Materials: Alloys, Ceramics, Glasses and Composites
Resources, Processes & Materials Engineering
LECTURE Materials M2 Dr Garry Leadbeater
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Lecture focus
Reproduced from “Materials and Man’s Needs”, National Academy of Sciences, Washington D.C., 1974.
Lecture Outline
o Alloy development and Phase Equilibrium Diagrams
• Phaseequilibriumdiagrams
• Heattreatmentsandcasestudies–steelandaluminiumalloysinthetransportandaerospace industries
o Non-Metals
• Polymers–typesandproperties
• Ceramics and Glasses – types properties, limitations, Weibull statistics • Strengtheningandtoughening
• Composites–LawofMixtures,designofproperties
• Examples and case studies – GFRP, CFRP, concrete
Alloy Development and Phase Equilibrium Diagrams
Alloy Systems
Definition: a metallic alloy is a mixture of a metal with other metals or non- metals.
Alloy System – that which describes all possible permutations of alloys, phases, compositions and temperatures for mixtures of two or more elements.
usually represented by a phase equilibrium diagram.
Alloy components – the chemical elements that make up the alloy. Number of components described by type of alloy or alloy system, for instance, binary alloy – 2 components, ternary alloy – 3 components
taking only 45 of the most common metals, any combination of two give 890 binary systems.
since many commercial alloy systems contain many elements, engineers have very many systems available to them.
Alloy Development and Phase Equilibrium Diagrams ( L2 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: • Phasespresent
• Weightfractionofeachphase • Compositionofeachphase
Phase is a portion/region of material that has uniform physical and chemical characteristics
• water,brine,ice
• pureiron,purecopper
• solidsolutionofZninCu • solidsolutionofCinFe
Alloy Development and Phase Equilibrium Diagrams
Phase Equilibrium Diagrams
• Thevariousalloysystemsaregraphicallyrepresentedbydiagramsknowasequilibriumorphase diagrams.
• Basically,thediagramsarecoolingcurvesderivedunderequilibriumconditions,i.e.,underextremely slow heating and cooling conditions.
• Thediagramalsoindicatesthesolubilityofelementsineachotherandstructureofphasechanges, which occur for various alloy compositions.
• Theinformationgainedbytheunderstandingofthesediagramsisthereforeessentialformaterials engineers, particularly when considering heat treatment and structural aspects of alloys
Alloy Development and Phase Equilibrium Diagrams
Complete solid solubility
A substitutional solid solution is the only phase formed with this system. The cooling curves and the equilibrium diagram are shown in the figures above. The letters A and B represent the pure metals.
Between the liquidus and solidus lines there exists a two-phase region. Any alloy in this region will consist of a mixture of A and B in liquid form and a solid solution.
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.
Alloy Development and Phase Equilibrium Diagrams
For Alloy 7.5% B
To determine the relative amounts of the two phases in equilibrium at a specific temperature requires a vertical line to be drawn on the diagram at the particular composition. This is shown in the figure opposite.
At temperature t2
Two phases present – solid α and Liquid
Solid α Composition 4.5% B
Liquid Composition 14.5% B
ProportionofSolidα BC/AC=(14.5-7.5)/(14.5- 4.5)= 70%
Proportion of Liquid AB/AC= 30%
Learning Outcome Check 1
What does a Phase Equilibrium Diagram represent?
What are the parameters on the axes of the diagram?
– an alloy component?
– an alloy system? – a phase?
When describing an alloy system, what might be the constituents of the alloy?
What is the Lever Law?
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 (green) Cu-40 wt % Zn.
Simple to Complex Phase Equilibrium Diagrams
Partial solubilities and Multi-Phase Alloys
This is the Lead-Tin alloy system – Solder.
A heat treatment process known as age hardening or precipitation hardening can be applicable which will provide a strengthening effect.
Simple to Complex Phase Equilibrium Diagrams
Partial Solubilities and Multi-Phase Alloys
Using phase equilibrium diagrams to develop improved properties by heat treatments This is from the Ai-Cu alloy system, commonly used in aircraft manufacture
Typical two phase microstructure
Case Study – Precipitation Hardening in Aircraft Alloys
Precipitation or Age Hardening Heat Treatment
Complete dissolution of 2nd phase
Supersaturated solid solution formed
Fine dispersion of 2nd phase produced
Case Study – Precipitation Hardening in Aircraft Alloys
Schematic representation of the precipitation hardening process and stages
(after Higgins: Engineering Metallurgy)
Case Study – Precipitation Hardening in Aircraft Alloys
Make the aircraft components from this condition alloy
W.F. Smith, Foundations of Materials Science and Engineering, McGraw‐Hill, Inc., 2nd Ed, , 1993.
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?
Case Study – Heat Treatment of Steels
Case Study – Heat Treatment of Steels
Iron – Carbon phase diagram The Steel part
α-ferrite – solid solution of C in BCC Fe Stable form of iron at room temperature. The maximum solubility of C is 0.022 wt% Transforms to FCC γ‐austenite at 912 °C
γ-austenite – solid solution of C in FCC Fe
The maximum solubility of C is 2.14 wt %. Transforms to BCC δ‐ferrite at 1394°C
Is not stable below the eutectic temperature (723 °C) unless stabilized with alloying additions
δ-ferrite – solid solution of C in BCC Fe The same structure as α‐ferrite Stable only at high T, above 1394 °C Melts at 1538 °C
cementite (iron carbide or Fe3C )
An intermetallic compound
of iron and carbon with the chemical formula Fe3C. C content is around 6.67%.
