Materials M2

3 Lecture focus Reproduced from “Materials and Man’s Needs”, National Academy of Sciences, Washington D.C., 1974.

6 Alloy Development and Phase Equilibrium Diagrams (Ashby GL2 1-27) Phase Equilibrium Diagrams Alloy composition (concentration) – an alloy’s composition is described by presenting the concentration of each component in weight% (or atomic%) Alloy Constitution – described by: • Phases present • Weight fraction of each phase • Composition of each phase Phase is a portion/region of material that has uniform physical and chemical characteristics e.g., • water, brine, ice • pure iron, pure copper • solid solution of Zn in Cu • solid solution of C in Fe M2

9 Alloy Development and Phase Equilibrium Diagrams By understanding the concepts of alloy phase equilibrium diagrams and being able to read and extract data, important information about the specific alloys can be obtained. For example, the alloy constitution in terms of phase compositions and proportions can be determined, and these have a direct influence on the properties of the alloy. M2

12 Simple to Complex Phase Equilibrium Diagrams The Copper-Zinc binary phase diagram (Brass) is another example of a complex phase diagram which shows many invariant reactions. Two common alloy compositions are shown, Cartridge Brass ( red ) which is Cu- 30 wt % Zn, and Muntz Metal ( green ) Cu-40 wt % Zn.

15 Case Study – Precipitation Hardening in Aircraft Alloys Complete dissolution of 2 nd phase Supersaturated solid solution formed Fine dispersion of 2 nd phase produced Precipitation or Age Hardening Heat Treatment

18 Learning Outcome Check 2 ❑ What is complete solid solubility? ❑ What is partial solid solubility? ❑ What is a dual-phase (duplex) alloy? ❑ Why might a duplex alloy be stronger than a single phase alloy? ❑ What process is used to create precipitation hardening?

21 Case Study – Heat Treatment of Steels Five basic heat treatments for steel Full Annealing Heat to the austenitic range (~900°C) and allow a very slow cooling rate Process Annealing Heat to below the austenitic range (~650°C) and allow a very slow cooling rate Normalising Heat to the austenitic range (~900°C) and allow an intermediate cooling rate (cool in air) Hardening Heat to the austenitic range (~900°C) and allow a rapid cooling rate (quench in water) Forming very hard metastable phase - Martensite Tempering After the hardening treatment above, re- heat to between discrete temperature between 250° - 600°C for controlled period of time

22 Learning Outcome Check 3 ❑ Iron is an allotropic element. What does this mean? ❑ What is the difference between ferrite and austenite? ❑ What is - a eutectic? - a eutectoid? - the eutectoid phase in the iron-carbon system called? ❑ Describe the basic heat treatment required to create martensite.

23 Non Metals • Covalent Ceramics (Si, diamond): Motion difficult - directional (angular) bonding • Metals (Cu, Al): Dislocation motion easiest - non-directional bonding - close-packed directions for slip electron cloud ion cores + + + + + + + + + + + + + + + + + + + + + + + + • Ionic Ceramics ( NaCl): Motion difficult - need to avoid nearest neighbours of like sign (- and +) + + + + + + + + + + + - - - - - - - - - - Review of bond types

24 Non Metals Polymers (Plastics) • Classification • Thermoplastic (Linear and non‐ crosslinked) • Thermosetting (‘Resins’) ( crosslinked or networked) • Elastomers (Rubbers) (lightly crosslinked, coiled and amorphous) • Natural – e.g. starch/cellulose made of sugar molecules thermoset thermoplastic elastomer Mechanical properties • Fracture strengths of polymers ~ 10% of those for metals • Deformation strains for polymers > 1000% • – for most metals, deformation strains < 10% • Strain rate and temperature dependence • Other environmental conditions: Lab A

25 Non Metals - Polymers Factors affecting Mechanical Properties of Polymers • Chemical compositions • Molecular weight/length (degree of polymerisation) • Structure • Tacticity • Polymer additives, such fillers, plasticisers … • Temperature • Strain rate • Environments Callister Pg.314

26 Non Metals - Polymers Structure – Tacticity and Crystallinity Stereoregularity or Spatial Arrangement of R Units Along Chain Isotactic – all R groups on same side of chain Syndiotactic – R groups on alternating sides Atactic – Random R group arrangement For same polymer, iso- and syndio-tactic are stronger than atactic – why? – facilitates stacking, or crystallinity Crystallinity in Polymers Increasing crystallinity in polymers can lead to improved strength, modulus, stiffness/brittle, lowering toughness. Crystallinity in polymers is determined by the polymer structure; complexity and order It can be induced by deformation or during manufacturing . Liquid crystal polymers – crystal formation promoted in liquid state (“self‐reinforced” plastics), high strength at high temperature. Crystalline zone Amorphous zone

27 Non Metals - Polymers T g’ Temperature Effects (Callister Pg. 462) Melting temperature - T m Glass Transition Temperature – T g What factors affect T m and T g ? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by presence of • Polar groups or side groups • Bulky side groups • Chain double bonds and aromatic chain groups Regularity of repeat unit arrangements – affects T m only Modulus – Temperature Diagrams Temperature dependence of mechanical property is more “visible” due to low Tm/Tg of polymers compared to metals and ceramics, i.e., closer to ambient. Temperature dependent modulus used to describe “engineering strength” (viability) at varying temperatures. Descriptive terms on modulus diagrams • glassy • leathery (visco-elastic) • rubbery (chain entanglements) • viscous • breakdown P5

28 Learning Outcome Check 4 ❑ What is the difference between thermoplastics and thermosets? ❑ Describe two mechanisms by which a thermoplastic polymer can be made stronger. ❑ How can cross-linking be used to control the engineering properties of elastomers ? ❑ What is the difference between glass transition temperature and melting temperature?

