TAFE NSW -Technical and Further Education Commission ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies) Lecture 8 – Alloys

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

TAFE NSW -Technical and Further Education Commission Alloys (Higgins 8.1) EMMAT101A Engineering Materials and Processes Note: Text (Higgins) is followed very closely in this chapter. An alloy is a mixture of two or more metals. The reason is usually to improve the properties of either metal. Often the alloy has properties not possessed by either of the metals in the pure state. Alloys are usually stronger than the original metals. (better for engineering) Alloys usually have a lower melting point than pure metals. (better for processing) Pure metals are usually better at conducting electricity (and heat). Another reason to alloy is to lower the cost – e.g. adding silver to gold.

TAFE NSW -Technical and Further Education Commission Solutions (Higgins 8.1.1) EMMAT101A Engineering Materials and Processes Solid solution is same idea as liquid solution – whether the 2 substances (or metals) will mix. Water and alcohol mix in any ratio. (soluble) Oil and water do not mix. (insoluble). Oil just floats on top. Likewise for molten metals; Molten lead and molten tin are soluble. (makes solder) Molten lead and molten zinc insoluble. Zinc just floats on top. Sn 60 Pb 40 solder Wikipedia

TAFE NSW -Technical and Further Education Commission Eutectics (Higgins 8.2) EMMAT101A Engineering Materials and Processes If a molten alloy of this composition is allowed to cool, it will remain completely liquid until the temperature falls to 140°C, when it will solidify by forming alternating thin layers of pure cadmium and pure bismuth (Figure 8.2) until solidification is complete. Higgins Figure 8.1 The freezing-points (melting- points) of both bismuth and cadmium are LOWER when alloyed to the other. A minimum freezing-point - or 'eutectic point' - is produced.

TAFE NSW -Technical and Further Education Commission Eutectics (Higgins 8.2) EMMAT101A Engineering Materials and Processes Higgins Figure 8.2 At a magnification of about ten million times (way too much for optical microscope), the arrangement of atoms of cadmium and bismuth would look something like that in the left-hand part of the diagram, except that the bands in the eutectic would each be many thousand atoms in width. Lamination makes the structure strong (like plywood). If one metal is ductile and the other strong, the eutectic structure tends to get both strength and toughness. (Ideal in most engineering applications)

TAFE NSW -Technical and Further Education Commission Eutectics EMMAT101A Engineering Materials and Processes Pearlite: Copyright unknown: A photo of a eutectic structure of steel known as pearlite. The name pearlite comes from the way it reflects light (like a pearl), due to the very fine bands. The white is iron, the black is iron-carbide (cementite), so not as simple as Cd-Bi eutectic.

TAFE NSW -Technical and Further Education Commission Solid Solutions (Higgins 8.3) EMMAT101A Engineering Materials and Processes As alloy cools, higher Melting Point metal forms dendrites first. Higgins Figure 8.3 The variations in composition in a cored solid solution. The coring can be dispersed by annealing.

TAFE NSW -Technical and Further Education Commission Solid Solutions (Higgins 8.3) EMMAT101A Engineering Materials and Processes Dendritic structure Higgins Figure 8.5 The dendritic structure brass. This is cast brass at a magnification of x39. The dendrites would not be visible were it not for the coring of the solid solution.

TAFE NSW -Technical and Further Education Commission Substitutional solid solution (Higgins 8.3.2) EMMAT101A Engineering Materials and Processes A substitutional solid solution, is where atoms of one metal been substituted for atoms of the other. This works best when the 2 atoms are nearly the same size: Complete solubility by substitution: E.g. copper/nickel, silver/gold, chromium/iron, Most alloys have limited solubility by substitution: E.g. copper/tin, copper/zinc, copper/aluminium, aluminium/magnesium. Higgins Figure 8.6 (i) A substitutional solid solution

TAFE NSW -Technical and Further Education Commission Interstitial solid solution (Higgins 8.3.3) EMMAT101A Engineering Materials and Processes An interstitial solid solution, is where atoms of one metal squeeze between the atoms of the other. This requires a big size difference in the atoms. The most famous example is carbon (small atom) sqeezing into the FCC structure of hot iron – which is how we get heat treatable steel. Heating the iron to promote diffusion of carbon into the FCC lattice is called carburising. Higgins Figure 8.6 (ii) An interstitial solid solution

TAFE NSW -Technical and Further Education Commission Diffusion The two main diffusion methods are: Vacancy Diffusion: A new atom works it’s way into the metallic lattice by taking vacant positions. Interstitial Diffusion: A new (small) atom migrates between atoms. Higher temperatures increase the rate of diffusion. Stress encourages diffusion by opening up more “gaps”. Diffusion of new type of atom into a metallic lattice. EMMAT101A Engineering Materials and Processes

TAFE NSW -Technical and Further Education Commission Diffusion (Higgins 8.3.5) EMMAT101A Engineering Materials and Processes Solid solutions give the best strength, ductility and toughness, so most useful metallic alloys are basically solid solution in structure. This is because distortions in the crystal structure hinder slip (increasing the yield strength). Slip is still possible (ductility) but at a higher stress (now it is toughness) Figure 8.8 Crystal lattice distortions caused by the presence of solute atoms: (i) a large substitutional atom, (ii) a small substitutional atom, (iii) an interstitial atom. In each case, the distortion produced will oppose the passage of a dislocation through the system.

