighbours.
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