It is metastable, it remains as a compound indefinitely at room T, but decomposes
(very slowly, within several years)
into α‐Fe and C (graphite) at 650 ‐ 700 °C.
It is hard and brittle
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)
Heat to the austenitic range (~900°C) and allow a rapid cooling rate (quench in water) Forming very hard metastable phase – Martensite
After the hardening treatment above, re- heat to between discrete temperature between 250° – 600°C for controlled period of time
Learning Outcome Check 3
Iron is an allotropic element. What does this mean?
What is the difference between ferrite and austenite?
– a eutectic?
– a eutectoid?
– the eutectoid phase in the iron-carbon system called?
Describe the basic heat treatment required to create martensite.
Non Metals
Review of bond types
• Metals (Cu, Al):
Dislocation motion easiest – non-directional bonding – close-packed directions
• Covalent Ceramics
(Si, diamond): Motion difficult
– directional (angular) bonding
• Ionic Ceramics (NaCl): Motion difficult
– need to avoid nearest
neighbours of like sign (- and +)
electron cloud
++++++++ ++++++++
+ – -+-+-+-
– + +-+-+-+
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
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:
thermoplastic
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
Non Metals - Polymers
Structure – Tacticity and or Spatial Arrangement of R Units Along Chain Atactic – Random R group arrangement Isotactic – all R groups on same side of chain Syndiotactic – R groups on alternating sides
For same polymer, iso- and syndio-tactic are stronger than atactic – why? – facilitates stacking, or crystallinity
Crystallinity in Polymers
Crystalline zone
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.
Amorphous zone
Non Metals - Polymers
Temperature Effects
Melting temperature - Tm
Glass Transition Temperature – Tg
What factors affect Tm and Tg?
Both Tm and Tg 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 Tm 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)
• breakdown
Learning Outcome Check 4
What is the difference between thermoplastics and thermosets?
Describe two mechanisms by which a thermoplastic polymer can be made
How can cross-linking be used to control the engineering properties of
elastomers?
What is the difference between glass transition temperature and melting temperature?
Non Metals - Ceramics and Glasses
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, Al2O3, ZrO2. • Covalent ceramics : Typically compounds of metalloid or non-metals e.g. SiO2, SiC
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+, Ca2+, Al3+, and B3+
(*Organic glasses do exist – e.g., Perspex (PMMA), polycarbonate.
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.
Weibull Statistics ( . 214)
Flaw Size (c/μm)
Al2O3 (crystalline)
Al2O3 (sintered), 5% porosity
SiC (sintered), 5% porosity
Silica Glass
Weibull Modulus (M)
M indicates how rapidly strength falls (confidence) approaching σ0.
Low M – greater variability – low design strength.
High M – more stability, more confidence.
Vitreous ceramics:
Engineering ceramics:
Non Metals – Ceramics and Glasses
Thermal Shock
• Whenmaterialpassesthroughtemperaturerange,fracturecanoccur. • Duetostressesresultingfromdifferentshrinkageorexpansionof
surface layer and inner part on cooling or heating (poor conductivity). • Parametersaffectingthermalshock
• Coefficient of thermal expansion [CoE] (α)
α(∆T) – strain in surface and S – elastic stress constant
Sα(∆T)≥σf -> fracture
For materials with lower α – better thermal shock resistance (TSR) – measured or expressed in °K
CoE (μm/m°K)
How do we toughen these materials?
Ceramics – make composites
Glasses – make composites and also process treatments – Tempering
Non Metals – Glasses
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.
SiO2 1710 109 As2O3 309 106 B2O3 450 105
BeF2 540 ˃106 Tg’ GeO2 1115 107
Glass Formers
Viscosity (Poise)
Compare to other materials
H2O 0 0.02 Na 98 0.01 Zn 420 0.03 Fe 1535 0.07
Most commercial glasses based on silicates – SiO2
Soda-lime (window) – 75 SiO2, 10 CaO, 15 Na2O Borosilicate (pyrex) – 80 SiO2, 15 B2O3, 5 Na2O
Glass forming capabilities relate to viscosity values at/around the melting point (softening point).
Non Metals – Thermal Toughening of Glass
Heat Treating Glass
Annealing:
Removes internal stresses caused by uneven cooling
Tempering:
puts surface of glass part into compression suppresses growth of cracks from surface scratches.
Critical aspect :
Low thermal conductivity of glass
before cooling
initial cooling
at room temp.
compression
compression
Surfaces of glass are in compression but subsurface in tension.
Crack growth suppression
If compressive stress in surface penetrated glass fails catastrophically.
Compression is produced to finished size/shape prior to process.
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?
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).
Composite Classifications
Components – Matrix and Reinforcement (‘fibre’) Continuous or discontinuous (oriented, random)
Geometries
Fibrous, Particulate, Structural and Natural
Non Metals – Composite Materials
History of Composites
•Biblical references – mud/straw constructions •Natural composites – materials of construction – wood
•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.).
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.
Ref: Budinski
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: EC = + Vm Em
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: σ =V σF +V σY
E – Modulus of composite
EC – Modulus of reinforcement (fibre) f
Em – – Volume fraction of reinforcement Vm – Volume fraction of matrix
Note: Vf = 1 – Vm
= Fracture stress of fibre
= Yield stress of matrix
Approximations on discontinuous and random orientations of fibres reduce first term of relationship:
Case Study – Concrete
Concrete essentially comprises:
•cement paste – manufactured from clay and limestone (subjected to heat).
•aggregate – sand, pebbles, rocks
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:
SiO2 + Al2O3 + CaCO3 →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.
Case Study – Concrete
Cement Paste – Stage 1
1.Paste suspended in 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
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