29 Non Metals - Ceramics and Glasses Ceramics Compounds of metallic and non‐metallic elements, in which the interatomic bonding is ionic (predominant) or covalent. The atomic structure is ordered or crystalline. Ceramics encompass materials with highest hardness and melting point in nature – diamond . Most are element combinations with metals, non metals or metalloids • Ionic ceramics : Typically compounds of metal with non‐ metal e.g., MgO, Al 2 O 3 , ZrO 2 . • Covalent ceramics : Typically compounds of metalloid or non‐metals e.g. SiO 2 , SiC Glasses A combination of metallic and non‐metallic elements, in which the interatomic bonding is ionic or covalent. The atomic structure is random or amorphous (usually silicate based). Glass is a (inorganic*) product of fusion which has been cooled to a rigid condition without crystallising. Fused silica is SiO2 to which no impurities have been added Other common glasses contain impurity ions such as Na + , Ca 2+ , Al 3+ , and B 3+ (*Organic glasses do exist – e.g., Perspex (PMMA), polycarbonate. P5

30 Non Metals - Ceramics Mechanical Properties High values of Young’s Modulus Diamond approximately 3 × Alumina and Alumina approximately 2 × steels Low ductility, low or no tendency for plastic deformation due to the nature of the atomic bonding. Brittle nature , related to the presence of flaws limits “engineering” strength . The size and distribution of flaws significantly affect the strength of ceramics . Flaws difficult to detect: once fracture initiates it is catastrophic. The limitation is not on the average properties, but on the severity of the worst (i.e., largest) flaw. Material σ f ( Nm ‐2 ) Flaw Size (c/ μm ) Al 2 O 3 (crystalline) 7 × 10 8 0.8 Al 2 O 3 (sintered), 5% porosity 2.8 × 10 8 5.0 SiC (sintered), 5% porosity 1.7 x 10 8 7.0 Silica Glass 1 × 10 8 6.5 Griffith Crack Theory Weibull Statistics (Ashby Pg. 214 ) Weibull Modulus (M) M indicates how rapidly strength falls (confidence) approaching σ 0 . Low M – greater variability – low design strength. High M – more stability, more confidence. Vitreous ceramics: M ≈ 3 -5 Engineering ceramics: M ≈ 10 Metals: M ≈ 100

31 Non Metals – Ceramics and Glasses Thermal Shock • When material passes through temperature range, fracture can occur. • Due to stresses resulting from different shrinkage or expansion of surface layer and inner part on cooling or heating (poor conductivity). • Parameters affecting thermal shock • ∆T (K) • Coefficient of thermal expansion [CoE] (α) α(∆T) – strain in surface and S – elastic stress constant S α (∆T) ≥ σ f -> fracture For materials with lower α – better thermal shock resistance (TSR) – measured or expressed in °K Material CoE ( μm /m°K) TSR (K) Diamond 1.2 1000 Soda Glass 8.5 80 Alumina 8.5 150 Sialon 3.2 500 How do we toughen these materials? ➢ Ceramics – make composites ➢ Glasses – make composites and also process treatments - Tempering P5

32 Non Metals - Glasses T g’ Glass - actually a ‘supercooled liquid’ - not a solid Tg – Glass Transition Temperature (when viscosity is so high, the glass can be considered solid) Tg’ – Lower GTT – can be achieved by slower cooling rate – stabilised glass. Glass Formers T M (°C) Viscosity (Poise) SiO 2 1710 10 9 As 2 O 3 309 10 6 B 2 O 3 450 10 5 BeF 2 540 ˃ 10 6 GeO 2 1115 10 7 Compare to other materials H 2 O 0 0.02 Na 98 0.01 Zn 420 0.03 Fe 1535 0.07 Most commercial glasses based on silicates – SiO 2 Soda-lime (window) – 75 SiO 2 , 10 CaO, 15 Na 2 O Borosilicate (pyrex) – 80 SiO 2 , 15 B 2 O 3 , 5 Na 2 O Glass forming capabilities relate to viscosity values at/around the melting point (softening point). P5

33 Non Metals - Thermal Toughening of Glass Annealing: Heat Treating Glass Removes internal stresses caused by uneven cooling Tempering: puts surface of glass part into compression suppresses growth of cracks from surface scratches. sequence: before cooling initial cooling at room temp. compression tension compression cooler hot cooler hot Crack growth suppression Surfaces of glass are in compression but subsurface in tension. If compressive stress in surface penetrated glass fails catastrophically. Compression is produced to finished size/shape prior to process. Critical aspect : Low thermal conductivity of glass P5

34 Learning Outcome Check 5 ❑ What is the difference between ceramics and glasses? ❑ What is thermal shock? ❑ What parameter is modified to help control thermal shock resistance? ❑ How can heat treatment be used to control the engineering properties (toughness) of glass?