TAFE NSW -Technical and Further Education Commission Slip… (again) Higgins Animation of slip by dislocation glide. Dislocation glide allows plastic deformation to occur at a much lower stress than would be required to move a whole plane of atoms at once. A perfect crystal (in theory) would be 1000 times stronger. dislocation_glide.php Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non- Commercial-Share Alike licence / / You Tube Offline (mp4) EMMAT101A Engineering Materials and Processes

TAFE NSW -Technical and Further Education Commission Diffusion: Carburising Mild steel cannot be hardened unless there is carbon in the lattice. Adding carbon to steel is called carburising. There are several ways to do this, but the oldest and simplest is to heat the mild steel in the presence of carbon (charcoal) – for a long time at high temperature. This allows carbon to diffuse into the surface for a mm or so. Pack Carburising. A few minutes excerpt from BBC Video Heat Treatment: Heat treatment [videorecording] / producer Brian Davies. [B.B.C.], Video: Discusses the use of heat which changes the properties of metals. Outlines different techiques including hardening, tempering, annealing, normalising as well as a non-heat process, coldworking. EMMAT101A Engineering Materials and Processes

TAFE NSW -Technical and Further Education Commission Diffusion to Dislocations Dislocation slip can be hindered: Here, an interstitial atom migrates into the stress zone, hindering dislocations. Otherwise the slip continues to the grain boundary, which distorts the grain, hence plastic deformation. Dislocation and the effect of migration of interstitial atoms You Tube Offline (mp4) EMMAT101A Engineering Materials and Processes

TAFE NSW -Technical and Further Education Commission Intermetallic Compounds (Higgins 8.4) EMMAT101A Engineering Materials and Processes Metallic oxides, sulphides and chlorides are ionic compounds formed by the attraction between positive and negative ions. Sometimes two metals when melted together will combine to form a chemical compound called an intermetallic compound, where one of the two metals has strongly positive ions and the other weakly positive ions. A normal solid solution acts similar to the parent metals, but intermetallic compounds are dramatically different (usually brittle) and have a fixed chemical formula. Tends to act like little bits of ceramic mixed into the metal. Cementite is an intermetallic compound in steel alloys with the chemical formula Fe 3 C. This phase has a specific chemical formula, unlike most phases which have ranges of chemical composition. Cementite is hard and brittle. IMAGE: Journal of Molecular Catalysis A: Chemical Volume 269, Issues 1–2, 18 May 2007, Pages 169–178Journal of Molecular Catalysis A: Chemical

TAFE NSW -Technical and Further Education Commission Alloys: Summary (Higgins 8.5) EMMAT101A Engineering Materials and Processes A phase is a chemically stable single homogeneous constituent in an alloy. A phase may be a solid solution, an intermetallic compound or a pure metal. The three main types of solid phases are; Solid solutions are subsitutional (similar atom size) or interstitial (dissimilar size atoms) larger atoms of the other metal. Solid solutions are more useful in engineering because slip is hindered, but without eliminating ductility. Intermetallic compounds are formed by chemical combinations, and like ceramic, tend to be hard and brittle. Eutectics are formed when two metals, soluble when molten become insoluble when in solid. They form alternate layers or bands of each metal. This occurs at a fixed temperature, lower than the melting-point of either of the two pure metals. Eutectics can also form with layers being solid solutions, or even an intermetallic compound.

TAFE NSW -Technical and Further Education Commission Alloys: Summary 2 (Higgins 8.5) EMMAT101A Engineering Materials and Processes Engineering alloys can combine six or even more metals, although there is usually a dominant metal (solvent) into which the lesser metals (solutes) dissolve. E.g. Thus, the stainless steel 347S17 is composed almost entirely of a solid solution in which iron has dissolved 18 per cent chromium, 10 per cent nickel, 1 per cent niobium and 0.8 per cent manganese - a residual 0.04 per cent carbon existing as a few scattered undissolved carbide particles. 347S17: Austenitic chromium-nickel stainless steel with moderate strength and niobium stabilised (347 type) with moderate corrosion resistance. For aerospace and defence components including weldments.

TAFE NSW -Technical and Further Education Commission EMMAT101A Engineering Materials and Processes Graphical comparison of materials properties. Wikipedia: Materials properties Online Properties Resources. Forming: Forging, Rolling, Extrusion, Machining DoITPoMS: Dissemination of IT for the Promotion of Materials Science

TAFE NSW -Technical and Further Education Commission GLOSSARY Alloy Binary Alloy Soluble Solvent Solute Substitutional Interstitial Carburising Phase Intermetallic Crystal Grain Eutectic Laminated grain structure Phase Dentritic structure EMMAT101A Engineering Materials and Processes Cementite Pearlite

TAFE NSW -Technical and Further Education Commission QUESTIONS Callister: NA Moodle XML: Processing 1.Define all the glossary terms. 2.Explain why alloys are usually more useful in engineering than pure metals. 3.Describe two ways that lattice distortion can occur with a binary alloy. 4.How does lattice distortion increase strength? 5.The intermetallic compound Cementite is deliberately employed in steel. Why does this hard and brittle material (which is really a ceramic) not destroy the properties of the steel? 6.In carburising, what kind of diffusion is taking place? 7.(Research) High speed steel AS 1239 grade M2 contains 0.85% carbon, 4.0% chromium, 5.0% molybdenum, 6.0% tungsten and 2.0% vanadium. What is the solvent? Which are the solutes? Research high speed steel and list the main reasons each of these solute metals is added. EMMAT101A Engineering Materials and Processes