35 Non Metals - Toughening - by Producing Composite Materials Composite Materials Composite Materials effectively form the fourth classification of materials, and are essentially born of the first three classifications, namely metals and alloys, polymers and ceramics and glasses. A composite material can be made up of any combination(s) of the materials, within certain guidelines. Guidelines A composite material is a mixture of two or more distinct constituents or phases, whereby: Both constituents are present in reasonable proportions e.g., >5%. Constituent phases must have noticeably different properties. In man-made composites, the composite material is produced by intimate mixing and consolidation of the constituents (not by the development of one constituent from another within the process eg phase nucleation in metals and alloys). A viable composite material will have properties superior to those of the individual constituents (properties described by the law of mixtures). Components - Matrix and Reinforcement (‘fibre’) Continuous or discontinuous (oriented, random) Geometries Fibrous, Particulate, Structural and Natural Composite Classifications P5

36 Non Metals - Composite Materials History of Composites • Biblical references – mud/straw constructions • Natural composites – materials of construction – wood and bone. • Man- made composites in “modern” materials (1900’s → present). • Future – increased use of composite materials for • special & extraordinary functions in all industries • (aero, auto, medical, etc.). Ref: Budinski Man-Made Fibre Composites Essentially, a composite material reinforced with randomly shaped and oriented or randomly dispersed fibres. Most common examples ‐ GFRP (Fibre Glass) and CFRP (Carbon Fibre) Man-Made Particle Composites Essentially, a composite material reinforced with randomly shaped and randomly dispersed particles. Most common example – Concrete.

37 Non Metals - Composite Materials Law of Mixtures To describe and develop the properties of composite materials the law of mixtures is used. In terms of modulus, when two linear elastic solids of a different moduli are combined, composite moduli (in longitudinal direction) given by: E C = V f E f + V m E m E C – Modulus of composite E f – Modulus of reinforcement (fibre) E m – Modulus of matrix V f – Volume fraction of reinforcement V m – Volume fraction of matrix Note: V f = 1 – V m s F Y Adaptation of Law of Mixtures Variations on above raw formula apply for tensile strength , yield strength and for variations of reinforcement orientation. For tensile strength in a continuous fibre reinforced composite: s C = V f s f + V M M  F f = Fracture stress of fibre Y M = Yield stress of matrix Approximations on discontinuous and random orientations of fibres reduce first term of relationship: s P5

38 Case Study – Concrete Concrete essentially comprises: • cement paste - manufactured from clay and limestone (subjected to heat). • aggregate – sand, pebbles, rocks • water • pores How concrete is made: by mixing the above constituents in appropriate proportions Concrete formation undergoes 2-stages: • Plastic Stage – ease of deformation and forming into various shapes. • Formation of hard rigid structure – can withstand many severe environments. Cement paste allows the plastic stage and then forms the hard rigid phase. The constituents of cement paste are: SiO 2 + Al 2 O 3 + CaCO 3 →heat calcium silicate + calcium aluminate When water is added, hydration products form leading to setting and hardening. Four stages of the “setting” of cement occur.

39 Case Study - Concrete Cement Paste – Stage 1 1. Paste suspended in water water cement paste crystals Cement Paste – Stage 2 1. After a few minutes ● Hydration products eat into the cement crystals ● Needle-like hydration products Cement Paste – Stage 3 1. After a few hours ● Crystals now interlocking ● Joining together of cement paste crystals, hardening starts Cement Paste – Stage 4 1. After a few days ● Further development of needle crystals ● Excess water in pores drains away. Throughout process heat is generated and presence of water is essential

40 Case Study - Concrete Strength of Concrete Porosity is the main reason for the lack of strength a. Stress raisers source of cracks, hence material tends to be brittle (cemented paste is 50% stronger in compression than tension). b. Pores allow creep to occur (high rate of creep, this can reduce crack propagation). -causing non-elastic deformation under load - The closure of pores (voids), allows the “squeezing out” of water from the voids. Concrete is very strong in compression, but has very limited strength in tension To improve the tensile (including bending) response of concrete, for example, in the construction of bridges, the concrete has to be reinforced. There are three main methods - by the introduction of: • Reinforcing steel bar (rebar) • Prestressed rebar • Fibre reinforcement

41 Learning Outcome Check 6 ❑ What is the basic law used to describe the mechanical properties of composites? ❑ Give two functions of the matrix in a carbon fibre reinforced plastic (CFRP). ❑ What are cements? Name one example of such a material. ❑ What are the constituents of concrete? ❑ Give three methods of reinforcing concrete.